Patent Publication Number: US-9421963-B2

Title: Materials handling vehicle having a control apparatus for determining an acceleration value

Description:
This application is a divisional application of prior application U.S. Ser. No. 13/788,683, filed Mar. 7, 2013, and entitled “A MATERIALS HANDLING VEHICLE HAVING A CONTROL APPARATUS FOR DETERMINING AN ACCELERATION VALUE,” which is a continuation of prior application U.S. Ser. No. 12/360,353, filed Jan. 27, 2009, and entitled “A MATERIALS HANDLING VEHICLE HAVING A CONTROL APPARATUS FOR DETERMINING AN ACCELERATION VALUE,” which claims the benefit of: U.S. Provisional Application No. 61/026,151, filed Feb. 5, 2008 and entitled “A MATERIALS HANDLING VEHICLE HAVING A STEER SYSTEM INCLUDING A TACTILE FEEDBACK DEVICE”; U.S. Provisional Application No. 61/026,153, filed Feb. 5, 2008 and entitled “A MATERIALS HANDLING VEHICLE HAVING A CONTROL APPARATUS FOR DETERMINING AN ACCELERATION VALUE”; U.S. Provisional Application No. 61/049,158, filed Apr. 30, 2008 and entitled “A MATERIALS HANDLING VEHICLE HAVING A STEER SYSTEM INCLUDING A TACTILE FEEDBACK DEVICE”; U.S. Provisional Application No. 61/055,667, filed May 23, 2008 and entitled “A MATERIALS HANDLING VEHICLE WITH A MODULE CAPABLE OF CHANGING A STEERABLE WHEEL TO CONTROL HANDLE POSITION RATIO,” the disclosures of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a materials handling vehicle having a control apparatus for controlling the operation of a traction motor and more specifically to such a vehicle having a control apparatus capable of determining a traction motor acceleration value. 
     BACKGROUND OF THE INVENTION 
     U.S. Pat. No. 6,564,897 discloses a steer-by-wire system for a materials handling vehicle. The vehicle comprises a steering tiller. The tiller, however, is not mechanically coupled to a steered wheel. A motor or an electromagnetic brake is used to provide a counter steering resistive force. 
     SUMMARY OF THE INVENTION 
     In accordance with a first aspect of the present invention, a materials handling vehicle is provided comprising: a frame; wheels supported on the frame; a traction motor coupled to one of the wheels to effect rotation of the one wheel; a speed control element operable by an operator to define a speed control signal corresponding to a desired speed of the traction motor; a system associated with a steerable wheel to effect angular movement of the steerable wheel; and control apparatus coupled to the speed control element to receive the speed control signal, and coupled to the traction motor to generate a drive signal to the traction motor in response to the speed control signal to control the operation of the traction motor. The control apparatus may use points from one or more curves each defining an acceleration value that varies based on one of an angular position of the steerable wheel, the speed of the traction motor and the speed control signal to determine an acceleration value for the traction motor. 
     The system may comprise a sensor generating signals indicative of an angular position of the steerable wheel. 
     The materials handling vehicle may further comprise a sensor associated with the traction motor for generating signals indicative of a speed of the traction motor. 
     In accordance with a second aspect of the present invention, a materials handling vehicle is provided comprising: a frame; wheels supported on the frame; a traction motor coupled to one of the wheels to effect rotation of the one wheel; a speed control element operable by an operator to define a speed control signal corresponding to a desired speed of the traction motor; a system associated with a steerable wheel to effect angular movement of the steerable wheel; and control apparatus coupled to the speed control element to receive the speed control signal, and coupled to the traction motor to generate a drive signal to the traction motor in response to the speed control signal to control the operation of the traction motor. The control apparatus may determine acceleration values for the traction motor based on an angular position of the steerable wheel, a speed of the traction motor and a current position of the speed control element as defined by the speed control signal. 
     The control apparatus may use points from curves to define the acceleration values based on the angular position of the steerable wheel, the speed of the traction motor and the current position of the speed control element. 
     In accordance with a third aspect of the present invention, a materials handling vehicle is provided comprising: a frame comprising an operator&#39;s compartment; wheels supported on the frame; a traction motor coupled to one of the wheels to effect rotation of the one wheel; a system associated with a steerable wheel to effect angular movement of the steerable wheel about a first axis, the system comprising a control handle capable of being moved by an operator to define a current desired angular position of the steerable wheel; and control apparatus varying a drive signal to the traction motor based on a steerable wheel error. 
     The steerable wheel error may be determined by comparing the current desired angular position of the steerable wheel to a current calculated actual position of the steerable wheel. 
