Abstract:
Motion of a hydraulically driven machine component is controlled in response to a velocity command that indicates a desired velocity for the machine component. A method for detecting a velocity fault involves determining an actual velocity at which the machine component is moving, and producing a velocity error value based on a difference between the velocity command and the actual velocity. The velocity error value is integrated, such as by a low pass, biquadratic filter function, to produce an integrated value. The integrated value is compared to one or more thresholds to determine whether a velocity fault has occurred.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit of U.S. Provisional Patent Application No. 60/821,877 filed on Aug. 9, 2006. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to hydraulic power systems with electrically operated valves that control fluid flow to hydraulically drive actuators, and more particularly to mechanisms that detect faults occurring in such systems. 
     2. Description of the Related Art 
     A wide variety of machines have moveable elements that are driven by a hydraulic actuator, such as a cylinder and piston arrangement. For example, a telehandler has a tractor on which a telescopic boom is mounted with a load carrier pivotally attached to the remote end of the boom. The telescopic boom and the load carrier are moved with respect to the tractor by hydraulic actuators. The flow of fluid to and from each hydraulic actuator is governed by a valve assembly controlled by the machine operator. 
     There is a present trend away from manually operated hydraulic valves toward electrical controls and the use of solenoid valves. For example, the operator sitting in a cab of the telehandler manipulates a joystick that produces an electrical signal designating a velocity desired for an associated element, such as the boom or load carrier. An electronic controller responds to the joystick signal by applying electric current to the valve assembly so that the proper amount of fluid is supplied to the respective hydraulic actuator to move the machine component at the desired velocity. 
     It is important to detect velocity faults or errors between the actual velocity of a machine component and the desired velocity. Such errors may result in an unsafe operation of the machine and thus require corrective action. On a telehandler, for example, it is desirable to detect a sudden drop of the boom which could occur due to a burst hose or other event. Upon detection of a velocity error, corrective action, such as operating a secondary isolation valve, can be performed. 
     Therefore, it is desirable to detect a velocity error of a machine component in order to take proper corrective action. However, such detection must be sufficiently robust to avoid erroneously declaring a fault condition because taking corrective action during normal machine action also may have adverse consequences. 
     SUMMARY OF THE INVENTION 
     A method is provided for detecting a velocity fault of a machine component that is hydraulically driven. This method comprises receiving a velocity command that indicates a desired velocity for the machine component and determining an actual velocity at which the machine component is moving. A velocity error value is produced based on a difference between the velocity command and the actual velocity and the velocity error value is integrated to produce an integrated value. Then the integrated value is analyzed to determine whether a velocity fault has occurred. 
     Integrating the velocity error value ensures that an over speed or an under speed condition must persist for a defined period of time before a velocity fault is declared. In a preferred implementation, the integrating is accomplished by a biquadratic filter function which decreases the integrated value for error frequencies that are below a cutoff frequency. 
     To determine whether a velocity fault has occurred, the integrated value preferably is analyzed by a threshold operation. The preferred threshold operation compares the integrated value to an over speed threshold and an under speed threshold. An over speed fault is declared when the integrated value is greater than the over speed threshold, and an under speed fault is declared when the integrated value is less than the under speed threshold. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a telehandler that incorporates a hydraulic system which employs the present invention; 
         FIG. 2  is a schematic diagram of the hydraulic system; and 
         FIG. 3  is a control diagram depicting a velocity fault detection mechanism. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With initial reference to  FIG. 1 , a telehandler  10  is an example of a machine on which the present invention can be used, with the understanding that the invention has application to a wide variety of machines. The telehandler  10  has a carriage  11  with an operator cab. The carriage  11  supports an engine or battery powered motors (not shown) for driving the wheels across the ground and for powering a hydraulic system. A boom assembly  12  comprises a boom  14 , an arm  15 , and a load carrier  16 . The boom  14  is pivotally attached to the rear of the carriage  11  and is raised and lowered by a boom hydraulic actuator  21 , in this case a pair of boom cylinders  22  each having a piston rod  24  (only one cylinder/piston rod arrangement is visible in  FIG. 1 ). An arm hydraulic actuator  26  causes the arm  15  to slide telescopically within the boom  14  thereby extending and retracting the length of the boom assembly  12 . The load carrier  16  is pivotally mounted at the remote end of the arm  15  and may comprise any one of several structures for carrying a load  20 . The load carrier  16  is tilted up and down by a load carrier hydraulic actuator  28 . 
     The present hydraulic system controls boom motion in terms of the boom hydraulic actuator  21 . For that purpose, a conventional sensor produces an electrical signal in response to motion of the boom assembly with respect to the carriage  11  in order to provide an indication of the actual velocity of the boom hydraulic actuator  21 . For example, a linear transducer  18  indicates the extension distance of a piston rod  24  from one of the boom cylinders  22  wherein that position signal is differentiated to derive the boom velocity. Alternatively, a velocity sensor could directly sense the boom hydraulic actuator velocity. The boom velocity also could be calculated from sensing the fluid flow to or from the boom cylinders. As an alternative sensor, an accelerometer  17  may be mounted on the boom  14  with its signal being integrated to produce a boom velocity signal, which then is converted trigonometrically into the corresponding boom hydraulic actuator velocity. Similarly, a resolver or encoder  19  can be attached to the pivot shaft  13  of the boom with its position signal being differentiated into a boom velocity value that then is converted into the velocity of the boom hydraulic actuator  21 . The velocity of the boom hydraulic actuator is arbitrarily defined as being positive when the boom is being raised and being negative when lowering the boom. As a further alternative, the hydraulic system  30  could control the motion in terms of velocity of the boom  14  thereby enabling velocity values from sensors on the boom assembly to be used without conversion. Thus the component of the telehandler  10 , the velocity of which is being controlled, may be the actuator or the element that is moved by the actuator, e.g. the boom  14 . 
     With additional reference to  FIG. 2 , the telehandler  10  has a hydraulic system  30  that controls movement of the boom  14 , the arm  15 , and the load carrier  16 . Hydraulic fluid is held in a reservoir, or tank,  32  from which the fluid is drawn by a conventional variable displacement pump  34  and fed through a check valve  36  into a supply line  38 . Alternatively, a fixed displacement pump may be utilized with an unloader valve at its outlet to control the supply line pressure. A tank return line  40  also runs through the telehandler  10  and provides a conduit for the hydraulic fluid to flow back to the tank  32 . A pair of pressure sensors  42  and  44  provide electrical signals that indicate the pressure in the supply line  38  and the tank return line  40 , respectively. 
     The supply line  38  furnishes hydraulic fluid to a first control valve assembly  50  comprising a Wheatstone bridge configuration of four electrohydraulic proportional (EHP) valves  51 ,  52 ,  53  and  54  which control the flow of fluid to and from the two boom hydraulic cylinders  22 . A separate EHP isolation valve  60  or  62  is located immediately adjacent each boom cylinder  22  and connect the first control valve assembly  50  to the respective cylinder&#39;s head chamber  57 . Each of these EHP valves  51 - 54 ,  60 ,  62  and other electrohydraulic proportional valves in the system  30  preferably are bidirectional poppet valves, thereby controlling flow of hydraulic fluid flowing in either direction through the valve. These EHP valves may be the type described in U.S. Pat. No. 6,328,275, for example, however other types of control valves, including an electrically operated spool valve, can be used. 
     A first pair of the EHP valves  51  and  52  governs the fluid flow from the supply line  38  into the head chamber  57  on one side of the piston in the boom cylinder  22  and from a rod chamber  55 , on the opposite side of the piston, to the tank return line  40 . This action extends the piston rod  24  from the boom cylinder  22  which raises the boom  14 . A second pair of EHP valves  53  and  54  controls the fluid flow from the supply line into the rod chamber  55  and from the head chamber  57  to the tank return line, which retracts the piston rod into the cylinder  22  thereby lowering the boom  14 . By controlling the rate at which pressurized fluid is sent into one cylinder chamber and drained from the other chamber, the boom  14  can be raised and lowered in a controlled manner. A first pair of pressure sensors  58  and  59  provide electrical signals indicating the pressure in the two chambers of the boom cylinder  22 . 
     A second control valve assembly  66 , similar to the first control valve assembly  50 , controls the flow of hydraulic fluid into and out of the arm hydraulic cylinder  26 . Operation of the second control valve assembly  66  extends and retracts the arm  15  with respect to the boom  14 . A third control valve assembly  68  controls fluid flow to and from a load carrier cylinder  28  that tilts the load carrier  16  up and down with respect to the remote end of the arm  15 . 
     With continuing reference to  FIG. 2 , operation of the hydraulic system  30  is governed by a system controller  70  that includes a microcomputer  71  connected by conventional signal busses  72  to a memory  73  in which software programs and data are stored. The set of signal busses  72  also connects input circuits  74 , output circuits  76  and valve drivers  78  to the microcomputer  71 . The input circuits  74  interface a joystick  79 , the boom motion sensor, and the pressure sensors and other devices to the system controller. The output circuits  76  provide signals to devices that indicate the status of the hydraulic system  30  and the functions being controlled. 
     A set of valve drivers  78  in the system controller  70  responds to commands from the microcomputer  71  by generating pulse width modulated (PWM) signals that are applied to the EHP valve assemblies  50 ,  66  and  68 . Each PWM signal is generated in a conventional manner by switching a DC voltage at a given frequency. When the hydraulic system is on a vehicle, such as a telehandler, the DC voltage is supplied from a battery and an alternator. By controlling the duty cycle of the PWM signal, the magnitude of electric current applied to a given valve can be varied, thus altering the degree to which that valve opens. This proportionally controls the fluid flowing through the valve to or from the associated hydraulic actuator. 
     To raise or lower the boom  14 , the machine operator moves the joystick  79  in the appropriate direction to produce an electrical signal indicating the desired velocity for the boom cylinder  22 , and indirectly the boom assembly  12 . The system controller  70  responds to the joystick signal by generating a velocity command and from that command derives current commands that designate electric current magnitudes for driving selected EHP valves  51 - 54  in order to apply fluid to the two boom cylinders  22  and produce the desired motion. Those current commands are sent to the valve drivers  78  which apply the appropriate electric current magnitudes to the selected EHP valves  51 - 54 . Thus, the hydraulic valves in assembly  50  are opened and closed to various degrees by varying the electric currents applied to those valves. Current commands also are sent to the valve drivers  78  to open fully the two isolation valves  60  and  62 . The control technique described in U.S. Pat. No. 6,775,974 may be used by the controller. 
     For example, when the machine operator desires to extend the rods  24  from the boom cylinders  22  and raise the boom assembly  12 , the electric current commands open the first and second EHP valves  51  and  52  by amounts that enable the proper level of fluid flow. Opening the first EHP valve  51  sends pressurized hydraulic fluid from the supply line  38  into the boom cylinder head chambers  57  and opening the second EHP valve  52  allows fluid from the rod chambers  55  to flow to the tank  32 . The system controller  70  monitors the pressure in the various hydraulic lines to properly operate the valves. To retract the rods  24  into the boom cylinders  22  and lower the boom assembly, the system controller  70  opens the third and fourth EHP valves  53  and  54 , which sends pressurized hydraulic fluid from the supply line  38  into the boom cylinder rod chambers  55  and exhausts fluid from the head chambers  57  to tank  32 . The force of gravity aids in lowering the boom assembly  12 . 
     With reference to  FIG. 3 , the system controller  70  continuously executes a velocity fault detection routine  80  as part of the software for controlling the telehandler  10 . That routine receives the velocity command produced in response to the signal from the joystick  79  and also receives the signal from the sensor  17 ,  18  or  19  which indicates the actual velocity of the boom assembly  12 . Those signals are applied to an arithmetic function  81  which produces a velocity ERROR value by calculating the difference between the velocity command (a desired velocity) and the actual boom velocity. The velocity ERROR, or difference, value is applied to two branches  82  and  83  of the velocity fault detection routine  80 . The first branch  82  is active when a positive velocity of the boom is commanded, whereas the second branch  83  is active for negative velocity commands. Two branches are provided so that an over speed or an under speed condition in one direction does not affect operation in the other direction. Note that the velocity of the boom has been arbitrarily defined as being positive when the boom is being raised. 
     The first branch  82  commences at a first selection function  84  where a determination is made whether the velocity command is positive, i.e. to raise the boom. If so, the velocity ERROR value is passed to the output of the first selection function  84 , otherwise the output is set to zero, thereby effectively disabling the first branch  82 . Assuming that the velocity command is positive, the velocity ERROR value is adjusted by a first multiplier function  85  which multiplies the velocity ERROR by minus one (−1), so that a positive velocity ERROR value represents an over speed condition. The adjusted velocity ERROR value from the first multiplier function  85  is applied to the input of a first dead band function  86 , so that relatively small velocity errors will be ignored and a fault condition will not be declared as a result. The first dead band function  86  produces a zero output when the adjusted velocity ERROR value is within a predefined range of values centered about zero, otherwise the adjusted velocity ERROR value is passed after being offset by an amount equal to the upper or lower limit of the dead band. 
     Simply determining when the adjusted velocity ERROR value exceeds a given threshold is not robust enough to avoid erroneously declaring a velocity fault condition. Large velocity errors typically occur for short durations when the boom-arm assembly strikes an object. A sizeable momentary over speed condition also exists immediately after the operator commands the boom motion to stop and a momentary under speed condition also occurs immediately after the operator commands the boom motion to commence from a stop. Therefore, the two branches  82  and  83  of the fault detection routine employ integration so that an over speed or under speed condition must persist for a period of time before declaring a fault condition. 
     Therefore, the output of the first dead band function  86  is applied to a first integration function  87  which forms a leaky integrator that is functionally equivalent to low pass filter with a very low cutoff frequency (e.g. 0.05 Hz) and high gain. The first integration function  87  preferably is implemented by a biquadratic filter having a filter function given by the expression: 
               y   ⁡     (   n   )       =         B   ⁢           ⁢   0   *     x   ⁡     (   n   )         +     B   ⁢           ⁢   1   *     x   ⁡     (     n   -   1     )         +     B   ⁢           ⁢   2   *     x   ⁡     (     n   -   2     )               A   ⁢           ⁢   1   *     y   ⁡     (     n   -   1     )         +     A   ⁢           ⁢   2   *     y   ⁡     (     n   -   2     )                   
where y(n) is the filter function output referred to as an integrated value, A 1 , A 2 , B 0 , B 1  and B 2  are filter coefficients, x(n) is the present output value from the dead band function  86 , x(n−1) and x(n−2) are the previous two dead band function output values, and y(n−1) and y(n−2) are the last two integrated values from the filter. At low frequencies, below a cutoff frequency defined by the filter coefficients, the filter leaks (i.e. decays) which drives the integrated value to zero over time, whereas above the cutoff frequency the filter act as an integrator. That integration converts error indication from a velocity value to a position value.
 
     That position value is applied to a first unit conversion function  88  where it is multiplied by a conversion factor  89  to convert the position error into the desired units of distance. The resultant position value then is applied to a first threshold operation  90  to an over speed threshold and an under speed threshold. Specifically the first threshold operation  90  comprises a first threshold function  91  that compares the position value to a positive over speed threshold, and a second threshold function  92  that compares the position value to a positive under speed threshold. When the positive over speed threshold is exceeded, a positive over speed fault is declared by first over speed function  91 . Similarly, if the speed is below the positive under speed fault threshold, a positive under speed fault is declared by a first under speed function  92 . These fault signals are binary thereby indicating whether a fault is or is not occurring. 
     If the first dead band function  86  was eliminated and a true integrator used in the first integration function  87 , small errors continue to accumulate over time until an error threshold eventually is reached. However, when determining the fault thresholds for functions  91  and  92 , it is helpful to consider the leaky first integration function  87  as a true integrator over a relatively small time period. In this way, the thresholds can be considered as a maximum allowable distance error after the velocity error is integrated. The dead band limits, cutoff frequency and gain of the first integration function  87 , and the fault detection thresholds are parameters that are determined and adjusted for the particular type of machine in order to ensure proper operation. 
     A clear ERROR command can be produced by the system controller  70  and applied to reset the first integration function  87  to zero. This avoids the accumulation of errors over a prolonged period of machine operation from producing continuous speed fault declarations. 
     The second branch  83  is similar to the first branch  82 , except the second selection function  93  renders the second branch active only when the velocity command is negative (i.e. a boom lower command). In other words, the velocity ERROR value is passed into the second branch  83  only upon occurrence of a negative velocity command, otherwise a zero value is applied to the downstream components in the second branch which thereby is disabled from indicating a fault condition. The output of the second selection function  93  is applied unadjusted to a second dead band function  94  which produces an output that is applied to a leaky second integration function  95 . The results of that latter function are then applied to a second units converter  96  to generate a signal representing the error in terms of a position. That position error then is compared by third and fourth threshold functions  97  and  98  of a second threshold operation  99  to a negative over speed threshold and a negative under speed threshold, respectively. When one of those negative thresholds is exceeded, a negative over speed fault indication or a negative under speed fault indication is generated. 
     It also should be understood that a particular machine may not require all the positive and negative over speed and under speed fault indications, as one or more of them may not correspond to a potentially hazardous condition. 
     The system controller  70  responds to the fault indications from the velocity fault detection routine  80  by taking the appropriate corrective action. For example, in response to an over speed fault condition, the system controller  70  operates the two isolation valves  60  and  62  for the boom cylinders  22 , thereby stopping any motion of the boom assembly  12 . As noted, these isolation valves are located in close proximity to the respective boom cylinders  22  and thus prevent fluid from exiting the head chambers  57  should the hose connecting those cylinders to the valve assembly  50  burst. A similar safeguard occurs if the first or fourth valve  51  or  54  fails in the open position. In place of the isolation valves, a mechanical stop on the load holding side of the actuator could be activated to arrest motion of the boom assembly  12 . 
     The foregoing description was primarily directed to a preferred embodiment of the invention. Although some attention was given to various alternatives within the scope of the invention, it is anticipated that one skilled in the art will likely realize additional alternatives that are now apparent from disclosure of embodiments of the invention.