A capacitive position sensor is a device that measures the capacitance of an object.
Capacitive sensors measure the change in capacitance due to a change in the distance between electrodes, which can be physical objects or electrical signals. Capacitance is a property of any dielectric material, such as air or plastic. An electric field exists between two closely spaced metal electrodes and causes a voltage difference across these surfaces when an electric potential is applied. The voltage will increase with increasing the electrodes’ spacing and decreasing material thickness between these electrodes. The capacitance of an object can be found by using a voltage to the electrodes and measuring the resulting capacitive voltage.
The output signal is used to determine the position of an object based on its capacitance. The greater its capacitance, the less sensitive it is too small changes in distance from one electrode. The electrode that produces maximum sensitivity varies with material composition, dielectric thickness, and the contact area between the bottom electrode and sample surface. A lower distance value from this electrode results in higher output signal frequency.
In the simplest form of a capacitive position sensor, an object is placed on a sensing pad (plastic sheet, metal plate). This sheet has two closely spaced metal electrodes. An electric potential is applied to one electrode, and the other electrode moves toward or away from the first electrode as the foil pad moves across the surface and its distance from both electrodes changes, its capacitance changes. This change in the capacitance of a capacitor produces a voltage that can be measured by connecting it to a voltage source. The measurement circuit can determine how far the foil pad has moved away from both electrodes.
The distance between two closely spaced electrodes determines how sensitive their resulting voltages are too localized differences in capacitance.
Capacitive position Sensors Working Principle
Capacitive position sensors work in two ways:
- The capacitance of the object to be measured is altered by a change in the distance between two electrodes placed on either side of the object. This change causes a voltage to be generated across the two electrodes. The voltage is proportional to the capacitance of the thing being measured.
- The sensitivity or resolution of a capacitive position sensor depends on its distance range from both electrodes. If a capacitor’s distance from its electrodes increases, its output signal frequency decreases because it is no longer as sensitive to small changes in capacitance caused by local surface irregularities.
Capacitive Proximity Sensor Design
Capacitive proximity sensors are usually composed of one or a combination of the following components:
- The same general construction as known capacitive position sensors. Capacitive proximity sensors require some slight differences, which will be described in this article, to become fully functional.
- An object (also known as “target” or “host”) is placed on a capacitor sheet. When this sheet is moved across the object, its capacitance changes and an electric potential is formed across the two electrodes on either side. The change in capacitance, if any, causes an electric signal to be produced at the two electrodes, which can be measured by connecting them to a voltage source. The capacitance and distance of the object on the sensing sheet can then be found by using this electric signal.
- The above measurement is done by using a microcontroller, which can only be done if there is a constant change in capacitance as a result of the movement of the sensing sheet across the object (like from one side to another). To get such a signal to work with, it must first go through an amplifier circuit which acts as a voltage amplifier. This amplifier will amplify the small signal produced at each electrode and help produce a significant change in its output signal frequency (due to small distance changes).
Major Features of Capacitive Position Sensors
Capacitive sensors are generally categorized as either sealed or unsealed. A sealed sensor has no “air” or another electrical insulator between the electrodes, whereas an unsealed sensor has some air between the electrodes.
Sealed sensors have higher sensitivity and resolution than unsealed sensors.
The time constant of a capacitor is the time required for the capacitor to charge by 60% after a sinusoidal voltage of amplitude equal to its capacitance has been applied to it for an interval equal to one period of that voltage. The time constant is directly proportional to the capacitance and inversely proportional to the frequency of the voltage.
Capacitive proximity sensors that use a sinusoidal voltage supply may have a narrow or wide frequency range. The band of frequencies over which the sensor is sensitive depends on its time constant, which varies with material thickness and area. For example, a ceramic disk has a very high time constant, so it is less sensitive to small changes in distance than an aluminum disk with a low time constant. A ceramic disk needs more current to produce an output signal that can be measured by a microcontroller (this will be discussed later).
Some capacitive proximity sensors are designed specifically for use with a microcontroller. This makes it easier to measure variations in capacitance that occur due to small changes in the distance of an object. The microcontroller is connected to the sensor using two wires and measures the time the capacitor needs to charge from zero (or some other starting voltage) to some final value within a certain range. This time is used to determine the actual distance between the sensing sheet and the object being measured.