,MICROCOMPUTER CONTROL OF A PNEUMATIC \

BLOOD PUMP, ;

by

Douglas Eugene\, Hux,1

Thesis submitted to the Graduate Faculty of the

Virginia Polytechnic Institute and State University

in partial fulfillment of the requirements for the degree of

MASTER OF SCIENCE

in

Mechanical Engineering

Endorsements:

L ~ J. f/Arp f cf irrnan

P. R. Rony H. H. Mabie

June, 1978

Blacksburg, Virginia

ACKNOWLEDGEMENTS

I would like to express my sincere appreciation to the

following people who assisted in this research:

Dr. Leon Arp, who served as committee chairman and

motivated my best efforts,

Dr. Peter Rony, for his patient instruction in the art of

microcomputer interfacing and programming,

Dr. Hamilton Mabie, for serving on the graduate committee,

Dr. Christopher Titus, of Tychon, Inc., for his generous

loan of the MicroDesigner, and,

My parents, for their patience and

encouragement throughout this graduate work.

This thesis is dedicated to

who sparked my interest in engineering twelve years ago, and hasn't

allowed it to fade.

ii

TABLE OF CONTENTS

Acknowledgements

List of Figures

List of Tables

I. Introduction

II. Control Strategy Overview The 8080 Microcomputer The Blood Pump The Flow Transducer System Operation

III. Implementation • Overview Interface Hardware and Software

Device Select Pulses Keyboard Displays Valve Drive Analog-to-Digital Converter

System Integration Closed-Loop Control

IV. Calibration and Tuning Overview Calibration Tuning and Response

V. Conclusions and Reconnnendations

References Cited

Appendix

Vita

A. B.

Abstract

Software Listing Construction Details

iii

ii

iv

• vii

1

3

3 3 9

13 13

14 14

. 15 • 15

15 23 23

• 34 • 43 • 64

• 66

• 66 • 66 • 70

• 91

• 96

98 136

144

Figure

1

2

3

4

5

6

7a

7b

8

9a

9b

10

11

12

13

14

15

16

17

18

19

20

LIST OF FIGURES

Blood Pump Monitoring and Control System •

8080 Computer Organization .

Page

4

6

The Arp Blood Pump Chamber

Details of Blood Pump Chamber

. . . . . . . . 10

Generation of Input Device Select Pulses .

Generation of Output Device Select Pulses

Keyboard Encoder • •

Keyboard Interface

Keyboard Service Routine .

Seven Segment Display

Inversion of Display Select Pulses • •

Interrupt Instruction Interfacing

Interrupt-Driven Valve Sequence

Eight-Bit Programmable Interval Timer

System Clock . • • • • .

Valve Drive Circuitry

Instrumentation Amplifier .••...•.

Scaling Amplifier . • . • • • . . • • •

11

. . . . 16

17

18

20

22

24

25

27

29

30

32

33

35

• • • • 36

Averaging Circuit • • • • • 39

Analog-to-Digital Conversion Strategy

Digital-to-Analog Converter

Analog-to-Digital Converter Interface

iv

40

41

42

LIST OF FIGURES (cont.)

Figure

21 Overall Controller Operation

22 Initialization Routine • . . •

23 Service Routine for Pumping Rate Key .

24 Service Routine for Per Cent Systolic Time Key

Service Routine for Set Point Key

Service Routine for Numeric Key

Service Routine for Enter Key

Pumping Rate Error Check . • .

Per Cent Systolic Time Error Check

Set Point Check

Service Routine for Go Key .

Part of Go Routine • • .

Service Routine for Stop Key

. .

25

26

27a

27b

27c

27d

28a

28b

29

30a

30b

31

32

33

34

35

36a

36b

Service Routine for Analog-to-Digital Converter ••

Part of Analog-to-Digital Conversion Routine .

Closed-Loop Control Indicator

Blood Pump Flow Circuit

Top View of Interface Board

Open-Loop System Response . . . . . . . . . . Performance Criteria . . . . . . . Response to Large Set Point Changes . . . . . Response to Small Set Point Changes

v

. .

. .

. .

. .

. .

. .

. .

1.'age

44

45

47

48

49

51

52

53

54

55

58

59

61

62

63

65

67

68

73

76

79

80

LIST OF FIGURES (cont.)

Figure Page

37a Response to Large Pumping Rate Changes . . . . . . 82 37b Response to Small Pumping Rate Changes 83

38a Response to Large Load Changes 84 . . . . . 38b Response to Small Load Changes 85 . . . . . 39a Response to Large Set Point and Small Pumping Rate

Changes . . . . . . . . . . . 87 . . . . . . . . . . 39b Response to Small Set Point and Large Pumping Rate

Changes . . . . . . . . . . . . . . . . . . . . . . . 88 39c Response to Large Set Point and Large Pumping Rate

Changes . . . . . . . . . . . . . . . . . . . . . . . 89 39d Response to Small Set Point and Small Pumping Rate

Changes . . . . . . . . . . . . . . 90 Bl Display Board ,138

B2 Interface Board . . . . . .141 B3 Interface Socket . . . . . .142 B4 Keypad Cabinet . . . . . . . . .143

vi

Table

1

2

3

Bl

LIST OF TABLES

Key Codes Available to Keyboard Service Routine • • . 21

Parameter Ranges

Model Parameters at Various Pumping Rates

MicroDesigner @Bus Configuration • • • •

vii

• • 56

74

139

I. INTRODUCTION

Approximately one-half of one per cent of the infants born each

year succumb to some form of respiratory distress (1). Respiratory

distress results in an inadequate transfer of carbon dioxide from

the blood and oxygen to the blood which initiates a reversion of the

cardiopulmonary circulation to that of the fetal state.

The survival rate of those infants afflicted was raised from 63%

to 86% by the development of the Arp Infant Respirator, which supplies

a carefully controlled air-oxygen mixture to the patient (2, 3).

It is possible that the survival rate could be raised further by

supporting some of these infants with either a partial or total

cardiopulmonary bypass during the critical days following birth (4).

Dr. Leon J. Arp, of the Mechanical Engineering Department of

Virginia Polytechnic Institute and State University, is currently

developing an infant sized perfusion system for this application.

The unit consists of a tubular membrane blood oxygenator, a

pneumatically operated, diaphragm-type blood pump, and associated

monitoring instrumentation and drive circuitry. Both the oxygenator

and the pump have been designed to minimize the lethal and sub-lethal

blood damage associated with long term cardiopulmonary perfusion.

An extended cardiopulmonary bypass, with perfusion times measured

in terms of days instead of hours, would require that special

consideration be given to the manner in which blood was pumped to the

patient. It has been generally accepted that a pulsatile pumping

1

2

action improves kidney function over that observed under steady

flow conditions (5). In addition, the blood flow rate must be main-

tained at a level which would insure that an adequate supply of oxygen

would be delivered to the tissues by the blood. This thesis describes

the design of a device to monitor and control the operation of the

Arp diaphragm-type blood pump.

The output of a diaphragm-type pump is determined by the product

of its pumping rate and stroke volume. Norman, et al (6, 7)

developed a control system which used capacitance measuring techniques

to determine when a preset stroke volume was reached and ejection of

the blood was to begin. Pumping rate, and hence the pump output, were

thereby automatically determined by the pump inflow. Average pump

output was obtained as a derived quantity.

The control system described in this thes~f.~~mo~~t

II. CONTROL STRATEGY

Overview

A schematic representation of the monitoring and control system

developed for the Arp diaphragm-type blood pump is shown in Fig. 1.

The microcomputer synchronizes data entry from the keyboard, the r presentation and updating of display information, \,the analog-to-

digital conversion) and (input o~--~!~~--'=~~~--~:1.~~~aEi~~--_!.~om.E.~~ electromagnetic flow meter, \)and the timing oV events during each

-·--•-·•••--'-'''"····~··"~· .. ··c·.7 , .. pump cycle. The basic operation of the microcomputer and the hard-

ware and software development techniques used will be discussed next,

followed by a description of the Arp bloocy"p-~p) and the electro-\,_ ___ /

magnetic :j:low meter!. "-~, ... ,.__ -- / . . ____ ,/,/

The specific operation of the blood pump "'1.;-::··--·-·

control sys~em will then be presented. __ ---......._ _______ .. /

The 8080 Microcomputer

A major advantage in using a microcomputer to control blood

pump operation is the reduction in the number of hardware devices

required and the concomitant increase in reliability obtained. The

computer does require hardware to operate; however, this hardware

simply provides data to and accepts data from external devices such

as the displays and the keyboard. The software Erogram then ----·-------- -·-·--manipulates this data and directs all pump operations.

A symbolic representation of an 8080 based microcomputer used

in this design is reproduced from The Bu;book(IDIII, by Rony, Larsen,

3

KEYBOARD

8080 MICRO-

COMPUTER ,...,-----·-·--·----....

/ " ( : ,y~Y-~---DRIVE RCUITRY

.i:-

A/D &~?

FLOW CONVERTER TRANSDUCER

FIG. I BLOOD PUMP MONITORING AND CONTROL SYSTEM

5

and Titus (8), and presented as Fig. 2. Program instructions are

retrieved from memory and sequentially executed by the central

processing unit (CPU). As the microcomputer was supplied complete,

including m~9_;-y, this discussion will concen~ra.E_~- o~--~-~=--t:.=-~h-~-~9ues /,...------ -~---....,., -. ____ ,,,. used to( inJerfac~)the microcomputer to the real world. Of particular

~....:...--;;:::-.:::::::-::.~:-~-~::~-::;::·:.:.".'.::-:::~::.::-·:--~---··-----------··~·-.---:··-···"·::::::::.:.::-:-... ··-· ·: - ,-:-·:~:-~-~-,:.~.-~::::.::;-

concern to this thesis, then, are the output and input processes and ----···-·-----·---•-..M.•~---~ .. ~-~-····-~---··--•-·' .... --~--~·-·~,_,, .... , .. _-.C7~·-·-~·-·,,.~-·~ ...... -~-·,_,.,_,_.,,.,.

the s~rvicing of interrupt requests.

