Abstract:
Wear of a pump is estimated using a process that involves operating the pump to drive a hydraulic actuator that moves a member. The actual speed of the member is determined and the speed of the pump is sensed. Pressure of fluid conveyed from the pump to the hydraulic actuator also is sensed. A predicted speed of the member is calculated based on the speed of the pump and the pressure of the fluid. The predicted speed is compared to the actual speed and the result is employed to provide an indication of a degree of wear of the pump. The difference between the predicted speed and the actual speed increases as the pump wear increases.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not Applicable 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to hydraulic pumps, and more particularly to techniques for detecting wear of hydraulic pumps. 
     2. Description of the Related Art 
     Hydraulic pumps are used in a wide variety of equipment to provide a source of pressurized hydraulic fluid that then is controlled to operate hydraulic actuators, such as hydraulic motors and hydraulic cylinder and ram assemblies. Over time, the internal components of a pump may wear, thereby leaking fluid which decreases the magnitude of fluid flow produced by the pump. Such leakage not only slows the motion of the hydraulic actuator, it wastes power and raises the temperature of the hydraulic fluid which also are disadvantageous. Over time, the actuator operation degrades to a point where either maintenance or replacement of the pump is necessary. 
     It is, therefore, desirable to detect if excessive wear of a pump occurs and be able to take remedial action. 
     Previous techniques for determining excessive pump wear involved sensing an amount of fluid flowing through a drain outlet in the case of the pump. Because pump wear introduces metal particles into the hydraulic fluid, another method periodically measured the size and concentration of solid particles in the fluid. The noise produced by a pump also has been used to detect excessive leakage. 
     SUMMARY OF THE INVENTION 
     A pump is connected to a hydraulic actuator that moves a member on a material handling vehicle. Pump wear is estimated by operating the pump to drive the hydraulic actuator to move the member. The actual speed of the member is determined, such as by one or more sensors, and the speed of the pump also is detected. Pressure of fluid conveyed from the pump to the hydraulic actuator is sensed. 
     The speed of the pump and the pressure of the fluid are employed to calculate a predicted speed. The temperature of the fluid optionally also may be used to calculate a predicted speed. The predicted speed is compared to the actual speed of the member. As the pump wear increases, a difference between the predicted speed and the actual speed increases. An indication of a degree of wear of the pump is produced in response to the comparison of the speeds. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a material handling vehicle incorporating the present invention; 
         FIG. 2  is a schematic diagram of the control system for the material handling vehicle; and 
         FIG. 3  is a flowchart of a method for monitoring performance of the hydraulic pump in the control system. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring initially to  FIG. 1 , a material handling vehicle  10 , such as a lift truck, includes main body  14  mounted on wheels  16  and  17  for movement across a floor of a warehouse or a factory, for example. The body includes an operator compartment  18  with an opening  20  for entry and exit of the operator. The operator compartment  18  contains a multi-function control handle  22  and a deadman switch  24  positioned on the floor  26 . The deadman switch  24  must be closed by the operator&#39;s foot before any of the motors on the material handling vehicle can operate, which prevents run away operation of the vehicle. A steering wheel  28  is also provided in the operator compartment  18 . Although the material handling vehicle  10  is shown by way of example as having a standing, fore-aft operator stance configuration, the present invention is not limited to vehicles of this type, and can also be used with other types of material handling vehicles including, without limitation, pallet trucks, platform trucks, fork material handling vehicles, counterbalanced fork lift vehicles, and other powered vehicles used in a warehouse or a factory to transport, store, and retrieve items. 
     The material handling vehicle  10  includes a vertical mast  30  secured to the body  14  with a carriage  32  is slideably mounted to the mast for vertical movement between different positions. A pair of forks  34  extends from the carriage  32  to support a load  50  ( FIG. 2 ) that is being transported by the material handling vehicle. By manipulating the multi-function control handle  22 , the operator controls the raising and lowering of the carriage  32  on the vertical mast  30 . 
     With reference to  FIG. 2 , the multi-function control handle  22  and steering wheel  28  are part of a control system  40  for the material handling vehicle  10 . The control system  40  includes a vehicle controller  42  that is a microcomputer based device that executes software which controls operation of other components on the vehicle. A conventional information display  41  and a keyboard  43  enable the operator to interface with the vehicle controller  42 . The vehicle controller  42  also receives operator input signals from the multi-function control handle  22 , the steering wheel  28 , a key switch  45 , and the deadman switch  24 . In response to those received signals, the vehicle controller provides command signals to a lift motor control  44  and a drive system  47  that includes both a traction motor control  46  and a steer motor control  48 . The drive system  47  provides a motive force for driving and steering the material handling vehicle  10  in a selected direction, while the lift motor control  44  governs motion of the carriage  32  along a mast  30  to raise or lower the load  50 , as described below. The material handling vehicle  10  and its control system  40  are powered by one or more batteries  38 , coupled to the vehicle controller  42 , drive system  47 , and lift motor control  44  through a bank of fuses or circuit breakers  52 . Although a battery powered material handling vehicle is being used in the disclosure herein, the present invention also can be used on a vehicle that is powered by an internal combustion engine or a fuel cell. 
     The traction motor control  46  activates an electric traction motor  54  which is connected to the wheel  16  to provide motive force to the material handling vehicle  10 . The speed and direction of the traction motor  54  are selected by operation of the multi-function control handle  22 . The wheel  16  is also connected to friction brake  56  through the traction motor  54 , providing both a service and parking brake function for the material handling vehicle  10 . The steer motor control  48  is connected to operate a steer motor  58  and associated steerable wheel  59 , in response to the operator rotating the steering wheel  28 . The direction of rotation of the steerable wheel  59  and the travel control command from multi-function control handle  22  determine the direction of motion of the material handling vehicle. 
     Of particular significance to the present invention is that the lift motor control  44  controls application of electric current to a hydraulic lift motor  60  which is connected to a hydraulic circuit  62 . The hydraulic circuit  62  propels the carriage  32  and forks  34  along the mast  30 , thereby moving the load  50  up or down, depending on the direction selected at the multi-function control handle  22 . The lift motor  60  drives a fixed positive displacement pump  64  that produces flow of fluid from a reservoir  66  to a hydraulic cylinder and ram assembly  68  connected between the body  14  of the material handling vehicle and the carriage  32 . A solenoid operated, bidirectional control valve  67  couples the outlet of the hydraulic pump  64  to the hydraulic cylinder and ram assembly  68 . A pressure relief valve  65  provides a release path to the reservoir  66  in the event that excessive pressure exists in the pump outlet line. 
     The hydraulic circuit  62  includes a pressure sensor  70  and a temperature sensor  72  that respectively sense the pressure and temperature of the fluid flowing between the hydraulic pump  64  and the hydraulic cylinder and ram assembly  68 . The pressure sensor  70  and a temperature sensor  72  produce electrical signals that are applied to inputs of the vehicle controller  42 . A speed sensor  74  is connected to the lift motor  60  and provides a measurement of the speed of the pump to the vehicle controller  42 . Because the pump  64  is connected directly to the lift motor  60 , the rotational speed of both devices is the same. That may not be true for other transmissions that couple the motor to the pump, in which situations the speed sensor  74  is attached directly to the hydraulic pump  64 . 
     Lower and upper mast switches  76  and  78  are located along the path of travel of the carriage  32  on the mast  30  and are closed when the carriage is at the respective position of the switch. The lower mast switch  76  is closed when the carriage  32  is at the lower extremity of travel along the mast. The distance between lower and upper mast switches  76  and  78  is known and fixed. As will be described, the mast switches  76  and  78  provide a means by which the actual speed of travel of the carriage  32  can be measured by the vehicle controller  42 . 
     As noted previously, the vehicle controller  42  responds to input signals via devices  22  and  28  from the operator indicating functions to be performed by the material handling vehicle  10 . One of those functions is to raise and lower the load  50  by moving the carriage  32  along the mast  30  as commanded by the operator manipulating the multi-function control handle  22 . The vehicle controller responds to that operator command by appropriately operating the lift motor control  44  and the solenoid operated, bidirectional control valve  67 . To raise the carriage  32 , the control valve  67  is opened and the lift motor control  44  is commanded to apply electric current to the lift motor  60  which drives the pump  64  to send fluid from the reservoir  66  through the control valve  67  to the cylinder and ram assembly  68 . To lower the carriage  32 , the control valve  67  is opened which allows fluid to be forced from the cylinder and ram assembly  68  by gravity acting on the carriage and any load that is present. The fluid flows backward through the pump to the reservoir  66 . Alternatively a three-position, three-way control valve may be used to provide a separate direct path from the cylinder and ram assembly  68  to the reservoir  66 . 
     In addition to controlling the pump  64 , the vehicle controller  42  executes a pump monitoring routine that examines the performance of the hydraulic system to determine whether the pump has experienced an excessive amount of wear and thereby requires maintenance or replacement. With reference to  FIG. 3 , the pump monitoring routine  100  is executed periodically based on a real time clock of the vehicle controller. The execution commences at step  101  at which the vehicle controller  42  waits until closure of the lower mast switch  76  occurs, which indicates that the carriage  32  is located at the bottom of the mast  30 . Then at step  102 , the vehicle controller  42  waits for a command from the operator to raise the carriage  32 . Upon that occurrence, the vehicle controller  42  clears and starts a software implemented timer at step  103  to measure the travel time of the carriage on the mast  30 . Next at step  104 , the vehicle controller  42  reads and records the measurements from the pressure sensor  70 , the fluid temperature sensor  72 , and the lift motor speed sensor  74 . 
     At step  105 , the vehicle controller  42  reads the input signal from the upper mast switch  78  to determine whether that switch is closed, as occurs when the carriage  32  reaches the position of that switch. It should be understood that the upper mast switch  78  is located at a position along the mast to which the carriage  32  is frequently raised. If the switch is not found closed, the program execution branches to step  106  to determine whether the value of the timer is greater than an predefined amount of time T. That amount of time T is longer than the maximum time that it should ever take the carriage  32  to be raised to the position of the upper mast switch  78  under the heaviest allowable load and worst case normal operating conditions expected for the hydraulic system. This test at step  106  resets the pump monitoring routine  100  when the carriage  32  is not being raised sufficiently high to reach the upper mast switch  78 . In that event, the process returns to step  101  to wait for the lower mast switch  76  to close, which occurs when the carriage  32  has been lowered to the bottom of the mast  30 . From that point, the process will resume again when another operator command to raise the carriage is received. 
     If, however, the timer has not reached the value of T at step  106 , the program execution returns to step  105  to examine the status of the upper mast switch  78 . Thus, while the mast is raising, the pump monitoring routine  100  continues to loop through steps  105  and  106  until the closure of the upper mast switch  78  is detected or until the timer times out, i.e., reaches the value of T. 
     Assuming that the carriage  32  continues raising upward and eventually reaches the upper mast switch  78 , the closure of that switch causes the pump monitoring routine to branch from step  105  to step  108  where the timer is stopped and its elapsed time recorded. 
     Although the remaining steps of the pump monitoring routine  100  can be performed by the vehicle controller  42 , alternatively the recorded time can be uploaded into a computer in the facility where the material handling vehicle  10  is operating. In that latter case, the computer performs those remaining steps. 
     The monitoring of pump wear is premised on the concept that the lift speed of the carriage  32  is a function of the pump output flow minus any leakage which is expressed as Lift Speed=Pump Output−Leakage. For some pumps, the leakage can be modeled as flow through an orifice. In that case, a Predicted Lift Speed value is calculated at step  110  according to the equation: 
                     PREDICTED   ⁢           ⁢   LIFT   ⁢           ⁢   SPEED     =       K   *   R   ⁢           ⁢   P   ⁢           ⁢   M     -     M   *       PRESSURE   TEMPERATURE                   (   1   )               
where K is the pump displacement, RPM is the measured speed of the pump  64  from sensor  74 , M is a constant, PRESSURE is the pressure at the outlet of the pump  64  as measured by sensor  70 , and TEMPERATURE is the temperature of the fluid at the pump outlet as measured by sensor  72 . The values for K and M are derived for a specific type of pump on a particular model of material handling vehicle and stored in the memory of the vehicle controller  42  of each material handling vehicle  10  of that model. Alternatively, values for K and M can be derived for each particular material handling vehicle  10  at time of manufacture.
 
     For other pump designs the leakage flow can be modeled flow down a narrow tube instead of through orifice. In this case, the Predicted Lift Speed value is calculated at step  110  according to the alternative equation:
 
PREDICTED LIFT SPEED= K *RPM− M *TEMPERATURE*√{square root over (PRESSURE)}  (2).
 
     The appropriate equation and values for terms K and M are derived for a specific type of pump on a particular model of material handling vehicle and stored in the memory of the vehicle controller  42  of each material handling vehicle  10  of that model. Alternatively, values for K and M can be derived for each particular material handling vehicle  10  at time of manufacture. Those values are derived as follows. 
     K is the pump displacement that results from one cycle of the pump, e.g., produced by one rotation of the pump shaft. The pump displacement depends on the volume change of the hydraulic actuator, such as the cylinder and ram assembly  68 . So for a particular cylinder diameter and piston displacement of the cylinder and ram assembly  68 , each meter of motion is equivalent to a volume of fluid that flows into the cylinder. If to lift the carriage  32  one meter per minute (Lift Speed) requires X amount of fluid per minute and one pump rotation produces Y amount of fluid, then the pump speed (RPM) needed to provide that fluid flow rate is given by X/Y. The values of X and Y can be determined empirically for a given model of material handling vehicle while lifting its carriage one meter per minute. Therefore, for a new pump with zero leakage, the expression Lift Speed=K*RPM is rewritten as K=(Lift Speed)/RPM=(Lift Speed)/(X/Y) and the latter equation is solved using the measured values. 
     To derive a value for the constant M, the leakage flow for a new pump is modeled by flow through a small orifice in a larger pipe which is given by the expression: 
                   Q   =       C   d     ⁢   A   ⁢       1     1   -     β   4           ⁢         2   ⁢     (       P   ⁢           ⁢   1     -     P   ⁢           ⁢   2       )       ρ                 (   3   )               
where Q is the amount of flow through the orifice, C d  is a coefficient of discharge, A is the area of the orifice, β is the ratio of the diameter of the orifice to the diameter of the pipe in which the orifice is located, (P 1 -P 2 ) is a pressure drop across the orifice, and ρ is the density of the hydraulic fluid.
 
     Applying this model to pump leakage, the change in fluid density is relatively small for the range of pressures and temperatures experienced by a typical material handling vehicle. As a result, the effects of pressure and temperature may sometimes be ignored, thereby making fluid density a constant within a nominal range of temperatures. In addition, the ratio β of the leakage orifice diameter to the overall diameter of the pump outlet is relatively small and its effect becomes an even smaller factor when raised to the fourth power. Therefore, the square root term containing β can be considered as a constant value of one. As a consequence, the pump leakage is a strong function of the area (A) of the leakage path and that area is a squared term, e.g., if the pump wear increases a leakage gap by a factor of two, the influence on leakage flow increases by a factor of four. As with most turbulent flows, the leakage flow is a function of the square root of the pressure drop (P 1 -P 2 ) across the leakage orifice. That pressure drop in a typical pump is the difference between the inlet and outlet pressure and the inlet pressure in many systems can be considered equal to atmospheric pressure. Therefore, the leakage pressure drop (P 1 -P 2 ) in Equation (3) can be considered as only the outlet pressure (PRESSURE) of the pump  64 . This enables the leakage equation to be simplified to:
 
Q=M√{square root over (PRESSURE)}  (4)
 
where M incorporates the values of A, C d , β, and √{square root over (2/ρ)}.
 
     As noted M is derived for a new pump. Over time as the leakage area A increases, the actual value of M changes. By keeping the value of M constant when calculating the Predicted Lift Speed in Equation (1) or (2), an indication of pump wear is provided by comparing the Predicted Lift Speed to the actual measured lift speed. 
     Referring again to the flowchart of  FIG. 3 , after the Predicted Lift Speed has been calculated, the pump monitoring routine  100  advances to step  111  at which the Actual Lift Speed is derived. That derivation is based on the timer value recorded at step  108  and the fixed distance between the upper and lower mast switches  76  and  78  (Actual Lift Speed=Distance/Timer Value). It should be appreciated that other mechanisms, such as a velocity sensor, can be used to provide the Actual Lift Speed of the carriage  32 . 
     At step  112 , the difference ΔS between the Predicted Lift Speed and the Actual Lift Speed is calculated. Then, the newly calculated lift speed difference ΔS is applied to a rolling average of a plurality of lift speed differences to derive the average lift speed difference ΔS AVE  at step  114 . For a new pump, the average lift speed difference is near zero, i.e., within a relatively small standard deviation. Over time, wear of the hydraulic pump  64  results in an increase in the difference between the Predicted Lift Speed and the Actual Lift Speed. As a result, the average lift speed difference increase provides an indication of the amount of pump wear. Furthermore, average lift speed difference ΔS AVE  reaching a predefined threshold value ΔS MAX  denotes that excessive wear has occurred. That threshold value ΔS MAX  can be determined empirically by intentionally operating the vehicle hydraulic circuit  62  until the actual lift speed fails to meet the minimum requirements set by the model specifications. During that operation the parameters of the pump monitoring system are recorded to provide a series of values for the average lift speed difference. 
     This enables, the pump wear to be indicated as a percentage based on the amount that the presently derived value for the average speed difference ΔS AVE  is of the threshold value ΔS MAX . That wear percentage is calculated at step  116  of the pump monitoring routine  100 . Next at step  118 , the new wear percentage is compared to determine whether it exceeds a given percentage amount S % at which it is desirable to provide a warning to the operator of the material handling vehicle or to maintenance personnel at the facility where the vehicle is operating. The warning indicates that significant pump wear has occurred and that the personnel should consider performing maintenance or replacement of the pump before a catastrophic failure occurs. Such a warning, if necessary, is issued at step  120  before the pump monitoring routine ends. For example the warning can be a message presented on the information display  41  of the material handling vehicle  10 , however other visual or audible annunciators can be used. 
     The rate of change of the difference AS between the Predicted Lift Speed and the Actual Lift Speed or the rate of change of the average lift speed difference ΔS AVE  also can be used as an indication of excessive pump wear. Typically those rates of change increase as the amount of wear becomes more severe. A high rate of change indicates that preventative maintenance (new filter, flush &amp; replace hydraulic oil, etc.) should be done to reduce over all costs. 
     Alternatively, the use of temperature in the previously described pump monitoring method may be eliminated and still provide an indication of the amount of pump wear. In this alternative, the temperature sensor  72  can be eliminated from the hydraulic system and the pump monitoring routine simplified by not having to read and utilize the temperature in calculating the predicted lift speed in Equation (1). In this alternative embodiment, the equation used to calculate the Predicted Lift Speed becomes:
 
PREDICTED LIFT SPEED= K *RPM− M *√{square root over (PRESSURE)}  (5)
 
The remaining steps of the process, such as in the pump monitoring routine  100 , are the same as described previously.
 
     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. Accordingly, the scope of the invention should be determined from the following claims and not limited by the above disclosure.