Theory and application of a capacitive sensor for safeguarding in industry

A capacitive sensor for the safeguarding of automatically working machines in industry has been developed. It can be used as a complement to more expensive pressure sensitive mats, light curtains or fences. The sensor consists of two electrically conducting planes, insulated electrically from the ambient. The planes are placed opposite each other, one in the floor and one in the ceiling of for example a robot cell. One of the planes is used as the antenna of the transmitter and the other works as the antenna of the receiver. When conductive conditions in the space between the two planes are altered, e.g. a person enters, the signal from the receiver changes and indicates the presence of the person. By the use of a current/voltage-converter followed by a lock-in amplifier as receiver an excellent signal-to-noise ratio is achieved. A sine wave modulated by bandwidth limited white Gaussian noise is used as transmitting signal. This signal reduces the strength of the electric field per frequency unit, radiated by the sensor. The theory for the sensor and the sensor signal handling is presented and some practical experiments are described. The theory is well verified by the experimental results