Servo driver

Abstract

The present invention reduces heat generation of at least some switching elements among the switching elements included in an inverter. This servo driver comprises: an inverter that generates a three-phase AC drive current by on / off controlling a plurality of switching elements; a determination unit that determines, on the basis of a command signal received from a higher-level device and information pertaining to the operation of a motor, a carrier wave used for generating a PWM signal that turns on and off the plurality of switching elements and a modulated wave for each phase of the three-phase AC; a correction unit that corrects the modulated wave of the each phase determined by the determination unit so that the on-time of at least some switching elements among the plurality of switching elements is shortened in a period less than one cycle of the electrical angle of the motor; and a supply unit that generates the PWM signal on the basis of the carrier wave and the modulated wave corrected by the correction unit and supplies the generated PWM signal to the plurality of switching elements.

Description

Servo driver

[0001] The present invention relates to a servo driver.

[0002] A motor control device that controls a motor using a pulse width modulation signal (PWM signal) is used. Patent Document 1 proposes a technology for suppressing torque unevenness that can occur when a brushless DC motor is rotated at low speed by the motor control device.

[0003] Japanese Patent Application Laid-Open No. 2003-199381

[0004] Some motors operate at power levels above their rated capacity for short periods of time. When controlling such motors with a servo driver, uneven loss (heat generation) can occur between the switching elements in the inverter of the servo driver. To correct this uneven heat generation, heat dissipation components have become larger, making it difficult to miniaturize the servo driver.

[0005] An object of one aspect of the disclosed technique is to provide a servo driver that can reduce heat generation from at least some of the switching elements of an inverter.

[0006] One aspect of the disclosed technology is exemplified by the following servo driver: The servo driver includes: an inverter that generates a three-phase AC drive current by controlling the on / off of a plurality of switching elements, a determination unit that determines a carrier wave and a modulated wave for each phase of the three-phase AC used to generate a PWM signal that turns on and off the plurality of switching elements based on a command signal received from a higher-level device and information related to motor operation, a correction unit that corrects the modulated wave for each phase determined by the determination unit so as to shorten the on-time of at least some of the plurality of switching elements within a period that is less than one cycle of the electrical angle of the motor, and a supply unit that generates the PWM signal based on the carrier wave and the modulated wave corrected by the correction unit, and supplies the generated PWM signal to the plurality of switching elements.

[0007] According to this servo driver, the modulation waves of the phases are corrected so that the ON time of at least some of the switching elements is shortened, thereby reducing heat generation from at least some of the switching elements of the inverter.

[0008] The servo driver may further include the following feature: an identifying unit that identifies a heat-generating switching element that generates more heat than the other switching elements among the plurality of switching elements; and the correction unit corrects the modulated wave of each phase so as to shorten the on-time of the heat-generating switching element. By including such a feature, the servo driver can suppress heat generation in the heat-generating switching elements, thereby more efficiently reducing heat generation in the switching elements of the inverter.

[0009] The servo driver may further include the following feature: the determination unit determines a first PWM signal based on information related to the operation of the motor and the command signal; the identification unit calculates a current value of each of the plurality of switching elements based on the first PWM signal and a current value of each phase of the three-phase AC, and identifies the heat-generating switching element based on the calculated current values ​​of each of the plurality of switching elements; and the servo driver includes such a feature that the heat-generating switching element can be identified without an additional instrument.

[0010] The servo driver may further include the following feature: an ammeter that measures a current value of a current flowing through each of the plurality of switching elements; and the identification unit identifies the switching element with the largest current value as the heat-generating switching element based on the current value measured by the ammeter. It is considered that the larger the current flowing through a switching element, the greater the heat generation. By including such a feature, the servo driver can identify the heat-generating switching element using the current value.

[0011] The servo driver may further include the following feature: a thermometer that measures the temperature of each of the plurality of switching elements. The identification unit then identifies the switching element with the highest temperature as the heat-generating switching element based on the temperatures measured by the thermometer. By including this feature, the servo driver can identify the heat-generating switching element even when the switching element is subjected to heat from other heat-generating components within the servo driver.

[0012] The servo driver may further include the following feature: a storage unit that stores a correspondence relationship between heat received from a heat-generating component disposed within the servo driver and a correction value for each of the plurality of switching elements; and the identification unit corrects the heat generated by the plurality of switching elements based on the correspondence relationship to identify the heat-generating switching elements. By including such a feature, the servo driver can identify the heat-generating switching elements while taking into account the influence of heat from the heat-generating components.

[0013] The servo driver may further include the following features: The correction unit corrects the modulated wave of each phase when an index related to the motor output is equal to or greater than a predetermined threshold. It is considered that uneven heat generation among the switching elements occurs when the index related to the motor output is equal to or greater than the predetermined threshold. By including these features, the servo driver can suppress uneven heat generation among the switching elements in situations where uneven heat generation among the switching elements is likely to occur. Here, examples of the index related to the motor output include the drive current supplied to the motor and the torque command value indicated by the command signal. Furthermore, the predetermined threshold is determined based on, for example, the rating of the motor.

[0014] The servo driver may further include the following feature: the correction unit determines a correction amount for the modulated wave of each phase in accordance with the speed of the motor. It is considered that the heat generation amount of each switching element varies in accordance with the speed of the motor. By including such a feature, the servo driver can correct the modulated wave in accordance with the speed of the motor.

[0015] The servo driver may further include the following feature: when the speed of the motor is equal to or less than a first threshold, the correction unit corrects the modulated wave of each phase with a first correction amount; when the speed of the motor is equal to or greater than a second threshold that is greater than the first threshold, the correction unit corrects the modulated wave of each phase with a second correction amount that is smaller than the first correction amount; and when the speed of the motor is greater than the first threshold but less than the second threshold, the correction amount decreases as the motor speed increases, within a range that is less than the first correction amount and greater than the second correction amount. By including this feature, the servo driver can suppress sudden fluctuations in the correction amount. Note that the determination of the correction amount is not limited to being based on the speed of the motor, and may be based on the position of the motor.

