Stroke sensing apparatus for a variable displacement compressor

Details Stroke sensing apparatus for a variable displacement compressor Stroke sensing apparatus for a variable displacement compressor

1999-07-06

2001-06-19

Walberg, Teresa (Department: 3742)

Pumps

Condition responsive control of drive transmission or pump...

Adjustable cam or linkage

C417S063000, C062S228500

Reexamination Certificate

active

06247900

ABSTRACT:

TECHNICAL FIELD
This invention relates to motor vehicle air conditioning systems including a variable displacement refrigerant compressor mechanically coupled to a rotary shaft of the vehicle engine, and more particularly to a non-intrusive sensing apparatus for determining operating parameters of the compressor.
BACKGROUND OF THE INVENTION
Variable displacement refrigerant compressors have been utilized in automotive air conditioning systems, with the displacement regulated in accordance with cooling demand via either a hydraulic control valve or solenoid control valve. In a typical arrangement, the compressor includes one or more pistons coupled to a tiltable wobble plate or swash plate, and the control valve adjusts a differential pressure acting on a wobble plate control mechanism to vary the wobble plate tilt angle, and hence the compressor displacement or stroke.
Various sensing devices have been proposed for determining the compressor speed and stroke, either for control or diagnostic purposes. In general, the sensing devices include a magnet mounted on a reciprocating element of the compressor, and a magnetic sensor mounted in or on the compressor housing in proximity to the reciprocating magnet. As the magnet reciprocates, the sensor develops a pulse or quasi-sinusoidal voltage waveform. The frequency of the waveform is typically independent of compressor stroke, and can be used as a measure of compressor speed, whereas the duty cycle of the waveform varies with the angle of the wobble or swash plate, and can be used as a measure of compressor stroke. Generally speaking, these devices are problematic because (1) they require changes in the mechanical design of the compressor, (2) the stroke measurement is non-linearly related to the actual stroke, and (3) the duty cycle measurements are subject to significant variation due to noise in the sensor output signal. As a result, a sensing system based on the known techniques is both costly and unreliable.
SUMMARY OF THE INVENTION
The present invention is directed to an improved low cost sensing apparatus for a swash or wobble plate compressor that requires no modifications in compressor design or operation, and which provides a repeatable and accurate measure of compressor stroke.
The apparatus of the invention comprises just two elements: a sensor module and a stroke sensing circuit. The compressor has an outer housing formed of aluminum or other non-magnetic material, as is customary in automotive air conditioning systems, and the sensor module includes a magnetic field responsive sensor such as a Hall Effect or magneto-resistive (MR) sensor. The sensor module is attached to the periphery of the housing in proximity to a reciprocating ferrous element such as a bushing shoe on the periphery of the swash or wobble plate assembly. Thus, no changes in the mechanical design of the compressor are required. The sensor produces a quasi-sinusoidal output voltage signal having a frequency proportional to compressor speed, and a stroke sensing circuit determines the compressor stroke by band pass filtering, amplifying, and peak detecting the signal. Additionally, the signal can be compensated for sensor non-linearities, if required. The output of the stroke sensing circuit is substantially insensitive to noise, and the relationship between it and the compressor stroke is substantially linear, resulting in a reliable, accurate and inexpensive sensing apparatus.


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