Circuit and method for control of a stepper motor

A circuit for the control of a stepper motor having a rotor configured as a permanent magnet and a stator configured of at least first and second energized excitation coils enclosing the rotor. Each end terminal of the first excitation coil is connected to the positive pole of the supply voltage over a switching element and each end terminal of a second excitation coil is connected to the negative pole of the supply voltage over a switching element. The central contacts of both excitation coils are connected to each other.

BACKGROUND

1. Field of the Invention

The invention relates to a circuit and a method for the control of a stepper motor.

2. Related Technology

Stepper motors are used in units and modules in the fields of automotive or office communication, medical engineering, toolmaking, consumer electronics, building services or measurement technology. They are suitable drives for the transformation of digital information to mechanical movement.

Due to the fast development of the semiconductor industry, particularly because of the greater variety of the transistors, diodes and integrated circuits, stepper motors are used not only in price-sensitive applications, but also where the mechanical commutation system of the dc motors limits the reliability or life of a drive. Stepper motors are more and more used also as controlled auxiliary drives.

Stepper motors have usually a stator winding with several separately switchable stator coils the number of which corresponds to the step angle of the stepper motor. Speed and position of the rotor are controlled by an open control chain. Bipolar driver ICs as well as discrete unipolar transistors are used for the control. Therefore the control unit can always determine the current position of the rotor and, dependent on the result of the determination, move to a new position.

Different operational conditions can be realized dependent on the wiring and control of the coils. On principle, stepper motors may be classified as unipolar and bipolar stepper motors.

In unipolar stepper motors only one coil half of the stator coils is energized. While the central taps of the coils are connected to the supply voltage, the end terminals of the coils each are grounded over an associated switch so that for the unipolar operational mode with two coils, five connection lines to the motor and four switches for control are necessary.

In bipolar stepper motors the coils are energized over their total length. In this case the coils of the stepper motor are operated in an H-bridge circuit each so that the current direction in the magnets can be changed for polarity reversal of the magnetic fields. When two coils are used, four supply lines to the motor and eight switches are necessary.

A plurality of circuit configurations are known in the state-of-the-art.

DE 102 374 34 A1, e.g., discloses a voltage supply for electromotors whereby the output voltage of a dc voltage source with a positive terminal and a negative terminal is halved by means of a voltage phase-balance circuit which in addition to the positive terminal and the negative terminal has a zero potential output that is connected to a terminal of each connected electromotor. At least one further terminal of each connected electromotor is connected to two driver transistors of a half-bridge in each case, which is connected, on the one hand, to the positive terminal and, on the other hand, to the negative terminal.

In DE 199 25 451 A1, an apparatus for driving a stepper motor using a simplified circuit structure and a bipolar driver system is described. It is characteristic of this invention that the stepper motor has two excitation coils driven by the respective bridge circuits. The transistors provided on two sides of a bridge circuit are used by both bridge circuits in common. Therefore the driver circuit can be designed having a reduced number of transistors.

Another driver circuit for a stepper motor is disclosed in WO 01/31773. The driver circuit consists of one or several pairs of excitation coils, whereby each excitation coil has two end contacts and a central tap with the central taps of the excitation coils connected to each other. Moreover, switching elements are provided that are designed such that the positive terminal of the supply voltage is energized over an end contact of a first excitation coil and the negative terminal of the supply voltage is energized over a second excitation coil.

In U.S. Pat. No. 4,558,268 a circuit for the control of stepper motors is described that is provided with several half-bridges for two stepper motors. In this way the number of the needed switching elements can be reduced, but the stepper motors, however, cannot be operated independently of each other.

In EP 0 657 990 A1, however, a method for the simultaneous control of several stepper motors is disclosed that uses a diode multiplex. Also, here switching elements can be saved, but additional diodes are required at the stepper motors. Moreover, also this method does not allow to operate the stepper motors independently of each other.

It is the object of the invention to propose a circuit and a method for the control of a stepper motor that require only a small number of switching elements and connection lines and, in addition, ensure to operate serially connected stepper motors independently of each other.

SUMMARY

It is provided according to the invention that the circuit for the control of a stepper motor includes a rotor configured as a permanent magnet and a stator configured of at least two energized excitation coils enclosing the rotor. Each end terminal of a first excitation coil is connected to the positive pole of the supply voltage over a switching element and each end terminal of a second excitation coil is connected to the negative pole of the supply voltage over a switching element. The central contacts of both excitation coils are connected to each other.

The advantage of this circuit arrangement consists in that only four switching elements and four connection lines are needed for the control of the stepper motor. The advantages of the bipolar stepper motor, which needs only four connection lines, and of the unipolar stepper motor, which needs only four switching elements, are combined in the circuit arrangement of the invention.

Both end terminals of a first coil are connected to the positive terminal of the supply voltage by means of a supply line and using one switching element in each case. Both end terminals of a second coil, however, are connected to the negative terminal of the supply voltage by means of a supply line and using one switching element in each case. As it will be described further down, four different magnetic states can be generated in the stepper motor by means of a defined switching sequence of the four switching elements used.

