Patent Publication Number: US-2009237026-A1

Title: System and Method for Drive Controller Anti-Backlash Control Topology

Description:
CLAIM TO PRIORITY 
     This application claims the benefit of co-pending United States provisional patent application entitled “System And Method For Anti-Backlash Control Topology For Use With Drives” filed Mar. 18, 2008 and assigned Ser. No. 61/037,581, which is incorporated by reference herein. 
    
    
     BACKGROUND OF THE DISCLOSURE 
     1. Field of the Invention 
     The invention relates to electric drive controllers for controlling electric drives, and particularly a method and system for backlash control in motion-reversing gear trains driven by electric drives. The drive controller of the present invention provides for backlash control in gear trains that are driven by at least a pair of first and second drives without the need for external or auxiliary electro-pneumatic or electro-mechanical gear tensioners or preloaders. 
     The present invention is suitable for high or variable load transport applications that require ability to change motion direction, such as by way of non-limiting example precision handling cranes, drag lines and winches. Such load transport mechanical systems employ direction-reversing gear trains powered by one or more electric drives. The electric drives are controlled by a drive controller. The electric drive is coupled to a driving gear of the gear train that in turn is capable of moving one or more driven gears. 
     2. Description of the Prior Art 
     Typical gear trains include at least one driving gear and at least one driven gear coupled to each other in series, in parallel or a combination of both. Gear trains may be constructed to provide rotary or linear gross motion or a combination of both. Gears in a gear train may be coupled directly (i.e., tooth-to-tooth contact) or through intermediate linear drive elements, such as belts or chains. 
     Meshing gears generally have gap, called a backlash, between opposing teeth surfaces, resulting from, among other things, gear element machining variances, operational wear, compensation for thermal expansion and gear element deformation under varying loads. Backlash, especially when multiple serial or parallel gear elements in the gear train are interacting, reduce predictability and precision in gear train motion and cause a phase delay in motion response. In order to transfer motion from a driving gear element to a driven gear element the backlash must be taken up so as to allow direct contact between the respective gear element (or intermediate linear drive element) tooth surfaces. 
     In the past, backlash take-up has been accomplished by fitting gear trains with external or auxiliary gear tensioners or preload devices that bias the respective gear element teeth in direct contact with each other. The biasing elements commonly have employed mechanical springs, pressurized fluid cylinders or suspended weights. Such auxiliary tensioners add additional mechanical complexity and expense to a gear train. 
     Other backlash take-up solutions have been proposed in the past, primarily for machine tool motion control applications, to use a pair of opposed-motion electric drives interacting on a set of driving/driven gears or on a ball screw drive element. Generally such solutions have employed electric drives capable of self-reversing motion, i.e., each drive being able to rotate under power in either clockwise or counter-clockwise direction to take up backlash and then mutual cooperation to move the gear train or ball screw assembly in the desired motion direction. 
     In such machine tool motion control applications the drive controller separately controls motor speed or phase in a pair of drives that, through driven gears or ball screws, momentarily cause driven gear motion in opposing directions to take up backlash. Thereafter the pair of drives cooperate to move the driven gear in the desired direction of motion. When utilizing phase or speed control backlash take-up, a first drive motor will be driven at a desired speed or phase angle position and the second drive motor will be driven at a slower speed or different phase angle position so that each drive gear effectively pretensions the corresponding driven gear or screw. Generally, driving two drive motors at different speeds for a set time period to pretension drive and driven gears, then coordinating rotation in a common direction is not as precise as phase angle control. Matching the desired variable speeds to have sufficient dwell time to take up backlash, but not so much as to generate large opposing counter forces in the counter-rotating drives becomes an educated guess for setting control parameters in the drive controller. The prior alternative solution of precise phase angle control and feedback sensors may not be suitable for some heavy-load transmission construction and mining applications as compared to a relatively clean environment factory floor normally encountered in machine tool motion control applications. A machine screw backlash take-up utilizing independently-reversible ball screw drive motors with torque-controlled ball screw pretension for part of an operating cycle has been proposed in the past. 
