Capacitive distance- and force-sensor
Fig. 1: Assembled sensor (without spring-element)


1) Principle of operation

This sensor measures the capacity between its top-electrode and a grounded metal or any other high-conductive plate. This capacity C depends on the distance between object and electrode d and the size of the electrode A and calculates as follows:

When the resulting capacitor forms an RC-oscillator, the frequency f is proportional to 1/RC, so the resulting frequency will be:

Which means that the frequency is proportional to the distance between the sensor electrode and the object. The problem is, that any additive parasite capacity will degrade the linearity of the sensor. But there are many applications where a high linearity is not required or can be compensated.

2) Circuit

The circuit is quite simple. A Schmitt-trigger gate (IC3C), the sensor-capacity (connected to pin SENSOR) and a resistor (R1) form an oscillator and generate the output frequency. R1 can be used to determine the center-frequency. Values between 10k and 1M can be used.
The generated signal is fed into the other five gates of the IC to get enough output-current to drive a correct terminated signal-line. IC1, D1 C1 and C2 generate a stable 5V supply from the input-voltage of 9 to 24V.
Attention: since HC-gates are used here, the edges of the output-signal are very steep and a properly shielded and terminatied cable is essential to get correct results and to avoid interferences. Fig.2 shows the schematic.


Fig. 2: Schematic

3) Board

The board can be built in many sizes. For my prototype, I chose a very small size of 20x50mm. In the center of the board you can see a hole, that carries the the spring element and - in case of a single sided board - connects the ground of the circuit to the case when a screw is inserted. As sensor electrode you will need another board of the same size and with a hole in its center that has to be big enough that the spring elements fits and has enough space to move. I use a thin (0.5mm) board with its copper-side oriented towards the sensor, so I don't have problems when the measured object accidently hits the sensor. This adds addition non-linearity, but that's no problem in my application. The sensor-plate can be placed on the sensor by using (plastic!) distance-rolls.
Fig. 3 shows the layout, fig. 4 the placement. Use the printable version (original size when printed with 600dpi) or the Eagle board-file and the Eagle schematic to construct the board.

Fig. 3:Layout
Fig 3: Layout

Fig. 4: Placement
Fig. 4: Placement

4) Typical Application

A typical application may be the bumper-ring of a mobile robot. Two sensors per edge can be placed under the bumper and in case of a collision, the position of contact can be determined from the ratio of force on the two sensors. Fig. 5 shows the assembly of such a configuration.

Fig. 5: Typical application

Have a lot of fun!

If you think: "Ees dis English?" and you are a native speaker, feel free to send me a corrected version of this page. :-)

Important note: These plans are provides without any warranty. Use at your own risk!

last change 2001-09-09 Archi