GB2304475A

GB2304475A - Reversible two phase motor controller

Info

Publication number: GB2304475A
Application number: GB9516726A
Authority: GB
Inventors: Marc Alfred Heyraud
Original assignee: Johnson Electric SA
Current assignee: Johnson Electric SA
Priority date: 1995-08-15
Filing date: 1995-08-15
Publication date: 1997-03-19
Also published as: GB9516726D0

Abstract

A controller for a bi-directional two phase motor incorporates an anti-dead point circuitry to prevent the motor locking at a dead point when the direction of rotation of the motor is reversed without bringing the motor to a standstill. The anti-dead point circuitry comprises two timers TMR1, TMR2. TMR1 counts down a time period when it is triggered by either a signal from a Hall sensor or by a direction changed signal. Timer 2 times down a preset time period once timer 1 has timed out. During the timing out of timer 2, the motor is switched off. In an alternative embodiment (Fig. 4) by resetting the first timer with a signal from the Hall sensor turning the motor off momentarily should the time taken to complete a revolution exceed a predetermined time period thereby the number of false triggerings by the anti-dead point circuitry is reduced. In a further aspect a two phase bi-directional motor comprising a stator having first and second phase windings, (L1, L2, Fig. 2) a permanent magnet rotor, control circuitry for selectively energizing the phase windings including a Hall sensor (H1) positioned adjacent the permanent magnet rotor for detecting rotation of the rotor, power MOSFETs (DT1, DT2) for connecting the phase windings to a source of power and a direction selection switch, the direction selection switch provides a logical signal indicative of desired direction and this signal is used in the control of the power MOSFETs.

Description

Reversible Two Phase Motor Controller This invention relates to a controller for and a method of controlling a two phase reversible motor, in particular, a bi-directional brushless d.c. motor.

Generally, bi-directional two phase motors are not reversible, meaning that for reliable operation, the motor should be brought to a standstill before the motor is started in the opposite direction. These motors generally have a two phase stator winding and a permanent magnet rotor with a standing or stationary rotor position with the rotor poles positioned between the stator poles of the two phases. Thus, at start up from a stationary position, direction can be selected by energizing the appropriate phase.

However, this offsetting of the stator and rotor poles produces a natural conflict between the electromagnetic forces created between the stator and rotor poles (EMF) and the magnetic force or cogging torque created between the permanent magnets of the rotor and the stator core such that for each phase, there is a position at which the cogging torque will equal the EMF in value but opposite in direction. Should the motor direction be reversed and the motor come to a standstill at or near that position, the rotor will freeze or lock and the motor will not self start. This does not happen during normal operation because the momentum of the rotor will carry the rotor pass its dead point, even at start up from rest.However, while the possibility of stopping at the dead point may not seem particularly great, in practice it does occur often enough to prevent simply reversing the motor without stopping it as an acceptable operating method.

The present invention provides a method and a controller for reversing the direction of the bi-directional two phase motor. This is achieved by reversing the direction for a pre-determined period of time and then removing the power from the motor for a predetermined period of time before starting the motor as normal. Alternatively, the motor can be monitored to determine if after the first time period, the motor is rotating at an acceptable speed in which case, direction change is assumed successful and power to the motor is not interrupted.

According to a first aspect, the present invention provides a method of reversing a bidirectional two phase motor comprising the steps of: during operation of the motor, in a first direction, switching the phases to run the motor in the second direction; after a predetermined period time T1, removing the power from the motor; after a second predetermined period of time T2, reconnecting the power to the motor to run in a second direction.

Preferably, the first time period Tl is selected depending on the speed of operation of the motor and the inertia of the rotor such that T1 represents slightly more than the time required for the rotor to go from full speed to standstill.

Preferably, the second time period T2 is selected as the time taken for the rotor to rotate from being stationary at the dead point position towards the rest position sufficiently to allow the motor to start normally.

Preferably, the method ftinher comprises the steps of monitoring rotation of the motor and if at the end of the predetermined time period T1, the motor is rotating, maintaining operation ofthe motor during the time period T2.

