Secure starting of ac motor

Abstract

A method (400) for safely starting an alternating current, AC, motor from a preloaded rest state, the method comprising: applying (402) a brake to immobilize a shaft of the AC motor; feeding (404) a predefined drive signal waveform configured to not generate torque to the AC motor, and sensing the resulting stator current; comparing (406) the sensed stator current with a reference current that is associated with the drive signal; and permitting (408) release of the brake in response to finding that the sensed stator current matches the reference current.

Description

Technical Field

[0001] The present disclosure relates to the field of electric motors, and more specifically, to a method for safely starting an alternating current (AC) motor from a pre-loaded stationary state. Background Art

[0002] Various types of internal faults can cause an electric motor to fail to deliver its rated torque. The severity of these faults depends on the nature of the application in which the electric motor is installed. For example, if the failed motor drives a fan, pump, or blower, the consequences of the failure may be limited. In contrast to such use cases, an electric motor failure in a robot, lift, or crane motor could be very harmful and even lead to physical injury. In the robotics use case, potential sources of failure can include:

[0003] - The insulated gate bipolar transistor (IGBT) used to generate voltage is damaged,

[0004] - Wrong motor cable connected between controller and robot,

[0005] -The motor cable between the controller and the robot is broken,

[0006] - Motor windings are damaged,

[0007] - The current measuring sensor is broken,

[0008] -The stator resistance is too large,

[0009] -Leakage inductance.

[0010] US20180254729A1 discloses a method for ensuring that an electric motor provides sufficient starting torque before releasing the brake. This is achieved by applying a compensation voltage to compensate for the lack of starting torque to the extent caused by the voltage drop.

[0011] Similarly, JP2011254596A discloses a method for improving magnetic flux rise in an induction motor and starting the induction motor in an optimal state for generating sufficient torque.

[0012] While these prior art approaches suggest ways of ensuring that the desired torque is generated in the braking state of an electric motor, it is desirable (especially for some use cases in robotics) to perform a health check on the motor while in a passive state. Summary of the invention

[0013] One object of the present disclosure is to propose a motor control method and a motor controller suitable for safely starting an AC motor from a preloaded static state. In this sense, after the AC motor successfully passes a health check, it can be considered safe to start the AC motor. Another object of the present disclosure is to propose a motor control method and a motor controller, by which the correct operation of the AC motor can be verified when the AC motor is in a torque-free state. Yet another object is to propose a motor control method and a motor controller suitable for starting an AC motor installed in an industrial robot.

[0014] At least some of the above objects are achieved by the invention as defined in the independent claims. The dependent claims relate to advantageous embodiments of the invention.

[0015] In a first aspect of the present disclosure, a method for safely starting an AC motor from a preloaded stationary state is provided. The method comprises: applying a brake to immobilize a shaft of the AC motor; feeding a predefined drive signal waveform to the AC motor, the predefined drive signal waveform being configured to generate no torque, and sensing a resulting stator current during the feeding; comparing the sensed stator current with a reference current, the reference current being associated with the drive signal; and permitting release of the brake if the sensed stator current is found to match the reference current.

[0016] According to the first aspect, since the AC motor is fed with a drive signal waveform configured not to generate torque, a comparison as to whether the sensed stator current matches a reference current can be performed in a non-rotating and substantially torque-free state of the AC motor, the reference signal being associated with the drive signal. Therefore, the applied brake does not have to absorb any electrically induced torque and is not exposed to mechanical wear that may be caused. Therefore, the brake is primarily used to immobilize the shaft against the action of torque applied by the deadweight of the machine on which the AC motor is mounted and / or from external forces on such a machine. When the machine is a robot arm, the external forces may include gravity or various elastic forces acting on the load during clamping of a workpiece.

[0017] As used herein, the action of "permitting release of the brake" may correspond to allowing the (human or automated) operator of the AC motor the ability to release the brake at the operator's discretion. Permitting release should not be understood to imply that the brake must be released when a match is found, which may be unsafe unless the AC motor is controlled to apply an appropriate starting torque. In this sense, a starting torque value may be considered appropriate if it roughly corresponds to the load on the motor shaft currently held by the brake.

[0018] If the sensed stator current is found to match the reference current, some important fault classes in the AC motor can be ruled out, and it can therefore be considered safe to start the motor. As used in this disclosure, the qualifier "safe" should not be understood as safe in an absolute sense, or should not refer to the certainty that every fault scenario can be ruled out if the comparison is successful.

[0019] In some embodiments, the predefined drive signal waveform is provided based on the following condition: the Q component in the rotor synchronous DQZ reference frame should be substantially equal to zero. By definition, the rotor synchronous DQZ reference frame (or rotating reference frame) is always aligned with the rotor phase: it rotates with the rotor of the AC motor and is stationary when the AC motor is stationary. In these embodiments, the non-zero component of the predefined drive signal waveform can have either polarity. More precisely, in a permanent magnet motor (PMSM), the non-zero component can be parallel or anti-parallel to the rotor magnet, thereby enhancing or weakening the permanent magnetic field.

[0020] In some embodiments, release of the brake is permitted in response to finding that the absolute error between the sensed stator current and the reference current is below a threshold. This provides a simple and robust criterion for determining whether the sensed stator current matches the reference current. If the absolute error between the sensed stator current and the reference current is found to exceed the threshold, release of the brake remains prohibited.

[0021] In some embodiments, an error indication may be provided in response to finding that the sensed stator current does not match the reference current.The error condition may be a human-perceivable signal or a message sent to a control processor or software application.

[0022] In some embodiments, comparing the sensed stator current to the reference current includes converting the stator current from a stationary reference frame to a rotor synchronous reference frame (or rotating reference frame), such as the DQZ reference frame mentioned above. In other words, when the shaft of the AC motor is stationary, the DQZ reference frame is stationary. Converting to the DQZ reference frame allows for an accurate and meaningful comparison of the actual stator current and the reference current associated with the predefined drive signal waveform.

[0023] In a second aspect of the present disclosure, a motor controller is provided, the motor controller being arranged to control an electric drive unit feeding an AC motor. The motor controller has a processing circuit configured to apply a brake to immobilize a shaft of the AC motor; feed a predefined drive signal waveform configured to generate no torque to the AC motor, and sense a resulting stator current; compare the sensed stator current with a reference current, the reference current being associated with the drive signal; and permit release of the brake in response to finding that the sensed stator current matches the reference current.

[0024] The invention also relates to a computer program comprising instructions which cause a computer, in particular a motor controller, to perform the above-described motor control method. The computer program may be stored or distributed on a data carrier. As used herein, a "data carrier" may be a temporary data carrier, such as a modulated electromagnetic wave or light wave, or a non-temporary data carrier. Non-temporary data carriers include volatile and non-volatile memories, such as permanent and non-permanent storage media of the magnetic, optical or solid-state type. Such memories remain within the scope of a "data carrier" and may be fixedly mounted or portable.

[0025] Generally, unless otherwise expressly defined herein, all terms used in the claims are to be understood according to their original meaning in the technical field. Unless otherwise expressly stated, all references to "an element, device, component, mode, step, etc." are to be understood as representing at least one instance of the element, device, component, mode, step, etc. Unless expressly stated, the steps of any method disclosed herein do not have to be performed in the exact order disclosed. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Various aspects and embodiments will now be described by way of example with reference to the accompanying drawings, in which:

[0027] Figure 1 is the cross section of a permanent magnet synchronous motor (PMSM);

[0028] Figure 2 The various physical quantities in the rotor synchronous DQZ reference frame are introduced;

[0029] Figure 3 is a block diagram representation of a motor controller connected to an AC motor via an electric drive unit;

[0030] Figure 4 is a flow chart of a motor control method according to an embodiment of the present invention; and

[0031] Figure 5 Shown in Figure 4 The following are the successive signal processing steps that occur in the motor control method. DETAILED DESCRIPTION

[0032] Various aspects of the present disclosure will now be described more fully below with reference to the accompanying drawings, in which certain embodiments of the present invention are shown. However, these aspects may be embodied in many different forms and should not be construed as limiting; rather, these embodiments are provided by way of example so that the present disclosure will be thorough and complete and fully convey the scope of all aspects of the present invention to those skilled in the art. Similar reference numerals are used throughout the specification to refer to similar elements.

[0033] Figure 11 is a cross section of a permanent magnet synchronous motor (PMSM) 100. A PMSM is an alternating current (AC) motor within the scope of the present disclosure. A PMSM can generally be classified as a surface mounted PMSM or an internal mounted PMSM. Figure 1 1 is a simplified diagram of an example surface mounted PMSM that includes a rotor 108 configured to rotate within a stator 102. The stator 102 includes a plurality of electrical windings 104 arranged around the rotor 108. For a surface mounted PMSM, permanent magnets 106 are mounted on the surface of the rotor 108. During operation, the current through the windings 104 establishes a magnetic field in the air gap 110 between the rotor 108 and the stator 102, and the interaction between the magnets 106 and the magnetic field causes the rotor 108 to rotate, thereby generating torque. The speed and direction of the rotor 108 can be controlled by controlling the current through the stator windings 104. An interior mounted PMSM is similar to a surface mounted PMSM, except that the permanent magnets 106 are buried within the rotor 108 instead of being mounted on the surface of the rotor 108. Figure 1 An example PMSM 100 is shown having a single rotor pole and three stator poles. The axial direction of the PMSM 100 may be imagined as being orthogonal to the plane of the drawing.

[0034] The teachings of the present disclosure may also be advantageously applied to several other types of AC motors, including induction motors. As previously described, the rotor magnetization in an induction motor is not static, but is induced by a current that opposes the magnetic field of the stator. The rotor current flows in short-circuited rotor windings, which may have a wound or squirrel cage circuit topology. As previously described, unlike synchronous motors like PMSMs, the rotation of an externally loaded induction motor is slightly slower than the rotation of the stator magnetic field (slip).

[0035] Figure 2 is a schematic diagram of a rotor synchronous DQZ reference frame, which is a way of representing the flow of current in an AC motor (such as a PMSM or an induction motor). The AC motor has three equidistant stator windings A, B, C arranged at equal pole angles, in which stator currents i A ,i B ,i C The AC motor also includes a rotor 108. The DQZ reference frame can also be used with AC motors having a greater number of stator poles, where each stator current can be used as i A ,i B ,i C The orthogonal axes α, β are stationary relative to the stator (i.e., stator synchronous), and the orthogonal direct and quadrature axes d, q are stationary relative to the rotor 108 (i.e., rotor synchronous). The direction of the d-axis is parallel to the magnetization of the rotor 108 in the PMSM.

[0036] The instantaneous stator current triplet without any common mode component (i A ,i B ,i C ) can be represented by the vector i in the d,q plane = (i D ,i Q In the present disclosure, the first category and the second component of i will be referred to as the direct axis (D) component and the quadrature axis (Q) component. Figure 2 The angles shown in are:

[0037] - Instantaneous rotor angle θ e (i.e. the angles between the α, β and d, q reference frames),

[0038] -The stator current phase angle θ in the stator reference frame αβ ,as well as

[0039] -The stator current phase angle θ in the rotor reference frame dq .

[0040] The stator current triplet with common mode variation (i A ,i B ,i C ) will also include a positive or negative so-called zero axis (Z) component i Z Transformation between ABC reference frame and DQZ reference frame

[0041]

[0042] Corresponding to the matrix

[0043]

[0044] Among them, θ e is the instantaneous rotor angle. The inverse transform corresponds to For further details, reference is made to B. Adkins and RG Harley, The General Theory of Alternating Current Machines: Application to Practical Problems, Chapman and Hall, London, 1975.

[0045] Figure 3An example configuration of a motor controller 310 is shown. The motor controller 310 may be implemented as part of a motor drive (e.g., a variable frequency drive) that controls the motion of the AC motor 100 based on a speed reference signal ωRef provided by a supervisory motion control application or system (not shown). In other configurations, the motor controller 310 may be implemented on one or more processing chips as part of an embedded system for controlling the AC motor 100. In yet another configuration, the motor controller 310 may be implemented as part of a motor control module of an industrial controller for controlling the AC motor 100 used in an industrial motion control system. It should be understood that the technology disclosed herein is not limited to these implementations.

[0046] In this example, the AC motor 100 is a sensorless motor whose motion is controlled by the motor controller 310. In operation, the motor controller 310 uses a flux control loop and a torque control loop to control the AC motor 100. The torque reference IsqRef and the flux reference IsdRef represent the target references for the quadrature (Q) and direct (D) components of the stator current, respectively. In order to provide feedback for the flux and torque control loops, the motor controller 310 measures the three stator currents (i A ,i B ,i C ),like Figure 3 As shown in the lower right portion of . Alternatively, two of the three-phase AC power delivered to the AC motor 100 may be measured, wherein the current of the third phase is calculated based on the values ​​of the other two phases. A transformation block 324 transforms the stator current measurements from the three-phase A, B, C reference system to a stationary α, β coordinate system (e.g., by Clarke transformation) to produce Isα and Isβ. Another transformation block 322 transforms Isα and Isβ to a rotor synchronous d, q coordinate system (e.g., Park transformation) to produce Isq and Isd. The Iq control block 314 and the Id control block 316 compare the values ​​of Isq and Isd with their corresponding reference values ​​IsqRef and IsdRef, and adjust the reference voltage values ​​Vsq and Vsd based on any errors detected between the measured values ​​Isq and Isd and their corresponding reference values ​​IsqRef and IsdRef. The adjustment may follow a P, PI, PD, PID, or similar control law.

[0047] The transformation block 318 transforms Vsq and Vsd from the rotating d, q reference frame to the stationary α, β reference frame (e.g., inverse Park transformation) to generate Vsα and Vsβ. Based on these values, the control signal output block 320 such as a space vector modulation (SVM) component or a pulse width modulation (PWM) component controls the AC output of the electric drive unit 330, thereby indirectly controlling the movement of the AC motor 100. The electric drive unit 330 can be powered by direct current Vdc.

[0048] If closed-loop sensorless control is used, during operation, the estimation component 326 estimates the speed of the AC motor 100 based on the measured stator currents Isα and Isβ and the reference voltage values ​​Vsα and Vsβ. The estimated speed ωEst is compared to a speed reference ωRef (received from an external source, such as an operator interface or a separate motion control application), and the speed control component 304 adjusts IsqRef as needed based on the detected error between the speed reference ωRef and the estimated speed ωEst. The optional field weakening (flux weakening) control component 306 controls the value of the flux reference IsdRef. In addition, the estimation component 326 provides a value for the rotor angle θ e The estimate θEst is obtained and fed to the transformation blocks 318 , 322 .

[0049] As an alternative to sensorless control, the motor controller 310 may directly measure the actual speed of the AC motor 100, rather than estimating the speed using the estimation component 326. For such sensing, an angle sensor (rotary encoder) may replace the estimation component 326.

[0050] It should be understood that Figure 3 The various components and blocks in the present invention may correspond to corresponding dedicated hardware units (eg, chipsets) or software units (eg, programs, modules), or to multi-functional hardware or software units.

[0051] This disclosure provides a reference Figure 3 An exemplary type of method 400 of operating a motor controller 310 to facilitate safe startup of a connected AC motor 100 is provided. The method 400 may be implemented at least in part by an operator, or may be implemented automatically. For example, the method 400 may be expressed as machine-readable instructions to be executed by the motor controller 310.

[0052] like Figure 4 As shown, method 400 includes an initial step 402 of applying a brake to immobilize the shaft of the AC motor. The brake can be a friction brake, such as a drum brake or a disc brake, or a form-locking brake. The brake need not be designed for dissipative braking (e.g., having the ability to thermally dissipate a large amount of kinetic energy); rather, it has the ability to apply a static or holding force sufficient for the purpose of performing the present method 400. More precisely, the activation of the brake can occur under non-rotating conditions of the shaft of the AC motor. The brake can be a component in an integrated robot arm servo motor that is operable to apply torque at the robot arm joint when energized, and to hold the joint stationary at other times.

[0053] In a second step 404, a predefined drive signal waveform is fed to the AC motor 100, which is configured not to generate torque. While applying the drive signal, the sensed stator current (i A ,i B ,i C ). As mentioned above, the stator currents of all three phases can be sensed, or the stator currents can be derived from two phases. Figure 3 , the drive signal can be represented by (IsqRef, IsdRef), (Vsq, Vsd) or (Vsα, Vsβ) components, that is, in the rotor synchronous or stator synchronous reference frame.

[0054] In a rotor synchronous reference frame such as DQZ, zero torque generation can be ensured by providing a drive signal waveform based on a zero set point value of the quadrature axis component (i.e., Vsq=0 or IsqRef=0). At the same time, with respect to torque generation, the polarity of the direct axis component Vd or IsdRef is generally arbitrary. The polarity can be assigned based on the situation that is considered appropriate for the overall AC motor device. In the specific case of PMSM, the predefined drive signal waveform can be field-enhanced (Vsd≥0) or field-weakened (Vsd≤0), i.e., a vector that is parallel or anti-parallel to the permanent magnetization of the rotor 108 is generated.

[0055] In the third step 406, the sensed stator current (i A ,i B ,i C ) is compared with a reference current, which is associated with the drive signal. For example, if configured accordingly, the comparison task can be performed by the Iq control block 314 and the Id control block 316.

[0056] In order to make a more meaningful comparison, the stator current (i A ,i B ,i C ) is converted (406.1) from a stationary reference frame to a rotor synchronous reference frame aligned with the current position of the rotor 108. For example, this conversion (or transformation) may be performed by the transformation blocks 322, 324 described above, which output a pair (Isd, Isq). This pair (Isd, Isq) is compared (406) to a pair (IsdRef, IsqRef) used as a reference current. Alternatively, the reference voltage values ​​(Vsd, Vsq) may be scaled with a known or estimated stator resistance to calculate the reference current.

[0057] In a fourth step 408 of method 400, it is determined whether the sensed stator current (after conversion in step 406.1 if included) matches the reference current. If the determination returns a positive result, the AC motor 100 is deemed safe to start and release of the brake is permitted.

[0058] The comparison 406 before the determination in step 408 may be based on one or more thresholds L, L of the absolute errors of these currents. d ,L q The absolute error can be a collective error, such as

[0059] (Isd-IsdRef) 2 +(Isq-IsqRef) 2 ≤L,

[0060] or point-by-point errors, such as

[0061]

[0062] Here, the threshold L,L d ,L q The value of can be determined by simulations of normal and faulty AC motors of the type under consideration. Alternatively, the threshold can be calculated from measurements of samples of this AC motor type with and without known defects.

[0063] In some embodiments of method 400, Figure 4 As shown, a negative result of the comparison 406 may trigger providing 410 an error indication. The error indication may be provided in a human or machine readable form, whichever is more appropriate for the current application scenario.

[0064] Figure 5 Successive signal processing steps that may be performed during performance of the motor control method 400 are shown.

[0065] In an initial step 510, a configuration is provided. The configuration may identify all drive axes connected to the drive system. In the case of an industrial robotic device, this may include robot axes and additional drive axes.

[0066] In the next step 512, the reference current IsdRef in the magnetizing direction is calculated for all connected drive axes. The reference current IsqRef in the orthogonal direction may be set to zero.

[0067] In a further step 514, the non-zero reference current direct-axis component IsdRef is converted (transformed) from the rotor synchronous reference frame to the stationary reference frame using an inverse Park transform.

[0068] Next, in step 516, a voltage is generated to be applied to the AC motor 100. The voltage may be generated using PWM.

[0069] In the subsequent step 518, the phase current (i A ,i B ,i C ).

[0070] Then, in step 520, the stator currents are converted from the stationary reference frame to the rotor synchronous reference frame using Clark and Park transformation.

[0071] In a comparison step 522 , the absolute error between the D component of the reference current IsdRef and the D component of the measured stator current is calculated.

[0072] At step 524, if the absolute error is found to be below a predefined threshold L (which may be configured to take into account variations in local conditions or tolerances), the control loop is deemed fully functional and release of the brake(s) currently immobilizing the shaft of the AC motor 100 is permitted. Conversely, if the absolute error exceeds the threshold L, the control loop has failed the health check and therefore the brake(s) should not be released.

[0073] Aspects of the disclosure have mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than those disclosed above are equally possible within the scope of the invention as defined by the appended patent claims.

Claims

1. A method (400) for safely starting an alternating current (AC) motor from a preloaded stationary state, the method comprising: applying (402) a brake to immobilize a shaft of the AC motor; feeding (404) the AC motor with a predefined drive signal waveform configured to generate no torque, and sensing a resulting stator current; comparing (406) the sensed stator current to a reference current, the reference current being associated with the drive signal; as well as Responsive to finding that the sensed stator current matches the reference current, release of the brake is permitted (408).

2. The method (400) of claim 1, wherein: The predefined drive signal waveform has a zero Q component in a rotor synchronous DQZ reference frame.

3. The method (400) according to claim 1 or 2, wherein: The motor is a permanent magnet synchronous motor, PMSM.

4. The method (400) of claim 3, wherein: The predefined drive signal waveform is field enhancing with respect to a rotor (108) of the PMSM.

5. The method (400) of claim 3, wherein: The predefined drive signal waveform is field weakening relative to a rotor (108) of the PMSM.

6. The method (400) according to any one of the preceding claims, wherein: The motor (100) is an induction motor.

7. The method (400) according to any one of the preceding claims, wherein: Responsive to finding that an absolute error between the sensed stator current and the reference current is below a threshold, releasing of the brake is permitted ( 408 ).

8. The method (400) according to any one of the preceding claims, further comprising: In response to finding that the sensed stator current does not match the reference current, an error indication is provided (410).

9. The method (400) according to any one of the preceding claims, wherein: The comparing (406) of the sensed stator current to the reference current includes converting (406.1) the stator current from a stationary reference frame to a rotor synchronous reference frame.

10. The method (400) according to any one of the preceding claims, wherein: The feeding (404) of a predefined drive signal waveform to the AC motor includes performing (404.1) pulse width modulation, PWM.

11. The method (400) according to any one of the preceding claims, wherein: The AC motor (100) is installed in an industrial robot.

12. A motor controller (310) arranged to control an electric drive unit (330) configured to feed an alternating current (AC) motor (100), The motor controller comprises processing circuitry configured to perform the method of any preceding claim.

13. A computer program comprising instructions for causing a motor controller according to claim 12 to perform the steps of a method according to any one of claims 1 to 11.

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