High-resolution differential-pressure indicator for the low-speed wind tunnel.

Mechanical Engineering Technical Report 2013/17P. A. Jacobs

School of Mechanical and Mining EngineeringThe University of Queensland.

December 2013

Contents

1 Hardware 2

2 Firmware 7

Abstract

This report describes the hardware and firmware for a differential-pressure indicator for the low-speed wind tunnel. It isessentially a two-channel high-resolution voltmeter attached to a pair of Sensirion pressure sensors but, being built around amicrocontroller running FlashForth, it is programmable.

1

1 Hardware

Inside the box, the prototype system is built on a single strip board, with a Microchip PIC18F2520 microcontroller [1] talking

to a 2-channel high-resolution analog-to-digital converter (MCP3422 [2]) and a Modtronix serial-enabled LCD[3]. Photographs

of the components are shown in Fig. 1 and schematic layouts of the circuit are shown on the following pages.

Figure 1: The photograph on the left shows the main circuit board with microcontroller (centre), MAX232 serial-port levelconverter (lower right) and the MCP3244 analog-to-digital converter (lower left). The potentiometer in view was used tosimulate a pressure sensor during firmware development. The photograph on the right shows the indicator boxed, sitting on theside of the low-speed wind tunnel, with two Sensirion SDP1108-R pressure sensors [4] mounted on the front panel. The uppersensor is attached to the pair of static-pressure rings on the contraction leading into the test-section of the wind tunnel and thelower sensor is connected to a Pitot-static probe.

2

Although the microcontroller is really quite capable and includes a 10-bit analog-to-digital converter, we really don’t use a lot of

its capability. Instead, we use its I2C bus to glue together the analog-to-digital converter chip (which we’ve been wanting to try

for some time) and the LCD display. As seen in the photograph, there are very few pins in use on the microcontroller and this

is confirmed on sheet 1 of the schematics, where more than half of the MCU pins are labelled as not connected. The principal

work of the microcontroller is to get the digital values from the MCP3422, do some arithmetic to convert the values from counts

of voltage into units of Pascals, and display them on the LCD.

The outputs of the Sensirion SDP1108-R pressure transducers can produce up to 4 V (for 500 Pa) and need to see a fairly

high input impedance (100kΩ is recommended [4]) at the data converter. The MCP3422 has an input range of ±2.048 V atthe channel input pins. To satisfy both of these requirements, a pair of 56 kΩ resistors is used as a voltage divider for each

channel (Schematic sheet 3). The 1µF capacitor provides some low-pass filtering for the incoming signal but also provides a low

impedance for the high-frequency (but small) currents that the switched-capacitor input stage of the MCP3422 requires during

the conversion process.

Note that the simple two-resistor voltage divider at the MCP3422 input is serviceable but not ideal. Using signed 16-bit values

from the MCP3422 means that the least-significant bit corresponds to 2 × 62.5 microvolts at the input to the voltage divider.Even with the input to the divider tied to ground, a leakage current of 25 nA through the protection diodes of the MCP3422

results in a 16-bit converter count of about 6 rather than (the ideal value of) zero, however, a count of 6 corresponds to about

0.7 millivolts and is not a problem in this application.

Communication with the microcontroller is provided by a minimal (3-wire) RS232 serial port. This port was used for the

development of the Forth firmware and is available for data logging. A suitable computer, with a serial-communications terminal

program, is needed to make use of this facility.

The current-limited supply (black box in Fig. 1) shows that, with the back-light on, the LCD draws about 50 mA. In contrast,

the microcontroller and data converter require only a few milliamps. The photograph shows the LCD backlight on, and this is

fine when operating from a DC plugpack. If you wish to run the system from a battery, the firmware should be changed to turn

the backlight off.

