GB2317058A

GB2317058A - Superimposed variations on back emf sensing angular position

Info

Publication number: GB2317058A
Application number: GB9618650A
Authority: GB
Inventors: Alfons Stockschlager
Original assignee: Draftex Industries Ltd
Current assignee: Laird Holdings Ltd
Priority date: 1996-09-06
Filing date: 1996-09-06
Publication date: 1998-03-11
Anticipated expiration: 2016-09-06
Also published as: GB2317058B; GB9618650D0

Abstract

Windings 24A, 24B ... produce a varying back emf as the rotor rotates causing a corresponding variation in motor current at a frequency dependent on the number of windings. One of the windings is wound with for example fewer turns so as to superimpose a cyclic variation on the back emf related to angular rotor position. This is detected and used to count the number of revolutions of the motor in a vehicle window winding arrangement so as to determine the fully closed/open positions. Alternatively, a resistor may be connected across the windings or the winding spacing may be varied.

Description

ELECTRIC MOTORS The invention relates to electric motors. Electric motors embodying the invention, and to be described below in more detail by way of example only, are small direct current (DC) electric motors such as for use in motor vehicles for raising and lowering panes of window glass.

According to the invention, there is provided an electric motor in which a varying back emf is generated as the rotor of the motor rotates, the back emf causing a corresponding variation in the motor current, the motor including means superimposing a cyclic variation on the back emf which is related to the angular position of the rotor.

According to the invention, there is also provided an electric motor in which a varying back emf is generated as the rotor of the motor rotates, the back emf causing a corresponding variation in the motor current together with noise which is associated with the rotation of the rotor, the motor including superimposing means operative in response to rotation of the rotor to superimpose a regular variation on the back emf which is distinguished by its magnitude and frequency from the noise.

According to the invention, there is further provided an electric motor having a rotor carrying winding means energisable to produce a magnetic flux for driving the rotor and also producing a back emf as the rotor rotates, a portion of the winding means in a predetermined angular position on the rotor being modified to superimpose a cyclic variation on the back emf which is related to the predetermined angular position.

According to the invention, there is still further provided an electric motor having a rotor carrying a plurality of windings distributed around the rotor and energisable to produce a magnetic flux for driving the rotor and also producing a back emf as the rotor rotates, the back emf varying at a frequency dependent on the number of windings, at least one of the windings being arranged to produce a magnetic flux different from the others so that it superimposes a cyclic variation on the varying back emf as the rotor rotates, the cyclic variation being related to the angular position of the rotor.

DC electric motors embodying the invention, and for use in raising and lowering panes of window glass in a motor vehicle, will now be described, by way of example only, with reference to the accompanying diagrammatic drawings in which: Figure 1 is a perspective view of a simplified version of the rotor of the motor, incorporating the armature windings; Figure 2 is a perspective view of the stator housing of the motor including permanent magnets for generating the stator field; Figure 3 is an end view of the stator housing of Figure 2; Figure 4 is a schematic representation of the armature windings; Figure 5 shows waveforms for explaining the operation of the motors; Figure 6 is a block circuit diagram of a detecting circuit for use with the motors; and Figure 7 corresponds to Figure 4 but shows a modification.

The rotor shown in Figure 1 comprises a shaft 10 carrying the commutator 12 and connected to the main rotor body 14 by an intermediate shaft 16. The main rotor body 14 is made up of ten (in this example) steel segments 18, of which six referenced 18A to 18F are visible in the Figure. The segments 18 are separated by slots 20, of which seven, referenced 20A to 20G, are visible in the Figure. The commutator is made up of ten copper segments 22, of which six referenced 22A to 22F, are visible in the Figure. The commutator segments are of course insulated from each other.

The rotor 14 carries the armature windings indicated generally at 24. Each winding comprises a plurality of turns (e.g. twenty or more), although for simplicity only two turns for each winding are shown in Figure 1. Considering winding 24A, one end of this winding is connected to commutator segment 22A by a connection 26A. The turns of this winding encompass four of the rotor segments 18. Thus, the turns of this winding are laid in slot 20A and extend along this slot and then emerge at the opposite end 28 of the rotor and are looped back through slot 20E. The other end of the winding 24A is connected to commutator segment 22B by a connection 26B.

The next winding 24B is wound in the same way. One end of this winding is connected by a connection 26C to commutator segment 22B. The turns of winding 24B extend in slot 20B to the end 28 of the rotor and return in slot 20F. The opposite end of winding 24B is connected to commutator segment 22C by a connection 26D.

The remaining windings 24 are connected in the same way.

Connections are made to the windings 24 via the commutator segments 22 and a pair of brushes (not shown) which are fixedly mounted in the stator housing (to be described in more detail below) at positions diametrically opposite each other with respect to the axis of the shaft 10.