     In accordance with a fourth aspect of the present invention, a materials handling vehicle is provided comprising: a frame comprising an operator&#39;s compartment; wheels supported on the frame; a traction motor coupled to one of the wheels to effect rotation of the one wheel; a system associated with the steerable wheel to effect angular movement of the steerable wheel about a first axis. The system may comprise a control handle capable of being moved by an operator to define a desired angular position of the steerable wheel. Further provided is a control apparatus to vary a drive signal to the traction motor based on one of the desired angular position of the steerable wheel, a calculated actual position of the steerable wheel, a steerable wheel error, and a steer rate of the control handle. The control apparatus may determine a first traction motor speed limit based on the desired angular position of the steerable wheel, a second traction motor speed limit based on the calculated actual position of the steerable wheel, a third traction motor speed limit based on the steerable wheel error and a fourth traction motor speed limit based on the steer rate of the control handle. The control apparatus may select the smallest of the first, second, third and fourth traction motor speed limits and may use the smallest limit when generating the drive signal to the traction motor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a materials handling vehicle in which the present invention is incorporated; 
         FIG. 1A  is an exploded view of a portion of an operator&#39;s compartment including a floorboard from the vehicle illustrated in  FIG. 1 ; 
         FIG. 2  is a schematic block diagram of a control apparatus from the vehicle illustrated in  FIG. 1 ; 
         FIGS. 3-5  are perspective views of a power unit of the vehicle in  FIG. 1  with covers removed from the power unit; 
         FIG. 6  is a view of a tactile feedback device of the vehicle illustrated in  FIG. 1 ; 
         FIG. 6A  is a view, partially in cross section, of a pin extending down from a control handle base, a spring and a block fixed to a steering column plate; 
         FIGS. 7 and 8  are perspective views of the control handle of the vehicle illustrated in  FIG. 1 ; 
         FIG. 9  is a view, partially in section, of the control handle and the tactile feedback device; 
         FIG. 10  illustrates a first curve C 1  used to define a steering motor speed limit based on a current traction motor speed when the vehicle is being operated in a power unit first direction and a second curve C 2  used to define a steering motor speed limit based on a current traction motor speed when the vehicle being operated in a forks first direction; 
         FIG. 11  illustrates a curve C 3  plotting a first traction motor speed limit or a second traction motor speed limit as a function of a desired steerable wheel angular position or a calculated actual steerable wheel angular position; 
         FIG. 11A  illustrates a curve C A  used to define a third traction motor speed limit based on steerable wheel error; 
         FIG. 11B  illustrates a curve C B  used to define a fourth traction motor speed limit based on steer rate; 
         FIG. 11C  illustrates a curve C c  used to determine a first acceleration reduction factor RF1 based on a calculated current actual angular position of the steerable wheel; 
         FIG. 11D  illustrates a curve C D  used to determine a second acceleration reduction factor RF2 based on a traction speed; 
         FIG. 12  illustrates a curve C 4  used to determine a first tactile feedback device signal value based on traction motor speed; 
         FIG. 13  illustrates a curve C 5  used to determine a second tactile feedback device signal value based on steerable wheel error; and 
         FIG. 14  illustrates in block diagram form steps for determining a tactile feedback device signal setpoint TFDS. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A materials handling vehicle constructed in accordance with the present invention, comprising a pallet truck  10  in the illustrated embodiment, is shown in  FIG. 1 . The truck  10  comprises a frame  20  including an operator&#39;s compartment  30 , a battery compartment  40  for housing a battery  42 , a base  52  forming part of a power unit  50  and a pair of load carrying forks  60 A and  60 B. Each fork  60 A,  60 B comprises a corresponding load wheel assembly  62 A,  62 B. When the load wheel assemblies  62 A,  62 B are pivoted relative to the forks  60 A,  60 B, the forks  60 A,  60 B are moved to a raised position. The operator&#39;s compartment  30  and the battery compartment  40  move with the forks  60 A,  60 B relative to the power unit  50 . 
     The operator&#39;s compartment  30  is defined by an operator&#39;s backrest  32 , a side wall  44  of the battery compartment  40  and a floorboard  34 . An operator stands on the floorboard  34  when positioned within the operator&#39;s compartment  30 . In the illustrated embodiment, the floorboard  34  is coupled to a frame base  20 A along a first edge portion  34 A via bolts  134 A, washers  134 B, nuts  134 C, spacers  134 D and flexible grommets  134 E, see  FIG. 1A . A second edge portion  34 B of the floorboard  34 , located opposite to the first edge portion  34 A, rests upon a pair of springs  135 . The floorboard  34  is capable of pivoting about an axis A FB , which axis A FB  extends through the first edge portion  34 A and the flexible grommets  134 E. A proximity sensor  36 , see  FIGS. 1A and 2 , is positioned adjacent to the floorboard  34  for sensing the position of the floorboard  34 . When an operator is standing on the floorboard  34 , it pivots about the axis A FB  and moves towards the proximity sensor  36  such that the floorboard  34  is sensed by the sensor  36 . When the operator steps off of the floorboard  34 , the floorboard  34  is biased in a direction away from the sensor  36  by the springs  135  such that it is no longer sensed by the sensor  36 . Hence, the proximity sensor  36  generates an operator status signal indicating that either an operator is standing on the floorboard  34  in the operator&#39;s compartment  30  or no operator is standing on the floorboard  34  in the operator&#39;s compartment  30 . A change in the operator status signal indicates that an operator has either entered or exited the operator&#39;s compartment  30 . 
     The power unit  50  comprises the base  52 , a side wall  54  and a steering column  56 , see  FIGS. 3-8 . The base  52 , side wall  54  and steering column  56  are fixed together such that the steering column  56  does not rotate or move relative to the side wall  54  or the base  52  in the illustrated embodiment. First and second caster wheels, only the first caster wheel  58  is illustrated in  FIG. 1 , are coupled to the base  52  on opposing sides  52 A and  52 B of the base  52 . 
     The power unit  50  further comprises a drive unit  70  mounted to the base  52  so as to be rotatable relative to the base  52  about a first axis A 1 , see  FIGS. 4 and 5 . The drive unit  70  comprises a support structure  71  mounted to the base  52  so as to be rotatable relative to the base  52 , a traction motor  72  mounted to the support structure  71 , and a driven steerable wheel  74  mounted to the support structure  71 , see  FIGS. 3-5 . The steerable wheel  74  is coupled to the traction motor  72  so as to be driven by the traction motor  72  about a second axis A 2 , see  FIG. 1 . The steerable wheel  74  also moves together with the traction motor  72  and the support structure  71  about the first axis A 1 . 
     An encoder  172 , see  FIG. 2 , is coupled to an output shaft (not shown) of the traction motor  72  to generate signals indicative of the speed and direction of rotation of the traction motor  72 . 
