Data acquisition measurements

- an overview or tutorial about the measurements that are made for data acquisition, the techniques used and avoiding the problems that can be encountered.

There is a large variety of measurements that can be made by data acquisition or DAQ systems. This wide variety of data acquisition measurements makes DAQ systems very flexible and useful in a variety of applications from general monitoring to making specific measurements in applications including production control and monitoring in every industry from chemical engineering to electronics or mechanical production, and many more varied applications including geographical or seismic monitoring.


Basic data acquisition measurements

Whatever the application used for the data acquisition system, a huge number of measurements can be made depending upon the sensors being used, but the basic measurements can be simplified down to some of the basic elements:

  • Voltage
  • Digital signals
  • Frequency or time interval

The sensors that are used in data acquisition measurements often return values of voltage in particular that can then be converted to the values of displacement, temperature, or whatever is being measured.

These data acquisition measurements that may be termed primary measurements will be looked at in turn, and then their applications in making other measurements in data acquisition systems will be covered.


Voltage

Voltage is one of the most commonly made measurements in any data acquisition system. Not only is it used as a data acquisition measurement in its own right, but it is also used for measurements with many other sensors including thermocouples, strain gauges, gas concentration probes and many more.

Voltages that are presented to a voltage measuring device is essentially an analogue quantity. However as data acquisition uses computer techniques, the analogue voltage needs to be converted into a digital format. The voltage measurement will use a form of digital to analogue converter to convert the analogue voltage into a digital representation of its value. The greater the number of bits, the greater the resolution of the measurement.

Typically the voltages that are required to be measured in data acquisition systems range from a few millivolts up to a few volts. Voltages that are much higher than this need to be reduced before they are measured.

Some sensors used for data acquisition require voltage measurements to be made with a very high impedance device because their source impedance is high. Any current drawn from the sensor will apply a load which will distort the reading. The types of sensor that fall into this category are the glass electrodes types that are used for measuring pH or gas concentration. Special high impedance voltmeters are available for these measurements.


Digital signals

Often in any data acquisition system it is necessary to measure the status of an indicator. This will typically be one of two states. Under these circumstances it is not necessary to measure the actual voltage, although this is one way to do it, but rather measure whether the line is above or below a given voltage level. This can be done with a simple comparator.

In many data acquisition applications the particular sensor may not be a logic circuit, but a simple switch. To generate a logic signal a voltage (often 5 V so that it is TTL compatible) is applied through a current limiting resistor and the resulting voltage or absence of it is measured. It is worth noting that the circuit with the switch should be configured so that when the switch is open circuit, the sense line is not left open circuit otherwise stray pick-up may be a problem. This can be done simply by providing the voltage through a suitable resistance and then having the switch taken to ground. Thus when the switch is closed the sense line will be shorted to ground and when it is open it will see the supply voltage through the current limiting resistor.

When a the number of instances a digital signal changes state in a given time, or the interval between them needs to be measured, then frequency and time interval measurement techniques are needed


Frequency and time interval measurements

In many data acquisition applications, frequency or time interval measurements are required. The two measurements are very similar. They are simply the reciprocal of each other. Time interval is 1 / f.

Data acquisition modules that can count time intervals or measure frequency are widely available. The time interval measurement measures the time taken between a logic state crossing from one state to another. A frequency measurement counts the number of state changes in a given time.. Some simple switching in the circuitry enables both time interval and frequency to be measured.

One of the key elements with any frequency or time interval measurement is to ensure that the counter timer sees all the required pulses and no more. This can sometimes be more difficult than it may appear at first sight. Long lines can alter the shape of the pulse, slowing it down so that the counter timer does not trigger. Alternatively for frequency measurements, stray pick-up can introduce additional pulses. To overcome these problems, lines should be kept short and the line impedances should be low to reduce the effect of pick-up. By using these and other basic precautions counts should be accurate and repeatable.


Data acquisition derived measurements

Using the basic voltage, digital and frequency / timer measurements, it is possible to perform an enormous variety of other measurements in a data acquisition system. Many sensors, for example, provide an analogue voltage proportional to the quantity they are measuring. This means that the voltage measurement in a data acquisition system is one of the most important techniques to master.

  • Current measurements     Current is a measurement that is commonly made within the wider range of electronics test. It is also widely used for data acquisition measurement applications as well. Although there may not be many applications where it is necessary to monitor the current being consumed by a unit, current is often used in data acquisition applications to transmit signals in noisy environments. The reason for this is that current measurements are generally much lower impedance and they are less affected by pick-up of any electrical noise in the environment.

    In order to sense or measure the current, the signal is converted into a voltage by placing a small resistor in the circuit and then measuring the voltage across it. This resistor should be a high precision type as its accuracy will have a direct bearing on the accuracy of the measurement. The actual tolerance or precision requirement for the resistor is determined by the level of accuracy needed for the overall measurement.

    The values of current that are normally used may range up to 20 milliamps, and the resistors used may be of the order of 100 ohms, but these figures will depend upon the given application. Tolerances on the resistors used are often as tight as 0.01%
  • Temperature measurement with thermocouples     Temperature measurement is one that is often needed in data acquisition systems. Thermocouples consist of a junction of two metals that produce a small voltage dependent upon the temperature difference between the thermocouple junction itself and the point where the thermocouple wires terminate. This is known as the cold junction. The way in which this occurs is beyond this page on the website, but it is worth noting that the voltages produced are small. As a result care is required to ensure that there is no stray pick-up from the environment and that there are no DC offsets on the system. Either of these could produce significantly erroneous results.
  • Resistance measurements     Another measurement that is widely used in data acquisition systems is the resistance measurement. This again uses a voltage as the basis of the measurement. Essentially it is done by using a current source with an accurately defined level of current, and then measuring the resultant voltage across the resistance under test. This technique can then be used not just for determining the resistance of particular elements but as the basic for other parameters that are required within data acquisition systems.

    To prevent errors when the values of resistance are small, the leads connecting the element to be measured to the system must have a much lower level of resistance. Accordingly these need to be kept short and sufficiently low resistance.
  • Strain gauges and strain measurement     Strain measurement is used in many data acquisition applications. These data acquisition systems may be required for geographical applications as well as monitoring strain on vessels in engineering manufacturing applications.

    Strain measurement can be considered as a special case of resistance measurement. As the changes in value are very small, a Wheatstone bridge arrangement is normally used, and in view of the variations that may be present, the changes in strain are measured as deviations from initial values. A voltage is applied to the bridge circuit and the voltage across the required element is measured, being converted from a voltage to a digital value in the normal way. Changes are generally small and therefore reasonably high levels of resolution are needed.

    As a result of the fact that only the changes are measured it is necessary to determine the initial values by some other means. Additionally as the values obtained from the strain gauge are dependent upon the supplied voltage, this needs to be measured and accurately maintained, especially if the system is to be used over a period of time where voltages, etc may drift.

Summary

Data acquisition is a very important area of the test and measurement industry. Data acquisition systems are required in many applications from electronics manufacturing to chemical engineering, mechanical manufacture as well as more diverse applications such as monitoring geographical data from mountains and volcanoes as well as many other interesting and diverse uses.

There are naturally many different measurements that can be made by data acquisition systems. Those mentioned above are just a few of the possibilities, but many more exist.

In view of this there are many data acquisition products available on the market that may use data acquisition cards directly included in PXI or VXI chassis, or in computers. In some instances bus systems may be incorporated into the data acquisition system - GPIB, USB, and RS232 ware widely used. Wireless systems are also used in many applications. With this level of flexibility it is possible to create systems that can be tailored to a given applications to make the measurements that are required.

By Ian Poole


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