The output process of an 8080 based microcomputer can be

sununarized as follows. tWlien an OUT instruction is encountered,

the contents of the accumulator are placed on the bidirectional

¥t~ -b~ and ~1:_ eight-b~_: __ C!~~e, used to select one of 256 possible , ___ ~ -----------------

output devices, is plac;d-~~=~~~--a~~~~-ss~~~s. The eight-bit device '---------- . ../ --* code is decoded and gated with the OUT ---·-·--------- --.... "·---. -

!lnic.fiie--output d~~.:f..-~E? .~E?:I..~_i;.t puls~~ The ' ·----- .. ----------------·-=::..:::-.:::...-:=---.'O'."!;"'~-:.::·.::':::':.::-::-::~ .. -

function pulse to produce a --------------OU tpu t d~~Tc:;-ser&t:-~~'> ---------------

is used to _ _!_i!,;sLI11~ . ......dat.a~-b.y.te.av~_gable on the bidirectional data ~------- - ··~------ ·----- ._ .. __ ----

bus into an external device. ,-------------·--·--·-----~-,-- -·--·---... -~-·--

The input process of an 8080 based microcomputer is analogous

to the output process described above. When an IN instruction is ------------

EXTERNAL DEVICE 1/0

INTERRUPT SERVICE

8 BITS DATA INPUT

8 BITS DATA OUTPUT

INTERRUPT

INTERRUPT ACKNOWLEDGE

FIG. 2

MEMORY UP TO 65,536

8 BIT WORDS

~ ~I I~

- a: a:: 3: 0 ........ m ::E Cl ILi

7

The ~~l?...~--~~~~~ity of an 8080 based microcomputer provides

a way for an external input or output device to gain the attention

of the computer quickly. r When an/interrupt ~ign51>1 is received' the - ..---------··--·--~}

computer finishes executing the current instruction and saves the

address of the next instruction of the main program. The

interrupt is then answered with an tfnt~~i~.P-~~:;led;;-s-ig-~~i-~:· _,_. __ __,,_ .. _ .. __ -'~----~ - - -. --------........ " ----~·-----·-- - -

used to c~~~~?~_,_,t,~~-J~.8.:E.~.1:1.&. of one of eight one-byte jump inst:_~C:t.~ons, -.,._. _________ , .,._, .... -... , - .- ... --.-·• , .. -.. ----

known as restart instructions, from external circuitry onto the ·~-~--~-----... -------

bidirectional data bus. The restart instruction causes program

execution to be transfered from the main program to a device service

routine located at a specific and predefined point in memory. When

the device service routine has been completed, the attention of the

computer is directed back to the main program, where execution

continues from the point of the interrupt.

The microcomputer systems chosen for this application are the

Mini-MicroDesigne1® * and the 1'{:i.c-~oDesf.g~'¢r, @ * produced by E & L ------- ,·' _________ .-· Instruments, Inc., Derby, Connecticut. Both units use the popular

eight-bit 8080-A microprocessor chip developed by Intel Corporation.

These microcomputers were chosen for their powerful set of 78 ,,...---_--------------~-,.---~--·

instructions, capability to be easily interfaced to external circuitry,

and excellent documentation.

The Mini-MicroDesigner -~ loaned to us by Dr. Peter Rony of the Chemical Engineering Department of Virginia Polytechnicinstitute

and State University, is a single board computer complete with 512 ~ -- ·--~··· - .... -·-· -····• ____ ,,.-------·--~__...,,_.........--... --~- .••-~•- __ _.._..,_._._..c--•~--·---.• ..... -~-

*Registered trademarks of E & L Instruments, Inc., Derby, Conn.

8

bytes of read/wr:l:te .. m~1:I.H?.b:Y- ?nd 256_.b.ytes of read/onl.y me.w.ry. An on-~--------·----· - ----··--···-\

I I board, octal coded keyboard and a keyboard executive routine JQ_th~_.h.a,_~_!_~-h~_r~ware in tw.e_c:~!L.~~

9

developed on the Mini-MicroDesigner@ into a functional blood pump

controller. The final interface circuitry was wire wrapped on circuit

boards designed to be inserted directly into the MicroDesigner(!}bus.

Subroutines, written in assembly language mnemonic code established by

Intel (9), were combined and assembled using the Virginia Tech

Conversational Monitoring System, and the MAC80 cross-assembler

program. The machine language output of the cross-assembler was

punched onto paper tape using a teletypewriter tape punch, and

loaded into the read/write memory of the MicroDesigner ~ Software and hardware were tested at the normal operating speed of

the computer. A complete listing of all SQ~!w~re used in the -------------------------- - ---

blood pump control algorithm appears as Appendix A • ...---------~--·.

The Blood Pump

The Arp diaphragm-type blood pump chamber used in this

application is shown in Fig. 3. Figure 4 illustrates the basic

configuration in detail. Co~~ed air and vaculJDl are alternately

applied to one side of the silicon rubber diaphragm. Blood is forced

through the pump chamber, by the motion of the diaphragm, in a

direction determined by the one way, ball-type check valves. Two ------·-· ------- . . . -- - ----- -- --. -- ___________ ..., ______ __

such chambers are connected in parallel, using Y connectors, to form

the blood pump. The chambers were driven alternately, one in /--- c ~ol' while the other was in diastole. E~_:..!.~~I~-~..P-~!~~d

. __ _,,

solenoid valves control the switching of the compressed air and

II

G) . (]) 5 0 0 "lJ c ~ "lJ (") ::c l> ~ (]) IT1 :::0

,---

-, ... ~-

--' -

-

-.-,.,

--.

---

-;:

;--.

_. -

---

-~--·

\..I

·-

---.

J-

-

'-./

r -

--

_.,,

'-

--

--

--

/ "'

1------'

I \

I I

I I

OT

/ I \

'

I l>

r ,, CJ) N !Tl

11

FULL SIZE

DIAPHRAGM

~CHECK VALVE

PUMP CHAMBER

.___CHECK VALVE

FIG. 4 DETAILS OF BLOOD PUMP CHAMBER

12

As mentioned previously, t~~loutput of._a diaphragm..""1:.yp.e-p.ump

is equal to t_!:1=-__p_~~~:1_C._~-~-f t;he P~E:i.ng rate and th~ s~E_2~~ _YQ.1-~9 /

The stroke volume, in turn, is dependent on the rate of filling of

the pump chamber, and the length of time the chamber is allowed to

fill before systole. This length of time is referred to as the

diastolic time, and the length of time that the blood is b_i:!!:!K . ...---·- --· -· - - -- ·-. --·· ~------- --- ·-···---.-.,. .. -.----···- -~----·"'·~---.~··•'·_.....,,_.._.,,.._,..,._....,... __ .

ejected from the pump chamber is referred to as the !systolic time. -"""".•c--~--··''''·'"''-•-·,-·••.,.••,•·-·•••-•- ,_,.•,-." .. _'""'"''"""'»•''•-•.-~."·"'-" .•-.c• .. , ., , , ,-,•, _, ••. , .. ,,.• •' ,.,, ., --·· -,_,., > ····,_,,, .-..~. ''-'' , • ""'• •'····~···''•-''-"';·.·· ~- "'*

The ratio of the systolic time to the diastolic time is called the

systolic/diastolic ratio, and can be of magnitude greater than,

less than, or equal to one. By normalizing this ratio with respect

to the total time required to complete one cycle of systole and ,,-----------------.

diastole, \the d~-~~--~!-~fe, measured in per cent, is obtained

chamber. -rhe-o~tput of this single chamber pump, then, can

for one

be

regulated by adjusting either the pumping rate or the duty cycle

parameter.

Recall that the blood pump used consists of two parallel,

connected, alternately driven pump chambers. The duty cycle parameter

discussed for one pump chamber, therefore, does not constitute

an easily used parameter for control of the blood pump by an attendant. (,--·---~ .. ,. .. _______ .......... -· ......... ··---- . .

A new parameter, th~_p~;:_-~nt systol~0ime, will be defined as the

13

The Flow Transducer ,,,,..,,..---·-·---·--·---.... _~

/ ....... ,, The Statham Macroflo electromagnetic blood \flow meter~\ model

... _____ . ---------·----·····-J

E-3002, and the model E-3006 flow transduc~r, was used as the flow --· .. -- ; rate sensing device. The (aj..ectromagnetic. blood flow me!;_~x:._.o:perates ~--------· '- ·-·····. ···----··-··-···· ...... • ····------·--·· b~~qring_the _ vqll~gft. .. ip,g~~-~d __ w:h.en __ blo.o.d.,_an.. .. tl_~~E.:_:!:.~.~y

C9Il.~~£E.iL~f.~ f~ow~~_l_!;i:9_i,1_gh .ii· ~~gnetrc-·r:relCl} o~e~~2,):>e~~en- ~ --·--·-------~-~--... ___ ,....,....-... --..,,,.-·~-., -- ···~"'-~,_ ... _... .. ., ...

dicular to the direction of flow (10). Both a large front panel meter --~---~---------------·----~-------··--···-···------···

display and a 'differential chart recorder output are provided. For

specific information on the operation and use of theiStatham flow

met~, refer to the Statham operator's manual (11) • . \ '-J

System Operation

As mentioned previously, data entry to the controller is made

via the keyboard. ~he operat_?_:_ ... ~~~--~~!=~~ -~~-~~--E-~~~~-~~~~-=~,E,..E!_ and

the_p_eE.~~~t sy~tolic time parameter of the blood pump {in the free-- ·-- .. -----------· .... :.:,-::::--..;~=::..-::-.;-·,".'.:...-::-::::~..:=.~::=-:.·.::::':'--------~~-------------·---H----... ····---·---~----·-••• . .""••'

running, open-loop mod~ \The __ clos~c!-=-~-~C!_e.__!llod~--~~-~~!-~~-~-1'.:~--~L-entering a desired flow rate. In this mode, the operator._ cl!.~. -------=====-~~=:-:-::.:::::==:;::_-:.::::-....:.::.::.-=.::::.:::.:.:~-=:.-·

select only the pumping rate desired, as the controller automatically ------------- ·----·----·;f--..c:;=o=:--="'"····

ad_J ~-~t~--~~-':-. .P.~.!:_-~-~~~---~Y~!=-~!.!£.!!.1!1~ .. P.~:l'.~~-t-~L_ tq_ maint.a:(,g_!;:.h.~u;l_e.s_i,red )

f~ow :at~. In both the open-loop and closed-loop modes, the current

pumping rate, per cent systolic time, and flow rate are continuously

updated and displayed.