     The truck  10  comprises a steer-by-wire system  80  for effecting angular movement of the steerable wheel  74  about the first axis A 1 . The steer-by-wire system  80  comprises the control handle  90 , a tactile feedback device  100 , biasing structure  110 , a steer motor  120  and the steerable wheel  74 , see  FIGS. 3, 4, 6 and 9 . The steer-by-wire system  80  does not comprise a mechanical linkage structure directly connecting the control handle  90  to the steerable wheel  74  to effect steering of the wheel  74 . The term “control handle” is intended to encompass the control handle  90  illustrated in  FIG. 1  and like control handles including steering tillers and steering wheels. 
     The control handle  90  is capable of being rotated by an operator approximately +/−60 degrees from a centered position, wherein the centered position corresponds to the steerable wheel  74  being located in a straight-ahead position. The control handle  90  is coupled to the tactile feedback device  100 , which, in turn, is coupled to a plate  56 A of the steering column  56  via bolts  101 , shown in  FIG. 6  but not shown in  FIG. 9 . The bolts  101  pass through bores in the plate  56 A and engage threaded bores in a boss  106 , shown in  FIG. 9 , of the tactile feedback device  100 . The tactile feedback device  100  may comprise an electrically controlled brake capable of generating a resistance or counter force that opposes movement of the control handle  90 , wherein the force varies based on a magnitude of a tactile feedback device signal, which signal will be discussed below. For example, the electrically controlled brake may comprise one of an electrorheological device, a magnetorheological device, and an electromagnetic device. In the illustrated embodiment, the tactile feedback device  100  comprises a device commercially available from the Lord Corporation under the product designation “RD 2104-01.” 
     As illustrated in  FIG. 9 , the control handle  90  is fixedly coupled to a shaft  102  of the tactile feedback device  100  such that the control handle  90  and the shaft  102  rotate together. A magnetically controllable medium (not shown) is provided within the device  100 . A magnetic field generating element (not shown) forms part of the device  100  and is capable of generating a variable strength magnetic field that changes with the tactile feedback device signal. The magnetically controllable medium may have a shear strength that changes in proportion to the strength of the magnetic field, and provides a variable resistance or counter force to the shaft  102 , which force is transferred by the shaft  102  to the control handle  90 . As the variable resistance force generated by the tactile feedback device  100  increases, the control handle  90  becomes more difficult to rotate by an operator. 
     The tactile feedback device  100  further comprises a control handle position sensor  100 A, shown in  FIG. 2  but not shown in  FIG. 9 , which senses the angular position of the control handle  90  within the angular range of approximately +/−60 degrees in the illustrated embodiment. The control handle position sensor  100 A comprises, in the illustrated embodiment, first and second potentiometers, each of which senses the angular position of the shaft  102 . The second potentiometer generates a redundant position signal. Hence, only a single potentiometer is required to sense the angular position of the shaft  102 . The angular position of the shaft  102  corresponds to the angular position of the control handle  90 . An operator rotates the control handle  90  within the angular range of approximately +/−60 degrees in the illustrated embodiment to control movement of the steerable wheel  74 , which wheel  74  is capable of rotating approximately +/−90 degrees from a centered position in the illustrated embodiment. As the control handle  90  is rotated by the operator, the control handle position sensor  100 A senses that rotation, i.e., magnitude and direction, and generates a steer control signal corresponding to a desired angular position of the steerable wheel  74  to a steering control module or unit  220 . 
     The biasing structure  110  comprises a coiled spring  112  in the illustrated embodiment, see  FIGS. 6, 6A and 9 , having first and second ends  112 A and  112 B. The spring  112  is positioned about the boss  106  of the tactile feedback device  100 , see  FIG. 9 . A pin  92 , shown in  FIGS. 6 and 6A  but not shown in  FIG. 9 , extends down from a base  94  of the control handle  90  and moves with the control handle  90 . When the control handle  90  is located in its centered position, the pin  92  is positioned between and adjacent to the first and second spring ends  112 A and  112 B, see  FIG. 6A . The spring ends  112 A and  112 B engage and rest against a block  115 A fixed to and extending down from the plate  56 A of the steering column  56  when the control handle  90  is in its centered position, see  FIGS. 6 and 6A . As the control handle  90  is rotated by an operator away from its centered position, the pin  92  engages and pushes against one of the spring ends  112 A,  112 B, causing that spring end  112 A,  112 B to move away from the block  115 A. In response, that spring end  112 A,  112 B applies a return force against the pin  92  and, hence, to the control handle  90 , in a direction urging the control handle  90  to return to its centered position. When the operator is no longer gripping and turning the control handle  90  and any resistance force generated by the tactile feedback device  100  is less than that of the biasing force applied by the spring  112 , the spring  112  causes the control handle  90  to return to its centered position. 
     The steering column  56  further comprises a cover portion  56 B, shown only in  FIGS. 7 and 8  and not in  FIGS. 6 and 9 , which covers the tactile feedback device  100 . 
     The steer motor  120  comprises a drive gear  122  coupled to a steer motor output shaft  123 , see  FIGS. 3 and 4 . The drive unit  70  further comprises a rotatable gear  76  coupled to the support structure  71  such that movement of the rotatable gear  76  effects rotation of the support structure  71 , the traction motor  72  and the steerable wheel  74  about the first axis A 1 , see  FIGS. 3-5 . A chain  124  extends about the drive gear  122  and the rotatable gear  76  such that rotation of the steer motor output shaft  123  and drive gear  122  causes rotation of the drive unit  70  and corresponding angular movement of the steerable wheel  74 . 
     The vehicle  10  further comprises a control apparatus  200 , which, in the illustrated embodiment, comprises a traction control module  210 , the steering control module  220  and a display module  230 , see  FIGS. 2, 3 and 7 . Each of the modules  210 ,  220  and  230  comprises a controller or processor for effecting functions to be discussed below. The functions effected by the modules  210 ,  220  and  230  may alternatively be performed by a single module, two modules or more than three modules. The traction control module  210  is mounted to the side wall  54 , the steering control module  220  is mounted to the base  52  and the display module  230  is mounted within the steering column  56 . 
