Patent Publication Number: US-2009220366-A1

Title: Slide valve position indicator

Description:
FIELD OF THE INVENTION 
     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. 
     BACKGROUND OF THE INVENTION 
     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. 
     SUMMARY OF THE INVENTION 
     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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings wherein similar characters of reference denote corresponding parts in each view: 
         FIG. 1  is, in upper front perspective view, a magnetic sensor slide valve position indicator according to one embodiment of the present invention mounted on the outer face of a compressor. 
         FIG. 2  is, in upper rear perspective view, the magnetic sensor slide valve position indicator according to  FIG. 1  with its disc-mounted helically inclined ramp rotated to the angular position spacing the sensor head most closely to the ramp surface. 
         FIG. 3   a  is, in plan view, the slide valve position indicator of  FIG. 2  with the helically inclined ramp rotated to correspond to a slide valve closed position. 
         FIG. 3   b  is the view of  FIG. 3   a  with the helically inclined ramp rotated to a mid position corresponding to a partially open slide valve. 
         FIG. 3   c  is the slide valve of  FIG. 3   a  with the helically inclined ramp rotated to a maximum spacing between the ramp and the magnetic sensor to correspond to a fully open position of the slide valve. 
         FIG. 4  is the view of  FIG. 2  with the disc-mounted helically inclined ramp rotated to the angular position spacing the sensor head furthest from the ramp surface. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     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  14   a  mountable to the outside face  16  of a compressor  18  and an arm  14   b  cantilevered outwardly from base plate  14   a . Arm  14   b  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  14   a  is mounted thereto. 
     A threaded barrel  20   a  of magnetic sensor  20  is journalled in an aperture in the distal end of arm  14   b  so as to extend generally orthogonally to outside face  16  when base plate  14   a  is mounted thereto. The position of threaded barrel  20   a  relative to arm  14   b  may be adjusted by the use of threaded nuts  22 . A protective resilient end cap  20   b  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  26   a  which is mounted onto the outermost end of shaft  24 , and a round gauge face-plate  26   c , optionally having a position indicator arrow  28  marked on the outer surface thereof. Sandwiched between base collar  26   a  and gauge face-plate  26   c  is a smoothly helically inclined cam defined by an annular ramp flange  26   b . Helically inclined ramp flange  26   b  defines a smoothly inclined annular helical ramp face  30   a  adjacent base collar  26   a . An oppositely disposed planar face  30   b  is adjacent gauge face-plate  26   c.    
     Ramp face  30   a  extends 270 degrees about shaft  24  and axis B between a first end  30   a ′ and an opposite second end  30   a ″. When shaft  24  is rotated so as to bring first end  30   a ′ closely adjacent magnetic sensor  20 , the cam surface is most closely spaced relative to the head or outer end  20   c  of magnetic sensor  20 . That is, at first end  30   a ′, ramp flange  26   b  is at its thickest. At the opposite second end  30   a ″ ramp flange  26   b  is at its thinnest so that when second end  30   a ″ is rotated so as to be most closely adjacent to head  20   c  of magnetic sensor  20 , the distance d between ramp face  30   a  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  20   c  of magnetic sensor  20  and ramp face  30   a  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  20   c  of magnetic sensor  20  and ramp face  30   a . A cut-out  32  may be formed in disc  26  between ends  30   a ′ and  30   a″.    
     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  20   c  of magnetic sensor  20  is one (1) mm from the opposed facing ramp face  30   a , and a 20 milliamp signal when spacing d is five (5) mm. The signal is carried by wire  20   d . 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  20   c  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.