Abstract:
A motorized pallet truck includes an angular indicator on the steering arm mechanism providing an internal controller with an angular position of the steering arm or tiller. The controller applies a brake when the tiller is in either of a substantially vertical or substantially horizontal position, limits the speed of the truck for a predetermined rotational movement from the vertical position, and allows full speed of the vehicle when the tiller arm is pulled into a predefined fast driving arc.

Description:
FIELD OF THE INVENTION 
   The present invention relates to material handling vehicles and, more particularly, to a material handling vehicle which is steered with a steering tongue or tiller arm including an angle detector for limiting the speed of the vehicle based on the angle of the tiller arm. 
   BACKGROUND OF THE INVENTION 
   Industrial material handling vehicles such as fork lift trucks or motorized hand pallet trucks are commonly found in warehouses, factories, shipping yards, and, generally, wherever pallets, packages, or loads of goods are required to be moved from place to place. Pallet trucks typically include a load bearing fork or lift arm for lifting packages or pallets to a height sufficient for transporting, an electric drive motor for driving the vehicles, a steering control mechanism, and a brake. These vehicles can include an operator station, on which the operator stands as the pallet truck moves, or can be designed for the operator to walk behind the vehicle at the end opposite the forks. 
   The steering mechanism for a common type of pallet truck includes a movable arm or tiller and a control handle mounted at the end of the tiller. The tiller is rotatable right and left to steer the vehicle, while a rotatable thumb wheel or twist grips on the handle control the speed and direction of the truck, selecting between a forward and a reverse direction. To prevent movement of the truck when the operator has left the vehicle, the steering tiller arm is typically spring loaded. When the tiller is released, it is forced by the spring to a near vertical position outside of a defined operating arc. In the vertical position, a spring-applied “deadman” brake mechanism is automatically activated to prevent further motion of the vehicle. 
   To activate the deadman brake quickly and to limit use of the vehicle when the tiller is in a near vertical position where the mechanical advantage for steering is typically poor and the potential speed of the vehicle is nonetheless relatively high, prior art material handling vehicles were constructed to require the tiller arm to be moved a relatively large angle from the vertical position prior to releasing the brake. The steering arm or tiller therefore had to be pulled a significant distance toward the horizontal before operation of the vehicle was allowed. Limiting operation in this way, however, poses problems for pallet trucks used in narrow lanes and, in an increasingly common mode of operation, inside of trailers and other large containers for moving goods. In these applications, the horizontal space available for swinging the tiller to the right and left within the operating arc is limited, and it can therefore be difficult to steer the vehicle when the tiller is pulled too far from the vertical position. In such applications, it is therefore desirable to allow the vehicle to be controlled with the tiller in a nearly vertical position. 
   One solution to this problem has been to provide switches in conjunction with the steering tiller which are activated to indicate a transition to a first angle at which a slow mode is entered and a second angle at which a fast mode is entered. In these prior art devices, the pallet truck transitions to a slow mode in which the maximum speed of the vehicle is restricted whenever the first switch is activated, and to a fast mode whenever the second switch is activated, irrespective of the position of the other switch. While typically providing the appropriate functions, these prior art devices suffer from a number of disadvantages. Specifically, when using this type of switching system, it is possible to identify only three driving states, even though two braking states, a fast, and a slow mode are required. Furthermore, as state changes occur irrespective of the position of the other switch, it is difficult to determine when a failure has occurred, or to adequately monitor changes in driving states. 
   SUMMARY OF THE INVENTION 
   In one aspect, the present invention provides a material handling vehicle comprising a drive system, a brake coupled to the drive system to prevent motion of the material handling vehicle, and a steering mechanism for selecting a direction of motion. The steering mechanism is moveable along an arc between a substantially horizontal position and a substantially vertical position. An angular position indicator is activated by the steering mechanism as the steering mechanism is rotated, and provides a control signal indicating the angle of rotation of the steering mechanism. A controller receives the control signal and selectively places the pallet truck in one of a plurality of successive driving states based on the angle of rotation. The driving states typically include a top (vertical) brake mode, a slow speed mode, a fast mode, and a bottom (horizontal) brake mode. 
   In another aspect, the present invention provides pallet truck in which the steering mechanism includes a cammed surface which selectively activates a switching device to produce a control signal indicating at least a first, a second, and a third angle of rotation of the steering mechanism. A controller receives the control signal and changes a driving state of the pallet truck progressively from a top braking mode, to a slow speed mode, to a fast speed mode, to a bottom braking mode as the steering mechanism is rotated from the vertical to the horizontal position. During the transitions, the controller monitors the changes to determine whether a valid transition has occurred and activates the brake if the transition is invalid. 
   In yet another aspect, the present invention provides a pallet truck including first and second switching devices which are each selectively activated by the steering mechanism as the steering mechanism is rotated in a vertical plane to produce a two bit state code, the two bit state code providing four possible sequential driving states. A controller is electrically connected to switching devices, receives the two bit state code, compares the two bit code to the present driving state code, determines if the transition to the state represented by the two bit code is a sequential transition, and enters an error mode if the transition is not sequential. If the transition is sequential, the controller transitions to the driving state represented by the two bit code. 
   These and other aspects of the invention will become apparent from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention and reference is made therefore, to the claims herein for interpreting the scope of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of a pallet truck; 
       FIG. 2  is a perspective showing the details of a steering handle for a hand/rider truck; 
       FIG. 3  is an exploded view of the tiller arm and associated switches; 
       FIG. 4  is a side view of the tiller arm of  FIG. 1 ; 
       FIG. 5  is a back view of the tiller arm of  FIG. 4 ; 
       FIG. 6  is a cutaway view of the tiller arm of  FIG. 5  taken along line  6 — 6 ; 
       FIG. 7  is a cutaway view of the tiller arm of  FIG. 5  taken along the line  7 — 7 ; 
       FIG. 8  is an exploded view of the tiller arm and associated cam surfaces; 
       FIG. 9  is a block diagram of the control circuit of the pallet truck of  FIG. 1 ; 
       FIG. 10  is a flow chart illustrating transitions between driving states as provided by the control circuit of  FIG. 9 ; 
       FIG. 11  is a flow chart illustrating an error check for returning the pallet truck to a braked state when stopped; and 
       FIG. 12  is a side view of the pallet truck of  FIG. 1  illustrating a driving arc of the tiller arm and associated angles for changing driving states. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring now to the figures and more particularly to  FIG. 1 , a pallet truck  10  constructed in accordance with the present invention is shown. The pallet truck  10  comprises forwardly extending forks  12 , a drive motor ( FIG. 9 ) provided in a motor compartment  14 , a battery ( FIG. 9 ) provided in battery compartment  16 , and a steered wheel  18 . The steered wheel  18  is coupled to a steering mechanism  23  which includes both a tiller arm  22  and an operator control handle  24 . The steering mechanism  23  is rotatable to the right and left to change the direction of the pallet truck  10  and is further movable in an arc between a substantially vertical position and a substantially horizontal position. When in either of the substantially horizontal position or the substantially vertical positions, a deadman brake  84  ( FIG. 9 ) is activated, as described below. To assure that the truck  10  is stopped when the operator leaves the vehicle, the steering mechanism  23  is spring loaded such that it is forced into a vertical position when released. 
   Referring now to  FIGS. 1 and 2 , the tiller arm  22  is pivotally mounted to a swiveling mount  27 , which is coupled to the transmission, including steered wheel  18 , of the pallet truck  10 . The tiller arm  22  can swivel the swiveling mount  27  to the right and left, thereby allowing the operator to change the direction of the lift truck  10  by moving the steered wheel  18 . The tiller arm  22  is further pivotable around a pivoting axis  26 , and is moveable through a “driving arc” extending from a substantially horizontal to a substantially vertical position around this axis, as shown in  FIG. 12 . Referring now also to  FIGS. 3 and 4 , a bottom distal end  28  of the tiller arm  22  is mounted adjacent a frame member  34  mounting first and second switches  36  and  37 . The surface of the tiller arm  22  at the distal end  28  is cammed to selectively activate the switches  36  and  37  as the tiller arm  22  is rotated about the pivot point  26  to provide an indication of the angle of rotation and to switch between driving states, as described below. Each of the cammed surfaces  30  and  32  are positioned adjacent a lever arm  60  with a roller activator for activating the respective switch. Although a number of lever and roller activated switches are available, one switch useful in the present invention is the V4N snap-action series of microswitches commercially available from Saia-Burgess USA of Vandalia, Ohio. 
   Referring now to  FIG. 4 , the tiller arm  22  includes an aperture  31  for receiving a shaft for rotating the arm  22  around the axis  26 . At the bottom distal end  28  of the tiller arm  22  adjacent the aperture  31  the tiller arm  22  is generally cylindrical in shape, but, as described above, includes a cammed surface for activating the switch. Referring now also to  FIG. 5  the bottom surface of the distal end  28  of the tiller arm  22  includes a first cammed surface  30  provided along a first side of the tiller arm  22  and a second cammed surface  32  provided along the opposing side of the tiller arm  22 . The cammed surfaces  30  and  32  are sized and dimensioned to selectively activate and deactivate the adjacent switches  36  and  37  ( FIGS. 2 and 3 ) at selected angles, as the tiller arm  22  is moved between a horizontal and a vertical mechanical stop point (not shown) providing a substantially horizontal and a substantially vertical tiller arm  22  position. The ON/OFF state of the switches are monitored by the control system ( FIG. 9 ) of the pallet truck  10  to switch between four modes of operation or driving states as indicated by a two bit code developed by the switches ( 37 / 36 ), as follows: a top brake mode ( 0 / 0 ), a bottom brake mode ( 1 / 0 ), a slow mode ( 0 / 1 ), and a fast mode ( 1 / 1 ), as described below. The cammed surfaces  30  and  32  are configured to turn the switches  36  and  37  off in the default top brake mode ( 0 / 0 ), the most likely failure mode for the switches. The fast mode is assigned to the ( 1 / 1 ) state, the least likely failure state. Failures in the switching devices  36  and  37  are identified by evaluating the sequencing through the drive states, as described below. 
   Referring now to  FIG. 6  a sectional view of the tiller arm  22  taken along the line  6 — 6  of  FIG. 5  is shown, illustrating the cammed surface  30 . The cammed surface  30  extends from a vertical mechanical stop (not shown) at end  50  (a substantially vertical tiller position) on a front side of the tiller arm  22  to a horizontal mechanical stop (not shown) at end  48  (a substantially horizontal tiller position) on the back side of the tiller arm  22 . From the vertical mechanical stop, the camming surface  30  extends inward toward the vertical center line  40  at a radius selected to prevent actuation of the associated switch, and ramps upward at transition point  46  to a radius  47  selected to activate the switch  37 . The camming surface  30  follows the radius  47  to a second transition  44  at which the radius is reduced to deactivate the switch. The radius  47  therefore provides a portion of the camming surface  30  during which the switch  37  is active. 
   Referring now to  FIG. 7  a sectional view of the tiller arm  22  taken along the line  7 — 7  of  FIG. 5  is shown, illustrating the cammed surface  32 . Like the cammed surface  30 , the cammed surface  32  extends between the horizontal mechanical stop at end  48  and the vertical mechanical stop at end  50 . From end  48  the cammed surface extends toward the vertical center line  40  at a radius selected to prevent activation of the switch, and then ramps upward to a transition point  54 . From the transition point  54  to an opposing transition point  51 , the cammed surface  32  follows a radius  53  selected to activate the associated switch  36 . 
   Referring now to  FIG. 8  a side view of the tiller arm  22  illustrating the aligned cammed surfaces  30  and  32  is shown. Extending from the first end  50 , each of the cammed surfaces  30  and  32  maintains the associated switch  37  and  36 , respectively, in an off position, providing the top brake mode  116 . At transition point  51 , the cammed surface  32  activates the switch  36 , and the slow speed mode  120  is entered. The slow speed mode  120  continues until the transition point  46 , at which the cammed surface  30  activates the switch  37  to enter the fast mode  122 . The fast mode  122  continues until transition point  54 , at which the cammed surface  32  deactivates the switch  36  to enter the bottom brake mode  124 . The cammed surfaces  30  and  32  therefore act together to provide an indication of changes in angular position of the tiller, which are then associated with driving states of the vehicle. 
   Referring now to  FIG. 9  a block diagram of a control system of the pallet truck  10  of the present invention is shown. Power is applied to the pallet truck  10  by activation of a main on/off switch  76  and a key switch  74 , which activates the control handle  24 . The control system comprises a controller  106  which receives input control signals from each of the switches  36  and  37 , as well as from the control handle  24  via a CAN (controller area network) bus  104 . The control handle  24  includes a controller  107  connected to the CAN bus, as well as switches and activators providing lift  70  and lower  68  controls for the fork  12  ( FIG. 1 ), an emergency reverse button  64 , a horn switch  62  for activating the horn  88 , and a display  72  which can provide information such as battery state of charge, hour meter, or other operational information, as well as error information, as described below. The control handle  24  further includes a directional and speed control, preferably in the form of a thumbwheel or twist grip  66 , which is selectively activated by an operator in a first direction to provide a control signal for motion in the forks first direction and in a second direction to provide a control signal for motion in the forks trailing direction. Based on the received inputs, the controller  106  activates a horn  88 , a lift motor solenoid  82 , a fork lowering valve solenoid  86 , a deadman brake coil  84 , and a main control contactor  80 . The controller  106  further controls an electrical drive mode  90  by applying a selected voltage across field coils  92 , as described below. 
   The key switch  74  is activated to apply power to the control handle  24 , putting the pallet truck  10  into an operational mode. Once the key switch  74  is activated, the operator can provide directional and functional control information to the pallet truck  10  through the controls on the control handle  24 , as described above. Upon a power request, the controller  106  pulls in the main contactor solenoid  80 , closing normally open contact  100 , and therefore allowing power to be applied to the circuit from the battery  96 . 
   Referring now to  FIG. 10 , in operation, controller  106  monitors input signals from each of the control switches  36  and  37  and, based on the states of these switches, allows drive. Initially, upon activation of the key switch  74  (step  110 ), the controller  106  monitors both the switches  36  and  37  to determine whether both switches  36  and  37  are off (step  112 ), and the truck  10  is therefore in the top brake mode  116 . If not, the controller keeps the brake applied  84  and provides an error message on the display  72 . The controller  106  then goes into a delay mode preventing further action by the truck  10  until the tiller arm  22  is returned to the vertical position ( 114 ) and re-enters the top brake mode ( 116 ). 
   Referring now to  FIGS. 9 and 10 , when both switches  36  and  37  are off, the pallet truck  10  enters the top brake state  116 . From the top brake mode  116 , the controller  106  monitors the states of the switches  36  and  37 . As described above, during operation the cammed surfaces  30  and  32  assure that, barring a failure, state transitions occur in a given order, specifically from top brake  116  ( 0 / 0 ), to slow mode ( 0 / 1 )  120 , to fast mode ( 1 / 1 )  122 , to bottom brake ( 1 / 0 )  124 , as the tiller arm  22  is moved from the substantially vertical to the substantially horizontal position in the driving arc. From the default top brake mode  116 , for example, the pallet truck  10  must next enter the slow state  120  as indicated by the transition of the switch  36  from an off condition to an on condition. If, however, the switches  36  and  37  indicate a transition from the top brake mode  116  to the fast mode  122  or the bottom brake mode  124 , the controller  106  determines that an invalid transition has been made, and the controller  106  enters a sequence error state  118 , applies the brake  84 , and provides a sequence fault error message on the display  72 . 
   As described above, when each of the switches  36  and  37  is off, the truck  10  is in the top brake state  116  and the controller  106  deactivates the brake  84  by removing the voltage directly across the brake coil  84 , preventing motion of the pallet truck  10 . When the switch  36  is on and the switch  37  is off, the controller  106  determines whether the previous state was the top brake state  116 , as expected. If the transition state is correct, the tiller arm  22  has moved out of the vertical position but is still at a relatively high angle, and the controller  106  allows a transition to the slow mode  120 . In the slow mode, the controller limits the maximum speed of the pallet truck  10  to approximately one mile per hour. Therefore, speed signals received from the hand grips  66  on the control handle  24  and transmitted to the controller  106  through the CAN bus  104  are scaled by the controller  106  to limit the speed of the pallet truck  10  to the pre-selected maximum speed, and an appropriate voltage is applied across the field coils  92  to drive the drive motor  90  at the selected speed. 
   From the slow mode  120 , if the switch  37  is activated, the tiller arm  22  has been rotated to a position closer to horizontal in which steering is easier, and the controller  106  allows operation of the pallet truck  10  in a fast mode  122 . Here, the controller  106  allows the pallet truck  10  to travel up to a predetermined maximum speed, typically 3.5 miles per hour. Again, the controller  106  scales the input signals received from the CAN bus  104  and applies a voltage across the field coil  92  to drive the motor  90  at the selected speed. From the slow mode  120 , the tiller arm  22  can also be rotated back toward a vertical position, deactivating the switch  36  and returning the pallet truck  10  to the top brake mode  116 . Other transitions result in a sequence error  118 . 
   From the fast mode  122 , deactivation of the switch  36  indicates that the tiller arm  22  has been moved to a near horizontal position, resulting in a transition to the bottom brake mode  124 , wherein the controller  106  applies the brake  84 . Deactivation of the switch  37 , on the other hand, indicates that the tiller arm  22  has been moved toward the vertical, resulting in a transition to the slow mode  120 . From the bottom brake state  124 , the tiller arm  22  can only be moved toward the vertical, resulting in an actuation of the switch  36  and entry into the fast mode  122 . Any other transition results in a sequence error  118 . 