III. Implementation

Overview

The following sections decribe the development of the pneumatic

blood pump control strategy in both hardware and software. Basic

components of the controller, the keyboard, displys, valve drive, and

flow transducer, are discussed in terms of the interaction between

the electronic circuitry and the software routines used to service this

circuitry. The overall operation of the pump controller is then

developed by describing the integration of the basic components into

a functional unit. Finally, the techniques used to obtain closed-

loop control of the pump output are presented and discussed in detail.

The software used to direct and coordinate all blood pump

operation is presented in Appendix}•· The development of

facilitated by th~:..;~gbog.k@ ~~1, V, and VI, --~~·--·>

the

by Rony,

Larsen, and Titus (8, 12, 13), which give numerous progrannning examples

and techniques. Flow charts are labeled with the appropriate

subroutine title and memory location as taken from the program

listing in Appendix A.

Actual wire-wrap board layout and other construction details are

presented in Appendix B. The Texas Instruments TTL Data Book (14)

and Linear Interface Circuits text (15), as well as the National

Semiconductor Linear Integrated Circuits handbook (16), were used to

obtain specifics on integrated circuit function and pinouts. The

Bugbook~eries was also consulted for its interface circuitry suggestions and diagrams.

14

15

In the following discussions of circuit functions, r,-high' refers

to a logic level 1 state of +5V, and 'low' refers to a logic level

0 state of 0 V. All resistances are given in ohms:__J

Interface Hardware and Software

Device Select Pulses

The gating of information into and out of the 8080 based Micro-

Designer ®requires input and output device select pulses to be used.

Figures 5 and 6 show how these pulses are generated. The four least --------~--~-

significant lines of the address bus are decoded into one of sixteen

mutually exclusive outputs, considered active when low, when the strobe

input G2 is taken low by the IN or OUT function pulse. The device

select pulses are labeled according to a mnemonic code established

as part of the pump control software in Appendix A.

Keyboard

Figure 7a shows the circuitry used to encode one of sixteen r---~

normally open keyboard switches into a four-bit binary number and a

priority flag used to indicate when a key is depressed. The circuit

is identical to ~hat used in the Mini-MicroDesignerQYfor data entry. The labels listed under KEY represent the particular function of the

blood pump activated when that key is depressed. Encoder operation

will now be described for one key closure.

+5V

I 12 -:! 9 24 A3 > 201 D A2 21 c

ADDRESS ~ 22 74154 BUS B IN Al

AO 23 A

IN> 191 G2 21 3 --} INPUT 18 GI > COMP DEVICE

SELECT - . ·2 "" KBDI PULSES

FIG. 5 GENERATION OF INPUT DEVICE SELECT PULSES

f-' O'\

+5V I ~

24 12

15 17 LOFL MIFL 201 0 14 16 HIFL

A3 > 13 15

LOST 12 I 4 =:Jc 74154 I 3 MIST A2 OUT 11 II

LORT ~ OUTPUT ADDRESS ~ B

10 10 MIRT DEVICE

I-' SELECT

-....J

BUS Al

9 9 HIRT PULSES

231 A 8 8 CNVC

AO > 7

OAC 7 6 6 CTRL 191 G2 5 OUT > 5 RSTR 4

TIMR 4 181 GI 3 3 KBDC 2

VALV I 2 I -

FIG. 6 GENERATION OF OUTPUT DEVICE SELECT PULSES

K-EY -0 ..---- --

~-I--4 5 6 l -

-----

1----

-8---9----

PUMPING RATE - --SYSTOLIC TIME ---

SET POINT - --ENTER---

GO - -STOP----

+5V

16

10 0 AO

74148-1 Al A2 GS

2 5

3 6 EI 7

10 0 I AO

12 2 13 7414 8 -1 A I

I 3 A2 4 GS 5

6 EO 4

7 16

-

1"5V

FIG. 7a

7400-2

9

7

14

-

9

14 PINl4=+5 PIN 7= GND

15

5

KEYBOARD ENCODER

A

B

c

PF

v

D

TO KEYBOARD INTERFACE I-' co

19

When the 'O' key is depressed, input 0 of 74148-1 is taken

low, forcing outputs AO, Al, and A2 high, and GS low. Outputs

AO, Al, A2, and GS of 74148-2 remain high and output EO remains

low because all the ungrounded inputs to this chip float to a logic

1. The AO, Al, A2, and GS outputs of 74148-1 are logically NANDed

to the corresponding outputs of 74148-2, producing a logic 0 at

the outputs of 7400-2 a, b, and c, and a logic 1 at the output of

7400-2d. The four bit binary code thus produced when the 'O' key

is depressed is 00002 . The PF priority flag is high, indicating

that a key is pressed.

Figure 7b shows the circuitry used to input key codes into the

microcomputer. The PF flag, delayed by C2 to reduce race problems,

is inverted by 7400-3a and used to preset outputs Q of 7476-la to a

logic 0 and Q to a logic 1. Output Q is used to latch the four-bit

binary key code into 7475-10. Output Q is used as a flag to indicate

that the keyboard requires servicing by the computer. Table 1

provides a listing of the codes produced for each key closure and

made available to the keyboard service routine.

Figure 8 shows the operation of the keyboard service routine

used to gather data from the keyboard. The keyboard service routine

was adapted from the KEX monitor routine furnished with the Mini-

MicroDesigner ~ The six bits of data are acquired from 8095-1 when

an IN KBDI instruction is executed. The service request flag is

examined to determine if a key closure has occurred. If a key closure

has not occurred, program execution is transferred away from the

+5V

~ 2

IQ 16 A ID

B 3 20 2Q 15

6 30 3Q 10 c

D 7 4D 4Q 9

7475-10

+5V

l. ' -8 ENI EN2 41 113

2 INI OUTI 3 DO 4 IN2 OUT2 5 DI 6 IN3 OUT3

7 D2 ~ INPUT 10

IN4 OUT4 9 D3 BUS

14 INS OUT6 13

06 12 IN5 OUT5

II 07 J N 0

8095-1 11 D1s1

DIS2 - i 15

+5V

+ 5V M Q 10 a pa 71PR

111 91 Q 7400-3 I 7476-la +c2

PF ........_ l.Olµt

KBDI

KBDC

FIG. 7b KEYBOARD INTERFACE

21

Table 1 Key Codes Available to Keyboard Service Routine

Key

0

1

2

3

4

5

6

7

8

9

Pum ping Rate

Per T

Set

Ent

Go

cent ime

Point

er

Sto p

Systolic

Service Request Flag

D7(l)

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

(1) Most significant bit of data

(2) I.east significant bit of data

Priority Flag

D6

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

Binary Key Code

D3 D2 Dl D0(2)

0 0 0 0

0 0 0 1

0 0 1 0

0 0 1 1

0 1 0 0

0 1 0 1

0 1 1 0

0 1 1 1

1 0 0 0

1 0 0 1

1 0 1 0

1 0 1 1

1 1 0 0

1 1 0 1

1 1 1 0

1 1 1 1

KYBD: Hi 04ll Lo 81H

No

D6 = 1

22

50 ms De bounce

Input Keyboard

Data

Yes

Clear Flags & Store

Key Code

No

D7 0

Figure 8 Keyboard Service Routine

23

keyboard service routine. If a key closure has occurred, the priority

flag is repeatedly examined to determine if the key has been released.

When the key is released, the service request flag is reset by

executing an OUT KBDC instruction which clears output Q of 7476-la

to a logic O. The service request and priority flags are then removed

from the six bits of data acquired, and the four-bit binary code

representing the key depressed is placed in temporary storage.

Displays

Figure 9a shows the circuitry used to display pump operating

parameters. Eight such display circuits were constructed, three

each for the pumping rate and flow rate displays, and two for the

per cent systolic time display. A four-bit binary coded decimal

(BCD) number is latched, at the discretion of the control algorithm,

into one of the eight 7475 latches by the appropriate device select

pulse. Figure 9b shows the pin connections made to 7404-1 and -2

to invert each device select pulse. The latched outputs are applied

to a 7447 decoder-driver which translates the four-bit BCD number

into the code required to drive the seven segment displays. Resistors

Rl-R56 are used to limit the current in each display segment to

approximately 15 ma. Appendix A lists the display mnemonic codes used.

Valve Drive

The sequencing of the solenoid valves, which switch the com-

pressed air and vacuum lines to drive the pump, is accomplished through

+5V r i +5V I I +IOV 0 -::!::-5 12 16 8 31 19 114

t 7 40 4Q 9 6 D 0 14 II --OUTPUT 02 6 30 3Q 10 2 c f 15 2. BUS DI 3 20 2Q 15 I B 9 ~

7

1...00 2. ID IQ 16 7 A :1',~ 8 3 LT 10 7475- 4 BI b112 13 (!-8) W's

E2 El 5 RBI 13 a a A t 1 a

4 13 Rl-R56

7447- 620 MANI-N ~

+5V (1-8) EACH (1-8)

* HIFL ':> 11~ CIRCUIT REPEATED FOR

7404-1 ALL DISPLAYS *SEE FIG. 9b

FIG. 9a SEVEN- SEGMENT DISPLAY

MIFL~

LOFL~

LOST~

MIST~ 7404-1

HIRT~

MIRT~

LORT~ 7404-2

NOTE: FOR 7404 PIN 14 + 5V PIN 7 GND

FIG. 9b INVERSION OF DISPLAY SELECT PULSES

N U'I

26

the use of the /interrupt servicing :technique gj,i?glJ§..$~.4.J-!1 C::l;tapter 2. ~----·-··- .. \ ........... . Interrupt servicing was chosen because the timing of pump events, i.e.,

the switching on and off of the solenoid valves, is dependent on

the pumping rate, per cent systolic time, and desired pump output

entered by the operator. The need to switch a solenoid valve on or

off could, therefore, occur at any point during the execution of the

main program.

