Abstract:
A position sensor senses the position of an article that moves over a short predetermined path. The position sensor is contained in a housing which is mounted in a fixed location adjacent the article and has an internal cavity in which a valve follower travels guided by the walls of the cavity to describe a predetermined path in the axial direction. The valve follower is operatively coupled with the article and carries a sensed magnetic element. A sensor is mounted in the housing adjacent the path of the valve follower, such that the sensed element moves laterally under the sensor and the sensor generates a signal indicative of its position.

Description:
INTRODUCTION  
       [0001]     The present invention relates generally to the field of hydraulic controls and in particular, the invention provides an improved hydraulic valve position-monitoring sensor which improves the reliability of valve monitoring.  
       BACKGROUND TO THE INVENTION  
       [0002]     Hydraulic valves are used in machinery to control motive force by controlling the flow of hydraulic fluid into rams, pistons, valves and hydraulic motors used for both motion of the machine itself, and/or motion of implements or other moving components of the machine such as press components, digging implements, etc.  
         [0003]     Generally, hydraulic machinery is provided with safety systems to prevent undesired movement of the machine or machine components at various points in the operation of the machine. For example, it is common to provide a safety screen on a hydraulic press to prevent access while the press is operating and to have interlocks on the screen, which prevent operation whenever the safety screen is not in its closed position. It is also common to put pressure sensors on the downstream side of a hydraulic valve to detect pressure in the hydraulic circuit of a machine and to prevent certain activities from occurring if the hydraulic circuit is pressurised. However, when the machine is initially started, the pressure sensor will read 0 (zero) pressure because the hydraulic pump is not operating and therefore there is no pressure on either side of the hydraulic valve. The pressure sensor will not read a pressure sufficient to indicate a dangerous situation, until such time as the hydraulic fluid has passed through the valve to create pressure in the downstream side of the circuit. During the instant while the pressure is building up to the level where the pressure sensor will trip, and due to delays and inertia after the sensor has detected a pressure, there will also be motion of the equipment driven by that hydraulic circuit and this can, in some circumstances, be quite dangerous and result in the accidental injury of a worker who might not have expected the equipment to move when the hydraulic pump was started. Every year there are a significant number of deaths in industry caused by unintentional movement of a machine at start-up and the device of the present invention in intended to reduce the possibility of occurrence of such accidents.  
         [0004]     It is also known to provide position monitors on valve spools to detect when a valve is open, however, such monitors detect only an open or closed condition and not the extent of opening of the valve and, in some circumstances fail to detect opening of the valve where the opening is slight. In particular, the accuracy of such prior art monitors depends on physical tolerances of the valve, the sensor and the fitment of the sensor to the valve and the operating temperature. Therefore, such arrangements are prone to false sensing of (for example) a closed position when the sensor is incorrectly adjusted or tolerances are exceeded.  
       SUMMARY OF THE INVENTION  
       [0005]     According to a first aspect, the present invention provides a valve spool monitor comprising position sensing means arranged to be mounted adjacent to a valve spool of a hydraulic valve and arranged to measure the absolute position of the spool within the valve and to provide an output signal indicative of the absolute position.  
         [0006]     According to a second aspect, the present invention provides a method of monitoring a valve spool, the method comprising locating a position sensing means adjacent to a valve spool of a hydraulic valve, to measure the absolute position of the spool within the valve and to provide an output signal indicative of the absolute position.  
         [0007]     In a preferred embodiment, the position sensing means comprises a first, sensed, component mounted in the valve spool or on a member operatively coupled with and moving in unison with the valve spool, and a second, sensor, element located adjacent a path described by the sensed component when the valve spool travels through its stroke, such that the sensed component passes under the sensor element as it travels along its path.  
         [0008]     According to a third aspect, the present invention provides a position sensor for sensing the position of an article that moves over a short predetermined path, the position sensor comprising a housing, mounting means arranged to permit mounting of the housing in a fixed location adjacent the article, the housing having an internal cavity having a substantially constant cross section along an axial direction of the cavity and in which is located a travelling element configured to cooperate with the walls of the cavity to be guided to describe a predetermined path in the axial direction, coupling means arranged to operatively couple the travelling element with the article, a sensed element being mounted on the travelling element and a sensor being mounted in or on the housing adjacent the path of the travelling element, such that the sensed element moves laterally under the sensor and whereby the sensor senses the position of the sensed element and generates a signal indicative of its position.  
