Capacitive sensors are used millionfold, but for industrial applications, e.g. with robots, a much higher sensitivity is required.
Our capacitive sensor uses a conductive surface to detect the distance to its surroundings. A carrier frequency is applied to this surface over a very high impedance (a very small capacitance). The change of the capacity of the surface versus its surroundings changes the voltage of the surface. The higher the impedance (the smaller the capacity) feeding the carrier frequency, the larger the voltage swing will be.
The machine to which the sensor is mounted usually dominates the capacity of the sensors surroundings simply because it is both large and close. Thus an approaching hand is much less “visible” to the sensor. Therefore, a screening electrode is placed between sensor and machine (ground).This electrode is connected to the output of an amplifier and follows exactly the same amplitude the sensor would have in free air. This makes the machine “invisible” to the sensor. That improves the sensitivity versus a hand approaching the sensor by more than a factor of 1000.
The only wanted capacity is the one of the operator who must be protected. The input capacity of the electronics is not desired. I invented an amplifier with an extremely low input capacitance, far below 0.1 pF. You may read a bit more on this in the chapter Temperature Stability.
For maximum sensitivity the amplifier works with feedback just above the point of self-oscillation. This way, a constant sensitivity can be maintained in different situations, e.g with adjacent sensors.
If the need arises to use multiple sensors in close vicinity – as with robots – the signals of all sensors are synchronized exactly down to their phases. This generates a protective skin covering the whole robot.
The feedback is realized by the properly adjusted operational amplifier and the capacity between the shielding electrode and the sensor electrode being connected to the input of the operational amplifier. The following image shows the basic setup.
The result of these efforts is a range of about 40 cm (16″) in which a hand is detected. Our video shows a model with a 30 cm (12″) range.