Figure 2 shows the stator 30 with a hollow steel housing 32 generally of part cylindrical shape with flattened sides 33, in which are fixed permanent magnets 34 and 36 having the polarities N and S as shown. The armature of Figure 1 is in use rotatably mounted within the cylindrical gap between the magnets 34 and 36 in the stator housing 32 and is supported at each end of the stator by suitable bearings not shown which are mounted in end caps also not shown. The shaft 10 extends outwardly of one of these end caps.

Figure 4 shows a diagrammatic electrical representation of the armature of Figure 1. Figure 4 shows the ten segments 22A to 22J of the commutator 12 and the ten windings 24A to 24J connected between them. Figure 4 also shows the stationery brushes 40 and 42 which are mounted within the housing 32 of the stator (Figure 2) and make electrical contact with succeeding pairs of the commutator segments as the rotor rotates. As shown in Figure 4, the brushes 40 and 42 are making electrical contact with the commutator segments 22D and 22I respectively. Clearly, if the rotor rotates clockwise (as viewed in Figure 4), the brushes 40 and 42 will next make contact with the commutator segments 22C and 22H respectively, and so on for further rotation.

It will be noted from Figure 4 that the connection of the windings 24 is such that they are divided into two groups by the brushes 40 and 42, each group comprising five windings. The five windings in each group are connected in series with each other and the two groups of windings are connected electrically in parallel to each other between the brush terminals 40A and 42A.

In use, of course, DC electrical energisation is applied to the brush terminals 40A and 42A to energise the armature windings 24A to 24J which thus produce corresponding electrical fields interacting with the field produced by the permanent magnets 34 and 36. The interaction between the fields causes the rotor to rotate.

As is well known, rotation of the rotor also causes generation of a "back emf" or "counter emf" in the armature windings 24A to 24J, because of the voltage induced into the armature windings by the rotation of the energised windings through the magnetic field of the stator. This back emf opposes the voltage applied across the terminals 40A and 42A.

Although the armature windings 24A to 24J overlap each other, the magnetic flux produced by the windings will not of course be constant around the circumference of the rotor but will fluctuate in magnitude regularly around the circumference, being at any circumferential point the resultant of the flux produced at that point by the overlapping windings. The result will therefore be that the back emf will fluctuate correspondingly as the rotor rotates. The back emf opposes the applied voltage and the motor current will thus fluctuate in anti-phase to the back emf.

Figure 5A diagrammatically shows how the motor current will fluctuate regularly in a normal motor of this type. Each fluctuation of Figure 5A represents angular movement of the rotor through one-tenth (in this example) of a complete revolution.

In certain applications of electric motors, it may be desirable to detect the total number of revolutions through which the rotor of the motor has turned - so as (for example) to be able to detect the exact extent of movement of an output member which is driven by the shaft 10 (through a non-slip drive arrangement).

For example, if the motor is connected through a suitable nonslip geared drive arrangement to raise and lower a pane of window glass in a motor vehicle window, it is desirable to be able to detect when the motor has been rotated in the appropriate direction through such number of revolutions from a datum point that the window glass has reached the top of the window. The motor can then be de-energised. Similarly, the instant when the motor has fully lowered the window glass can then be detected, and the motor again de-energised.

In accordance with a feature of the invention, the windings on the rotor are modified to enable the fluctuating motor current to be used as a means of counting the number of rotor revolutions from a datum point.

In one embodiment of the invention, one of the windings (winding 24A in this example) is wound with fewer turns than all the other windings. It may have, say, twenty per cent fewer windings than the others. It will thus produce a correspondingly reduced magnetic flux, thereby correspondingly reducing the back emf which it produces. Because the windings overlap, the fewer number of turns in the winding 24A will not produce an abrupt reduction in magnetic flux coinciding with this winding; instead, the reduction in magnetic flux will be a maximum at a position centrally coinciding with this winding but there will be a progressively lesser reduction in flux at positions increasingly angularly displaced on either side of winding 24A.

The result is to produce a resultant waveform for the motor current (through terminals 40A and 42A) as shown in Figure SB.

The waveform comprises the fluctuations shown in Figure 5A on which is now superimposed a waveform having one-fifth of the frequency of the waveform 5A. This one-fifth-frequency waveform has positive and negative peaks P1,P2,P3 and P4. During one complete revolution of the rotor, one peak (e.g. positive peak P1) will be produced when the flux produced by the winding 24A (the winding with the fewer turns) coincides with the mid-point of the N permanent magnet 34 (Fig. 2), the next peak (e.g.

negative peak P2) will be produced when the flux produced by winding 24A coincides with the mid-point of the gap 44 or 46 (Fig. 2) depending on the direction of movement of the rotor, the third peak (positive peak P3) will be produced when magnetic flux produced by winding 24A coincides with the mid-point of the S permanent magnet 36, and, finally, the fourth peak (positive peak P4) will be produced when the magnetic flux produced by winding 24A coincides with the other gap (44 or 46) between the permanent magnets 34 and 36.

