Controlling a voltage output of a two-phase stepper motor

Abstract

The invention relates to a method for controlling a voltage output of a two-phase stepper motor (1) having a stator which has two stator coils (Ca, Cb), the coil voltages (Ua, Ub) of which are each limited in terms of magnitude by a maximum voltage (Ud). In the method, a target voltage, which is the square root of the sum of the squares of the two coil voltages (Ua, Ub), is used as the desired voltage of the controller. In a stator-fixed Cartesian coordinate system having two coordinate axes, each of which corresponds to one of the coil voltages (Ua, Ub), a required space vector (Z) is defined, the length of which equals the target voltage and the polar angle (φ) of which is incremented by a step angle at equidistant time intervals, modulo a maximum value of the polar angle (φ). If the target voltage can be achieved by the required space vector (Z), the coil voltages (Ua, Ub) of the required space vector (Z) are realised. Otherwise, the required space vector (Z) is replaced by a substitute space vector (Z') which is closest to the required space vector (Z) among all realisable space vectors with a length equal to the target voltage, and the coil voltages (Ua, Ub) of the substitute space vector (Z') are realised.

Description

[0001] Description

[0002] Controlling a voltage output of a two-phase stepper motor

[0003] The invention relates to a method for controlling a voltage output of a two-phase stepper motor. Furthermore, the invention relates to a two-phase stepper motor.

[0004] The two-phase stepper motor has a stator with two stator coils, the coil voltages of which are each limited by a maximum voltage.

[0005] One parameter for the operation of a stepper motor is the stepping mode. In so-called full-step mode, a pole pitch is divided into only four angular positions. This results in operation with a rectangular current. This leads to unstable operation of the stepper motor at low speeds.

[0006] To improve smoothness and positioning accuracy, the number of substeps within a pole pitch is increased until a sinusoidal signal is achieved in so-called microstep operation. However, this degrades the performance at the stepper motor's voltage limit.

[0007] The effective output voltage in full-step mode is approximately 80% higher than when controlled with sinusoidal voltages. This is due, on the one hand, to the geometric addition of the component voltages of the subsystems formed by the stator coils and, on the other hand, to the exaggeration of the fundamental oscillation in the amplitude spectrum of a square-wave signal compared to its amplitude. Stepper motors generally have a very high inductance, so this full-step control does not produce excessively large current peaks. Therefore, higher stepper motor speeds can be achieved with full-step control than with microstep control.

[0008] The invention is based on the object of providing an improved control of a voltage output of a stepper motor which combines the advantages of a microstep operation and a full step operation of the stepper motor.

[0009] The object is achieved according to the invention by a method having the features of claim 1 and a stepper motor having the features of claim 6. Advantageous embodiments of the invention are the subject of the dependent claims.

[0010] In the method according to the invention, a voltage output of a two-phase stepper motor with a stator having two stator coils, the coil voltages of which are each limited in magnitude by a maximum voltage, is controlled, wherein

[0011] - a target voltage is used as the control setpoint voltage, which is a root of the sum of the squares of both coil voltages,

[0012] - in a stator-fixed Cartesian coordinate system with two coordinate axes, each corresponding to one of the coil voltages, a required space vector is defined, the length of which is the target voltage and the polar angle of which is increased by one step angle modulo a maximum value of the polar angle at equidistant times, and,

[0013] - if the target voltage can be realized by the required space vector, the coil voltages of the required space vector are realized and

[0014] - otherwise, the required space vector is replaced by a substitute space vector which is closest to the required space vector among all realizable space vectors whose length is the target voltage, and the coil voltages of the substitute space vector are realized.

[0015] In the method according to the invention, coil voltages of the stator coils of a stepper motor are realized as a function of a target voltage, which is the square root of the sum of the squares of both coil voltages. Since the magnitudes of both coil voltages are limited by a maximum voltage, the target voltage is limited by a maximum target voltage value, which is the maximum voltage of the coil voltages multiplied by 2. To define coil voltages by which a target voltage is realized, a stator-fixed Cartesian coordinate system with two coordinate axes, each corresponding to one of the coil voltages, is used. In this coordinate system, a "required space vector" is defined for a target voltage, the length of which is the target voltage and the polar angle of which is time-dependent, with the polar angle being increased by one step angle at equidistant times.