   As described above, if the controller  106  determines at any time that a transition from one driving state to another driving state is incorrect, a sequence error  118  has occurred and the controller  106  applies the brake  84  and provides an error message on the display  72  through the CAN bus  104 , as described above. Typically, the error is caused by a failure of one of the switches  36  and  37 , and can be rectified with maintenance. 
   Referring now to  FIG. 11 , during operation, the controller  107  also continually monitors the input control signals from the CAN bus  104  (step  130 ) to determine if the pallet truck  10  is still active or has stopped. The controller monitors the driving status (step  132 ) and, if no driving occurs, the controller  107  increments a counter calculating the amount of time the pallet truck has been inactive or off (step  134 ), and compares the off time to a predetermined stop time (step  136 ) selected empirically as representative of an unmanned vehicle. If the off time is greater than or equal to the predetermined stop time, the controller  107  determines that the pallet truck  10  is unmanned, and checks switches  36  and  37  to verify that the tiller arm  22  has been returned to the vertical position. If the truck  10  is unmanned and the tiller arm  22  has not returned to the top brake mode  116 , the controller  107  enters a static return to off error mode (step  130 ), applies the brake  84 , and provides an error message on the display  72 . As the tiller arm is spring-loaded it should return to the top brake mode  116  automatically, and failure to do so could be caused by a mechanical problem by the associated spring or other component. If driving occurs, the controller  107  re-sets the off time to zero (step  140 ). 
   Referring now to  FIG. 12 , a side view of the pallet truck  10  illustrating the preferred angles for switching between driving states is shown. As described above, the tiller arm  22  is movable up and down through a driving arc that ranges from a nearly horizontal position to a substantially vertical position, and is spring loaded to drive the steering arm  22  to a default position in the substantially vertical position. In a preferred embodiment of the invention, the tiller arm  22  is moveable between mechanical stops provided at angles of −5 degrees and 80 degrees, as measured versus a line drawn through the center of the tiller arm  22  to the control handle  24 . The cammed surfaces  30  and  32  are positioned to provide angle indications for switching the operation of the vehicle between the bottom (horizontal) brake position  124 , a fast speed mode  122 , the slow speed mode  120 , and the top (vertical) brake mode  116 , as described above. 
   Starting from the vertical mechanical stop at −5 degrees, each of the switches  36  and  37  are maintained in an off position until the tiller arm  22  is rotated to an angle of approximately zero degrees. As the tiller arm  22  is rotated toward the horizontal position to an angle greater than zero degrees, the cammed surface  32  activates the switch  36 , and the pallet truck  10  enters the slow mode  120  of operation in which the maximum speed of the pallet truck is limited, preferably to a speed of approximately one mile per hour. As the tiller arm  22  continues to rotate to an angle of about forty degrees, the switch  37  is activated by cammed surface  30 , and the controller  106  transitions the pallet truck  10  from the slow mode  120  of operation to the fast mode  122  of operation. Here, the limitation of the speed of the vehicle is dropped, and the controller  106  allows the pallet truck to drive up to the maximum speed, which is approximately 3.5 miles per hour. As the tiller arm  22  continues to rotate downward to an angle of about 70 degrees, the switch  36  is deactivated and the bottom brake  124  state is entered in which the controller  106  again applies the brake  84 . Finally, at 80 degrees a mechanical down stop is reached. 
   Although preferred switching angles and speed levels have been described, the cammed surfaces  30  and  32  can be configured to provide switching at any number of angles. Furthermore, although specific speed levels have been described, it will be apparent that variations can be made to the selected speed levels without exceeding the scope of the invention. Additionally, although a specific type of switch has been described, it will be apparent that various switches and other types of actuators could also be used. Furthermore, other methods of determining an angle of rotation of a steering mechanism and providing a control signal will be known to those of skill in the art. 
   Furthermore, although the invention has been described for use with a four state control system, it will be apparent that the principled of the invention could be applied to provide various driving functions at various additional angles. For example, a pallet truck could be constructed to include a plurality of different speed ranges as the steering mechanism is rotated, as well as one or more stop position. 
   Additionally, although the invention has been described with reference to a pallet truck, the principles described could also be applied to various other types of material handling vehicles. 
   The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.