Rudolf Žitný, Ústav procesní a zpracovatelské techniky ČVUT FS 2010

Data acquisitionProtocolsA/D conversionLabview

Experimental methods E181101 EXM7

Rudolf Žitný, Ústav procesní a zpracovatelské techniky ČVUT FS 2010

Data transferEXM7

Typical arrangement of measuring chain consists in probes (Pt100, thermocouples, strain gauges…), each driven by its own module (e.g. current source, Wheatstone bridge). Analog signals (voltages 0-10V, current 4-20 mA) are connected to combined multiplexer (switching channels) and analog/digital converter 12-18 bits. This device is controlled via serial or parallel bus USB,RS232,IEEE488 to a PC controller.

Instrumentmass balancespH meterpower meterthermostat

PC

interfaceIAE232,425IEE488

Pt100 bridge MultimeterMUX+A/D converter

PC

interfaceIAE232,425IEE488

Pt100 bridge

Pt100 bridge

ANALOG PART - current loop 4-20 mA, voltage 10V

DIGITAL PART – transmission SERIAL/PARALLEL, by wires, optical fibres, IR, elmag.waves (Bluetooths)

Multiplexer MUXEXM7I0

I1

out

sel

MUX selects one of many analog input signals and forwards the selected input into a single line. A multiplexer of 2n inputs has n select lines, which are used to select required connection.

EMR (electro mechanical relays) or SSR (solid state relays –e.g. using MOSFET

transistors) are used for switching of input/output connections

SSR exhibits a longer operational life (EMR typically only 106 cycles, SST 1010).

SSR will almost always exhibit higher input to output isolation voltages than an EMR. Important e.g. in telecom applications design.

Package Dimensions are less for SSR.

EMR have resistance in the range of 100 milliohms, whereas SSRs have an On Resistance in the range of 10 Ohms. This is the reason why EMR allows for greater load current capability and less signal attenuation.

EMR have an output capacitance of less than 1 picoFarad, whereas SSRs typically have a capacitance of greater than 20 picoFarads. EMRs are therefore a better option for HF(high frequency) applications.

A/D conversionEXM7

12bits resolution 2.4 mV 14bits resolution 0.61 mV 16bits resolution 0.15 mV 18bits resolution 0.04 mV

ADCIN OUT

Analog signal (voltage in the nominal range 10V) is converted to integer number represented by 8 to 24 bits. Number of bits determines resolution of A/D converter (for 10V range):

There are several ways how to realize this conversion: for example using a discharged capacitor with continuously decreasing voltage that is compared with measured voltage at precise time clocks. Number of clocks corresponding to equality of voltages is the result of conversion.

Example: NI USB-6281 18 bit16 analog inputs (18-bit), 625 kS/s single-channel (500 kS/s aggregate) 2 analog outputs (16-bit, 2.8 MS/s); 24 digital I/O (8 clocked); 32-bit counters Accuracy 1 mV at +-10 V range Accuracy 0.028 mV at +-10 mV range.

The USB connects peripheral devices, such as keyboards to PCs. The USB is a Plug and Play bus that can handle up to 127 devices on one port, and has a theoretical maximum throughput of 480 Mb/s. Only 4 wires (supply 5V, +data,-data, ground), cable length restricted. Like RS-232, USB is useful for applications in a laboratory

setting that do not require a rugged bus connection.

Data transferEXM7

RS-232 is a specification for serial communication with scientific instruments (multimeters, weights, analyzers,…) and peripherals such as printers using only 3-5 wires (transmit, receive, ground). It is possible to connect and control only one device at a time. Slow interface with typical data rates of less than 20 kbytes/s.

The General Purpose Interface Bus (GPIB) is an IEEE-488 (a standard created by the Institute of Electrical and Electronics Engineers) standard parallel interface used for attaching sensors and programmable instruments to a computer. GPIB is a digital 8-bit parallel communications interface capable of achieving data transfers of more than 8 Mbytes/s. It allows daisy-chaining up to 14 instruments to a system controller using a 24-pin connector. It is one of the most common I/O interfaces present in instruments and is designed specifically for instrument control applications.

Serial data transfer 8bits (character) are converted to sequence by hardware

Parallel data transfer (8 wires for data, another wires for periphery addressing)

LabviewEXM7

Hopper

LabviewEXM7

OSI Open System Interconnection model (International Organization for Standardisation) sub-divides a communications system into smaller parts called layers. A layer is a collection of conceptually similar functions that provide services to the layer above it and receives services from the layer below it. The lowest physical layer describes specific protocols (RS 232…), hardware connections (voltage, pins, cables…). The highest application layer represents interface to user (example is TCP/IP).

Variety of transmission protocols and broad variation of hardware complicate development of software that should be user friendly and portable.

1. International standards represent systems of hierarchically arranged standardized layers

2. There exist proprietary tools (not standardized by international committees) making use programming of instrument control quite easy for beginners.

Example is LabVIEW (Laboratory Virtual Instrumentation Engineering Workbench) - a visual programming language from National Instruments. LabVIEW is commonly used for data acquisition, instrument control, and industrial automation on a variety of platforms including Microsoft Windows, UNIX, Linux. Problems are described by the structure of a graphical block diagram (the LV-source code) on which the programmer connects different function-nodes by drawing wires. These wires propagate variables and any node can execute as soon as all its input data become available (Labview is an interpret but the executable code can be generated).

Labview exampleEXM7

Hardware configuration

NI

NI USB-6281 Measured voltage

USB connector

Labview example (1/7)EXM7

Create new VI (Virtual Instrument)

projectSelect driver for active

instrument NI 6281 (it will be automatically

identified)

Labview example (2/7)EXM7

Icon of Digital Acquisition Unit

appears

Specification of signal (voltage, resistance…)

Select channel (16 possible channels)

Labview example (3/7)EXM7

Range and units of measured

voltage

Specify frequency of sampling

Other option is CONTINUOUS

sampling

Labview example (4/7)EXM7

Right click on the DATA item

of the DAQ icon

DATA output will be redirected to the GRAPH icon

Labview example (5/7)EXM7

Front view panelwith representation of

GRAPH and STOP button

RUN/ABORT buttons

Block Diagram for graphical

programming

Labview example (6/7)EXM7

A SPOOL appears at the position of locator on the

connection wire. CREATE e.g. numeric indicator

Actual value is indicated here

Move the DAQ Icon to the right: the connecting WIRE

appears

Labview example (7/7)EXM7

CREATE CONTROL to change the sampling rate

If you need to delete something

use Edit Ctrl X

CREATE box (express function) for

signal processing

Hint: Right Click when outside of the block

diagram submits menu of express functions

Labview project syringeEXM7

R

Rliquid

V3 V2 V1

V3-voltage drop on fixed resistor

V2-voltage drop in liquid

V1-output of pressure transducer

Rudolf Žitný, Ústav procesní a zpracovatelské techniky ČVUT FS 2010

Labview project syringeEXM7

Comments are written to data file that complicates

MATLAB processing