About Capacitive sensors

Bare Touch Sensors expanded

Circuit, sensor, capacitive sensing

How do capacitive sensors work?

First, let’s discuss what a capacitor is. In an electrical circuit, there are three fundamental components: resistors, capacitors and inductors. A resistor is a passive element that dissipates energy. Inductors and capacitors, however, are active elements, they store energy, and it’s capacitors that allow you to do capacitive sensing. Vice versa, you can do inductive sensing with inductors.

A capacitor in its simplest form compromises two conductive plates that are separated by a dielectric medium, which can be air, but in most cases is porcelain, glass or plastic. When a voltage is applied across the capacitor, they store a charge and an electric field forms across the dielectric. Capacitors can be used to smooth out the output voltage of a power supply or for filtering in signal processing. The amount of charge a capacitor can store is dependent on its capacitance and the capacitance of a capacitor is dependent on the area and distance of the plate and the dielectric constant. Capacitive sensing in turn is detecting the change in capacitance. An inductive sensor is similar to a capacitive sensor, but uses the inductance of a sensor to detect or measure objects.

A touch capacitive sensor is therefore a sensor that uses the change in capacitance to sense touch or the presence of bulk material or fluids. Proximity capacitive sensors work like touch capacitive sensors, except they are more sensitive. There are two common ways to do capacitive sensing. One is self-capacitance and we use this sensing with our technology. On our boards, there are a number of electrodes that are used for capacitive sensing. You can think of them as one plate of a parallel-plate capacitor. The other plate is the surface of the earth – very large, but quite far away. When you bring part of your body close to one of these electrodes, that body part acts as the other plate of the capacitor. It takes over from the earth, as the body part is much closer and so has a bigger effect, and this increases the capacitance measured on that electrode. This is a proximity sensor and this change is registered by the board. If you bring your hand so close that you actually touch the electrode, the effect is even bigger than if you are just close to it. If you look at the output from that electrode on our Grapher, you will notice that the line goes down, not up – that is because the chip that measures capacitance on our boards has an output that is inversely proportional to capacitance – if the capacitance goes up, the output goes down. Each electrode of our boards acts as a capacitive touch sensor or as a capacitive proximity sensor. This method of capacitive sensing is called self-capacitance and each electrode is working independently. Here the capacitance is measured with respect to earth ground.

Another method is mutual capacitive sensing, which is the one used in touch screens of smartphones. With mutual capacitive sensing, there are two grids of capacitors overlaying each other. Instead of having two capacitive plates that don’t face each other, the plates lie next to each other. When a voltage is applied, you have a grid of capacitor plates that generate an electric field in-between them. When you approach them with your finger, it changes the characteristic of the field and that change is registered as a touch. This method of capacitive sensing allows you to detect more than just one capacitive touch and instead it can detect multiple touches.

Another important type of capacitive sensing is a capacitive displacement sensor. These are mostly used in manufacturing and are used for the measurement of distances and any displacement. In a capacitive displacement sensor, you have one part of the capacitor in a cylindrical shape, which forms the first plate of your capacitor. The other plate is formed by the area that is being measured. Since the area and the dielectric constant of the sensor remain constant, the distance can be measured by detecting changes in the capacitance allowing one to sense the position of the object.

With capacitive sensing, the user is able to detect and measure touch or proximity, the presence of a bulk material or fluid levels, and much more. You can use a capacitive sensor to create touch interfaces, like the touch screen on a smartphone, or as a proximity sensor, where you can create a switch, by hovering your hand over the sensor.

Grapher

A