Issue 1: Interfacing Transducers
The Windmill software suite is now available bundled with the new Microlink 552 transducer monitoring card, from Biodata. This card plugs into the PC and lets you monitor current, load, pH, pressure, resistance, strain, temperature and voltage. For more details of the Microlink 552 package contact Biodata at [email protected].
ABOUT THE WINDMILL DATA ACQUISITION AND CONTROL SOFTWARE SUITE
Windmill is a suite of software which can handle over 100 input and output channels. It integrates Ethernet, GPIB (IEEE-488), ISA-bus, USB, Modbus, RS232 and RS485 devices, from many different manufacturers. Windmill modules include logging, charting, alarm monitoring, NetDDE, PID, virtual instrument generation and sequence control. For more details see http://www.windmill.co.uk/windmill.html
In this section we highlight things to think about when specifying the measurement hardware to interface your transducers.
Many transducers need a power supply. For most the power supply will be a low voltage dc (direct current), but for transducers based on capacitance measurement an ac (alternating current) supply may be required.
Check that your measurement hardware can be equipped with power supplies suitable for providing the excitation voltage(s) you need. Because the choice of power supply will depend on the number and type of transducers, it's best to draw up a list of transducers and their excitation requirements. The information needed includes voltage range and current requirements, and for ac excitation voltages, the frequency used.
For some transducers you need to measure resistance. To do this you use a current source and a voltage input. The current flows through the unknown resistance and you measure the voltage drop across the resistance. For example, a 1 mA current source will provide a voltage of 100 mV across the 100 ohm of a Pt100 temperature measuring device at 0 oC.
When the resistance to be measured is small, the resistance in the leads to the device to be measured can be a significant source of error. To deal with this problem connection arrangements are available which allow the lead resistance to be measured and compensated for (2-wire, 3-wire, 4-wire and 4-wire compensated). More connections are required for these arrangements; so for the same number of connections, half the number of measurement channels are available compared to straightforward voltage measuring channels. If you think, therefore, that lead resistance may be a problem in your measurements, make sure you ask your hardware supplier about their lead arrangement options.
Resistance considerations apply to, amongst others, temperature measurement using RTDs, displacement measurement using resistive linear position and light level measurement using photo-resistors.
Strain measurement is a special case of resistance measurement. A Wheatstone bridge arrangement is used to measure the resistance of a gauge, which varies as the gauge is distorted by the strain applied. You should take into account that the gauges may be not at their nominal resistance value, either because of variability in the gauges or strains arising out of their attachment. Consequently strain measurement is usually the measurement of deviations from initial values, not an absolute measurement. You therefore need to know the initial strain values. These may be very much larger than the subsequent changes caused by strain, so you need to make sure either that your analogue-to-digital converter has a high enough resolution to give the dynamic signal range required, or that you can zero out the initial, unstrained, bridge output. Check with your hardware supplier. (See future issues for a discussion of analogue-to-digital converters.)
The measurement of strain is affected by changes in the excitation voltage applied to the Wheatstone bridge. For long-term measurement, where component values may drift with time and temperature, the excitation voltage should be measured and any variations compensated for in software.
More details of strain measurement are given in Issue 17 of Monitor.
Sensors for Measurement and Control by Peter Elgar; 176 pages; 1999.
A comprehensive and practical introduction to the increasingly important topic of sensor technology. It also serves as an extensive reference to sensors and their applications. Order from UK; Order from US
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