To ensure that the output signal of the capacitive safety sensor is correct, it must be continuously monitored. Luckily, it is extremely easy to monitor capacitive sensors. It beats me why nobody before had this idea.
A small insulated electrode (test electrode) attached to the sensor electrode will not influence anything as long as it is not connected with anything. Grounding of the test electrode activates its capacity and consequently the sensor’s voltage drops. The design and the mounting of the test electrode guarantee the required sensitivity. Turning the grounding on and off periodically means dynamizing – as the safety specialists call it – the sensor signal at the first thinkable point. The function of the sensor itself is tested continuously. This allows the performance level PL d according to DIN EN ISO 13849 which was unprecedented with capacitive sensors. With an appropriate processing electronics even PL e should be possible.
The above scope plot shows the sensor signal in yellow. The blue signal switches the grounding of the test electrode.
To supervise the actual function of the sensor is much more challenging with all other technologies.