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Microlink Measurement and Control Systems

Technical Notes: Data Acquisition Techniques


Data acquisition and control systems need to get real-world signals into the computer. These signals come from a diverse range of instruments and sensors, and each type of signal needs special consideration. This page highlights points to think about, and should help you identify the most suitable interface for your measurements. The following techniques are covered:


Voltage Signals

The most commonly interfaced signal is a voltage signal. Thermocouples, strain gauge bridge circuits and gas concentration probes, for example, all produce voltage signals. Data acquisition hardware will condition the signal (amplify it or filter it for instance) and then convert it to a digital number using an analogue-to-digital converter. This digital value is then stored or processed by the computer.

Data acquisition systems are usually capable of directly handling low-voltage inputs. By low-voltage we mean from a few millivolts up to a few volts.

There are 3 major aspects of your voltage signal that you need to consider.
1. Amplitude
2. Frequency
3. Duration

1. Amplitude

If the signal is smaller than a few millivolts you may need to amplify it. If it is larger than the maximum range of your analogue input hardware (typically ±10 V) you will have to divide the signal down using a resistor network.

2. Frequency

You should decide the highest frequency you want to record. Higher frequencies may be present as noise and you will need to be aware of these. You can remove noise by filtering the signal before digitising it.

The highest frequency component of the signal determines how often you should sample the input (at least twice the highest component). If you have more than one input, but only one A-D converter, the overall sampling rate goes up in proportion to the number of inputs.

If you want to plot the signal showing the high frequency components you will need 10 to 20 points in a cycle to get a reasonable picture of a sinusoid. For slower changing, essentially DC signals, you need only consider the minimum time for a significant change in the signal.

3. Duration

How long do you want to sample the signal for? If you are recording the data the duration determines the storage required, which may be in computer memory or on disk. The format of the stored data also affects the amount of storage space required. Data stored in ASCII format, for example, takes more space than data stored in binary format.


pH electrode with BNC connector

High Impedance Probes

Certain types of transducer have a very high output impedance and are not able to supply enough current to use a normal voltage input. When connected to a normal amplifier, the currents drawn from the transducer can seriously distort the input signal. Typically the glass electrodes used to measure pH, or gas concentration probes, are of this type. You should connect them to a voltage measuring circuit with a very high input impedance. The Microlink 3800 module has 4 high impedance amplifiers and is suitable for conditioning this type of signal. You can then measure it with a normal voltage input. For pH measurements we also offer a USB unit and a small Ethernet data logger. Both of these are available on-line from Windmill Software.


Current Signals

Current is often used to transmit signals in noisy environments because it is much less affected by environmental noise pick-up. The full scale range of the current signal is normally either 4-20 mA or 0-20 mA. A 4-20 mA signal has the advantage that even at minimum signal value there should be a detectable current flowing. The absence of this indicates a wiring problem. Before A-D conversion the current signals are usually turned into voltage signals by a current-sensing resistor. The resistor should be of high precision (consider how much resolution the A-D converter will give you), and should match the signal to an input range of the analogue input hardware. For 4-20 mA signals a 50 ohm resistor will give a voltage of 1 V for a 20 mA signal. Choose 0.03% or 0.01% resistor accuracy.

Use of a current-sensing resistor gives you a normal voltage input and the voltage considerations apply.


Power Signals

You can monitor signals from electrical power supplies when the current signal is sensed with a current-sensing resistor, and high voltage signals are divided down using resistive dividers. Software scaling functions turn your low-voltage readings back to the original power supply voltage and current values.

Since many power supplies provide an AC signal, a signal conditioning input which provides a DC signal proportional to the root mean square (rms) amplitude of the input signal would be suitable.


Thermocouples

Thermocouples provide a low-voltage signal, typically a few millivolts. The relationship between temperature and voltage is non-linear. The voltage depends on:

  1. The temperature difference between the thermocouple junction itself and the point where the thermocouple wires terminate - the cold junction.
  2. The temperature of the cold junction.

Temperature measurement systems using thermocouples need to provide a means of measuring the cold junction temperature (often using a platinum resistance device) and an algorithm for linearising the voltage readings to temperature. Microlink hardware runs the Interface Management Language (IML) whose linearisation functions currently support B, E, J, K, N, R, S and T type thermocouples.

Thermocouple accuracy varies according to the type of the thermocouple you are using.


Resistance

Resistance measurements are made using a current source and a normal voltage input. The current flows through the unknown resistance and the voltage drop across the resistance is measured. 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°C.

When the resistance to be measured is small, then 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. 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.


Strain Gauge Bridges

Strain measurement is a special case of resistance measurement. A Wheatstone bridge arrangement is used to measure the resistance of the gauge, which varies as the gauge is distorted by the strain applied. The gauges may be not at their nominal resistance value, either because of variablility in the gauges or strains arising out of their attachment. Consequently strain measurement is usually concerned with the measurement of deviations from initial values, not absolute measurements. The initial values therefore need to be known. These may be very much larger than the subsequent changes in bridge imbalance caused by imposed strain, so an analogue-to-digital converter with high resolution is normally used to give the dynamic signal range required.

The measurement of bridge imbalance is affected by changes in the excitation voltage applied to the bridge. For long-term measurement, where component values may drift with time and temperature, the excitation voltage must be measured and any variations compensated for.

There are several Microlink methods of measuring strain.


Excitation

Many transducers need a power supply. The signal from these transducers is either a voltage or a mA current. For many transducers the supply will be low voltage DC, but for transducers based on capacitance measurement an AC supply may be required.

Microlink systems can be equipped with power supplies suitable for providing the excitation voltage(s) required. Because the choice of power supply will depend on the number and type of transducers, you should 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.


LVDTs (Linear Variable Displacement Transducers)

LVDTs normally use an AC excitation, and give an AC output signal. AC energised LVDTs therefore require additional oscillator and demodulation circuits. Given the specification of the LVDT, these circuits are supplied in the appropriate physical format. DC energised LVDTs are available with the oscillator-demodulator circuit built-in. These require an excitation supply and give a normal low voltage DC output signal. The Microlink 3806 module provides the excitation and signal conditioning for AC LVDT probes.


Encoders

Encoders used for monitoring angular position have two pulse train outputs, A and B, with the A phase shifted by 90º relative to B. The relative timing of the A and B pulses determine direction of rotation. A third pulse train, C, provides a synchronisation pulse once per revolution (in phase with channel A). The output of an encoder can be used as an input to a counter for angular position logging. Additional circuitry may be required to detect up/down counting (ie direction of rotation) and reset pulses based on channel C output.


Counter-Timers

Digital pulses can be counted, the frequency measured, or the time between pulses measured. For simple counting applications the major concern is that counts shouldn't be missed. When the count is read regularly for long periods it is sensible to reset the counter to 0 when it is read. You should also consider whether the count will exceed the capacity of the counter. A 16-bit counter will count up to 65535. Cascadable counters allow a carry count to be used to join two counters together: two 16-bit counters become one 32-bit counter. Frequency measurement works by counting pulses over a defined gate time, typically between 0.1 and 10 seconds. A period timer measures the time taken for a number of cycles of the input signal to occur.


Digital Signals

Digital outputs from switches and so on are treated as logic signals. They are sensed as on (1) or off (0). For volt-free contacts, where no external voltage is being switched, a small sensing voltage is applied to determine the switch state. This will often be 5 V to be compatible with TTL levels.

Where voltage is being switched the voltage level itself can be used to determine the logic state. The voltage levels determine the type of input required: TTL, up to 12 V logic or 24 V DC for example.

When digital signals are changing rapidly and become pulse trains you should consider counter-timer type inputs.


You can find these and other technical notes in the Microlink Measurement and Control Systems Catalogue which is available free from Biodata.


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