The waveform of Figure SB therefore now enables the number of revolutions of the motor to be detected and counted from a known angular datum position of the rotor; for example, the datum position corresponding to peak P1.

Figure 6 shows, in block diagram form, a circuit which can be used to process the waveform of Figure 5B.

The waveform of Figure SB, corresponding to the motor current, is fed into a low pass filter 50. The low pass filter removes the fluctuations on the motor current at the frequency shown in Figure SA. The waveform at the output of the low pass filter 50 therefore has the form shown in Figure 5C. This waveform is then normalised in an automatic gain control circuit 52 and fed to a pulse shaping circuit 54 which converts the waveform shown in Figure SC to the rectangular waveform shown in Figure SD. The rectangular waveform pulses of waveform 5D can then be passed into a counter 56 for counting thereby.

In this way, a count of the number of positive pulses of waveform SD, divided by two, will represent the number of revolutions of the motor shaft since the start of the count. In the particular application being considered, where the motor is used to raise and lower the window pane of a motor vehicle window, a count of the number of pulses of Figure SD indicates (to an accuracy of one half of a revolution of the motor) the distance which the window pane has travelled - assuming, of course, that there is a geared or other form of non-slip drive arrangement between the motor shaft and the window pane. In this way, therefore, the output of the counter can be used to determine when the window pane has reached the fully raised or fully lowered positions, whereupon the motor current can be switched off to halt the motor. Such an arrangement is advantageous because it does not require the use of limit switches or other force-responsive devices to determine when the window pane is fully raised or fully lowered. It also differs from systems which measure the magnitude of the mean motor current to detect when the window pane is fully raised or fully lowered by the resultant increase of this current when the motor becomes stalled on reaching the end of its travel. The system embodying the invention is particularly advantageous for incorporation in window safety systems having means for detecting when an object has been trapped between the rising window glass and the window frame and for immediately de-energising the motor. Because the system embodying the invention detects the fully raised and fully lowered window positions only by counting pulses, it more easily enables these two positions to be distinguished from any force exerted on the window glass by a trapped object.

The system embodying the invention and described above is also advantageous in that it requires no separate connections to the motor; it operates solely by monitoring the current energising the motor. It is therefore not necessary for the detecting, processing and counting circuitry to be mounted in the door of a motor vehicle. The circuitry can be mounted in any suitable place in the vehicle where the supply connections to the motor can be monitored.

However, the system described is not limited to use with electric motors for operating windows in motor vehicles. It can be used to count the number of revolutions of motors used for any other purpose.

Although it is in principle possible to count the pulses of waveform 5A, thus giving a pulse count from which the number of revolutions of the motor shaft can be derived (by dividing the count by ten in this example), in practice this is difficult or impossible. This is because the pulses of waveform 5A are produced with the accompaniment of considerable noise of such magnitude and frequency that it cannot easily be removed. The system embodying the invention avoids this difficulty by creating a pulse train (Figure 5B) containing a waveform (Figure 5C) which can much more easily be distinguished from the noise. The noise is omitted from the waveforms of Figures SA and 5B for the purposes of clarity.

In the embodiment described above, one of the windings (winding 24A) is modified so as to have fewer turns than the others.

However, instead other modifications can be made to one or more of the windings to achieve the same end. For example, two or more adjacent windings can be modified so as to have fewer turns than the others. Instead, one or more of the windings can be modified so as to have a greater number of turns, instead of fewer than the others. Another possibility, involving more modification to the rotor, is to provide a local increase in spacing between two of the windings.

However, any other means of modifying the magnetic flux produced by the windings on the rotor so that the magnetic flux produced by a portion of all the windings is differentiated from the magnetic flux produced by the remainder of the windings can be used instead.

Figure 7 shows one such other method in which a resistor 60 is connected between two of the commutator segments (segments 22B and 22J in this example) so as to provide a shunt path for the motor current, thus reducing the magnetic flux produced by the shunted windings 24A and 24J. The resistor 60 can be replaced by any other suitable electrical component. It can be connected across more or less than two windings shown in Figure 7. An arrangement such as shown in Figure 7 produces a waveform for the motor current which is similar to that shown in Figure SC.

It may be necessary or advisable to add local mass to the rotor to ensure its correct balance. Thus, local mass may be added to offset the reduction in mass caused by the reduction of the number of turns of one of the windings. In the case where a resistor is connected as shown in Figure 7, local mass may be added on the opposite side of the rotor to counter-balance the mass of the resistor.

