Hydraulic cylinder with position encoder

Abstract

An actuator has a position encoder provided by a helical groove formed in the piston rod. The groove is filled with a material of different magnetic characteristics to provide a smooth exterior surface and a varying discernible signal as the rod moves relative to the cylinder. An array of Hall effect sensors is provided around the rod to provide phase shifted signals as the rod moves so that the signal of one sensor may be correlated by the signals of other sensors.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:  
     
         1 . An actuator having a cylinder and a piston assembly moveable within said cylinder, said piston assembly projecting from said cylinder and having a surface that presents to a predetermined location on said cylinder a periodically varying discernible characteristic as said surface moves relative to said cylinder, and a sensor assembly to monitor said discernible characteristic and measure changes therein to provide an indication of movement of said piston assembly relative to said cylinder, said varying discernible characteristic being provided by an interface on said piston assembly exhibiting a change in said discernible characteristic, said interface being inclined to the direction of travel of the rod whereby the distance between said interface and said transducer varies progressively and continuously as said piston assembly moves relative to said cylinder.  
     
     
         2 . An actuator according to  claim 1  wherein said discernible characteristic is magnetic permeability.  
     
     
         3 . An actuator according to  claim 1  wherein said predetermined location is positioned adjacent the exit of said piston assembly from said cylinder.  
     
     
         4 . An actuator according to  claim 1  wherein said discernible characteristic is provided by a continuous groove formed in and extending along and about said surface.  
     
     
         5 . An actuator according to  claim 4  wherein said surface is cylindrical and said groove is wound about said surface.  
     
     
         6 . An actuator according to  claim 5  wherein said sensor assembly includes a plurality of sensors circumferentially spaced about said surface to provide a plurality of similar but phase shifted signals as said surface moves past said location.  
     
     
         7 . An actuator according to  claim 6  wherein a signal from one of said transducers is correlated to predetermined signals from other of said transducers.  
     
     
         8 . An actuator according to  claim 1  wherein said piston assembly includes a piston rod projecting from said cylinder and having a cylindrical outer surface and said interface is provided by flanks of a helical groove formed in said outer surface.  
     
     
         9 . An actuator according to  claim 8  wherein said groove is filled with a material different to that of said piston to provide a smooth outer surface.  
     
     
         10 . An actuator according to  claim 9  wherein said rod is supported in a bearing in said cylinder and a seal assembly encompasses said rod, said sensor assembly being located adjacent said bearing assembly.  
     
     
         11 . An actuator according to  claim 10  wherein said sensor assembly includes a plurality of sensors circumferentially spaced about said rod.  
     
     
         12 . An actuator according to  claim 11  wherein each of said sensors produces a similar but phase shifted signal as said piston rod moves past said sensor assembly, said signals being correlated to one another to provide verification of a signal obtained from one of said sensors by reference to other of said signals.  
     
     
         13 . An actuator according to  claim 12  wherein said helical groove is in the form of a screw thread having tips adjacent to said cylindrical outer surface, each of said sensors providing a peak signal upon a tip passing thereby.  
     
     
         14 . An actuator according to  claim 13  wherein the number of peak signals received from a sensor are monitored to provide an indication of movement of said rod relative to said cylinder.  
     
     
         15 . An actuator according to  claim 14  wherein identification of a peak signal is verified by correlation to signals from other of said sensors.  
     
     
         16 . An actuator according to  claim 14  wherein peak signals are monitored from one of said sensors when an error signal representing the difference between a desired position and an actual a position is greater than a predetermined value and peak signals from a plurality of sensors are monitored when said error signal is less than said predetermined value.  
     
     
         17 . An actuator according to  claim 11  wherein said discernible characteristic is the magnetic permeability and said sensors are Hall effect sensors.  
     
     
         18 . An actuator according to  claim 1  wherein said interface is provided by a tapered surface on said piston assembly.  
     
     
         19 . An actuator according to  claim 18  wherein said tapered surface progressively and continuously decreases in diameter along said piston assembly in the direction of relative movement between said piston assembly and said cylinder assembly.  
     
     
         20 . An actuator according to  claim 19  wherein said tapered surface is coated to provide a cylindrical outer surface for said piston assembly.  
     
     
         21 . An actuator according to  claim 20  wherein said discernible characteristic is the magnetic permeability.  
     
     
         22 . An actuator according to  claim 21  wherein said interface is formed between a magnetically reactive body and a magnetically non-reactive coating.  
     
     
         23 . An actuator according to  claim 22  wherein said sensor assembly includes a Hall effect sensor mounted on said cylinder adjacent said piston assembly.  
     
     
         24 . An actuator according to  claim 23  wherein said sensor assembly further includes a magnet and said Hall effect sensor is interposed between said magnet and said piston assembly.  
     
     
         25 . An actuator according to  claim 22  wherein said sensor assembly is mounted on a support on said cylinder, said support being radially adjustable relative to said cylinder to accommodate radial movement between said piston assembly and said cylinder.