Recall the sequence of.J;W.eP.:t.& in interrupt servicing. An ------·---~--··-·· ··--~- •,,·.~- --· '•'·-· -· ·-'"-·"• - . ' . ., •,' .. --·· - '.-.... -···· ·-····-··-..... --·-, .. ,, . .,.,_.,_,_ ...... ,._, .. · ~ -~---- __ , __ -

external deyJ_~~,..:;:~_gl!es.ts .. .the .. at:t~µt_ion of the C()lllPl!t~r J>y_generating .... ------~~-- ... ----·--· .... -.... -.... . - '. - ·- ' __ ,, _______________ ~·-·-····--·-····· .. ·-··

ag._J,J1.S,err.up.t r.eq.uest.sign,al. The computer responds with an interrupt

acknowledge signal which controls the gating of a restart instruction

into the computer.\"Program execution is then directed to the

appropriate service routine, and the main task is resumed after the

service routine has been executed , . .J

Ellis (17) has developed a unique technique for servicing

interrupt requests used to sequence the operation of some device.

As part of one interrupt service routine, the next interrupt service

routine to be executed in the sequence is determined. Figure 10

shows the hardware needed to use this technique. The appropriate

restart instruction for the next service routine in the sequence

is output to and latched by 74100-1. In response to the next

interrupt request, the interrupt acknowledge signal IAK is generated

from the computer control lines INTA.and DBIN as shown. The !AK

signal strobes 8212-1 to input the restart instruction latched by

74100-1.

+5V -!- +5V n ) L2

-24 7 24

07 204 2Q4 17 22 ors 008 21 07 06 203 2Q3 18 20 DI7 007 19 06

05 10 202 2Q2 9 18 0!6 006 7 05 04 11 201 2QI 8 16 Dl5 005

15 04 ~ INPUT OUTPUT ~

BUS 03 2.1 104 IQ4 20 9 Dl.4 004 10 03 BUS 02 22 103 IQ3 19 7 013 003 8 02 DI 3 102 IQ2 4 5 012 002 6 DI DO 2 IOI IQI 5 3 Dll DOI 4 DO

74100-1 I I 2 MD ~ 8212-1 DS2 I N E2 Et I I 1 14 CLR -....! 121 123 1 II STB

RSTR~ I I I I OSI

7404- 2 .. +5V I INTA c

IAK 10

lg

28

Figure 11 shows how this technique is used to sequence the

solenoid valves used in the pump drive. Pumping action is initiated

by calling the service routine that activates solenoid valve number

one and thereby forces blood out of pump chamber number one. The

service routineloads a programmable interval timer with avalue

representing the duration of systole for pump chamber one, activates

solenoid valve one, provides the restart instruction for the next

service routine in the sequence, and returns to main program

execution. The timer waits the desired length of time and then

.interrupts the main program execution. This interrupt request

calls the service routine which loads the timer with avalue

representing the duration of diastole for pump chamber one, turns

solenoid valve one off, provides the restart instruction for the

next service routine in the sequence, and returns to the main

program. Solenoid valve two is activated and deactivated in a

similar fashion, and the sequence is repeated starting again with

solenoid valve one.

Figure 12 shows the eight-bit programmable interval timer

developed for this application. An eight-bit quantity is loaded into

the timer by strobing the LOAD pins of cascaded binary counters

74193-1 and 74193-2 with device select pulse TIMR. TIMR also presets

the Q output of 7476-lb to a logic 1, thereby gating a train of

clock pulses through 7400-la to the COUNT DOWN pin of 74193-1. The

counters then begin to count down from the eight-bit number loaded, at

a rate determined by the clock frequency. When the count has reached

SST 1: Hi 08H Lo 07H

Restart 1

Save All Registers

Load PIT w/ Systolic Time

& Activate Valve 1

Output RST2

Restore All Registers

And Return

DST 1: Hi 08H Lo lEH

Restart 2

Save All Registers

Load PIT w/ iastolic Time & Turn Both Valves Off

Output RST3

Restore All Registers

And Return

SST 2: Hi 08H Lo 35H

Restart 3 '-

1

Save All Registers

, . Load PIT w/

Systolic Time & Activate

Valve 2

, Output RST4 l

, I

Restore All Registers

And Return

Figure 11 Interrupt Driven Valve Sequence

DST 2: Hi 08H Lo ACH

Restart 4

Save All Registers

Load PIT w/ Diastolic Tim

& Turn Both Valves Off

Output RSTl

Restore All Registers

and Return

N \.0

+ 5V +5V

8 J l1 J .. - .. TIMR ;;-

II 8 9 LOAD

~ STB y

74193-2 8200-2

D7 - 9 D QD 7 10 Al Bl 4

D6 :::-- 10 c QC 6 II A2 B2 3 D5 > I B QB 2 12 A3 B3 2 D4 ~ 15 At tQA 3 13 A4 B4 I I b \_3

21 .....

4 5 f --== 7400-1

13 12 BOR CAR

8 STB y 9 TIMR > II LOAD ...L 74193-1 8200-1

D3 ::- 9 D QD 7 10 Al Bl

4

D2 -10 c QC 6 II A2 B2 3

DI> I B QB 2 12 A3 B3 2

DO, 15 A QA 3 13 A4 B4 I 5V t t + 15 113

...L. 4 5 -.:

Q 15 13 ) a }1L +5V 12

7476-1 7400-1 b CLOCK I

CLR PR ~TIMR 21

FIG. 12 EIGHT BIT PROGRAMMABLE INTERVAL TIMER

. EXT

w 0

31

zero, the Y outputs of the four-bit comparators 8200-1 and 8200-2

go high and are logically NANDed to produce the interrupt request

signal EXT. At this time, EXT is used to clear the Q output of

7476-lb to a logic 0 which sto~s the train of clock pulses to 74193-1.

Figure 13 shows how the clock pulses are generated. Schmitt

triggers 7413a and b, Cl, R57, and Pl form a pulse generator of

frequency l/3RC, where R is the sum of R57 and Pl. For the component

values used, oscillator frequency could be varied from approximately

10 Hz to 150 Hz. This is the CLOCK 1 pulse train used to drive the

programmable interval timer. Decade counter 7490 divides the CLOCK

1 frequency by ten to produce a CLOCK 2 frequency. The CLOCK 2

frequency can be further divided by two by 7476-2b to produce a

CLOCK 3 frequency. Either CLOCK 2 or CLOCK 3 pulses, selected by a

jumper wire, are applied to a 74121 monostable multivibrator used as a

shaping circuit to provide clock pulses of approximately 35 ns

duration to the flow rate sampling interface discussed in the next

section.

The circuitry used to activate the solenoid valve is shown in

Fig. 14. The two-bit binary code used to switch the solenoid valve is

output to and latched by 7475-9. Assume for discussion purposes that

this binary code is 012, indicating that solenoid valve one is to be

switched on and solenoid valve two to be switched off. Output lQ

of 7475-9 is then high, forward biasing diode Kl, and switching

transistor Tl on. Transistor Tl is used in the emitter follower

configuration to supply adequate base current to switch transistor

R57~ 220

OSCILLATOR Pl 5K

I~~~· 7413

NOTE: FOR 7413 Pl4 = + 5V P7 = GND

+5V

l10

-

8 QAl12 1----------4 CK

7490

~:',~ 6

7476-2b

-

FIG. 13 SYSTEM CLOCK

T.P. CLOCK I

CLOCK 2

13 7

01 15 3 IAI CLOCK

-1 I Q TO ;)

3 74121

2 B Ri 41 I 19

5

+5V

SAMPLE FLAG w

N

+5V

LED- 2(3)

+5V

DO ') 2 1 ID IQ i.!..-...J

>..

R58 (61) 220

+IOV

r, ~= .. K2. (4)

Kl (3) R59 (62.)

10 K

OUTPUT BUS 7475-9 IQ~6

} TO IDENTICAL

DI ____ 31120 ".)..--

10 VALV ~ 1

7404-2

2Q 14 2.Q 15

El 13

FIG. 14

DRIVE CIRCUIT ' FOR VALVE 2

(COMPONENT NUMBERS IN PARENTHESIS)

C3(4)

.01µ1 I DO I DI II VALVE I I VALVE 2

I I 0 0 o I o

ON OFF OFF

OFF ON

OFF

R76 (77) IK

VALVE DRIVE CIRCUITRY

r-----, I +14V

ARP PUMP Q I CONTROLLER I I I

I I

Tl (3) L MOT 524

T2. (4) MOT 52.5

I I

VALVE I I

- J

(....) (....)

34

T2 on and activate the solenoid valve. Resistors R59 and R60 limit

current, respectively, in the base and collector circuits of Tl.

Resistor R76 insures that Tl remains off unless driven by 7475-9

or the manual systole function. Radio frequency interference is

reduced by C3. Output lQ of 7475-9 is at logic 0, lighting LED-1

to indicate that pump chamber one is in systole. The solenoid valve

can also be activated manually as shown. In this example, the

transistors driving solenoid valve two and the associated systole

indicator are off by similar reasoning.

Analog-to-Digital Converter

Recall that the electromagnetic blood flow meter provides a

differential chart recorder output of the flow rate measurement.

