Electronic systems of any kind need to receive input from the actual world in order to be useful. Whether it’s the process control system for a chemical plant or the HVAC system in your house, input must also be in a signal or form that the system can understand, process, and act on. This is where sensors and transducers come into use.
In a broad sense, sensors and transducers are devices that detect some form of physical quantity and convert it into another. The temperature in a room, for example, can be sensed and converted into an electrical signal to trigger a relay that starts your air conditioning unit. The terms sensor and transducer are often used interchangeably to describe the same devices, but they do have somewhat different meanings.
A sensor is a device that detects some form of physical quantity (acceleration, speed, heat, light, etc.) and converts it into an electric signal that may be suitable for processing by an outside system, or is simply displayed at the input location or remotely. So, a sensor is a “detector”.
A transducer converts energy from one form of physical quantity into another form of readable signal that is in relation to the input. A transducer reprocesses the information from a sensor into something that is useful. Transducers can also produce output signals other than electric, including hydraulic or pneumatic. Some devices, including thermocouples, loudspeakers, and microphones can also be termed transducers. So, a transducer is a “translator”.
Other than electronic circuitry, there is no single explanation for how all sensors or transducers work. Both sensors and transducers work based on numerous different technologies that are matched exactly to what is being detected. Photo sensors use optoelectronics. Smoke sensors use optical technology. Force sensors can use piezo electric technology. Proximity sensors can use sound waves, or a mechanical link. Gas sensors use catalytic, electrochemical or infrared technology.
The common denominator is that detection of a stimulus, either through the presence of the stimulus, or a change in the concentration or rate of the stimulus, produces a signal that the sensor then passes on as an indication of activity. Both sensors and transducers are then typically paired with actuators to activate a response to affect the initial quantity that was detected.
The types of external signals that both sensors and transducers can receive include mechanical, electric, chemical, thermal, optical, magnetic, radiation, and others. And new designs are constantly being introduced based on detection needs and features of new systems
Sensors can be classified in several different ways:
- Active Sensors: require an external excitation or power signal to produce an output signal
- Passive Sensors: do not require an external power signal, directly generate an output response to an external stimulus
- Analog Sensors: produce a continuous output signal in proportion to the measured input quantity
- Digital Sensors: produce a discrete digital output signal that is a binary representation of the quantity being measured at any one moment
Transducers can also be broadly classified as follows:
- Active Transducers: generate current themselves in response to a stimulus
- Passive Transducers: produce a change in a passive electrical quantity, such as resistance, as a response to a stimulus (passive transducers often require additional electricity to operate)
- Classified by Input: converts input energy into an electrical signal
- Classified by Output: takes electricity and converts it into another form of energy
Sensors and transducers can also be classified by the type of physical quantity that they measure, including: temperature, proximity, acceleration, motion, pressure, sound, radiation, light, touch, color, magnetic fields, humidity, tilt/angle, flow/level, smoke/gas, chemicals, and others.
When choosing a sensor or transducer for a design, production run, or replacement, some factors to consider are the device range, sensitivity, response time, frequency response, ruggedness, stability, susceptibility to interference, and actual size. For example, digital sensors have better accuracy and throughput than analog sensors.
Concurrent measurement factors in the system being measured can include humidity and corrosion, system working temperatures, power consumption limits, electrical or RF interference, corrosive atmospheres, radiation, etc.
As electronic devices and systems in the modern world have gotten more complicated, feature-rich, and function-dedicated, sensors and transducers have played an instrumental role. They are employed in industrial, consumer, medical, military, aerospace, automotive and commercial systems and products. Modern automobiles commonly use over 20 individual sensors per vehicle. All you have to do to see one at work is to tilt your smart phone and watch the internal accelerometer adjust the display.