Sensor :- Capacitive Position Sensors

Capacitive Sensors are non-contact type of devices used for precision measurement of a target position if the target is conductive in nature, or used for measurement of thickness and density of a material if the target is non-conductive in nature. When used with conductive targets, they work irrespective of the material of the target as all conductors look same to a capacitive sensor. The thickness of the target is also not important as the sensors sense the surface of the target. They are mainly used in disk drives, semiconductor technology and high precision manufacturing industries where high accuracy and frequency response are important. When used with non-conductive targets, they are generally used in label detectors, coating thickness monitors, and paper and film thickness measuring units.

They are primarily used in measurement of linear displacement from a few millimeters to nanometers. Capacitive sensors make use of the electrical property conductance for measuring position. The ability to store electric charge by a body is capacitance. The most common device used to store charge is a parallel plate capacitor. The capacitance of a parallel plate capacitor is directly proportional to the surface area of the plates and dielectric constant, and inversely proportional to the distance between the plates. Hence, when the spacing between the plates is changed, there is a change in its capacitance and capacitive sensors make use of this property.

The capacitance,

C = (εr εo A) / d

Where

εr is relative permittivity of dielectric

εo is permittivity of free space

A is the overlap area of plates

And d is distance between the plates

A typical capacitive sensing model consists of two metal plates with air in between them acting as dielectric. The sensor or probe is one of the metal plates and the target object which is conductive, is the other plate.

When a potential is applied to the plates of the conductor, an electric field is created between them by causing positive charges to collect on one plate and negative charges on other.

Capacitive sensors use alternating voltage. Alternating voltage causes the charges to continually reverse their positions. The alternating electric field between the capacitive probe and the target is monitored for changes and is used in measuring the capacitance between the probe and target. Capacitance is determined by the area of the surfaces, dielectric constant and spacing of the surfaces. In the majority of capacitive sensing applications, the size and area of the capacitive sensor and the target do not change. The dielectric material between the conductive surfaces doesn’t change. The only factor responsible for any changes in the capacitance is the distance or spacing between the capacitive sensor and the target.

Hence the capacitance is an indicator of the target’s position. Capacitive sensors are calibrated to produce an output voltage corresponding to the change in distance between the probe and target which causes the capacitance to change. This is called the sensitivity of the capacitive sensor. Sensitivity of a capacitive sensor is the amount of voltage change to a defined amount of distance change. Generally used sensitivity setting is 1 V / 100 µ m i.e. the output voltage changes 1 V for every 100 µ m change in distance.

A capacitive sensor probe consists of three components: sensing area, guard and body.

The potential is applied to the sensing area. There is a problem of spreading of an electric field to areas on the target other than the defined sensing area and the target. To prevent this from happening, a technique called guarding is used. In this technique, a guard area is created by surrounding the sides and back of the sensing area and is maintained at a potential same as the sensing area. As the guard and the sensing area are at the same potential, there will be no electric field between them. Any other conductors in the proximity other than the sensing area will form an electric field with the guard. The sensing area and the corresponding target are undisturbed.

Because of this guard, the electric field projection of the sensing area will be conical in nature. The electric field from the probe covers an area on the target of approximately 30 % larger that the sensor area. Therefore, it is essential to have a minimum target diameter as 30 % of the sensing areas’ diameter for standard calibration.

The range of a sensing probe is directly proportional to the size of the sensing area. Smaller probes must be placed closer to the target in order to achieve the desired amount of capacitance. The maximum allowable gap between the probe and the target is approximately 40% of the diameter of the sensing area. Beyond this, the probe becomes useless. There are applications where multiple probes are simultaneously used. In these applications, it is essential to synchronize the excitation voltage of all the probes. If the voltages are not synchronized, the probes interfere with each other as one probe may try to increase the electric field while the other decreases it. This gives a false reading.

Capacitive Sensors can also be used with non-conducting targets. The dielectric constant of the non-conductive target is the basis for the operation. The dielectric constant of the non-conductive materials like plastic is different than air. When the non-conducting material is used as a dielectric medium between two conducting plates, its dielectric constant will determine the capacitance between the conductors.

The two conducting plates are sensor probe and a grounded conductive reference. The changes in capacitance and hence the output of the sensor will be corresponding to the changes in material thickness, density or composition.

There are high precision and high performance capacitive sensors that can measure a displacement of the order of nanometers. These high performance sensors are stable to changes in temperature, produce a linear output and have high resolution.

The advantages of capacitive sensors over other non-contact devices are high resolution, inexpensive and not sensitive to the material of the target. The capacitive sensors are not suitable in conditions where the environment is dry or wet and the distance between the probe and the target is large.