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A data acquisition and control system consists of many components and subsystems that are integrated to:
For example, to regulate the temperature in your house, you need to continuously measure the room temperature using a thermocouple (TC); condition the TC signal for acceptance by an analog input board; convert this signal from an analog to a digital format readable by your PC; use software to compare this actual temperature data against the desired temperature; and cause the PC to respond via a digital output to turn the heating/cooling circuit on or off.
In order to sense and measure physical variables such as pressure, flow, and motion, we use transducers (sensors) which convert the physical variables into electrical signals and transmit these electrical signals either to a signal conditioning device or directly to your data acquisition board.
The signal conditioning device amplifies and filters the sensor signal so it can be used by an analog input board. Additional information can be found in the Tech Note on pg 54, in our Signal Conditioning Chapter.
After signal conditioning, the sensor signal is passed to the Analog Input (A/D) Board. The A/D Board converts the conditioned voltage or current signal into a digital format which is readable by your PC. (See Schematic 1.) The A/D Board will typically incorporate several of the capabilities below:
An analog signal is a continuous-time function with a physical parameter defined for every instance of time. This signal must be converted into a discrete-time signal so that it can be used by the computer to depict the original signal. Analog to Digital conversion is a ratio operation, where the input signal is compared to a reference, and converted into a fraction, which is then represented as a coded digital number. To optimize measurement accuracy, there is a minimum and a maximum number of data points that need to be acquired.
Sampling Rate: One of the most critical factors when selecting an A/D Board is sampling rate (speed). The sampling rate is a measure of how rapidly your A/D board can scan the input channel and identify the discrete value of the signal present with respect to a reference signal. In order to better understand sampling rate, consider this example: If you want to acquire a sine wave that has a frequency of 1Hz (1 cycle per second), how many data points will be necessary to approximate the waveform?
If the sample rate is too slow, then a completely different waveform of a lower frequency is constructed from the data acquired. This effect is called aliasing. To avoid aliasing, it is necessary that the sample rate be at least twice the highest expected frequency input, and the signal should be bandlimited. Thus to sample a 1Hz Sine Wave, the sample rate should be at least 2 Hz. However, a sampling rate of 8 - 16Hz would result in a more accurate representation of the signal being acquired. Diagram 1 illustrates the effect of sampling at different rates. Low-pass filters may be employed to eliminate high-frequency transients that could corrupt the data.
Throughput is another important selection factor. If your A/D Board has 4 input channels with a maximum throughput of 4Hz,when you are sampling on a single channel you will be able to acquire 4 samples per second. However, if you want to sample on all 4 channels, you will only acquire 1 sample per second per channel, a sampling rate of 1Hz. The charts on pp. 4-7 show the maximum throughput for each A/D Board. Thus:
Resolution defines the number of divisions into which a full-scale input range can be divided to approximate an analog input voltage. For example: If you want to measure a 0-10V signal, and your A/D Board has 8-bit resolution, you can measure the input signal in steps of 10/2 8 = 10/256 = 0.039V. Thus a 10V input is equal to the digital number 255, and a 0V input equals 0. This A/D Board would be capable of detecting only input changes greater than 0.039V. Each 0.039V change in the input is indicated by adding or subtracting 1 from the previous number, i.e 9.961V is digitally represented by 254. The true resolution of an A/D board can be as much as 2 bits lower than the manufacturer's specification. This means that a 16-bit board may be accurate only to 14 bits.
You have 2 basic options when connecting your input signals: Single-Ended and Differential. Single-ended (SE) inputs offer the lowest cost per input. However, Differential (Diff) inputs offer greater noise immunity for more accurate readings. (A typical A/D board offers 16 SE or 8 Diff input channels.)
Single-Ended Inputs should be utilized whenever analog measurements are to be made with respect to a common external ground, and there is no practical way to bring both a remote ground and the analog ground back to your DAS system's front end.
A Differential Input Configuration should be considered: