Sensors and transducers are crucial in instrumenting an engineering system. Sensors may be used in an engineering system for a variety of purposes.
A measuring device passes through two main stages while measuring a signal. First, the measurand is felt or sensed by the sensing element. Then, the sensed signal is transduced (or converted) into the form of the device output.
In fact, the sensor, which senses the response automatically, converts (i.e., transduces) this signal into the sensor output the response of the sensor element.
Essentially, sensors are needed to monitor and learn about the system. This knowledge will be useful in many types of applications, including the following:
- Process monitoring
- Operating or controlling a system
- Experimental modeling (i.e., model identification)
- Product testing and qualification
- Product quality assessment
- Fault prediction, detection, and diagnosis
- Alarm and warning generation
- Surveillance
Specifically in a control system, sensing is used for such purposes as
- Measuring the system outputs for feedback control
- Measuring some types of system inputs (unknown inputs, disturbances, etc.) for feedforward control
- Measuring output signals for system monitoring, parameter adaptation, self-tuning, and supervisory control
- Measuring input and output signal pairs for experimental modeling of the plant (i.e., for system identification)
- The terms sensor and transducer are often used interchangeably to mean the same thing. However, strictly, a sensor senses the quantity that needs to be observed or measured (called measurand) while the transducer converts into a form that can be observed or used in a subsequent operation.
- Sensor and transducer stages are functional stages, and sometimes it is not easy or even feasible to draw a line to separate them or to separately identify physical elements associated with them. Furthermore, this separation is not very important in using existing devices.
- Sometimes, we may have to design and develop new sensors or modify existing sensors, depending on the needs of the specific application. Such activities are based on a set of performance specifications for the required sensors.
Terminology
Potentiometers, differential transformers, resolvers, strain gauges, tachometers, piezoelectric devices, gyros, bellows, diaphragms, flowmeters, thermocouples, thermistors, and resistance temperature detectors (RTDs) are examples of sensors used in engineering systems.
Measurand and Measurement
- The variable that is measured is termed the measurand. Examples strain in a structural member, temperature and pressure of a process plant, and vibration of a motor. The output of the sensor unit is the measurement.
- The nature of the measurand and the nature of the sensor output are typically quite different. For example, while the measurand of an LVDT is the displacement, the LVDT output is voltage.
- Similarly, the measurand of a strain-gauge bridge is a strain and the bridge output is a voltage. However, the sensor output can be calibrated in the units of the measurand (e.g., in displacement in mm or strain units).
Sensor Signal Conditioning
- A complex measuring device can have more than one sensing stage. Often, the measurand goes through several transducer stages before it is available for practical purposes.
- Furthermore, filtering may be needed to remove measurement noise and other types of noise and disturbances that enter into the true measurand (including process noise and external disturbance inputs). Hence, signal conditioning is usually needed between the sensor and the application.
- Charge amplifiers, lock-in amplifiers, power amplifiers, switching amplifiers, linear amplifiers, pulse-width modulation (PWM) amplifiers, tracking filters, low-pass filters, high-pass filters, band-pass filters, and notch filters are signal-conditioning devices that are used in sensing and instrumentation applications.
- Note that it is somewhat redundant to consider electrical-to-electrical sensors–transducers as measuring devices because electrical signals need conditioning only before they are used to carry out a useful task. In this sense, electrical-to-electrical transduction should be considered as a conditioning function rather than a measuring function.
- Additional components, such as power supplies, isolation devices, and surge-protection units are often needed in the instrumentation of engineering systems, but they are only indirectly related to the functions of sensing and actuation.
- Relays and other switching devices and modulators and demodulators may also be included under signal conditioning (more correctly, as signal conversion).
- Modern sensors–transducers may have signal conditioning circuitry integrated into them, particularly in the monolithic-integrated circuit (IC) form. Then it is rather difficult to physically separate the sensor, transducer, and signal conditioner in the overall hardware unit.
A schematic representation of the process of sensing and its application is given in Figure.
Difference between Sensors and Transducers
Sr. No. | Sensor | Transducer |
1 | A sensor senses the quantity that needs to be observed or measured. | The transducer converts quantity into a form that can be observed or used in a subsequent operation. |
2 | A sensor does not have any other component on it. | The components of a transducer are sensor, signal conditioning device and output device. |
3 | All the sensors are not transducers. | A sensor is the part of all the transducers. |
4 | A sensor detects the change in the physical parameter of quantity to produce corresponding electrical signal. | Transducer converts the energy into a different form. |
5 | Sensor only measures the change in the physical quantity and does not provide any feedback to the system. | Transducer generally provides a feedback to the system through the output device after processing. |
6 | For example: smoke detector, proximity sensor, photo sensor, thermistor, etc. | For example: strain gauge, LVDT, microphone, piezoelectric transducers, etc. |
