GB2350500A

GB2350500A - Driver circuit for a stepper motor

Info

Publication number: GB2350500A
Application number: GB9925685A
Authority: GB
Inventors: Christopher Stuart Brown
Original assignee: AB Automotive Electronics Ltd
Current assignee: AB Automotive Electronics Ltd
Priority date: 1999-10-29
Filing date: 1999-10-29
Publication date: 2000-11-29
Also published as: AU1044601A; WO2001031773A1; GB9925685D0

Abstract

In a driver circuit for operating a stepper motor having one or more pairs of windings, one end of each winding is connected to each other to provide a centre tapped winding, the other ends of each winding are connectable by a switch A, B, C, D to a voltage source. By connecting the centre tap of the windings together enables the motor to be operated from a supply voltage (24 volt) higher than a nominal voltage (12 volt). Instead of using centre tapped windings two separate windings (Fig 6) may be connected by switches to provide such a centre tapped connection.

Description

1 2350500 1 STEPPER MOTOR AND DRIVER The present invention relates to

stepper motors and in particular drives for controlling stepper motors.

Stepper motors are well-known in the art and are particularly useful in applications where precise control or position of the motor and the object or device being driven by the motor is required. Because of their accurate control of position, stepper motors have been used increasingly in the automotive industry in a number of applications such as vent controls in ventilation systems, seat position adjustment and so on. As a consequence, the cost and availability of 12 volt stepper motors has improved considerably as a result of economies of scale of manufacture.

However, the largest part of the market has been in motors for use in cars and small commercial vehicles where the electrical system is operated from a 12 volt supply. Consequently whilst stepper motors operating at 12 volts are commonly available motors operating on other voltages are produced in much smaller numbers and consequently do not benefit from the economies of scale of manufacture. In addition, with 24V motors, to keep the power dissipation the same as a 12V motor, the current has to be reduced and the number of turns on the windings increased to give the same torque. This tends to mean smaller diameter wires must be used and a larger motor. As a result of this and the lower production volumes, motors operating on voltages other than 12 volts tend to be considerably more expensive than their 12 volt counterparts.

This is a particular problem in the large commercial vehicle market where the power supply in such vehicles is generally 24 volts or higher.

2 This leaves the manufacture of such vehicles with little choice than to use the more expensive 24 volt stepper motors resulting in higher manufacturing cost. One solution to this problem is to provide a separate 12 volt power source within the vehicle from the main 24 volt (or higher) power source. However, such voltage converters are relatively expensive and consequently the overall cost is higher than operating 12 volt stepper motors on a 12 volt system.

Stepper motors come in two main varieties: Bipolar and Unipolar. Unipolar stepper motors have a centre-tapped winding on each pole of the stator of a motor. By connecting the centre tap of the winding to the positive voltage supply and selectively connecting one of the ends of the winding to the negative voltage supply, current can be made to flow through one half of the winding. Consequently, the direction of magnetic flux can be chosen.

In a bipolar stepper motor, the poles of the motor are provided with a single winding, the ends of which are connected to an H-bridge driver circuit which allows current to be selectively passed through the winding in either direction.

According to the present invention there is provided a driver circuit for use with a stepper motor having one or more pairs of centre-tapped windings, each centre-tapped winding having end connections and a centretapped connection, the centre-tapped connection of each winding being connected to a common connection, the driver circuit comprising: switching means for selectively connecting, in use, a positive voltage to one of the end connections of a first winding and a negative voltage to one of the end connections of the other winding of a pair of centre- tapped windings and wherein: said switching means alternates which of the ends of the first and second windings the positive and negative voltages respectively are applied to.

An embodiment of the present invention will now be described in detail with reference to the attached drawings in which:

3 Figure 1 is an example of the conventional arrangement for driving a unipolar stepper motor; Figure 2 shows a representation of a unipolar stepper motor and the connections applied thereto; Figure 3 shows the switching sequence for a conventional unipolar stepper motor; Figure 4 shows the switching sequence of a stepper motor arranged in accordance with the present invention; Figure 5 shows the arrangement for driving a stepper motor in accordance with the present invention; and Figure 6 shows an arrangement for driving a bi-polar motor in accordance with the present invention.

As shown in Figure 1, a conventional unipolar stepper motor comprises a number of poles, each provided with a centre-tapped winding. In a conventional 12 volt supply system, in order to cause the rotor of the motor to rotate, the windings are selectively energised for example as shown in Figure 3. The unipolar motor provides advantages over the bipolar stepper motor in that the driving system requires only two switches for example A and B in order to energise the windings on 1 pole in both directions. In contrast in a bipolar motor four switches are needed to provide an H-bridge drive circuit.

A typical unipolar stepper motor would be provided with five connections to the drive circuit as shown in Figure 2. These include 4 connections to each of the ends of the windings A, B, C, D in addition to the 12 volt common line which is connected to the centre taps of each winding Without any modification to the 12 volt motor itself, the system can be modified so as to be operated from a 24 volt supply without the need to use expensive and complex voltage converters to convert the higher voltage supply to a lower voltage supply. As shown in Figure 4, the same sequence of coil energisations can be achieved using a 24 4 volt supply. Instead of providing the 12 volt supply to the common centre tap connections of the motor, this connection is left unconnected externally and the 24 volt supply is applied via one of two switching elements in the drive unit (see Figure 5). In this way the 24 volt supply is used to drive current through two different windings in the stepper motor resulting in half the voltage being dropped across each of the two windings which the supply is connected across.

Referring to Figure 3, it can be seen that in the conventional 12 volt supply system, when switch Sc is closed, current flows through the winding C from the centre tap, inducing a magnetic field. Similarly when switch SD is closed, current flows through winding D from the centre tap causing a magnetic field to be induced in the opposite direction to that produced by closing switch Sc. However, in the present invention, as shown in Figure 4, 24 volt is supplied to the windings C or D when the switch Sc or SD is closed, which causes current to flow towards the centre tap, i.e. in the opposite sense to that of the 12V system.

As the current flows towards the centre tap, a magnetic field is induced in the opposite direction to that when the switches were connected to the 0 volt supply. Consequently the sequence of closing of the switches Sc and SD must be reversed to provide the appropriate sequence of coil energisations for the motor to turn correctly.

It will be appreciated that the switching sequence can be modified in different ways to overcome this same effect.

The arrangement of the present invention finther means that no connections between the common centre tap connections and the drive circuit is required. This means that fewer, i.e. four, interconnections between the motor and the drive circuit can be used, further reducing the cost of production and installation of the motors. Alternatively, the common centre tap can be used to provide diagnostic information. The signal from a connection to the centre tap can be used to detect short circuits to the supply, short circuit to ground, short circuited coil, open circuit terminal or open circuit coil. Even if the common terminal is used for diagnostic purposes, only a small low- current sensing wire is needed.

Whilst the above embodiment of the present invention has been described in respect of operating a nominally 12 volt motor from a 24 volt supply system, the present invention is equally applicable to operating stepper motors designed to operate at any nominal voltage with a supply voltage of approximately twice the nominal voltage of the stepper motor. The present invention is similarly not limited to stepper motors with two centretapped windings. The invention can equally be applied to motors having more poles.

The same advantageous effect of the present invention can be achieved in motors having a higher number of poles by connecting the supply across two serially connected groups of several such windings connected in parallel within the group so long as the number of windings in each group is matched. For example, the invention is applicable to three- phase motors which can be driven with two phases switch on.

Whilst the advantages of the above invention have been described in respect of unipolar motors, the invention may be applied to bi-polar motors. Figure 6 shows an example of an arrangement for a bi-polar motor having two windings. Again, a conventional motor can be driven from a higher supply voltage by arranging for the supply voltage to be passed across both windings. As can be seen in Figure 6, no additional switching elements are needed. Figure 6 shows how the individual switches are closed over the four stage cycle. For example in step one all the switches marked I are closed, then in step two all the switches marked 2 are closed and so on. Also, the circuit could operate from either a higher voltage or the nominal voltage by changing the switching sequence (e.g. for a winding with a 12V nominal voltage, the motor could be used with both 24V and 12V supplies).

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Claims

CLAIMS:

1. A driver circuit for use with a stepper motor having one or more pairs of centretapped windings, each centre-tapped winding having end connections and a centretapped connection, the centre-tapped connection of each winding being connected to a common connection, the driver circuit comprising: switching means for selectively applying, in use, a voltage source across one of the end connections of a first winding and one of the end connections of the other winding of a pair of centre- tapped windings and wherein said switching means alternates which of the ends of the first and second windings the voltage source is respectively applied to.

2. A stepper motor in combination with a drive circuit according to claim 1.

3. A method of operating a stepper motor having one or more pairs of centretapped windings each centre-tapped winding having end connections and a centretapped connection, the centre-tapped connection of each winding being connected to a common connection, the method comprising: applying a voltage source across one of the end connections of a first winding and one of the end connections of the other winding of a pair of centre-tapped windings; and alternating which of the ends of the first and second windings the voltage source is respectively applied to.

4. A driver circuit for use with a stepper motor having one or more pairs of windings, the driver circuit comprising switching means for selectively applying, in use, a voltage source across one of the end connections of a first winding and one of the end connections of the other winding of a pair of centre-tapped windings and connecting the unconnected ends of the first and second windings together, wherein said switching means alternates which of the ends of the first and second windings the voltage source is respectively applied to.

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5. A drive circuit substantially as described herein with reference to Figures 4 and 5 of the drawings.

6. A method of operating a stepper motor substantially as described herein with reference to Figures 4 and 5 of the drawings.