Preferably field effect transistors, bipolar transistors, an integrated circuit (IC) or a relay are used as switching elements.

Another substantial advantage of this invention is that when there are several serially connected stepper motors operation of each individual stepper motor independent of the others can be ensured without multiplex operation.

In serial connection of stepper motors, the excitation coils of the stepper motors connected to the positive pole of the supply voltage are connected to each other forming a connection node. In addition, the connection nodes between neighboring coils and the free end terminals of the outer excitation coils are led to the positive pole of the supply voltage over switching elements. The excitation coils led to the negative pole of the supply voltage are also connected to each other forming a connection node. In addition, the connection nodes between neighboring excitation coils and the free end terminals of the outer excitation coils are led to the negative pole of the supply voltage.

In the serial circuit according to the invention, the number ASof the switching elements required is reduced to the equation AS=2*(n+2), wherein n is the number of the stepper motors. For 5 serially connected stepper motors, e.g., only 14 switching elements are needed.

The method for the control of a stepper motor includes using a rotor configured as a permanent magnet and a stator configured of at least two energized excitation coils completely enclosing the rotor. A first end terminal of a first excitation coil is connected over a first switching element and a second end terminal of the first excitation coil is connected over a second switching element to the positive pole of the supply voltage. A first end terminal of a second excitation coil is connected over a third switching element and the second end terminal of a second excitation coil is connected over a fourth switching element to the negative pole of the supply voltage. The central contacts of both excitation coils are connected to each other, whereby for the control of the stepper motor to rotate in clockwise direction, the following process steps repeating themselves are provided:

a) closing of the first and the fourth switching element and opening of the second and the third switching element;

b) closing of the second and the fourth switching element and opening of the first and the third switching element;

c) closing of the second and the third switching element and opening of the first and the fourth switching element; and

d) closing of the first and the third switching element and opening of the second and the fourth switching element.

For persons skilled in the art it is evident that the enumerated process steps only relate to a single 360°-rotation of the rotor and that the process steps a) to d) continuously repeat themselves to ensure continuous operation of the stepper motor.

Rotation of the rotor configured as a permanent magnet in anticlockwise direction is realized in an especially simple manner by that the process steps a) to d) are executed in reverse order.

The significant advantages and features of the invention over the state-of-the-art include the combination of a bipolar and a unipolar operated stepper motor by that only one supply line and one switching element are assigned to each end terminal of both coils, whereby the end terminals of a first coil are led to the positive pole and the end terminals of a second coil are led to the negative pole of the supply voltage and the central contacts of both coils connected to each other, and totally independent operation of several serially connected stepper motor without multiplexing.

DETAILED DESCRIPTION

FIG. 1illustrates a typical unipolar operated stepper motor1′ of the state-of-the-art. It is seen that the stepper motor1′ has a rotor2′ structured as a permanent magnet and two excitation coils4.1,4.2structured as stator3. Each excitation coil4.1′,4.2′ includes two end terminals5.1′,5.2′ and a central contact6′. All end terminals5.1′,5.2′ of both excitation coils4.1′,4.2′ are led to the negative pole10′, or ground, of the supply voltage over connection lines7.1′ to7.4′ using a switching element8.1′ to8.4′ in each case. The central contacts6′ of both excitation coils4.1′,4.2′ are connected to each other and connected to the positive pole9′ of the supply voltage by a common switching line11′. For this circuit arrangement, therefore, four switching elements8.1′ to8.4′, four connection lines7.1′ to7.4′ and one switching line11are required.

InFIG. 2a typical circuit arrangement of a bipolar operated stepper motor of the state-of-the-art is shown. As opposed to the circuit arrangement illustrated inFIG. 1, first, the central contacts6″ of both excitation coils4.1″,4.2″ are not connected to each other. Second, the end terminals5.1″,5.2″ of both excitation coils4.1″,4.2″ are connected over connection lines7.1″ to7.4″ to a switching line11″, each switching line11″ extending between the positive pole9″ and the negative pole10″ of the supply voltage. Each switching line11″ is provided with two switching elements8″, whereby a first one of these two switching elements8″ is placed at the positive pole9″ and a second one of these two switching elements8″ is placed at the negative pole10″ of the supply voltage. Thus, for this circuit arrangement four connection lines7.1″ to7.4″ to the stepper motor1″ and eight switching elements8″ are required.

The circuit arrangement according to the invention for the control of a stepper motor1is shown inFIG. 3. Both end terminals5.1,5.2of a first excitation coil4.1are led to the positive pole9of the supply voltage, each by one connection line7.1to7.4and each by use of one switching element8.1,8.2. Both end terminals5.1,5.2of a second excitation coil4.2, on the other hand, are led to the negative pole10of the supply voltage each by one connection line7.1to7.4and each by use of one switching element8.1,8.2. The central contacts6of both excitation coils4.1,4.2are connected to each other. Four different magnetic states can be generated in the stepper motor1by means of a defined switching sequence for the control of the four switching elements8.1to8.4used, as described in the FIGS.4.1-4.4. The number of the required switching elements8and the number of the required connection lines7are reduced to four.