[0016] The invention takes into account that not every required space vector defined in this way can actually be realized, because it can happen that the magnitude of a component of the required space vector exceeds the maximum voltage of the coil voltages. However, this can only happen if the target voltage is greater than the maximum voltage of the coil voltages. As long as the target voltage does not exceed the maximum voltage of the coil voltages, however, any required space vector can be realized, and according to the invention, the required space vector is then also realized. Since the polar angle increases as a function of time, the required space vector rotates with advancing time in the stator-fixed coordinate system, and its components change sinusoidally as a function of time as long as the target voltage does not change, or quasi-sinusoidally when the target voltage changes.As long as the target voltage does not exceed the maximum voltage of the coil voltages, the stepper motor is operated in microstep mode.

[0017] If the target voltage is greater than the maximum voltage of the coil voltages, only certain required space vectors can be realized. An unrealizable required space vector is then replaced by an equivalent space vector that is closest to the required space vector among all realizable space vectors whose length is the target voltage. The components of the equivalent space vector are then realized as coil voltages. This means that the realized coil voltages, depending on time, only have a sinusoidal or quasi-sinusoidal characteristic in individual time intervals, while their magnitudes between these time intervals are constant or quasi-constant. The time intervals with sinusoidal or quasi-sinusoidal curves of the coil voltages become increasingly shorter as the target voltage increases, so that the temporal curves of the coil voltages increasingly approach rectangular voltages as the target voltage increases.

[0018] When the target voltage reaches its maximum value, the target voltage can only be realized using four space vectors, and the time profiles of the coil voltages are square-wave voltages. The stepper motor then operates in full-step mode.

[0019] The method according to the invention thus enables a continuous transition from microstep operation to full step operation and vice versa.

[0020] In one embodiment of the method according to the invention, the maximum voltage of the coil voltages is a constant intermediate circuit DC voltage of an intermediate circuit in which the stator coils are arranged. The intermediate circuit DC voltage can be applied to the stator coils, for example, by opening and closing switches.

[0021] In a further embodiment of the method according to the invention, the coil voltages are each generated from the maximum voltage by pulse-width modulation. The coil voltages are thus generated by temporally averaging or smoothing voltage pulses of variable duration applied to the stator coils. In a further embodiment of the method according to the invention, the output voltage of the stepper motor is used as a manipulated variable for controlling the output current of the stepper motor. The output current of the stepper motor corresponds to the speed of the stepper motor, so that the speed of the stepper motor is also controlled by controlling the output current.

[0022] A two-phase stepper motor according to the invention comprises

[0023] - a stator having two stator coils, the coil voltages of which are each limited by a maximum voltage, and

[0024] - a control unit configured to control a voltage output of the stepper motor, wherein

[0025] - a target voltage is used as the control setpoint voltage, which is a root of the sum of the squares of both coil voltages,

[0026] - in a stator-fixed Cartesian coordinate system with two coordinate axes, each corresponding to one of the coil voltages, a required space vector is defined, the length of which is the target voltage and the polar angle of which is increased by one step angle modulo a maximum value of the polar angle at equidistant times, and,

[0027] - if the target voltage can be realized by the required space vector, the coil voltages of the required space vector are realized and

[0028] - otherwise, the required space vector is replaced by a substitute space vector which is closest to the required space vector among all realizable space vectors whose length is the target voltage, and the coil voltages of the substitute space vector are realized.

[0029] In one embodiment of the stepper motor according to the invention, the maximum voltage of the coil voltages is a constant intermediate circuit DC voltage of an intermediate circuit in which the stator coils are arranged.

[0030] In a further embodiment of the stepper motor according to the invention, the control unit is configured to generate the coil voltages from the maximum voltage by pulse width modulation.

[0031] In a further embodiment of the stepper motor according to the invention, the control unit is configured to use the output voltage of the stepper motor as a manipulated variable for controlling the output current of the stepper motor. The maximum value of the polar angle of a required space vector is, for example, 360 degrees.

[0032] The features of a stepper motor according to the invention correspond to the features of the method according to the invention. Therefore, the advantages of a stepper motor according to the invention also correspond to the above-mentioned advantages of the method according to the invention.

[0033] The above-described properties, features, and advantages of this invention, as well as the manner in which they are achieved, will become clearer and more readily understood in connection with the following description of exemplary embodiments, which are explained in more detail in conjunction with the drawings.

[0034] FIG 1 shows a circuit diagram of an embodiment of a two-phase stepper motor,

[0035] FIG 2 realized coil voltages of a stepper motor for a first target voltage,

[0036] FIG 3 realized coil voltages of a stepper motor for a second target voltage,

[0037] FIG 4 realized coil voltages of a stepper motor for a third target voltage,

[0038] FIG 5 Time courses of coil voltages of a stepper motor with increasing target voltage.