         [0009]     According to a fourth aspect, the present invention provides a method of sensing the position of an article that moves over a short predetermined path comprising mounting a housing enclosing a monitor assembly in a fixed location adjacent the article, providing the housing with an internal cavity and internal guiding surfaces extending in an axial direction of the cavity, locating a travelling member between the guiding surfaces to be guided to describe a predetermined path in the axial direction, operatively coupling the travelling member with the article, locating a sensed element relative to the travelling member and locating a sensor adjacent the path of the travelling member, such that the sensed element moves laterally under the sensor and whereby the sensor senses the position of the sensed element and generates a signal indicative of its position, to indicate the position of the article.  
         [0010]     Preferably, the sensed component is a magnet mounted on a member extending in the axis of the valve spool and coupled to the valve spool to move therewith, and the sensor is preferably a magnetic field angle sensor located adjacent to a path described by the magnet, whereby the magnetic field angle sensor determines valve position by monitoring the change in angle of the magnetic field of the magnet at the sensor as the magnet moves past the sensor.  
         [0011]     Preferably also, the valve spool monitor or position sensor further includes processing means to convert the sensor output signal to an assembly output signal whereby the sensor output signal is indicative of the instantaneous position of the valve and the assembly output signal, includes discrete levels which indicate closed and opened position signals. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     An embodiment of the invention will now be described by way of example with reference to the accompanying drawings, in which:  
         [0013]      FIG. 1  shows a partial cutaway side view of a hydraulic valve with a first embodiment of a sensor assembly added according to an embodiment of the present invention;  
         [0014]      FIG. 2  is an end view of the sensor assembly of  FIG. 1 , showing the interface with the hydraulic valve;  
         [0015]      FIG. 3  is a schematic illustration of the electronic and magnetic function of the sensor assembly of  FIG. 1 ; and  
         [0016]      FIG. 4  shows a partial cutaway side view of a hydraulic valve with a second embodiment of a sensor assembly added.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0017]     A first embodiment of a valve sensor is illustrated in  FIGS. 1 and 2 , which show a partial cutaway side view and an end view respectively, of a hydraulic valve with a valve sensor assembly attached. In  FIG. 1 , a valve body  11 , housing a valve spool  12 , having valve porting  13  is provided with a sensor assembly  22 , attached to the rear of the valve housing. The sensor assembly  22  comprises a sensor housing  14  in which is housed a valve spool extension member  15  operatively coupled with the valve spool  12  to act as a valve spool follower and, in this case, physically connected to the valve spool  12  by a linkage  16 . The valve spool extension member  15  travels in a cavity  24  in the sensor housing  14 , and has mounted on one side, a magnet element  17  which is located adjacent to one side of the cavity  24 . The magnetic element  17  travels backwards and forwards along a surface of a wall  25  of the sensor housing  14  when the valve spool  12  moves in the hydraulic valve body  11  such that the magnetic element  17  moves under a sensor element  19 , housed in a second chamber  26  of the sensor assembly  22 . The sensor element  19  is mounted to a sensor electronics board  18 , also mounted within the chamber  26  and a signal cable  21  extends from the electronics board  18  through an aperture in an end cap  27  of the chamber  26  and then runs to a control system of the machine to which the hydraulic valve is connected.  
         [0018]     Because the sensor relies on field direction, it is independent of magnet strength and temperature and therefore its accuracy is relatively independent of operating conditions.  
         [0019]     A seal  23  is provided between the valve body  11  and the sensor housing  14  and extends around the spool  12  to prevent loss of hydraulic fluid from the valve into the chamber  24  of the sensor body  14 .  
         [0020]     Referring to  FIG. 3 , a second embodiment of a valve sensor is illustrated in which a partial cutaway side view of a hydraulic valve with a sensor attached is again shown. The valve body  11  in  FIG. 3  is similar to that shown in  FIGS. 1 &amp; 2  and houses a valve spool  12 , having valve porting  13 . The  FIG. 3  valve body  11  is provided with a sensor assembly  122 , attached to the rear of the valve housing. The sensor assembly  122  which comprises a sensor housing  114  in which is housed a valve spool extension member  115  operatively coupled with the valve spool  112  to act as a valve spool follower and is biased into engagement with the valve spool  12  by a spring  116 . A guide  129  is screwed through the housing  114  and extends down the centre of the spring  116  to maintain the spring in alignment and to act as a stop for the valve spool extension member  115  to prevent over-compression of the spring  116  and to calibrate the position of the valve spool,  12 . The valve spool extension member  115  travels in a cavity  124  in the sensor housing  114 , and is retained by a screw  140  extending through the side of the housing  114  and into a slot  141  in the side of the valve spool extension member, which prevents rotation of the valve spool extension member.  
