This transducer is used for displacement measurement. It is done by calculating the change in inductance in a single coil according to the variation in inductance. A schematic of the linear motion variable inductance transducer is shown below.
- Linear Motion Variable Inductance Transducer
The device consists of an arm that moves linearly according to the displacement produced. It also consists of a single coil wound on a former with ‘N’ number of turns. The end of the arm is connected to a soft iron core which moves linearly along the axis of the former. Thus, reluctance ‘R’ will be produced due to the flux path. The coil inductance of the device can be written by the equation, L= N2 /R.
A linear movement of the arm to the right decreases the reluctance ‘R’ of the flux path. Thus, according to the equation given above, the inductance increases due to the decrease in reluctance and vice versa. This inductance ‘L’ can be calculated or recorded with the help of an inductance bridge or through a recorder. Thus the measure of the displacement of the arm can be obtained from the corresponding change in inductance.
If the transducer is connected to the input of an oscillator tank circuit, the change in frequency ‘f’ of the oscillator could be taken as the measurement for the corresponding change in the displacement of the arm. A displacement of the arm changes the inductance and hence the frequency. Thus, the output can be measured in terms of inductance and frequency.
1. There will not be problems due to mechanical hysteresis.
2. Provides a good response to static as well as dynamic measurements.
3. Provides a high output.
1. The frequency response is controlled by the construction of force ring members.
2. Accuracy errors may occur due to the interference of external magnetic field.
A linear potentiometer transducer consists of a potentiometer, which is short circuited by a slider. The other end of the slider is connected to a slider arm. The force summing device on the slider arm causes linear displacement of the slider causing the short circuit of a certain portion of the resistance in the potentiometer. Let the whole resistance positions on the potentiometer be ABC. Let the resistance position caused by the slider movement be BC. As the movement of the slider moves further to the right, the amount of resistance increases. This increase in resistance value can be noted according to the corresponding change in the linear displacement of the slider. The change in resistance can be calculated with the help of a Wheatstone bridge.
Another easy method than calculating the resistance with the help of a bridge connection is to connect a constant current source in series with the potentiometer. Thus a voltage will be developed. This voltage can be measured and hence the resistance, R = V/I.
- Linear Potentiometer Transducer
Some of the most commonly used potentiometers for this purpose and their basic working is explained below.
1. Wire-Wound Potentiometer – The most commonly used resistance elements in this potentiometer are nickel, chromium or nickel copper. As these materials have a very low temperature coefficient of resistance, they can be used to handle large currents and also can be used up to 5 Hertz. They are also very cost effective. The winding of the resistance wire will depend on the different types of resistance changes due to the slider motion like linear, arithmetic, logarithmic and so on.
2. Cermet Potentiometer – This potentiometer is made from a material called Cermet which is made by mixing a paste of precious metal particles and a ceramic. Some of the most common mixtures used are palladium silver glass and palladium oxide glass. This device is used mostly for ac purposes as it has a low temperature coefficient of resistance and huge power ratings at high temperatures. Out of the lot, this device is mostly used as it is cost-effective.
3. Hot-Moulded Carbon Potentiometers – As the name implies, it is made by depositing a thin film of carbon and a thermosetting plastic binder. This device is mostly used for alternating current (ac) purposes.
4. Carbon Film Potentiometers – This potentiometer is made by coating a thin layer of carbon film on a non-conductive base. The temperature coefficient of resistance of this device is 1000 x 10-6 ohms/degree Celsius.
5. Thin Metal Film Potentiometer – This device is in the form of a thin vapour deposited layer of metal on glass or ceramic base. This is also used for ac applications.
- Simple design and simple working
- Can be used for measuring even large displacements.
- The device produces a large output and hence can be used for control purposes without further amplification steps. Thus the whole operation is bounded to a single device.
- Can produce a high electrical efficiency.
- All devices other than wire-wound potentiometer can be used for a large frequency range.
- Except wire wound, all other potentiometers can provide excellent resolutions.
- A huge force may be required for the slider movement.
- Can produce unwanted noise due to alignment problems, wear and tear of the sliding contact. This may also affect the total life of the device.
A transducer is a device that is used to convert a physical quantity into its corresponding electrical signal.
In most of the electrical systems, the input signal will not be an electrical signal, but a non-electrical signal. This will have to be converted into its corresponding electrical signal if its value is to be measured using electrical methods.
The block diagram of a transducer is given below.