[0016] According to the disclosed technique, it is possible to reduce heat generation from at least some of the switching elements of the inverter.

[0017] FIG. 1 is a diagram illustrating an example of a servo system according to an embodiment. FIG. 2 is a diagram illustrating an example of an inverter. FIG. 3 is a diagram illustrating waveforms of drive voltages and drive currents for each phase output by an inverter of a servo driver according to an embodiment. FIG. 4 is an enlarged view of a rectangular area in FIG. 3. FIG. 5 is a diagram comparing current waveforms of switching elements. FIG. 6 is a diagram illustrating an example of a processing block of a control unit according to an embodiment. FIG. 7 is a diagram illustrating an example of a change in ON time of a switching element due to zero-phase signal addition according to an embodiment. FIG. 8 is a diagram illustrating an example of an inverter when Method 2 is adopted. FIG. 9 is a diagram illustrating an example of an inverter when Method 3 is adopted. FIG. 10 is a diagram illustrating an example of an inverter when Method 3 is adopted. FIG. 11 is a diagram illustrating an example of a processing flow of a control unit 21 according to an embodiment. FIG. 12 is a diagram illustrating an example of a processing block of a control unit according to a first modification. FIG. 13 is a diagram illustrating an example of a correspondence relationship between a coefficient used to correct a zero-phase signal and the speed of a servo motor 3 according to the first modification.

[0018] <Application Example> An application example of the present invention will be described. An application example of the present invention is, for example, a servo driver 2 illustrated in Fig. 1. The servo driver 2 controls the servo motor 3 based on a command signal from the PLC 1 and information related to the operation of the servo motor 3.

[0019] The servo driver 2 controls the on / off of each of the switching elements Q1, Q2, Q3, Q4, Q5, and Q6, thereby supplying a three-phase AC drive current to the servo motor 3. Based on the command signal from the PLC 1 and information related to the operation of the servo motor 3, the servo driver 2 determines the carrier wave and the modulation wave of each phase used to generate a PWM signal that turns on / off each of the switching elements Q1, Q2, Q3, Q4, Q5, and Q6.

[0020] Some servo motors 3 can obtain an output (torque, etc.) greater than the rated value by being supplied with a drive current greater than the rated value for a short period of time. When such a servo motor 3 is caused to output an output greater than the rated value, the heat generated by each of the switching elements Q1, Q2, Q3, Q4, Q5, and Q6 is averaged out and made uniform over one period of the electrical angle of the servo motor 3. However, within less than one period of the electrical angle of the servo motor 3, the heat generated by each of the switching elements Q1, Q2, Q3, Q4, Q5, and Q6 is not averaged out, and some switching elements may generate more heat than the other switching elements.

[0021] Therefore, in this application example, the servo driver 2 corrects the modulation wave of each phase so as to shorten the on-time of at least some of the switching elements Q1, Q2, Q3, Q4, Q5, and Q6 in a period less than one cycle of the electrical angle of the servo motor 3. Then, the servo driver 2 determines a PWM signal using the carrier wave and the corrected modulation wave, and supplies the determined PWM signal to the switching elements Q1, Q2, Q3, Q4, Q5, and Q6.

[0022] According to this application example, the ON time of at least some of the switching elements is shortened, so that heat generation of at least some of the switching elements Q1, Q2, Q3, Q4, Q5, and Q6 can be reduced.

[0023] <Embodiments> Hereinafter, embodiments will be described with reference to the drawings. Fig. 1 is a diagram showing an example of a servo system 100 according to an embodiment. The servo system 100 includes a PLC 1, a servo driver 2, and a servo motor 3. The servo driver 2 is arranged to drive and control the servo motor 3. An object to be driven by the servo motor 3 may be connected to an output shaft 32 of the servo motor 3 via a coupling or the like. Thus, in the servo system 100 illustrated in Fig. 1, one drive shaft driven by the servo motor 3 is provided, but two or more drive shafts may be provided.

[0024] The PLC 1 outputs a command signal via a control line S1 to the servo driver 2. The PLC 1 functions as, for example, a monitor for the servo driver 2 by executing processing according to a program prepared in advance.

[0025] The control unit 21 of the servo driver 2 is a computer equipped with a processor and a memory. The control unit 21 performs predetermined processing by having the processor execute a computer program stored in the memory. The inverter 22 is a circuit that supplies three-phase AC driving power to the servo motor 3 by controlling the switching of multiple switching elements.

[0026] The control unit 21 receives a command signal from the PLC 1 via the control line S1. The control unit 21 also receives a feedback signal from the servo motor 3. The control unit 21 causes the inverter 22 to output a drive current for driving the servo motor 3 based on the command signal from the PLC 1 and the feedback signal from the servo motor 3. The servo driver 2 is formed with a servo system that performs feedback control using a position controller, a speed controller, a current controller, etc., and these signals are used to servo-control and drive the servo motor 3.

[0027] The servo motor 3 includes a motor main body 30 and an encoder 31. The servo motor 3 is, for example, an AC servo motor. The servo motor 3 is supplied with a driving current from the servo driver 2 via a power line P1. The encoder 31 detects the displacement of an output shaft 32 of the motor main body 30. Examples of the displacement of the output shaft 32 detected by the encoder 31 include the direction of rotation, amount of rotation, and rotation speed of the output shaft 32. The encoder 31 outputs a feedback signal indicating the detected displacement to the servo driver 2 via an encoder cable S2. The information indicated by the feedback signal is an example of "information related to the operation of the motor."

