WO2001031773A1

WO2001031773A1 - Driver for operating a stepper motor at a higher voltage than the nominal voltage

Info

Publication number: WO2001031773A1
Application number: PCT/GB2000/004164
Authority: WO
Inventors: Christopher Stuart Brown
Original assignee: Ab Automotive Electronics Ltd
Priority date: 1999-10-29
Filing date: 2000-10-30
Publication date: 2001-05-03
Also published as: AU1044601A; GB2350500A; GB9925685D0

Abstract

The invention relates to a driver circuit to allow a stepper motor, or multiple stepper motors, having windings arranged to operate at a nominal voltage, to operate from a supply voltage higher than the nominal voltage.

Description

DRIVER FOR OPERATING A STEPPER MOTOR AT A HIGHER VOLTAGE THAN THE NOMINAL VOLTAGE

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. ι

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 centre-tapped 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.

The present invention also provides a system for the operation of multiple stepper motors. Such a system contains a control unit with first and second switching elements. The first switching elements make it possible to select at least one current supply cable, which leads to at least one stepper motor. The second switching element transmits the control signals required for the stepper motors via a number of control cables. The advantage of such a device lies in the reduced use of wiring, compared to individually wiring stepper motors, combined with the use of stepper motors rated to only half the supply voltage, and with similar torque and power dissipation to a conventional circuit operated at the motors^" rated voltage.

On a four phase unipolar stepper motor, two phases at a time are energised in parallel for full-stepping operation. (Energising one phase at a time gives full-stepping at a lower torque). This invention, when applied to four phase unipolar stepper motors, energises two phases in series, instead of in parallel. The common connection between the series phases is made at the motor terminal previously used for the supply cable.

The first advantage of this drive control device for stepper motors is that it results in reduced usage of wiring, compared to a separate wiring of each motor to the control unit. A further advantage of this drive control device for stepper motors is that the cost of the electronics is reduced because it is not necessary to include individual switching arrangements for each stepper motor phase. Also, the common terminal of each motor can be monitored for diagnostics and also for stall detection.

A useful further development of this control is provided by fitting diodes, or equivalent devices, to the wiring of the individual windings in the stepper motors, which are switched to the first supply. These diodes prevent the formation of parasitic circuits, in which the flow of current from the windings of one stepper motor could interfere with the corresponding windings of another motor. Whilst such diodes are preferably fitted to the stepper motors themselves, they may alternatively be provided on the output connections of the driver circuit for use with stepper motors which are not fitted with such diodes. The devices based on this invention are particularly suitable for use in vehicles. In this application, the advantages of this cost-effective device will be noticeable. A particularly suitable application within the vehicle is the use of the device as part of a climate control system, which can contain several stepper motors for activating air flaps, depending on the design.

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

Figure 1 shows a representation of a unipolar stepper motor and an example of the conventional drive arrangement:

Figure 2 shows the arrangement for driving a stepper motor in accordance with the present invention;

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 multiple stepper;

Figure 6 shows the signals used with the arrangement of Figure 5 in order to drive one of the motors clockwise:

Figure 7 shows the signals used with the arrangement of Figure 5 in order to drive one of the motors anti-clockwise;

Figure 8 shows one possibility for the signals used with the arrangement of Figure 5 in order to keep a motor almost stationary (± 1 step);

Figure 9 shows another possibility for the signals used with the arrangement of Figure 5 in order to keep a motor stationary;

Figure 10 shows one possibility for the signals used with the arrangement of Figure 5 in order to keep all of the motors stationary:

Figure 1 1 shows the optional harnessing and control circuitry for monitoring the common node voltage at a motor, and for monitoring the motor current; and

Figure 12 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 1. 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 both windings.

