Abstract:
The present method can determine the inertia of an electric motor and the load driven by the electric motor in situations where the electric motor accelerates or decelerates in a non-linear manner. During acceleration and deceleration, the torque produced by the electric motor is sampled periodically and the average values for the acceleration torque and deceleration are calculated. The rates of acceleration and deceleration also are determined. These values are employed to derive a value indicating the inertia.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     Not Applicable  
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     Not Applicable  
       BACKGROUND OF THE INVENTION  
       [0003]     1. Field of the Invention  
         [0004]     The present invention relates to apparatus and methods for controlling operation of electric motors, and more particularly to determining motor inertia.  
         [0005]     2. Description of the Related Art  
         [0006]     Some industrial electric motors are operated by a motor drive which responds to a velocity command by applying electricity to the motor in a manner that causes the motor to operate at the commanded velocity. In a typical motor drive, the velocity command is compared to a measurement of the actual velocity of the motor to produce a commanded torque indicating how the motor&#39;s operation needs to change in order to achieve the commanded velocity. For example, to accelerate the motor a positive commanded torque is produced, whereas a negative commanded torque is required to decelerate the motor.  
         [0007]     The amount of torque that is required to produce a given change in velocity is a function of the inertia of the motor and the mechanical apparatus being driven. The inertia in a typical industrial installation is determined and programmed into the motor drive upon commissioning the motor. Thus it is desirable to provide a mechanism for accurately estimating that inertia.  
         [0008]     The traditional process for estimating the motor system inertia involves operating the motor through a linear acceleration/deceleration profile. If the velocity changes at a constant rate, the motor torque is constant during both acceleration and deceleration and it is relatively straight forward to calculate the inertia. This is the case with drive systems that have regenerative capabilities, i.e. where the electric current induced in the stator coils during deceleration is able to flow unrestricted back into the DC supply bus for the motor drive. However, many present day motor drives incorporate a bus regulator which limits the voltage on the DC supply bus and thus restricts the regenerative capability. As a consequence, the deceleration is non-linear which can adversely affect traditional inertia estimating techniques.  
         [0009]     Therefore, a different method for estimating the inertia is needed, one that does not require a linear acceleration/deceleration profile.  
       SUMMARY OF THE INVENTION  
       [0010]     The inertia of an electric motor and a load connected to the motor is estimated by accelerating the motor from a first velocity to a second velocity. The acceleration rate is determined and the amount of torque produced by the motor is ascertained. Thereafter, the motor is decelerated from a third velocity to a fourth velocity. As the motor decelerates, the rate of deceleration is determined and an average amount of torque produced by the motor during the deceleration is detected. Specific procedures for determining the rates of acceleration and deceleration and the torque values are described herein.  
         [0011]     An inertia value is calculated as a function of the amount of torque, the average amount of torque, the rate of acceleration, and the rate of deceleration. In the preferred embodiment, the inertia value is computed by dividing a sum of the amount of torque and the average amount of torque by a sum of the magnitudes of the acceleration and deceleration rates. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  is a functional block diagram of motor drive for controlling operation of an electric motor;  
         [0013]      FIG. 2  is a functional block diagram of an inertia module in the motor drive;  
         [0014]      FIG. 3  is a flowchart of the present method for estimating the motor inertia; and  
         [0015]      FIGS. 4A  and B are graphs depicting variation of velocity and torque, respectively, as a motor accelerates and the decelerates. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]     With initial reference to  FIG. 1 , a motor drive  10  controls a three-phase electric motor  12 , which drives mechanical elements on a machine. The motor drive  10  has a control panel  14  which serves as a user interface by providing a keypad and a display through which an operator can enter commands and receive information about the motor&#39;s performance. The control panel  14  is connected to a system controller  16  which governs the operation of the motor drive. The system controller may have additional inputs and outputs through which commands and motor performance data are exchanged with external control devices. The system controller  16  supplies control signals via line  15  to other components of the motor drive  10 , as will be described. The functions of the system controller  16  and the other components of the motor drive are performed by one or more microcomputers which execute software programs that implement those functions.  
         [0017]     In response to these commands, the system controller  16  produces a velocity command ωc that indicates a desired velocity for the motor  12 . The velocity command ωc is applied to a standard velocity regulator  18  which also receives a position signal Mp from an encoder  20  attached to the motor  12 . The encoder  20  provides either a digital word indicating absolute angular position of the shaft of the motor or a series of pulses indicating incremental motion and direction. By monitoring the change of that position signal Mp with time, the velocity regulator  18  is able to determine the actual motor velocity. The velocity regulator  18  produces a commanded torque τc in response to the relationship between the commanded velocity and the actual velocity. The commanded torque is generated in a conventional manner and indicates how the motor  12  should be operated in order to achieve the commanded velocity. For example, if the motor is operating slower than the commanded velocity, a positive torque has to be generated in the motor in order to increase its velocity. Similarly, a negative commanded torque is generated when the motor is operating faster then desired.  
         [0018]     The commanded torque produced on line  22  is applied to an input of a conventional motor control  24  which responds by producing a set of control signals for a standard PWM inverter  26 . The control signals operate the PWM inverter  26  which switches DC voltage from an AC-DC converter  25  to generate PWM waveforms that are applied to the stator coils of the three-phase electric motor  12 . The PWM waveforms are varied to control the motor velocity, as is well understood by those skilled in motor control.  
         [0019]     In addition to receiving the velocity command ωc and the encoder signal, the velocity regulator  18  also receives a value indicating the inertia Ĵ of the motor  12  and the mechanical system driven by the motor, hereinafter collectively referred to as the “motor system”. The inertia is used to set circuit gains in the velocity regulator  18 . The value of the inertia is supplied by an inertia module  28 , the details of which are shown in  FIG. 2 . The inertia module  28  contains a velocity detector  30  which processes the position signal Mp from the encoder  20  to determine the actual velocity of the motor  12 . A torque sampler  32  periodically acquires samples of the commanded torque τc produced by the velocity regulator  18 . A timer  34  is provided to measure time intervals between certain events which will be described. The data produced by the velocity detector  30 , the torque sampler  32 , and the timer  34  are sent via a data bus  31  to a storage device  35 , such as the memory of the microcomputer that implements the inertia module  28  functionality. An arithmetic and control unit  36  controls operation of the via control lines  33  and derives the estimate of the motor system inertia from the stored data, as described subsequently. The resultant inertia value Ĵ is held in an output register  38  from which it is communicated to the velocity regulator  18 .  
         [0020]     The inertia of the motor system is relatively constant and needs to be determined only upon initial commissioning of the motor  12  or whenever changes are made to the motor system which affect its inertia. On those occasions, the operator enters the appropriate command into the control panel  14  which causes the motor drive to commence an inertia determination procedure. That operator command causes the system controller  16  to issue a control signal over lines  15  which instructs the inertia module  28  to enter the determination mode. The inertia can be determined even when a constant load is applied to the motor.  
         [0021]     The inertia determination procedure  40  is depicted by the flowchart in  FIG. 3  and operates the motor through a velocity profile shown in  FIG. 4A  which has an acceleration phase  70  and a deceleration phase  72 . This procedure begins at step  42  where the system controller  16  in  FIG. 1  produces a velocity command ωc to operate the motor  12  at a relatively slow initial velocity Vi. The velocity regulator  18  responds to this velocity command by issuing a corresponding torque command τc to the motor control  24 . This causes the motor control  24  to operate the PWM inverter  26  in a manner that applies electricity to accelerate the motor  12  to the initial velocity Vi at profile plateau  74 . Operating the motor at a relatively slow initial velocity prior to acquiring data for the inertia computation ensures that any lost motion in a transmission or mechanical linkage of the motor system occurs before data acquisition. At step  44 , the arithmetic and control unit  36  detects when this initial velocity has been achieved.  
         [0022]     When that occurs, a control signal is sent to the system controller  16  which responds at step  46  by issuing a new commanded velocity designating a test velocity Vt to which the motor  12  is to accelerate. Then at step  48 , the timer  34  is reset and started to measure the amount of time to accelerate to the test velocity. Thereafter the arithmetic and control unit  36  examines the output data from the velocity detector  30  at step  50  to determine whether the motor  12  has reached the test velocity. If that is not yet the case, the inertia determination procedure  40  branches to step  52  at which the torque sampler  32  stores the value of the commanded torque τc in storage device  35 .  FIG. 4B  illustrates the variation in torque during the exemplary velocity profile in  FIG. 4A . Because of the nature of the feedback control loop that governs motor operation, it can be accurately assumed that the actual torque produced by the motor equals the commanded torque τc, eliminating the need to measure the torque of the motor directly. Therefore the sampling of the commanded torque can be considered as sampling the motor torque. The inertia determination procedure continues to loop through steps  50  and  52  taking samples periodically until the motor  12  reaches the test velocity Vt at point  76  on velocity profile of  FIG. 4A . When the motor reaches the test velocity Vt, the value of the timer  34 , corresponding to the interval between times T 1  and T 2 , is stored in the storage device  35  at step  54 .  
         [0023]     Then at step  55  the inertia determination procedure  40  delays for a brief period to ensure that the motor operation stabilizes at the test velocity Vt.  
         [0024]     Following that delay, the inertia module&#39;s arithmetic and control unit  36  resets the timer  34  to measure the duration of the deceleration phase  72  at step  56  and issues a control signal which causes the system controller  16  to produce a zero velocity command (ωc=0) at step  57 . A velocity regulator  18  responds to the zero velocity command by producing a negative commanded torque τc to stop the motor  12 . In many motor drives the velocity command during deceleration is limited in response to various control parameters. For example, the AC-DC converter  25  includes a regulator which limits the voltage on the DC supply bus between the AC-DC converter and the PWM inverter  26 . During deceleration, electric current that is induced in the motor&#39;s stator coils by the rotating magnetic field flows into the DC supply bus. If that current produces an over voltage condition the AC-DC converter  25  activates the torque limiter  23  to reduce the commanded torque during deceleration. Thus although the velocity regulator is producing a constant negative commanded torque, the torque command value at the input of the motor control  24  varies due to system limiters. This dynamic limiting results in a non-linear deceleration and a varying motor torque as shown in  FIGS. 4A and 4B .  
         [0025]     Next at step  58 , the arithmetic and control unit  36  begins examining the velocity signal from the encoder  20  to determine whether the motor has stopped, i.e., reached zero velocity. Until that occurs, the procedure periodically acquires samples of the torque command which are placed into storage at step  60 . Eventually when the motor  12  stops, the inertia determination procedure branches to step  62  at which the value of the timer  34 , corresponding to the interval between times T 3  and T 4 , is stored in storage device  35  which completes the data acquisition.  
         [0026]     Then at step  64 , the stored torque samples and timer readings are employed to calculate the inertia of the motor system. The inertia is derived according the following equation, which is solved by the inertia module&#39;s arithmetic and control unit  36 :  
               J   ^     =                1   N     ·       ∑     i   =   1     N     ⁢     τ   ⁢           ⁢     a   ⁡     (   i   )                  +            1   M     ·       ∑     k   =   1     M     ⁢     τ   ⁢           ⁢     d   ⁡     (   k   )                               Δ   ⁢           ⁢   Va     ta          +            Δ   ⁢           ⁢   Vd     td                      (   1   )             
 
 where Ĵ is inertia in seconds, N is the number of torque samples acquired during the acceleration phase  70 , τa(i) is the ith torque sample acquired during motor acceleration, M is the number of torque samples acquired during the deceleration phase  72 , τd(k) is the kth torque sample acquired during motor deceleration, ΔVa is the net velocity change (Vt−Vi) during acceleration, ΔVd is the net velocity change (−Vt) during deceleration, ta is the acceleration time (T 2 −T 1 ), and td is the deceleration time (T 4 −T 3 ). The above equation employs the absolute values of the terms. 
 
         [0028]     It should be appreciated that the average torque during acceleration is a positive value, whereas the average torque during deceleration is a negative value. Similarly the change in velocity ΔVa during acceleration phase is positive, and the velocity change ΔVd during the deceleration phase is negative. Therefore, equation (1) may be rewritten as:  
               J   ^     =           1   N     ·       ∑     i   =   1     N     ⁢     τ   ⁢           ⁢     a   ⁡     (   i   )             -       1   M     ·       ∑     k   =   1     M     ⁢     τ   ⁢           ⁢     d   ⁡     (   k   )                     Δ   ⁢           ⁢   Va     ta     -       Δ   ⁢           ⁢   Vd     td                 (   2   )             
 
 in which the absolute values are not required and the plus signs have been replaced with minus signs (subtracting a negative value is equivalent to adding the absolute value of that negative value). Both methods can be generically referred to as summing the magnitudes of the respective values. 
 
         [0030]     Therefore, the determination of the inertia Ĵ separately averages torque of the motor during the acceleration phase and the deceleration phase and then sums the magnitudes of those averages. The change in velocity during acceleration is divided by the acceleration time to derive the rate of acceleration. A similar derivation of the rate of deceleration involves dividing the change in velocity during the deceleration phase by the deceleration time. The inertia Ĵ of the motor system is the average torque magnitude sum divided by the sum of the magnitudes of the acceleration rate and the deceleration rate.  
         [0031]     In installations where the motor always will accelerate at a constant rate, i.e. the acceleration phase is linear, a single torque sample can be acquired. The value of that single torque sample τa 1  then is used in place of the average of the acceleration torque in the computation of the inertia as given by the expression:  
               J   ^     =                τ   ⁢           ⁢   a1          +            1   M     ·       ∑     k   =   1     M     ⁢     τ   ⁢           ⁢     d   ⁡     (   k   )                               Δ   ⁢           ⁢   Va     ta          +            Δ   ⁢           ⁢   Vd     td              .             (   2   )             
 
         [0032]     Once the motor system inertia has been determined, it is stored in an output register  38  of the inertia module  28  and applied as an input to the velocity regulator  18 .  
         [0033]     The foregoing description was primarily directed to a preferred embodiment of the invention. Although some attention was given to various alternatives within the scope of the invention, it is anticipated that one skilled in the art will likely realize additional alternatives that are now apparent from disclosure of embodiments of the invention. Accordingly, the scope of the invention should be determined from the following claims and not limited by the above disclosure.