Method for detecting a standstill and electric motor

Abstract

The invention relates to a method for detecting a standstill (26) of a rotor standstill of a rotor (18) relative to a stator (12) of a respective electric motor (10), comprising the steps of providing (28) the electric motor (10) with the stator (12) having at least one stator coil (14) and the rotor (18) which can be rotated relative to the stator (12) at least depending on an electric current (Is) by applying an electric supply voltage (Us) to the stator coil (14), operating the electric motor (10) by modulation cycles (Z, Z1, Z2, Zn) which have multiple temporally successive respectively predefined current values (Ir1, Ir0) of the electric current (Is) and which divide a periodic electrical full cycle (24) multiple times, determining (30) at least one average supply voltage (Un) applied to the stator coil (14) at least during one modulation cycle (Z1) of the modulation cycles (Z, Z1, Z2, Zn), calculating (32) an average voltage value (Um) from the average supply voltage (Un), comparing (36) the average voltage value (Um) with a voltage threshold value (38), identifying (40) a standstill of the rotor (18) based on the comparison (36). The invention also relates to an electric motor (10).

Description

[0001] Verfahren zur Stillstandserkennung und Elektromotor

[0002] Description introduction

[0003] Die Erfindung betrifft ein Verfahren zur Stillstandserkennung nach Anspruch 1. Weiterhin betrifft die Erfindung einen Elektromotor.

[0004] US 2009 153 093 A1 describes a method for detecting the standstill of a stepper motor that is operated in microsteps over a full electrical cycle and in which a standstill of the rotor relative to a stator is detected by switching off the stator coil of the stator in several steps during the full cycle by decoupling it from the supply voltage and sampling it, for example by measuring the back EMF.

[0005] Die Aufgabe der vorliegenden Erfindung liegt darin, den Stillstand des Rotors genauer und zuverlässiger zu erfassen.

[0006] At least one of these tasks is solved by a standstill detection method with the features according to claim 1. This allows the standstill of the rotor to be detected more accurately and reliably, in particular independently of a motor load, while the operation of the electric motor is affected or influenced as little as possible.

[0007] The electric motor can have at least two stator coils. The electric motor can be a stepper motor. The stepper motor can be configured to adjust an expansion valve, for example, in an air conditioning compressor. The electric motor can be installed in a vehicle.

[0008] The electric motor can be operated with current control. The specified current value at the stator coil can serve as the controlled variable. This specified current value can be regulated by pulse width modulation of the supply voltage.

[0009] The average supply voltage of the stator coil, used to calculate the average voltage value, can be influenced by the resistive component (i.e., the electrical resistance of the stator coil and its connections), the inductive component of the stator coil, and the electromagnetic force (back EMF) acting on the stator coil when the rotor rotates. The amplitude of the back EMF can be influenced by the rotor's rotational speed.

[0010] In an unloaded state, an electric motor can have a phase shift of approximately 90° between the current at the stator coil and the back EMF. This phase shift can decrease as the electric motor becomes more heavily loaded.

[0011] The average supply voltage can be calculated and provided using information from a control circuit of the electric motor.

[0012] In a preferred embodiment of the invention, it is advantageous if the detection process infers that the rotor is stationary when the average voltage value is below the voltage threshold. With the rotor stationary, the average supply voltage required to build up the current in the stator coil to reach the predetermined current value is lower because there is no back EMF. This back EMF, in turn, is independent of the motor load and dependent on the rotor speed.

[0013] This allows standstill detection to be carried out independently of the motor load by comparing the average voltage value with the voltage threshold.

[0014] It is also possible to conclude that the rotor has stopped if the average voltage value reaches the voltage threshold from above.

[0015] A preferred embodiment of the invention is advantageous in which the electric motor is a stepper motor operated in microstepping mode, in which the full cycle is divided into several microsteps, each containing several modulation cycles. The full cycle can, for example, be divided into 128 microsteps.

[0016] In a preferred embodiment of the invention, several average supply voltages present during respective modulation cycles are determined, and the average voltage value is calculated as the average of the average supply voltages of the respective modulation cycles. The average supply voltages considered in the calculation of the average voltage value can be those of all stator coils of the electric motor. The average voltage value can thus be calculated as the average of the average supply voltages of the modulation cycles of one stator coil and the average supply voltages of the modulation cycles of at least one further stator coil, preferably all stator coils, of the electric motor, particularly for each full cycle.A preferred embodiment of the invention is advantageous in which, when calculating the average voltage value, a maximum of the average supply voltages of the modulation cycles of a full cycle are included. The average voltage value can be calculated definitively for each full cycle and assigned to that full cycle.

[0017] In a specific embodiment of the invention, it is advantageous if the average voltage value is assigned to a full cycle, but for the calculation of the average voltage value, a maximum proportion of the total modulation cycles of the full cycle are taken into account. This simplifies the overall calculation of the average supply voltages.

[0018] In an advantageous embodiment of the invention, the calculation comprises only those modulation cycles that exhibit a slow decay current cycle for controlling the respective predetermined current value. This simplifies the overall calculation of the average supply voltages. For example, if modulation cycles with a fast decay current cycle were included, the active counter-voltage that reduces the current, for instance by inverting the supply voltage, would be factored into the calculation of the average supply voltage. The slow decay current cycle can be implemented by electrically short-circuiting the stator coil.

[0019] In a preferred embodiment of the invention, the component comprises the modulation cycles during which the predetermined current value increases over the modulation cycles during the full cycle. The component can include at least, and in particular at most, the modulation cycles of the microsteps in which the predetermined current increases towards the next microstep.

[0020] In an advantageous embodiment of the invention, it is provided that the proportion comprises a maximum of half of the modulation cycles of the total modulation cycles of a full cycle.

[0021] Furthermore, within the scope of the invention, an electric motor with the features according to claim 10 is proposed to solve at least one of the previously specified problems.

[0022] Further advantages and advantageous embodiments of the invention will become apparent from the description of the figures and the illustrations. Description of Figures

[0023] The invention is described in detail below with reference to the illustrations. These show, in detail:

[0024] Figure 1: An electric motor in a special embodiment of the invention.

[0025] Figure 2: A circuit diagram for the electrical operation of a stator coil of an electric motor in a special embodiment of the invention.

[0026] Figure 3: A time course of an electric current applied to the stator coil of the electric motor during a full cycle.

[0027] Figure 4: An enlarged view of section A from Figure 3.

[0028] Figure 5: A method for standstill detection in a special embodiment of the invention.

[0029] Figure 6: A time course of characteristic parameters during execution of the standstill detection method in a special embodiment of the invention.

[0030] Figure 1 shows an electric motor in a specific embodiment of the invention. The electric motor 10 comprises a stator 12 with a stator coil 14 and a further stator coil 16 offset therefrom by 90°, and a rotor 18 rotatable relative to the stator 12. The rotor 18 may have a permanent magnet 20. Preferably, the stator coil 14 has two radially opposite coil elements connected in series, and the further stator coil 16 has two radially opposite coil elements connected in series.

[0031] The rotor 18 is rotatable relative to the stator 12 depending on an electric current Is in the stator coil 14 and in the further stator coil 16. The electric current Is depends on the respective electrical supply voltage Us at the stator coil 14 and the further stator coil 16.

[0032] Figure 2 shows a circuit configuration for the electrical control of a stator coil of an electric motor in a specific embodiment of the invention. The circuit configuration comprises an H-bridge 22, which electrically connects the stator coil 14 to the supply voltage Us via several switching elements S, in particular field-effect transistors comprising a first switching element S1, a second switching element S2, a third switching element S3, and a fourth switching element S4. For example, the supply voltage Us is applied to the stator coil 14 when the electric motor is operating if the first switching element S1 and the fourth switching element S4 are closed and the second switching element S2 and the third switching element S3 are open.

[0033] Figure 3 shows the time course of an electric current applied to the stator coil of the electric motor during a complete cycle. The electric motor can, for example, be a stepper motor in which the complete rotation of the rotor relative to the stator is divided into several complete electrical cycles. For example, one complete electrical cycle 24 can correspond to a rotation of the rotor relative to the stator of 1.8°. The complete cycles each exhibit a periodic course of the predetermined current value Ir at the stator coil during current-controlled operation of the electric motor. The electric motor can be operated in microstepping mode, in which the individual complete electrical cycle 24 is divided into several microsteps M per complete cycle 24. This allows for an approximately harmonic, in particular sinusoidal, course 25 of the predetermined current value Ir at the stator coil.

[0034] During a full cycle 24, there are microsteps M in which the specified electric current value Ir increases towards the next microstep M in time, as in the first quarter B1 and the third quarter B3 of the period, or in which the specified electric current Ir decreases towards the next microstep M in time, as in the second quarter B2 and the last quarter B4.