Fig. 15 shows the instrumentation amplifier used to provide a

single ended output. Operational amplifiers (op amps) 324-a and

324-b are configured as unity gain voltage followers used to present

a high input impedance to the flow meter output circuitry. Op amp

324-c is configured as a differential, balanced-to-ground amplifier

with a gain, equal to R66/R64, of approximately ten. The single

ended output of this amplifier ranges from -0.29V for no measurable

flow to -0.BOV for a full scale reading of 500 ml/minute.

Figure 16 shows the scaling amplifier used to shift the single

ended output from the range -0.29V to -0.BOV to the range O.OV to

l.OV for the given flow rates. Op amp 324-d is configured as an

LM324

+ .>-------! FROM FLOW

METER

GND

LM324

~ ---~

14

R64 IOK

6

R67 IOOK

R66 IOOK

7 > • ~ TO SCALING AMPLIFIER

NOTE: FOR LM324 PIN 4 + 15V PIN II - 15V

FIG. 15 INSTRUMENTATION AMPLIFIER

VJ l.J1

-15V

R70 2K

R71 IK

FROM INSTRUMENTATION

AMPLIFIER

+5V

IOK OFFSET

R68 150K

R69 150K

P2 500 K GAIN

FIG. 16 SCALING AMPLIFIER

w "'

37

inverting summing amplifier with a gain, equal to P2/R68, variable

from about one-half to three. The quantities summed are the analog

signal representing the measured flow rate from the instrumentation

amplifier and some positive reference voltage used to shift the range

of this analog signal. The design of the scaling amplifier will now

be considered.

Let X be the reference voltage, Ei be the analog signal from

the instrumentation amplifier and G be the gain associated with the

scaling amplifier. The output voltage E is then given by, 0

U>

For the shift from -0.29V to O.OV for the no flow condition, Eq. 1

becomes,

0.0 = -G(X - 0.29). (2)

Similarly, for the shift from -0.80V to l.OV for the maximum flow

condition, Eq. 1 becomes,

1.0 = -G(X - 0.80). (3)

Solving Eq. 2 and 3 simultaneously for X and G yields a reference

voltage of 0.29V and a gain of 1.96 which are required.to shift ranges

as described.

The positive reference voltage· is adjusted by varying potentio-

meter P3. Resistors R70 and R71 form a voltage divider that supplies

approximately -SV to one end of the potentiometer while the other end

38

of the potentiometer is connected to the +SV supply. Resistors R68

and R69 insure that both the reference voltage and the analog flow

rate signal are weighted equally in the swnmation. Amplifier gain

is adjusted be varying P2. A detailed procedure for making these

adjustments that also corrects for component mismatch and the

attenuation caused by the averaging circuit presented next will be

discussed in the following chapter on calibration.

Figure 17 shows the circuit used to average the variations in

flow rate that occur during each cycle of systole and diastole.

The values of resistance and capacitance were varied empirically to

produce a signal that did not change appreciably during each pump

cycle and did not seriously affect the response time of the blood flow

meter. Op amp 741 is configured as a unity gain voltage follower to

buffer the output of and avoid loading the averaging circuit.

The strategy actually used to perform the analog-to-digital

conversion is shown in Fig. 18; the hardware required is detailed in

Fig. 19 and 20. A staircase-ramp-type conversion is used where the

ramp is created by incrementing a computer register. The contents of

the register are output to 74100-2 and applied to the digital-to-analog

converter 3711. The current output of the digital-to-analog converter

is dropped by R74 to produce a nominal full range voltage output

of O.OV to l.OV.

The analog signal from the averaging circuit, representing the

magnitude of the flow rate, and the output of the digital-to-analog

converter are compared by op amp 301A. Op amp 301A is configured as

FROM SCALING

AMPLIFIER

R72 22K

C51+

30~fr

R73 22K

+I CG

130~1

? G • 7 TO COMPARATOR T. P.

NOTE: FOR LM 741 PIN 7 +15V PIN 4 -15V

FIG. 17 AVERAGING CIRCUIT

VJ

'°

No

C=O

40

Initialize Ramp &

Clear Sample Flag

Increment Ramp & Check

Com arator

Yes

Save Result

No

C=O

Yes

Z=l

Display 'U' on Flow Rate Display

Figure 18 Analog-to-Digital Conversion Strategy

+5V +15 -15

I - . I 7 I 5 II 16

D7 16 204 2Q4 17 14 2-3 I

06 15 203 2Q3 18 13 2-4

D5 IO 202 2Q2 9 12 2-IS

D4 II 201 201 8 II 2-e

104 20 10 2 -7

103 19 9 2 -e 03 21 104

02 22 103

3 ~ TO ouT I T.P. TcoMPARATOR

DI 3 102 IQ2 4 8 2-9

DO 2 IOI IQI 5 7 2-10

74100-2 16 2-•

I 15 2 -2 E2 El

12 23 DAC371I-10

DAC ~ - I 8

7404-1

R 74 IK

FIG. 19 DIGITAL TO ANALOG CONVERTER

~ I-'

LM301A +15V -15V

FROM AMP +5V

FROM DAC K5 I 1~ -5.6V 8095-2 b }

5 DO INPUT ~IN2 OUT2 7 D? BUS I:; 61IN3 OUT3 . . Wo1s1

·~v I .l l ~ 13 -=

s~~Jc~M : :1-C p-1:.:::...R----.Qr T. P. COMP CNVC 7476-2a

FIG. 20 ANALOG TO DIGITAL CONVERTER INTERFACE

43

a comparator whose output is clamped to TTL levels by zener diode KS.

When the output from the ramp-driven digital-to-analog converter

equals the output from the averaging circuit, the comparator output

changes state from a logic level 0 to a logic level 1.

The conversion process is initiated when the computer senses,

through 8095-2, that the Q output of 7476-2a has been set to a logic

level 1 by the falling edge of the CLOCK 2 or the CLOCK 3 signal. The

CLOCK 2 or CLOCK 3 frequency thus determines the sample rate.

This sample flag is immediately reset by executing an OUT CNVC

instruction. The staircase ramp is incremented, as described

previously, when the status of the comparator changes state from a

logic 0 to a logic 1, indicating that the analog flow rate signal is

equal to the output of the digital-to-analog converter, the con-

version process is stopped. The binary equivalent of the magnitude

of the flow rate, i.e., the contents of the computer register, is then

placed in temporary storage. An overflow is generated if more than

250 steps in the ramp are required to match signal levels.

System Integration

The overall operation of the blood pump control system is

shown schematically in Fig. 21. After an initialization process,

detailed in Fig. 22, the keyboard status and the analog-to-digital

converter status are alternately examined. When the keyboard

requires servicing, the key code is obtained, as discussed previously,

Yes

Decode and Service Key

Closure

44

Input Keyboard

Data

No

Input A/D Converter Status

Service A/D Converter and

Control Algorithm

Figure 21 Overall Controller Operation

No

START: Hi 04H Lo 28H

45

Jump Around Vectored

Interrupts

Load Controller

Gains

Clear Flags Counters And

Hardware

Figure 22 Initialization Routine

46

and decoded. When the sample flag indicates that it is time to per-

form an analog-to-digital conversion, the converter routine is

executed. In either case, after the appropriate service has been

performed, the status flags are examined again until one device or

the other requires attention. Recall that the pump chambers are

sequented independently of the main controller program.

The sixteen key codes from the keyboard represent three functional

groups which govern data entry to and the operation of the blood

pump controller. The PUMPING RATE, PER CENT SYSTOLIC TIME, and

SET POINT keys are defined as parameter keys that indicate to

the controller which of the operating parameters is being entered.

The numeric keys, ZERO through NINE, are used to specify the value

of this parameter. The ENTER, GO, and STOP keys are defined as

function keys that indicate what the controller is to do with the

data presented. The procedure for operating the blood pump con-

troller, then, is to select a parameter, key in its value, and

load it using the ENTER function. When all parameters have been

loaded, the GO function is used to initiate pumping action. Pumping

action is halted with the STOP function. The specific routines used

to service these key codes will be presented next.

Figures 23, 24, and 25 show the service routines executed when

one of three parameter keys is pressed. The important features of

these routines are that an instruction code is generated that directs

two or three digits to be entered to the appropriate display,

and that the display selected by the parameter key is indicated to

RKEY: Hi 04H Lo B2H

Number

C=l

47

Yes

Load Instruction

Code

Initialize igit Counter

Reset RFLG And Clear

Digit Storag

No

Z=O

Figure 23 Service Routine for Pumping Rate Key

SKEY: Hi 04H Lo DAH

48

Yes

Loa Instruction

Code

Initialize Digit Counter

Reset SFLG And Clear

Digit Storage

No

Z=O

Figure 24 Service Routine for Per Cent Systolic Time Key

FKEY: Hi 04H Lo FDH

49

Yes

Load Instruction

Code

Initialize Digit Counter

Clear Digit Storage

No

Z=O

Figure 25 Service Routine for Set Point Key

50

the operator by showing all zeroes. Other initializing functions are

performed as shown.

The numeric keys are serviced by the routine shown in Fig. 26.

The instruction code is examined to determine which parameter is

being entered, and the digits are placed in temporary storage and

routed to the proper display. The weighting of each digit entered is

noted by a digit counter in software. If a numeric key is pressed

out of sequence, i.e., before a parameter key, no instruction code

is available and therefore the digits are not displayed.

The ENTER key is serviced by the routine detailed in Fig. 27a,

b, c, and d. The ENTER function can be summarized by saying .that it

converts the parameter value entered as binary coded decimal (BCD)

digits into its binary equivalent, and then checks to be sure the

result is within certain predefined limits. The BCD-to-binary con-

version is accomplished by multiplying each BCD digit by the

appropriate power of ten and summing the products. The instruction

code is examined to determine which parameter was entered and thereby

determine which error checking routine should be used. If the ENTER

key is pressed out of sequence, i.e., before a parameter or numeric

key, no instruction code is available and therefore the key closure

is ignored.

