Patent Application
20090220366
This invention relates to the field of valve position sensors and in particular to a slide valve position sensor employing an inclined ramp displacement indicator cooperatively coupled with a magnetic sensor mounted in spaced apart relation to the ramp.
It is well known in the prior art that it is advantageous to be able to determine the position of a slide valve of a compressor so as to allow the calculation of the loaded percentage of the compressor. However, in the prior art, because such compressors are typically employed in environments subjected to dirt, vibration and/or heat, it has been found that conventional potentiometer-type position sensors lacked in longevity and durability and have typically caused shut downs of the compressor in order to exchange the sensors when they malfunction.
Also in the prior art, applicant is aware of various attempts to provide slide valve position indicators such as in, for example, U.S. Pat. No. 5,257,921 which issued to Clark et al on Nov. 2, 1993 for an Electronic Slide Valve Position Indicator. In their patent, Clark et al disclose the use of a closed stainless steel tube to provide pressure and media isolation between the core and coils of a sensor in the form of a variable core transformer. As taught, the core is connected to and moved with a slide valve of a screw compressor and its actuating piston, and the current output from the secondary coil converted into a voltage which when linearized and communicated to a microprocessor is used for controlling the position of the slide valve and thereby deloading of the compressor.
Also in the prior art, applicant is aware of U.S. Pat. No. 5,081,876 which issued to Marshall on Jan. 21, 1992 for a Slide Valve Position Indicator and Magnetic Coupler. Marshall discloses in his patent that movement in a pressurized environment such as within a screw compressor, is transmitted to a non-pressurized environment, for example, outside of the screw compressor, by means of a magnetic coupling whereby linear movement of a magnet in the pressurized environment causes linear movement of a surrounding magnet in the non-pressurized environment which is then used to drive a linear-to-rotary motion converter. The rotary output drives a rotary potentiometer.
None of the prior art which applicant is aware teaches or suggests the rugged simplicity of the slide valve position sensor according to the present invention as described better below.
In summary, the slide valve position indicator according to one aspect of the present invention may be characterized as including:
a) a magnetic sensor having a magnetic sensor head at a first end thereof and adapted to output an electrical signal to a processor;
b) a rigid mount for rigidly mounting the magnetic sensor to a screw compressor in spaced apart relation to a slide valve position indicating shaft of the screw compressor; and
c) a ferrous metal cam mountable to the shaft, the cam having an inclined cam surface formed on the cam in opposed facing spaced relation to the sensor head spaced apart by a spacing distance within the magnetic detection range of the sensor, herein otherwise referred to as a sensor range spacing.
Wherein the inclined cam surface is formed so as to reduce the sensor range spacing at a first end of the cam surface and to increase the sensor range spacing at a second end of the cam surface and so as to linearly vary the sensor range spacing therebetween as the shaft moves the cam relative to the sensor head in linear correspondence with a corresponding slide valve position.
In one embodiment the inclined cam surface is a smoothly helical surface and the cam is formed on a disc mountable to a distal end of the shaft so that the disc is orthogonal to the shaft. The cam surface may be formed around an annular perimeter of the disc and the helical surface varies by the sensor range spacing relative to the sensor head. As noted above, advantageously the sensor range spacing varies over a range corresponding substantially to a detection range of said sensor.
In a preferred embodiment, the mount is a rigid bracket, for example L-shaped, mountable to the compressor adjacent the shaft. The sensor may be elongate and held by the mount so as to be substantially parallel to the shaft and substantially orthogonal to the disc.
According to one aspect of the present invention, the helical surface extends substantially 270 degrees around the shaft and the sensor range spacing is substantially in the range of 1-5 mm.
In the drawings wherein similar characters of reference denote corresponding parts in each view:
a is, in plan view, the slide valve position indicator of
b is the view of
c is the slide valve of
The slide valve position indicator 10, according to one embodiment of the present invention, includes a sensor mounting bracket 12 which itself comprises a base plate 14a mountable to the outside face 16 of a compressor 18 and an arm 14b cantilevered outwardly from base plate 14a. Arm 14b may be L-shaped so as to dispose a distal end of the arm substantially parallel to outside face 16 of the compressor when base plate 14a is mounted thereto.
A threaded barrel 20a of magnetic sensor 20 is journalled in an aperture in the distal end of arm 14b so as to extend generally orthogonally to outside face 16 when base plate 14a is mounted thereto. The position of threaded barrel 20a relative to arm 14b may be adjusted by the use of threaded nuts 22. A protective resilient end cap 20b may be slideably mounted over the exposed outward end of magnetic sensor 20.
