Abstract:
A method of positioning a load wheel in a tire uniformity machine to load a tire for testing, the method comprising providing a drive assembly including a drive motor and a drive controller, electrically connected to each other, said drive controller being adapted to switch between analog velocity control and position control; moving the load wheel with the drive motor using analog velocity control; and switching to position control to cause said motor to maintain a selected position of said load wheel without a mechanical brake.

Description:
RELATED PATENT APPLICATIONS 
   This application claims the benefit of U.S. Provisional Application No. 60/548,291 filed on Feb. 27, 2004. 

   TECHNICAL FIELD 
   In general, the present invention relates to tire uniformity machines. In particular, the present invention relates to the load wheel drive in a tire uniformity machine. More particularly, the present invention relates to a load wheel drive having a servo motor connected to a gear reduction that drives the load wheel and eliminates the need for a brake motor. 
   BACKGROUND OF THE INVENTION 
   Tire uniformity machines are well known in the art. In general, these machines test characteristics of a tire by placing the tire on a chuck and contacting it with a rotating load wheel that is equipped with load cells to measure characteristics of the tire. Typically, the load wheel is in a retracted position while the tire is being conveyed into the machine and mounted on the chuck. In this retracted position, the load wheel is moved laterally outward relative to the chuck assembly until the load wheel is more or less outside of the machine proper. Once the tire has been chucked, the load wheel is driven inwardly to contact the tire. The position of the load wheel relative to the tire is important in terms of applying a specified load to the tire and ensuring accurate test results. 
   In one known system, the motion of the load wheel is controlled by a D.C. motor via a 15:1 gear box reduction and a 54 T-23T chain-sprocket drive increase with a resulting reduction of 6.39:1 to a 0.25 inch pitch ball screw. The maximum speed of such a system is about 1.1 inches per second. Loading is accomplished by using a 4-quadrant D.C. drive that controls the velocity of the load wheel carriage. Using the velocity controlled method, the load wheel is initially advanced at full speed until its load cells detect an increase in the instantaneous load. When the measured load is within 200 pounds of a set point load, the analog velocity set point to the D.C. drive is reduced as the “load error” decreases. In this system, it is necessary to load to an average radial force, which is calculated over one or more revolutions of the tire. When the load is within the desired tolerance, the drive motor is disabled and a mechanical brake is engaged to maintain the final position of the load wheel. 
   In existing systems, the mechanical brake mechanism has been a critical component. During loading, the electrically released brake mechanism must fully release or else it can prevent proper loading and can cause excessive brake wear. At the end of the loading cycle, the brake release mechanism is de-energized and the brake must fully resist the back driving force generated by a loaded tire. Even a small amount of brake slippage results in degraded measurement results for the tire uniformity machine. 
   SUMMARY OF THE INVENTION 
   In light of the foregoing, it is an object of the present invention to provide an improved load wheel drive system. 
   In light of this object, the present invention generally provides a load drive motor assembly that is retrofit to a conventional load wheel drive having a gear box and ball screw assembly connected to each other by a chain, the load drive motor assembly, including a servo motor; an adapter plate fastenable to the gear box on one side and fastenable to the servo motor on the other side; and wherein the servo motor is switchable between analog velocity control and a fixed position control; an enable relay connected to the servo drive, the enable relay having an energized state and a de-energized state, wherein the energized state switches the servo-drive to the analog velocity control, and wherein the de-energized state switches the servo drive to the fixed position control being adapted to maintain a current position of the load wheel. 
   The present invention further provides a method of retrofitting a servo motor to a load wheel drive having a dc motor, gear box, and motor controls the method including removing the dc motor; attaching an adapter to the gear box, providing receivers on the adapter that align with receivers on the servo motor; fastening the servo motor to the adapter plate at the aligned receivers; connecting the servo motor to the motor controller at a relay on the servo motor; programming the relay to cause the servo motor to hold a position if the relay is de-energized. 