[0028] FIG. 2 is a diagram illustrating an example of an inverter 22. In the inverter 22, a positive voltage relative to the electric wire L2 is supplied to the electric wire L1. In the inverter 22, for example, an AC voltage from a system power supply is rectified by a rectifier diode to generate a DC voltage, which is then supplied to the electric wires L1 and L2. The inverter 22 includes a U-phase leg U1, a V-phase leg V1, a W-phase leg W1, and a power line P1 connected in parallel between the electric wires L1 and L2. The U-phase leg U1 includes switching elements Q1 and Q2 connected in series. The V-phase leg V1 includes switching elements Q3 and Q4 connected in series. The W-phase leg W1 includes switching elements Q5 and Q6 connected in series. The switching elements Q1, Q3, and Q5 are connected to the electric wire L1 side, and the switching elements Q2, Q4, and Q6 are connected to the electric wire L2 side. Hereinafter, in this specification, the electric wire L1 side will also be referred to as the high side, and the electric wire L2 side will also be referred to as the low side.

[0029] Furthermore, power line P1 includes a U-phase power line U2, a V-phase power line V2, and a W-phase power line W2. A connection point U3 between switching element Q1 and switching element Q2 connected in series in U-phase leg U1 is connected to one end of U-phase power line U2. A connection point V3 between switching element Q3 and switching element Q4 connected in series in V-phase leg V1 is connected to one end of V-phase power line V2. A connection point W3 between switching element Q5 and switching element Q6 connected in series in W-phase leg W1 is connected to one end of W-phase power line W2.

[0030] The inverter 22 converts DC power supplied from the DC power supply into AC power and supplies it to the servo motor 3. The inverter 22 outputs a U-phase current from a U-phase power line U2, a V-phase current from a V-phase power line V2, and a W-phase current from a W-phase power line W2 by controlling the on / off of each of the switching elements Q1, Q2, Q3, Q4, Q5, and Q6 using a control signal (PWM signal) supplied from the control unit 21.

[0031] 3 is a diagram illustrating waveforms of the drive voltage and drive current for each phase output by the inverter 22 of the servo driver 2 in this embodiment. In FIG. 3, the upper diagram illustrates the waveform of the drive voltage, and the lower diagram illustrates the waveform of the drive voltage. The drive voltage and drive current having the waveforms illustrated in FIG. 3 are supplied to the servo motor 3 by the inverter 22.

[0032] Fig. 4 is an enlarged view of rectangular region R1 in Fig. 3. Fig. 4 also illustrates the switching duties of the U, V, and W phases. In Fig. 4, the upper diagram illustrates the waveform of the drive voltage, the middle diagram illustrates the waveform of the drive current, and the lower diagram illustrates the switching duties of the high-side switching elements Q1, Q3, and Q5 of the U, V, and W phases. In the example of Fig. 4, within the range illustrated by rectangle R2, the servo motor 3 is driven at low rotation speed, and for a short period of time, the inverter 22 outputs to the servo motor 3 a drive current several times the rated current.

[0033] Referring to the rectangle R2 in Fig. 4, it can be seen that the current of the U phase is larger than that of the other phases, and that the ON signal of the High-side switch (switching element Q1) is longer than the OFF signal. Fig. 5 is a diagram comparing the current waveforms of switching element Q1 and switching element Q2. The vertical axis of Fig. 5 represents current, and the horizontal axis represents time.

[0034] The upper part of FIG. 5 illustrates a positive current waveform for switching element Q1, and the lower part of FIG. 5 illustrates a negative current waveform for switching element Q2. Referring to FIG. 5 , it can be seen that a larger current flows through switching element Q1 than switching element Q2 because the ON signal for switching element Q1 is longer than the OFF signal. Switching element Q1, through which a larger current flows, may generate more heat than switching element Q2. That is, when inverter 22 outputs a drive current several times the rated current to servo motor 3 for a short period of time, some switching elements may generate high heat for a portion of one cycle of the PWM signal. An example of a servo motor 3 in which some switching elements tend to generate high heat is a press machine, which continuously applies a constant torque while servo locked (the rotational speed of servo motor 3 is "0" or extremely low). In this embodiment, the following configuration is adopted to suppress heat generation in such switching elements.

[0035] 6 is a diagram showing an example of a processing block of the control unit 21 according to the embodiment. The inverter 22 includes a determination unit 211, an identification unit 212, a correction unit 213, and a supply unit 214. The control unit 21 executes the processes of each unit, such as the determination unit 211, the identification unit 212, the correction unit 213, and the supply unit 214, by having the processor of the control unit 21 execute a program stored in the memory of the control unit 21.

[0036] The determination unit 211 determines the carrier wave and the modulating wave of each phase used to generate the PWM signal to be supplied to the inverter 22 based on the command signal from the PLC 1 and the feedback signal from the encoder 31 of the servo motor 3 .

[0037] When the torque indicated by the command signal (torque command) received from the PLC 1 is equal to or greater than a predetermined threshold, the identifying unit 212 identifies a switching element that generates more heat than other switching elements in the inverter 22. Here, the predetermined threshold is, for example, a value greater than the rated torque of the servo motor 3. The method of identifying a switching element by the identifying unit 212 will be described in detail later.

[0038] The correction unit 213 adds a zero-phase-sequence signal to the modulated wave determined by the determination unit 211 for a period less than one period of the electrical angle of the servo motor 3 so as to shorten the time during which the switching elements identified by the identification unit 212 are turned on. The zero-phase-sequence signal is, for example, a signal that turns on all of the high-side switching elements Q1, Q3, and Q4, or a signal that turns on all of the low-side switching elements Q2, Q4, and Q5. By adding the zero-phase-sequence signal, for example, the time during which the high-side switching elements Q1, Q3, and Q4 are turned on is adjusted.

[0039] The supply unit 214 generates a PWM signal based on the carrier wave determined by the determination unit 211 and the modulated wave to which the zero-phase signal has been added by the correction unit 213, and supplies the generated PWM signal to the inverter 22. When the identification unit 212 has not identified a switching element that generates more heat than other switching elements (for example, when all switching elements generate heat equally), the supply unit 214 generates a PWM signal based on the carrier wave and modulated wave determined by the determination unit 211, and supplies the generated PWM signal to the inverter 22.

