Abstract:
An integrated gearbox/encoder and control system that includes: a gearbox with an output shaft connected to a mechanical load; a first sensor detecting the rotary position of the output shaft; a motor; a second sensor detecting the rotary position of the motor; and a system controller controlling motive drive to the motor. The two rotary position sensors permit direct determination of gearbox backlash which can be used in motor control. A drive current sensor similarly permits determination of a vibration signature for comparison with a standard.

Description:
CLAIM OF PRIORITY 
       [0001]    This application claims priority under 35 U.S.C. 119(e)(1) to U.S. Provisional Application No. 61/139,765 filed Dec. 22, 2008. 
     
    
     TECHNICAL FIELD OF THE INVENTION 
       [0002]    The present invention relates to motor control systems and particularly an integrated gearbox/encoder and control system. 
       BACKGROUND OF THE INVENTION 
       [0003]    It is common practice to combine electric motors and gear reduction systems to reduce system speed to a desired operating point. All gear systems have an internal loss of rotational motion called backlash. Errors created by system backlash make it impossible for a controller to accurately position a load connected to the gearbox if the only source of position information is attached to the motor. A common solution adds an additional position feedback device on the load side as the master position reference. This configuration of motor and sensors is widely used, it does not take full advantage of the information available to improve the control system performance. 
       SUMMARY OF THE INVENTION 
       [0004]    This invention is an integrated gearbox/encoder and control system that includes: a gearbox with a first output shaft that couples to a mechanical load; a first integrated sensor that determines the position of a first output shaft; a motor with a second output shaft; a second sensor that determines the rotary position of the second output; and a system controller coupled to motor drive electronics and the first second sensors. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]    These and other aspects of this invention are illustrated in the drawings, in which: 
           [0006]      FIG. 1  is a schematic diagram of the invention; and 
           [0007]      FIG. 2  illustrates the dynamic compensation provided by the disclosed invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0008]    This invention is an integrated gearbox/encoder and control system. This application describes numerous specific details in order to provide a thorough understanding of the present invention. One skilled in the art will appreciate that one may practice the present invention without these specific details. Additionally, this disclosure does not describe some well known items in detail to not obscure the present invention. 
         [0009]      FIG. 1  illustrates the parts of this invention. Gearbox or jack screw  1  has an output shaft coupled to mechanical load  2 .  FIG. 1  illustrates no particular geometry for gearbox  1 . The position of mechanical load  2  is to be controlled. Sensor  3  is integrated into gearbox  1 . Sensor  3  may be magnetic, optical or based on strain. Sensor determines the rotary position of the output shaft of gearbox  1 . Motor  4  has a rotary position sensor  5  rigidly coupled to the shaft, a controller  6  and motor drive power electronics  7  commanded by controller  6 . Motor  4  may be electric, pneumatic or hydraulic. Rotary position sensor  5  may be may be magnetic, optical or based on strain. Controller  6  is preferably a proportional, integral and derivative controller. Current sensor  8  measures the drive current from motor drive electronics  7  to motor  4 . 
         [0010]    In any similarly configured system the two position sensors providing position information leads to several benefits permitting the creation of new control software. Such new control software would improve performance by incorporating information into the control system model and making adjustments to the velocity, position or toque values. 
         [0011]    On startup controller  6  can automatically advance and reverse the motor at slow speed and low torque just enough to cause engagement of the gears in either direction. Controller  6  compares the motion of the motor measured by rotary position sensor  5  to the motion of the load measured by sensor  3 . Controller  6  can thus measure the actual mechanical backlash and save this measured value. Controller  6  may periodically measure this backlash. An increase in the measured value of the backlash over time indicates wear in the system. Controller  6  may be programmed with a limit for the wear as a criteria for repair or replacement. 
         [0012]      FIG. 2  illustrates a flow chart example of this backlash determination. Flow chart  200  begins with start block  201 . In the preferred embodiment controller  6  makes a backlash determination upon each initial application of electric power, start up. It is also possible to periodically make this backlash determination following start up. Flow chart  200  advances motor  4  at low speed and low torque. 
         [0013]    Test block  203  determines if motion is detected in load sensor  3 . If no motion is detected in load sensor  3  (No at test block  203 ), then test block  203  repeats. This repeated test takes place while controller  6  continues to control motor  4 . If motion is detected in load sensor  3  (Yes at test block  203 ), then block  204  stores the detected motor position M 1  and the detected load position L 1  and reverses the drive to motor  4 . 
         [0014]    Test block  205  determines if motion is detected in load sensor  3 . If no motion is detected in load sensor  3  (No at test block  205 ), then test block  205  repeats. This repeated test takes place while controller  6  continues to control motor  4 . If motion is detected in load sensor  3  (Yes at test block  205 ), then block  206  stores the detected motor position M 2  and the detected load position L 2  and stops the drive to motor  4 . 
         [0015]    Block  207  calculates the backlash. This backlash calculation is based upon the difference in the change in the motor sensor  5  detected positions and the change in the load sensor  3  detected position. Thus the backlash BL is give by: 
         [0000]    
       
         
           
             
               
                 
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         [0000]    The control of flow chart  200  continues via continue block  208 . This determined backlash can be stored and used in later control as outlined below. 
         [0016]      FIG. 3  illustrates flow chart  300  of an embodiment of the use of measured backlash in this invention. Flow chart  300  begins with start block  301 . Block  302  performs a backlash measurement. This backlash measurement could be made as illustrated in  FIG. 2 . As previously noted, this backlash measurement could be preformed upon each initial application of electric power to controller  6  or at periodic intervals during system operation. 
         [0017]    Test block  302  compares the current measured backlash cBL with a prior stored initial backlash iBL. Test block  302  determines whether the current backlash cBL is less than or equal to the sum of the initial backlash iBL and an empirically determined constant α. If this is true (Yes at test block  303 ), flow chart  300  continues at continue block  304 . If this is not true (No at text block  303 ), then block  305  flags a remedial operation such as repair or replacement. As previously noted, this increase in the current measured backlash above an initial backlash indicates wear in the drive system indicating a need of remedial action. 
         [0018]      FIG. 4  illustrates flow chart  400  showing another use of the measured backlash. Flow chart  400  begins at start block  401 . Test block  402  determines whether controller  6  orders a motor direction reverse. This test takes place while controller  6  continues to control motor  4 . If there is no direction reverse (No at text block  402 ), then flow chart  400  continues at continue block  403 . If controller  6  orders a motor direction reversal (Yes at text block  403 ), then block  404  alters the control program. Block  404  adds the current measured backlash to the position term in the control program of controller  6 . This addition of the backlash amount accounts for the amount of rotation of motor  4  before motion at the output of gearbox  1 . This alternation of the control program of controller  6  permits controlled operation through the backlash of the motor reversal. 
         [0019]    In any motion system there are inflection points where the acceleration force changes from a non-zero value to a zero value.  FIG. 5  illustrates an example of a prior art control sequence commanded by controller  6 .  FIG. 5  illustrates a velocity profile over time. Region  501  is before the motor is controlled. During region  501  the position is constant and both the velocity and acceleration are 0. Region  502  is an acceleration region. During region  502  the load position changes. The load velocity is increasing and the acceleration is greater than 0. Region  502  represents an initial acceleration of the load.  FIG. 5  illustrates a constant acceleration by the constant velocity slope. Such a constant acceleration is not required. The load acceleration may vary during region  502 . Region  503  represents a constant velocity region. The load position changes continually, the load velocity is constant and the load acceleration is 0. Region  504  is a deceleration region. During region  504  the load position changes. The load velocity is decreasing and the acceleration is less than 0. Region  504  represents deceleration of the load.  FIG. 5  illustrates a constant deceleration by the constant velocity slope. Such a constant deceleration is not required. The load deceleration may vary during region  504 . Region  505  is after the controlled operation. In region  505  the position is constant and the velocity and acceleration are 0. 
         [0020]      FIG. 5  illustrates several inflection points of changing acceleration. The first of these is at point  511  where the acceleration changes from 0 to a non-zero positive value. When the load has been accelerated up to a constant target speed, the acceleration then changes from non-zero to 0 with the load running at the desired speed. This occurs at point  512 . At point  513  the acceleration changes from 0 to a non-zero negative value. Finally at point  514  the acceleration changes from a negative value to 0. Following point  514  the load is a rest at a constant position with both the velocity and acceleration 0. 
         [0021]    At these inflection points  511 ,  512 ,  513  and  514  gearbox  1  becomes unloaded. Normally there is some instability in the control system usually resulting in momentary overshoot. Generally there is a high impact load called jerk that can damage gear teeth as the gears return to a meshed state. Motor control can be improved by taking into account the dynamic property of the backlash by adjusting the torque and speed loops to smoothly engage the gears as the system transitions through inflection points, thereby eliminating stress. 
         [0022]    In the displacement profile illustrated in  FIG. 5  the inflection points are easy to identify. When the system is at rest, zero velocity, it is impossible to determine the engagement of the gears. As the motor starts, backlash is immediately taken up by the rotation of the motor causing jerk or impact stress as the gears begin to move. When the system stops accelerating, and tries to achieve constant speed, another inflection point occurs during which the load can overrun slightly and the gears become momentarily disengaged. In this way, every time acceleration changes from a zero to a non-zero state, inflection points occur which present abnormal stresses to the gears and which cannot be adequately compensated in the control system. 
         [0023]      FIG. 6  illustrates flow chart  600  of a proposed embodiment of the invention including a second control loop around the two feedback devices. This permits precise regulation of the stress created by the backlash in its various states. Flow chart  600  begins with start block  601 . Test block  602  determines if the control profile is near points  511  or  513 . Each of these control points are where the acceleration changes from zero to non-zero. If this is true (Yes at test block  602 ), then control switches from the programmed command profile such as illustrated in  FIG. 5  to a second control loop. Block  603  controls motor  4  at a low acceleration until the position change detected by motor sensor  5  moves an amount corresponding to the previously measured backlash BL past the position change detected by load sensor  3 . Block  603  exits the second control loop and returns to the initial command profile upon reaching this point. Reaching this point assures that the backlash is wound out of gearbox  1 . Command control can proceed with the assurance that there will be a minimal jerk within gearbox  1 . 
         [0024]    Whether test block  602  did not detect proximity to points  511  or  513 , or block  603  completes, test block  604  determines if the control profile is near points  512  or  514 . Each of these control points are where the acceleration changes from non-zero to zero. If this is true (Yes at test block  604 ), then control switches from the programmed command profile such as illustrated in  FIG. 5  to a second control loop. Block  604  controls motor  4  to reduce the acceleration upon nearing the inflection point  512  or  514 . The interval before the inflection point for acceleration reduction depends on the amount of backlash BL. A large BL requires a large offset from the inflection point. A small BL requires a smaller offset. This torque reduction reduces or eliminates the torque overrun upon reaching a steady state condition reducing the jerk upon remeshing the gears. Whether test block  604  did not detect proximity to points  512  or  514 , or block  604  completes, control continues with the main command profile at continue block  606 . 
         [0025]    Normal torque control in a motor drive involves providing sufficient current to allow the motor to turn the attached load. Since most motor control systems involve monitoring of current the motor current is readily available to the control system.  FIG. 1  illustrates current sensor  8  as part of motor drive electronics  7 . Mechanical loads that have eccentric properties, such as cams or rotary knives, require current compensation to maintain commanded speed. 
         [0026]    By precisely measuring the torque loop waveforms via current sensor  8  it is possible to measure vibration signatures in the gearbox. Subtle vibration in the gears are commonly described in manufacturer&#39;s specifications as torque ripple. Since the torque ripple of a gear system is a function of the mechanical construction details, a signature waveform will appear as noise in the torque loop of the control system. 
         [0027]    If the vibration levels exceed some determined normal operating range, it would indicate an impending failure in the mechanism. This out of tolerance condition can be reported through the control system and acted upon as a preventive maintenance event instead of a catastrophic failure. 
         [0028]      FIG. 7  illustrates flow chart  700  of an embodiment of the use of measured vibration profile in this invention. Flow chart  700  begins with start block  701 . Block  702  performs a torque profile measurement. 
         [0029]    Test block  702  compares the current measured torque profile with a prior stored initial torque profile. Test block  702  determines whether the torque profile is within predetermined tolerances of the prior torque profile. If this is true (Yes at test block  703 ), flow chart  700  continues at continue block  704 . If this is not true (No at text block  703 ), then block  705  flags a remedial operation such as repair or replacement. As previously noted, this out of tolerance torque profile indicates an impending failure. 
         [0030]    Gear reducers and other rolling mechanisms have a certain deflection characteristic called torsion which is intrinsic to the materials used in their design. Over time, repeated cycling will load the materials and they will fatigue. This fatigue is a unique material property which substantially reduces the deflection of the parts. By comparing the measured flexure of the system and observing the change in the deflection of the gear train, a threshold of performance can be established that indicates that catastrophic failure may be imminent. Thus the measurement of deflection by the control system comparing the motor position with the gear system output position leads to a new and novel means of preventing fatigue failure in the gear components. 
         [0031]      FIG. 8  illustrates flow chart  800  of an embodiment of the use of measured gearbox flexure/deflection in this invention. Flow chart  800  begins with start block  801 . Block  802  performs a flexure/deflection measurement. 
         [0032]    Test block  802  compares the current measured flexure/deflection with a prior stored initial flexure/deflection. Test block  802  determines whether the flexure/deflection is within predetermined tolerances of the prior flexure/deflection. If this is true (Yes at test block  803 ), flow chart  800  continues at continue block  804 . If this is not true (No at text block  803 ), then block  805  flags a remedial operation such as repair or replacement. As previously noted, this out of tolerance flexure/deflection indicates an impending failure.