Without any modification to the 12 volt motor itself, the system can be modified according to the invention so as to be operated from a 24 volt supply, as shown in figure 2. 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 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 internally provides a connection between the centre taps. The 24 volt supply is applied via one of two switching elements S_A. S_B in the drive unit. In this way the 24 volt supply is used to drive current through two separate 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 S_A is closed, current flows through the winding A from the centre tap, inducing a magnetic field. Similarly when switch S_B is closed, current flows through winding B from the centre tap causing a magnetic field to be induced in the opposite direction to that produced by closing switch S_A. However, in the present invention, as shown in Figure 4. 24 volt is supplied to the windings A or B when the switch S_A or S_B respectively 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 S_A and S_B 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 and. for example, to vary the rotation direction.

The arrangement of the present invention further means that no connections between the common centre tap connections and the drive circuit are 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. This represents a significant advantage in the some industries such as the vehicle manufacturing industry where costs are determined based upon the number of wires and connections required for each part. 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; a short circuit to ground; a short circuited coil: an open circuit terminal or an open circuit coil. Even if the common terminal is used for diagnostic purposes, only a small low-current sensing wire is needed.

As shown in Figure 5. a conventional four-phase motor is provided with two phase coils 41. 43 and two inverse phase coils 42. 44. Each phase coil and its respective inverse phase coil is wound to generate torque to act on a rotor in reverse directions. The common nodes of each pair of phases are connected together. The rotor is a permanent magnet type. When any motor is to be moved, signal lines A and B are alternately switched to the supply rail (e.g. 24V) by the drive circuit 1. Diodes 3 are optionally included to prevent back feed from signal line A through the motor coils 41. 42 to signal line B, and vice versa.

To move one or more motors simultaneously, the motor signals X_n and Y_n of each of n motors are alternately switched to the opposing supply rail (e.g. 0V) by drive circuit 2 at 90° out of phase with signals A and B. If X_π leads A by 90° then the motor will move in one direction and if X_n lags A by 90° then the motor will move in the opposite direction. Waveforms for these two directions of operation are illustrated in Figures 6 and 7.

If one of X_n and Y_n is held on with the other off. as in Figure 8, then the motor will oscillate back and forth by 1 step as the signals A and B are alternately connected to the supply rail. Alternatively, to reduce motor noise, the motor could be driven back and forth by several steps at a time dependent upon the application. This can be used to keep the final position of the motor within ±1 step of the desired position.

An alternative to this would be to turn off both X_n and Y_n. as in Figure 9, preventing any current flow through the motor windings and thus leaving one or more motors unpowered. whilst one or more of the other motors are moving.

When no motors need to be moved then one of signals A and B, and one of signals X_n and Y_n can be switched to their supply rails to step-lock the motors, as shown in Figure 10. In this case the motors will be held in position by force of the motor preventing free-wheeling of the motor.

Figure 1 1 shows an optional feedback signal. Z_n. connected to the common node of the motor. If this connection is included, then as indicated above, the control unit 5 can be used to detect faults such as open circuits and short circuits and also changes in the waveform mav be used to detect a motor stall. Figure 1 1 also shows an optional current sense circuit 6 connected in series with the switching elements of one motor. This can be used to detect overcurrent and undercurrent faults and changes in the waveform may be used to detect a motor stall.

Although it is suggested above that the positive supply be connected to the first switching elements and the negative supply to the second switching elements, it is perfectly feasible to reverse this and connect the first switching elements to the negative rail and vice versa.

Flyback diodes to recirculate inductive energy are omitted from the drawings for clarity.

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 centre tapped 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 switched on.

Whilst the advantages of the above invention have been described in respect of unipolar motors, the invention may be applied to single bi-polar motors. Figure 12 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 12, no additional switching elements are needed. Also, the circuit could operate from either a higher voltage or the nominal voltage simply by changing the switching sequence (e.g. for a winding 12V nominal voltage, the motor could be used with both 24V and 12V supplies). The reference numbers against the switches on Figure 12 show which switches need to be closed at each step in the sequence 1 - 2 - 3 - 4 - 1. where a 12V motor is used with a 24V supply.