     The control handle  90  further comprises first and second rotatable speed control elements  96 A and  96 B forming part of a speed control apparatus  96 . One or both of the speed control elements  96 A,  96 B may be gripped and rotated by an operator to control a direction and speed of movement of the vehicle  10 , see  FIGS. 2, 7 and 8 . The first and second speed control elements  96 A and  96 B are mechanically coupled together such that rotation of one element  96 A,  96 B effects rotation of the other element  96 B,  96 A. The speed control elements  96 A and  96 B are spring biased to a center neutral or home position and coupled to a signal generator SG, which, in turn, is coupled to the traction control module  210 . The signal generator SG, for example, a potentiometer, forms part of the speed control apparatus  96  and is capable of generating a speed control signal to the traction control module  210 . The speed control signal varies in sign based on the direction of rotation of the speed control elements  96 A,  96 B, clockwise or counterclockwise from their home positions, and magnitude based on the amount of rotation of the speed control elements  96 A,  96 B from their home positions. When an operator rotates a control element  96 A,  96 B in a clockwise direction, as viewed in  FIG. 7 , a speed control signal is generated to the traction control module  210  corresponding to vehicle movement in a power unit first direction. When the operator rotates a control element  96 A,  96 B in a counter-clockwise direction, as viewed in  FIG. 7 , a speed control signal is generated to the traction control module  210  corresponding to vehicle movement in a forks first direction. 
     The control handle  90  further comprises a speed selection switch  98 , see  FIGS. 2, 7 and 8 , which is capable of being toggled back and forth between a high speed position corresponding to a “high speed” mode and a low speed position corresponding to a “low speed” mode. Based on its position, the speed selection switch  98  generates a speed select signal to the traction control module  210 . If the switch  98  is in its low speed position, the traction control module  210  may limit maximum speed of the vehicle  10  to about 3.5 MPH in both a forks first direction and a power unit first direction. If the switch  98  is in its high speed position, the traction control module  210  will allow, unless otherwise limited based on other vehicle conditions, see for example the discussion below regarding  FIGS. 11, 11A and 11B , the vehicle to be operated up to a first maximum vehicle speed, e.g., 6.0 MPH, when the vehicle is being operated in a forks first direction and up to a second maximum vehicle speed, e.g., 9.0 MPH, when the vehicle is being operated in a power unit first direction. It is noted that when an operator is operating the vehicle  10  without standing on the floorboard  34 , referred to as a “walkie” mode, discussed further below, the traction control module  210  will limit maximum speed of the vehicle to the maximum speed corresponding to the switch low speed position, e.g., about 3.5 MPH, even if the switch  98  is located in its high speed position. It is noted that the speed of the vehicle  10  within a speed range, e.g., 0-3.5 MPH, 0-6.0 MPH and 0-9.0 MPH, corresponding to one of the low speed mode/walkie mode, the high speed mode/first maximum vehicle speed, and the high speed mode/second maximum speed is proportional to the amount of rotation of a speed control element  96 A,  96 B being rotated. 
     The steer motor  120  comprises a position sensor  124 , see  FIG. 2 . As the steer motor output shaft  123  and drive gear  122  rotate, the position sensor  124  generates a steer motor position signal to the steering control unit  220 , which signal is indicative of an angular position of the steerable wheel  74  and the speed of rotation of the steerable wheel  74  about the first axis A 1 . The steering control unit  220  calculates from the steer motor position signal a current actual angular position of the steerable wheel  74 , and the current speed of rotation of the steerable wheel  74  about the first axis A 1 . The steering control unit passes the calculated current angular position of the steerable wheel  74  and the current speed of rotation of the steerable wheel  74  to the display module  230 . 
     The steering control unit  220  also receives the steer control signal from the control handle position sensor  100 A, which, as noted above, senses the angular position of the control handle  90  within the angular range of approximately +/−60 degrees in the illustrated embodiment. The steering control unit  220  passes the steer control signal to the display module  230 . Since a current steer control signal corresponds to a current position of the control handle  90  falling within the range of from about +/−60 degrees and the steerable wheel  74  is capable of rotating through an angular range of +/−90 degrees, the display module  230  converts the current control handle position, as indicated by the steer control signal, to a corresponding desired angular position of the steerable wheel  74  by multiplying the current control handle position by a ratio of equal to or about 90/60 in the illustrated embodiment, e.g., an angular position of the control handle  90  of +60 degrees equals a desired angular position of the steerable wheel  74  of +90 degrees. The display module  230  further determines a steer rate, i.e., change in angular position of the control handle  90  per unit time, using the steer control signal. For example, the display module  230  may compare angular positions of the control handle  90  determined every 32 milliseconds to determine the steer rate. 
     As noted above, the proximity sensor  36  generates an operator status signal indicating that either an operator is standing on the floorboard  34  in the operator&#39;s compartment  30  or no operator is standing on the floorboard  34  in the operator&#39;s compartment  30 . The proximity sensor  36  is coupled to the traction control module  210  such that the traction control module  210  receives the operator status signal from the proximity sensor  36 . The traction control module  210  forwards the operator status signal to the display module  230 . If an operator is standing on the floorboard  34  in the operator&#39;s compartment  30 , as indicated by the operator status signal, the display module  230  will allow movement of the steerable wheel  74  to an angular position falling within a first angular range, which, in the illustrated embodiment, is equal to approximately +/−90 degrees. If, however, an operator is NOT standing on the floorboard  34  in the operator&#39;s compartment  30 , the display module  230  will limit movement of the steerable wheel  74  to an angular position within a second angular range, which, in the illustrated embodiment, is equal to approximately +/−15 degrees. It is noted that when an operator is standing on the floorboard  34  in the operator&#39;s compartment  30 , the vehicle is being operated in a rider mode, such as the high speed or the low speed mode noted above. When an operator is NOT standing on the floorboard  34  in the operator&#39;s compartment  30 , the vehicle may be operated in the “walkie” mode, where the operator walks alongside the vehicle  10  while gripping and maneuvering the control handle  90  and one of the first and second rotatable speed control elements  96 A and  96 B. Hence, rotation of the steerable wheel  74  is limited during the walkie mode to an angular position within the second angular range. 
