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Sensors
A sensor converts the physical phenomena of interest into a signal that is input into your data acquisition hardware. There are two main types of sensors based on the output they produce: digital sensors and analog sensors.
Digital sensors produce an output signal that is a digital representation of the input signal, and has discrete values of magnitude measured at discrete times. A digital sensor must output logic levels that are compatible with the digital receiver. Some standard logic levels include transistor-transistor logic (TTL) and emitter-coupled logic (ECL). Examples of digital sensors include switches and position encoders.
Analog sensors produce an output signal that is directly proportional to the input signal, and is continuous in both magnitude and in time. Most physical variables such as temperature, pressure, and acceleration are continuous in nature and are readily measured with an analog sensor. For example, the temperature of an automobile cooling system and the acceleration produced by a child on a swing all vary continuously.
The sensor you use depends on the phenomena you are measuring. Some common analog sensors and the physical variables they measure are listed below.
When choosing the best analog sensor to use, you must match the characteristics of the physical variable you are measuring with the characteristics of the sensor. The two most important sensor characteristics are
Sensor Output
The output from a sensor can be an analog signal or a digital signal, and the output variable is usually a voltage although some sensors output current.
Current Signals. Current is often used to transmit signals in noisy environments because it is much less affected by environmental noise. The full scale range of the current signal is often 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 conversion by the analog input subsystem, the current signals are usually turned into voltage signals by a current-sensing resistor. The resistor should be of high precision, perhaps 0.03% or 0.01% depending on the resolution of your hardware. Additionally, the voltage signal should match the signal to an input range of the analog input hardware. For 4-20 mA signals, a 50 ohm resistor will give a voltage of 1 V for a 20 mA signal by Ohm's law.
Voltage Signals. The most commonly interfaced signal is a voltage signal. For example, thermocouples, strain gauges, and accelerometers all produce voltage signals. There are three major aspects of a voltage signal that you need to consider:
The amplitude is related to the sensitivity (resolution) of your hardware. Refer to Accuracy and Precision for more information about hardware sensitivity.
The highest frequency component of the signal determines how often you should sample the input. If you have more than one input, but only one analog input subsystem, then the overall sampling rate goes up in proportion to the number of inputs. Higher frequencies might be present as noise, which you can remove by filtering the signal before it is digitized.
If you sample the input signal at least twice as fast as the highest frequency component, then that signal will be uniquely characterized. However, this rate might not mimic the waveform very closely. For a rapidly varying signal, you might need a sampling rate of roughly 10 to 20 times the highest frequency to get an accurate picture of the waveform. For slowly varying signals, you need only consider the minimum time for a significant change in the signal.
The frequency is related to the bandwidth of your measurement. Bandwidth is discussed in the next section.
Sensor Bandwidth
In a real-world data acquisition experiment, the physical phenomena you are measuring has expected limits. For example, the temperature of your automobile's cooling system varies continuously between its low limit and high limit. The temperature limits, as well as how rapidly the temperature varies between the limits, depends on several factors including your driving habits, the weather, and the condition of the cooling system. The expected limits might be readily approximated, but there are an infinite number of possible temperatures that you can measure at a given time. As explained in Quantization, these unlimited possibilities are mapped to a finite set of values by your data acquisition hardware.
The bandwidth is given by the range of frequencies present in the signal being measured. You can also think of bandwidth as being related to the rate of change of the signal. A slowly varying signal has a low bandwidth, while a rapidly varying signal has a high bandwidth. To properly measure the physical phenomena of interest, the sensor bandwidth must be compatible with the measurement bandwidth.
You might want to use sensors with the widest possible bandwidth when making any physical measurement. This is the one way to ensure that the basic measurement system is capable of responding linearly over the full range of interest. However, the wider the bandwidth of the sensor, the more you must be concerned with eliminating sensor response to unwanted frequency components.
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