DE102011075238B4 - Method and device for monitoring the angular position of a rotor in an electric machine - Google Patents

Abstract

Method for monitoring an angular position signal detected by an angular position detection device (3) in an electric machine (1), wherein the method comprises:
- repeated detection of an angular position signal (21) of the angular position detection device (3) indicating a detected angular position of the rotor;
- Determining a difference value Δa between values of two successively determined angular position signals (21); and outputting an error signal (29) if the difference value Δa is greater than a preset threshold value Δα _max , wherein an error signal (29) is output if the difference value Δa is greater than six times the product of the number of machine poles p of the electric machine times the maximum permissible machine speed n _max times the time interval Δt between successive determinations of angular position signals (21),
-Forwarding the monitored angular position signal (17) to a control unit (9), provided no fault signal is output,
wherein the angular position signal (21) is determined periodically,
where the angular position of the rotor is determined at equal time intervals.

Description

AREA OF INVENTION

The present invention relates to monitoring the angular position of a rotor in an electric machine.

STATE OF THE ART

Electric machines, such as synchronous machines, are frequently used in electric or hybrid vehicles. The electric machine can often function both as a motor to propel the vehicle and as a generator to convert kinetic energy into electricity when the vehicle decelerates. The electric machine generally has a stator in which a rotor can rotate.

In order to correctly control the electric machine for motor or generator operation using a control unit, it is generally necessary to provide the control unit with information about the current angular position of the rotor.

From the DE 10 2009 029 396 A1 A method for operating an electric machine is known in which an angular position is determined by means of a resolver formed by at least one rotor winding of the rotor and at least one stator winding of the stator. The printed materials EP 2 000 873 A1 and EP 2 063 339 A1 disclose a method and a device for monitoring the angular position of a rotor.

It has been observed that errors can occur when determining an angular position, which can lead to incorrect control of the electric machine.

REVELATION OF THE INVENTION

In contrast, the method presented here, or a correspondingly operated monitoring device or electric machine as defined in the independent claims, enables reliable monitoring of the angular position of a rotor in an electric machine. Advantageous embodiments are defined in the dependent claims.

The proposed monitoring procedure comprises the following steps: An angular position signal, indicating a detected angular position of the rotor, is repeatedly determined. After an angular position signal has been determined, a difference value is calculated between two consecutively determined angular position signals; that is, the difference between the value of the currently determined angular position signal and the value of the immediately preceding angular position signal. If this difference value exceeds a preset threshold, an error signal is output.

The monitoring method according to the invention is based, among other things, on the following findings and ideas. Various methods and devices are known for detecting the current angular position of a rotor in an electric machine. For example, the rotor can interact with an angular position detection device, such as a resolver. A resolver is an electromagnetic transducer capable of converting the rotor's angular position into an electrical quantity. Alternatively, other angular position detection devices, such as potentiometer encoders, incremental encoders, and absolute encoders, can be used. As a further alternative, a separate angular position sensor can be dispensed with, and instead, the angular position can be detected using the rotor and stator windings of the electric machine and measurements performed on them.

It has been observed that an angular position signal detected by such a device may be erroneous with a certain probability. This can be due, for example, to the fact that the angular position signal is determined from detected current intensities and/or detected phase shifts, for instance, within the windings of the resolver or the electric machine itself, and errors can occur in such a determination. In particular, it is possible that the angular position may be determined incorrectly by 180°.

Since the detected angular position signal is conventionally used to control the electric motor, errors in determining the angular position can lead to incorrect control of the electric motor. In particular, if the electric motor and the resolver have the same number of pole pairs, a rotor angular position determined incorrectly by 180°, for example, can cause the electric motor to be driven in reverse instead of forward.

To reliably avoid such incorrect control, it is now proposed to continuously monitor an angular position signal detected by an angular position detection device for errors.

For this purpose, the angular position signal can be determined at short time intervals and compared with a previous angular position signal. Preferably, the angular position signal is determined periodically, that is, the angular position of the rotor is determined at regular time intervals.

To validate the angular position detected by the angle detection device, a comparison is subsequently performed for each detected angular position with the angular position detected in the previous determination step. For this purpose, a value from the previously detected angular position is subtracted from the value of the currently detected angular position, and it is checked whether the resulting difference is greater than a predefined threshold. If this is the case, it is assumed that an error occurred during the current angular position detection, and an error signal is output.

The error signal can, for example, be used within a monitoring device to filter out the currently detected angular position signal. The monitoring device can then normally forward the angular position signal to a control unit, allowing the control unit to control the electric motor based on the monitored angular position signal output by the monitoring device. However, if an excessively large difference is detected between two consecutively measured angular position signals, thus triggering an error signal, the most recently detected angular position signal may not be forwarded to the control unit. Consequently, the control unit will not attempt to control the electric motor based on the faulty angular position signal.

The preset threshold can be chosen, in particular, such that an error signal is output if the difference between successively determined angular position signals is greater than six times the product of the number of machine poles p of the electric machine multiplied by the maximum permissible machine speed n _max (measured in revolutions per minute) multiplied by the time interval Δt (measured in seconds) between successive determinations of angular position signals. In other words, the specified threshold α _max should satisfy the following inequality: $${\alpha }_{\text{max}}\le n_{\text{max}}\cdot p\cdot \Delta t\cdot \frac{360}{60}$$

In other words, the threshold value α_{_max} can be chosen to specify the angle by which the rotor rotates further at the maximum permissible machine speed within the period between two successive angular position signal measurements. The maximum permissible machine speed can, for example, be greater than the rated machine speed by a tolerance value, so that it can be assumed that the electric machine will never exceed this maximum permissible machine speed during normal operation. Therefore, if the difference between successively measured angular position signals is greater than such a threshold value α_{_max} , it can be assumed that an error in the detection of the angular position can be assumed.

Assuming that a 180° error in angular position detection, in particular, should be reliably detected, the above equation can also be used to specify an upper limit for the time interval Δt between successive determinations of angular signals. Here, it should be... $$\Delta t<\frac{180\cdot 60}{360\cdot n_{\text{max}}\cdot p}$$ In other words, the time interval Δt should be less than 30 divided by the maximum permissible machine speed multiplied by the number of machine pole pairs.

To account for a transition of the rotor angle from 360° to 0° when monitoring the rotor's angular position, the proposed monitoring method can be designed to ignore negative difference values. Alternatively, monitoring can be temporarily suspended or the output of an error signal suppressed at the relevant times.

Features and advantages of embodiments of the invention are described herein partly with regard to the proposed monitoring method and partly with regard to the proposed monitoring device or an electric machine equipped with such a monitoring device. A person skilled in the art will recognize that the features can be combined or transferred analogously in order to achieve further embodiments and the associated synergistic effects.

BRIEF DESCRIPTION OF THE DRAWINGS

• 1 zeigt eine Elektromaschine mit einer Überwachungsvorrichtung gemäß einer Ausführungsform der vorliegenden Erfindung.
• 2 veranschaulicht ein Überwachungsverfahren gemäß einer Ausführungsform der vorliegenden Erfindung.

Embodiments of the invention are described below with reference to the accompanying drawings, whereby neither the description nor the drawings are intended to limit the scope of the invention.

• 1 shows an electric machine with a monitoring device according to an embodiment of the present invention.
• 2 illustrates a monitoring method according to an embodiment of the present invention.

The figures are merely schematic and not to scale.

FORMS OF EXECUTION OF THE INVENTION

1 Figure 1 shows an electric machine 1, such as one that can be used as a synchronous machine in electric vehicles. A rotor of the electric machine, rotatable within a stator, drives a shaft 5.

To detect the current angular position of the rotor, an angular position detection device 3 in the form of a resolver is provided on the electric machine 1. This resolver can, for example, have two stator windings offset by 90° within a cylindrical housing. These stator windings enclose a rotor mounted in the housing, which is coupled to the rotor of the electric machine 1. When the rotor rotates, an alternating voltage, preferably sinusoidal, is excited in each of the two stator windings. The alternating voltage of the two offset stator windings is excited with a phase shift of, for example, 90°. The phase angle of the voltage induced in the rotor winding then depends on the position of the rotor or the rotor winding. During one revolution of the rotor, the resolver therefore supplies an alternating voltage whose phase angle rotates from 0° to 360°. The phase relationship of the rotor's output voltage with respect to the excitation voltage at the stator windings can serve as a measure of the rotor's angular position.

An angular position signal 21 generated in this way can be transmitted by the angular position detection device 3 to a monitoring device 7 via an interface 11 at periodic time intervals Δt, or read out by the monitoring device 7. Additionally, information can be transmitted from the electric machine 1 to the monitoring device 7 via an interface 15.

In the monitoring device 7, after each transmission of an angular position signal 21, a difference value is calculated between its value and the value of the angular position signal transmitted in the last transmission using a calculation device 25. In other words, the device determines by how much the detected angular position of the rotor has changed since the last measurement.

As in 2 As shown, the measured values of the angle signals 21 change linearly with time t. A transition from 360° to 0° is ignored.

If the difference value Δα determined by the monitoring device 7 remains below a predefined threshold value α _max , the angular position of the rotor is assumed to have been correctly detected and the monitored angular position signal 17 is forwarded to a control unit 9. Based on the angular position signal 17 thus monitored, the control unit 9 can then transmit a suitable signal 13 for controlling the electric machine 1.

As in 2 However, in rare cases, a faulty angular position signal 23 may be detected. Such a faulty angular position signal 23 results in the difference value Δα to the previously detected angular position signal being greater than the threshold value α _max . This is detected by the monitoring device 7, and a corresponding error signal 29 is output via an output device 27. The error signal 29 can either be used within the monitoring device 7 to suppress the transmission of the faulty angular position signal 23 to the control unit 9. Alternatively, the error signal 29 can be transmitted to the control unit 9 or another device to trigger suitable countermeasures against incorrect control of the electric machine 1.

With the described monitoring method or monitoring device, the angular position signal 21 determined by an angular position detection device 3 can be continuously checked for plausibility and thus false detections of the angular position can be detected, so that a false control of the electric machine 1 can be reliably prevented.

Claims (8)

Method for monitoring an angular position signal detected by an angular position detection device (3) in an electric machine (1), the method comprising: - repeatedly determining an angular position signal (21) from the angular position detection device (3) indicating a detected angular position of the rotor; - determining a difference value Δa between values of two successively determined angular position signals (21); and outputting an error signal (29) if the difference value Δa is greater than a preset threshold value Δα _max , wherein an error signal (29) is output if the difference value Δa exceeds a preset threshold value Δα max. limit value Δa is greater than six times the product of the number of machine poles p of the electric machine times the maximum permissible machine speed n _max times the time interval Δt between successive determinations of angular position signals (21), -forwarding the monitored angular position signal (17) to a control unit (9), provided no error signal is output, wherein the angular position signal (21) is determined periodically, and the angular position of the rotor is determined at equal time intervals. Procedure according to Claim 1 , where negative difference values are ignored. Procedure according to one of the Claims 1 until 2 , where the time interval Δt between successive determinations of angular position signals (21) is less than 30 times the inverse value of the number of machine poles p of the electric machine and the maximum permissible machine speed n _max . Monitoring device (7) for an electric machine (1) for monitoring an angular position of a rotor in the electric machine (1), wherein the monitoring device (7) is designed to perform a method according to one of the Claims 1 until 3 to execute. Monitoring device according to Claim 4 , comprising an interface (11) for transmitting an angular position signal (21) determined by an angular position detection device (3); a calculation device (25) for calculating a difference value Δa between values of two successive determined angular position signals (21); and an output device (27) for outputting an error signal when the difference value Δa is greater than a preset threshold value a _max . Electric machine (1) with a stator, a rotor and a monitoring device (7) according to Claim 4 or 5 . electric motor according to Claim 6 , further comprising an angular position detection device (3) that interacts with the rotor. electric motor according to Claim 6 or 7 , further comprising a control unit (9) wherein the control unit is designed to control the electric machine (1) on the basis of a monitored angular position signal (17) issued by the monitoring device (7).