[0040] (Change in ON Time of Switching Elements Due to Zero-Sequence Signal Addition) FIG. 7 is a diagram showing an example of change in ON time of switching elements due to zero-phase signal addition in the embodiment. The upper part of FIG. 7 illustrates time-series changes of the carrier wave N1, the U-phase PWM-modulated wave N2, the V-phase PWM-modulated wave N3, and the W-phase PWM-modulated wave N4. The lower part of FIG. 7 illustrates an ON signal for the U-phase high-side switching elements (switching elements Q1, Q3, and Q5) and an ON signal for the U-phase low-side switching elements (switching elements Q2, Q4, and Q6). In FIG. 7 , it is assumed that the correction unit 213 adds a zero-phase signal at time T1. The addition of the zero-phase signal in FIG. 7 is performed on the U-phase PWM-modulated wave N2, the V-phase PWM-modulated wave N3, and the W-phase PWM-modulated wave N4 so that the ON times of the U-phase high-side switching elements and the U-phase low-side switching elements are equal. As a result, the U-phase PWM-modulated wave N2, the V-phase PWM-modulated wave N3, and the W-phase PWM-modulated wave N4 are offset by "α".

[0041] In a rectangular region R3 before time T1 when the zero-phase signal is added, the on-time of the U-phase high-side switching element is longer than the on-time of the U-phase low-side switching element. On the other hand, in a rectangular region R4 after time T1 when the zero-phase signal is added, the on-time of the U-phase high-side switching element and the on-time of the U-phase low-side switching element are equal. In other words, the on-times of the high-side and low-side switching elements are adjusted by adding the zero-phase signal.

[0042] (Method for Identifying Switching Elements by Identifying Unit 212) Various methods can be applied to the identification of switching elements that generate more heat than other switching elements by the identification unit 212. Hereinafter, a method for identifying switching elements that generate more heat than other switching elements by the identification unit 212 will be described.

[0043] (Method 1) In Method 1, the identifying unit 212 identifies a switching element that generates more heat than other switching elements using a motor voltage equation. The motor voltage equation can be, for example, Equation 1 below.

[0044] Equation 1 is the d-axis voltage V when the servo motor 3 is a Surface Permanent Magnet (SPM) motor. ｄ and the q-axis voltage V ｑ is the motor voltage equation. ｄ , L ｑ , ω, K ｅｍｆ , I ｄ , I ｑ, s are the motor phase resistance, d-axis inductance, q-axis inductance, electrical angular velocity, induced voltage constant, d-axis current, q-axis current, and differential term, respectively. The d-axis current value and q-axis current value are determined so as to follow the command signal from PLC 1. The current values ​​of each phase of the three-phase AC (U-phase current value, V-phase current value, W-phase current value) and the PWM signals of each switching element Q1, Q2, Q3, Q4, Q5, and Q6 are determined from the determined d-axis current value and q-axis current value. That is, the current values ​​and switching duties of the currents flowing through each switching element Q1, Q2, Q3, Q4, Q5, and Q6 are determined.

[0045] The identification unit 212, for example, identifies the switching element through which the largest effective current flows as the switching element generating the most heat, based on the current value and switching duty of the current flowing through each of the determined switching elements Q1, Q2, Q3, Q4, Q5, and Q6.

[0046] In addition, the d-axis voltage V ｄ and the q-axis voltage V ｑ Various motor voltage equations have been proposed to represent this equation depending on the type of motor, axis conversion, etc. A motor voltage equation corresponding to the servo motor 3 connected to the servo driver 2 may be stored in advance in the memory of the servo driver 2, and the identification unit 212 may use the motor voltage equation stored in the memory to identify the switching element that is generating the most heat. Alternatively, the identification unit 212 may store the calculation results using the motor voltage equation (correspondence between the rotational position, rotation speed, torque, and switching element that is generating the most heat of the servo motor 3) as a table in the memory of the servo driver 2, and identify the switching element that is generating the most heat by referring to the table.

[0047] (Method 2) In method 2, the identification unit 212 acquires the measured current values ​​flowing through each of the switching elements Q1, Q2, Q3, Q4, Q5, and Q6, and identifies the switching element that generates more heat than the other switching elements based on the acquired current values.

[0048] FIG. 8 is a diagram illustrating an example of the inverter 22 when Method 2 is employed. When Method 2 is employed, the inverter 22 is provided with ammeters A1, A2, A3, A4, A5, and A6 that measure the current values ​​flowing through the switching elements Q1, Q2, Q3, Q4, Q5, and Q6. The ammeter A1 measures the current value of the current flowing through the switching element Q1 and is disposed between the electric wire L1 and the switching element Q1. The ammeter A2 measures the current value of the current flowing through the switching element Q2 and is disposed between the electric wire L2 and the switching element Q2. The ammeter A3 measures the current value of the current flowing through the switching element Q3 and is disposed between the electric wire L1 and the switching element Q3. The ammeter A4 measures the current value of the current flowing through the switching element Q4 and is disposed between the electric wire L2 and the switching element Q4. The ammeter A5 measures the current value of the current flowing through the switching element Q5 and is disposed between the electric wire L1 and the switching element Q5. The ammeter A6 is an ammeter that measures the current value of the current flowing through the switching element Q6, and is disposed between the electric wire L2 and the switching element Q6. The current values ​​measured by the ammeters A1, A2, A3, A4, A5, and A6 are input to the control unit 21.

