Abstract:
A pallet truck including a controller that calculates the height of the fork carriage without the use of mechanical devices such as switches or sensors. A calculated carriage height is derived by adding the amount of time that a carriage is commanded to be raised and offsetting that by the amount of time that the carriage lower button is depressed. The calculated carriage height is essentially an accumulated lift time where the time that the carriage is lifting is a positive value and the time that the carriage is lowering is a negative value. When these values are combined, the result is the present calculated carriage height. The controller further monitors the calculated carriage height such that if the carriage is too close to the ground, the maximum speed of the pallet truck is limited to prevent damage to the forks or ground due to striking or scraping between them.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    Not applicable. 
       STATEMENT CONCERNING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not applicable. 
       FIELD OF THE INVENTION 
       [0003]    This invention relates to material handling apparatus, and more particularly, to a pallet truck that calculates the height of a vertically moveable fork carriage without the use of mechanical devices such as switches or sensors. 
       BACKGROUND OF THE INVENTION 
       [0004]    Electrically powered low-lift pallet trucks are common in the warehousing industry. These trucks are designed to travel at varying speeds while carrying a load supported by a pallet on a vertically displaceable fork carriage. Driving a pallet truck while the fork carriage is close to the ground may result in damage to the warehouse floor, the fork carriage or pallet caused by inadvertent contact, such as striking or scraping, with the floor. When operating on an uneven or rough floor, the pallet truck or its load may wobble or shift, increasing the potential for scraping between the floor and fork carriage. Further, contact with the floor while traveling may cause the pallet or load to shift and even spill. 
         [0005]    To prevent this situation, a number of pallet truck manufacturers limit travel speed of the pallet truck when the fork carriage is below a minimum height. Although designed to prevent a different problem, a number of forklift manufacturers also limit travel speed, though only when the fork carriage is too high and may cause the forklift to tip over. Despite the differences, both applications use the height of the fork carriage to limit truck performance. It is therefore important to have an accurate and reliable means for determining the carriage height of material handling vehicle. 
         [0006]    In response, a number of height determining technologies or algorithms have been employed with varying results. One common approach is to use limit switches, proximity sensors, or the like mounted at fixed heights that directly sense the position of the fork carriage. This information is then inputted to a vehicle controller. In one control methodology for a pallet truck, when the fork carriage is sensed to be above a predetermined height, high speed operation is enabled. When the carriage is sensed to be lower than the predetermined height, the maximum allowable travel speed of the truck is limited appropriately. 
         [0007]    This type of control methodology works well as long as all of the height sensors or switches are functioning properly. However, mechanical sensors and switches may wear out, become misaligned, suffer physical abuse, have signal wiring become disconnected either intentionally or unintentionally, and generally suffer from various problems known to both designers and users of forklifts and pallet trucks alike. These problems are exacerbated by the continuous use, constant abuse, hostile environments, and limited maintenance that many pallet trucks endure. Furthermore, sensors and switches, especially when multiples ones are used in redundant systems, increase the manufacturing and maintenance costs associated with pallet trucks over their serviceable life. 
         [0008]    The present invention addresses these issues. 
       SUMMARY OF THE INVENTION 
       [0009]    In one aspect, the present invention provides a pallet truck comprising a power unit including a traction motor for driving the truck, a vertically displaceable carriage having a pair of load bearing forks or platform coupled to the power unit, and an actuator for raising the carriage vertically. A controller is configured to store a value corresponding to a present height of the carriage, monitor the actuator for an input signal to drive the carriage, and direct a vertical movement of the carriage in response to the input signal. The controller measures a length of time the carriage is directed to move in a vertical direction, and adjusts the stored value corresponding to the height of the carriage by an amount corresponding to the length of time the carriage is directed to move. The value corresponding to the height of the carriage is compared to a predetermined minimum height value, and the maximum travel speed of the pallet truck is limited if the value corresponding to the carriage height is less than the minimum height value. 