     In the previously proposed backlash-take-up by counter-rotating drives solutions primarily for machine tool motion control applications, the independent drives then have coordinated rotations to cause desired translation of the driven gear. After initial pretensioning, translation of the driven gear in the desired direction is then accomplished by powering both drive motors in the same rotational direction. Generally such self-reversible drives for motion control applications are not suitable for the much higher load applications demanded by precision cranes, drag lines, winches or the like. In such high-load applications it is preferable to have a drive dedicated to rotation in a single direction. By way of example, a first drive causes motion of a driven gear in a first rotational direction and a second drive causes motion of the driven gear in the opposite rotational direction. In a precision crane application, the first motor may cause the gear train to lift the load and the second motor may cause the gear train to lower the load. 
     Thus, a need exists in the art for a method and system to control gear backlash in an application having a pair of drives that are always powered in opposite rotational directions, without the need for auxiliary gear pretensioning systems. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the invention is to control gear backlash in a gear train, without the need for separate gear tensioners or preloader apparatus. 
     These and other objects are achieved by the method and system of the present invention, for backlash control in gear trains that are driven by electric drives controlled by a drive controller. The drive controller causes the drives to generate continuously opposing torques and adjusts torque rotational offsets so as to maintain desired backlash torque and gross motion of the driven gear. 
     One aspect of the present invention is directed to a method for operating a drive controller to control gear backlash in a gear train having at least a pair of first and second driving gears and at least one commonly driven gear, the driving gears being powered by respective first and second drives that are coupled to the drive controller, comprising simultaneously powering the first drive to cause first gear rotation only in a first positive rotational direction and the second drive to cause second gear rotation only in an opposite negative rotational direction, selectively varying the respective drive torque outputs with the drive controller in order to generate continuously opposing rotational torques and adjusting torque rotational offsets so as to maintain desired backlash torque among the respective gears during driven operation of the gear train and desired gross motion of the driven gear. 
     Another aspect of the present invention is directed to a drive controller adapted to couple to at least one pair of first and second drives that are in turn coupled to respective first and second driving gears that form a gear train with at least one commonly driven gear, the drive controller comprising circuitry that simultaneously powers the first drive to cause first gear rotation only in a first positive rotational direction and the second drive to cause second gear rotation only in an opposite negative rotational direction, selectively varies drive torque outputs of the respective first and second drives so that they generate continuously opposing rotational torques and that adjusts torque rotational offsets, so as to maintain desired backlash torque among the respective gears during driven operation of the gear train and desired gross motion of the driven gear. 
     An additional aspect of the present invention is directed to a gear train backlash control system comprising a gear train having at least a pair of first and second driving gears and at least one commonly driven gear, with first and second drives coupled to the respective first and second driving gears. A drive controller is coupled to the first and second drives, having circuitry that simultaneously powers the first drive to cause first gear rotation only in a first positive rotational direction and the second drive to cause second gear rotation only in an opposite negative rotational direction. The drive controller selectively varies drive torque outputs of the respective first and second drives, so that they generate continuously opposing rotational torques. The drive controller adjusts torque rotational offsets, so as to maintain desired backlash torque among the respective gears and gross motion of the driven gear during driven operation of the gear train. 
     The present invention is also directed to drive controller software code stored in an electronic storage medium that when run by a processor of the drive controller enables the drive controller to control gear backlash in a gear train having at least a pair of first and second driving gears and at least one commonly driven gear, where the driving gears are powered by respective first and second drives that are coupled to the drive controller. The software run by the processor enables the drive controller to power simultaneously the first drive to cause first gear rotation only in a first positive rotational direction and the second drive to cause second gear rotation only in an opposite negative rotational direction. The drive controller running the software code selectively varies the respective drive torque outputs in order to generate continuously opposing rotational torques and adjusts torque rotational offsets so as to maintain desired backlash torque and gross motion among the respective gears during driven operation of the gear train. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a schematic perspective view of the backlash control system of the present invention, as applied to a rotary-motion gear train; 
         FIG. 2  is a schematic perspective view of the backlash control system of the present invention, as applied to a linear-motion gear train; 
         FIG. 3  is a schematic perspective view of the backlash control system of the present invention, as applied to a gear train including linear drive elements, such as cogged belts or chains; 
         FIG. 4  is a block diagram of the drive controller architecture of the present invention; 
         FIG. 5  is a block diagram of the drive controller control topology; and 
         FIG. 6  is an exemplary operational speed and torque profile diagram of the backlash control system of the present invention. 
