Control device for brushless motor and control method for brushless motor

Abstract

A control device for a brushless motor according to the present invention controls a brushless motor in a sensorless manner by switching between a square-wave driving system and a sine-wave driving system. In such a case, when switching from the sine-wave driving system to the square-wave driving system, the control device of the brushless motor advances, by a prescribed angle, the motor angle estimated from the phase voltage and the phase current of the brushless motor in the sine-wave driving system, and initiates control of the brushless motor according to the square-wave driving system.

Description

Brushless motor control device and brushless motor control method

[0001] The present invention relates to a control device for a brushless motor and a control method for a brushless motor.

[0002] In the control of brushless motors, a technology is known in which sensorless control is performed by switching between a rectangular wave drive method (120° energization method) and a sine wave drive method (180° energization method), as described in Japanese Patent Laid-Open No. 2014-79041 (Patent Document 1).

[0003] JP 2014-79041 A

[0004] In a sensorless sine wave drive system, the motor angle of a brushless motor is estimated from the three-phase voltage and three-phase current of the brushless motor, and a sine wave voltage corresponding to this motor angle and a target rotation speed is applied to drive the brushless motor. In a sensorless square wave drive system, the motor angle of a brushless motor can be detected in 60° increments, and the current application mode is sequentially switched every 60° to drive the brushless motor. When switching the drive system of a brushless motor from a sine wave drive system to a square wave drive system, for example, if the motor angle estimated in the sine wave drive system is just before 180°, it may not be possible to detect that the motor angle has reached 180° in the first control cycle after switching to the square wave drive system. In this case, the 180° angle cannot be detected until the brushless motor has rotated one more time, which could result in inappropriate control of the brushless motor and loss of synchronization.

[0005] Therefore, an object of the present invention is to provide a brushless motor control device and a brushless motor control method that make it less likely for step-out to occur when the drive method of a brushless motor is switched from a sine wave drive method to a square wave drive method.

[0006] The brushless motor control device controls the brushless motor in a sensorless manner by switching between a square wave drive system and a sine wave drive system. When switching from the sine wave drive system to the square wave drive system, the brushless motor control device advances the motor angle estimated from the phase voltage and phase current of the brushless motor in the sine wave drive system by a predetermined angle and starts control of the brushless motor using the square wave drive system.

[0007] According to the present invention, in a brushless motor control device and a brushless motor control method, it is possible to make it difficult for step-out to occur when the drive system of a brushless motor is switched from a sine wave drive system to a square wave drive system.

[0008] FIG. 1 is a schematic diagram showing an example of a motor system mounted on a vehicle. FIG. 2 is an explanatory diagram of the internal structure of an example of a microcomputer. FIG. 3 is an explanatory diagram of problems that occur when switching the drive system from a sine wave drive system to a square wave drive system. FIG. 4 is an explanatory diagram of a method for resolving problems that occur when switching the drive system from a sine wave drive system to a square wave drive system. FIG. 5 is an explanatory diagram of a method for calculating a voltage equivalent to the q-axis voltage in vector control. FIG. 6 is an explanatory diagram of a method for calculating a predetermined rotation speed at which the control system is switched from the sine wave drive system to the square wave drive system. FIG. 7 is an explanatory diagram of another method for calculating a predetermined rotation speed at which the control system is switched from the sine wave drive system to the square wave drive system. FIG. 8 is an explanatory diagram of a map for dynamically setting a predetermined rotation speed. FIG. 9 is an explanatory diagram of a method for updating the predetermined rotation speed by learning.

[0009] Hereinafter, an embodiment for carrying out the present invention will be described in detail with reference to the accompanying drawings. Fig. 1 shows an example of a motor system 100 mounted on a vehicle, such as a passenger car, bus, truck, or construction machine, to which the present embodiment is applied. Note that the motor system 100 described below is merely an example showing one example of the present embodiment, and should not be construed as being limited to its configuration.

[0010] The motor system 100 includes a three-phase brushless motor 200 and a motor controller 300. Here, the three-phase brushless motor 200 is given as an example of a brushless motor.