[0035] Figure 4 shows an enlarged view of section A from Figure 3. Each microstep M is assigned a current value Ir for the electric current in the stator coil. For example, the microstep M1 shown here has a current value Ir1 that is higher than a given current value IrO of a preceding microstep MO. The microsteps M exhibit several modulation cycles Z of the electric current in the stator coil. These modulation cycles Z can include pulse-width modulation of the supply voltage and thus of the electric current in the stator coil.

[0036] Upon transition from the preceding microstep MO to microstep M1, the current is increased by applying the supply voltage to the stator coil in a first modulation cycle Z1 of microstep M1 until the predetermined current value Ir1 is reached. Subsequently, during the first modulation cycle Z1, a slow current decay cycle C follows, in which the supply voltage is decoupled from the stator coil and, for example, as shown in Figure 2, the third and fourth switching elements are closed, thus short-circuiting the stator coil. After completion of modulation cycle Z1, an average supply voltage is calculated and provided, depending on the magnitude of the supply voltage and the ratio of the length of the time interval T1, during which the predetermined current value Ir1 is driven by the application of the supply voltage, to the time interval T2 of the slow current decay cycle C.

[0037] In the subsequent modulation cycles Zn following the first modulation cycle Z1, the electric current is increased up to the specified current value I r1 and then decreased again by the slow current decay cycle C, until the current value is changed again with a modulation cycle Z2 of the subsequent microstep M2.

[0038] Figure 5 shows a method for standstill detection in a special embodiment of the invention. The method for standstill detection 26 of a rotor standstill of a rotor relative to a stator of an electric motor 10 comprises providing 28 the electric motor 10 with the stator having at least one stator coil and the rotor which is rotatable relative to the stator at least dependent on an electric current by applying an electrical supply voltage to the stator coil. The electric motor 10 is in particular a stepper motor which is operated in a current-controlled mode by a microstepping mode 29 in which the full cycle 24 is divided into several microsteps M, each with several modulation cycles, each again with pulse width modulation of the supply voltage at the stator coil.

[0039] Furthermore, the average supply voltages 11n applied to the stator coil during each modulation cycle are determined. These average supply voltages 11n are calculated and provided after the completion of each modulation cycle.

[0040] Subsequently, an average voltage value Um is calculated as the average of the average supply voltages Un of the modulation cycles over a full cycle 24. In calculating the average voltage value Um, a maximum of the average supply voltages Un of the modulation cycles of a full cycle 24 can be included. The average voltage value Um is thus preferably assigned to a full cycle 24. For calculating the average voltage value Um, a maximum proportion 34 of the total modulation cycles of the full cycle 24 can be considered. This proportion 34 can exclusively include those modulation cycles that exhibit a slow current decay cycle, in particular by short-circuiting the stator coil, for controlling the respective specified current value, for example, the modulation cycles from the first quarter B1 and the third quarter B3 of the period of the full cycle 24.Subsequently, a comparison 36 of the average voltage value Um with a predefined voltage threshold 38 is performed, and a rotor standstill is detected 40 depending on the comparison 36. Upon detection 40, a rotor standstill is inferred if the average voltage value Um is below the voltage threshold 38. For example, a standstill signal 42 indicating the rotor standstill can be provided.

[0041] Figure 6 shows a time course of characteristic parameters during the execution of the standstill detection method in a specific embodiment of the invention. The common time course indicates the average voltage value Um relative to a predetermined voltage threshold 38 in the upper diagram, the standstill signal 42 in the middle diagram, and a target rotational position Ds of the rotor and an actual rotational position D of the rotor in the lower diagram over time t.

[0042] In the time sequence shown, the electric motor, for example a stepper motor, is moved at a speed of 50 full cycles per second against a mechanical end stop. As shown in the lower diagram based on the actual rotational position D, the end stop is reached earlier. This premature standstill of the electric motor is detected by setting the standstill signal 42 to 1, thus determining that the rotor has stopped as soon as and as long as the average voltage value Um is below the voltage threshold 38.

