Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Distributive type parameters
Patent
1991-10-03
1995-11-28
Wieder, Kenneth A.
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Distributive type parameters
324636, 324644, 73579, G01R 2704
Patent
active
054711479
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates generally to an apparatus and method for determining the position of a piston and piston rod within a housing of a hydraulic steering cylinder and, more particularly, to an apparatus and method for determining the position of the piston and piston rod over the full extension range of the cylinder.
BACKGROUND ART
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.
One attempt to provide a sensor which is particularly suitable for the heavy equipment industry uses radio frequency (RF) signals. 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, a hydraulic cylinder is almost never a perfect geometric shape (for example, a perfect cylinder.) This may result in portions of the linear extension of the cylinder where the position is inaccurate or totally unreadable. Such portions are called deadband.
For example, in Bitar, a pair of antennas are located at one end of the coaxial resonant cavity. There is a portion (deadband), starting from the antennas and extending out into the cylinder for which the position cannot be determined. The length of the deadband portion varies from cylinder to cylinder.
In many applications, this deadband portion is not critical either because for the given application the cylinder does not extend into the deadband portion or the accuracy is not needed. However, in some applications cylinder extension is needed for the full length of the cylinder.
The present invention is directed at overcoming one or more of the problems as set forth above.
DISCLOSURE OF THE INVENTION
In one aspect of the present invention, an apparatus for detecting a linear position of a piston and a piston rod is provided. The piston and piston rod are movable within a housing and form first and second coaxial resonant cavities. The apparatus includes RF sections for determining the resonant frequencies of the cavities and a sensor controller for determining the position of the piston and piston rod.
In another aspect of the invention, a method for detecting a linear position of a piston and a piston rod is provided. The piston and piston rod are movable within a housing and define first and second coaxial resonant cavities. The method includes the steps of determining the resonant frequency of one of the cavities and responsively determining a first location of the piston. If the first location is invalid then the resonant frequency of the other cavity is determined and a second location is responsively determined.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates a block diagram of an radio frequency (RF) linear sensor for a steering cylinder, according to one embodiment of the present invention;
FIG. 2 illustrates a block diagram of a multiplexed RF linear position sensor system with a transmitting section, a receiving section, and a sensor controller, according to an embodiment of the present invention;
FIG. 3
REFERENCES:
patent: 3514703 (1970-05-01), Soga
patent: 3522527 (1970-08-01), Williams
patent: 3737770 (1973-06-01), Masson
patent: 4364046 (1982-12-01), Ogasawara
patent: 5351541 (1994-10-01), Petrovich
Allen William E.
Bitar Ali A.
Herold Robert A.
Caterpillar Inc.
Solis Jose M.
Wieder Kenneth A.
Yee James R.
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