3

FILE: REVISION:

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TITLE

2

4

6

1

3

5

CONN_ICSP

+5V

Vss

!MCLR

VDD

VSS

DATA

CLK

NC

1N4148

10k

15pF

15pF

Vss

4MHz

Low−speed wind−tunnel −− MCU

Peter Jacobs

100n

470

Vss

1 3

PIC18F2520

!MCLR/VPP/RE31

RA0/AN02

RA1/AN13

RA2/AN2/VREF−/CVREF4

RA3/AN3/VREF+5

RA4/T0CKI/C1OUT6

RA5/AN4/!SS/HLVDIN/C2OUT7

VSS8

OSC1/CLKI/RA79

OSC2/CLKO/RA610

RC0/T1OSO/T13CKI11

RC1/T1OSI/CCP212

RC2/CCP113

RC3/SCK/SCL14

RC4/SDI/SDA15

RC5/SDO16

RC6/TX/CK17

RC7/RX/DT18

VSS19

VDD20

RB0/INT0/FLT0/AN1221

RB1/INT1/AN1022

RB2/INT2/AN823

RB3/AN9/CCP224

RB4/KBI0/AN1125

RB5/KBI1/PGM26

RB6/KBI2/PGC27

RB7/KBI3/PGD28

U101

+5V

Vss

Vss

SCL

2

1

CONN_RESET

47k

02−Nov−2013

MCU_TX

SDA

MCU_RX

Vss

NC

NC

NC

NC

NC

NC

NC

NC

NC

NC

NC

NC

NC

NC

NC

330

330

+5

V

4

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Low−speed wind−tunnel −− power and comms

Peter Jacobs

ST232CN

C1+1

V+2

C1−3

C2+4

C2−5

V−6

T2OUT7

R2IN8

R2OUT9

T2IN10

T1IN11

R1OUT12

R1IN13

T1OUT14

GND15

VCC16

device=MAX232EPE

U202

+5V

2

1

3

CONN_RS232

Vss

100uH

1u

1u

1u

1u

100n

Vss

Vss

10RMCU_TX

MCU_RX

TX

RX

GND

NC

NC

NC

NC

32

+5V

Vss

2

1

CONN_POWER

1N4004

1N4004

1210u

121u

IN OUT

7805

GND

1

2

3

U201

+9 to 12V

0V

02−Nov−2013

3

2

4

1

CONN_I2C

2k2

2k2

Vss

+5V

SDA

SCL

+V

GND

SDA

SCL

5

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Low−speed wind−tunnel −− 18−bit data converter

Peter Jacobs33

02−Nov−2013

MCP3422

CH1+1

CH1−2

VDD3

SDA4

SCL5

VSS6

CH2+7

CH2−8

U301

SDA SCL

Vss

2

1

3

CONN_CH1

VDD 5.0V

GND

Sensor signal 0.25−4.0V

CH2+

CH1+

Vss

Vss

100n

+5V

Vss

Vss

CH1+

2

1

3

CONN_CH2

Vss

CH2+

VDD 5.0V

GND

Sensor signal 0.25−4.0V

100uH

1210u

Vss

56k

56k

56k

56k1u

1u

Vss

Vss

6

2 Firmware

The firmware running within the microcontroller is written in Forth, with the FlashForth interpreter [5] being previously pro-

grammed into the MCU. During development, interaction with the microcontroller was through the RS232 serial port and the

application code was built interactively, in the four stages.

lswt.txt

The first stage was to talk to the MC3422, via the I2C bus, and get the converted values as integer counts. These are reported

via the RS232 serial port.

1 \ lswt.txt

2 \ Low -speed wind -tunnel board using a PIC18F2520 and FlashForth

3 \ with mcp3422 and divide -by two analog input ,

4 \ ready for Sensirion pressure sensors.

5 \ PJ, 09-Nov -2013

6

7 -lswt

8 marker -lswt

9

10 : mcp3422 -go -1 ( -- )

11 \ $d0 is default mcp4322 address for writing

12 \ 16-bit one -shot conversion of ch 1

13 $d0 i2cws %10001000 i2c! spen

14 ;

15

16 : mcp3422 -go -2 ( -- )

17 $d0 i2cws %10101000 i2c! spen

18 ;

19

20 : mcp3422@2byte ( -- n f ) \ Read the 16-bit result as 2 bytes

21 $d1 i2cws i2c@ak $8 lshift \ MSB

22 i2c@ak or \ LSB into same cell

23 i2c@nak $80 and 0= \ leave true if result is latest

24 ;

25

26 : report -counts ( n f -- )

27 if ." new " else ." old " then

28 . ." counts "

29 ;

30

7

31 : run -with -counts ( -- )

32 decimal

33 i2cinit

34 begin

35 mcp3422 -go -1 #100 ms mcp3422@2byte

36 cr ." chan 1 " report -counts

37 mcp3422 -go -2 #100 ms mcp3422@2byte

38 cr ." chan 2 " report -counts

39 #800 ms

40 key? until

41 hex

42 ;

lcd2s.txt

The second stage was to talk to the Modtronix LCD, via the same I2C serial bus, and display text.

1 \ lcd2s.txt

2 \ Words for the Modtronix LCD2S display

3 \ Peter J. 26-Oct -2013

4

5 \ Assume that i2c_base has been loaded.

6

7 -lcd2s

8 marker -lcd2s

9

10 : lcdc! ( c -- ) $50 i2cws i2c! spen ;

11

12 : remember ( -- ) $8d lcdc! ;

13

14 : blight -off ( -- ) $20 lcdc! ;

15 : blight -on ( -- ) $28 lcdc! ;

16

17 : lcd -clear ( -- ) $8c lcdc! ;

18

19 : lcd -curs -pos ( row col -- )

20 swap \ now have S: col row

21 $50 i2cws $8a i2c! i2c! i2c! spen

22 ;

23

24 : lcd -go-line -2 ( -- )

8

25 $2 $1 lcd -curs -pos

26 ;

27

28 : lcd -type ( c-addr n -- ) \ send string to be parsed

29 $50 i2cws $80 i2c!

30 for c@+ i2c! next

31 spen drop

32 ;

33

34 : lcd -hello ( -- )

35 i2cinit

36 lcd -clear

37 s" Hello" lcd -type

38 lcd -go -line -2

39 s" World !" lcd -type

40 ;

lswt-1.txt

We now need to convert from integer counts to voltage knowing that the sensitivity is 125µV for the least significant bit of theADC result. Since we would like to do the calculation with integers on the microcontroller, it is convenient to work with scaledunits of tenths of a millivolt and write the conversion as

Vout

[mV

10

]= count ∗ 125

100

To avoid losing too much precision, this calculation is done with the scale operator for mixed-precision operands, m*/. Once wehave the voltage value in tenths of a millivolt, the output word, (d.1), displays the integer value in a format that puts a decimalpoint in front of the last digit, thus making the displayed result appear as a fractional number.

1 \ lswt -1. txt

2 \ Low -speed wind -tunnel board firmware

3 \ extended to report millivolts to the LCD.

4 \ Load lswt.txt and lcd2s.txt before this file.

5 \ PJ, 09-Nov -2013

6

7 -lswt -1

8 marker -lswt -1

9

9

10 : mv10 ( n -- d )

11 \ Convert 16-bit count to tenths of millivolts.

12 \ With the divide -by-two resistors on the analog input ,

13 \ the least -significant bit corresponds to 62.5*2 microvolts

14 s>d #125 #100 m*/

15 ;

16

17 : (d.1) ( d -- )

18 swap over dabs

19

20 ;

21

22 : report -mv ( n f -- ) \ Assuming decimal , print millivolt value

23 if ." new " else ." old " then

24 mv10 (d.1) type space ." mV "

25 ;

26

27 : report -mv -lcd ( n -- ) \ Display millivolt value to LCD

28 mv10 (d.1) lcd -type s" mV" lcd -type

29 ;

30

31 : run -with -volts ( -- )