[0043] Reference symbol list 0 Electric motor 2 Stator 4 Stator coil 6 Additional stator coil 8 Rotor 0 Permanent magnet 2 H-bridge setup 4 Full cycle 5 Course 6 Standstill detection method 8 Provision 9 Microstepping 0 Determine 2 Calculate 4 Proportion

[0044] 36 Comparison

[0045] 38 Voltage threshold

[0046] 40 Capture

[0047] 42 Stop signal

[0048] B1 first quarter

[0049] B2 second quarter

[0050] B3 third quarter

[0051] B4 last quarter

[0052] C slow current decay cycle

[0053] D Actual rotation position

[0054] Ds target rotation position Ir current value

[0055] IrO current value

[0056] Ir1 current value

[0057] Is electricity

[0058] M microstep

[0059] MO microstep

[0060] M1 microstep

[0061] Mn microstep

[0062] S switching element

[0063] 51 first switching element

[0064] 52 second switching element

[0065] 53 third switching element

[0066] 54 fourth switching element

[0067] Average voltage value

[0068] Unaverage supply voltage

[0069] Us supply voltage

[0070] Z modulation cycle

[0071] Z1 first modulation cycle

[0072] Z2 modulation cycle

[0073] Zn further modulation cycle

Claims

Patent claims 1. Method for detecting standstill (26) of a rotor standstill of a rotor (18) relative to a stator (12) of each of an electric motor (10), comprising the steps Providing (28) the electric motor (10) with the stator (12) having at least one stator coil (14) and the rotor (18) which is rotatable relative to the stator (12) at least depending on an electric current (Is) by applying an electric supply voltage (Us) to the stator coil (14), Operation of the electric motor (10) by several successive, predetermined current values ​​(Ir1 , IrO) of the electric current (Is) and modulation cycles (Z, Z1 , Z2, Zn) that subdivide a periodic full electric cycle (24) multiple times, Determine (30) at least one average supply voltage (Iln) applied to the stator coil (14) at least during one modulation cycle (Z1) of the modulation cycles (Z, Z1 , Z2, Zn), Calculating (32) an average voltage value (Um) from the average supply voltage (Un), Comparison (36) of the average voltage value (Um) with a voltage threshold value (38), Detecting (40) a standstill of the rotor (18) depending on the comparison (36).

2. Method for standstill detection (26) according to claim 1 , characterized in that during detection (40) a standstill of the rotor (18) is inferred if the average voltage value (Um) is below the voltage threshold value (38).

3. Method for standstill detection (26) according to claim 1 or 2, characterized in that the electric motor (10) is a stepper motor which is operated in a microstepping mode (29) in which the full cycle (24) is divided into several microsteps (M, MO, M1 , Mn) each with several of the modulation cycles (Z, Z1 , Z2, Zn).

4. Method for standstill detection (26) according to one of the preceding claims, characterized in that a determination (30) of several during respective The average supply voltages (Un) applied during modulation cycles (Z, Z1, Z2, Zn) are calculated, and the average voltage value (Um) is calculated as the average of the average supply voltages (Un) of the respective modulation cycles (Z, Z1, Z2, Zn).

5. Method for standstill detection (26) according to one of the preceding claims, characterized in that, in the calculation of the average voltage value (Um), a maximum of the average supply voltages (Un) of the modulation cycles (Z, Z1, Z2, Zn) of a full cycle (24) are included.

6. Method for standstill detection (26) according to one of the preceding claims, characterized in that the average voltage value (Um) is assigned to a full cycle (24), however, for the calculation of the average voltage value (Um) a maximum proportion (34) of modulation cycles (Z, Z1 , Z2, Zn) of the total modulation cycles (Z, Z1, Z2, Zn) of the full cycle (24) are taken into account.

7. Method for standstill detection (26) according to claim 6, characterized in that the portion (34) comprises exclusively the modulation cycles (Z, Z1, Z2, Zn) which have a slow current decay cycle (C) for controlling the respective predetermined current value (IrO, Ir1).

8. Method for standstill detection (26) according to claim 6 or 7, characterized in that the portion (34) comprises the modulation cycles (Z, Z1, Z2, Zn) during the full cycle (24) with a predetermined current value (IrO, Ir1) increasing over the modulation cycles (Z, Z1, Z2, Zn).

9. Method for standstill detection (26) according to one of claims 6 to 8, characterized in that the portion (34) comprises at most half of the modulation cycles (Z, Z1 , Z2, Zn) of the total modulation cycles (Z, Z1, Z2, Zn) of a full cycle (24).

10. Electric motor (10) with a stator (12) and a rotor (18) rotatable relative to the stator (12), the standstill of which can be detected by a standstill detection method (26) according to one of the preceding claims.

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