- Transducer Block Diagram
A transducer will have basically two main components. They are
1. Sensing Element
The physical quantity or its rate of change is sensed and responded to by this part of the transistor.
2. Transduction Element
The output of the sensing element is passed on to the transduction element. This element is responsible for converting the non-electrical signal into its proportional electrical signal.
There may be cases when the transduction element performs the action of both transduction and sensing. The best example of such a transducer is a thermocouple. A thermocouple is used to generate a voltage corresponding to the heat that is generated at the junction of two dissimilar metals.
Selection of Transducer
Selection of a transducer is one of the most important factors which help in obtaining accurate results. Some of the main parameters are given below.
- Selection depends on the physical quantity to be measured.
- Depends on the best transducer principle for the given physical input.
- Depends on the order of accuracy to be obtained.
Some of the common methods of classifying transducers are given below.
- Based on their application.
- Based on the method of converting the non-electric signal into electric signal.
- Based on the output electrical quantity to be produced.
- Based on the electrical phenomenon or parameter that may be changed due to the whole process. Some of the most commonly electrical quantities in a transducer are resistance, capacitance, voltage, current or inductance. Thus, during transduction, there may be changes in resistance, capacitance and induction, which in turn change the output voltage or current.
- Based on whether the transducer is active or passive.
The applications of transducers based on the electric parameter used and the principle involved is given below.
1. Passive Type Transducers
a. Resistance Variation Type
- Resistance Strain Gauge – The change in value of resistance of metal semi-conductor due to elongation or compression is known by the measurement of torque, displacement or force.
- Resistance Thermometer – The change in resistance of metal wire due to the change in temperature known by the measurement of temperature.
- Resistance Hygrometer – The change in the resistance of conductive strip due to the change of moisture content is known by the value of its corresponding humidity.
- Hot Wire Meter – The change in resistance of a heating element due to convection cooling of a flow of gas is known by its corresponding gas flow or pressure.
- Photoconductive Cell – The change in resistance of a cell due to a corresponding change in light flux is known by its corresponding light intensity.
- Thermistor – The change in resistance of a semi-conductor that has a negative co-efficient of resistance is known by its corresponding measure of temperature.
- Potentiometer Type – The change in resistance of a potentiometer reading due to the movement of the slider as a part of an external force applied is known by its corresponding pressure or displacement.
b. Capacitance Variation Type
- Variable Capacitance Pressure Gauge – The change in capacitance due to the change of distance between two parallel plates caused by an external force is known by its corresponding displacement or pressure.
- Dielectric Gauge – The change in capacitance due to a change in the dielectric is known by its corresponding liquid level or thickness.
- Capacitor Microphone – The change in capacitance due to the variation in sound pressure on a movable diagram is known by its corresponding sound.
c. Inductance Variation Type
- Eddy Current Transducer – The change in inductance of a coil due to the proximity of an eddy current plate is known by its corresponding displacement or thickness.
- Variable Reluctance Type – The variation in reluctance of a magnetic circuit that occurs due to the change in position of the iron core or coil is known by its corresponding displacement or pressure.
- Proximity Inductance Type – The inductance change of an alternating current excited coil due to the change in the magnetic circuit is known by its corresponding pressure or displacement.
- Differential Transformer – The change in differential voltage of 2 secondary windings of a transformer because of the change in position of the magnetic core is known by its corresponding force, pressure or displacement.
- Magnetostrictive Transducer – The change in magnetic properties due to change in pressure and stress is known by its corresponding sound value, pressure or force.
d. Voltage and Current Type
- Photo-emissive Cell – Electron emission due to light incidence on photo-emissive surface is known by its corresponding light flux value.
- Hall Effect – The voltage generated due to magnetic flux across a semi-conductor plate with a movement of current through it is known by its corresponding value of magnetic flux or current.
- Ionisation Chamber – The electron flow variation due to the ionisation of gas caused by radio-active radiation is known by its corresponding radiation value.
2. Active Type
- Photo-voltaic Cell – The voltage change that occurs across the p-n junction due to light radiation is known by its corresponding solar cell value or light intensity.
- Thermopile – The voltage change developed across a junction of two dissimilar metals is known by its corresponding value of temperature, heat or flow.
- Piezoelectric Type – When an external force is applied on to a quartz crystal, there will be a change in the voltage generated across the surface. This change is measured by its corresponding value of sound or vibration.
- Moving Coil Type – The change in voltage generated in a magnetic field can be measured using its corresponding value of vibration or velocity.