         [0010]    In another aspect of the invention, a method for operating a pallet truck without the use of height-indicating devices is provided. The method comprises the steps of obtaining a fork carriage height expressed as a function of time, tracking an increment of time a control device commands a vertical carriage movement, and adjusting the fork carriage height by the increment of time the control device commands a vertical carriage movement. The fork height is compared to a predetermined minimum carriage travel height, and the travel speed of the pallet truck is limited if the fork carriage height is less than the minimum carriage travel height. 
         [0011]    In another aspect the present invention provides a pallet truck having a controller configured to calculate the height of the carriage without the use of mechanical devices such as switches and sensors. The controller maintains a calculated carriage height, expressed as an amount of carriage lift time, for example, two seconds. Each duration of time that the operator requests a carriage lift and lower command is tracked and the calculated carriage height is adjusted upwards or downwards accordingly. Alternatively, each duration of time that the controller directs the carriage to be lifted or lowered is tracked and the calculated carriage height is adjusted accordingly. 
         [0012]    According to a preferred embodiment, the pallet truck includes a microprocessor having an internal memory and a timer. The microprocessor controls the operation of the pallet truck, including monitoring for inputs and commanding output devices, by executing operating code stored in the memory. The microprocessor monitors the status of the carriage lift and carriage lower command buttons and tracks either the length of time one is depressed or the length of time the carriage is commanded to be lifted or lowered. The microprocessor adjusts the calculated carriage height stored in memory at regular increments by adding each increment of time to the calculated carriage height stored in memory. In some applications, the height value can also be adjusted down by subtracting from the height value during a lowering movement. The calculated carriage height can also be retained in non-volatile memory so that it is available upon pallet truck startup. The microprocessor further limits the maximum allowable speed of the pallet truck if the carriage is too close to the ground. 
         [0013]    In another aspect of the invention, if the carriage is commanded to be lowered for a length of time greater than that required to fully lower the carriage, the microprocessor sets the calculated carriage height to zero. Similarly, if the carriage is commanded to be lifted for a length of time greater than that required to fully raise the carriage, the microprocessor sets the calculated carriage height to the maximum carriage height. In a similar aspect, an initialization procedure may lower (or raise) the fork carriage for a period of time longer than that required to fully lower (or raise) the carriage. The microprocessor subsequently initializes the calculated carriage height to the appropriate amount of lift time. 
         [0014]    The foregoing and other objects and advantages of the invention will appear in the detailed description which follows. In the description, reference is made to the accompanying drawings which illustrate a preferred embodiment of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a perspective view of a pallet truck; 
           [0016]      FIG. 2  is a block diagram of a portion of the control circuit of the pallet truck of  FIG. 1 ; and 
           [0017]      FIG. 3  is a flow chart illustrating a process for inferring the height of the fork carriage of the pallet truck of  FIG. 1  and controlling the travel speed accordingly. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0018]    Referring to  FIG. 1 , a motorized hand/rider low-lift pallet truck  10  is comprised of fork carriage  12  having a pair of load bearing forks  14  that are coupled to a power unit  11 . The power unit  11  includes a housing  22  that houses a hydraulic lift motor pump  16  ( FIG. 2 ) and traction motor  24  ( FIG. 2 ), a drive wheel (not shown), and a battery housing  18  that houses a battery  20 . The drive wheel is coupled to a steering mechanism  26  having a tiller arm  28  and an operator control handle  30 . The steering mechanism  26  is rotatable to the right and left to control the steering of the pallet truck  10 . Although the battery  20  is shown here as provided within a housing  18 , the battery can also be mounted directly to the pallet truck  10 , without a housing  20 . 
         [0019]    The fork carriage  12  has a vertical span of several inches, traveling up and down between ground level and the maximum height. The pallet truck  10  is designed such that the forks  14  are inserted under a load to be moved such as a pallet of goods and the fork carriage  12  lifts the load completely off of the ground. The pallet truck  10  may be driven to another location where the fork carriage  12  is lowered to place the load on the ground and the forks  14  are withdrawn from the load. 
         [0020]    Referring now also to  FIG. 2 , a block diagram of one embodiment of a control system  40  of the pallet truck  10  is shown. The control system  40  is powered by the battery  18  and activated by a key switch  42 . The control system  40  further includes a microprocessor  44  contained within the operator control handle  30  and a motor controller, or power amplifier,  46 . The microprocessor  44  and motor controller  46  communicate through a CAN (controller area network) bus  48 . 
