Method for the open- / closed-loop controlling of a stepper motor, control unit and stepper motor

Abstract

The invention relates to a method for the open- / closed-loop controlling of a stepper motor. The stator windings of the stepper motor are controlled via a full-bridge circuit with switching elements. The switching elements can be controlled in such a way that a rapid-decay mode or a slow-decay mode can be set. A charge cycle time in a pulse width modulation (PWM) period is stored, and the rapid-decay mode or the slow-decay mode for the next PWM period is set according to the charge cycle time. The invention additionally relates to a control unit, and a stepper motor comprising the full-bridge circuit and the control unit.

Description

[0001] Method for controlling / regulating a stepper motor, control unit and stepper motor

[0002] The present invention relates to a method for controlling / regulating a stepper motor whose stator windings are controlled via a full-bridge circuit with switching elements, wherein the switching elements can be controlled such that a fast decay mode or a slow decay mode can be selected. Furthermore, the invention relates to a control unit configured and programmed to carry out the method, as well as to a stepper motor with the full-bridge circuit and the control unit.

[0003] A stepper motor can, for example, be used to control an expansion valve of a heat pump. To control the stepper motor, sinusoidal currents are applied to the stator windings of the stepper motor.

[0004] A common problem is choosing between different decay modes of a full-bridge circuit used to control the stator windings of the stepper motor when the target current is decreasing. While a slow decay mode generates the smallest possible current fluctuation, the current is not reduced quickly enough, especially at high gradients and small output amplitudes. While the fast decay mode allows the current to decay significantly faster, it usually does so too rapidly. This leads to significantly higher current fluctuations and possibly the need to recharge the current in the stator windings. However, the current fluctuations around the target current should be kept as small as possible to avoid noise and unnecessary losses.

[0005] Control circuits and control methods for stepper motors are known, for example, from DE 102016 000 847 A1, DE 11 2015 005 791 T5, US 2015 / 0 015 177 A1 and US 2010 / 0 157 635 A1. In the known state of the art, a predetermined component in the fast or slow decay mode is controlled in the second half of the falling sine quadrant. Furthermore, it should be possible to set the modes with as little prior knowledge as possible, e.g., about current amplitude, speed, motor parameters, load, temperature, etc., since, due to the nature of the system, often only a small amount of information or

[0006] Intervention options are available. Intervention options include setting a current limit for the hardware-based current regulator, which aborts the charging mode, specifying the decay mode for the PWM off cycle, and the option to set the decay mode directly or at the end of the PWM period. Information is available about the point in time within the PWM period at which the current limit was reached, thus switching to the set decay mode.

[0007] The electronics utilize a current limiting circuit for current regulation, which is programmed with a current limit, i.e., a maximum current. The current itself cannot be measured. However, the point in time within a PWM period at which the current limiting circuit shuts down can be determined.

[0008] Based on this, it is the object of the invention to provide a further developed technology for controlling / regulating a stepper motor whose stator windings are controlled via a full-bridge circuit with switching elements, wherein the switching elements are controllable in such a way that a fast decay mode or a slow decay mode can be set.

[0009] This object is achieved by the subject matter having the features according to the independent claims. Advantageous embodiments are the subject matter of the dependent claims.

[0010] A method for controlling / regulating a stepper motor is disclosed. Stator windings of the stepper motor are controlled via a full-bridge circuit having switching elements. The switching elements are controllable such that a fast decay mode or a slow decay mode can be set. In the method, a charging cycle time is stored in a pulse-width modulation (PWM) period. The charging cycle time corresponds to a duration during which the full-bridge circuit ("PWM-on") is switched on, so that current flows through the stator windings. The fast decay mode or the slow decay mode for the next PWM period is then set according to the charging cycle time, i.e., the duration of PWM-on. Accordingly, it is possible to detect in a simple and robust manner when the fast decay mode is required or the slow decay mode is sufficient for the next PWM period.

[0011] The fast decay mode or the slow decay mode can be set in a software interrupt. The software interrupt can be triggered when a current limit is reached during the PWM period or a target current is not reached during the PWM period. Accordingly, the appropriate decay mode is set for the next PWM clock cycle.

[0012] In particular, the software interrupt can be configured to fast decay mode if the charging cycle time is less than a first threshold. Falling below the first threshold means that the current in the stator windings is already in the current limit range (or even above it). By setting the fast decay mode, a faster current decay is achieved, especially in the second quadrant of the sine wave. Consequently, the sinusoidal shape of the current can be more accurately imprinted.

[0013] Advantageously, the fast decay mode can only be activated if the charging cycle time is a predefined number of times less than the first threshold. For example, the predefined number of times can be set to a value of 3 or greater. If the number of times is exceeded, the decay mode of the last PWM period can be retained. This ensures that the fast decay mode is not activated immediately upon the first undershoot of the first threshold.

[0014] Furthermore, the software interrupt can be configured to slow decay mode if the charging cycle time exceeds a second threshold. Exceeding the second threshold means that rapid decay is no longer necessary because current had to be rebuilt. Setting the slow decay mode then ensures that rapid discharge does not occur during a period of a low PWM level ("PWM off") in the PWM period.

[0015] The second threshold can be higher than the first threshold. This maintains control hysteresis and prevents constant switching between decay modes.

[0016] Preferably, a counter or debounce counter can be reset to record the specified number of times when the charging cycle time is greater than the second threshold. The counter is then reinitialized so that a correct transition to fast decay mode occurs when the current limit is reached again.

[0017] Furthermore, the decay mode of the last PWM period can be maintained in the software interrupt if the charging cycle time is greater than or equal to the first threshold and less than or equal to the second threshold. As already mentioned, this avoids unnecessary switching between the decay modes.

[0018] Also disclosed is a control unit configured and programmed to execute the method according to the above aspects. This control unit can receive or determine the charging cycle time and control the switching elements of the full-bridge circuit according to the set decay mode.

[0019] Furthermore, a stepper motor with a full-bridge circuit that controls the stator windings of the stepper motor and has switching elements is disclosed. The switching elements are controllable such that a fast decay mode or a slow decay mode can be selected. The stepper motor has the control unit according to the above aspect.

[0020] The method, the control unit, and the stepper motor can be used to control an expansion valve of a heat pump. The heat pump can preferably be used in a vehicle. The present invention is described in detail below with reference to the figures. They show:

[0021] Fig. 1 is a flowchart for generating a PWM control signal for controlling / regulating a stepper motor; and

[0022] Fig. 2 is a flowchart of a software interrupt in which a decay mode is set for a next PWM period.

[0023] The present invention will be described below using preferred embodiments with reference to the figures. However, the description of the embodiments should not be considered exhaustive.

[0024] The method is used to control or regulate a stepper motor. For this purpose, the stator windings of the stepper motor are controlled via a full-bridge circuit comprising switching elements. The switching elements can be controlled in such a way that a fast decay mode or a slow decay mode can be selected. Such devices are already known from the prior art.

[0025] Fig. 1 shows a flowchart for generating a PWM control signal for controlling / regulating the stepper motor. After a start, initialization with the slow decay mode occurs in step S1. In a subsequent step S2, a PWM period is started and the charging cycle time is set to zero. In step S3, a check is made to determine whether a current limit, i.e., a maximum current, has been reached.

[0026] If the current limit is not reached in step S3, step S4 is executed, in which the charging cycle time is increased. Consequently, the full bridge is in "PWM-On" mode as long as the current limit is not exceeded. In this way, the current in the stepper motor's stator windings builds up toward a saturation value.

[0027] In the subsequent step S5, it is checked whether the PWM period has been completed. If the PWM period has not been completed, the system returns to step S3, where it is checked whether the current limit has been reached.

[0028] If the PWM period is complete, the process proceeds to step S100, where the charging cycle time is saved and a software interrupt is triggered. The software interrupt in step S100 is described in detail below. After step S100, the process returns to step S2, and a new PWM period is started.

[0029] If the current limit is reached in step S3, step S100 is executed, where the charging cycle time is saved and the software interrupt is triggered. As already mentioned, the software interrupt in S100 is described in detail below. If the current limit is exceeded, the full-bridge circuit is switched to "PWM off" mode, and the current in the stator windings is reduced according to the set decay mode.

[0030] Next, step S6 is executed, in which the decay mode is queried and set in the full-bridge circuit. Step S7 checks whether the PWM period has been completed.

[0031] If the PWM period is not completed in step S7, the process returns to step S6. If the PWM period is completed in S7, the process returns to step S2 and starts a new PWM period.

[0032] The software interrupt in S100 is described below with reference to the flowchart shown in Fig. 2. As already mentioned, before the software interrupt is triggered, the charging cycle time of the current PWM period is saved. In the triggered software interrupt, the decay mode is then selected depending on the charging cycle time and a counter (hereinafter referred to as the debounce counter). The background is that a short charging cycle time means that the current could not be reduced far enough, while a long charging cycle time means that the current is being reduced quickly enough. In step S101, the charging cycle time is first compared with a first threshold value. If the charging cycle time is below the first threshold value, the debounce counter is incremented by one in step S102. In step S103, the debounce counter is compared with a predetermined number of times.If the debounce counter exceeds the preset number of times, the software interrupt proceeds to step S104 and the decay mode is set to the fast decay mode. The software interrupt then terminates and returns to the flowchart in Fig. 1.

[0033] If the debounce counter is less than the predetermined number of times in step S103, the current debounce mode is maintained according to step S106. Subsequently, the software interrupt is terminated, and the flowchart of Fig. 1 is returned to.

[0034] If it is determined in step S101 that the charging cycle time is greater than or equal to the first threshold, a check is made in step S105 to determine whether the charging cycle time is greater than a second threshold. If the charging cycle time is less than or equal to the second threshold, the software interrupt proceeds to step S106, and the current decay mode is maintained. The software interrupt is then terminated, and the flowchart in Fig. 1 is returned to.

[0035] If the charging cycle time is greater than the second threshold in S105, the software interrupt proceeds to step S107, where the slow decay mode is set. Furthermore, the debounce counter is reset in step S107. After completion of step S107, the software interrupt is terminated, and the process returns to the method shown in Fig. 1.

[0036] Using the procedure described above, a decay mode can be appropriately set so that the sinusoidal shape can be maintained, especially in a second (and fourth) sinusoidal quadrant.

Claims

Claims 1 . A method for controlling / regulating a stepper motor whose stator windings are controlled via a full-bridge circuit with switching elements, wherein the switching elements are controllable such that a fast decay mode or a slow decay mode can be set, characterized in that a charging cycle time is stored in a pulse width modulation (PWM) period, and the fast decay mode or the slow decay mode is set for the next PWM period according to the charging cycle time.

2. Method according to claim 1, characterized in that the fast decay mode or the slow decay mode is set in a software interrupt which is triggered when a current limitation is reached during the PWM period or a target current is not reached during the PWM period.

3. Method according to claim 2, characterized in that the fast decay mode is set in the software interrupt if the charging cycle time is less than a first threshold value.

4. The method according to claim 3, characterized in that the fast decay mode is set only if the charging cycle time is a predetermined number of times less than the first threshold, and otherwise the decay mode of the last PWM period is maintained.

5. Method according to one of claims 2 to 4, characterized in that the slow decay mode is set in the software interrupt if the charging cycle time is greater than a second threshold value.

6. Method according to claim 3 or 4 and claim 5, characterized in that the second threshold value is greater than the first threshold value.

7. The method according to claim 6, characterized in that a counter for detecting the predetermined number of times is reset when the charging cycle time is greater than the second threshold value.

8. Method according to claim 6 or 7, characterized in that in the software interrupt the decay mode of the last PWM period is maintained if the charging cycle time is greater than or equal to the first threshold and less than or equal to the second threshold.

9. Control unit configured and programmed to carry out the method according to one of claims 1 to 8.

10. Stepper motor with a full-bridge circuit that controls stator windings of the stepper motor and has switching elements, wherein the switching elements are controllable such that a fast decay mode or a slow decay mode can be set, characterized by the control unit according to claim 9.

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