It will be appreciated that the invention is not only applicable to motors with permanent magnet stators but can be used with motors having electrically energised field windings. The system described is also not limited to DC motors.

Claims

1. An electric motor in which a varying back emf is generated as the rotor of the motor rotates, the back emf causing a corresponding variation in the motor current, the motor including means superimposing a cyclic variation on the back emf which is related to the angular position of the rotor.

2. An electric motor in which a varying back emf is generated as the rotor of the motor rotates, the back emf causing a corresponding variation in the motor current together with noise which is associated with the rotation of the rotor, the motor including superimposing means operative in response to rotation of the rotor to superimpose a regular variation on the back emf which is distinguished by its magnitude and frequency from the noise.

3. A motor according to claim 1 or 2, in which the motor incorporates a plurality of windings mounted at different angular positions around the axis of the rotor, and in which the superimposing means is constituted by at least one of the windings in a predetermined angular position which is arranged to produce a magnetic flux the strength of which differs from the magnetic flux produced by the other windings.

4. A motor according to claim 3, in which the said at least one winding has a number of turns which differs from the number of turns of the other windings.

5. An electric motor having a rotor carrying winding means energisable to produce a magnetic flux for driving the rotor and also producing a back emf as the rotor rotates, a portion of the winding means in a predetermined angular position on the rotor being modified to superimpose a cyclic variation on the back emf which is related to the predetermined angular position.

6. A motor according to claim S, having a permanent magnet stator.

7. A motor according to claim 5 or 6, in which the winding means comprises a plurality of windings angularly displaced around the rotor, and in which the said portion of the winding means comprises at least one of the windings which has a different number of turns from the other windings.

8. An electric motor having a rotor carrying a plurality of windings distributed around the rotor and energisable to produce a magnetic flux for driving the rotor and also producing a back emf as the rotor rotates, the back emf varying at a frequency dependent on the number of windings, at least one of the windings being arranged to produce a magnetic flux different from the others so that it superimposes a cyclic variation on the varying back emf as the rotor rotates, the cyclic variation being related to the angular position of the rotor.

9. A motor according to any one of claims 3,4,7 and 8, in which the said at least one winding has fewer turns than the other windings.

10. A motor according to claim 5 or 6, in which the winding means comprises a plurality of windings angularly distributed around the rotor, and in which the said portion of the winding means comprises at least one of the windings whose energisation is arranged to differ from the energisation of the other windings.

11. A motor according to claim 3 or 8, in which the current energising the said at least one winding is different from the current energising the other windings.

12. A motor according to claim 10 or 11, in which the current energising the said at least one winding is arranged to be at a lower level than the energisation of the other windings.

13. A motor according to claim 12, in which the said at least one winding is electrically shunted.

14. A motor according to claim 13, in which the said at least one winding is electrically shunted by resistive means.

15. A motor according to claim 3 or 4, in which the windings are carried by the rotor.

16. A motor according to claim 15, having a permanent magnet stator.

17. A motor according to any one of claims 5 to 8, 15 and 16, in which the windings are energised through a commutator.

18. A motor according to any one of claims 3,4 and 7 to 17, in which the windings partially overlap each other.

19. A motor according to any preceding claim, including detecting means for detecting the cyclic variation superimposed on the back emf, whereby to detect each revolution of the motor.

20. A motor according to claim 19, in which the detecting means comprises means responsive to the current energising the motor to detect the cyclic variation in the motor current corresponding to the cyclic variation of the back emf.

21. A motor according to claim 20, in which the detecting means includes filter means for filtering the motor current to produce a signal corresponding to the said cyclic variation in the motor current, pulse shaping means for shaping the said signal to produce a corresponding pulse train, and counting means for counting the pulses in the pulse train.

22. A motor according to any one of claims 1 to 18, in combination with a non-slip driving connection between the rotor and a window glass raising and lowering mechanism for the window of a motor vehicle whereby energisation of the motor causes the motor to raise or lower the window glass according to the motor's direction of motion, and including detecting means responsive to the magnitude of the motor current to detect cyclic variation thereof corresponding to the said cyclic variation on the back emf in the motor, the detecting means including means responsive to the detected cyclic variation in the motor current to count the number of revolutions of the motor, and control means responsive to the count of the number of revolutions of the motor to de-energise the motor when the count corresponds to the fully raised or fully lowered position of the window glass.

23. An electric motor, substantially as described with reference to Figures 1 to 6 of the accompanying drawings.

24. An electric motor, substantially as described with reference to Figures 1 to 5 and 7 of the accompanying drawings.