[0011] Three-phase brushless motor 200 has a substantially cylindrical stator 200S around which U-phase coil 200U, V-phase coil 200V, and W-phase coil 200W are wound in a star connection, and rotor 200R rotatably disposed on the inner circumferential surface of stator 200S. The rotary drive shaft of rotor 200R protrudes from the axial end of stator 200S, and a rotary drive force is output from this shaft to the outside.

[0012] The motor controller 300 includes an inverter 320 that drives the three-phase brushless motor 200, a shunt resistor 340, a voltage sensor 360, and a microcomputer 380. Here, the microcomputer 380 is an example of a control device for a brushless motor.

[0013] Inverter 320 is a power supply circuit that converts DC current supplied from a DC power source VB such as a battery mounted on the vehicle into AC current and selectively energizes U-phase coil 200U, V-phase coil 200V, and W-phase coil 200W of three-phase brushless motor 200, thereby driving three-phase brushless motor 200. Inverter 320 is a three-phase bridge circuit configured by appropriately connecting switching elements 320A to 320F such as N-channel MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) having freewheeling diodes.

[0014] Specifically, inverter 320 has a U-phase arm in which switching elements 320A and 320B are connected in series, a V-phase arm in which switching elements 320C and 320D are connected in series, and a W-phase arm in which switching elements 320E and 320F are connected in series. Note that switching elements 320A to 320F are not limited to N-channel MOSFETs, and may also be P-channel MOSFETs, NPN transistors, PNP transistors, IGBTs (Insulated Gate Bipolar Transistors), or the like.

[0015] In the U-phase arm, the drain of upper switching element 320A is connected to DC power supply VB, the source of upper switching element 320A is connected to the drain of lower switching element 320B, and the source of lower switching element 320B is connected to ground GND. An electric circuit located between upper switching element 320A and lower switching element 320B is connected to U-phase coil 200U of three-phase brushless motor 200 via U-phase drive line 320U. In the following description, upper switching element 320A will be referred to as "U-phase upper FET 320A," and lower switching element 320B will be referred to as "U-phase lower FET 320B."

[0016] In the V-phase arm, the drain of upper switching element 320C is connected to DC power supply VB, the source of upper switching element 320C is connected to the drain of lower switching element 320D, and the source of lower switching element 320D is connected to ground GND. An electric circuit located between upper switching element 320C and lower switching element 320D is connected to V-phase coil 200V of three-phase brushless motor 200 via V-phase drive line 320V. In the following description, upper switching element 320C will be referred to as "V-phase upper FET 320C," and lower switching element 320D will be referred to as "V-phase lower FET 320D."

[0017] In the W-phase arm, the drain of upper switching element 320E is connected to DC power supply VB, the source of upper switching element 320E is connected to the drain of lower switching element 320F, and the source of lower switching element 320F is connected to ground GND. An electric circuit located between upper switching element 320E and lower switching element 320F is connected to W-phase coil 200W of three-phase brushless motor 200 via W-phase drive line 320W. In the following description, upper switching element 320E will be referred to as "W-phase upper FET 320E," and lower switching element 320F will be referred to as "W-phase lower FET 320F."

[0018] The shunt resistor 340 is disposed on a common electric path connecting the U-phase lower stage FET 320B, the V-phase lower stage FET 320D, and the W-phase lower stage FET 320F to the ground GND, i.e., on the bus bar of the inverter 320. The shunt resistor 340 detects three-phase currents flowing through the U-phase coil 200U, the V-phase coil 200V, and the W-phase coil 200W of the three-phase brushless motor 200.

[0019] The voltage sensor 360 is arranged on the U-phase drive line 320U, the V-phase drive line 320V, and the W-phase drive line 320W of the inverter 320, and detects the three-phase voltage or the voltage between two phases of the U-phase coil 200U, the V-phase coil 200V, and the W-phase coil 200W, with the ground GND as the reference voltage.

[0020] As shown in FIG. 2, the microcomputer 380 incorporates a processor 380A, a non-volatile memory 380B, a volatile memory 380C, a communication circuit 380D, an input / output circuit 380E, and an internal bus 380F that connects these circuits so that they can communicate with each other.