Alternatively, the method flirther comprises monitoring rotation of the rotor and resetting the predetermined time period T1 on each detected rotation of the motor.

According to a second aspect, the present invention provides a controller for a bidirectional two phase motor comprising a Hall sensor for detecting rotation of the rotor; a control circuit responsive to the Hall sensor for controlling energization of the phase windings to effect operation of the motor; direction changing means for effecting changes to the energizing sequence of the phase windings to change the direction of rotation of the motor; a first timer to time a predetermined time period Ti from initialization of direction change; a second timer to time a predetermined time period T2 from the end of the time period T1; isolating means responsive to the second timer for isolating the motor during the time period T2.

Preferably, the isolating means inhibits energization of both phase windings.

Alternatively, the first timer for timing a predetermined time period T1 and being responsive to the Hall sensor whereby each revolution of the rotor causes the first timer to be reset.

Preferably, operation of the isolation means is inhibited during initial time period T3 following turning on ofthe motor.

According to a third aspect, the present invention provides a two phase bi-directional motor comprising a stator having first and second phase windings; a permanent magnet rotor; control circuitry for selectively energizing the phase windings including a Hall sensor positioned adjacent the permanent magnet rotor for detecting rotation of the rotor, powder MOSFETs for connecting the phase windings to a source of power and a direction selection switch wherein the direction selection switch provides a logical signal indicative of desired direction and this signal is used in the control of the power MOSFETs.

Preferably, the signal from the direction selection switch is combined with the output from the Hall sensor using exclusive OR gates to change the energization sequence of the phase windings.

Two embodiments of the invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 is a graph of magnetic torque vs. rotational position of the rotor of a two phase bi-directional motor; Figure 2 is a circuit diagram of a control circuit for a two phase bi-directional motor incorporating anti-dead point circuitry; Figure 3 is a circuit diagram of the anti-dead point circuitry of Figure 2 according to a first embodiment; Figure 4 is a circuit diagram of an alternative anti-dead point circuitry according to a second embodiment; and Figure 5 is a graph against time of various motor signals during operation of the embodiment of Figure 4.

The principle of operation of the invention is to insert a break or pause in the energization of the stator windings, after a sufficient time has elapsed from initiating a direction change that the motor should have reversed direction, to allow the motor, if it has been locked at a dead point, to move away from the dead point sufficiently to allow the motor to start normally.

In the simplest form, every time the motor direction is reversed, the motor power will be momentarily interrupted regardless of whether the motor is locked. Indeed, this momentary interruption may occur when the motor is started initially, i.e., from standstill. This creates a false start every time which although not harmful, may be perceived as a fault by users ofthe final appliance. To overcome this ever present false start, an improved version is described in which the anti-dead point circuitry only causes the motor to be switched off momentarily if, after a predetermined time period, the motor speed has not reached a minimum.

Figure 1 is a graph of magnetic forces within a two phase bi-directional motor vs.

rotor position and illustrates the mechanism of the dead point. Curve A is the magnetic force or EMF generated by the phase one winding. Curve B is the EMF generated by the phase two winding. Curve C is the magnetic force or cogging torque generated between the magnets of the rotor and the stator core.

At point 1, the cogging torque and phase one EMF are both maximum = and and in the same direction, complementing each other. At point 2, the cogging torque is zero and phase one EMF is high and positive At point 3, the phase one EMF and the cogging torque have the same absolute value but the cogging torque is in the opposite direction, effectively canceling the phase one EMF resulting in zero forces on the rotor. At point 4, the EMF for phase one is zero and the cogging torque is a maximum in the reverse direction. It is in the region of point 4 that commutation occurs resulting in phase two winding being energized. Point 5 is a second dead point where the cogging torque and phase two EMF are equal in value but opposite in direction.At point 6, the cogging torque returns to zero and phase two EMF is approaching a maximum, and so on and so forth. Hence, for each full cycle, there are four dead points or two dead points per phase.