     Typically, an operator does not request that the control handle  90  be turned to an angular position greater than about +/−_45 degrees from the centered position when the vehicle  10  is operating in the walkie mode. If a request is made to rotate the control handle  90  to an angular position greater than about +/−45 degrees and the vehicle  10  is being operated in the walkie mode, the display module  230  will command the traction control module  210  to cause the vehicle  10  to brake to a stop. If the display module  230  has caused the vehicle  10  to brake to a stop, the display module  230  will allow the traction motor  72  to rotate again to effect movement of the driven steerable wheel  74  after the control handle  90  has been moved to a position within a predefined range such as +/−40 degrees and the first and second speed control elements  96 A and  96 B have been returned to their neutral/home positions. 
     As noted above, the steering control unit  220  passes the calculated current angular position of the steerable wheel  74  and the current speed of rotation of the steerable wheel  74  to the display module  230 . The steering control unit  220  further passes the steer control signal to the display module  230 , which module  230  converts the steer control signal to a corresponding requested or desired angular position of the steerable wheel  74 . If an operator is standing on the floorboard  34  in the operator&#39;s compartment  30 , as detected by the proximity sensor  36 , the display module  230  forwards the requested angular position for the steerable wheel  74  to the steering control unit  220 , which generates a first drive signal to the steer motor  120  causing the steer motor  120  to move the steerable wheel  74  to the requested angular position. If an operator is NOT standing on the floorboard  34  in the operator&#39;s compartment  30 , as detected by the proximity sensor  36 , the display module  230  will determine if the requested angular position for the steerable wheel  74  is within the second angular range, noted above. If so, the display module  230  forwards the requested angular position for the steerable wheel  74  to the steering control unit  220 , which generates a first signal to the steer motor  120  causing the steer motor  120  to move the steerable wheel  74  to the requested angular position. If the requested angular position for the steerable wheel  74  is NOT within the second angular range, the display module  230  limits the angular position for the steerable wheel  74  forwarded to the steering control unit  220  to the appropriate extreme or outer limit of the second angular range. 
     As noted above, the encoder  172  is coupled to the output shaft of the traction motor  72  to generate signals indicative of the speed and direction of rotation of the traction motor  72 . The encoder signals are provided to the traction control module  210  which determines the direction and speed of rotation of the traction motor  72  from those signals. The traction control module  210  then forwards traction motor rotation speed and direction information to the display module  230 . This information corresponds to the direction and speed of rotation of the steerable wheel  74  about the second axis A 2 . 
     The display module  230  may define an upper steering motor speed limit based on a current traction motor speed using linear interpolation between points from a curve, which points may be stored in a lookup table. When the truck  10  is being operated in a power unit first direction, points from a curve, such as curve C 1  illustrated in  FIG. 10 , may be used to define a steering motor speed limit based on a current traction motor speed. When the truck  10  is being operated in a forks first direction, points from a curve, such as curve C 2  illustrated in  FIG. 10 , may be used to define a steering motor speed limit based on a current traction motor speed. In the illustrated embodiment, the steering motor speed upper limit decreases as the speed of the traction motor increases beyond about 2000 RPM, see curves C 1  and C 2  in  FIG. 10 . As a result, the steering motor responsiveness is purposefully slowed at higher speeds in order to prevent a “twitchy” or “overly sensitive” steering response as an operator operates the vehicle  10  at those higher speeds. Hence, the drivability of the vehicle  10  is improved at higher speeds. It is noted that the steering motor speed limits in curve C 2  for the forks first direction are lower than the steering motor speed limits in curve C 1  for the power unit first direction. An appropriate steering motor speed limit based on a current traction motor speed is provided by the display module  230  to the steering control module  210 . The steering control module  210  uses the steering motor speed limit when generating the first drive signal to the steer motor  120  so as to maintain the speed of the steer motor  120  at a value equal to or less than the steering motor speed limit until the steerable wheel  74  has been moved to a desired angular position. Instead of storing points from curve C 1  or curve C 2 , an equation or equations corresponding to each of the curves C 1  and C 2  may be stored and used by the display module  230  to determine a steering motor speed limit based on a current traction motor speed. 
     As noted above, the steering control unit  220  passes the steer control signal to the display module  230 , which module  230  converts the steer control signal to a corresponding desired angular position of the steerable wheel  74 . The steering control unit  220  also passes the calculated current actual angular position of the steerable wheel  74  to the display module  230 . The display module  230  uses the desired angular position for the steerable wheel  74  to determine a first upper traction motor speed limit using, for example, linear interpolation between points from a curve, such as curve C 3 , illustrated in  FIG. 11 , wherein the points may be stored in a lookup table. The display module  230  further uses the calculated actual angular position for the steerable wheel  74  to determine a second upper traction motor speed limit using, for example, linear interpolation between points from the curve C 3 . Instead of storing points from a curve C 3 , an equation or equations corresponding to the curve may be stored and used by the display module  230  to determine the first and second traction motor speed limits based on a desired angular position for the steerable wheel and a calculated current angular position of the steerable wheel. As is apparent from  FIG. 11 , the first/second traction motor speed limit decreases as the desired angular position/calculated angular position for the steerable wheel  74  increases so as to improve the stability of the vehicle  10  during high steerable wheel angle turns. 