Shaft 24 extends from face 16 of compressor 18. The amount of rotation of shaft 24 relative to the compressor face 16 indicates the position of the compressor slide valve. A steel disc 26, for example formed from ¾ inch steel plate, is rigidly mounted onto the outermost end of shaft 24 so as to lie generally in a plane orthogonal to shaft 24 and perpendicular to outside face 16. Disc 26 includes a base collar 26a which is mounted onto the outermost end of shaft 24, and a round gauge face-plate 26c, optionally having a position indicator arrow 28 marked on the outer surface thereof. Sandwiched between base collar 26a and gauge face-plate 26c is a smoothly helically inclined cam defined by an annular ramp flange 26b. Helically inclined ramp flange 26b defines a smoothly inclined annular helical ramp face 30a adjacent base collar 26a. An oppositely disposed planar face 30b is adjacent gauge face-plate 26c.
Ramp face 30a extends 270 degrees about shaft 24 and axis B between a first end 30a′ and an opposite second end 30a″. When shaft 24 is rotated so as to bring first end 30a′ closely adjacent magnetic sensor 20, the cam surface is most closely spaced relative to the head or outer end 20c of magnetic sensor 20. That is, at first end 30a′, ramp flange 26b is at its thickest. At the opposite second end 30a″ ramp flange 26b is at its thinnest so that when second end 30a″ is rotated so as to be most closely adjacent to head 20c of magnetic sensor 20, the distance d between ramp face 30a and magnetic sensor 20 is at a maximum. Thus as shaft 24 is rotated in direction A about its longitudinal axis of rotation B, the spacing d between the outermost end 20c of magnetic sensor 20 and ramp face 30a varies linearly with the angular rotational displacement of shaft 24 about axis B. Thus the rotational position of optional indicator arrow 28 indicates the relative position of the slide valve as does distance d between head 20c of magnetic sensor 20 and ramp face 30a. A cut-out 32 may be formed in disc 26 between ends 30a′ and 30a″.
In a preferred embodiment, magnetic sensor 20 is a linear sensor producing a current output as manufactured by Baumer Electric (www.baumerelectric.com) under model no. IWRM 1219704. The sensor measures in 0.01 mm increments and produces a 4 milliamp signal when head 20c of magnetic sensor 20 is one (1) mm from the opposed facing ramp face 30a, and a 20 milliamp signal when spacing d is five (5) mm. The signal is carried by wire 20d. Thus, for example, when spacing d is 1 mm, the 4 milliamp signal corresponds to a 0% slide valve opening, and when spacing d is 5 mm, the 20 milliamp signal corresponds to a 100% open slide valve position.
The ruggedness comes from, in part, having only a single moving element which does not contact any stationary element and is constructed of relatively robust components the device thus operates irrespective of the operative environment in which the compressor is situate including beat, cold, moisture, dust, vibration, etc. Thus because of the ruggedness of the sensor mechanism, for example as compared to the moisture sensitive capacitive sensor type, applications such as engines benefit from less down time and, for example, the reduction in cyclical variation in compressor pressure. For example, where the maximum discharge pressure for an engine may not exceed 1500 KPa, the programmable logic controller (PLC) cooperating with the magnetic sensor slide valve position indicator according to the present invention, causes the slide valve to back off, that is to reduce the slide valve position so as to slightly close the valve at a set point pressure of 1450 KPa and thereafter keep the pressure close to the desired 1460 KPa pressure set point by small adjustments to the slide valve position. Thus, the set point may, for example, be set at the 99% open position for the associated slide valve, and the magnetic sensor according to the present invention will provide for monitoring the set point and backing off the slide valve at the set point, that is, at the 99% position so as to avoid the maximum discharge pressure of the compressor.
Other benefits include ease of calibration and that the compressor does not have to be shut down for exchange of the sensors.
In an alternative embodiment, the helically inclined ramp may be replaced with a linear inclined slide, the operative face of which is again spaced apart by distance d from head 20c of magnetic sensor 20. Embodiments employing the linear inclined slide instead of the helically inclined ramp of the illustrated embodiment, will be useful depending on the type of slide valve as would be known to one skilled in the art, choosing as between for example, Howden™, Frickυ, Ariel™, Sullair™, Mycom™, and Kobelco™ screw compressors.
As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the spirit or scope thereof. Accordingly, the scope of the invention is to be construed in accordance with the substance defined by the following claims.