[0021] The processor 380A is an electronic device that executes an instruction set written in an application program, and is composed of, for example, a CPU (Central Processing Unit). The non-volatile memory 380B is a semiconductor storage element that can retain data even when the power supply is cut off, and is composed of, for example, an EEPROM (Electrically Erasable Programmable Read Only Memory) or a flash ROM (Read Only Memory). The volatile memory 380C is a semiconductor storage element that loses data when the power supply is cut off, and is composed of, for example, a DRAM (Dynamic Random Access Memory) or an SRAM (Static Random Access Memory).

[0022] The communication circuit 380D is an electronic device for communicating with other controllers via an in-vehicle network such as a Controller Area Network (CAN), and is composed of, for example, a CAN transceiver. The input / output circuit 380E is an electronic device for reading analog or digital signals from various sensors and switches, and for outputting analog or digital drive signals to various actuators, and is composed of, for example, an A / D converter, a D / A converter, a D / D converter, etc. The internal bus 380F is a path for exchanging data between devices, and includes an address bus for transferring addresses, a data bus for transferring data, and a control bus for exchanging timing and control information for actual input / output via the address bus and data bus.

[0023] The output signal of shunt resistor 340 and the output signal of voltage sensor 360 are input to microcomputer 380 via its input / output circuit 380E. Microcomputer 380 receives the target rotation speed of three-phase brushless motor 200 from a higher-level controller via communication circuit 380D, and controls three-phase brushless motor 200 in a sensorless manner by switching between a rectangular wave drive method and a sine wave drive method depending on the target rotation speed.

[0024] An electric circuit located between the DC power supply VB and the inverter 320 and an electric circuit located between the shunt resistor 340 and the ground GND are connected by an electric circuit on which a smoothing capacitor 400 is disposed. The smoothing capacitor 400 keeps the output voltage of the DC power supply VB substantially constant, prevents damage to the U-phase upper stage FET 320A, the U-phase lower stage FET 320B, the V-phase upper stage FET 320C, the V-phase lower stage FET 320D, the W-phase upper stage FET 320E, and the W-phase lower stage FET 320F due to a surge, and reduces radiation noise.

[0025] Here, we will explain an overview of the sine wave drive method and square wave drive method for sensorless control of three-phase brushless motor 200, which are executed by microcomputer 380 of motor controller 300 in accordance with an application program stored in non-volatile memory 380B. Note that since the sine wave drive method and square wave drive method are well known to those skilled in the art, only the key points will be briefly explained below.

[0026] <Sine Wave Drive Method> When three-phase brushless motor 200 is controlled without a sensor, the motor angle cannot be directly detected by a sensor. Therefore, microcomputer 380 estimates the motor angle at each predetermined control period from the three-phase voltage detected by voltage sensor 360 and the three-phase current detected by shunt resistor 340. Microcomputer 380 then generates sinusoidal voltages, each 120 degrees out of phase, to be applied to U-phase coil 200U, V-phase coil 200V, and W-phase coil 200W of three-phase brushless motor 200, based on the motor angle estimated from the three-phase voltage and three-phase current. Microcomputer 380 then applies the generated sinusoidal voltages to U-phase coil 200U, V-phase coil 200V, and W-phase coil 200W, respectively. These three sinusoidal voltages can be generated by microcomputer 380 controlling inverter 320 using pulse width modulation (PWM). The sine wave drive system is superior to the square wave drive system in terms of control precision, efficiency, vibration and noise, but has the characteristic that the system is complicated and expensive.

[0027] <Square Wave Drive Method> When three-phase brushless motor 200 is controlled without a sensor, as in the case of the sine wave drive method, the motor angle cannot be directly detected by a sensor. However, microcomputer 380 can detect the motor angle every 60 degrees (0 degrees, 60 degrees, 120 degrees, 180 degrees, 240 degrees, 300 degrees, and 360 degrees), for example, by detecting the zero-crossing points of the induced voltage. Therefore, upon detecting the motor angle in a predetermined control cycle, microcomputer 380 generates complementary square wave voltages that are turned on for a 120-degree period and that energize two phase coils out of U-phase coil 200U, V-phase coil 200V, and W-phase coil 200W of three-phase brushless motor 200. Microcomputer 380 then energizes two phase coils out of U-phase coil 200U, V-phase coil 200V, and W-phase coil 200W with the square wave voltages thus generated. Similar to the sine wave drive method, such complementary square wave voltages can be generated by the microcomputer 380 performing PWM control on the inverter 320. The square wave drive method is inferior to the sine wave drive method in terms of control accuracy, efficiency, vibration, and noise, but has the advantage that the system is simple and inexpensive.