     The display module  230  compares a current desired angular position of the steerable wheel  74  to a current calculated actual position of the steerable wheel  74  to determine a difference between the two equal to a steerable wheel error. Since the control handle position and the steerable wheel position are not locked to one another, steerable wheel error results from a delay between when an operator rotates the control handle  90  to effect a change in the position of the steerable wheel  74  and the time it takes the steer motor  120  to effect corresponding movement of the steerable wheel  74  to move the steerable wheel  74  to the new angular position. 
     The display module  230  uses the steerable wheel error to determine a third upper traction motor speed limit using, for example, linear interpolation between points from a curve, such as curve C A , illustrated in  FIG. 11A , wherein the points may be stored in a lookup table. Instead of storing points from a curve, an equation or equations corresponding to the curve C A  may be stored and used by the display module  230  to determine the third traction motor speed limit based on steerable wheel error. As is apparent from  FIG. 11A , the third traction motor speed limit generally decreases as the steerable wheel error increases. 
     The display module  230  uses the steer rate to determine a fourth upper traction motor speed limit using, for example, linear interpolation between points from a curve, such as curve C B , illustrated in  FIG. 11B , wherein the points may be stored in a lookup table. Instead of storing points from a curve, an equation or equations corresponding to the curve C B  may be stored and used by the display module  230  to determine the fourth traction motor speed limit based on steer rate. As is apparent from  FIG. 11B , the fourth traction motor speed limit generally decreases as the steer rate increases. 
     The display module  230  determines the lowest value from among the first, second, third and fourth traction motor speed limits and forwards the lowest speed limit to the traction control module  210  for use in controlling the speed of the traction motor  72  when generating a second drive signal to the traction motor  72 . 
     The display module  230  may generate a high steerable wheel turn signal to the traction control module  210  when the steer control signal corresponds to a steerable wheel angular position greater than about +/−7 degrees from its straight ahead position. When the display module  230  is generating a high steerable wheel turn signal, the vehicle is considered to be in a “special for turn” mode. 
     In the illustrated embodiment, the traction control module  210  stores a plurality of acceleration values for the traction motor  72 . Each acceleration value defines a single, constant rate of acceleration for the traction motor  72  and corresponds to a separate vehicle mode of operation. For example, a single acceleration value may be stored by the traction control module  210  for each of the following vehicle modes of operation: low speed/walkie mode, forks first direction; low speed/walkie mode, power unit first direction; high speed mode, forks first direction; high speed mode, power unit first direction; special for turn mode, forks first direction; and special for turn mode, power unit first direction. The traction control module  210  selects the appropriate acceleration value based on a current vehicle mode of operation and uses that value when generating the second drive signal for the traction motor  72 . 
     The display module  230  determines, in the illustrated embodiment, first, second and third acceleration reduction factors RF1, RF2 and RF3. 
     As noted above, the steering control unit  220  passes the calculated current actual angular position of the steerable wheel  74  and the current speed of rotation of the steerable wheel  74  to the display module  230 . The display module  230  may use the calculated current actual angular position of the steerable wheel  74  to determine the first acceleration reduction factor RF1 using, for example, linear interpolation between points from a curve, such as curve C c , illustrated in  FIG. 11C , wherein the points may be stored in a lookup table. Instead of storing points from a curve, an equation or equations corresponding to the curve C c  may be stored and used by the display module  230  to determine the first acceleration reduction factor RF1. As is apparent from  FIG. 11C , after a steered wheel angle of about 10 degrees, the first acceleration reduction factor RF1 decreases generally linear as the steerable wheel angle increases. 
     As discussed above, the traction control module  210  forwards traction motor rotation speed and direction information to the display module  230 . The display module  230  may use the traction motor speed to determine the second acceleration reduction factor RF2 using, for example, linear interpolation between points from a curve, such as curve C D , illustrated in  FIG. 11D , wherein the points may be stored in a lookup table. Instead of storing points from a curve, an equation or equations corresponding to the curve C D  may be stored and used by the display module  230  to determine the second acceleration reduction factor RF2. As is apparent from  FIG. 11D , the second acceleration reduction factor RF2 generally increases as the traction motor speed increases. 
     As noted above, an operator may rotate one or both of the first and second speed control elements  96 A,  96 B causing the signal generator SG to generate a corresponding speed control signal to the traction control module  210 . The traction control module  210  forwards the speed control signal to the display module  230 . As also noted above, the speed control signal varies in magnitude based on the amount of rotation of the speed control elements  96 A,  96 B from their home positions. Hence, the speed control signal is indicative of the current position of the speed control elements  96 A,  96 B. The display module  230  may determined the third acceleration reduction factor RF3 using the speed control signal. For example, the third acceleration reduction factor RF3 may equal a first predefined value, e.g., 10, for all speed control signals corresponding to a position of each speed control element  96 A,  96 B between a zero or home position and a position corresponding to 80% of its maximum rotated position and may equal a second predefined value, e.g., 128, for all speed control signals corresponding to a position of each speed control element  96 A,  96 B greater than 80% of its maximum rotated position. 
     The display module  230  determines which of the first, second and third reduction factors RF1, RF2 and RF3 has the lowest value and provides that reduction factor to the traction control module  210 . The traction control module  210  receives the selected reduction factor, which, in the illustrated embodiment, has a value between 0 and 128. The module  210  divides the reduction factor by 128 to determine a modified reduction factor. The modified reduction factor is multiplied by the selected acceleration value to determine an updated selected acceleration value, which is used by the traction control module  210  when generating the second drive signal to the traction motor  72 . The reduction factor having the lowest value, prior to being divided by 128, effects the greatest reduction in the acceleration value. 