The circuit diagram of Figure 2 shows a simple motor controller with an anti-dead point circuitry shown as a black box 20. In the control circuitry of Figure 2, a Hall sensor H1 senses rotation of the rotor by changing its output as the north and south poles of the rotor magnet passes by. The output from the Hall sensor H1 passes through logic gates to control the drive MOSFETs DT1 and DT2 for energizing the stator windings L1 and L2. The logic gates combine the Hall sensor output with a direction signal DIR, on/off signal and the output ADP of the anti-dead point circuitry 20. By using digital logic gates for controlling the drive MOSFETs DT1, DT2, the direction selection controller can be simple logic gates, in this case, an exclusive OR gate U3:A, U3:B, for each phase. The output from the directional logic gates is passed through AND gates combining the output from the anti-dead point circuitry which can thus turn off both drive MOSFETL Anti-dead point circuitry according to a first embodiment is shown in Figure 3. This is the simpler or basic version. The dead point circuitry 20 comprises essentially two timers and a logical AND gate U2:D. The first timer TMR1 is set to time 1.5 seconds (in this example) from a trigger provided on input line on/off. The signal for this line is generated by a switching circuit, not shown, which provides a logical HI when the motor is switched on.A logical LO is pulsed on this line when the direction is reversed. This can be achieved, for example, by providing a three position switch with clockwise direction in position 1, anti-dockwise direction in position 3 and off in position 2, resulting in the motor being switched off momentarily when switching between directions. The switch functions as both an on/off switch and a direction selection switch.

Timer 1 (tor1) times out the first preset time period which is calculated to be slightly more than the time required to being the motor to a standstill from maximum speed by reversing direction ofthe motor. In this example, the time is set at 1.5 seconds. Timer 2 OUR2) then times out the second preset time period. This time is taken as the time required for the rotor when stopped in a dead point position to rotate towards the rest position sufficiently to allow the motor to restart normally, once the power is turned off: In this example, T2 is chosen as 0.25 seconds.

The inverse output of TMR2 is combined with the on/off signal by an AND gate U2:D to provide the output signal ADP from the anti-dead point circuitry 20 to turn off the motor for 0.25 seconds, 1.5 seconds after it has started operation or switched to the opposite direction.

In the embodiment of Figure 4, the anti-dead point circuitry 20 is basically similar, having two timers, TMR1' and TMR2'. Again, TMR2' is initiated by the timing out ofTMR1'. However, Th(Ri' is reset by the pulses generated by the Hall sensor H1.

In this manner, each revolution of the rotor resets TMR1' and the output from TMR2' switches the motor off for a predetermined time period, in this example, again 0.25 seconds. Ideally, the predetermined time period for TMR1' is set to slightly longer than the expected maximum time between pulses from the Hall sensor when the rotor changes direction of rotation without locking, in this example also 0.25 seconds.

Shorter time periods may be used but the shorter the time period the greater the risk of a false trip.

Thus, at start up, as the rotor turns, it causes the Hall sensor H1 to pulse, resetting the first timer TMR1'. TMR1' counts down until the time expires or it is reset by the next pulse from the Hall sensor H1 which would occur first under normal conditions.

However, should the rotor stop at the dead point position during reversal of rotation, TMR1' would time out, activating TMR2' to turn the motor off for 0.25 seconds allowing the rotor to relax to the normal resting position from which it can start normally once TMR2' has timed out. The sequence of events is more clearly shown in Figure 5.

The motor, initially at rest, is turned on as indicated by on/off signal going HI. H1 generates pulses representing the time taken for the rotor to rotate. At an arbitrary point in time, direction signal DIR is changed indicating a desired change in motor direction. The controller reverses phase one phase two windings and the motor slows down, as shown by longer pulses from H, 0.25 seconds after the last pulse from H1, when the motor locks preventing further pulses, TMR1' times out, triggering TMR2' which triggers on/off 2 signal to go HI, causing the motor to be isolated for 0.25 seconds. After TMR2' times out, its output goes LO and the motor is restarted as normal from a resting position.