[0028] If the sine wave drive system is switched to the square wave drive system just before the motor angle reaches a detectable value every 60 degrees under the square wave drive system, it may not be possible to detect that the motor angle has exceeded the detectable motor angle during the first control cycle of the square wave drive system. To explain this using a specific example, as shown in FIG. 3 , if the motor angle estimated under the sine wave drive system is switched to the square wave drive system just before reaching 180 degrees, the motor angle may exceed 180 degrees before the first control cycle of the square wave drive system. As a result, it may not be possible to detect that the motor angle has exceeded 180 degrees (crossover detection). In this case, because the microcomputer 380 cannot detect that the motor angle has exceeded 180 degrees, it maintains the motor angle at the time of switching from the sine wave drive system to the square wave drive system until the three-phase brushless motor 200 makes another rotation, as indicated by the dashed line in the figure. Therefore, the complementary square wave voltage generated in response to this motor angle corresponds to the motor angle recognized at the time the drive system was switched, which may result in inappropriate control of the three-phase brushless motor 200, resulting in loss of synchronization. In FIG. 3, the motor angles indicated by white circles in the square wave drive method indicate actual motor angles.

[0029] Therefore, in this embodiment, as shown in FIG. 4 , when switching from the sine wave drive method to the square wave drive method, the motor angle estimated from the three-phase voltage and three-phase current in the sine wave drive method is advanced by a predetermined angle to start control of the three-phase brushless motor 200 using the square wave drive method. The predetermined angle can be set to a value that does not exceed the motor angle that can be detected between the last control cycle in the sine wave drive method and the first control cycle in the square wave drive method. Specifically, the predetermined angle can be set between a minimum value (motor rotation speed [degrees / second] × control cycle [seconds] at which the drive method is switched) and a maximum value (60 degrees). Note that the control cycle for switching the drive method is not limited to one control cycle, nor is it limited to a fixed value. Therefore, the predetermined angle can be a fixed worst-case value or calculated during control.

[0030] 4, when the motor angle estimated under the sine wave drive method is switched to the square wave drive method just before reaching 180 degrees, microcomputer 380 advances the motor angle recognized at the time of control method switching by 30 degrees and begins control of three-phase brushless motor 200 using the square wave drive method. In this case, microcomputer 380 can detect that the motor angle has exceeded 180 degrees in the first control cycle after switching to the square wave drive method, and can therefore supply complementary square wave voltages corresponding to this motor angle to two of U-phase coil 200U, V-phase coil 200V, and W-phase coil 200W of three-phase brushless motor 200.

[0031] Therefore, by appropriately setting the predetermined angle, it is possible to detect that three-phase brushless motor 200 has exceeded the motor angle that can be detected every 60 degrees immediately after switching the control method, and to optimize the complementary square wave voltages that are applied to two of U-phase coil 200U, V-phase coil 200V, and W-phase coil 200W. By optimizing the square wave voltages, it is possible to suppress loss of synchronization when switching the drive method.

[0032] Incidentally, when the drive method of three-phase brushless motor 200 is switched from the sine wave drive method to the square wave drive method, in addition to the problems described above, if the square wave voltage immediately after switching to the square wave drive method deviates from the sine wave voltage of the sine wave drive method, there is a possibility that the rotation speed of three-phase brushless motor 200 will change suddenly. This is because the three-phase current changes suddenly before and after the drive method of three-phase brushless motor 200 is switched due to the deviation between the square wave voltage and the sine wave voltage.

[0033] Therefore, in this embodiment, when switching from the sine wave drive method to the square wave drive method, control of three-phase brushless motor 200 is started using the square wave drive method with a voltage (initial output voltage) equivalent to the q-axis voltage of vector control in the sine wave drive method. Here, the q-axis voltage is the voltage of the torque generation component of three-phase brushless motor 200. In this way, when switching from the sine wave drive method to the square wave drive method, control is started with an initial output voltage equivalent to the q-axis voltage, which is the voltage of the torque generation component of three-phase brushless motor 200, so that sudden changes in the three-phase current and rotational speed can be suppressed and loss of synchronization can be prevented.

[0034] The initial output voltage equivalent to the q-axis voltage in vector control can be calculated from V2 = V1 × (square root of 3), taking into consideration that V1 is a phase voltage referenced to the neutral point NP in the sine wave drive system and V2 is an inter-phase voltage between two phase coils in the square wave drive system, as shown in Figure 5. Note that this calculation formula is merely an example and may be changed depending on the control system.

[0035] The drive system switches from the sine wave drive system to the square wave drive system when the rotation speed of the three-phase brushless motor 200 drops to a predetermined rotation speed. Since the sine wave drive system is superior to the square wave drive system in terms of control accuracy, efficiency, vibration, and noise, it is desirable to set the predetermined rotation speed as small as possible. Therefore, it is desirable to set the predetermined rotation speed to a rotation speed at which the motor angle estimation accuracy can be maintained in the sine wave drive system, or to an angle at which the three-phase brushless motor 200 does not stop when switching from the sine wave drive system to the square wave drive system.

[0036] Considering the sinusoidal wave drive system, as the motor rotation speed decreases, the error in the estimation accuracy of the motor angle increases. For this reason, the predetermined rotation speed is set to be equal to or less than the error allowable in the motor system 100. If the motor rotation speed is ωr, the error in the estimation accuracy of the motor angle is Δθ, the d-axis current is Id, the q-axis current is Iq, the motor phase resistance is R, the induced voltage constant is Ke, and the q-axis inductance is Lq, then the relationship shown in the following equation holds. Note that the subscript c in the following equation indicates a numerical value recognized by the microcomputer 380. Therefore, the predetermined rotation speed can be determined by calculating the motor rotation speed ωr from the following equation.

[0037] Also, from the perspective of the square wave drive system, as shown in Figure 6, immediately after switching from the sine wave drive system to the square wave drive system, the rotational speed of three-phase brushless motor 200 continues to decrease rapidly, and in the worst case, reaches zero. For this reason, immediately after switching from the sine wave drive system to the square wave drive system, a predetermined rotational speed is set so that the rotational speed of three-phase brushless motor 200 does not reach zero, that is, a predetermined rotational speed so that three-phase brushless motor 200 does not stop. Taking into account that the characteristics differ depending on motor system 100, the predetermined rotational speed can be determined, for example, by experiments using an actual machine or simulations using a computer.

[0038] The predetermined rotation speed is then selected to be the larger of the two rotation speeds determined as described above, thereby achieving the performance requirements for both the sine wave drive method and the square wave drive method.

[0039] ​Furthermore, the predetermined rotational speed is not limited to a fixed value set as described above, but may be dynamically set as follows. As shown in FIG. 7 , when the target rotational speed of three-phase brushless motor 200 decreases due to a step response, if the drive mode is switched due to the step response of the target rotational speed, the amount of undershoot of the motor rotational speed after the step response is affected by the deviation between the actual rotational speed before the step response and the target rotational speed after the step response. Therefore, when the target rotational speed undergoes a step response, as shown in FIG. 8 , a map is referenced in which predetermined rotational speeds are set according to the actual rotational speed before the step response and the target rotational speed after the step response, and the predetermined rotational speed at which the drive mode is switched can be dynamically set. This reduces the amount of undershoot when the target rotational speed undergoes a step response, thereby making it less likely for step-out to occur. The map shown in FIG. 8 can be obtained, for example, by experiments using an actual machine or computer simulations.

[0040] The map for dynamically setting the predetermined rotational speed need not be configured with pre-determined fixed values, but may be sequentially updated through learning. When the target rotational speed of three-phase brushless motor 200 decreases due to a step response, the actual amount of undershoot in the motor rotational speed is measured. Then, as shown in FIG. 9 , a margin for the minimum drive rotational speed of three-phase brushless motor 200 is determined, and a new predetermined rotational speed is calculated from this margin to update the map. Specifically, the new predetermined rotational speed can be set to the larger of "current predetermined rotational speed - margin" and "minimum drive rotational speed." In this way, even if the characteristics of motor system 100 change over time, this change can be corrected to continue to perform the original function.