     Based on the position of the speed selection switch  98 , the operator status signal, whether a high steerable wheel turn signal has been generated by the display module  230 , the sign and magnitude of a speed control signal generated by the signal generator SG in response to operation of the first and second rotatable speed control elements  96 A and  96 B, an acceleration value corresponding to the current vehicle mode of operation, a selected acceleration reduction factor, a current traction motor speed and direction as detected by the encoder  172 , and a selected traction motor speed limit, the traction control module  210  generates the second drive signal to the traction motor  72  so as to control the speed, acceleration and direction of rotation of the traction motor  72  and, hence, the speed, acceleration and direction of rotation of the steerable wheel  74  about the second axis A 2 . 
     Instead of determining first, second and third reduction factors, selecting a lowest reduction factor, dividing the selected reduction factor by 128 and multiplying the modified reduction factor by a selected acceleration value to determine an updated selected acceleration value, the following steps may be implemented by the display module  230  either alone or in combination with the traction control module  210 . Three separate curves are defined for each vehicle mode of operation, which modes of operation are listed above. The first curve defines a first acceleration value that varies based on the calculated current actual angular position of the steerable wheel  74 . The second curve defines a second acceleration value that varies based on traction motor speed. The third curve defines a third acceleration value that varies based on the speed control signal from the signal generator SG. The display module and/or the traction control module determines using, for example, linear interpolation between points from each of the first, second and third curves corresponding to the current vehicle mode of operations, wherein the points may be stored in lookup tables, first, second and third acceleration values, selects the lowest acceleration value and uses that value when generating the second drive signal to the traction motor  72 . 
     As noted above, the tactile feedback device  100  is capable of generating a resistance or counter force that opposes movement of the control handle  90 , wherein the force varies based on the magnitude of the tactile feedback device signal. In the illustrated embodiment, the display module  230  defines a setpoint TFDS for the tactile feedback device signal, communicates the setpoint TFDS to the steering control module  220  and the steering control module  220  generates a corresponding tactile feedback device signal, e.g., a current measured for example in milliAmperes (mA), to the tactile feedback device  100 . 
     In the illustrated embodiment, the display module  230  defines the tactile feedback device signal setpoint TFDS as follows. The display module  230  constantly queries the traction control module  210  for speed and direction of rotation of the traction motor  72 , which information is determined by the traction control module  210  from signals output by the encoder  172 , as noted above. Based on the traction motor speed, the display module  230  determines a first tactile feedback device signal value TFD1, see step  302  in  FIG. 14 , using, for example, linear interpolation between points from a curve, such as curve C 4 , illustrated in  FIG. 12 , wherein the points may be stored in a lookup table. Instead of storing points from a curve, an equation or equations corresponding to the curve C 4  may be stored and used by the display module  230  to determine the first value TFD1. As can be seen from  FIG. 12 , the first value TFD1 generally increases with traction motor speed. 
     As noted above, the display module  230  compares the current desired angular position of the steerable wheel  74  to a current calculated actual position of the steerable wheel  74  to determine a difference between the two equal to a steerable wheel error. Based on the steerable wheel error, the display module  230  determines a second tactile feedback device signal value TFD2, see step  302  in  FIG. 14 , using, for example, linear interpolation between points from a curve, such as curve C 5 , illustrated in  FIG. 13 , wherein the points may be stored in a lookup table. Instead of storing points from a curve, an equation or equations corresponding to the curve C 5  may be stored and used by the display module  230  to determine the second value TFD2. As can be seen from  FIG. 13 , the second value TFD2 generally increases with steerable wheel error. 
     In the illustrated embodiment, the display module  230  sums the first and second values TFD1 and TFD2 together to determine a combined tactile feedback device signal value TFDC, see step  304  in  FIG. 14 , and multiplies this value by a reduction factor based on a direction in which the vehicle  10  is moving in order to determine the tactile feedback device signal setpoint TFDS, see step  306  in  FIG. 14 . If the vehicle  10  is being driven in a forks first direction, the reduction factor may equal 0.5. If the vehicle  10  is being driven in a power unit first direction, the reduction factor may equal 1.0. Generally, an operator has only one hand on the control handle  90  when the vehicle  10  is moving in the forks first direction. Hence, the reduction factor of 0.5 makes it easier for the operator to rotate the control handle  90  when the vehicle  10  is traveling in the forks first direction. 
     The display module  230  provides the tactile feedback device signal setpoint TFDS to the steering control unit  220 , which uses the setpoint TFDS to determine a corresponding tactile feedback device signal for the tactile feedback device  100 . Because the tactile feedback device signal is determined in the illustrated embodiment from the first and second values TFD1 and TFD2, which values come from curves C 4  and C 5  in  FIGS. 12 and 13 , the tactile feedback device signal increases in magnitude as the traction motor speed and steerable wheel error increase. Hence, as the traction motor speed increases and the steerable wheel error increases, the counter force generated by the tactile feedback device  100  and applied to the control handle  90  increases, thus, making it more difficult for an operator to turn the control handle  90 . It is believed to be advantageous to increase the counter force generated by the tactile feedback device  100  as the traction motor speed increases to reduce the likelihood that unintended motion will be imparted to the control handle  90  by an operator as the vehicle  10  travels over bumps or into holes/low spots found in a floor upon which it is driven and enhance operator stability during operation of the vehicle. It is further believed to be advantageous to increase the counter force generated by the tactile feedback device  100  as the steerable wheel error increases so as to provide tactile feedback to the operator related to the magnitude of the steerable wheel error. 
