Abstract: An apparatus for determining the position of a work implement relative to an earthworking machine. An antenna is mounted to the machine and is oriented to transmit a high frequency signal toward the implement and receive a reflected signal back from the implement. An oscillator is electrically coupled to the antenna and energizes the antenna with electrical energy to generate the high frequency signal. The oscillator produces an output signal having a frequency responsive to the inductance of the antenna. A microprocessor based circuit receives the output signal of the oscillator and responsively determines the position of the work implement relative to the machine. The position of the work implement is a function of the frequency of the output signal.

Abstract: A measurement circuit which is rendered insensitive to temperature and other environmental quantities comprising first and second electrically identical sensor elements (10, 12) which are alternately connected through multiplexed gates (G1, G2) into the tank circuit of an oscillator (14) to generate first and second signal frequencies f.sub.1 and f.sub.2. The first frequency f.sub.1 varies in the same sense with both temperature and the measured signal quantity but the second signal f.sub.2 varies in one sense with temperature and in the opposite sense with the measured signal quantity. A logic circuit (18) is provided for generating an output signal proportional to the ratio of f.sub.1 and f.sub.2. The ratio of the two signals is achieved by frequency dividing f.sub.1 and using the result as a variable length gating signal for f.sub.2. A non-multiplexed system using matched oscillators is also disclosed.

Abstract: A compensated measurement system utilizing dual sensing elements (10, 12) and a single variable frequency oscillator (14). A common operative association between the sensing elements (10, 12) and the oscillator (14) is achieved by multiplexing. Temperature and supply voltage variations are compensated by generating an output (f.sub.0) which is proportional to the ratio of the two generated frequencies (f.sub.1, F.sub.2).