Electronic instrumentation called data acquisition systems ((DAQs or DASs) acquire data from sensors. They are often extended to be instrumentation and control systems. This is a tutorial presentation of principles of instrumentation that is typically multi-channel, medium to high resolution (12 to 20 bits) and relatively slow - slower than oscilloscopes in sample rate. DAQ principles - mostly basic, with some subtleties and simplifying concepts - and sensors will be emphasized.
To give the presentation a concrete context, the application example will be a rocket test system as used to verify the performance of small rockets fired statically on test stands. Test firings must be sequenced by a controller and a DAS acquires the sensor data. The rocket test control system must be informed about what is going on in the rocket, and the instrumentation subsystem provides it. Sensors convert quantities of interest, such as tank pressures or acceleration, to electrical form. The data acquisition system converts these electrical quantities to digital form compatible with the control computer as input.
Data Acquisition System
The numerical values of measured quantities, or data, are usually converted into digital form for the control computer by a DAS. A typical DAS is shown below.
Sensor waveforms enter antialiasing filters that remove high frequencies from them. This is sometimes necessary to prevent aliasing, the generation of spurious waveforms. A common example of aliasing is the backward rotation of wheel spokes in movies or on television. The successive picture frames of a movie or television signal are not continuous and cause difference (or “beat”) frequencies that result in these spurious images. Spurious waveforms are generated in DASs when the sensor waveforms are not "slowed down" sufficiently to remove the faster changes in them that cause aliasing. Any process that samples a continuous quantity outputs a discrete sequence of values and is subject to aliasing. To avoid it, all frequencies that are at least half the sampling rate are removed by the filters.
The MUX is an analog multiplexer, an electronic switch, like a television channel switch. It is commanded by the microcomputer (μC) to switch to a particular sensor input channel. Each channel is selected in turn for measurement. The PGA is a programmable-gain amplifier. Different sensors require different amounts of amplification of their waveforms, and the PGA gain is commanded by the μC. The A/D converter, or ADC, converts the filtered and amplified analog sensor waveform into a digital count that can be input to the μC.
The number of discrete values of analog input voltage that the ADC can distinguish between is its resolution, measured in the unit of bits. For N bits, the number of different outcomes is 2N. A 12-bit ADC can discriminate between 212, or 4096, different analog input values. If its full-scale range is 4.096 V, each of the 4096 input levels is 1 mV apart. The 12-bit digital output of the ADC consequently has a resolution of 1 mV per count, or 1 mV per least-significant bit (LSB). The resolution of the ADC is 1 mV/LSB.
The computer further processes the sampled sensor quantities from the ADC, but in digital form. The ADC counts are unprocessed or raw data that must be corrected for offset and gain (slope) errors caused by inaccuracies in both the sensors and analog DAS circuits preceding the ADC. Then the result must be corrected for sensor nonlinearity, if appreciable, and the result accurately scaled.
Sensors typically used in rocket flight or testing are:
- temperature sensors: thermocouples, RTDs, thermistors, solid-state.
- pressure sensors: silicon or sapphire.
- flow sensors: turbine, ultrasonic doppler.
- inertial sensors: rate and vertical gyros, solid-state acceleration and rotational sensors, tilt switches.
- proximity sensors: microswitches.
- electrical sensors: voltage and current sensing.
- cryogenic sensors: cryogenic thermistors.
Most sensors output a voltage that is scaled to the measured quantity, to give a conversion factor (gain) in the form of, for example, V/kN, for a force sensor, or V/oC for a temperature sensor. Voltages occur across two circuit nodes (as "across" quantities). If one node is the zero-volt reference node for the system, or ground, then the sensor output is a voltage relative to ground. Voltages measured on nodes relative to ground are single-ended.
Some sensors have two terminals across which their output voltage appears and neither is ground. These are differential voltages because they are the difference between the voltages on each terminal, as measured with respect to ground. They are also sometimes referred to as “floating”.
Differential outputs are typical for sensors that are part of a common instrumentation circuit called a bridge. The "Sensor Bridge Circuit" diagram below illustrates this for a strain-sensor bridge. The bridge output voltage is the difference in voltages between the AIN+ and AIN– nodes, measured with respect to ground. In other words, a voltmeter with its – input connected to the ground terminal can be used to measure the voltage at AIN–. This reading is subtracted from the voltage measured at AIN+ with respect to the same ground node for the bridge output voltage.
Examples of bridge-based sensors are RTDs (temperature), pressure and strain or force (load cell) sensors. The resistance of these sensors varies with the measured quantity. In the diagram below, a load cell is configured from two strain gages with opposite polarity drives.
Bridge circuits consist of two voltage dividers driven by the bridge supply. Each two-resistance divider is a half-bridge. Bridge output sensitivity is proportional to bridge excitation voltage. For half-bridge sensors, the other half bridge is a divide by 2 voltage divider, consisting of precisely matched equal-value resistors.
The two strain gages are attached to opposite sides of a cantilever, so that when it is flexed, the top strain-gage increases in resistance (by +ε) and the bottom one decreases (ε). When not flexed, both sensors have nominally the same resistance and the voltage at AIN+ is half the bridge voltage, Vbr. For zero differential input voltage at zero scale, the other voltage divider consisting of stable, equal-value (R) resistors divides Vbr in half at AIN-. The output voltage of AIN+ varies around half the bridge voltage, resulting in a bipolar (+/-) differential output.