     In a further embodiment, a pressure transducer  400 , shown in dotted line in  FIG. 2 , is provided as part of a hydraulic system (not shown) coupled to the forks  60 A and  60 B for elevating the forks  60 A and  60 B. The pressure transducer  400  generates a signal indicative of the weight of any load on the forks  60 A and  60 B to the display module  230 . Based on the fork load, the display module  230  may determine a third tactile feedback device signal value TFD3 using, for example, linear interpolation between points from a curve (not shown), where the value TFD3 may vary linearly with fork load such that the value TFD3 may increase as the weight on the forks  60 A and  60 B increases. The display module  230  may sum the first, second and third values TFD1, TFD2 and TFD3 together to determine a combined tactile feedback device signal value TFDC, which may be multiplied by a reduction factor, noted above, based on a direction in which the vehicle  10  is moving in order to determine a tactile feedback device signal setpoint TFDS. The display module  230  provides the tactile feedback device signal setpoint TFDS to the steering control unit  220 , which uses the setpoint TFDS to determine a corresponding tactile feedback device signal for the tactile feedback device  100 . 
     As discussed above, the proximity sensor  36  outputs an operator status signal to the traction control module  210 , wherein a change in the operator status signal indicates that an operator has either stepped onto or stepped off of the floorboard  34  in the operator&#39;s compartment  30 . As also noted above, the traction control module  210  provides the operator status signal to the display module  230 . The display module  230  monitors the operator status signal and determines whether an operator status signal change corresponds to an operator stepping onto or stepping off of the floorboard  34 . An operator stops the vehicle before stepping out of the operator&#39;s compartment. When the operator leaves the operator&#39;s compartment, if the tactile feedback device signal is at a force generating value, e.g., a non-zero value in the illustrated embodiment, causing the tactile feedback device  100  to generate a counter force to the control handle  90 , the display module  230  decreases the tactile feedback device signal setpoint TFDS at a controlled rate, e.g., 900 mA/second, until the tactile feedback device signal setpoint TFDS, and, hence, the tactile feedback device signal, equal zero. By slowly decreasing the tactile feedback device signal setpoint TFDS and, hence, the tactile feedback device signal, at a controlled rate and presuming the control handle  90  is positioned away from its centered position, the biasing structure  110  is permitted to return the control handle  90  back to its centered position, i.e., 0 degrees, without substantially overshooting the centered position after the operator has stepped off the floorboard  34 . The tactile feedback device signal setpoint TFDS, and, hence, the tactile feedback device signal, are maintained at a zero value for a predefined period of time, e.g., two seconds. Thereafter, the display module  230  determines an updated tactile feedback device signal setpoint TFDS and provides the updated tactile feedback device signal setpoint TFDS to the steering control unit  220 . It is contemplated that the display module  230  may only decrease the tactile feedback device signal setpoint TFDS if, in addition to an operator leaving the operator&#39;s compartment and the tactile feedback device signal being at a force generating value, the control handle  90  is positioned away from its centered position. It is further contemplated that the display module  230  may maintain the tactile feedback device signal setpoint TFDS at a zero value until it determines that the control handle  90  has returned to its centered position. 
     If, while monitoring the operator status signal, the display module  230  determines that an operator status signal change corresponds to an operator stepping onto the floorboard  34 , the display module  230  will immediately increase the tactile feedback device signal setpoint TFDS for a predefined period of time, e.g., two seconds, causing a corresponding increase in the tactile feedback device signal. The increase in the tactile feedback signal is sufficient such that the tactile feedback device  100  generates a counter force of sufficient magnitude to the control handle  90  to inhibit an operator from making a quick turn request via the control handle  90  just after the operator has stepping into the operator&#39;s compartment  30 . After the predefined time period has expired, the display module  230  determines an updated tactile feedback device signal setpoint TFDS and provides the updated tactile feedback device signal setpoint TFDS to the steering control unit  220 . 
     Also in response to determining that an operator has just stepped onto the floorboard  34  and if a steer request is immediately made by an operator via the control handle  90 , the display module  230  provides an instruction to the steering control module  220  to operate the steer motor  120  at a first low speed, e.g., 500 RPM and, thereafter, ramp up the steer motor speed, e.g., linearly, to a second higher speed over a predefined period of time, e.g., one second. The second speed is defined by curve C 1  or curve C 2  in  FIG. 10  based on a current traction motor speed. Hence, the first drive signal to the steer motor  120  is varied such that the speed of the steer motor  120 , i.e., the rate of speed increase, gradually increases from a low value after the operator enters the operator&#39;s compartment in order to avoid a sudden sharp turn maneuver. 
     It is further contemplated that the steerable wheel may not be driven. Instead, a different wheel forming part of the vehicle would be driven by the traction motor  72 . In such an embodiment, the traction control module  210  may generate a second drive signal to the traction motor  72  so as to control the speed, acceleration and direction of rotation of the traction motor  72  and, hence, the speed, acceleration and direction of rotation of the driven wheel based on the position of the speed selection switch  98 , the operator status signal, whether a high steerable wheel turn signal has been generated by the display module  230 , the sign and magnitude of a speed control signal generated by the signal generator SG in response to operation of the first and second rotatable speed control elements  96 A and  96 B, an acceleration value corresponding to the current vehicle mode of operation, a selected acceleration reduction factor, a current traction motor speed and direction as detected by the encoder  172 , and a selected traction motor speed limit. 
     It is still further contemplated that a vehicle including a mechanical or hydrostatic steering system may include a traction motor  72  controlled via a traction control module  210  and a display module  230  as set out herein presuming the vehicle includes a control handle position sensor or like sensor for generating signals indicative of an angular position of the control handle and its steer rate and a position sensor or like sensor for generating signals indicative of an angular position of a steerable wheel and a speed of rotation of the steerable wheel about an axis A 1 . 
     While particular embodiments of the present invention have been illustrated and described, it would be obvious to those skilled in the art that various other changes and modifications can be made without departing from the spirit and scope of the invention. It is therefore intended to cover in the appended claims all such changes and modifications that are within the scope of this invention.