   The present invention further provides a load wheel drive assembly for an existing tire uniformity machine having a load wheel that is driven by a ball nut connected to a gear box, the load wheel drive assembly including a servo motor attachable to the gear box and operable therewith to control movement of the load wheel, the servo motor having a relay adapted to switch the servo motor between a velocity mode and a position mode of operation. 
   The present invention further provides a load wheel drive assembly that is retrofit to a conventional load wheel drive having a gear box that interconnects with a ball screw used to drive a load wheel, wherein movement of the load wheel is controlled by a controller that receives force feed back from at least one load cell associated with the load wheel, the load wheel drive assembly including a servo motor attachable to the gear box and operatively connected therewith; a drive controller electrically connected to the controller and the servo motor, the drive controller having at least one programmable input adapted to switch operation of the servo motor between analog velocity control and position control, wherein the servo motor drives the load wheel at an average velocity when in the analog velocity control mode and wherein the servo motor maintains a selected position when in the position control mode; wherein the programmable input is in communication with the controller and is programmed to switch to position control upon receiving a signal from the controller indicating that the load wheel is in a selected position, whereby the servo motor being in a position control mode holds the selected position. 
   The present invention further provides a method of positioning a load wheel in a tire uniformity machine to load a tire for testing, the method including providing a drive assembly including a drive motor and a drive controller, electrically connected to each other, the drive controller being adapted to switch between analog velocity control and position control; moving the load wheel with the drive motor using analog velocity control; and switching to position control to cause the motor to maintain a selected position of the load wheel without a mechanical brake. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a complete understanding of the objects, techniques and structure of the invention, reference should be made to the following detailed description and accompanying drawings wherein: 
       FIG. 1  is a front side elevational view of a tire uniformity machine having a load wheel drive system according to the present invention; 
       FIG. 2  is an enlarged side elevational view as might be seen along line  2 - 2  in  FIG. 1 , partially sectioned to show details of a servo drive load wheel drive assembly; 
       FIG. 3  is a side elevational view as might be seen along line  3 - 3  in  FIG. 2 ; and 
       FIG. 4  is a partially schematic side elevational view similar to  FIG. 3 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   A tire uniformity machine according to the concepts of the present invention is generally indicated by the numeral  10 . The basic components of the tire uniformity machine are familiar to the those of ordinary skill in the art, and, thus, will be described in general terms only. A tire uniformity machine  10  includes a box-like frame  11  having a plurality of upstanding members  12  capped by a top frame member  14  defining the machine proper, generally indicated by the numeral  15 , therein. A first conveyor  16  brings tires into the machine proper  15  where they are chucked on a chuck assembly, generally indicated by the numeral  20 . After the tire T has been tested, an exit conveyor carries the tire T from the machine proper  15  to downstream applications. 
   The chuck assembly  20  may include two halves  21 ,  22  that clamp the tire T from either side in preparation for testing. The chuck assembly  20  is rotatable and a motor  23  may be used to rotate the chuck assembly  20 . 
   A load wheel assembly, generally indicated by the numeral  25  is located to one side of the machine proper  15  and is mounted on a carriage  26  that allows the load wheel to be driven inward toward the center C of the machine proper  15 . In this way, the road surface  27  of the load wheel  25  may be brought into contact with the tread portion of the tire T. The load wheel  25  may be freely rotatable such that rotation of the tire T causes the load wheel  25  to rotate. It will be appreciated that instead of driving the chuck assembly  20 , the load wheel  25  may be driven with a freely rotatable chuck assembly  20 . Once testing is complete, the load wheel  25  is retracted clearing the machine proper  15  for the next tire T. 
