Patent Publication Number: US-10784803-B2

Title: Position control device

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present invention claims priority under 35 U.S.C. § 119 to Japanese Patent Application No. 2018-119061 filed on Jun. 22, 2018, the entire content of which is incorporated herein by reference. 
     TECHNICAL FIELD 
     The present specification discloses a position control device that controls current for an electric motor (for example, a synchronous motor or an induction motor) and thereby controls a speed or rotation angle (position) of a feed axis or main axis and adopts a PWM invertor as a power converter. 
     BACKGROUND 
     A position control device that controls the speed or rotation angle (position) of the feed axis or main axis of a numerical control machine tool often controls a three-phase synchronous motor or induction motor (hereinafter also referred to as a motor) to drive a load such as a table or workpiece connected to the motor. In this configuration, a PWM inverter is usually adopted as the power converter. 
     Conventionally, motor control requires use of a two-phase rotational coordinate system (generally, a magnetic pole direction is referred to as a d-axis and its electrically orthogonal direction as a q-axis) and calculation of two-phase voltage command values (vd*, vq*) for controlling motor current, as well as application of three-phase conversion and determination of three-phase control voltage command values (vu*, vv*, vw*). Furthermore, the three-phase control voltage command values are power-amplified by the PWM inverter to serve as phase voltages (vu, vv, vw) for driving the motor. 
     However, conventional position control devices have a problem in that a voltage output becomes discontinuous when a current direction reverses due to an impact of a dead time that occurs at the time of switching of a power conversion element such as a transistor adopted in the PWM inverter, and a DIF ripple of a current direction reversal number occurs in position deviation DIF, disturbing a work machining surface, whereby machined surface quality decreases. 
     For the problem of decrease in the machined surface quality caused by the DIF ripple due to the dead time, the impact has been conventionally reduced by dead time compensation as described above. However, it is not easy to accurately detect the current direction and precisely switch a dead time compensation value, and there has been a limit to the DIF ripple reduction by the dead time compensation. 
     The present specification discloses a position control device that reduces the DIF ripple due to the dead time by improving voltage disturbance suppression performance and that achieves improvement of command following performance and optimization of command response timing. 
     SUMMARY 
     A position control device disclosed herein includes an error amplifier that amplifies a q-axis current error by I-P (Integration-Proportion) control, and adds a q-axis voltage feedforward amount corresponding to a q-axis current command time derivative value to a q-axis voltage command value, and adds a q-axis current compensation amount for optimizing response timing of q-axis current to a q-axis current error. 
     By configuring a q-axis current control loop by the I-P (Integration-Proportion) control, it is possible to improve voltage disturbance suppression performance and reduce a DIF ripple due to the dead time. In addition, by the q-axis voltage feedforward corresponding to a jerk command value, it is possible to compensate for a q-axis voltage error and improve position command value following performance. Furthermore, since the q-axis current compensation amount allows for adjusting q-axis current response timing, DIF occurrence during acceleration/deceleration operation can be suppressed, and a positional synchronous relationship with other control axes operated by PI control can be maintained. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Embodiment(s) of the present disclosure will be described based on the following figures, wherein: 
         FIG. 1  is a block diagram illustrating an exemplary configuration of a position control device; 
         FIG. 2  is a graph showing an example of a speed loop frequency characteristic (ωm/ωm*) in the position control device of  FIG. 1 ; 
         FIG. 3  is a graph showing an example of a position loop frequency characteristic (iq/vdis) in the position control device of  FIG. 1 ; 
         FIG. 4  is a graph showing an example of a DIF ripple due to a dead time in the position control device of  FIG. 1 ; 
         FIG. 5  is a graph showing an example of a following error during acceleration/deceleration in the position control device of  FIG. 1 ; 
         FIG. 6  is a graph showing an example of a following error during acceleration/deceleration when a q-axis current compensation amount is adjusted; 
         FIG. 7  is a block diagram illustrating an exemplary configuration of a position control device of a comparative example; 
         FIG. 8  is a graph showing an example of a speed loop frequency characteristic (ωm/ωm*) in the position control device of the comparative example; 
         FIG. 9  is a graph showing an example of a position loop frequency characteristic (iq/vdis) in the position control device of the comparative example; 
         FIG. 10  is a graph showing an example of a DIF ripple due to a dead time in the position control device of the comparative example; 
         FIG. 11  is a graph showing an example of a following error during acceleration/deceleration in the position control device of the comparative example; 
         FIG. 12  is a circuit diagram illustrating a voltage output unit for one phase of a PWM inverter; 
         FIG. 13  is a time chart illustrating a control voltage command value and an output phase voltage for one phase of the PWM inverter; 
         FIG. 14  is a diagram illustrating an input-output relationship (control voltage command value-output phase voltage) for one phase of the PWM inverter. 
     
    
    
     EMBODIMENTS 
     First, a DIF ripple will be explained with reference to a comparison example.  FIG. 12  is a circuit diagram illustrating a voltage output unit for one phase of a PWM inverter. By performing high-speed switching control of on/off of a transistor Tr 1  and a transistor Tr 2 , a phase voltage V output from a circuit of DC voltage Vdc can be changed between 0 and Vdc in average. A control voltage command value vo is to command on/off of the two transistors, V=Vdc while the TR 1  is on, and V=0 while the TR 2  is on. A diode D 1  and a diode D 2  are for reflux. 
       FIG. 13  shows an output phase voltage V when the transistor Tr 1  and the transistor Tr 2  are switched by a certain control voltage command value vo. The upper side of  FIG. 13  is an example of the control voltage command value vo. A dead time Td is a pause time for preventing both transistors from turning on simultaneously and causing short circuit breakdown. 
     The lower side of  FIG. 13  is an output phase voltage V of the PWM inverter at this time. During current outflow OUT, the transistor Tr 1  is off and current flows through the diode D 2 , and the output voltage becomes 0, and during current inflow IN, the transistor Tr 2  is off and current flows through the diode D 1 , and the output voltage becomes Vdc. That is, even if the same control voltage command value vo is applied to the transistors, a difference due to current directions occurs in an output average phase voltage V. 
       FIG. 14  is a diagram illustrating an input-output relationship for one phase of the PWM inverter with the control voltage command value vo on the horizontal axis and the output phase voltage V on the vertical axis. However, the control voltage command value vo and output phase voltage V are both expressed as an average voltage corresponding to an on/off duty. A dotted line indicates an input-output relationship during the current outflow OUT, and an alternate long and short dashed line indicates an input-output relationship during the current inflow IN. A solid line represents an ideal linear characteristic. 
     Since the output phase voltage V changes as indicated by an arrow  300  when the current direction changes from the current inflow IN to the current outflow OUT, and changes as indicated by an arrow  301  when the current direction changes from the current outflow OUT to the current inflow IN, the output phase voltage V becomes discontinuous, as a result a current response delays, and positional deviation DIF occurs as the position control device. 
     Therefore, dead time compensation processing is conventionally performed so that the output phase voltage V is on the solid line showing the ideal linear characteristic by previously adding, to the control voltage command value vo, a dead time compensation value −vod during the current inflow IN and a dead time compensation value +vod during the current outflow OUT and setting a resulting value as an input voltage to the PWM inverter. 
       FIG. 7  is a block diagram illustrating an exemplary configuration of a position control device  200  of a comparative example. A position command value X is output from a higher device (not shown) to the position control device  200  of the example. A rotation angle θm of a motor  100  output from a position detector  101  is a position detection value that indicates the position of a load  102  connected with and driven by the motor, and is subtracted from the position command value X by a subtracter  50  whose output is the position deviation DIF. 
     The position deviation DIF is amplified by position loop gain Kp times by a position deviation amplifier  52 . In the example, a feedforward configuration is adopted in order to accelerate a command response. Therefore, the position command value X is differentiated by time by a differentiator  51  and results in a speed feedforward amount ωf. The speed feedforward amount ωf is amplified by total inertia moment J times including the load after being differentiated by time by a differential amplifier  56 , and results in a torque feedforward amount τf necessary for acceleration/deceleration operation corresponding to acceleration of the position command value. Note that “s” of the differentiator  51  denotes a differential operator of Laplace transform. 
     An adder  53  adds the output of the position deviation amplifier  52  and the speed feedforward amount ωf to output a speed command value ωm*. On the other hand, a differentiator  54  differentiates a position detection value θm by time to output a speed detection value ωm. A subtracter  55  subtracts the speed detection value ωm from the speed command value ωm*, and speed deviation which is an output of the subtracter  55  is amplified by PI (Proportional Integration) by a speed controller  57 . An output of the speed controller  57  and the acceleration/deceleration torque feedforward amount τf are added by an adder  58  to result in a torque command value τc* of the motor. 
     A current vector control calculation unit  59  is a calculation unit that calculates and outputs a q-axis current command value iq*, a d-axis current command value id*, and a power supply angular frequency ω. In a magnet type synchronous motor (PMSM) such as a surface magnet type (SPM), an embedded magnet type (IPM), or a reluctance type, the current vector control calculation unit  59  calculates the q-axis current command value iq* and d-axis current command value id* from an N−τ (speed-torque) characteristic of the motor and the speed detection value ωm with respect to the torque command value τc*. 
     In an induction motor (IM), the current vector control calculation unit  59  calculates the d-axis current command value id* from the N−τ (speed-torque) characteristic of the motor and the speed detection value ωm and calculates the q-axis current command value iq* from the torque command value τc* and a d-axis current detection value id. Furthermore, the current vector control calculation unit  59  calculates a slip angular frequency ωs from the d-axis current detection value id and a q-axis current detection value iq, and adds it and a motor pair-pole number times the speed detection value ωm (not shown) to calculate the power supply angular frequency ω. 
     Generally, torque is controlled by the q-axis current in a two-phase rotational coordinate system, so the q-axis current is hereafter referred to as torque current or torque, depending on the situation. Note that the magnet type synchronous motor (PMSM) is controlled in the slip angular frequency ωs=0, so the power supply angular frequency ω output from the current vector control calculation unit  59  is the motor pair-pole number times the speed detection value ωm. An integrator  60  integrates the power supply angular frequency ω by time to output a power supply phase angle θ. 
     U-phase current iu and W-phase current iw of the motor are detected by current detectors  72  and  73 . Note that V-phase current iv can be calculated by iv=−(iu+iw). A three phase→d-q converter  65  calculates and outputs d-axis current id and q-axis current iq from the U-phase current iu, W-phase current iw, and power supply phase angle θ by coordinate conversion. 
     A subtracter  61  subtracts the d-axis current id from the d-axis current command value id* to calculate a d-axis current error Δid. A d-axis current controller  62  includes an error amplifier for amplifying the d-axis current error Δid by PI (Proportional Integration) and a non-interfering compensation unit (not shown) for compensating for an interference component with the q-axis from the q-axis current command value iq* and power supply angular frequency ω. The d-axis current controller  62  adds an error amplifier output and a non-interfering compensation value to output a d-axis voltage command value vd*. 
     A subtracter  63  subtracts the q-axis current iq from the q-axis current command value iq* to calculate a q-axis current error Δiq. A q-axis current controller  64  includes an error amplifier for amplifying the q-axis current error Δiq by the PI (Proportional Integration), a non-interfering compensation unit (not shown) for compensating an interference component with the d-axis from the d-axis current command value id* and power supply angular frequency ω, and an induced voltage compensation unit (not shown) for compensating for induced voltage of the motor. The q-axis current controller  64  adds an error amplifier output, a non-interfering compensation value, and an induced voltage compensation value to output a q-axis voltage command value vq*. 
     A d-q→three phase converter  66  calculates and outputs a U-phase control voltage command value vu*, a V-phase control voltage command value vv*, and a W-phase control voltage command value vw* from the d-axis voltage command value vd*, the q-axis voltage command value vq*, and the power supply phase angle θ by coordinate conversion. 
     A dead time compensation value calculation unit  67  performs d-q→three phase conversion (not shown) to calculate and output each phase current command value of the U phase, the V phase, and the W phase from the d-axis current command value id*, the q-axis current command value iq*, and the power supply phase angle θ by coordinate conversion, and calculates the above-described dead time compensation value (vud, vvd, and vwd) for each phase in order to compensate for phase voltage becoming discontinuous during direction reverse of the current command value. 
     An adder  68  adds the dead time compensation value vud to the U-phase control voltage command value vu* and outputs a U-phase control voltage command value vuo after the dead time compensation. Also, an adder  69  and an adder  70  add the dead time compensation values (vvd, vwd) to the phase control voltage command values (vv*, vw*) and output a V-phase control voltage command value vvo and a W-phase control voltage command value vwo after the dead time compensation, respectively. 
     As described above, a PWM inverter  71  inputs thereinto the phase control voltage command values (vuo, vvo, vwo) after the dead time compensation, power amplifies them, and outputs phase voltages (vu, vv, vw) for driving the motor. The output phase voltages are applied to the respective phases of the motor to generate respective phase currents. 
       FIG. 8  shows a speed loop frequency characteristic from the speed command value ωm* to the speed detection value ωm by suitably adjusting proportional gain and integral gain in a PI control system of the d-axis current controller  62 , the q-axis current controller  64 , and the speed controller  57  of the position control device  200  shown in  FIG. 7 . In this case, a speed control band (−3 dB down of gain) is about 200 Hz. (Note that this characteristic is evaluated assuming that the speed feedforward amount ωf, the torque feedforward amount τf, and a position loop gain Kp are 0.) 
     Next, the above-described impact of the dead time causes discontinuity of the output voltage as shown in  FIG. 14 , so it can be considered as voltage disturbance to the position control device  200 . Therefore, by forming the q-axis voltage command value vq* by adding voltage disturbance vdis (not shown) to the output of the q-axis current controller  64  of  FIG. 7  after suitably setting the position loop gain Kp, a response of the q-axis current iq to the voltage disturbance vdis is evaluated. 
       FIG. 9  shows a position loop frequency characteristic of voltage disturbance suppression performance (iq/vdis) of the position control device  200 . Because the q-axis current iq is proportional to the torque, a ripple of the q-axis current iq is proportional to a speed and position ripple; that is, a DIF ripple.  FIG. 10  shows a DIF ripple generated when the motor is operated at a constant speed at which the total of current direction reversal numbers of the phase currents is 20 times per second (20 Hz) after setting an appropriate dead time and dead time compensation value. In this case, a DIF ripple of 0.4-0.5 μm on both side amplitude in length conversion (20 mm per rotation of the motor) occurs. 
     For the problem of decrease in machined surface quality caused by the DIF ripple due to the dead time, the impact has been conventionally reduced by the dead time compensation as described above. It is not easy, however, to accurately detect the current direction and precisely switch the dead time compensation value, and there has been a limit to the DIF ripple reduction by the dead time compensation. 
     A position control device in  FIG. 1  is a position control device configured for reducing the DIF ripple as one advantage. This position control device will be described below with reference to figures.  FIG. 1  is a block diagram illustrating an exemplary configuration of the position control device. Hereinafter, only differences from the comparative example described above will be described. In addition,  FIG. 1  illustrates the case of the surface magnet type synchronous motor (SPM). 
     In the position control device  10  in  FIG. 1 , an adder  13  adds a q-axis current compensation amount iqc* to a q-axis current error Δiq and inputs a resulting value into a q-axis current controller  11 . The q-axis current compensation amount iqc* is a compensation input for adjusting a torque response time, and a calculation output of a later-described q-axis response compensation amount calculation unit  1 . In addition, the q-axis current iq is added to the input of the q-axis current controller  11 . 
     An error amplifier of the q-axis current controller  11  uses I-P (Integration-Proportion) control. The I-P control is a known control method and a calculation expression in this case is shown by an expression (1): 
     
       
         
           
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             where, a q-axis voltage error Δvq is an output of the I-P control, and Gcqi denoting integral gain and Gcqp denoting proportional gain are gain parameters involved in the I-P control. 
           
         
       
    
     The q-axis current controller  11  includes, other than the error amplifier for I-P (Integration-Proportion) amplification, a non-interfering compensation unit for compensating for an interference component with the d-axis from the d-axis current command value id* and power supply angular frequency ω and an induced voltage compensation unit for compensating induced voltage of the motor in the same manner as the comparative example. That is, the q-axis current controller  11  adds a non-interfering compensation value ωL·id and an induced voltage compensation value Ke(ω/p) to the q-axis voltage error Δvq which is an output of the I-P control and outputs a q-axis voltage command value vq*. This series of processing is expressed by an expression (2):
 
[Expression 2]
 
 Vq*=Δvq+ωLid+Ke (ω/ p )  (2),
         where p denotes a pair-pole number, L an inductance per phase, and Ke a torque constant.       

     An adder  12  adds a q-axis voltage feedforward amount vqf to the q-axis voltage command value vq* which is an output of the q-axis current controller  11  and forms a q-axis voltage command value which is an input to a d-q→three phase converter  66 . The q-axis voltage feedforward amount vqf is a q-axis voltage compensation value that compensates for the q-axis voltage error Δvq generated corresponding to a jerk command value to make Δvq=0 and is a calculation output of the later-described q-axis response compensation amount calculation unit  1 . 
       FIG. 1  shows the q-axis response compensation amount calculation unit  1  that has a block configuration in the case of the surface magnet type synchronous motor (SPM). A q-axis voltage vq generated by time change of the q-axis current iq is shown by expression (3).
 
[Expression 3]
 
 vq=sL·iq   (3)
 
     A differential amplifier  2  divides the acceleration/deceleration torque feedforward amount τf by a torque constant Ke for time differentiation and outputs a time derivative value s·iq of the q-axis current. Since the acceleration/deceleration torque feedforward amount τf is a torque command value proportional to an acceleration command value, this q-axis current is also proportional to the acceleration command value. 
     Since an amplifier  3  amplifies the output of the differential amplifier  2  by the inductance L times, its output becomes the q-axis voltage vq generated by time change of the q-axis current iq of the expression (3). Here, the q-axis current and torque and acceleration have a proportional relationship, the q-axis current time derivative value s·i is proportional to jerk. That is, the q-axis voltage vq becomes the q-axis voltage feedforward amount vqf corresponding to a jerk command value. 
     By performing the above-described compensation processing from a well-known dq coordinate system voltage equation of the SPM, the q-axis voltage error Δvq of the expression (1) can be made Δvq=0 including during acceleration/deceleration operation. On the other hand, in the I-P control, since command following performance in a high-frequency band is deteriorated as compared with the PI control, a q-axis current response; that is, a torque response tends to be delayed. 
     The q-axis current compensation amount iqc*, which is an output of the q-axis response compensation amount calculation unit  1 , compensates for the delay of the torque response. When the expression (1) is solved for the q-axis current error Δiq, an expression (4) is obtained because, in addition to Δvq=0, s·iq→s·iq* holds under constant jerk. 
     
       
         
           
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     Therefore, when the q-axis current compensation amount iqc* is set by an expression (5), the q-axis current error Δiq is expressed by an expression (6). 
     
       
         
           
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     In the expression (6), the q-axis current error Δiq is proportional to a q-axis current command time derivative value s·iq*. That is, under the constant jerk, a q-axis current response iq is delayed by a constant time with respect to the q-axis current command value iq*, which indicates that its q-axis current response delay time can be adjusted by an interpolation constant Giqc. 
     Because the output of the differential amplifier  2  can be regarded as the q-axis current command time derivative value s·iq* in  FIG. 1 , assuming that an amplification factor Cqr of an amplifier  4  is Cqr=Gcqp/Gcqi, the q-axis current compensation amount iqc* of the expression (5) can be derived by amplifying the output by the interpolation constant Giqc of an amplifier  5 . 
       FIG. 2  corresponds to  FIG. 8  of the comparison example, and shows a speed loop frequency characteristic from the speed command value ωm* to the speed detection value ωm by suitably adjusting the proportional gain Gcqp and integral gain Gcqi in the I-P control of the q-axis current controller  11  of the position control device  10  shown in  FIG. 1 . Note that the proportional gain and integral gain of the d-axis current controller  62  and the speed controller  57  are adjusted to the same values as in  FIG. 8 . In this case, a speed control band is about the same as in  FIG. 8 . 
       FIG. 3  corresponds to  FIG. 9  of the comparison example, and shows a position loop frequency characteristic of voltage disturbance suppression performance (iq/vdis) of the position control device  10  shown in  FIG. 1 . Note that the position loop gain Kp is adjusted to the same value as in  FIG. 9 . The DIF ripple in the example is shown in  FIG. 4 . 
     Here, the dead time, the dead time compensation value, and the motor speed are set to the same values as in  FIG. 10 . In  FIG. 4 , the DIF ripple is about 0.05 μm in both-side amplitude and can be suppressed by about 1/10 as compared with  FIG. 10  of the comparative example. This is the same for the DIF ripple generated due to the dead time. 
     Next, consideration is given to the position deviation DIF when an S-shaped acceleration/deceleration position command value X is input to the position control device  10  shown in  FIG. 1 .  FIG. 5  illustrates the case of setting the interpolation constant Giqc=0. Because the command following performance of the q-axis current (torque) is deteriorated in the high-frequency band by the I-P control, the DIF of about 1 μm on one side occurs during acceleration/deceleration. Note that in  FIG. 5  and later  FIG. 11 , and  FIG. 6 , a frequency DIF ripple component generated due to the dead time is removed. 
       FIG. 11  shows the position deviation DIF when the same S-shaped acceleration/deceleration position command value X is input into the position control device  200  in  FIG. 7  for comparison. Because of the PI control, the command following performance of the q-axis current (torque) is high and the DIF during acceleration/deceleration can be suppressed to about 0.2 μm on one side. 
       FIG. 6  shows an example when the response delay time of the q-axis current (torque) is changed by adjusting the interpolation constant Giqc so that the DIF during acceleration/deceleration is small in the position control device  10  shown in  FIG. 1 . The DIF during acceleration/deceleration can be suppressed to about 0.1 μm on one side and it is seen that a position synchronous relationship can be maintained even when simultaneously operated with other control axes operating under the PI control. 
     The above description is made on an example of the surface magnet type synchronous motor (SPM), but the technique disclosed herein is also applicable to the embedded magnet type (IPM) and reluctance type and induction motor (IM), and the same effect can be expected. 
     In the case of the embedded magnet type (IPM) and reluctance type synchronous motors, in the q-axis response compensation amount calculation unit  1 , the amplification factor of an expression (7) is set to the differential amplifier  2  in order to calculate the time derivative value s·iq of the q-axis current from the acceleration/deceleration torque feedforward amount τf. 
     
       
         
           
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             where p denotes the motor pair-pole number, Φf an interlinkage magnetic flux, Ld a d-axis inductance, and lq a q-axis inductance. Furthermore, an amplification ratio of the amplifier  3  becomes the q-axis inductance lq. 
           
         
       
    
     In the case of the induction motor (IM), the amplification factor of an expression (8) is set to the differential amplifier  2 . 
     
       
         
           
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             where M denotes a mutual inductance of one phase. Furthermore, the amplification factor of the amplifier  3  is a leakage inductance of one phase Lσ.