         [0021]     The valve spool extension member  115  has mounted on one side, a magnet element  117  which is located adjacent to one side of the cavity  124 . The magnetic element  117  travels backwards and forwards along a surface of a wall  125  of the sensor housing  114  when the valve spool  12  moves in the hydraulic valve body  11  such that the magnetic element  117  moves under a sensor element  119 , housed in a second chamber  126  of the sensor assembly  122 . The sensor element  119  is mounted to a sensor electronics board  118 , also mounted within the chamber  126  and a signal cable  121  extends from the electronics board  118  through a conduit  128  extending through an end plate  127  of the chamber  126  and then runs to a control system of the machine to which the hydraulic valve is connected.  
         [0022]     A seal  123  is provided between the valve body  11  and the sensor housing  114  as in the first embodiment and extends around the spool  12  to prevent loss of hydraulic fluid from the valve to atmosphere.  
         [0023]     Referring to  FIG. 4 , the electronic and magnetic function of the sensor assembly  22 ,  122  are schematically illustrated. For convenience, the description of the circuit of  FIG. 4  will refer to the elements described in relation to the embodiment of  FIGS. 1 &amp; 2  however this circuit will operate identically with the embodiment of  FIG. 3 . It will be seen in  FIG. 4  that the magnet  15  produces lines of magnetic field radiating out of the upper (eg; north) pole of the magnet and these lines pass through the sensor chip  19  mounted on the electronics board  18  of the sensor assembly.  
         [0024]     In the preferred embodiment, the chip is a Honeywell™ Linear/Angular/Rotary Displacement Sensor, model HMC1501 or HMC1512. Each of these devices operate on the effect of anisotropic magnetoresistance (AMR) which occurs in ferrous materials. AMR is a change in resistance which occurs when a magnetic field is applied in a thin strip of ferrous material such as a permalloy thin film (NiFe). The magnetoresistance is a function of Cos 2   where is the angle between magnetization M and current flow in the thin strip. When the magnetic field applied to the Honeywell™ HMC1501 or HMC1512 devices is greater than 80 Oe, the magnetization aligns in the direction of the applied field; this is called saturation mode. In this mode, is the angle between the applied field and the current flow.  
         [0025]     In the present application, the sensor chip  19  measures a field angle being the angle  34  between the axis of the chip and a field direction of field lines  35  passing through the chip. The circuit board  18  carries a circuit which interfaces the sensor chip output  37  of a signal representing the field angle to a microprocessor which converts the field angle signal into a digital position signal representing the linear position of the magnetic element  17  and hence the valve spool  12 . The microprocessor  36  then further processes the position signal to provide a value status output. In the preferred embodiment, the microprocessor is a Microchip™, PIC12CE674™. This 8 pin DIL packaged integrated circuit has analogue inputs, digital input/output and EEPROM data storage in which the calibration data is held. The microprocessor  36  outputs valve status information which is converted to a 4-20 mA signal  21  carried on a current loop circuit  31  to the control system  32  of the machine to which the valve is fitted. In the control system  32 , the 4-20 mA signal is typically passed through a 250 Ω resistor to convert it to a 1-5 volt signal.  
         [0026]     The 4-20 mA signal uses the following protocol to indicate value status: 
        i)&lt;4 mA—Fault condition     ii) 5 mA—Out of calibration     iii) 8 mA—valve open to right side     iv) 12 mA—valve closed     v) 16 mA—valve open to left side.        
 
         [0032]     After installation of a sensor unit, the sensor unit is calibrated. The valve is moved towards hydraulic crack point and then moved back till there is no flow. The equivalent absolute location is then read from the sensor and programmed into EEPROM of the microprocessor. Calibration is performed separately for the right and left crack point positions of the valve.  
         [0033]     It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.