- 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.
