HK40069169A - Control method of stepping motor - Google Patents

Description

Control method of stepping motor

Technical Field

The invention relates to a method for controlling an electrical commutation motor having at least two windings (each winding having two winding terminals) and a rotor, wherein the motor is a stepping motor, in accordance with a mechanical load. In particular, the present invention relates to an adaptive control method of a stepping motor, i.e., control according to need.

Background

Such methods are generally known from brushless direct current (BLDC) motor control with or without a sensor system. In the sensorless approach, the speed and phase of the control commutation scheme are typically set to best match the respective current phase of the rotor. Control is typically performed such that the control unit attempts to use the control variable phase and velocity to produce a phase that is always optimal. Initially, these systems simulated a brushed motor that reached its maximum speed as a function of mechanical load when a fixed voltage was applied. If speed control is required for application-related reasons, the speed of the brushless motor is typically controlled using a higher level second control by additionally modulating the energization amplitude (which is influenced by the above-described control).

Depending on the complexity of the power-up mode, this type of control is very complex in terms of computation time and memory requirements in the control microprocessor. This applies in particular to electrical machines which have to operate very quietly and smoothly, for example for reasons of their application. For smaller and cheaper motors, such as smaller stepper motors, this type of motor therefore significantly increases the effort related to the cost of the motor. To generate micro-steps and start and stop ramps, a small low cost microprocessor is typically used.

Disclosure of Invention

It is therefore an object of the present invention to provide a load-dependent adaptive motor energization which enables a very smooth operation with low production costs.

To achieve the object defined above, the invention provides a method for generating a control for an electrically commutated electric machine, which control is adapted to mechanical loads and does not exceed a predefinable maximum current supply, the electric machine having at least two windings and a rotor, each winding having two winding terminals, a possibly variable load torque acting on the electric machine from the outside, there being an actual angle between the electrical commutation and the rotor position assumed by the rotor in the process, which angle is substantially equal to a predetermined set angle by the control of the electric machine, wherein, in the method,

the electric machine is supplied by a high-side switch and a low-side switch, which are provided for each winding terminal and connect the winding terminals to the positive terminal or the negative terminal of the supply voltage, respectively, the switches being implemented in semiconductor technology (for example as MOSFETs) and being associated with voltage-limiting diodes, respectively;

-the high-side and low-side switches of all motor winding terminals are cyclically switched on or off according to a switching scheme having a predeterminable switching number, said switching scheme comprising a cyclically repeated switching number during which, for at least one winding terminal, both the high-side and the low-side switches are switched off during an off-interval period, i.e. the respective at least one winding terminal is switched to a high impedance;

at the beginning of each switching-off interval, a definable residual current is still present in the respective winding of the electric machine, i.e. the beginning of the switching-off interval lies chronologically before the current flowing in the respective winding is zero;

during this disconnection interval, the residual current decreases for a period of time due to the discharge of the electrical energy stored in the winding;

-during the time period in which said residual current falls to at least one threshold value, the amplitude variation of the voltage limiting diode of the winding terminal switched to high impedance is either higher than the potential of the positive terminal V + or lower than the potential of the negative terminal V-of the supply voltage and the voltage is limited to the limiting voltage by means of the voltage limiting diode;

-determining a voltage limitation period for which the voltage limitation lasts;

-resuming the predetermined cyclic switching scheme after the voltage limiting period has ended;

in a cyclically repeated switching scheme, the process is always repeated at the same location;

-comparing the determined values of the voltage limitation periods with each other for different winding terminals, and/or sequentially with each other and/or with an expected value for a respective one of the winding terminals,

the evaluation unit calculates a deviation dt from a respective desired value reexp, which is indicative of the magnitude of the deviation of the actual angle between the electrical commutation and the motor rotor position assumed during the commutation from a predeterminable set angle, and

-controlling the motor accordingly to reduce potential deviations of the actual angle from the set angle.

Similarly, the invention checks the behavior of the discharge current after the winding terminals have switched to a high impedance while a residual current is still present in the winding. When the winding terminals are switched to a high impedance, the inductance of the winding initially keeps the current flowing. As a result of the switching of the winding terminals to a high impedance, a voltage drop occurs across the high-side or low-side switch, which is determined by the voltage across the voltage-limiting diode, which is switched to its reversible breakdown. The voltage is higher than the positive supply voltage or lower than the negative supply voltage, depending on the zero crossing direction of the current from positive to negative or vice versa. The voltage across the respective switch is thus limited or "clamped", the duration of which is the current discharge period. This time can be determined. If the duration is seen to deviate from the expected value within a few switching cycles, this indicates that the motor operation has to be controlled in accordance with the changing load. Particular attention is paid here to compliance with a predetermined setting angle, which is the angular difference between the electrical commutation and the motor rotor position assumed in the electrical commutation. Potential deviations of the actual angle from the set angle should be avoided as much as possible, which is why the motor is controlled accordingly.

The method according to the invention is based on a traditional uncontrolled step motor energization scheme, comprising micro-steps adjusted according to the acoustic requirements of the system. Here, as is typical of the BLDC method of brushless dc motors, the speed of the motor is not controlled, but is fixedly predetermined (with start and stop ramps, if necessary). Further, computer-aided phase correction is not typically provided in stepper motor control.

The present invention thus describes a method which does not have a high requirement on computing power and which can be "imposed" on conventional control of conventional stepper motors. For this purpose, certain measured values are determined during conventional "classical" stepper motor control. These measured values can be used to achieve a simple control of the current value of the current scheme by means of a single additional controller. Traditionally, small low-cost microcontrollers have been used to control stepper motors with microstepping. The requirements on the additional controller are so low that adaptive current control can be imposed on existing microcontroller solutions with little extra cost. The hardware support requirements according to the invention are also low. Preferably, the hardware support is already contained in the semiconductor used to control the stepping motor winding.

According to the invention, when the motor is running, a particular cyclic repetition of the energization pattern modulates the energizing stepper motor (bipolar or three-phase) feeding back its instantaneous residual coil energy, so as to send (not by means of the generator effect) a voltage pulse that lasts until the residual energy remaining in the respective coil has been completely dissipated or a specific portion has been dissipated. The voltage pulse is detected by a comparator and its length is measured. The threshold of the comparator is located near the positive terminal V + or the negative terminal V-of the supply voltage, depending on whether the method is used in the high-side or low-side path. VBAT/2 comparators typically used in other methods are not at all suitable for the method according to the invention. The commercial advantage of the method according to the invention is that it accepts even very large comparator tolerances and everything after the comparator can be implemented in shrinkable digital logic and software.

Typically, existing Vds over-current comparators can be used, so that the method can be implemented with little increase in cost.

The time of the above measurement, sometimes also referred to as "recycle time", must not be confused with the BEMF measurement. Once the corresponding coil is discharged, the usual energization scheme continues. The BEMF measurement will only start at this time, i.e. will last longer.

It has been found that the measured recirculation time is a measure of the angle between the electrically applied rotating field and the mechanical position of the rotor. This angle is also commonly referred to as "drag loss".

In reference operation with and/or without controlled load, the target recirculation time is determined for different situations.

In the later operation of the stepper motor, the magnitude of the sinusoidal coil current, for example, is directly controlled without affecting the speed or the step frequency, so that the recirculation time is always obtained. These target recirculation times correspond to the required target resistance losses which, on the one hand, are so high as to obtain a significant energy saving effect in comparison with conventional stepper motor control, but which, on the other hand, are so small that even in the case of spontaneous load increases, no attention is paid to the motor runout of 360 ° (motoring). This would mean that the correct position information is lost.

By controlling the residual current at the beginning of the recirculation time measurement by this method, the dependence of the recirculation time on the amplitude level can be prevented. This is part of the "special cycle repetitive modulation of the power-on pattern" described above.

In a further advantageous embodiment of the invention it can be provided that the actual angle is greater than the set angle when the determined value of the voltage limitation period is below a desired value, wherein the electric machine is controlled by increasing the current in the winding, and that the actual angle is less than the set angle when the determined value of the voltage limitation period is above the desired value, wherein the electric machine is controlled by decreasing the current in the winding.

Alternatively, it may also be provided that the actual angle is greater than the set angle when the determined value of the voltage limitation period is lower than an expected value, wherein the motor is controlled by decreasing the frequency of the number of switching of the switching scheme, and that the actual angle is less than the set angle when the determined value of the voltage limitation period is higher than the expected value, wherein the motor is controlled by increasing the frequency of the number of switching of the switching scheme.

Alternatively or in addition, it can also be provided that the control adjusts the cyclically repeated switching pattern in accordance with its phase position in such a way that the actual angle approaches the set angle as closely as possible.

As described previously, for example, it has been proposed that the voltage limit is determined by a voltage detector, and the period of the voltage limit is determined using a voltage limit period measuring unit that determines the period in which the voltage detector generates an output signal representing that the voltage limit is reached.

In a further suitable embodiment of the invention, it can be provided that the switching threshold of the voltage detector is either close to the positive or to the negative terminal of the supply voltage, depending on the polarity of the cut-off voltage, in order to exclude potential interfering influences of generator effects, such as back emf, as far as possible.

As an alternative to the above, it may be proposed to include an a/D converter measuring unit at least one winding terminal for measuring the voltage limit period and then to make a digital measurement of the voltage limit period, wherein, depending on the polarity of the method, the decision threshold for determining the voltage limit period is either close to the positive terminal or close to the negative terminal of the supply voltage in order to exclude potential disturbing influences of generator effects, such as back emf, as far as possible.

The voltage limit period measuring unit may operate linearly, i.e. at a continuous counting speed.

It is further advantageous that the voltage limit period measurement unit operates logarithmically in order to speed up the subsequent digital processing and to reduce the digital bit width of the voltage limit periods to be processed.

In a further suitable embodiment of the invention, it can be provided that in a cyclically repeating switching scheme the switching of the winding terminals to a high impedance is omitted, wherein instead the polarity of the voltage drop is dependent on its sign over the respective number of switching times in the cyclically repeating switching scheme and is monitored at the high-side switch or the low-side switch of the respective winding terminal, the change in polarity of the voltage drop indicating a change in polarity of the current and determining when the residual current has dissipated completely or at least reduced to the residual value.

Advantageously, the voltage limiting diodes preferably form parasitic diodes of the high-side switch and the low-side switch, or are configured as diodes provided in addition to such parasitic diodes.

Drawings

The invention will be explained in more detail below with reference to two embodiments and the accompanying drawings. Specifically, the figures show:

FIG. 1 shows an example of a circuit with an adaptively energized bipolar stepper motor;

FIG. 2 shows an example of a circuit for a three-phase motor with adaptive energization;

FIG. 3 shows a signal time course to illustrate the determination of a voltage limit or clamp period, an

Fig. 4 shows a signal sequence for explaining the regulation of the energization of the electric motor.

Detailed Description

Fig. 1 shows an example of a circuit with an adaptively energized bipolar stepping motor 10' according to the method of the invention. At each connection/junction of the two windings a and B, a driver stage 12, 13, 14 and 15 is provided, which has a high-side switch 4 and a low-side switch 3, respectively. The two switches are implemented as MOSFET switches and are thus implemented in semiconductor technology, each having a parasitic diode 3.1, 4.1, the parasitic diodes 3.1, 4.1 acting as voltage limiting diodes when the winding terminals a0, a1 or B1, B0 are switched to high impedance, while a residual current is still present in the respective winding A, B. The diode may also be referred to as a clamping diode.

The current magnitude controller 2 controls the high-side and low-side switches 3, 4, while the voltage detector 7 responds when a voltage limitation occurs across the switches due to the voltage limiting diodes 3.1, 4.1. The duration for which the voltage limitation is valid is detected by the voltage detector 7, which sends a signal to a time measurement unit, i.e. a voltage limitation period measurement unit 9. The controller 11 controls the control unit 2 such that a desired current always flows through the winding A, B of the bipolar stepping motor 10'. The switching scheme and the switching times of the high-side switch 4 and the low-side switch 3 of the individual driver stages 12, 13, 14 and 15 are generated by the unit 1.

In the evaluation unit 10, the voltage limit period of each winding terminal is evaluated when it has been switched to a high impedance. The evaluation is performed in order to compare the respective measured voltage limit periods with a predetermined expected value. These comparisons may be made for each winding terminal a0, a1, B0, B1, or may be made "across winding terminals".

Now, the degree of deviation of the individual values of the periods from the expected values, and the direction of deviation from the expected values (up or down) are checked in the evaluation unit. The winding current is correspondingly readjusted by the controller 11 so that the bipolar stepper motor 10' again operates according to the present invention. According to the invention, the deviation of the individual voltage limiting cycles 26 from the expected value is evaluated as a change in the mechanical load acting on the electric machine. For whatever reason, if the mechanical load becomes excessive, there is a risk of exceeding the tolerable loss of the calculated drag angle, possibly resulting in the risk of not executing any more control steps of the bipolar stepping motor 10', which in turn may result in a loss of position accuracy and, in the worst case, in the actual position information.

Fig. 2 shows the circuit of a three-phase stepping motor 10 ″ with three windings S12, S23 and S31 and three connections P1, P2 and P3 of the three windings, here connected in a triangle as an example. The components of the circuit in fig. 2 that are the same or similar to those in fig. 1, or the circuit portions schematically shown, are identified in fig. 2 by the same reference numerals as in fig. 1.

As mentioned above, fig. 3 and 4 show different time courses of voltage and current during the energization of the bipolar stepping motor 10 ".

List of reference numerals

1 predeterminable cyclic switching scheme

2 current amplitude control

3 low side switch

3.1 parasitic diode or voltage limiting diode

4 high side switch

4.1 parasitic diode or voltage limiting diode

7 Voltage detector

9 Voltage limit period measuring unit

10 evaluation unit

10' bipolar stepping motor

10' three-phase stepping motor

11 control/controller

12 individual drive stages

13 individual drive stages

14 individual drive stages

15 individual drive stages

20 definable residual current

21V + positive supply voltage connection

22V-negative supply voltage connection

26 Voltage limiting period tclamp

27 common energization scheme

28 break interval

30 expected value tEXP

Deviation dt of 31 from the respective expected value tEXP

A0 winding terminal

A1 winding terminal

B0 winding terminal

B1 winding terminal

P1 winding terminal

P2 winding terminal

P3 winding terminal

S12 winding

S23 winding

S31 winding

A winding

B winding

Claims (11)

1. A method of generating a control for an electrically commutated electric machine, which control is adapted to mechanical loads and does not exceed a predefinable maximum energization, the electric machine having at least two windings and a rotor, each winding having two winding terminals, a possibly varying load moment acting on the electric machine from the outside, there being an actual angle between the electrical commutation and the rotor position assumed by the rotor in the process, which angle is approximately equal to a set angle predefined by the control of the electric machine, wherein, in the method,

the electric machine is supplied by a high-side switch and a low-side switch, which are provided for each winding terminal and connect the winding terminals to the positive terminal or the negative terminal of the supply voltage, respectively, the switches being implemented in semiconductor technology (for example as MOSFETs) and being associated with voltage-limiting diodes, respectively;

-the high-side and low-side switches of all motor winding terminals are cyclically switched on or off according to a switching scheme having a predeterminable switching number, said switching scheme comprising a cyclically repeated switching number during which, for at least one winding terminal, both the high-side and the low-side switches are switched off during an off-interval period, i.e. the respective at least one winding terminal is switched to a high impedance;

at the beginning of each switching-off interval, a definable residual current is still present in the respective winding of the electric machine, i.e. the beginning of the switching-off interval lies chronologically before the current flowing in the respective winding is zero;

during this disconnection interval, the residual current decreases for a period of time due to the discharge of the electrical energy stored in the winding;

-during the time period in which said residual current falls to at least one threshold value, the amplitude variation of the voltage limiting diode of the winding terminal switched to high impedance is either higher than the potential of the positive terminal V + or lower than the potential of the negative terminal V-of the supply voltage and the voltage is limited to the limiting voltage by means of the voltage limiting diode;

-determining a voltage limiting period tclamp for which the voltage limitation lasts;

-resuming the predetermined cyclic switching scheme after the end of the voltage limiting period tclamp;

in a cyclically repeated switching scheme, the process is always repeated at the same location;

-comparing the determined values of the voltage limiting period tclamp with each other for different winding terminals, and/or sequentially with each other and/or with an expected value tEXP for a respective one of the winding terminals,

the evaluation unit calculates a deviation dt from a respective desired value reexp, which is indicative of the magnitude of the deviation of the actual angle between the electrical commutation and the motor rotor position assumed during the commutation from a predeterminable set angle, and

-controlling the motor accordingly to reduce potential deviations of the actual angle from the set angle.

2. Method according to claim 1, wherein the actual angle is larger than the set angle when the determined value of the voltage limit period tclamp is lower than the desired value txep, wherein the motor is controlled by increasing the current in the winding; and wherein the actual angle is smaller than the set angle when the determined value of the voltage limit period tclamp is higher than the desired value, the motor is controlled by reducing the current in the winding.

3. Method according to claim 1, wherein the actual angle is greater than the set angle when the determined value of the voltage limit period tclamp is lower than the desired value tEXP, wherein the electric machine is controlled by reducing the frequency of the switching times of the switching scheme; and wherein when the determined value of the voltage limit period tclamp is higher than the desired value, the actual angle is smaller than the set angle, the motor is controlled by increasing the frequency of the switching times of the switching scheme.

4. A method according to claim 2 or 3, wherein said controlling is a cyclically repeated switching scheme adjusted accordingly according to its phase position to bring the actual angle as close as possible to the set angle.

5. The method of claim 1, wherein the voltage limit is determined by a voltage detector and a period of the voltage limit is determined using a voltage limit period measurement unit that determines a period during which the voltage detector generates the following output signals: the output signal indicates that the voltage limit is reached.

6. The method according to claim 5, wherein the switching threshold of the voltage detector is either close to the positive or to the negative terminal of the supply voltage, depending on the polarity of the cut-off voltage, to exclude as far as possible potentially disturbing influences of generator effects, such as back emf.

7. Method according to claim 5, wherein an A/D converter measurement unit at least one winding terminal is used to measure the voltage limit period and then to make a digital measurement of the voltage limit period, wherein depending on the polarity of the method the decision threshold for determining the voltage limit period is either close to the positive or the negative terminal of the supply voltage in order to exclude potential disturbing influences of generator effects, such as back EMF, as much as possible.

8. The method according to claim 6 or 7, wherein the voltage limit period measuring unit is capable of operating linearly, i.e. at a continuous counting speed.

9. A method according to claim 6 or 7, wherein the voltage limit period measurement unit operates logarithmically in order to speed up subsequent digital processing and reduce the digital bit width of the voltage limit period to be processed.

10. The method according to claim 1, wherein in a cyclic repeating switching scheme, switching of the winding terminals to a high impedance is omitted, wherein instead the polarity of the voltage drop depending on its sign is monitored at the high side switch or the low side switch of the respective winding terminal over the respective number of switches in the cyclic repeating switching scheme, the polarity change of the voltage drop indicating a polarity change of the current, and it is determined when the residual current is completely dissipated or at least reduced to a remaining value.

11. The method according to claim 1, wherein the voltage limiting diode preferably forms a parasitic diode of the high-side switch and the low-side switch or is configured as a diode provided in addition to such a parasitic diode.