32 decimal

33 i2cinit

34 begin

35 mcp3422 -go -1 #100 ms mcp3422@2byte 2dup

36 cr ." chan 1 " report -mv drop \ S: ch1

37 mcp3422 -go -2 #100 ms mcp3422@2byte 2dup

38 cr ." chan 2 " report -mv drop \ S: ch1 ch2

39 swap \ S: ch2 ch1

40 lcd -clear s" ch1 " lcd -type report -mv -lcd

41 lcd -go -line -2 s" ch2 " lcd -type report -mv -lcd

42 #800 ms

43 key? until

44 hex

45 ;

10

lswt-2.txt

The final stage of the firmware needs to do a bit more arithmetic to convert from voltage to pressure. If we continue to workin tenths of a millivolt

[mV10

]and tenths of a Pascal

[Pa10

], we can write the quadratic voltage-pressure relationship for the

transducer [4] as

∆p[Pa

10

]=

Vout[mV10

]− 2500

[mV10

]37500

[mV10

]2 × 5000 [Pa

10

]In the Forth code below, we do the calculation in two steps:

dV = V out[mV

10

]− 2500

[mV

10

]and, using two scale operations,

∆p[Pa

10

]= (dV ∗ 5

375) ∗ dV

3750

1 \ lswt -2. txt

2 \ Low -speed wind -tunnel board firmware , continued ...

3 \ We really want the pressure sensor values to be

4 \ reported in Pascals.

5 \ Builds on lswt -1. txt.

6 \ PJ, 09-Nov -2013

7

8 -lswt -2

9 marker -lswt -2

10

11 : mv10 -to-pa10 ( d1 -- d2 ) \ tenths of mV to tenths of Pa

12 \ Incoming voltage should be in range 0.25V to 4.0V

13 \ so d1 should be no bigger than 40000.

14 #2500. d-

15 2dup d0 < if 2drop 0. then \ S: d

16 2dup \ S: d d

17 #5 #375 m*/ drop \ S: d n1 ( max value is 533 )

18 #3750 m*/ \ S: d2 ( in tenths of Pa )

19 ;

20

21 : report -pa ( n -- ) \ Assuming decimal , print Pascal value

22 space mv10 mv10 -to-pa10 (d.1) type ." Pa"

23 ;

11

24

25 : report -pa -lcd ( n -- ) \ Display Pascal value to LCD

26 mv10 mv10 -to-pa10 (d.1) lcd -type s" Pa" lcd -type

27 ;

28

29 : run -with -pa ( -- )

30 decimal

31 i2cinit

32 begin

33 mcp3422 -go -1 #100 ms mcp3422@2byte 2dup

34 cr ." chan 1 " report -mv drop dup \ S: n1 n1

35 report -pa \ S: n1

36 mcp3422 -go -2 #100 ms mcp3422@2byte 2dup

37 cr ." chan 2 " report -mv drop dup \ S: n1 n2 n2

38 report -pa \ S: n1 n2

39 swap \ S: n2 n1

40 lcd -clear s" ch1 " lcd -type report -pa -lcd

41 lcd -go -line -2 s" ch2 " lcd -type report -pa -lcd

42 #800 ms

43 key? until

44 hex

45 ;

12

References

[1] Microchip Technology Inc. PIC18(L)F2420/2520/4420/4520 data sheet: 28/40/44-pin enhanced flash microcontrollers with10-bit A/D and nanowatt technology. Datasheet DS39631E, Microchip Technology Inc., www.microchip.com, 2008.

[2] Microchip Technology Inc. MCP3422/3/4: 18-bit, multi-channel ∆Σ analog-to-digital converter with I2C interface andon-board reference. Technical Report DS22088C, Microchip Technology Inc., www.microchip.com, 2009.

[3] Modtronix Engineering. Serial LCD display. Datasheet LCD2S Revision 1, Firmware V1.40, http://www.modtronix.com/,2009.

[4] Sensirion. SDP1108/SDP2108 low differential pressure sensor with fast response time. Datasheet, http://www.sensirion.com/,2013.

[5] Mikael Nordman. FLASHFORTH for the Microchip PIC 18, 24, 30, 33 series and Atmel Atmega (Arduino) series. URLhttp://flashforth.sourceforge.net, http://personal.inet.fi/private/oh2aun, 2014.

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