   Movement of the load wheel  25  into and out of the machine proper  15  is performed by a load wheel drive, generally indicated by the numeral  30 . Load wheel drive  30  is generally located adjacent the load wheel carriage  26  and in operable contact therewith to move the load wheel  25  into and out of contact with the tire T. The load wheel drive  30  generally includes a motor  31 , which may be a servo motor, as shown, a gear box  32  operatively connected with the motor  31 , and a ball screw  33  that is operatively connected with the gear box  32 . The gear box  32  typically performs a gear reduction to provide a mechanical advantage to the motor  31 . The gear box  32  may also provide a change in the direction of rotation. In the example shown, the gear box  32  provides a 90° change in the direction of the rotation. As will be understood, rotation of a motor shaft of the motor  31  causes the ball screw  33  to rotate clockwise do counterclockwise depending on the direction of rotation of the shaft of the motor  31  causing the ball screw  33  to advance or retract the load wheel  25 . Movement of the load wheel  25  may be controlled according to any method known in the art including, for example, the position-based method described in U.S. Pat. No. 6,578,275, which is incorporated herein by reference. In particular, when a tire T enters the machine proper  15  and is chucked, load wheel drive  30  positions the load wheel  25  in an initial load position (WLP). This position may be determined in a variety of ways. The initial load position may be calculated before testing and manually entered. Alternatively, when a single tire or a number of tires (one or more) of the same type have previously been tested, a final position of the last tire of the same type may be used as the initial load position for the present tire. Or, an historical average of previous tires of the same or similar type may be used to determine the initial load position. 
   With the initial load position determined, the load wheel drive  30  makes an initial move (IM) to the initial load position. Since the load wheel drive  30  is moving the carriage and load wheel  25  to a set position, this move may occur at relatively high speed, or, in other words greater than a one inch per second move typically performed in the art. Using the Indramat™ servo motor as an example, moves of up to about 4 to about 6 inches per second may be attained. It will be appreciated that substitution of other drive systems may permit moves at other speeds including speeds greater than 6 inches per second. 
   Once the load wheel  25  is in the initial load position, an average radial load (ARL) is computed from the feedback from the load wheel sensors. If the average radial load is within a selected tolerance, the load sequence is over. Otherwise, the load wheel drive  30  makes a corrective move (CM) to bring the average radial load within the tolerated level. 
   In performing the corrective move, the controller calculates a target offset position (TOP). This position is calculated based on the difference between the initial load position and the average radial load multiplied by the spring rate (K. T ) of the tire T under test.
 
TOP= ILP−K   T (ARL)  (1)
 
   Once the target offset position is determined, the load wheel drive  30  is commanded to make a relative position move in the correct direction equal to the target offset position or, in other words, absolute position move to a new position target, which is the initial load position plus the target offset position.
 
 NPT =( ILP +TOP)  (2)
 
   When this move is completed, the average radial load is recalculated with new data generated over one or more revolutions of the tire T. If the average radial load is within tolerance, the loading sequence is over. Otherwise, the corrective move sequence may be repeated until the load is within the desired tolerance. With an accurate value for the spring rate K T  of the tire T, the final load may be achieved on the initial move or after the first corrective move. 
   When the final load move has been completed, the servo control system may be used to maintain the final load position eliminating the need for a brake. 
   When loading the first tire of a new tire type it will be necessary to modify the sequence of operations since there is no historical load set point position established. One possibility is to use prior art methods to load the first tire. Alternatively, the load wheel may be advanced at a slower constant velocity, about one inch per second until the control system detects an instantaneous load of about 100 pounds force. At this point, the load wheel advancement is stopped and the average radial load is calculated with data generated during one or more revolutions of the tire T. If the spring rate K T  of the tire T is already known, the load correction move can be made in the same manner as previously described. If the spring rate K T  is not known, a second move of about 0.5 inches is commanded. Based upon a typical spring rate of about 1000 pounds per inch for passenger tires, the average radial load will increase by an additional 500 pounds force. The distance for the second move can be increased or decreased, when the load reading of the tire makes it necessary to use a smaller or larger second move. 