A specific embodiment has been described above, but further modification and variation of the invention is possible without departing from the scope of the invention as claimed in the appended claims.

Claims

CLAIMS:

1. A driver circuit for use with a stepper motor having one or more pairs of centre- tapped windings, each centre-tapped winding having two end connections and a centre- tapped connection, the centre-tapped connection of each winding being connected to a common connection, the driver circuit comprising switching means arranged to selectively apply the positive supply of a voltage source to one of the end connections of a first winding and the negative supply of the voltage source to one of the end connections of the other winding of a pair of centre- tapped windings, and wherein said switching means is adapted to alternate which of the ends of the first and second windings the positive and negative supplies are respectively connected to.

2. 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 centre- tapped 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.

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

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 windings and connecting the ends of the first and second windings not connected to the voltage supply together, wherein, in use, said switching means alternates which of the ends of the first and second windings the voltage source is respectively applied to.

5. A driver circuit for use with one or more stepper motors, each having one or more pairs of centre-tapped windings, each pair comprising a first and a second centre- tapped winding with each winding having end connections and a centre-tapped connection, the centre-tapped connection of each winding being connected to a common connection, and wherein one of the ends of each of the first windings are connected to a first common line and the other ends of each of the first windings are connected to a second common line, the driver circuit comprising switching means arranged to apply one terminal of a voltage source selectively to one of the first and second common lines and for the or each second winding to apply the other terminal of a voltage source selectively to one of the end. and wherein said switching means is adapted to alternate which of the first and second common lines said one terminal of the voltage source is applied to and for the or each second winding selectively alternating which of the ends of the winding said other terminal of the voltage source is respectively connected to.

6. A driver circuit according to claim 5 wherein the ends of the first windings are connected to the corresponding first and second common lines via a rectifier.

7. A driver circuit according to claim 6 wherein the rectifiers are formed as part of connectors for connection to a motor.

8. A driver circuit according to claims 5. 6 or 7 adapted for controlling a plurality of motors independently and simultaneously.

9. A driver circuit according to any preceding claim adapted to selectively energise two coils of a motor to provide torque which holds the motor in position.

10. A driver circuit according to any preceding claim further comprising a current monitor arranged to monitor the current in the second winding of one or more of the or each motor.

1 1. A driver circuit according to any preceding claim further comprising a centre-tap monitor attached to the common connection.

12. A driver circuit according to any preceding claim wherein the supply voltage is twice the nominal voltage of each half of each winding.

13. A climate control system comprising a driver circuit according to any preceding claim.

14. A method of operating a stepper motor having one or more pairs of centre- tapped windings each centre-tapped winding having end connections and a centre- tapped 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.

15. A method of operating a stepper motor having one or more pairs of centre- tapped windings each centre-tapped winding having two end connections and a centre- tapped connection, the centre-tapped connection of each winding being connected to a common connection, the method comprising: applying a positive supply of a voltage source to one of the end connections of a first winding and a negative supply of the voltage source to 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 positive and negative supplies are respectively connected to.

16. A method of operating a plurality of stepper motors, each stepper motor having at least one pair of centre-tapped windings with each centre-tapped winding having end connections and a centre-tapped connection, the centre-tapped connection of each winding being connected to a common connection in each stepper motor, the method comprising: applying one polarity of a voltage source to one of the end connections of a first winding of each stepper motor and the opposite polarity to one of the end connections of the other winding of a pair of centre-tapped windings; and alternating which end of the first windings the voltage source is respectively applied to for each stepper motor and selectively, for each motor to be driven, alternating which end of the second winding the voltage source is respectively connected to.

17. A drive circuit substantially as described herein with reference to Figures 2 and 4 to 12 of the drawings.

18. A method of operating a stepper motor substantially as described herein with reference to Figures 2 and 4 to 12 of the drawings.