[0041] Furthermore, a person skilled in the art will easily understand that new embodiments can be created by omitting parts of the various technical ideas of the above embodiments, combining parts appropriately, or replacing parts with well-known technology, provided that the required functions can be achieved.

[0042] As one example, the amount of change in the target rotation speed of three-phase brushless motor 200 may be limited to a predetermined amount or less. In this way, the amount of change in the target rotation speed of three-phase brushless motor 200 is limited, and therefore the amount of change in the motor rotation speed, three-phase voltage, and three-phase current when switching the drive method is suppressed, thereby enabling stable control of three-phase brushless motor 200.

[0043] Furthermore, the motor system 100 can control a three-phase brushless motor 200 mounted on a vessel, not limited to a vehicle.

[0044] 200...3-phase brushless motor (brushless motor), 380...microcomputer (control device)

Claims

1. A brushless motor control device that controls a brushless motor sensorlessly by switching between a square wave drive system and a sine wave drive system, configured to, when switching from the sine wave drive system to the square wave drive system, advance the motor angle estimated from the phase voltage and phase current of the brushless motor in the sine wave drive system by a predetermined angle, and start control of the brushless motor using the square wave drive system.

2. A brushless motor control device according to claim 1, configured to, when switching from the sine wave drive method to the square wave drive method, start control of the brushless motor using the square wave drive method with a voltage equivalent to the q-axis voltage in the sine wave drive method.

3. The brushless motor control device according to claim 1, wherein the switching from the sine wave drive method to the square wave drive method is performed when the rotation speed of the brushless motor drops to a predetermined rotation speed.

4. A brushless motor control device according to claim 3, wherein the predetermined angle by which the brushless motor is advanced is set based on the predetermined rotation speed at which the brushless motor is switched from the sine wave drive system to the square wave drive system, and the control period of the brushless motor.

5. A brushless motor control device as set forth in claim 3, wherein the predetermined rotation speed at which the brushless motor is switched from the sine wave drive system to the square wave drive system is set to a rotation speed at which the estimation accuracy of the motor angle in the sine wave drive system can be maintained, or a rotation speed at which the brushless motor does not stop when switching from the sine wave drive system to the square wave drive system.

6. The brushless motor control device according to claim 1, which is configured to limit the amount of change in the target rotation speed of the brushless motor to a predetermined amount or less.

7. The brushless motor control device according to claim 1, wherein the brushless motor is a three-phase brushless motor.

8. A method for controlling a brushless motor, wherein a control device that controls a brushless motor sensorlessly by switching between a square wave drive system and a sine wave drive system advances the motor angle estimated from the phase voltage and phase current of the brushless motor in the sine wave drive system by a predetermined angle when switching from the sine wave drive system to the square wave drive system, and starts control of the brushless motor using the square wave drive system.

9. The brushless motor control method according to claim 8, wherein when the control device switches from the sine wave drive method to the square wave drive method, the control device starts control of the brushless motor using the square wave drive method with a voltage equivalent to the q-axis voltage in the sine wave drive method.

10. The method for controlling a brushless motor according to claim 8, wherein the control device switches from the sine wave drive system to the square wave drive system when the rotation speed of the brushless motor drops to a predetermined rotation speed.

11. A method for controlling a brushless motor according to claim 10, wherein the predetermined angle by which the brushless motor is advanced is set based on the predetermined rotation speed at which the brushless motor is switched from the sine wave drive system to the square wave drive system, and the control period of the brushless motor.

12. A method for controlling a brushless motor as set forth in claim 10, wherein the predetermined rotational speed at which the brushless motor is switched from the sine wave drive system to the square wave drive system is set to a rotational speed at which the estimation accuracy of the motor angle in the sine wave drive system can be maintained, or a rotational speed at which the brushless motor does not stop when switching from the sine wave drive system to the square wave drive system.

13. The method for controlling a brushless motor according to claim 8, wherein the control device limits the amount of change in the target rotation speed of the brushless motor to a predetermined amount or less.

14. The method for controlling a brushless motor according to claim 8, wherein the brushless motor is a three-phase brushless motor.

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