In the field of hydraulic cylinders, a variety of systems have been proposed in which the displacement of the elements of the cylinder is electrically sensed and displayed or used to perform control functions. Conventional transducers have serious deficiencies owing in part to difficulties in mounting the units and the harsh environmental conditions to which they are exposed. Transducers used on hydraulic systems in the heavy equipment industry are particularly subject to damage from the severe work environment. Hydraulic cylinders are typically located in relatively unprotected areas and are typically subject to, for example, high g-forces, wide temperature variations, dust, water, debris, etc. which can result in both electrical and mechanical failure.
In one attempt to provide a sensor which is particularly suitable for the heavy equipment industry radio frequency (RF) signals are used. One such system is disclosed in U.S. Pat. No. 4,737,705 issued Apr. 12, 1988 to Bitar, et al. Bitar transmits a ramping RF signal into a coaxial resonant cavity formed by the hydraulic cylinder. When the cylinder's resonant frequency is transmitted, the signal detected by a receiving antenna reaches a peak. The resonant frequency has a one to one relationship with the cylinder's extension. Thus, by determining the cylinder's resonant frequency, the cylinder's linear extension can be determined.
However the resonant frequency is determined by detecting its first harmonic. Therefore, the sensor has to be capable of generating a large range of frequencies in order to detect the full range of harmonics and consequently, the full cylinder extension range. This dramatically increases the cost of the sensor, especially for larger cylinders where, for example, an oscillator capable of generating frequencies from 50 to 1800 MHz may be needed.
The present invention is directed at overcoming one or more of the problems as set forth above.