[0039] Figure 1 (FIG 1) shows a circuit diagram of an embodiment of a two-phase stepper motor 1. The stepper motor 1 of this embodiment has a stator with a first stator coil Ca and a second stator coil Cb, which are connected in a bipolar manner. Each coil end of each stator coil Ca, Cb can be connected to a ground potential or an intermediate circuit potential that is positive relative to the ground potential by one of the switches S1 to S8. As a result, the voltages Lid, 0, and -Lid can be realized between a first and second coil end of each stator coil Ca, Cb, where Lid denotes the (positive) intermediate circuit DC voltage of the intermediate circuit in which the stator coils Ca, Cb are arranged.Switches S1 to S8 are each, for example, a semiconductor switch such as an insulated-gate bipolar transistor (IGBT) or a metal-oxide-semiconductor field-effect transistor (MOSFET), for example based on silicon or gallium nitride. Stepper motor 1 further includes a control unit 3, which can control switches S1 to S8 independently of one another, so that they are each open or closed.The control unit 3 is configured to modulate the voltages present between the coil ends of each stator coil Ca, Cb by pulse width modulation and thereby generate a time-averaged or smoothed first coil voltage 11a between the coil ends of the first stator coil Ca and a time-averaged or smoothed second coil voltage 11b between the coil ends of the second stator coil Cb. The coil voltages 11a, 11b can therefore assume values ​​between -Ud and +Ud and are limited in magnitude by the intermediate circuit DC voltage Ud as the maximum voltage. The control unit 3 is configured to control a voltage output of the stepper motor 1 by the coil voltages 11a, 11b according to the method described below.

[0040] The control system can be clearly described using a stator-fixed Cartesian coordinate system with two coordinate axes. The values ​​of the first coil voltage 11a are plotted on the abscissa axis (right axis) of the coordinate system, and the values ​​of the second coil voltage 11b are plotted on the ordinate axis (vertical axis). Each pair of coil voltages 11a, 11b thus corresponds uniquely in this coordinate system to a space vector that points from the coordinate origin of the coordinate system to a point in the coordinate system with the coordinates (11a, 11b) and is referred to as the end point of the space vector. Instead of using the coordinates (11a, 11b) of its end point, a space vector can also be described using polar coordinates, i.e., its longitude and its polar angle.Each space vector thus uniquely defines a pair of coil voltages lla, llb, which are called coil voltages lla, llb of the space vector.

[0041] The setpoint voltage for controlling the voltage output of stepper motor 1 is a target voltage, which is the square root of the sum of the squares of both coil voltages 11a, 11b and thus defines the length of a space vector in the stator-fixed coordinate system. A required space vector Z is assigned to the target voltage in the stator-fixed coordinate system, the length of which is the target voltage and whose polar angle (p modulo 360°) is increased by one step angle at equidistant times. If the target voltage can be realized by the required space vector Z, the coil voltages Ua, Ub of the required space vector Z are realized. Otherwise, the required space vector Z is replaced by a substitute space vector Z' that is closest to the required space vector Z among all realizable space vectors whose length is the target voltage, and the coil voltages Ua, Ub of the substitute space vector Z' are realized.The distance between two space vectors of equal length is defined as the difference between the polar angles of the two space vectors. For polar angles for which there are two candidates for the equivalent space vector that, according to this distance definition, have the same distance from the required space vector Z, one of these two candidates is assigned as the equivalent space vector Z' using a suitable selection rule.

[0042] If the target voltage does not exceed the intermediate circuit DC voltage Lid, the required space vector Z can be realized for any value of the polar angle αp. Otherwise, the required space vector Z can only be realized for certain values ​​of the polar angle αp. Figures 2 to 4 illustrate this. In Figures 2 to 4, the stator-fixed coordinate system is shown on the left. A square Q, whose corner points P1, P2, P3, P4 have the coordinates (Ud,Ud), (-Ud,Ud), (-Ud,-Ud), (Ud,-Ud), shows the range of the realizable space vectors Z: every space vector whose end point lies in the square Q can be realized and every other space vector cannot be realized because the absolute value of at least one coil voltage lla, llb of a space vector whose end point lies outside the square Q is greater than the intermediate circuit DC voltage Lid. A circular line L is the edge of a circle whose center is the origin of coordinates and whose radius is the target voltage.The end point of a required space vector Z and the end point of a substitute space vector Z' therefore lie on the circular line L. Furthermore, in Figures 2 to 4, a diagram is shown on the right-hand side, which shows the coil voltages Ua((p), Ub(cp) of the realized space vector as a function of the polar angle cp of the required space vector Z.

[0043] Figure 2 (FIG 2) shows the case where the target voltage corresponds to the intermediate circuit DC voltage Ld. In this case, the circular line L on the coordinate axes touches one side of the square Q. The end point of each required space vector Z lies for each polar angle αp in the square Q and therefore each required space vector Z is feasible. The first coil voltage Ua(p) as a function of the polar angle αp is Ud cos(p), the second coil voltage Ub(p) as a function of the polar angle αp is Ud sin(p).

[0044] Figure 3 (FIG 3) shows a case in which the target voltage lies between the values ​​Ld and Ud. In this case, part of the circular line L runs outside the square Q and only space vectors with the length of the target voltage can be realized whose end point lies on one of the circular arcs L1, L2, L3, L4 that run within the square Q. As an example, a required space vector Z is shown which cannot be realized because the first coil voltage lla of this required space vector Z is greater than the intermediate circuit DC voltage Ld. This required space vector Z is replaced by a substitute space vector Z' whose end point lies on the circular arc L1 and which is closest to the required space vector Z. The same applies to other non-realizable space vectors. Each coil voltage Ua(p), Ub(p) as a function of the polar angle (p) exhibits plateaus and jumps.Each plateau corresponds either to a range of polar angles cp, to which the same equivalent space vector Z' is assigned, or to two adjacent ranges of polar angles cp, to which two different equivalent space vectors Z' with the same component 11a or 11b are assigned. A jump in a coil voltage Ua(p), Ub(p) occurs between two plateaus, which correspond to two equivalent space vectors Z', which differ from each other in the sign of their respective components 11a or 11b. The first coil voltage Ua(p) has jumps at cp=90° and 11p=270°, the second coil voltage Ub(p) has jumps at 11p=0° and 11p=180°. As the target voltage increases, the plateaus broaden and the coil voltages 11a, 11b of the plateaus approach the values ​​±Ud.

[0045] Figure 4 (FIG 4) shows the case where the target voltage assumes its maximum value ^2-Lid. In this case, the circular line L only touches square Q at its corners P1, P2, P3, P4 and otherwise runs outside square Q. Thus, only required space vectors with the length of the target voltage can be realized whose end point coincides with one of the corners P1, P2, P3, P4 of square Q. Analogous to Figure 3, an example of a required space vector Z is shown which cannot be realized because the first coil voltage lla of this required space vector Z is greater than the intermediate circuit DC voltage Lid. This required space vector Z is replaced by a substitute space vector Z' whose end point is the corner point P1 of square Q. Each coil voltage Ua((p), Ub((p) as a function of the polar angle (p) only exhibits plateaus with the values ​​-Lid and +Ud.

[0046] Figure 5 (FIG 5) shows curves lla(t), llb(t) of the coil voltages lla, llb as a function of time t, when the target voltage is continuously increased with increasing time t up to its maximum value ^2-Lid. Time t is plotted in seconds on the abscissa axis, and the ratios Ua / lld and Ub / lld of the coil voltages lla, llb to the intermediate circuit DC voltage Lid are plotted on the ordinate axis.

[0047] Initially, until approximately t=2s, the target voltage is less than or equal to Id, so that Ila(t) and Ilb(t) are sinusoidal. These curves Ila(t) and Ilb(t) correspond approximately to the curves Ua(p), Ub(p) in Figure 2, since the polar angle (p) of the required space vector (modulo 360°) increases linearly with time t. Therefore, until approximately t=2s, stepper motor 1 is operated in microstep mode.

[0048] Thereafter, between approximately t=2s and t=6s, the target voltage assumes values ​​between Ld and ^2 Ud. lla(t) and llb(t) behave similarly to Ua(p), Ub(p) in Figure 3, whereby the “plateaus” of Ua(t) and llb(t) are not flat like the plateaus of Ua(cp) and Ub(cp) in Figure 3, but somewhat slanted, since the target voltage increases with time t. Because of the increasing target voltage, the plateaus of lla(t) and llb(t) also broaden with increasing time t and the coil voltages lla, llb of the plateaus approach the values ​​±Ud. As time t increases, lla(t) and llb(t) therefore approach square wave voltages more and more.

