Introduction to measurement systems sensor classification
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A great number of sensors are available for different physical quantities. In order to study them, it is advisable first to classify sensors according to some criterion. In addition to the criteria mentioned here, several additional criteria are g...
A great number of sensors are available for different physical quantities. In order to study them, it is advisable first to classify sensors according to some criterion. In addition to the criteria mentioned here, several additional criteria are given in . In considering the need for a power supply, sensors are classified as modulating or self-generating. In modulating (or active) sensors most of the output signal power comes from an auxiliary power source. The input only controls the output. Conversely, in self-generating (or passive) sensors output power comes from the input.
Modulating sensors usually require more wires than self-generating ones because power is supplied by wires different from the signal wires. Moreover the presence of an auxiliary power source can increase the danger of explosion in explosive atmospheres. Modulating sensors have the advantage
that their overall sensitivity can be controlled by the power supply voltage. Some authors use the terms actiue for self-generating and passiue for modulating. To avoid confusion, we will not use these terms.
In considering output signals, we classify sensors as analog or digital. In analog sensors the output changes in a continuous way at a macroscopic
level. The information is usually obtained from the amplitude, although sensors with output in the time domain are usually considered as analog. Sen-
sors whose output is in the form of a variable frequency are called quasi-digital because it is very easy to obtain a digital output from them.
The output of digital sensors takes the form of discrete steps or states. Digital sensors do not require an ADC, and their output is easier to transmit than that of analog sensors. Digital output is also more repeatable and reliable, and often more accurate. But regrettably many physical quantities cannot be measured by digital sensors.
In considering the operating mode, sensors are classified in terms of their function in a deflection or a null mode. In deflection sensors the measured
quantity produces a physical effect that generates in some part of the instrument a similar but opposing effect that is related to some useful variable. For example, a dynamometer to measure force is a sensor where the force to be measured deflects a spring to the point where the force it exerts, which is proportional to its deformation, balances the applied force.
Null-type sensors attempt to prevent deflection from the null point by applying a known effect that opposes that produced by the quantity being
measured. There is an imbalance detector and some means to restore balance. In a weighing scale, for example, the placement of a load on a pan
produces an imbalance indicated by a pointer. The user has to place one or more calibrated weights on the other pan until a balance is reached, which
can be ascertained from the pointer's position.
Null measurements are usually more accurate because the opposing known effect can be calibrated against a high-precision standard or a reference quantity. The imbalance detector only measures near zero; therefore it can be very sensitive and not require any calibration. Nevertheless, null measurements are slow, and despite attempts at automation using a Servo-mechanism, their response time is usually not as short as that of deflection
In considering the input-output relationship, se.qsors can be classified as zero, first, second, or higher order (Section 1.5). The order is related to the
number of independent energy-storing elements present in the sensor, and this affects its accuracy and speed. Such classification is important when the sensor is part of a closed loop control system.
Table 1.1 compares all the classification criteria and gives examples for each type in different measurement situations. In order to study these myriad devices, it is customary to classify them according to what they measure. Consequently we speak of sensors for temperature, pressure, flow, humidity
and moisture, position, velocity, acceleration, force, torque, and so forth.
This classification, however, can hardly be exhaustive because of the seemingly unlimited number of measurable quantities. Electronic engineers prefer to classify sensors according to some variable quantity: resistance, capacitance, inductance, and then to add sensors generating voltage, charge, or current; and other sensors not inclucled in the preceding groups. We will adopt this approach here because it reduces the number of groups and leads to the direct study of the associated signal conditioners.
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