WO2007119094A2

WO2007119094A2 - Driving brushless dc motors

Info

Publication number: WO2007119094A2
Application number: PCT/IB2006/002724
Authority: WO
Inventors: Rumen Marinov Peev; Radostin Tsankov Tsankov; Dobromir Petkov Dobrev; Pavel Georgiev Nedev; Christophe Daniel Cot
Original assignee: Melexis Nv
Priority date: 2006-01-20
Filing date: 2006-10-02
Publication date: 2007-10-25
Also published as: GB0601172D0; WO2007119094A3

Abstract

When a stall-rotor event occurs in a BLDC motor (100), the controller unit (104) periodically alternates (at a selected frequency) the direction of current supplied to the coils and thus correspondingly varies the direction of the torque applied to the coils. The frequency of alternation is appropriately selected in order to lower the effective friction experienced by the rotor (102) and thus to maximize the possibility to overcome the stall-rotor event. The mean current flow applied to the coils is not equal in each direction. In one direction, corresponding to the desired direction of rotation of the rotor (102), a larger mean current is applied than in the other direction. Accordingly, the average value of the alternating rotor torque corresponds to the desired direction of rotation. Accordingly, this ensures that rotation occurs in the desired direction when the stall-rotor event is overcome.

Description

Driving Brushless DC Motors

The present invention relates to driving brushless DC motors and in particular relates to a method and apparatus for overcoming a stall-rotor event in such a motor.

A single-phase brushless DC (BLDC) electric motor comprises a stator and a rotor, the rotor being free to rotate with respect to the stator. The rotor typically comprises permanent magnets having 1, 2, 3 or more pole pairs. The stator typically comprises 1, 2, 3 or more coils supplied by electronically-controlled commutation system. The actual direction of the rotor magnetic field (depending on the rotor angular position) is sensed by a Hall-effect or other magnetic sensor. The controller monitors the sensed magnetic field to determine when to switch the coil current direction in order to maintain the desired (normal) direction of the torque applied to the rotor (and hence the desired rotation direction). Some particular applications of such motors lie in vibration motors such as those used in cellular phones, pagers, game system controllers, toothbrushes and the like.

A basic problem with single-phase BLDC motors is the so called "dead- point". At this point, the rotor torque is equal to zero (or is lower than the mechanical friction) when a current is applied to the coils. Accordingly, the motor cannot start to operate from this position. It is known to incorporate special constructional features (cogging plates) into the motor to try to ensure that the rotor does not stop at the "dead-point". Despite such precautions, in some cases (due to increased mechanical friction, for example) the motor does stop at or near to the dead point and accordingly will not start reliably. The state when the motor is powered up and the rotor is stuck is defined as a "stall-rotor event".

In one known single-phase BLDC motor system, the motor driver recognizes that a stall-rotor event has occurred when the magnetic sensor does not detect any change of the magnetic field direction for a predefined time period T₀. To start the motor during a stall-rotor event, the controller alternates the coil current direction after each subsequent period T₀ (which results in a commutation frequency of F= 1/(2T₀)). The current direction alternation sequence continues until the magnetic sensor detects a change of the magnetic field direction (rotation started). The rotation direction after stall-rotor event could be either the normal operation direction or a reversed operation direction. Often it is not desirable to start rotating the motor in reverse direction and therefore additional means must be provided to prevent the motor starting in reverse.

It is an object of the present invention to provide an alternative method of overcoming a stall-rotor event in a single-phase BLDC motor.

According to a first aspect of the present invention there is provided a method for starting a brushless DC electric motor when the rotor of said motor is stopped at or near to a dead point with respect to the stator comprising the steps of: alternating the direction of the current applied to the coils of the motor at a selected frequency; monitoring the direction of the magnetic field in one location adjacent to the rotor;

and returning the motor to normal operation once a change of magnetic field direction is detected wherein the mean coil current applied in the direction corresponding to a first direction of rotation of the rotor is greater than the mean current in the direction corresponding to the alternative direction of rotation of the rotor.

The alternation of the current results in the alternation of the direction of the torque applied to the rotor which helps to overcome the stall rotor event.

