Capacitive and Inductive Transducers

Capacitive Transducers:

Capacitive transducer is a measurement device in which variations in pressure upon a capacitive element proportionally change the element’s capacitive rating and thus the strength of the measured electric signal from the device. As we know here the capacitance of the transducer varies with respect to the external stimulas. Normally a capacitive transducer uses a stationary plate and a movable plate. The stationary and the movable plates are separated by aair or vacuum dielectric. The movable plate changes its position under the influence of an external stimulus. As we know the capacitance depends on the area of plates, the movement of plate causes a change in capacitance. This is the basic working principle of a capacitive transducer.



Advantages of Capacitive transducers:

Compared with optical, piezo-resistive and inductive transducers, capacitive transducers have many advantages.
1. Low cost,
2. Less power usage,
3. Good stability,
4. High resolution,
5. Better speed,
6. Good frequency response
7. They require very little force to operate..

Disadvantages of Capacitive transducers:

1. The performance is severely affected by dirt and contaminants as they change the dielectric constant.
2. They are sensitive to temperature variations
3. Metallic parts must be insulated from each other

Biomedical Application:

1. It is mainly used for blood pressure measurements.
2. Also used for displacement or position measurement.
3. They can detect motion, acceleration, flow and many other variables.

Inductive Transducers:

The inductance of a coil can be changed either by changing its physical dimension or by changing the effective permeability of its magnetic core. In transducers using inductive elements we normally use the second principle. The effective permeability can be changed by moving a core, which is having a permeability higher than, the air through the coil. But the disadvantage with basic type of inductive transducer is that the inductance of coil is not related linearly to the displacement of coil if large displacements occur accidently. It is to avoid this disadvantage that we use LVDTs. The existing inductive transducers have a relatively small output power and often require amplification of their outputs.