     
    
    
     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. 
     DETAILED DESCRIPTION 
     After considering the following description, those skilled in the art will clearly realize that the teachings of the invention can be readily utilized in making and using the backlash control system of the present invention. 
     General System Description 
     Referring to  FIG. 1 , a rotary gear train  10  is shown in simple schematic form, having a driven gear  12  that is coupled to a shaft  14 . It is understood by those skilled in the art that the shaft  14  depicts one of many ways to generate useful output work from the gear train, for example to drive a cable winch for raising and lowering loads in a precision crane (not shown). Driven gear  12  is driven by a pair of first and second driving gears,  16 ,  18 . In  FIG. 1  the gears  12 ,  16 ,  18  are shown as simple pinion gears for exemplary purposes. As one skilled in the art will appreciate, gear trains can comprise multiple gears arrayed in series, in parallel or a combination of both. In application, gears functioning as the driving gears or the driven gears may comprise multiple gears. 
     As shown in  FIG. 1 , the first driving gear  16  is coupled to and powered by first drive  20 , an electric motor. The electric motor, by way of example, may be an induction motor in vector-control with a speed control loop or in direct torque control with speed monitoring, a reluctance motor in speed and torque control mode or a permanent magnet motor in speed control mode. Likewise, the second driving gear  18  is powered by second drive  22 . While one exemplary pair of driving gears  16 ,  18  and drives  20 ,  22  are shown in  FIG. 1 , it should be understood that multiple pairs of driving gears and drives can be utilized to drive the gear train  10 , and the single driven gear  12  can comprise a series of gears, such as a planetary gear array (not shown). When applying the present invention, gears in a gear train need not be restricted to rotary motion or direct tooth-to-tooth driving applications. 
     For example,  FIG. 2 . shows a schematic representation of a rack and pinion linear drive system  10  that includes driving gears  16 ,  18  powered by respective drives  20 ,  22  similar to the system shown in  FIG. 1 . In  FIG. 2  the driven gear is a linear-motion rack  24  that is coupled to a table  25 . In  FIG. 3 , the gear train  10  includes exemplary linear drive element cogged drive belts  26 ,  28  intermediate the respective driving gears  16 ,  18  and driven gear  12 . As one can appreciate other direct contact surface to contact surface drive elements can be substituted for toothed gears and/or belts, such as chains and toothed sprockets (not shown). 
     Referring generally to all of the system embodiments shown in  FIGS. 1-3 , the first and second drives  20 ,  22  are powered by an electric power source. Drive controller  30  separately controls electric power application to the first and second drives  20 ,  22 , enabling the drives to impart mechanical motion to the respective driving gears  16 ,  18 , and they in turn to the driven gear  12 . Thus, through the drive controller  30  selectively varying power application to the drives  16 ,  18  the output driven gear  12  speed, direction of motion and output power can be selectively varied. As will be explained below in the operational description of the present invention, gear train  10  backlash and ultimate driven gear motion is selectively controlled by the drive controller  30 . 