         [0021]    The control handle  30  further includes a number of switches and actuators connected to or monitored by the microprocessor  44 . The operator controls include, but are not limited to, a carriage lift button  52 , a carriage lower button  50 , a horn button  54 , and a display  56 . The display  56  can provide pertinent data such as battery charge status, hour meter data, and other operational information. The control handle  30  also includes a directional (i.e., forward or reverse) and speed control actuator, such as a thumbwheel or twist grip  58 . The twist grip  58  is selectively actuated in a first direction to produce a control signal for movement of the pallet truck  10  in a forward direction and actuated in a second direction to produce a control signal for movement of the pallet truck  10  in a reverse direction. The twist grip  58  is selectively actuated in either direction through a range corresponding to a zero travel speed and the maximum travel speed of the pallet truck  10 . 
         [0022]    The status of each of the switches, buttons, and other actuators comprising the inputs in the control handle  30  are continuously monitored by the microprocessor  44 . The statuses of these inputs are regularly communicated from the microprocessor  44  to the motor controller  46  via the CAN bus  48 . Based on the status of the inputs, the motor controller  46  activates or deactivates a main contactor  60 , a horn  62 , a carriage lift contactor  64 , and a carriage lower solenoid  66 . The motor controller  46  further performs as a variable drive for the pallet truck  10  by regulating the speed output to the traction motor  24  in accordance with the position of the twist grip  58 . 
         [0023]    The key switch  42  is activated to apply logic power to the control handle  30  and the motor controller  46 , placing the pallet truck  10  in an operational mode. After the key switch  42  is activated, control handle  30  is energized, an operator is able to provide directional and functional control requests to the pallet truck  10  through the aforementioned controls. For example, a power on request selected by an operator is detected by the microprocessor  44  and communicated to the motor controller  46  over the CAN bus  48 . After the key switch  42  is activated, a capacitor bank (not shown) used for a high-power motor inverter stage is charged. When the bank is charged, the controller  46  energizes the main contactor  60 , closing normally open contact  70  and allowing full operational power from the battery  20  to be applied to the motor controller  46 . 
         [0024]    Upon detecting a lift request (i.e., carriage lift button  50  is depressed), the motor controller  46  energizes the lift contactor  64 , closing normally open contact  72  and allowing power to be applied to the hydraulic lift motor pump  16 . In one embodiment, vertical movement of the carriage  12  is accomplished by a hydraulic cylinder (not shown) and piston (not shown) connected to the carriage  12 , such as the hydraulic system disclosed in U.S. Pat. No. 5,341,695, incorporated herein by reference. When energized, the lift motor pump  16  pumps hydraulic fluid into the cylinder to lift the piston and carriage  12  to a desired height, at a rate that is dependent on the weight of the load. If the carriage lift button  52  is depressed after the carriage  12  is fully elevated, the lift motor pump  16  continues to operate but a hydraulic bypass line (not shown) is provided to prevent excessive pressure in the cylinder. 
         [0025]    When the lift motor pump  16  is de-energized, the carriage  12  is held in place by the static pressure in the cylinder. When the carriage lower button  50  is depressed, the carriage lower solenoid  66  is energized, opening a hydraulic fluid return path and allowing the fluid to be pushed out of the cylinder. The carriage  12  may lower at a constant rate or may lower at a variable rate depending on the weight of the load, for example, heavier loads being lowered faster by force of gravity. If the carriage lower button  50  is depressed after the carriage  12  is at the lowermost position, the solenoid  66  will be energized but the carriage  12  will not descend further as all of the hydraulic fluid will have been expelled from the cylinder. 
         [0026]    Referring now also to  FIG. 3 , in operation and upon activation of the key switch  42  (step  152 ), the microprocessor  44  starts executing a program of operating code stored in non-volatile memory. A previously calculated fork carriage height is retrieved from memory (step  154 ). Alternatively, an initialization procedure may be performed whereby the fork carriage  12  is lowered (or raised) for a period of time longer than that required to fully lower (or raise) the carriage  12  and subsequently the calculated carriage height is set to the appropriate initialized value. 
