Method and Control System for Controlling an Electric Motor

Abstract

A method for controlling an electric motor, the method including determining a planned reference speed of the electric motor; determining a pulse-width modulation (PWM) switching frequency based on the planned reference speed; and controlling the electric motor with an alternating current produced by PWM switching with the determined PWM switching frequency. A control system for controlling an electric motor and an industrial robot including a control system, are also provided.

Claims

1. A method for controlling electric motors the method comprising: determining a planned reference speed of a plurality of electric motors of an industrial robot; wherein: for each of the electric motors, determining a pulse-width modulation (PWM) switching frequency based on the planned reference speed of the electric motor; and controlling each of the electric motors with an alternating current produced by PWM switching with the determined PWM switching frequency.

2. The method according to claim 1, wherein the determination of the PWM switching frequency comprises determining a higher PWM switching frequency if the planned reference speed is relatively high, or determining a lower PWM switching frequency if the planned reference speed is relatively low.

3. The method according to claim 1, wherein the determination of the PWM switching frequency comprises setting a predetermined high PWM switching frequency when the planned reference speed is above a high speed threshold value.

4. The method according to claim 1, wherein the determination of the PWM switching frequency comprises setting a predetermined low PWM switching frequency when the planned reference speed is below a low speed threshold value.

5. The method according to claim 4, wherein the determination of the PWM switching frequency comprises setting a PWM switching frequency that is proportional to the planned reference speed when the planned reference speed is above the low speed threshold value.

6. The method according to claim 1, wherein the determination of the PWM switching frequency comprises: predicting a load on the electric motor based on the planned reference speed; and determining the PWM switching frequency based on the load on the electric motor.

7. The method according to claim 6, wherein the prediction of the load on the electric motor is made based on a mathematical model of the electric motor.

8. The method according to claim 1, further comprising adjusting one or more control parameters of a drive unit for outputting the alternating current, based on the planned reference speed.

9. A control system for controlling electric motors, the control system comprising a data processing device and a memory having a computer program stored thereon, the computer program comprising program code which, when executed by the data processing device, causes the data processing device to perform the step of determining a planned reference speed of a plurality of electric motors of an industrial robot; wherein the computer program further comprises program code which, when executed by the data processing device, causes the data processing device to perform the steps of: for each of the electric motors, determining a PWM switching frequency based on the planned reference speed, of the electric motor; and controlling each of the electric motors with an alternating current produced by PWM switching with the determined PWM switching frequency.

10. An industrial robot comprising a control system according to claim 9 and a manipulator having at least one electric motor.

11. The industrial robot according to claim 10, wherein the electric motor comprises at least six poles.

12. The industrial robot according to claim 10, wherein the control system comprises a path planner configured to determine the planned reference speed of the electric motor.

13. The method according to claim 2, wherein the determination of the PWM switching frequency comprises setting a predetermined high PWM switching frequency when the planned reference speed is above a high speed threshold value.

14. The method according to claim 2, wherein the determination of the PWM switching frequency comprises setting a predetermined low PWM switching frequency when the planned reference speed is below a low speed threshold value.

15. The method according to claim 2, wherein the determination of the PWM switching frequency comprises: predicting a load on the electric motor based on the planned reference speed; and determining the PWM switching frequency based on the load on the electric motor.

16. The industrial robot according to claim 11, wherein the control system comprises a path planner configured to determine the planned reference speed of the electric motor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Further details, advantages and aspects of the present disclosure will become apparent from the following embodiments taken in conjunction with the drawings, wherein:

[0037] FIG. 1: schematically represents a block diagram of an industrial robot comprising a control system and a manipulator having a plurality of electric motors.

DETAILED DESCRIPTION

[0038] In the following, a method for controlling an electric motor where a pulse-width modulation (PWM) switching frequency is determined based on a planned reference speed of the electric motor, a control system for controlling an electric motor, and an industrial robot comprising a control system, will be described. The same reference numerals will be used to denote the same or similar structural features.

[0039] FIG. 1 schematically represents a block diagram of one example of an industrial robot 10 comprising a control system 12 and a manipulator 14 having a plurality of electric motors 16. The industrial robot 10 according to FIG. 1 constitutes one of many possible implementations of a method for controlling an electric motor 16 according to the present disclosure.

[0040] In FIG. 1, the electric motors 16 of the manipulator 14 are used to control movements (e.g. rotational or translational) of a plurality of links (not shown) relative to each other. Each electric motor 16 is arranged to drive a joint (not shown) between two adjacent links. In the non-limiting example in FIG. 1, each electric motor 16 is constituted by a rotary electric servomotor having 16 poles. The manipulator 14 is illustrated as comprising six electric motors 16, but the number of electric motors 16 may be increased or reduced.

[0041] The manipulator 14 further comprises a plurality of position sensors 18, e.g. resolvers, associated with the electric motors 16. Each position sensor 18 is arranged for real-time detection of the rotational position of an associated electric motor 16. Signals representing the measured position 20 of each electric motor 16 are sent to the control system 12. Optionally, the manipulator 14 further comprises one or more speed detection sensors (not shown) for real-time detection of the rotational speed of each electric motor 16.

[0042] The control system 12 comprises a plurality of drive units 22. Each drive unit 22 is configured to produce an alternating current 24 with a certain electric frequency produced by PWM technique. In the example in FIG. 1, the control system 12 comprises one drive unit 22 associated with each electric motor 16. One drive unit 22 may however alternatively drive a plurality of electric motors 16.

[0043] Each drive unit 22 may comprise a rectifier for converting AC into DC, a frequency inverter, and a DC bus connected between the rectifier and the inverter. The inverter converts the DC current to a variable alternating current 24. Each drive unit 22 may further comprise an inverter control unit that controls switching of one or more switching elements of the inverter according to a commanded switching frequency. The variable alternating current 24 from the inverter of the drive unit 22 is supplied to an associated electric motor 16.

[0044] The inverter of the drive unit 22 may further comprise an IGBT (insulated-gate bipolar transistor) module. The IGBT module has a lifetime depending on the power cycling. If high PWM switching frequencies are used also at lower speeds of the associated electric motor 16, the lifetime of the IGBT module is reduced.

[0045] The control system 12 of this example further comprises a main computer 26. The main computer 26 comprises a data processing device 28 (e.g. a central processing unit, CPU) and a memory 30. A computer program is stored in the memory 30. A manipulator program, a model of the manipulator 14 and a path planner is implemented in the main computer 26, e.g. in the memory 30. The path planner plans the path of the manipulator 14. For each electric motor 16 of the manipulator 14, the path planner generates a signal representing planned reference speed 32 based on movement instructions from the manipulator program and the model of the manipulator 14. The control system 12 is thereby configured to determine a planned reference speed 32 of the electric motor 16.

[0046] The path planner may further generate a signal representing a reference position 34 based on movement instructions from the manipulator program and the model of the manipulator 14 for each electric motor 16. The planned reference speed 32, and optionally the reference position 34 for each electric motor 16, are sent to the associated drive unit 22.

[0047] The planned reference speed 32 for each electric motor 16 and the measured position 20 for each electric motor 16 are used by the associated drive unit 22 for close loop PID control of the electric motor 16. The control parameters of the PID control (K.sub.p, K.sub.i, K.sub.d) may be regulated with a function of the planned reference speed 32 and the PWM switching frequency.

[0048] The alternating current 24 may for example be generated by means of SVPWM (Space Vector PWM) and current loop PI control, implemented in each drive unit 22. The control parameters of the PI control (K.sub.p, K.sub.i) may be regulated with a function of the planned reference speed 32 and the PWM switching frequency.

[0049] The control system 12 is further configured to determine a PWM switching frequency based on the planned reference speed 32 of the associated electric motor 16. A high switching frequency may be set when the planned reference speed 32 is high and a low switching frequency may be set when the planned reference speed 32 is low. The alternating current 24, produced by PWM switching with the determined PWM switching frequency, is output to the electric motor 16 when driving the electric motor 16 at the planned reference speed 32.

[0050] The PWM switching frequency may be set to a predetermined low value (e.g. 2 kHz) when the planned reference speed 32 is below a low speed threshold value (e.g. 250 rpm). The PWM switching frequency may be set to a predetermined high value (e.g., 10 kHz) when the planned reference speed 32 is above a high speed threshold value (e.g. 6000 rpm). The PWM switching frequency may be set to an intermediate value (between the predetermined low value and the predetermined high value) proportional to the planned reference speed 32 when the planned reference speed 32 is between the low speed threshold value and the high speed threshold value.

[0051] While the present disclosure has been described with reference to exemplary embodiments, it will be appreciated that the present invention is not limited to what has been described above. For example, it will be appreciated that the dimensions of the parts may be varied as needed.