Additionally, the difference in current magnitude in each direction provides a bias in the average torque applied corresponding to the first direction of rotation, such that when the stall-rotor event is overcome the rotor will start rotating in the first direction.

This enables the initial direction of rotation after a stall-rotor event to be reliably controlled.

The BLDC motor may be a single-phase BLDC motor. The motor may be of the type comprising a stator and a rotor, the rotor comprising permanent magnetic means having one or more pole pairs and the stator comprising 1, 2, 3 or more coils supplied with current via an electronic commutation system. The electronic commutation system may be operable according to "single-coil" driving principle (using a full bridge coil driver) or the "two-coil" driving principle (using two open drain/collector coil drivers).

The method may include the further step of determining that the rotor has stopped at a dead point (a stall-rotor event has occurred). This determination may be made if no change in the magnetic field direction occurs for a predetermined time period (T₀). T₀ may be of the order of 100ms. The frequency of alternation of the direction of coil current may be selected in order to lower the effective friction experienced by the rotor. The frequency of

alternation of the direction of coil current may be substantially 1/(2T₀). The frequency of alternation of the direction of coil current may be greater than 1/(2T₀) and is preferably significantly greater than 1/(2T₀). Such an increase in frequency lowers the effective friction experienced by the rotor and thus makes it more likely that the stall-rotor event may be overcome.

The direction of the magnetic field may be determined using any suitable magnetic sensor. In one preferred embodiment, the magnetic sensor may be a Hall effect magnetic sensor. It may be determined that a stall-rotor event is overcome when the direction of the magnetic field sensed by the magnetic sensor varies, thus indicating that the rotor has moved in respect to the stator.

Preferably, the output of the magnetic sensor is monitored by a controller unit. The controller unit may also be operable to monitor the output of the magnetic sensor during normal operation and to determine the occurrence of a stall-rotor event.

Preferably, the controller controls the supply of current to the coils via the electronic commutation system.

According to a second aspect of the present invention there is provided a driver for a BLDC motor comprising means operable to carry out the method of the first aspect of the present invention. The driver of the second aspect of the present invention may incorporate any of the features described in relation to the first aspect of the present invention, as desired or as appropriate.

According to a third aspect of the present invention there is provided a BLDC motor comprising a driver according to the second aspect of the present invention.

The motor of the third aspect of the present invention may be used in a vibration motor. Such vibration motors may be incorporated into cellular phones, pagers, game controllers, toothbrushes and other devices with vibration function.

In order that the present invention is more clearly understood, one embodiment is described further herein, with reference to the accompanying drawing, being a schematic block diagram of a single-phase BLDC motor.

Referring now to the single figure, a BLDC motor 100 comprises a stator 101 and a rotor 102 provided such that it may rotate with respect to the said stator 101.

The rotor 102 comprises one or more permanent magnets arranged so as to provide one or more magnetic pole pairs. The stator 101 comprises 1, 2, 3 or more coils. A controller unit 104 is provided connected to the coils via a commutation means 103.

A magnetic sensor 105 is located adjacent to the rotor 102. The magnetic sensor 105 is typically a Hall Effect sensor. The magnetic sensor 105 is operable to sense variations in the magnetic field direction as the rotor 102 rotates. By monitoring these variations, the controller unit 104 is able to control the direction of current flow in the coils via the commutation means 103 such that rotation of the rotor

102 continues in the desired direction.

After the motor is switched off, no current flows in the coils and hence the rotor 102 comes to rest. It is possible for the rotor 102 to come to rest in a position, known as a dead-point wherein the net torque on the rotor 102 is zero (or is less than the torque required to overcome friction), when a current flows through the coils. In such circumstances, known as a stall-rotor event, the rotor 102 will not rotate. This condition is detected by the controller unit 104 monitoring the output of the magnetic sensor 105 means. If no change in the magnetic field direction is sensed within a first time interval T₀ (typically of the order of 100ms), then the controller unit 104 detects that a stall-rotor event has occurred.

When a stall-rotor event occurs the controller unit 104 supplies current to the coils. The direction of flow of the current supplied to the rotor 102 is periodically alternated hence the direction of the torque applied to the rotor 102 with respect to the stator 101 is correspondingly alternated. Eventually, this will result in some movement of the rotor 102 with respect to the stator 101 and rotation of the rotor 102 can then occur again, hi the present invention, in order that the rotation of the rotor 102 that occurs is in a desired direction when the stall-rotor event is overcome, the mean current flow applied to the coils is not equal in each direction. In one direction, corresponding to the desired direction of rotation of the rotor 102, a larger mean current is applied than in the other direction. As a result of this bias, the direction of the average value of the alternating rotor torque corresponds to the desired direction of rotation. Accordingly, when rotation of the rotor 102 restarts, the rotation occurs in the desired direction. This means that external or mechanical means provided to prevent rotation being initiated in a non-desired direction are not required.

The frequency of alternation of current direction may be around 1/(2T₀), but is preferably significantly higher. The high frequency of alternation of current direction lowers the effective friction experienced by the rotor 102 and thus maximises the possibility that the stall-rotor event may be overcome.

The controller unit 104 monitors the output of the magnetic sensor 105 to determine the end of the stall-rotor event. It is determined that the stall-rotor 'event is overcome when the rotor 102 has rotated a sufficient distance that the direction of the magnetic field sensed by the magnetic sensor 105 means varies. Once the stall-rotor event has ended, the controller unit 104 reverts to controlling the current supplied to the coils according to its usual mode of operation.

It is of course to be understood that the invention is not to be restricted to the details of the above embodiment, which is described by way of example only.

Claims

1. A method for starting a brushless DC electric motor when the rotor of said motor is stopped at or near to a dead point with respect to the stator comprising the steps of: alternating the direction of the current applied to the coils of the motor at a selected frequency; monitoring the direction of the magnetic field in one location adjacent to the rotor; and returning the motor to normal operation once a change of magnetic field direction is detected wherein the mean coil current applied in the direction corresponding to a first direction of rotation of the rotor is greater than the mean current in the direction corresponding to the alternative direction of rotation of the rotor.

2. A method as claimed in claim 1 wherein the BLDC motor is a single-phase BLDC motor.

3. A method as claimed in claim 2 wherein the motor comprises a stator and a rotor, the rotor comprising permanent magnetic means having one or more pole pairs and the stator comprising one or more coils supplied with current via an electronic commutation system.

4. A method as claimed in any preceding claim wherein the method includes the further step of determining that the rotor has stopped at a dead point.

5. A method as claimed in claim 4 wherein this determination is made if no change in the magnetic field direction occurs in a predetermined time period

(To)-

6. A method as claimed in claim 5 wherein T₀ is of the order of 100ms.

7. A method as claimed in claim 5 or claim 6 wherein the frequency of alternation of the direction of coil current is substantially 1/(2T₀).

8. A method as claimed in claim 5 or claim 6 wherein the frequency of alternation of the direction of coil current is significantly greater than 1/(2T₀).

9. A method as claimed in any preceding claim wherein the direction of the magnetic field is determined using a magnetic sensor.

10. A method as claimed in claim 9 wherein the magnetic sensor is a Hall effect magnetic sensor.

11. A method as claimed in claim 9 or claim 10 wherein the output of the magnetic sensor is monitored by a controller unit. r

12. A method as claimed in claim 11 wherein the controller unit is also operable to monitor the output of the magnetic sensor during normal operation and to determine the occurrence of a stall-rotor event.

13. A method as claimed in claim 11 or claim 12 wherein the controller controls the supply of current to the coils via the electronic commutation system.

14. A method as claimed in claim 13 wherein the electronic commutation system is operable on the "single-coil" driving principle.

15. A method as claimed in claim 13 wherein the electronic commutation system is operable on the "two-coil" driving principle.

16. A driver for a brushless DC motor comprising means operable to carry out the method of any one of the preceding claims.

17. A brushless DC motor comprising a driver according to claim 16.

18. A vibration motor comprising a brushless DC motor as claimed in claim 17.

19. A vibration motor as claimed in claim 18 wherein the vibration motors is incorporated into any one of: a cellular phone, a pager, a game controller, or a toothbrush.