[0049] In method 2, the identification unit 212 identifies the switching element through which the largest current is flowing based on the current values ​​input from the ammeters A1, A2, A3, A4, A5, and A6. Because the switching elements Q1, Q2, Q3, Q4, Q5, and Q6 generate heat in accordance with the current flowing therethrough, there is a high probability that the switching element through which the largest current is flowing is the switching element that is generating the most heat. Therefore, the identification unit 212 identifies the switching element through which the largest current is flowing as the switching element that is generating the most heat.

[0050] (Method 3) In method 3, the identification unit 212 acquires the measured temperatures of each of the switching elements Q1, Q2, Q3, Q4, Q5, and Q6, and identifies the switching element that generates more heat than the other switching elements based on the acquired temperatures.

[0051] 9 is a diagram showing an example of the inverter 22 when Method 3 is adopted. When Method 3 is adopted, the inverter 22 is provided with thermometers T1, T2, T3, T4, T5, and T6 that measure the temperatures of the switching elements Q1, Q2, Q3, Q4, Q5, and Q6. The temperatures measured by the thermometers T1, T2, T3, T4, T5, and T6 are input to the control unit 21.

[0052] In method 3, the identifying unit 212 identifies the switching element that generates the most heat based on the temperatures input from the thermometers T1, T2, T3, T4, T5, and T6.

[0053] (Method 4) In method 4, when a heat source is present near the inverter 22, the identifying unit 212 identifies a switching element disposed near the heat source as a switching element that generates more heat than other switching elements.

[0054] FIG. 10 is a diagram illustrating an example of an inverter 22 when Method 3 is employed. In the example of FIG. 10 , a heat-generating component H1 is disposed near a switching element Q5 of the inverter 22. The heat-generating component H1 is, for example, an electronic component within the inverter 22. Information indicating the switching element closest to the heat-generating component H1 may be stored in advance in the memory of the servo driver 2, and the identifying unit 212 may identify the switching element generating the most heat based on the information stored in the memory. Alternatively, a thermometer T5 may be disposed in the inverter 22 to measure the temperature of the switching element Q5 disposed near the heat-generating component H1. The identifying unit 212 may identify the switching element Q5 as the switching element generating the most heat when the temperature measured by the thermometer T5 is equal to or higher than a predetermined temperature threshold.

[0055] As an alternative to Method 4, the temperature rise of the switching elements Q1, Q2, Q3, Q4, Q5, and Q6 due to heat received from the heat-generating component H1 is verified in advance, and a correspondence relationship in which correction values ​​indicating the rate of temperature rise relative to when the heat-generating component H1 is not present are associated with each of the switching elements Q1, Q2, Q3, Q4, Q5, and Q6 is stored in advance in the memory of the servo driver 2. Furthermore, a correspondence relationship between the current values ​​of the currents flowing through the switching elements Q1, Q2, Q3, Q4, Q5, and Q6 and the temperatures of the switching elements Q1, Q2, Q3, Q4, Q5, and Q6 is stored in advance in the memory of the servo driver 2. The identification unit 212 determines the current values ​​of the currents flowing through the switching elements Q1, Q2, Q3, Q4, Q5, and Q6 using, for example, Method 1, and determines the temperatures of each switching element Q1, Q2, Q3, Q4, Q5, and Q6 based on the determined current values ​​and the correspondence relationship. The identifying unit 212 corrects the temperatures of the switching elements Q1, Q2, Q3, Q4, Q5, and Q6 by adding the increased temperatures of the switching elements Q1, Q2, Q3, Q4, Q5, and Q6 that are stored in advance in memory to the determined temperatures of the switching elements Q1, Q2, Q3, Q4, Q5, and Q6. The identifying unit 212 may identify the switching element that is generating the most heat based on the corrected temperatures.

[0056] <Processing Flow> Fig. 11 is a diagram showing an example of a processing flow of the control unit 21 according to the embodiment. Hereinafter, an example of the processing flow of the control unit 21 will be described with reference to Fig. 11 .

[0057] In step S1 , the determination unit 211 determines a carrier wave and a modulated wave for each phase based on a command signal from the PLC 1 and a feedback signal from the encoder 31 of the servo motor 3 .

[0058] In step S2, the determination unit 212 determines whether the torque indicated by the torque command for the servo motor 3 is equal to or greater than a predetermined threshold. If the torque is equal to or greater than the predetermined threshold (YES in step S2), the process proceeds to step S3. If the torque is less than the predetermined threshold (NO in step S2), the process proceeds to step S6.

[0059] In step S3, the identifying unit 212 identifies the switching element that generates the most heat among the switching elements Q1, Q2, Q3, Q4, Q5, and Q6. If the identifying unit 212 identifies the switching element that generates the most heat, i.e., if there is a switching element that generates more heat than the other switching elements (YES in step S3), the process proceeds to step S4. If the identifying unit 212 cannot identify the switching element that generates the most heat, i.e., if the switching elements Q1, Q2, Q3, Q4, Q5, and Q6 generate heat approximately equally (NO in step S3), the process proceeds to step S6.

[0060] In step S4, the correction unit 213 adds a zero-phase sequence signal to the modulated wave of each phase determined in step S1 so as to shorten the ON time of the switching element identified in step S3. For example, if the switching element Q5 is identified in step S3, the correction unit 213 adds a zero-phase sequence signal so that the ON time of the low-side switching element becomes relatively longer than the ON time of the high-side switching element, since the switching element Q5 is the high-side switching element.

[0061] In step S5, the supply unit 214 generates a PWM signal based on the carrier wave determined in step S1 and the modulated wave of each phase to which the zero-phase sequence signal has been added in step S4. Then, the supply unit 214 supplies the generated PWM signal to the switching elements Q1, Q2, Q3, Q4, Q5, and Q6.

[0062] In step S6, the supply unit 214 generates a PWM signal based on the carrier wave and the modulated wave of each phase determined in step S1. The supply unit 214 supplies the generated PWM signal to the switching elements Q1, Q2, Q3, Q4, Q5, and Q6.

[0063] <Effects of the embodiment> According to this embodiment, by adding a zero-phase-sequence signal to the modulation wave of each phase, the ON time of the switching element that generates more heat than the other switching elements is shortened. Therefore, according to this embodiment, heat generation of the switching element that generates more heat than the other switching elements is suppressed. Furthermore, in this embodiment, the modulation wave of each phase is uniformly changed (offset) by the zero-phase-sequence signal, so adding the zero-phase-sequence signal does not reduce the control accuracy of the servo motor 3.

[0064] In this embodiment, the identifying unit 212 identifies the switching element that generates the most heat, and the correcting unit 213 adds a zero-phase-sequence signal so as to shorten the on-time of the identified switching element. Therefore, according to this embodiment, heat generation in the switching element that generates the most heat can be efficiently suppressed. Note that the servo driver 2 may add a zero-phase-sequence signal to the modulated wave of each phase so as to shorten the on-time of at least one of the switching elements Q1, Q2, Q3, Q4, Q5, and Q6, without identifying the switching element that generates the most heat. By shortening the on-time of at least one switching element, heat generation in the switching element whose on-time is shortened is suppressed.

[0065] In this embodiment, a switching element that generates more heat than other switching elements is identified using, for example, a motor voltage equation as shown in Equation 1. Therefore, according to this embodiment, even a servo driver that does not have an ammeter or a thermometer can identify a switching element that generates more heat than other switching elements.

[0066] In this embodiment, for example, ammeters A1, A2, A3, A4, A5, and A6 are provided to measure the current flowing through each switching element, and the switching element generating the most heat is identified based on the current values ​​measured by the ammeters A1, A2, A3, A4, A5, and A6. It is believed that the heat generated by the switching elements Q1, Q2, Q3, Q4, Q5, and Q6 increases in accordance with the magnitude of the current flowing through them. Therefore, according to this embodiment, the switching element generating the most heat can be identified based on the current value.

[0067] In this embodiment, for example, thermometers T1, T2, T3, T4, T5, and T6 are provided to measure the temperatures of the switching elements, and the switching element generating the most heat is identified based on the temperatures measured by the thermometers T1, T2, T3, T4, T5, and T6. It is considered that the temperatures of the switching elements Q1, Q2, Q3, Q4, Q5, and Q6 may be affected by a heat-generating component H1 disposed around the switching elements Q1, Q2, Q3, Q4, Q5, and Q6. By using the thermometers T1, T2, T3, T4, T5, and T6, the switching element generating the most heat can be more accurately identified even when affected by the heat-generating component H1.

[0068] In the servo motor 3, the heat generation of the switching elements Q1, Q2, Q3, Q4, Q5, and Q6 may be more uneven when the torque of the servo motor 3 is equal to or greater than a predetermined threshold (e.g., greater than the rated torque) than when the torque of the servo motor 3 is less than a predetermined threshold (e.g., equal to or less than the rated torque). In this embodiment, a zero-phase-sequence signal is added to the modulation wave of each phase when the torque of the servo motor 3 is equal to or greater than the predetermined threshold. Therefore, this embodiment can suppress uneven heat generation among the switching elements that may occur when the torque of the servo motor 3 is equal to or greater than the rated torque. Note that in this embodiment, a zero-phase-sequence signal is added when the torque of the servo motor 3 is equal to or greater than the predetermined threshold. However, a zero-phase-sequence signal may also be added when the drive current supplied to the servo motor 3 is equal to or greater than the predetermined threshold.

[0069] <First Modification> In the embodiment described above, a zero-phase signal is added to the PWM signal so that the on-time and off-time of the switching element generating the most heat are equal. In the first modification, a configuration will be described in which the on-time of the switching element generating the most heat is adjusted according to the rotation speed or position of the servo motor 3. Components common to the embodiment will be assigned the same reference numerals, and their description will be omitted. Below, the first modification will be described with reference to the drawings.

[0070] 12 is a diagram showing an example of a processing block of a control unit 21A according to Modification 1. The control unit 21A differs from the control unit 21 according to the embodiment in that it includes a correction unit 213A instead of the correction unit 213.

[0071] The correction unit 213A corrects the zero-phase signal to be added to the modulated wave of each phase in accordance with the speed of the servo motor 3 (the rotation speed of the output shaft 32) indicated by the feedback signal received from the encoder 31. FIG. 13 is a diagram showing an example of the correspondence between the coefficient used to correct the zero-phase signal and the speed of the servo motor 3 in the first modified example. In FIG. 13, the vertical axis shows the coefficient used for correction, and the horizontal axis shows the speed of the servo motor 3. As shown in FIG. 13, the coefficient used for correction is "100%" while the rotation speed of the servo motor 3 is equal to or less than a threshold value TH1. The coefficient used for correction gradually decreases as the rotation speed of the servo motor 3 becomes greater than TH1, and becomes "0%" when the rotation speed of the servo motor 3 becomes equal to or greater than a threshold value TH2. The correspondence shown in FIG. 13 is stored in advance in the memory of the servo driver 2.

[0072] The correction unit 213A acquires a coefficient to be used for correction according to the speed of the servo motor 3 by referring to the correspondence relationship between the coefficient to be used for correction and the speed of the servo motor 3, which is stored in advance in the memory of the servo driver 2. The correction unit 213A corrects the zero-phase signal using the acquired coefficient. For example, if the zero-phase signal before correction is a signal that offsets the modulated wave of each phase by α, the correction unit 213A corrects the zero-phase signal by multiplying α by the coefficient to be used for correction according to the speed of the servo motor 3.

[0073] The heat generation of the switching elements Q1, Q2, Q3, Q4, Q5, and Q6 differs depending on the rotation speed of the servo motor 3. According to the first modification, the zero-phase signal is corrected depending on the rotation speed of the servo motor 3, thereby making it possible to suppress the heat generation of the switching elements Q1, Q2, Q3, Q4, Q5, and Q6 depending on the rotation speed of the servo motor 3. Note that in the first modification, the coefficient used for correction is determined depending on the rotation speed of the servo motor 3, but the coefficient used for correction may also be determined depending on the position of the servo motor 3 (the angle of the output shaft 32).

[0074] In addition, in the first variant, by using two thresholds, threshold TH1 and threshold TH2, sudden fluctuations in the coefficients used for correction (for example, the coefficients used for correction becoming 100% immediately after 0%) are suppressed.

[0075] <Other Modifications> In the embodiment described above, the servo motor 3 has the encoder 31 that outputs a feedback signal, but a motor without the encoder 31 may also be used. For example, if the motor does not have the encoder 31, the servo driver 2 may obtain the rotational position and speed of the motor by detecting the drive current supplied to the motor or by injecting a signal. The rotational position and speed of the motor are examples of "information related to the operation of the motor."

[0076] In the above-described embodiment, ammeters A1, A2, A3, A4, A5, and A6 are provided to measure the current values ​​flowing through the switching elements Q1, Q2, Q3, Q4, Q5, and Q6. However, some switching elements may be provided with ammeters and other switching elements may not be provided with ammeters. That is, the correspondence between the switching elements and the ammeters does not have to be one-to-one. For example, ammeters may be provided corresponding to the switching elements Q4, Q5, and Q6, thereby estimating the current values ​​flowing through the switching elements Q1, Q2, Q3, Q4, Q5, and Q6. Alternatively, an ammeter may be provided on the electric wire L2, and the current values ​​flowing through the switching elements Q1, Q2, Q3, Q4, Q5, and Q6 may be estimated based on the rotational position of the servo motor 3.

[0077] In the embodiment described above, thermometers T1, T2, T3, T4, T5, and T6 are provided to measure the temperatures of the switching elements Q1, Q2, Q3, Q4, Q5, and Q6, respectively. However, it is also possible to provide thermometers for some of the switching elements and not provide thermometers for other switching elements. In other words, the correspondence between the switching elements and the thermometers does not have to be one-to-one. For example, it is sufficient to place thermometers near some of the switching elements and estimate the temperature of each switching element based on the positional relationship between the switching elements and the thermometers and the measurement values ​​of the thermometers.

[0078] The embodiments and modifications disclosed above can be combined with each other.

[0079] <Supplementary Note 1> A servo driver (2) comprising: an inverter (22) that generates a three-phase AC drive current by controlling the on / off of a plurality of switching elements (Q1 to Q6); a determination unit (211) that determines a carrier wave and a modulated wave for each phase of the three-phase AC used to generate a PWM signal that turns on and off the plurality of switching elements (Q1 to Q6) based on a command signal received from a higher-level device (1) and information related to the operation of a motor (3); a correction unit (213) that corrects the modulated wave for each phase determined by the determination unit (211) so that an on-time of at least a switching element (Q5) of the plurality of switching elements (Q1 to Q6) is shortened within a period that is less than one cycle of the electrical angle of the motor (3); and a supply unit (214) that generates the PWM signal based on the carrier wave and the modulated wave corrected by the correction unit, and supplies the generated PWM signal to the plurality of switching elements (Q1 to Q6). <Supplementary Note 2> The servo driver (2) according to Supplementary Note 1, further comprising an identification unit (212) that identifies a heat-generating switching element (Q5) among the plurality of switching elements (Q1 to Q6) that generates more heat than the other switching elements, and the correction unit (213) corrects the modulated wave of each phase so as to shorten the on-time of the heat-generating switching element (Q5). <Supplementary Note 3> The servo driver (2) according to Supplementary Note 2, further comprising: ... <Supplementary Note 4> The servo driver (2) according to Supplementary Note 2, further comprising ammeters (A1 to A6) that measure current values ​​of currents flowing through the plurality of switching elements (Q1 to Q6), and the identifying unit (212) identifies the switching element (Q5) with the largest current value as the heat-generating switching element (Q5) based on the current values ​​measured by the ammeters (A1 to A6).<Supplementary Note 5> The servo driver (2) according to Supplementary Note 2, further comprising thermometers (T1 to T6) that measure the temperatures of the plurality of switching elements (Q1 to Q6), and the identification unit (212) identifies the switching element (Q5) with the highest temperature as the heat-generating switching element (Q5) based on the temperatures measured by the thermometers (T1 to T6). <Supplementary Note 6> The servo driver (2) according to Supplementary Note 2, further comprising a memory unit that stores a correspondence relationship between heat received from a heat-generating component (H1) arranged in the servo driver (2) and a correction value for each of the plurality of switching elements (Q1 to Q6), and the identification unit (212) corrects the heat generated by the plurality of switching elements (Q1 to Q6) based on the correspondence relationship to identify the heat-generating switching element (Q5). <Supplementary Note 7> The servo driver (2) according to any one of Supplementary Notes 1 to 6, wherein the correction unit (213) corrects the modulated wave of each phase when an index related to the output of the motor (3) is equal to or greater than a predetermined threshold. <Supplementary Note 8> The servo driver (2) according to Supplementary Note 7, wherein the index related to the output of the motor (3) includes the drive current supplied to the motor (3). <Supplementary Note 9> The servo driver (2) according to Supplementary Note 7, wherein the index related to the output of the motor (3) includes a torque command value indicated by the command signal. <Supplementary Note 10> The servo driver (2) according to any one of Supplementary Notes 1 to 9, wherein the correction unit (213) determines a correction amount for the modulated wave of each phase in accordance with a speed of the motor (3). <Supplementary Note 11> The correction unit (213) corrects the modulated wave of each phase with a first correction amount when the speed of the motor (3) is equal to or less than a first threshold, corrects the modulated wave of each phase with a second correction amount smaller than the first correction amount when the speed of the motor (3) is equal to or greater than a second threshold that is greater than the first threshold, and decreases the correction amount as the speed of the motor (3) increases, within a range that is less than the first correction amount and greater than the second correction amount when the speed of the motor (3) is greater than the first threshold and less than the second threshold. The servo driver (2) according to Supplementary Note 10.<Supplementary Note 12> The servo driver (2) according to any one of Supplementary Note 1 to Supplementary Note 9, wherein the correction unit (213) determines a correction amount for the modulated wave of each phase depending on a position of the motor (2). <Supplementary Note 13> The servo driver (2) according to Supplementary Note 12, wherein the correction unit (213) corrects the modulated wave of each phase with a third correction amount when the position of the motor (3) is equal to or less than a third threshold, corrects the modulated wave of each phase with a fourth correction amount smaller than the third correction amount when the position of the motor (3) is equal to or greater than a fourth threshold that is greater than the third threshold, and decreases the correction amount as the position of the motor (3) increases, within a range that is less than the third correction amount and greater than the fourth correction amount, when the position of the motor (3) is greater than the third threshold and less than the fourth threshold.