The error checking routines examine the parameter value to

d~termine if it falls within a range determined by both physiological

needs and equipment limitations. Table 2 lists the minimum and

maximum values allowed for the pumping rate, per cent systolic time,

DSPLA: Hi OSH Lo lFH

51

Fetch Instruction

Code

No

Yes

C=l

Yes

C=l

Yes C=l

Store & Display Pumping

Rate

Store & Display

Set Point

Figure 26 Service Routine for Numeric Key

ENTER: Hi 05H Lo DEH

52

Yes

BCD To Binary Conversion

No

Z=O

No

Z=l

Yes

Z=l

Yes

C=l

Figure 27a Service Routine for Enter Key

No

Set RFLG And Clear

Instruction Code

Store Pumping Rate

53

Yes C=O

Yes Flash

C=l

Figure 27b Pumping Rate Error Check

No

Set SFLG And Clear

Inst. Code

Store Systolic

Time

54

Yes C=l

Figure 27c Per Cent Systolic Time Error Check

No

Reset FFLG And Clear

Instruction Code

55

Store Set

Point

Set FFLG And Clear

Instruction Code

Figure 27d Set Point Check

56

Table 2 Parameter Ranges

Parameter Units Minimum Maximum Default

pumping rate beats/ 20 200 60 minute

per cent systolic per cent 5 95 50 time

flow rate ml/minute 0(1) 500 set point

(1) Entering this value places the controller in the open-loop mode

57

and the flow rate set point. If the pumping rate or per cent systolic

time ranges are exceeded, an error message is presented and the

appropriate display is zeroed, indicating that the operator should

try again. If the ranges are not exceeded, a memory flag is set

indicating that the parameter is valid and the parameter is placed

in storage. Entering a flow rate greater than zero places the

controller in the closed-loop mode by setting an appropriate memory

flag, while entering a flow rate equal to zero places the controller

in the open-loop mode by clearing this flag.

Figures 28a and b show the routine executed to service the GO

key. The GO function examines the parameter flags, set by the ENTER

routine, to determine if the pumping rate and per cent systolic

time are within their preset ranges. If the parameters are not within

range or have not been entered, the default conditions listed in

Table 2 are used. In either case, the interrupt timer inputs are

calculated and the pumping process is begun.

The eight-bit interrupt timer inputs, which determine the lengths

of systole and diastole for each pump chamber, are calculated in

unsigned, binary integer format from

where ITI = (T/100) x (F x 60)/N

T = either per cent systolic or diastolic time

F = clock frequency (Hz)

N =pumping rate (beats/minute).

(4)

GO: Hi 07H Lo 58H

58

No

Use 60 BPM

Figure 28a Service Routine for Go Key

59

Use 50%

on Time

is la

Calculate Timer Inputs

No

Set GFLG

Start Pump-ing Cycle

Yes C•l

Yes C=l

Figure 28b Part of Go Routine

60

In order to avoid overflow problems in the multiplication and

division routines used, the calculation is ordered and performed as

T x F + 100 x 60 + N. (5)

A clock frequency of 88 Hz was chosen to maximize the range of the

per cent systolic time parameter for the fixed range of pumping

rates given.

The service routine for the STOP key is shown in Fig. 29. The

pumping process is halted by turning both solenoid valves off and

disabling the interrupt service capability of the microcomputer.

Program execution then continues from the initialization routine of

Fig. 22.

The routine that services the analog-to-digital converter when

it is time to sample the flow rate is shown in Fig. 30a and b. The

staircase-ramp conversion technique used has been discussed previously

in connection with the analog-to-digital converter interfacing and

Fig. 18. Samples are not taken unless the pumping sequence has been

initiated by the GO function.

Figure 30b also shows how the per cent systolic time and flow

rate displays are continuously updated. The per cent systolic time

display is automatically changed enly when the controller is in the

closed-loop mode. The closed-loop mode of operation is selected by

entering a set point greater than zero. Both the per cent systolic

time and the flow rate are converted from binary code to binary coded

decimal notation by successive division by ten, and presented to their

STOP: Hi 08H Lo 63H

61

Halt Pumping

cycle

Figure 29 Service Routine for Stop Key

CNVRT: Hi 08H Lo 6AH

62

Yes

Clear Sample Flag

Perform Conversion

Scale and Store

Result

Figure 30a Service Routine for Analog To Digital Converter

No

Increinent -Sample Counter

63

C=l

Yes Z=l

Correct Systolic Time

Update Systolic Time

Display

Figure 30b Part of Analog to Digital Conversion Routine

64

respective displays each time a preset number of samples are taken.

Closed Loop Control

As mentioned previously, the closed-loop control mode is

selected by entering a flow rate set point greater than zero. A

memory flag is set as part of the ENTER function to indicate this

condition. This flag is examined as part of the analog-to-digital

conversion routine in Fig. 30b and used to call the closed-loop

control service routine that corrects flow rate variations by

adjusting the per cent systolic time parameter. The closed-loop con-

trol mode is indicated by the circuitry shown in Fig. 31. A logic

level 1 output to and latched by 7475-9 causes output 4Q to go low,

lighting LED-3.

The particular control algorithm used is the velocity form of the

proportional-integral-derivative algorithm, modified by Kattwinkel (18)

for implementation on an 8080 based computer. The modifications to

the algorithm and the tuning process will be discussed in the next

chapter. Kattwinkel's algorithm is used by pointing to a data file

in memory, containing the controller gains, set point, sampled data,

and a scaling factor, and calling the algorithm. The result of this

calculation is a signed change in the per cent systolic time parameter

which is added to or subtracted from the current value of the per cent

systolic time. A new set of interrupt timer inputs is calculated from

the new per cent systolic time and current pumping rate each time a

flow rate sample is taken. The PID algorithm is a very fast routine,

and is executed in approximately two milliseconds.

+5V

A LED-3

+ 5V

OUTPUT{oo > 7 140 4QI 8 I BUS

5 112 I

7475-9

E2 4

CTRL 12

7404-2

FIG. 31 CLOSED-LOOP CONTROL INDICATOR

C]\ V1

IV. CALIBRATION AND TUNING

Overview

The calibration and tuning of the blood pump controller was

accomplished using the flow circuit shown in Fig. 32. The diameters ' of the silicon rubber tubing were selected empirically to allow the

flow rate through the system to be varied from approximately 20 ml/

minute to 500 ml/minute by changing pump operating parameters and the

flow path through the tubing. A 36.7% solution of glycerin in water,

with enough salt added to insure proper operation of the electro-

magnetic flow meter, was used to simulate the viscosity of blood (19).

Calibration

The blood pump controller hardware and software are designed to

eliminate the need for external instrumentation during the calibration

procedure. The calibration procedure involves adjusting the CLOCK 1

frequency, which drives the progrannnable interval timer and thus

affects the pumping rate, and adjusting the offset and gain of the

scaling amplifier so that the digital flow rate display on the display

board and the analog flow rate display on the electromagnetic flow

meter agree. The procedure for operating the blood pump controller

has been detailed in Chapter 3. Figure 33 shows a top view of the

interface board and the location of the calibration potentiometers

and test points.

66

RESERVOIR 010

BYPASS

NOTE: ALL CONNECTING TUBING 5 mm. l.D.

BLOOD PUMP

FLOW I ,TRANSDUCER

FLUID FLUID RESISTANCE CAPACITANCE 3mm. l.D. x 300mm. LONG

FIG.32 BLOOD PUMP FLOW CIRCUIT

(j\ -....J

TEST POINT

SAMPLE FLAG

Pl I 7476-2

TEST POINT

CLOCK I

PS 7413b

Pl CLOCK I

TEST POINT FLOW RATE

OUTPUT P6 741

TEST POINT

DAC OUTPUT

P3 731 I

P3 OFFSET

P2 GAIN

FIG. 33 TOP VIEW OF INTERFACE BOARD

"' 00

69

The CLOCK 1 adjustment is initiated by pressing the GO key. This

causes the pumping action to commence at the default conditions of 60

beats per minute and SO per cent systolic time. Potentiometer Pl is

adjusted to set the pumping rate to 60 beats per minute, using a stop

watch as a reference. If an oscilloscope or event counter is available,

the CLOCK 1 frequency is adjusted to 88 Hz to maximize the range of

the per cent systolic time parameter without encountering overflow

problems in the interrupt timer calculation routine.

The scaling amplifier adjustments are made by varying the per cent

systolic time parameter to change the flow rate. The pumping rate

is not critical and can remain at 60 beats per minute. A large per

cent systolic time of about 95% is used to attain a high flow rate.

When the flow rate stabilizes, the gain potentiometer P2 is adjusted

until the digital and analog flow rate displays match. A low per cent

systolic time of about 5% is then used to attain a low flow rate.

When the flow rate stabilizes, offset potentiometer P3 is adjusted until

the digital and analog flow rate displays again match. This process

is repeated a sufficient number of times to accurately and precisely

set the full scale range of the digital flow rate display. The

instrument calibration procedure is now complete.

During the closed-loop testing process a fixed error of 18 ml/

minute between the desired set point and the flow rate actually obtained

was discovered. This error is most probably caused by round-off errors

in the calculation routines of the blood pump control algorithm. The

error was corrected by modifying the pump control algorithm to add

70

18 ml/minute to the set point entered. This correction is internal

to the blood pump control algorithm and does not affect the operator's

use of the instrument.

Tuning and Response

The discrete representation of the velocity form of the propor-

tional-integral-derivative (PID) contE~!--~-1:~~-~~-~y is used to adjust

the per cent systolic time parameter (the manipulated variable) to

maintain a specified flow rate (the controlled variable) despite

load fluxuations. S2_th_£20). ___ g:i_yf?~.,:th~.--a1gorithm-as

6m n T Td = K [(e - e 1) + ~ e + ~ (e - 2e 1 + e 2)] e n n- T. n T n n- n-

1

where 6m = change in the manipulated variable at sampling point n n e = error in controlled variable n K = proportional gain e

Ti = reset time

Td = derivative time

_ T = sampling _!=j,xne. -------·---~----~·--·---··--

r (6)) \ ____ /

The algorithm developed by Kattiwinkel (18) substitutes the difference

between the set point and the controlled variable for the error term

and combines the time constants for easier calculation yielding

6m = 2-P[K (C l - C) + K.(R n p n- n 1 . 'I i

where K = K p c KT c = -- = T.

l.