   During operation, tire T is brought into the tire uniformity machine  10  along conveyor  16 . Once the tire T is in registry with the chuck assembly  20 , the tire T is chucked and rotated by the motor  23 . The load wheel  25  is then brought in to contact with the periphery of the tire T, such that it rotates with tire T. Once the tire T is properly loaded by the load wheel  25 , the load cells  40  associated with the load wheel  25  measure forces (f(t)) transmitted from the tire T to the load wheel  25  and relay this information as shown schematically to controller  63 . 
   The controller  63  includes the necessary hardware, software and memory for receiving inputs and generating control signals to various components of the machine  10 . Indeed, the measured forces along with other measurements made by the tire uniformity machine  10 , allow the controller  63  to assess the characteristics of the tire T and make corrections as necessary. The controller  63  may be any controller used in connection with the operation of a tire uniformity machine and generally may include a programmable logic controller or processor having inputs and outputs for receiving respectively for receiving information from the tire uniformity machine&#39;s components and transmit control signals to control operation of the tire uniformity machine  10 . In the present example, controller  63  communicates with a servo drive controller, generally indicated at  84 . To that end, drive controller  84  has at least one output and at least one programmable input which will be described more completely below. 
   The load wheel drive assembly of U.S. Pat. No. 6,578,275 is advantageous, but is difficult to implement in existing tire uniformity machines because multiple mechanical components need to be replaced and the software controls and the loading sequence must be changed. In some existing tire uniformity machines, it is not possible to change the software and load sequence making it impossible to retrofit the drive assembly shown in U.S. Pat. No. 6,578,275 in these systems. 
   To accommodate the load wheel drive assembly used in U.S. Pat. No. 6,578,275, the chain sprocket must be removed from the output shaft of the existing right angle gear box  32 . Then, the existing right angle gear box, DC motor, and brake must be removed. The casting support  34  for the gear box is removed to access the gear&#39;s chain sprocket  35  which is removed. The screw chain sprocket  36  and chain  37  for the ball screw nut must be removed. Once all of these components are removed, a timing belt and timing belt sprocket for the screw are installed. An adapter assembly is installed in place of the existing right angle gear box  32 . And a timing belt pulley is installed on the output shaft. The support casting with adapter assembly is attached to the tire uniformity machine frame  11 . The servo motor is mounted to an adapter assembly and the timing belt is positioned between sprockets. After which, the timing belt is tensioned by moving the adapter assembly and making use of slots in the casting. Removing and replacing the components is time consuming and labor intensive. It is, therefore, desirable to provide a system that may be more easily retrofit to existing machines. 
   With reference to  FIGS. 1-4  the improved load wheel drive system may be retrofit to existing load wheel drives in the following manner. The existing DC motor and brake are removed from the right angle gear box  32 , and a servo motor  31  is attached. It may be necessary to use an adapter  38  ( FIG. 3 ) to account for a change in the motor mounts used with the DC and servo motors. As will be appreciated, if necessary, the gear box itself may be replaced with mounts acceptable for servo motor  31 . When using an adapter  38 , it may be necessary to provide a shaft extension  39  ( FIG. 3 ) for the servo motor  31  due to the thickness of the adapter  38  and allow for a change in the shaft diameter of the servo motor  31  to the shaft diameter required by the gear box  32 . 
   While existing load wheel drives use the mechanical brake whenever motion of the load wheel carriage is not desired, the brake may be omitted in the present system. In particular, as best shown in  FIG. 4 , the servo drive controller  84  may be provided with programmable digital inputs that can be used in conjunction with controller  63  to control a variety of functions. One suitable drive motor  31  is the Kollmorgen ServoStar 300/600 series servo drive or equivalent. One of the programmable inputs is programmed to switch the operating mode of the drive motor  31  from its primary mode of operation to a secondary mode of operation. The primary mode of operation is programmed for analog velocity control, and the secondary mode is programmed for position control, as described above. Consequently, by using the velocity control mode as the primary mode, the servo drive motor  31  may be retrofit to an existing system and operate without changing the drive control software. Without changing the software, the load wheel is advanced with the drive motor  31  operating in analog velocity control mode until proper loading of the tire T is achieved. Then, drive controller  84  switches to position control causing the servo motor  31  to hold the desired loading position without a mechanical brake. 
