A transducer is defined as a device that receives energy from one system and transmits it to another, often in a different form.
Broadly defined, the transducer is a device capable of being actuated by an energizing input from one or more transmission media and in turn generating a related signal to one or more transmission systems. It provides a usable output in response to a specified input measurand, which may be a physical or mechanical quantity, property, or conditions. The energy transmitted by these systems may be electrical, mechanical or acoustical.
The nature of electrical output from the transducer depends on the basic principle involved in the design. The output may be analog, digital or frequency modulated.
Basically, there are two types of transducers, electrical, and mechanical.
An electrical transducer is a sensing device by which the physical, mechanical or optical quantity to be measured is transformed directly by a suitable mechanism into an electrical voltage/current proportional to the input measurand.
An electrical transducer must have the following parameters:
- Linearity: The relationship between a physical parameter and the resulting electrical signal must be linear.
- Sensitivity: This is defined as the electrical output per unit change in the physical parameter (for example V/°C for a temperature sensor). High sensitivity is generally desirable for a transducer.
- Dynamic Range: The operating range of the transducer should be wide, to permit its use under a wide range of measurement conditions.
- Repeatability: The input/output relationship for a transducer should be predictable over a long period of time. This ensures reliability of operation.
- Physical Size: The Electrical Transducer Definition must have minimal weight and volume, so that its presence in the measurement system does not disturb the existing conditions.
Advantages of Electrical Transducer:
The main advantages of electrical transducer (conversion of physical quantity into electrical quantities) are as follows:
- Electrical amplification and attenuation can be easily done.
- Mass-inertia effects are minimized.
- Effects of friction are minimized.
- The output can be indicated and recorded remotely at a distance from the sensing medium.
- The output can be modified to meet the requirements of the indicating or controlling units. The signal magnitude can be related in terms of the voltage current. (The analog signal information can be converted in to pulse or frequency information. Since output can be modified, modulated or amplified at will, the output signal can be easily used for recording on any suitable multichannel recording device.)
- The signal can be conditioned or mixed to obtain any combination with outputs of similar transducers or control signals.
- The electrical or electronic system can be controlled with a very small power level.
- The electrical output can be easily used, transmitted and processed for the purpose of measurement.
Electrical transducer can be broadly classified into two major categories,
- Active transducer,
- Passive transducer.
An active transducer generates an electrical signal directly in response to the physical parameter and does not require an external power source for its operation. Active transducers are self generating devices, which operate under energy conversion principle and generate an equivalent output signal (for example from pressure to charge or temperature to electrical potential).
Typical example of active transducers are piezo electric sensors (for generation of charge corresponding to pressure) and photo voltaic cells (for generation of voltage in response to illumination).
Passive transducer operate under energy controlling principles, which makes it necessary to use an external electrical source with them. They depend upon the change in an electrical parameter (R, L and C).
Typical example are strain gauges (for resistance change in response to pressure), and thermistors (for resistance change corresponding to temperature variations).
Electrical transducer are used mostly to measure non-electrical quantities. For this purpose a detector or sensing element is used, which converts the physical quantity into a displacement. This displacement actuates an electric transducer, which acts as a secondary transducer and give an output that is electrical in nature. This electrical quantity is measured by the standard method used for electrical measurement. The electrical signals may be current, voltage, or frequency; their production is based on R, L and C effects.
A transducer which converts a non-electrical quantity into an analog electrical signal may be considered as consisting of two parts, the sensing element, and the transduction element.
The sensing or detector element is that part of a transducer which responds to a physical phenomenon or to a change in a physical phenomenon. The response of the sensing element must be closely related to the physical phenomenon.
The transduction element transforms the output of a sensing element to an electrical output. This, in a way, acts as a secondary transducer.
Transducers may be further classified into different categories depending upon the principle employed by their transduction elements to convert physical phenomena into output electrical signals.
The different electrical phenomena employed in the transduction elements of transducers are as follows.
- Electro magnetic
- Frequency generating
Selecting a Transducer:
The transducer or sensor has to be physically compatible with its intended application. The following should be considered while selecting a transducer.
- Operating range: Chosen to maintain range requirements and good resolution.
- Sensitivity: Chosen to allow sufficient output.
- Frequency response and resonant frequency: Flat over the entire desired range.
- Environmental compatibility: Temperature range, corrosive fluids, pressure, shocks, interaction, size and mounting restrictions.
- Minimum sensitivity: To expected stimulus, other than the measurand.
- Accuracy: Repeatability and calibration errors as well as errors expected due to sensitivity to other stimuli.
- Usage and ruggedness: Ruggedness, both of mechanical and electrical intensities versus size and weight.
- Electrical parameters: Length and type of cable required, signal to noise ratio when combined with amplifiers, and frequency response limitations.