71

proportional term constant

integral term constant

KT K cd d. . t d = -T- = eri.vati.ve term constan

R = set point

C = controlled variable at sampling point n n P = scaling factor.

Equation 7 _ _:!--~---E~~---~-!_g~:i::!.!'.1:1.!!!_ ~e._~Q__J.!LJ;h.~. l?JgQg ___ p_umR-~.Qn.t.r.oll.er_._ _____ The --=---,,:::::.:::;;;-....::;::--~::::.:~-·-·-··-,...,, .. __ ••.. ,. .. .---· "4•-- _,., ,_ - ·- -· ,_,,,.,,_ •..• ,

The integral control action gives weight to small errors persisting ~-~·-.........--·---------·----... ___ .. -----........ ---..---~,.---- ~-------------·

over time and eliminates the steady state/offset error associated ~------------·---·---------.----·F--·------!....:.----------------·-•'''•-,, __ __.--••-.,.,,

with proportional-only __ ~ol'.lt:i;gi.) The derivative term is sensitive c------·----·-----·----.... _______ .. ___ ._ .. -~ .. , ... ·,

to the rate of change of the controlled variable. LT!.1_:__ __ ~_:_1._:~:_i_cm ?~

the controller constants K , Ki, and Kd' then, determines the overall ----------·-· ··-·--·-·· ... - --···-... -·-- -···---·-··---l?.···--···-··--·--------· ............ ·-·-···--·--···-·-···· ··-·····---·-····· ........ ---·-·-····----·-----,. --·--·---------···---

control system response and accuracy ... ----··· ... -----····----··--• .. ·-····-· .... -··-· ····-·-· -·-----------·-·· ... .! ~

~--~-----·-- '" --·--·--·--------------------, . The technique used to !obtain the controller gain\and time para-------------------------·---------~-- ... -·-----+-------... ·-------------~-:::;::.:::::·::=.:.:;::-..;.."'?"·'~..C.:;:::::7".....:c::.::::::=;..-:;;:-;_";-:-----~---------,.,... .... -.~-·--~.,.. .... -- ----

meters used in Eq. 6 is described in Chapter 6 of reference 20. (The ----~---~----·.---·---·----- ---·--------------·-·-----.-.--·-----------··---~--------·-·--· ._, ---- --~·--~- ...... ._. --~--- -·.- , ....... ,.,, ......... ---

\

technique consists of modeling the blood pump and mock circulatory __ ,__ ___ ,.,...-...._ _________ -- -

system as a first-order- lag-p 1 us-dea9_,.,-time '--_ ~---~-~ .. ~ ... -------~-·"_.,, • .----~--··--· .J

where K = process gain

a = dead time T = time constant.

-es K lj\ e· G (s) = ---m T +l s ;,. L

system of the form

(8) )

72

\The controller gain and time parameters are .:hen calculaied fr~~ r:--1 (I LI

e~re_s_~-~~-~-~-1::-~l~~~~~-~!te proces~ 'gain/ dead time, sampling time and j---~,.\

time constant 'toithe proportional gain, ' 't ) -1 ( ke, _)

timeJ Finally the controller constants

reset time, and derivative T1 J T ~ ~

in Eq. 7 are obtaineJ using

the relationships shown in Eq. 7.

The first-order-lag-plus-dead-time model was obtained by -----•·----· ••"• .. ·-~- ·-- --•·--·•-· "-.--···-·--•-•-•--·--•·-•··••· .. ,_ ••·· -··--•r-.•----.-~~-·-• -··•·---~·-•·••p=···~

~---------~---._,_--..

; a~plying a step chang~ to the system;-from 20 per cent sy~~tolic time to, _, '-------===:::::::.-:-;---_::::::;:;o.:::=:::=.::.::_::::c:· _ _-,_~::·::-::·::.:::::c.::.:.:::·:~~-::.:::·.:c:.::::_.- ·\ ' -

-. z -~ . -E -

w I-

Table 3

~um~:!.!_l!-_Ra te (beats/minute)

20

65

110

155

200

Mean

Mean Not Including 20 BPM Results

74

Model Parameters at Various Pumping Rates i I

\'.

Gain K

2.36

4.84

5.28

5.24

5.04

4.55

5.10

r· . \ .. !

Dead Time e (sec)

0.890

2.46

2.32

2.21

2.11

2.00

2.28

Time Constant T (sec)

7.38

5.03

4.88

5.07

4.79

5.43

4.94

75

pumping rates shown and for the four highest pumping rates alone.

As the no~ ____ qp_e.:r:.at.i~Jt.!.~~ge of the blood J>1:1ml? would be from approxi-

mately 60 to 120 beats/minute, the average of the model. pa-rameters for .. ________ ,_,__ .. , _,._,..,... __________ .,,_._...._..__, .. -- ····-·····-··· --- --·--·. .. /

the four highest pumping rates W:?~-~-l?~e_d for tun~ll&.P~l'.'.PQSes •. r·--- ---------- .. ·---- ·-·-- .... -··· .

paramete,,rs selected charac.t_erize the first-order-lag-plus-dead-time '---------·- ---.._ .. ,,.,-

model of the blood pump and flow circuit operating at approximately 90 ~-

beats/minute.

The t~~--C:E~E~ria _t() be sat~sf~~~ is t}-ie _0~

de ado t/ime' and ti~ sons tan t to the

are controller gain and reset time r I ' \ i"'~

./ (~ KK ·= \ \ Cj

r!i·. '-----· =

T where

r 0 , -0.946 0.928 (-)

T

8,0.583 o. 928 (-)

T ~-·-:3.-i> ~ \~y~-~ c-...\, t-hj n 1

\ K :o(r_'>T \;,-,/) I

(12)

(13)

to account for the

---1' ' ' - f,1.~\.'-_,,·;I -.. '.) :: L (: .L1!'.i (14) 8 1 = 8 +-\ :::.2 .. ll)o 1. I \ \_'---- _ 2 1 r- \.!\.:[==t·is ,1.·:v1\":::: o.ll'r sample and hcild_e-ffect.,·: The derivative mode of

r--"<

control is not used. The average values of system gain, dead time,

and time constant discussed previously, and a sample time of 0.227 sec

(4.~ Hz CLOCK 3 signal) yield a controller gain and reset time of

,-K = 0.361 ·.l ~----·--·-···----··--··-·

\ T. = 3.00 seconds. ' -1 l --·---··· ·-····-_.,....,._...,,..._,., _____ _

The proportional term constant K and the integral term constant K. p 1

for the Kattwinkel algorithm are calculated from the relations under

j i

/

PERCENT OVERSHOOT DECAY RATIO = b/a

REF. 20: SMITH p. 170

105 °lo 100 °/o J / * \. / w., """""'"

I- 9 5 °/o ' T !""""'""" ____. ' :::> 0.. I-:::> 0

0 ~~~~~~~~~~~~-i-~~~~~~~~

0 SETTLING TIME

TIME

FIG. 35 PERFORMANCE CRITERIA

-...J 0\

Eq. 7 to be

77

K 0.361 p K. 0.0272.

1

The values obtained for K and K. will, when substituted back in-p 1

to Eq. 7 with a set point and flow rates less than 500 ml/minute, pro-

duce signed changes in the per cent systolic time on the order of one

per cent. The sign information appears in the most significant bit of

the sixteen-bit result of the computation, while the absolute value of

the per cent systolic time change appears in the other fifteen bits.

The maximum value of the per cent systolic time change that can be used

is 90%, since the values of percent systolic time are limited by software

to values between 5% and 95%, and thus is always contained in the least

significant seven bits of the result. To simplify the calculation of the

new per cent systolic time in the blood pump algorithm, it is desirable

to have both the sign and magnitude information in the same byte of the

result. This is accomplished by simply multiplying the proportional and

integral term constants by 256 which effectively shifts the result of the

algorithm computation eight places to the left. The sign and magnitude

information are now contained in one byte.

Kattwinkel's PID algorithm performs its internal calculations

using double precision arithmetic. To take advantage of the additional

accuracy thus afforded, the proportional and integral constants are

multiplied again, this time by 128. This produces a result, internal

to the PID computation, with a magnitude equivalent to shifting the

result an additional seven places to the left. The scaling factor

P in Eq. 7 is set to seven to shift the result of the PID computa-

tion back to the proper position. The controller constants

78

required in Eq. 7 to satisfy the one-quarter decay ratio criteria

are therefore

K 11829 p

Ki 891

Kd = 0

p = 7.

Figures 36a and b show the closed-loop response of the blood

pump control system, tuned with the parameters listed above, to

large and small increases and decreases in set point. The analog

signal representing the flow rate, used to drive the x-y plotter, was

taken from the flow rate test point. The pumping rate was set at

70 beats/minute. The bypass tube was clamped, directing all flow

through the four small tubes comprising the fluid resistance. The

large step increase in set point, from 50 ml/minute to 300 ml/minute,

produced no detectable overshoot and a settling time to within ±5%

of the final value within 13 seconds. The large step decrease in

set point, from 300 ml/minute back to 50 ml/minute, produced aper cent

undershoot of 6% and a settling time of 13 seconds. The small

step increase in set point, from 200 ml/minute to 250 ml/minute,

produced aper cent overshoot of 3% and a settling time of 6 seconds.

The small step decrease in set point, from 250 ml/minute back to 200

ml/minute, produced aper cent undershoot of 4% and a settling time of

7 seconds.