     As shown in  FIG. 4 , drive controller  30  preferably has at least one processor  32  that is coupled to memory  34  capable of storing software code  36  that is executable by the processor. The processor controls the drive inverter system  37  that in turn powers the gears/drives  16 / 20  and  18 / 22  The drive controller  30  preferably has at least selective connection to a human-machine interface (HMI)  38 , for example allowing an operator to specify operational parameters and/or monitor system operational performance. The HMI  38  or other communication systems, not shown, may also allow communication of the drive controller  30  to automation systems via a fixed or wireless communications network employing any communications protocol, including Internet protocols. Communication to and/or from the drive controller  30  enables remote operation, monitoring and reconfiguration, i.e., modifying software code stored in memory  34  and run by the processor  32 . 
     An exemplary drive controller  30  suitable for use in the present invention is the SINAMICS® drive controller family and torque-control operational software sold in the United States by Siemens Energy &amp; Automation, Inc. of Alpharetta, Ga., Internet website URL www.sea.siemens.com, though it should be understood that other drive controllers should be capable of being programmed to perform the gear train backlash control system and method of the present invention. In the following description, some control parameter reference designators may be those customarily used by those skilled in the art who are familiar with SINAMICS® brand drive controllers, but it should be understood that other manufacturers use other reference designations for the same control parameters in their product literature. While drive controller functions in the embodiments described herein are performed in a programmable electronic drive controller, one skilled in the art can appreciate that the operational control functions described below can be accomplished in an electro-mechanical control device or control relay employing electro-mechanical relays, dedicated-use processors, analog electronic relays, firmware controls and the like. 
     System Operational Description 
     Exemplary system operation is now described, with reference to the control topology block diagram,  FIG. 5  and operational speed and torque profile diagram,  FIG. 6 .  FIG. 5  shows that driven gear  12  is capable of clockwise or counter-clockwise desired gross motion through application of the first and second driving gears  16 ,  18 . For simplicity in understanding of  FIG. 5 , the respective first and second drive motors corresponding to the first driving gear  16  and second driving gear  18  are not shown, it being understood that reference to motor means a corresponding reference to the driving gear shown in the figure. 
     In the operating embodiments of the present invention described below, the drive controller  30  restricts first drive motor operation to clockwise rotation by application of only positive torque in the Torque CTRL  40 . Thus it follows that counterclockwise rotation of the driven gear  12  is generated by first drive motor and corresponding drive gear  16 . Similarly the drive controller  30  restricts second drive motor operation to counter-clockwise rotation by application of only negative torque in Torque CTRL  42 . It then follows that clockwise rotation of the driven gear  12  is generated by the second drive and corresponding drive gear  18 . 
     Referring again to  FIG. 5 , in order to control gear train  10  backlash, first and second drive gears  16 ,  18  preferably are powered in their respective positive and negative torque directions by application of equal absolute torque at the Additional Torque functional control block p 2900  that feeds a positive torque command p 1569  to Torque CTRL  40  and a corresponding negative torque command p 1569  to Torque CTRL  42 . In this manner gear teeth backlash is taken up between the respective gears  12 ,  16  and  18 , and the driven gear  12 , if desired, can be maintained in a stationary or neutral rotational position. In some applications applying constant equal and opposite torque may not be necessary to eliminate backlash under all operating conditions. It is again noted that the p 2900  and p 1569  designations are commonly used in SINAMIC® brand drive controller literature, and that other manufacturers use different designations in their literature. 
     When it is desired to cause operational rotational movement of driven gear  12 , such as to raise or lower a precision crane payload (not shown), as described in further detail below, the drive controller  30  through the respective Torque CTRL functions  40 ,  42  generates offsetting positive and negative torques on the driven gear  12 . All drive control preferably is effectuated through torque control. Desirably, torque control is further refined via known torque sensing feedback loops coupled to the drive controller  30 , so that torque outputs generated by each of the drives powering the driving gears  16 ,  18  is sensed by and is varied at least partially based on the sensed torque outputs. In this manner the differential between desired and sensed torque outputs is reduced. 
     As shown in  FIG. 5 , it is desirable to have the first and second drive gears  16 ,  18  and their drives controlled from the same drive controller that is capable of simultaneously controlling two drive axes and minimizing communication time between them. In this exemplary embodiment, first motor and drive gear  16  is speed controlled and the second motor and drive gear  18  is only torque controlled. As discussed above, the Torque Limit  46  is only positive for the first motor/drive gear  16  and Torque Limit  48  is only negative for the second motor/drive gear  18 . Rotational speed and direction of the driven gear  12  are specified by the output of the Speed CTRL  50 , which converts the desired rotational speed and direction to positive or negative torque commands r 1480 , that preferably are at least partially based on first drive/gear  16  speed. Desirably the drive controller  30  speed set point is compared with the actual speed sensed in motor/drive gear  16  via a known feedback loop. It follows that first motor/drive gear  16  only responds to positive torque commands and, correspondingly, second motor/drive gear  18  only responds to negative torque commands. Additional Torque p 2900  commands are fed to the respective Torque CTRL  40 ,  42  apparatus, thus generating overall cumulative offsetting torque commands to each of their respective first and second motor/drive gear pairs  16 ,  18 . 
     An exemplary inter-relationship between speed and torque set points in the respective first and second motor/drive gear  16 ,  18  pairs during acceleration, achievement of constant rotational speed, and reversal of rotational speed is shown in  FIG. 6 . During operational periods ( 1 ), ( 2 ) and ( 3 ), driven gear  12  speed has a counter-clockwise (CCW) direction, while during the ( 4 ), ( 5 ) and ( 6 ) periods the drive gear  12  speed is clockwise (CW). At all times both electric motor drives  20 ,  22  are commanded to develop an Additional Torque p 2900 . This p 2900  torque value is represented in the torque profile graph of  FIG. 6  by a dashed line and also as a cross-hatch when it overlaps with other commanded torque values on each of motor drives  20 ,  22 . 
     During the acceleration time period ( 1 ) the first gear  16 /motor drive  20  takes control and develops the inertial torque necessary to accelerate driven gear  12 . The inertial torque is added on top of the already present backlash compensation torque p 2900 . 
     At constant counter-clockwise speed shown in time period ( 2 ) the first gear  16 /motor drive  20  is producing enough torque to compensate for the backlash counter-torque generated by gear  18 /motor drive  22 , any mechanical power dissipation losses in the driven system, such as friction, and any load oscillations on the driven gear  12 . In this mode of operation the gear  16 /motor drive  20  is generating sufficient power necessary to maintain the desired system steady state. 
     Time period ( 3 ) of the motion profile refers to a deceleration. If the present invention were not practiced during deceleration, first drive/drive gear  16 / 20  normally should be commanded to brake the motion, because the speed controller output is a negative value. However, the drive control topology of the present invention has the two torque limitation blocks  46 ,  48  that prevent the torque in first drive/drive gear  16 / 20  to go negative and conversely prevents the second drive/drive gear  18 / 22  to go positive. During the operational period ( 3 ) deceleration, the second drive/drive gear  18 / 22  takes control of the motion and in fact accelerates the motion of driven gear  12  towards the clockwise direction. 
     The torque profile repeats in opposite direction during operational time periods ( 4 ), ( 5 ) and ( 6 ) with second drive/drive gear  18 / 22  being now the main actor in producing the motion of the driven gear  12 . It should be noted that the exemplary torque profile shown in  FIG. 6  is for an equal and opposite magnitude oscillating directional load. In other applications the torque profile will vary in direction, magnitude, acceleration and time periods. 
     Over the whole speed profile, the first and second drive motors  20 ,  22  are controlled to develop an opposing torque which in fact is keeping the teeth of the respective drive and driven gears  12 ,  16 ,  18  from losing direct contact and thus eliminating gear train backlash from the system. 
     Although various exemplary embodiments which incorporate the teachings of the present invention have been shown and described in detail herein, those skilled in the art can readily devise many other varied embodiments that still incorporate these teachings. Accordingly, it is intended that the scope of the present be defined by the accompanying claims given their broadest interpretation allowable by law, rather than being limited by the exemplary embodiments described above that are intended to help those skilled in the art understand how to make and use the subject invention.