[0080] DESCRIPTION OF SYMBOLS 1 PLC 2 Servo driver 3 Servo motor 21 Control unit 21A Control unit 22 Inverter 30 Motor body 31 Encoder 32 Output shaft 100 Servo system 211 Determination unit 212 Identification unit 213 Correction unit 213A Correction unit 214 Supply unit A1 Ammeter A2 Ammeter A3 Ammeter A3 Ammeter A4 Ammeter A5 Ammeter A6 Ammeter H1 Heat-generating component L1 Electric wire L2 Electric wire N1 Carrier wave N2 U-phase PWM modulated wave N3 V-phase PWM modulated wave N4 W-phase PWM modulated wave S1 Control line S2 Encoder cable T1 Thermometer T2...Thermometer T3...Thermometer T4...Thermometer T5...Thermometer T6...Thermometer P1...Power line Q1...Switching element Q2...Switching element Q3...Switching element Q4...Switching element Q5...Switching element Q6...Switching element U1...U-phase leg U2...U-phase power line U3...Connection point V1...V-phase leg V2...V-phase power line V3...Connection point W1...W-phase leg W2...W-phase power line W3...Connection point

Claims

1. A servo driver comprising: an inverter that generates a three-phase AC drive current by controlling the on / off of multiple switching elements; a determination unit that determines a carrier wave and a modulating wave for each phase of the three-phase AC to be used in generating a PWM signal that turns the multiple switching elements on and off based on a command signal received from a higher-level device and information related to the operation of a motor; a correction unit that corrects the modulating wave for each phase determined by the determination unit so that the on time of at least some of the multiple switching elements is shortened within a period of less than one cycle of the electrical angle of the motor; and a supply unit that generates the PWM signal based on the carrier wave and the modulating wave corrected by the correction unit, and supplies the generated PWM signal to the multiple switching elements.

2. The servo driver according to claim 1, further comprising an identification unit that identifies a heat-generating switching element among the plurality of switching elements that generates more heat than the other switching elements, and the correction unit corrects the modulated wave of each phase so as to shorten the on-time of the heat-generating switching element.

3. The servo driver according to claim 2, wherein the determination unit determines a first PWM signal based on information relating to the operation of the motor and the command signal, and the identification unit calculates a current value for each of the plurality of switching elements based on the first PWM signal and the current value of each phase of the three-phase AC, and identifies the heat-generating switching element based on the calculated current value for each of the plurality of switching elements.

4. The servo driver according to claim 2, further comprising an ammeter that measures the current value of the current flowing through each of the plurality of switching elements, and the identification unit identifies the switching element with the largest current value as the heat-generating switching element based on the current value measured by the ammeter.

5. The servo driver according to claim 2, further comprising a thermometer that measures the temperature of each of the plurality of switching elements, and the identification unit identifies the switching element with the highest temperature as the heat-generating switching element based on the temperatures measured by the thermometer.

6. A servo driver as described in claim 2, further comprising a memory unit that stores a correspondence relationship between heat received from heat-generating components arranged within the servo driver and a correction value for each of the plurality of switching elements, and the identification unit corrects the heat generated by the plurality of switching elements based on the correspondence relationship to identify the heat-generating switching element.

7. A servo driver according to any one of claims 1 to 6, wherein the correction unit corrects the modulated wave of each phase when an index relating to the output of the motor is equal to or greater than a predetermined threshold value.

8. The servo driver according to claim 7, wherein the indicator relating to the motor output includes the drive current supplied to the motor.

9. The servo driver according to claim 7, wherein the index relating to the motor output includes a torque command value indicated by the command signal.

10. A servo driver according to any one of claims 1 to 6, wherein the correction unit determines a correction amount for the modulated wave of each phase in accordance with the speed of the motor.

11. The servo driver described in claim 10, wherein the correction unit corrects the modulated wave of each phase with a first correction amount when the motor speed is equal to or less than a first threshold, corrects the modulated wave of each phase with a second correction amount smaller than the first correction amount when the motor speed is equal to or greater than a second threshold that is greater than the first threshold, and reduces the correction amount as the motor speed increases, within a range less than the first correction amount and greater than the second correction amount, when the motor speed is greater than the first threshold and less than the second threshold.

12. A servo driver according to any one of claims 1 to 6, wherein the correction unit determines a correction amount for the modulated wave of each phase depending on the position of the motor.

13. The servo driver described in claim 12, wherein the correction unit: when the motor position is equal to or less than a third threshold, corrects the modulated wave of each phase with a third correction amount; when the motor position is equal to or greater than a fourth threshold that is greater than the third threshold, corrects the modulated wave of each phase with a fourth correction amount that is smaller than the third correction amount; and when the motor position is greater than the third threshold and less than the fourth threshold, decreases the correction amount as the motor position increases, within a range that is less than the third correction amount and greater than the fourth correction amount.

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