[0049] At approximately t=6s, the target voltage reaches its maximum value ^2 Ud and then maintains this value. lla(t) and llb(t) now correspond to Ua(cp) and Ub(cp) in Figure 4 and are square-wave voltages. Therefore, starting at approximately t=6s, stepper motor 1 operates in full-step mode.

[0050] The control of the stepper motor 1 according to the method according to the invention thus enables a continuous transition from microstep operation to full step operation.

[0051] A unipolar stepper motor is controlled accordingly according to the invention, wherein the square Q of Figures 2 to 4 is replaced by a square whose vertices are the coordinates (0,0), (Ud,0), (Ud,Ud), (0,Ud), and the polar angles (p) of the required space vectors refer to a coordinate system whose coordinate origin is the center of this square.

[0052] Although the invention has been illustrated and described in detail by means of preferred embodiments, the invention is not limited by the disclosed examples and other variations can be derived therefrom by those skilled in the art without departing from the scope of the invention.

[0053] Regardless of the grammatical gender of a particular term, it includes persons with male, female or other gender identities.

Claims

Patent claims 1. Method for controlling a voltage output of a two-phase stepper motor (1) with a stator having two stator coils (Ca, Cb), the coil voltages (lla, llb) of which are each limited in magnitude by a maximum voltage (Lid), wherein - a target voltage is used as the control setpoint voltage, which is a root of the sum of the squares of both coil voltages (lla, llb), - in a stator-fixed Cartesian coordinate system with two coordinate axes, each corresponding to one of the coil voltages (lla, llb), a required space vector (Z) is defined, the length of which is the target voltage and the polar angle ( <p) modulo eines Maximalwertes des Polarwinkels (cp) zu äquidistanten Zeitpunkten um jeweils einen Schrittwinkel erhöht wird, und, - if the target voltage can be realized by the required space vector (Z), the coil voltages (lla, llb) of the required space vector (Z) are realized and - otherwise, the required space vector (Z) is replaced by a substitute space vector (Z') which is closest to the required space vector (Z) among all realizable space vectors whose length is the target voltage, and the coil voltages (Ua, Ub) of the substitute space vector (Z') are realized.

2. Method according to claim 1, wherein the maximum voltage (Ud) is a constant intermediate circuit DC voltage of an intermediate circuit in which the stator coils (Ca, Cb) are arranged.

3. Method according to claim 1 or 2, wherein the coil voltages (Ua, Ub) are each generated by pulse width modulation from the maximum voltage (Ud).

4. Method according to one of the preceding claims, wherein the output voltage of the stepper motor (1) is used as a manipulated variable for regulating an output current of the stepper motor (1).

5. Method according to one of the preceding claims, wherein the maximum value of the polar angle (cp) is 360 degrees.

6. Two-phase stepper motor (1), comprising - a stator having two stator coils (Ca, Cb), the coil voltages (Ua, Ub) of which are each limited in magnitude by a maximum voltage (Ud), and - a control unit (3) arranged to control a voltage output of the stepper motor (1), wherein - a target voltage is used as the control setpoint voltage, which is a root of the sum of the squares of both coil voltages (lla, llb), - in a stator-fixed Cartesian coordinate system with two coordinate axes, each corresponding to one of the coil voltages (lla, llb), a required space vector (Z) is defined, the length of which is the target voltage and the polar angle ( <p) modulo eines Maximalwertes des Polarwinkels (cp) zu äquidistanten Zeitpunkten um jeweils einen Schrittwinkel erhöht wird, und, - if the target voltage can be realized by the required space vector (Z), the coil voltages (lla, llb) of the required space vector (Z) are realized and - otherwise, the required space vector (Z) is replaced by a substitute space vector (Z') which is closest to the required space vector (Z) among all realizable space vectors whose length is the target voltage, and the coil voltages (Ua, Ub) of the substitute space vector (Z') are realized.

7. Stepper motor (1) according to claim 6, wherein the maximum voltage (Ud) is a constant intermediate circuit DC voltage of an intermediate circuit in which the stator coils (Ca, Cb) are arranged.

8. Stepper motor (1) according to claim 6 or 7, wherein the control unit (3) is arranged to generate the coil voltages (Ua, Ub) from the maximum voltage (Ud) by pulse width modulation.

9. Stepper motor (1) according to one of claims 6 to 8, wherein the control unit (3) is arranged to use the output voltage of the stepper motor (1) as a manipulated variable for regulating an output current of the stepper motor (1).

10. Stepper motor (1) according to one of claims 6 to 9, wherein the maximum value of the polar angle ( <p) 360 Grad beträgt.

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