Method and apparatus for detecting the position of a piston of a piston cylinder through the use of microwaves

Apparatus for measuring the position of a piston of a piston cylinder, comprising:

• a microwave transmitting device and a microwave receiving device which are at least partially arranged at the piston cylinder;
• the microwave transmitting device emitting transmit signals in a direction of the piston and the microwave receiving device receiving and detecting receive signals reflected from the piston;
• an evaluation device for performing an evaluation of a phase difference between evaluation transmit signals and the receive signals, the evaluation transmit signals corresponding in their characteristics to the transmit signals;
• the microwave transmitting device generates the transmit signals which comprise frequency-modulated signals having a base frequency (f0) sinusoidally modulated at a modulation frequency (fm);
• a first unit which performs a determination of a phase difference between the evaluation transmit signals and the receive signals at a baseband, the phase difference at the baseband being used for a fine determination of the piston position;
• and
• a second unit which performs a simultaneous determination of the phase difference between the evaluation transmit signals and the receive signals at at least one sideband, the phase difference at the at least one sideband being used for a coarse determination of the piston position;
• wherein:
• the first unit comprises a filter device which performs a filtering of the receive signals and evaluation transmit signals with respect to the base frequency or an intermediate frequency;
• the second unit comprises a filter device which performs a filtering of the receive signals and evaluation transmit signals with respect to the modulation frequency (fm);
• a phase of the modulated signal is of the form
• φT=2πf0t+η sin(2πfmt−φm0)
• where f0 is the base frequency, fm is the modulation frequency, η is a modulation index and φm0 is a constant.