Proximity sensor monitor

A proximity sensor monitor for sensing the proximity of a target and for coupling to a proximity sensor for sensing the proximity of the same target, the proximity sensor comprising:

• an electrical component having electrical properties that vary with the proximity of the target;
• an impedance component connected to the electrical component and having a known impedance;
• and a signal generator for generating an electrical signal for application to the connected electrical and impedance components, the proximity sensor signal generator being driven by a proximity sensor excitation frequency, the electrical and impedance components being arranged to provide first and second analogue voltage signals in response to the electrical signal from the signal generator;
• the proximity sensor monitor comprising:
• a first and second input for receiving, respectively, the first and second analogue voltage signals from the proximity sensor;
• a switch having first and second inputs coupled respectively to the first and second inputs of the proximity sensor monitor, and having an output, the switch being configured to switch between the first and second inputs of the proximity sensor monitor;
• an analogue-to-digital converter coupled to the output of the switch for receiving an analogue voltage signal and for generating a digital voltage signal;
• and
• a processor connected to the analogue-to-digital converter for receiving the digital voltage signal and for generating a proximity monitor signal, the processor being driven by a proximity sensor monitor excitation frequency,
• wherein the processor is configured to control the switch to connect to the first and second inputs sequentially to measure the first analogue voltage signal and the second analogue voltage signal respectively, and
• wherein the processor is configured to compensate for a frequency difference between the proximity sensor monitor excitation frequency used to generate the proximity monitor signal, and the proximity sensor excitation frequency used to generate the electrical signal applied to the connected electrical and impedance components by the signal generator of the proximity sensor, in order to generate the proximity monitor signal, the proximity sensor monitor processor compensating for the frequency difference by:
• determining a phase correction value, and
• applying the phase correction value to a measured voltage.