400 I

LARGE SET POINT CHANGE -300 ~

50 - 300 - 50 ml./MIN. . z PUMP RATE = 70 BPM - MINIMUM LOAD ~ ~ \ D. HUX 15 MAY 78

' . -E -w 200 ~ I I \ " 0::: '° 3: 100 0 ...J LL

0 0 10 20 30 40 50 60

TIME (SECONDS)

FIG. 36a RESPONSE TO LARGE SET POINT CHANGES

400 I I

SMALL SET POINT CHANGE 200 -250 - 200 ml./ MIN. - ~ I PUMP RATE = 70 BPM . MINIMUM LOAD z - D. HUX 15 MAY 78

~ 300 ' . -E -w 200 I-

81

The fluid pressure for the range of flow rates discussed was

measured using a mercury manometer connected between the flow trans-

ducer and the fluid resistance. The fluid pressure measured at a

flow rate of 300 ml/minute was approximately 75 mm Hg and the pressure

measured at a flow rate of 50 ml/minute was approximately 20 mm Hg.

These pressures are within the physiological range of pressures

expected to be measured in an infant.

Figures 37a and b show the closed-loop response to large and

small step increases in pumping rate. Loading conditions were not

changed from the set point variation testing. The flow rate was to

be maintained at 200 ml/minute. The large increase and decrease in

pumping rate, ·from 25 beats/minute to 150 beats/minute and back to

25 beats/minute, are well outside of the normal operating range during

a cardiopulmonary bypass procedure. These large pumping rate changes

are compensated for within 13 seconds. A small pumping rate change,

from 70 beats/minute to 80 beats/minute and back to 70 beats/minute,

produced no appreciable change in the flow rate maintained.

Figures 38a and b show the closed-loop response to large and

small step increases and decreases in load. The flow rate to be

maintained was 150 ml/minute at a pumping rate of 70 beats/minute.

The large increase and decrease in load was achieved by clamping and

unclamping two of the four small tubes in the fluid resistance,

thereby changing the resistance to flow by a factor of two. The

bypass tube remained clamped. The large step increase in load

produced a steady state error in the flow rate maintained of 6%. The

400 I

LARGE PUMP RATE CHANGE -300 J

25 - 150 - 25 BPM z SET POINT = 200 ml. I MIN. - MINIMUM LOAD ~ ' D. HUX 15 MAY 78 . -E -w t- 200

400 I

SMALL PUMP RATE CHANGE -300 J

70 --+ 80 - 70 BPM . z SET POINT= 200 ml./MIN. - MINIMUM LOAD ~ D. HUX 15 MAY 78

' . -E -w 200 .....

400 I r--

LARGE LOAD CHANGE -1

FACTOR OF 2 z PUMP RATE = 70 BPM - SET POINT = 150 ml./MIN. ::?! 300 D. HUX 15 MAY 78 ......... . -E -w ._. 200

400

- SMALL LOAD CHANGE . FACTOR OF 1.3 z PUMP RA TE = 70 BPM -~ SET POINT = 150 ml./MIN.

' 300 D. HUX 15 MAY 78 . -E -

w I- 200

86

large step decrease in load produced a per cent overshoot of 3% and a

settling time of 9 seconds. The small step changes in load were

achieved by clamping and unclamping only one of the four tubes in the

fluid resistance, thereby changing the resistance to flow by a factor

of one and one-third. The small step increase in load produced a

steady state error in the flow rate maintained of 3%. The small step

decrease in load causes no detectable overshoot and produced a settling time of 2 seconds.

Figures 39a, b, c, and d show the effects on the closed-loop

system response of increasing both the pumping rate and the set point

simultaneously. The bypass tube was clamped and the four tubes in the

fluid resistance open. The four possible combinations of large and

small increases in pumping rate and set point were investigated. A

steady state error of 3% was observed for each of the four combinations

and no appreciable overshoot was seen. The settling time was

determined by the magnitude of the set point change and was approxi-

mately the same value as determined previously. The pumping rate

changes did not significantly affect the closed-loop system response.

400

...._ . z -::? 300 ' . -E - I " ' LARGE SET POINT CHANGE LU

200 1 I 50 __... 300 ml./MIN.

I- SMALL PUMP RATE CHANGE

400

-. z -:E 300 ' . -E - SMALL SET POINT CHANGE w 200 - 250 ml./ MIN. 1-- 200 LARGE PUMP RATE CHANGE

400

-. z -~ 300 .......... . -E i I I LARGE SET POINT CHANGE - 50-+ 300 ml./ MIN. w

200 j I LARGE PUMP RATE CHANGE

I- 25 -+150 BPM

400

-. z -~ 300 ......... . E -w 200r SMALL SET POINT CHANGE ~ 200---... 250 ml./MIN.

V. CONCLUSIONS AND RECOMMENDATIONS

A microcomputer-based control system, designed to control the

operation of the Arp diaphram-type pneumatic blood pump, has been

developed and presented in this thesis. The controller hae .. two

mo_de and the cl()_i;ed-loop mode. --·----·---·--···--·-· ··---·-··--· -··-· .. ·---~- ·-·-· --· ......

IIn the ~y mode, the attendant sZle~t~.J:!t~ .. P.1:1111PJP.S. rate and per cent systolic time operating parameters of th~ J>J9od .. pump. The -----------~------------------·---··-~--·---·--· ,.;.·••"' -. ••• _._. __ ,.... •k_ ••••• -------... ---~-·-·>--.... ~.--........ -.. -·-·. ...

output of the pump is then dependent on these parameters and the

resistance to flow presente~lin the f:~d=ii~p mode, the ·.,,...__., ___ .,

attendant selects the pumping rate and the flow rate to be

maintained. This flow rate is maintained automatically, in spite of

variations in pumping rate and system load, by computer controlled

selection of the per cent systolic time parameter. In addition,

either chamber of the blood pump can be placed in systole, at any

time, by depressing one of two manual systole switches~

The time response of the closed-loop pump controller to

changes in set point and load is approximately 10 seconds and does

not appear to be affected by the use of average model parameters to

obtain the controller gain constants. Both the time response obtained

and the fluid pressures developed appear to be within physiologically

acceptable bounds.

T~e per cent overshoot incurred to achieve this settling time

should not present problems in an animal test of the instrument. A

more representative simulation of the circulatory system and the

91

92

perfusion circuit would be a prerequisite to any human testing,

however.

The relatively large per cent overshoot expected as a consequence

of the one-quarter decay ratio tuning criteria was not observed. This

suggests that the controller constants calculated, using the first-

order-lag-plus-dead-time model obtained for the blood pump and flow

circuit and the one-quarter decay ratio criteria, are conservative.

The proportional term constant K could be increased to speed up the p

response of the blood pump controller at the expense of a larger over-

shoot. The integral term constant Ki could then be adjusted to speed

up convergence on the set point. In addition, derivative control action

could be included by selecting a non-zero derivative term constant Kd.

The controller constants, and hence the response of the controller,

may be optimized for a particular pumping rate, set point, or load

condition empirically.

The per cent overshoot may also have been reduced by restricting

the operating range of the blood pump itself. The per cent systolic

time parameter was limited in software, as mentioned previously,

to values between 5% and 95% and thus did not drive the pump operation

into saturation. In addition, the maximum output of the blood pump

was restricted by the selection of tubing diameters in the mock circu-

latory system, to less than 500 ml/minute. This flow rate is signifi-

cantly less than the maximum output capacity of the blood pump.

As mentioned previously, the blood pump and flow circuit were

modeled as a first-order-lag-plus-dead-time system. The sample open-

loop response curve indicated that the observed dead time was approxi-

93

mately one-half of the calculated dead time. Other curves, not shown

in the thesis, behaved similarly. This effect was expected, as the

analytical solution for the dead time and time constant is described

by Smith as being "conservative" (20). Further experimentation is

suggested to refine the first order model developed and to explore a

second order approximation.

The closed-loop response of the blood pump controller could

possibly be improved by decreasing the sampling time. Smith suggests

that the ratio of one-half the sampling time to the time constant be

significantly less than the ratio of the dead time to time constant.

Using the average dead time calculated, the ratio of one-half the

sampling time to the dead time is less than the ratio of dead time

to time constant by a factor of twenty. If, however, the system dead

time obtained is not accurate, this factor would change. The SB:IDpling

time can be decreased by a factor of two by using the CLOCK 2 signal

to preset the sample flag instead of the CLOCK 3 signal. The controller

constants would have to be recalculated using this new sampling time.

The steady state error in flow rate attained of 18 ml/minute

was compensated for by internally adding 18 ml/minute to the set point

entered. This technique was adopted after empirical variation of the

proportional and integral term constants failed to eliminate the

offset error. The offset error is most probably due to round-off

errors in the various calculation routines employed in the blood pump

control algorithm, and merits additional investigation.

With any microcomputer-based device, there is a trade-off

between functions to be performed in hardware and functions to be

performed in software. The experience gained in this endeavor suggests

94

that the analog-to-digital conversion of the flow rate be performed

entirely in hardware by a hybrid ADC and the result strobed into

the microcomputer as part of an interrupt service routine. This would

entail the wiring of a priority interrupt circuit, as the solenoid

valves are also sequenced by interrupt service routines, but would

eliminate the software overhead presently required to perform the

analog-to-digital conversion and decrease the conversion time.

The method of specifying operating parameters could be improved

by replacing the keyboard with rotary thumbwheel switches. The

thumbwheel switches are available with BCD encoding and display a

decimal number representing the switch position. They would, there-

fore, perform the functions presently accomplished by the keyboard,

the keyboard encoder, and the seven segment displays, while sig-

nificantly reducing software overhead. The microcomputer could scan

the thumbwheel switches, examine an operator set flag, or respond to

an operator initiated interrupt to determine when new operating

parameters were specified. A pause function could be included that

would allow the operator to temporarily stop the pumping action. The

current values of the pumping rate, per cent systolic time, and flow

rate set point would be preserved on the thumbwheel switches and would

be used when pumping action was resumed.

A desireable function, not included in the scope of the thesis,

is the synchronization of blood pump events with cardiac events by

using the electrocardiogram (EKG) signal. This mode of operation

would be applicable whenever a partial cardiopulmonary bypass was

95

performed. The pumping rate of the extracorporeal blood pump would