         [0027]    The microprocessor  44  then compares the calculated carriage height to a predetermined minimum travel height (step  156 ). If the carriage  12  is above or equal to the minimum travel height, full speed operation is enabled (block  158 ). If, however, the calculated carriage height is less than the minimum travel height, a speed limit is imposed (step  160 ) on the pallet truck  10 . According to one embodiment, the microprocessor  44  controls the speed of the pallet truck  10  by communicating a travel speed command for the traction motor  24  to the motor controller  46 . For example, when full speed operation is enabled, the travel speed command communicated to the motor controller  46  ranges from zero to 4500 RPM. When a speed limit imposed, the truck travel speed command ranges from zero to 2000 RPM. 
         [0028]    During operation, the microprocessor  44  continuously monitors for input signals from the carriage lift button  52  and carriage lower button  50  (step  162 ). If an input signal is detected (step  164 ), for example, a discrete signal received from one of the buttons  50 ,  52 , the microprocessor communicates a command to the motor controller  46  to energize the carriage lift contactor  64  or the lower solenoid  66 , respectively. 
         [0029]    When an input signal is detected, the microprocessor  44  further begins to track the time that the input signal is detected (step  166 ). In one embodiment, the time is tracked by using a timer internal to the microprocessor  44 . The calculated carriage height is then incrementally adjusted upwardly (when lifting) or downwardly (when lowering) (step  168 ) each time the microprocessor  44  executes a cycle of the operating program, for example, every twenty milliseconds. After the calculated carriage height has been upwardly or downwardly adjusted, the microprocessor  44  compares the calculated carriage height to the predetermined minimum travel height (step  170 ). As described above, if the carriage  12  is above or equal to the minimum travel height, full speed operation is enabled (step  172 ). If the carriage  12  is below the minimum height, a speed limit is either imposed or maintained (step  174 ). After the input signal is no longer detected (step  176 ), the timer is stopped and cleared. While the pallet truck  10  is operational, the microprocessor  44  continues to monitor for input signals (step  162 ). 
         [0030]    Because the carriage height is calculated substantially in real time, the microprocessor  44  can respond to changes in carriage height even if the pallet truck  10  is moving quickly. For example, if the carriage  12  is lowered below the minimum travel height while the pallet truck  10  is traveling at full speed, a speed limit will be imposed and the pallet truck  10  will be slowed quickly, ideally before any damage-causing contact occurs between the carriage  12  and the ground. 
         [0031]    In a contemplated embodiment of the present invention, if the microprocessor  44  calculates a carriage height that is less than zero, such as when an operator continues to depress the carriage lower button  50  after the carriage  12  is completely lowered, the microprocessor  44  sets the calculated carriage height to zero. However, on a floor where the carriage  12  cannot be completely lowered, such as on an uneven or rough floor, such an action may result in a “zeroing out” effect at a slightly elevated height. Conversely, if the calculated carriage height is greater than the maximum possible carriage height, the microprocessor  44  sets the calculated carriage height to the maximum vertical displacement of the carriage  12 . 
         [0032]    In a further contemplated embodiment, the calculated carriage height is corrected by monitoring the current flow to the hydraulic lift motor pump  16 . An appreciably heavier load may cause the lift motor pump  16  to lift the carriage  12  at a slower rate. This can be detected by monitoring for an increase in current to the lift motor pump  16 . In such a case, the calculated carriage height is decreased by an amount corresponding to the reduced rate of lifting. Other parameters may also be used for adjusting the calculated carriage height including lift motor pump operational status, lift motor pump RPM, hydraulic fluid pressure, flow and temperature. 
         [0033]    Preferred embodiments and examples of the invention have been described in considerable detail. Many modifications and variations to the preferred embodiment described will be apparent to a person of ordinary skill in the art. It should be understood, therefore, that the methods and apparatuses described above are only illustrative and do not limit the scope of the invention, and that various modifications could be made by those skilled in the art that would fall within the scope of the invention. To apprise the public of the scope of this invention, the following claims are made: