Method and circuit arrangement for operating stepper motors

The invention relates to a method and a circuit arrangement for operating step motors or other suitably dimensioned synchronous motors.

As known, stepping motors can be moved and controlled in a controlled manner. The rotational position of a magnetic rotor is followed by a magnetic field which is produced by phase-shifted energization of a plurality of coils which are arranged around the rotor. If a stepping motor is to be used not only for relative but also for absolute positioning of an object, a reference position must first be determined, to which the absolute position can be referred. A controlled absolute positioning is then possible as long as the control of the stepping motor takes place taking into account its characteristic movement parameters such as the rotation angle, speed and acceleration.

Two alternatives are known for determining a reference position. These are, on the one hand, the mechanical reference travel, in which the motor is driven against a mechanical limit or stop serving as a reference position, and on the other hand the electrical reference travel, in which a sensor (for example, an electromechanical switch or a photoelectric barrier) A corresponding signal is generated.

Both alternatives have advantages and disadvantages. While the mechanical reference travel is expected to result in noise development and increased wear due to the mechanical load, the realization of the electrical reference travel due to the sensors is associated with higher costs, increased design complexity for the integration of the sensors into a mechatronic system and with additional cabling effort , Wherein the reliability of the sensors themselves can also pose a problem, in particular under harsh environmental conditions.

Furthermore, it must be taken into consideration that step losses can occur during certain operating states, such as suddenly occurring load changes due to obstacles or the like, during the continuous operation of a stepping motor, or the stepping motor can even stop, so that a further reference travel is necessary. However, there are applications in which reference travel is not possible during operation, so that, in addition to the one-time determination of the reference position, it is also desirable to monitor the operating state of the stepping motor during operation, but in particular without additional sensors.

From the <patcit id="pcit0001" dnum="EP0182490A"><text>EP 0 182 490</text></patcit> A method is known for driving a rotary detector circuit arrangement with which it is determined whether a driving pulse which is fed into a stepping motor causes the rotor to rotate in the correct direction. The rotation detection is effected essentially by electrically connecting a first or second switching element element to a winding of the motor, and first, second and third signals being fed into the first and second switching means to feed the element Low impedance from the winding while the high impedance element is connected to the winding at the time of the rotation detection.

From the <patcit id="pcit0002" dnum="EP1017159A"><text>EP 1 017 159</text></patcit> A method is known for controlling a voltage / frequency converter-controlled single-phase or multi-phase electric motor with which a phase shift between an EMF voltage and a BEMF voltage is determined by means of the deviation between the zero crossing of the phase current and the voltage generated by self-induction and the frequency Of the inverter. The self-induction is measured in the zero sequence of the current phase of the associated phase, during the measurement the phase being disconnected from the supply network.

The invention is based on the general object of providing a method and a circuit arrangement for operating a stepping motor (or another appropriately dimensioned synchronous motor), by means of which it is possible in a simple manner to recognize the lances and thus also to determine a reference position of the motor.

This object is achieved according to claim 1 with methods for operating step motors with a first operating mode for normal motor operation in which an alternating current is impressed into at least one of the coils of the stepping motor and a second operating mode for determining a reference position of the stepping motor by means of a drive driven by driving Of the stepping motor against a mechanical stop, by comparing the height of a measuring current flowing in the coil with at least one lower threshold value, the height of the measuring current being determined essentially by the phase of a voltage induced by a rotor of the motor in the coil; Wherein the reference position is defined when the measuring current becomes smaller than the lower threshold value and wherein the second operating mode for the coil is activated within a time window of the first operating mode either by the coil being short-circuited when the coil is in the coil Impressed alternating current at a zero crossing, or the direction of the alternating current impressed into the coil is reversed.

The object is further achieved with a circuit arrangement for operating a stepping motor according to claim 6.

A general advantage of these solutions is that no sensors are required and a relatively simple and cost-effective implementation is possible, in particular when the motor is controlled by pulse width modulation (PWM) of a voltage which impresses a corresponding current into the motor coils (coil current) The components required for evaluating the measuring current (short-circuit current) are already largely present in a PWM circuit.

A further advantage of this solution is that the start of the measuring current (current cipes) is very reproducible and largely independent of the magnitude of this voltage due to the regulation of the coil current (target current) by the PWM voltage. This results in at least a further independent independence of component tolerances.

Furthermore, it has been shown that there is at least a largely linear relationship between the level of the measuring current and the power which is removed by a mechanical load. This means, in particular, that the higher the mechanical load on the motor, the smaller the measuring current.

The background is the fact that the phase shift between the coil current impressed in the coil and the voltage which is induced by the rotor is reduced as the motor load increases and thus the load angle (angle between the rotor and the main direction of the magnetic field generated by the coils) increases Motor is zero).

Since the mechanical power of the motor is proportional to the motor force or the motor torque at a constant speed, the height of the measuring current (current lobe) directly represents the height of the torque of the motor load and, together with the torque characteristic of the motor concerned,

In particular, a reference position of the motor can also be detected sensor-free by driving the motor against a mechanical stop and the thus changed operating or load state. The disadvantages associated with a conventional sensorless determination of a reference position, such as mechanical wear and noise development, do not occur in this case or to only a much lesser extent.

An advantage of the almost direct load or torque measurement explained above is the (mechanical) reference travel in that the latter is largely independent of the elasticity module of the mechanical stop.

The subclaims have the advantage of further advantageous developments of the invention.

Further details, features and advantages of the invention will emerge from the following description of a preferred embodiment with reference to the drawings. It shows:<dl id="dl0001" compact="compact"><dt>FIG</dt><dd>1 shows a circuit diagram of a first circuit arrangement according to the invention;</dd><dt>FIG</dt><dd>A first diagram of the voltage and current paths in the region of a zero crossing of the coil current;</dd><dt>FIG</dt><dd>A second diagram of the voltage and current paths in the region of a zero crossing of the coil current;</dd><dt>FIG</dt><dd>A third diagram of the voltage and current paths in the region of a zero crossing of the coil current; and</dd><dt>FIG</dt><dd>2 shows a circuit diagram of a second circuit arrangement according to the invention.</dd></dl>

FIG. 1 shows a circuit diagram of a circuit arrangement which can be switched by a switch S between a first operating mode for normal motor operation (switch position 1) and a second operating mode for detecting an operating state and in particular a reference position of the motor (switch position 0) A mechanical load change during driving of the stepping motor against a mechanical stop is determined.

The stepping motor itself is only in the form of one of its coils L with a series internal resistance R_L , A plurality of these coils being arranged in a known manner around a magnetic rotor in a known manner. Alternatively, however, it may also be a linear stepping motor.

A voltage source for generating a supply voltage U.sub.L is connected in parallel to the coil L (or mutually assigned winding groups)_M (PWM voltage), with which, in the first operating mode, a specific target current (coil current) I_L Is impressed into the respective coil (s) L. Furthermore, in series with each coil L is a measuring resistor R_S, To which a measuring voltage U_S As a function of a (measuring) current actually flowing through the coil L.

Ideally, such a step motor driver operates as a quasi-ideal current source at a sufficiently high supply voltage, provided that the current reduction is not only passive ( "slow decay") but active ( "fast decay").

The circuit arrangement further comprises a measuring circuit M, at the input of which the measuring voltage U_S And which has a comparator K for comparing the measuring voltage U_S With a reference voltage U_R , And a digital / analog converter DAC for generating the comparison voltage from a digital signal supplied to the measuring circuit M. The reference voltage U_R Can also be supplied in a different manner or in an analogous form.

Furthermore, a control circuit C is provided, which generates the digital signal for the digital / analog converter DAC and to which the output signal of the comparator K is fed. The control circuit C can be controlled via an interface I in order to switch the switch S between its two switch positions 0 and 1 as a function of the output signal of the comparator K, or to provide this output signal for further processing.

The measuring circuit M as well as the control circuit C are in the case of a coil current I controlled by pulse width modulation (PWM)_L In general already present, so that the circuit arrangement according to the invention can be realized cost-effectively and with a relatively small additional expenditure.

In normal motor operation (first operating mode), the coils L are phase-shifted with alternating currents in switch position 1 such that a progressive magnetic field is produced in a known manner, followed by the magnetic rotor step by step or quasi-continuously in micro steps.

To achieve a defined target current I_L Into the coil L (coil current), the voltage applied to the measuring resistor R_S Falling measuring voltage U_S Is evaluated and, in a known manner, for controlling or regulating the voltage supply U_M Generating voltage source.

As a result of the movement of the rotor and the magnetic flux change caused thereby, an electrical voltage U.sub._EMF (Counter), which corresponds to the supply voltage U_M And is substantially dependent on the speed of the rotor and its instantaneous position relative to the coil in terms of its course. In the circuit diagram of FIG. 1, this voltage is represented by a voltage source which is connected in series with the coil L and which receives the voltage U_EMF generated.

Without mechanical load (ie at a load angle of substantially 0 ° and an idealized internal resistance R_L Of the coil of 0 Ohm) is the voltage U_L Across the terminals of the coil L opposite the impressed coil current I_L Is shifted by 90 ° (ie, with a power of zero). The electric power averaged over a period is then equal to zero. As the load angle increases, this phase shift between the voltage U decreases_L And the coil current I_L. At a maximum load angle (ie, just before a step loss), the phase shift is finally substantially zero and the power is maximum.

In the case of a mechanically unloaded stepping motor which is provided with sinusoidal and cosinusoidal coil currents I_L Constant frequency, the counter-induced voltages are U_EMF In the ideal case also sinusoidal and cosinusoidal and opposite the coil currents I_L In the respective coils L by 90 ° in the phase. Phase shifts of constant type and other variations of the voltages U_EMF But can be caused by the design of the motor and in particular the geometrical shape of the rotor and the stator as well as their magnetization.

Furthermore, in particular during a mechanical load on the motor, reduced phase shifts occur between the voltage U_EMF And the coil current I_L By a non-zero load angle (angle between the rotor and the main direction of the magnetic field).

If the stepping motor is mechanically stressed beyond a limit, it can also jump by one or more integer multiples of the current period (four steps in a 2-phase stepping motor) and thus lose steps. In this case, between the coil current I_L And the counter-induced voltage U_EMF Also has a phase shift, whereby the course of the voltage U_EMF Due to the jumps. The phase of the counter-induced voltage U_EMF Thus essentially represents the rotor position.

The time course of the target current I_L Through the coils L is not necessarily sinusoidal and cosinusoidal. Depending on the type of the stepping motor, a better running behavior can be achieved with trapezoidal or triangular current-flow patterns or mixing forms thereof.

In a second operating mode, the counter-induced voltage U is suitable_EMF In principle, however, for determining the movement and load states of the stepping motor explained above, ie, its operating state, and thus also for detecting a mechanical stop, for example at a reference position, since the magnitude of the voltage U.sub._EMF And the phase position to the coil current I_L In addition to the speed of the rotor, is dependent in particular on the load angle of the rotor and thus on the load state of the motor.

Specifically, the amplitude of the counter-induced voltage is U_EMF Proportional to the speed of the rotor. The phase of this voltage U_EMF Relative to the impressed coil current I_L Is essentially determined by the mechanical load. At a maximum possible load, this phase shift substantially decreases to the value 0. This is clear from FIGS. 2 to 4 and will be explained in more detail below.

The voltage U_EMF Could be measured directly over the terminals of the coil in the case of a non-energized coil L. However, since such a coil L does not contribute to the torque of the motor, this type of detection is not desirable. A rapid changeover between the energized state and the non-energized state is also undesirable because of the relatively high induction voltages occurring in this case.

According to the invention, therefore, the counter-induced voltage U_EMF In the second operating mode, by disconnecting the coil from the supply voltage U_M And short-circuiting the coil, in each case periodically within such time windows, in which the current I impressed into the coil L in question_L Is relatively low, ie shortly before and after the polarity change of this current or the supply voltage U which impresss the latter_M.

For this purpose, the switch S is switched to the switch position 0.

The start of the time windows is not required by monitoring the measuring voltage U_S And comparison with a reference voltage. Rather, the beginning and the length of the time windows is determined by the known or fixed course of the supply voltage U_M Or the course of the current I impressed into the coil in question_L , So that the control circuit C for switching the switch S can be activated directly via the interface I.

The counter-induced voltage U_EMF Drives a measuring current I in the winding circuit, which is thus short-circuited_{S, EMF} (Current lobe), which is connected to the measuring resistor R_S A corresponding voltage drop U_{S, EMF} generated.

This voltage U_{S, EMF} Is again evaluated with the measuring circuit M in order to determine the operating state of the motor and to determine whether the motor is running under a more or less high mechanical load or even against a mechanical stop in order to stop it immediately in order to avoid wear.

For this purpose the voltage U_{S, EMF} In the comparator K with different thresholds U_SO, U_SU , Which are determined as a function of the rotational speed of the motor and are supplied as digital values ​​via the interface I and the control circuit C to the digital / analog converter DAC (or analog).

This evaluation is to be explained with reference to the diagrams of FIGS. 2 to 4. The vertical axis indicates the magnitude of the current or the voltage, while the period of the period is plotted on the horizontal axis.

In the vicinity of the time window Z (U_M = 0) flowing through the coil L_L Is shown with a solid line. This coil current I_L Is zero within the time window Z, in which the winding circuit is short-circuited by switching the switch S into the switch position 0. Instead, substantially a current flowing through the counter-induced voltage U flows within the time window Z_EMF Called measuring current I_{S, EMF} (Shown by dashed lines, current lobes), through which the measuring resistor R_S A measuring voltage U_{S, EMF} .

Furthermore, the curve of the counter-induced voltage U 1 is also shown in these diagrams with a dashed line_EMF Is entered.

FIG. 2 shows the curves of these currents and voltages for a substantially unstressed state, that is to say at a minimum load angle, while FIG. 3 shows the curves for an average load and an average load angle. FIG. 4 finally shows the curves of the currents and voltages at a maximum load and a maximum load angle.

As already explained, the phase shift between the coil current I decreases with increasing load_L Or - during the second operating mode - between the measuring current I_{S, EMF} Through the coil L and the counter-induced voltage U_EMF.

For the sake of completeness, it should be mentioned that, as is generally known, the counter-induced voltage U_EMF With respect to their polarity in phase opposition to the supply voltage U_M Behavior.

The height of the measuring current I flowing within the time window Z_{S, EMF} (Current lobes) and thus the magnitude of the measuring voltage U generated by the latter within the time window Z._{S, EMF} Is dependent on the rotational speed and the load angle of the motor caused by a load, and thus also represents the received actual power.

A load change results in a phase shift of the counter-induced voltage U_EMF And thus to a change in the current driven by the latter in the winding circuit. This change again has a change in the measuring current I flowing within the time window Z._{S, EMF} (Current lobes), which are determined by evaluation of the measuring voltage U_{S, EMF} Is detected.

Thus, by comparing the measurement voltages U_{S, EMF} In successive time windows Z a change in the load of the motor can be detected. In particular, it is possible to detect the load change occurring during driving against a mechanical stop, and in this way to define or define a reference position.

Load changes are preferably made by comparing the measuring voltage U_{S, EMF} Are detected within the time slots Z with threshold values ​​which are determined as a function of the speed of the motor.

For example, according to FIGS. 2 to 4, an upper and a lower threshold value U_SO, U_SU For measuring voltage U_{S, EMF} In such a way that, at a certain speed and with a relatively low load on the motor, the measuring voltage U_{S, EMF} Is greater than the upper threshold U_SO, Whereas in the case of a mechanical load increase caused by driving against a mechanical stop, the measuring voltage U_{S, EMF} Below the lower threshold U_SU lies.

FIG. 2 shows the case in which, within the time window Z, a measuring current I_{S, EMF} Through the coil, through which a measuring voltage U_{S, EMF} Which corresponds to the upper threshold U_SO Respectively.

According to FIG. 3, a measuring current I flows_{S, EMF} Through the coil, through which a measuring voltage U_{S, EMF} Which corresponds to the lower threshold U_SU Respectively.

According to FIG. 4, the load is finally so large that the measuring current I_{S, EMF} And thus the voltage drop U_{S, EMF} Is substantially zero.

With the measuring circuit according to FIG. 1, the measuring voltage U_{S, EMF} Within a time window Z with the two threshold values ​​U_SO, U_SU , The measuring voltage being applied to an input of the comparator K and a respective threshold value at the other input of the comparator K. The thresholds U_SO, U_SU Are supplied via the interface I and the control circuit C to the digital / analog converter DAC.

If the measuring voltage U_{S, EMF} Is less than the lower threshold U_SU, A signal is generated via the control unit C and the interface I, which indicates the reaching of a reference position on a mechanical stop.

If the measuring voltage U_{S, EMF} Between the two thresholds U_SO, U_SU , A signal can be produced in a corresponding manner with which an increased engine load is indicated.

Finally, if the measuring voltage U_{S, EMF} Is greater than the upper threshold U_SO, A signal can be generated indicating that the engine is running at a relatively low load.

For the realization of the invention, the boundary conditions must be that the internal resistance R.sub._L Of the respective coils L in the order of magnitude of the measuring resistor R_S , So that the measuring voltage U_{S, EMF} Is sufficiently large in the load measurement, that is to say in the region of the measuring voltage U_S In normal operation. If a switchable measuring resistor R_S The solution according to the invention can also be used with high-resistance motors. If, on the other hand, R_L &gt;&gt; R_S The measuring circuit M should have a sufficiently high gain.

Particular advantages of the invention are that the measuring voltage U_{S, EMF} Independent of the supply voltage U_M Since it is detected during a time window in which the supply voltage (PWM) is not applied. The current loop resulting from the short circuit of the winding circuit is low-resistance, so that the measuring voltage U_{S, EMF} Is relatively insensitive to disturbances. In addition, the measuring resistor R causes_S In the case of a short-circuited winding circuit, a current limiter.

Due to the regulation of the coil current I_L During the first operating mode (normal operation) are used to measure the height of the measuring current I_{S, EMF} (Current lobes) during the second operating mode. If, on the other hand, the stepping motor were operated at a voltage source, the height of the current cinch would depend on the level of the supply voltage, which would have to be taken into account accordingly.

FIG. 5 shows, finally, a schematic circuit diagram of a second circuit arrangement according to the invention in which the same or corresponding components as in FIG. 1 are provided with the same designations.

In contrast to the embodiment according to FIG. 1, the PWM supply voltage U_M Can be reversed by means of a first and a second switch S1, S2, which are switched by the control circuit C. The second operating mode, during which the voltage U, which is induced by the counter-induced voltage U_EMF Generated measuring current I_{S, EMF} Is detected and is evaluated as described above, in this case immediately after the polarity reversal of the PWM supply voltage, that is to say immediately after the switching of the two switches S1, S2, is activated, the measuring current not being allowed to be adjusted before (slow decay mode) .

Furthermore, it is indicated in this embodiment that the measuring voltage conducted to the comparator is not necessarily caused by a voltage drop by means of a resistor R_S Must be generated. Rather, Hall sensors, current dividers in MOSFET switches or other elements, possibly at other locations in the circuit arrangement, can also be used with which a signal proportional to the measuring current through the coil L is generated which can be compared with the corresponding threshold values .

As already explained, with the illustrated circuit arrangements not only a mechanical stop but also a load and possibly a suddenly occurring obstacle can be detected, so that monitoring of the driving operation of the motor is possible. Likewise, a load angle measurement is possible in principle.

In contrast to numerous known possibilities for the detection of the bearings, it is not necessary here for the engine to spring back.

By the winding circuit which is short-circuited in the region of the zero crossing of the supply voltage or of the impressed coil current, any resonant vibrations (comparable to the principle of the eddy current brake) are dampened.

The phase shift as well as the amplitude reduction of the periodic coil current I_L React very sensitively to a load variation, but at the same time both sizes are very insensitive to other disturbances which are not associated with this, such as the irradiation of electrical energy. As a result, very accurate detection of low load changes is possible. In addition, the coil in the winding circuit smoothes any electrical disturbances, so that their influence is further reduced.

The measuring conditions are very well defined within the time slots Z and are therefore very reproducible. The current I flowing within the time window Z_{S, EMF} Is only dependent on the rotational speed of the motor, the load angle, as well as motor constants, but it is not, or only very slightly, dependent on parameters that drift or vary due to specimen scattering.

Finally, it should be pointed out that the method according to the invention and the circuit arrangement according to the invention is also suitable for application with other synchronous motors, provided these are dimensioned such that at least one of the winding circuits can be short-circuited.