The FIGS.4.1-4.4show the four magnetic states or phases of the rotor2according to the method of circuit arrangement of the invention. The circuit arrangement in this case corresponds toFIG. 3. For the control of the stepper motor1to rotate in clockwise direction the process steps described in the FIGS.4.1-4.4are continuously repeated.

InFIG. 4.1the first and the fourth switching element8.1,8.4are closed and the second and the third switching element8.2,8.3are opened. As indicated by the arrows the current flows starting from the positive pole9of the supply voltage over the closed first switching element8.1to a first end contact5.1of the first excitation coil4.1. After having passed the first excitation coil4.1the current is led over the central contact6of the first excitation coil4.1to the central contact6of the second excitation coil4.2. After having passed the second excitation coil4.2the current is led over a second end contact5.2of the second excitation coil4.2over the closed fourth switching element8.4to the negative pole10of the supply voltage.

InFIG. 4.2the second and the fourth switching element8.2,8.4are closed and the first and the third switching element8.1,8.3are opened. As indicated by the arrows the current flows starting from the positive pole9of the supply voltage over the dosed second switching element8.2to a second end contact5.2of the first excitation coil4.1. After having passed the first excitation coil4.1the current is led over the central contact6of the first excitation coil4.1to the central contact6of the second excitation coil4.2. After having passed the second excitation coil4.2the current is led over a second end contact5.2of the second excitation coil4.2over the closed fourth switching element8.4to the negative pole10of the supply voltage.

InFIG. 4.3the second and the third switching element8.2,8.3are closed and the first and the fourth switching element8.1,8.4are opened. As indicated by the arrows the current flows starting from the positive pole9of the supply voltage over the closed second switching element8.2to a second end contact5.2of the first excitation coil4.1. After having passed the first excitation coil4.1the current is led over the central contact6of the first excitation coil4.1to the central contact6of the second excitation coil4.2. After having passed the second excitation coil4.2the current is led over a first end contact5.1of the second excitation coil4.2over the closed third switching element8.3to the negative pole10of the supply voltage.

InFIG. 4.4the first and the third switching element8.1,8.3are closed and the second and the fourth switching element8.1,8.4are opened. As indicated by the arrows the current flows starting from the positive pole9of the supply voltage over the closed first switching element8.1to a second end contact5.2of the first excitation coil4.1. After having passed the first excitation coil4.1the current is led over the central contact6of the first excitation coil4.1to the central contact6of the second excitation coil4.2. After having passed the second excitation coil4.2the current is led over a second end contact5.2of the second excitation coil4.2over the closed third switching element8.3to the negative pole10of the supply voltage.

Dependent on the dosing position of the four switching elements8.1to8.4the permanent magnet configured as rotor2rotates by 90° after each switching.

A rotation of the rotor2in anticlockwise direction is made possible by that the process steps of theFIGS. 4.1to4.4are executed in reverse order.

If several stepper motors1are to be serially coupled to each other, they are switched according toFIG. 5. In this figure, three stepper motors1are connected to each other such that they can also be operated independently of each other despite of the use of the same switching elements.

In serial connection of stepper motors1the excitation coils4.1,24.1,34.1each connected to the positive pole9of the supply voltage, of the stepper motors are connected to each other forming a connection node. For that, the second end contact5.2of the first excitation coil4.1of the first stepper motor is connected to the first end contact25.1of the first excitation coil24.1of the second stepper motor and the second end contact25.2of the first excitation coil24.1of the second stepper motor is connected to the first end contact35.1of the first excitation coil34.1of the third stepper motor forming nodes. Between these nodes and the positive pole9of the supply voltage the switching elements8.2.1,8.3.1are placed. Further, the first end contact5.1of the first excitation coil4.1of the first stepper motor1and the second end contact35.2of the first excitation coil34.1of the third stepper motor are connected over the switching elements8.1.1,8.4.1to the positive pole9of the supply voltage. The central contacts6,26,36of the excitation coils4.1,4.2;24.1,24.2;34.1,34.2are connected to each other as in single operational mode of a single stepper motor1.

In addition, the first end contact5.1of the second excitation coil4.2of the first stepper motor is connected to the second end contact25.2of the second excitation coil24.2of the second stepper motor and the first end contact25.1of the second excitation coil24.2of the second stepper motor is connected to the second end contact35.2of the second excitation coil34.2of the third stepper motor forming nodes. Between these nodes and the negative pole10of the supply voltage the switching elements8.2.2and8.3.2are placed. Further, the second end contact5.2of the second excitation coil4.2of the first stepper motor1and the first end contact35.1of the second excitation coil34.2of the third stepper motor1are connected to the negative pole10of the supply voltage over the switching elements8.1.2,8.4.2.

In the example shown, the first stepper motor is controlled over the switching elements8.1.1,8.2.1,8.1.2and8.2.2and each n-th stepper motor is controlled over the switching elements8.n.1,8.(n+1).1,8.n.2and8.(n+1).2.