   Optionally, software for position control may be installed and the servo-drive operated in a position control mode. In this way, the system can achieve the increased speeds associated with the position control process without having to rebuild the load drive assembly. 
   The drive motor  31  ( FIG. 4 ) includes one or more feedback sensors  81  that communicate with drive controller  84  via a motor cable  82  and feedback cable  83 . In this application a Kollmorgen ServoStar 600 drive controller number S60300 and a Kollmorgen ServoStar motor number MT328A1-M2C1 with multi-turn encoder feedback was found suitable. Other drive controllers, servo motors, and feedback encoders can be used. 
   In accordance with the concepts of the present invention, in the embodiment depicted in  FIG. 4 , the drive controller  84  is powered as long as there are no emergency stop conditions on the tire uniformity machine  10  controller  63 . 
   After an emergency stop is reset, controller  63  supplies a 24 VDC control power, schematically shown at  71 , to the drive controller  84  causing it to boot-up. The drive controller  84  closes the RTR (ready to operate) contact, schematically represented at  68 , after it has successfully completed the internal boot-up cycle. The RTR contact output  68  is used to energize a 3-phase motor contact  69 , within controller  63 , that provides 460 VAC 3-phase power, schematically represented at  70 , to the drive controller  84  which in turn produces an internal DC bus voltage of about 680 DC that is transmitted to the servo drive  31  via cable  82 . The drive controller  84  is programmed to activate (open) a digital output  65 , which energizes a DC relay  66 , within controller  63 , when the internal DC bus voltage exceeds a minimum operational threshold. As a result, the normally open contact on relay  66  is closed and provides a 24 VDC signal  67  to the input  67 A (enable) on the drive. 
   When the enable relay  62  is de-energized, a normally closed contact provides a 24 VDC signal  64  to digital input  64 A on the servo drive controller. The result is the servo drive controller  84  is in position control mode with the task of maintaining the current motor position. This is the function that was previously provided by the electronically released mechanical brake. In other words, the servo motor holds the position without the need for a mechanical brake. 
   When the enable relay  62  is energized, the normally closed contact opens removing the 24 VDC signal  64  to digital input  64 A. The result is the drive controller  84  is in analog velocity mode being commanded to move the load wheel carriage  26  at the analog set point speed  61 , communicated from the controller  63  to the servo controller  84  at input  61 A. 
   Many servo motors are capable of speeds of 6,000 rpm or more. In order to prevent software changes to the existing controller  63 , the drive  31  is programmed to scale analog set point speed  61  to give the same scale factor that existed in the DC drive motor. The drive  31  may be given a larger scale factor to increase motor speed and improve cycle time. 
   Other digital inputs may be provided on drive controller  84 , and can be programmed for a variety of functions. These include but are not limited to: a) stopping motion using over travel limit switches; b) jogging the location of the load wheel while in position control mode; c) disabling the servo drive; and d) provide an alternative analog speed scaling. For example, controller  63  may activate the alternate scale input  61 A with an alternate scale to retract the loadwheel at a higher speed. In this way, an existing tire uniformity machine controller  63  could provide a shorter cycle time. It will be appreciated that any savings in cycle time is multiplied by the number of tires being tested and is of great value. 
   In light of the foregoing, it should thus be evident that a load wheel drive according to the concepts of the present invention substantially improves the art. While, in accordance with the patent statutes, only the preferred embodiment of the present invention has been described in detail hereinabove, the present invention is not to be limited thereto or thereby. It will be appreciated that various modifications may be made to the above-described embodiment without departing from the spirit of the invention. Therefore, to appreciate the scope of the invention, reference should be made to the following claims.