Optionally a signal A, preferably generated by a third timer (not shown) is used to mask the anti-dead point circuitry output signal at startup for a period of time, in this example, 0.4 seconds to prevent false switching by the anti-dead point circuitry at start up caused by stiction or initial high loading on the motor. This signal is combined with the output from TMR2' by a NAND gate U2:C producing an output signal for controlling the gates for the drive MOSFETs.

Various modifications to the described embodiments will be obvious to the reader and it is desired to include all such modifications as fall within the scope of the accompanying claims.

Claims

1. A method of reversing a bi-directional two phase motor comprising the steps of: during operation of a motor in a first direction, switching the phases to run the motor in a second direction; after a predetermined period oftime T1, removing the power from the motor; after a second predetermined period of time T2, re-energizing the motor to run in the second direction.

2. A method as defined in claim 1 wherein the first time period T1 is selected depending on the speed of operation of the motor and the inertia of the motor such that T1 represents slightly more than the time required for the motor to go from full speed to zero under reverse current.

3. A method as defined in claim 1 or claim 2 wherein the first time period T1 is about 1.5 seconds.

4. A method as defined in any one of the preceding claims wherein T2 is selected as the time taken for the rotor to rotate from being stationary at the dead point position towards the rest position sufficiently to allow the motor to start normally in either direction.

5. A method as defined in any one of the preceding claims further comprising the steps of: monitoring rotation of the motor and if at the end of the first predetermined time period T1, the motor is rotating, maintaining operation of the motor during the time period T2.

6. A method as defined in claim 1 further comprising monitoring rotation of the rotor and resetting the first time period T1 at each detected rotation ofthe motor.

7. A method as defined in claim 6 wherein the two time periods T1 and T2 are both set to 0.25 seconds.

8. A method of controlling a reversible two phase bi-directional motor substantially as hereinbefore described with reference to the accompanying drawings.

9. A controller for a bi.directional two phase motor comprising: a Hall sensor for detecting rotation of the motor; a control circuit responsive to the Hall sensor for controlling energization of the phase windings to effect operation ofthe motor; direction changing means for effecting changes to the energizing sequence of the phase winding to change the direction of rotation ofthe motor; a first timer to time a predetermined time period T1 from initialization of direction change; a second timer to time a predetermined time period T2 from the end of the first time period T1; and isolating means responsive to the second timer for isolating the motor during the time period T2.

10. A controller as defined in claim 9 wherein the isolating means inhibits initialization of both phase windings during the time period T2.

11. A controller as defined in claim 9 or claim 10 wherein time period T1 is set to 1.5 seconds and time period T2 is set to 0.25 seconds.

12. A controller for a bi-directional two phase motor comprising: a Hall sensor for detecting rotation ofthe motor; a control circuit responsive to the Hall sensor for controlling energization of the phase windings to effect operation ofthe motor; direction changing means for effecting changes to the energizing sequence of the phase windings to change the direction of rotation of the motor; a first timer to time a predetermined time period T1 in response to a trigger from the Hall sensor indicating a revolution of the rotor; a second timer to time a predetermined time period T2 from the end of the first time period T1; and isolating means responsive to the second timer for isolating the motor during the time period T2 wherein the first timer is reset on each complete revolution of the rotor.

13. A controller as defined in claim 12 wherein the operation of the isolation means is inhibited during an initial time period T3 following turning on ofthe rotor.

14. A controller as defined in claim 12 or claim 13 wherein T1 and T2 are set to 0.25 seconds and T3 is set to 0.4 seconds.

15. A controller for a bi-directional two phase motor substantially as hereinbefore described with reference to the accompanying drawings.

16. A two phase bi-directional motor comprising a stator having first and second phase windings; a permanent magnet rotor; control circuitry for selectively energizing the phase windings including a Hall sensor positioned adjacent the permanent magnet rotor for detecting rotation of the rotor, power MOSFETs for connecting the phase windings to a source of power and a direction selection switch wherein the direction selection switch provides a logical signal indicative of desired direction and this signal is used in the control of the power MOSFETs.

17. A motor as defined in claim 16 wherein the signal from the direction selection switch is combined with the output from the Hall sensor using exclusive OR gates to change the energization sequence of the phase windings.