The present invention relates to synchronous electrical motors, and more specifically to synchronous motors of the press construction type.

As is well known, synchronous motors can be operated to rotate by appropriate energisation of coils of the motor and thus it is possible to rotate the motor to a predetermined position by causing the wave forms applied to the coils to take up certain values, usually related to sinθ and cosθ.

In order that the problems and solutions with which this invention is concerned may be understood, the generalised theory- of synchronous motors will first be described, ^* as may be found in any relevant text book, e.g. Circuits, Devices and Systems, R.J. Smith, 1984.

In a typical synchronous motor ac voltages are applied to stator coils which thereby generate a rotating magnetic field. In high power applications there are commonly three or a multiple of three stator coils and the 3-phase mains supply is used accordingly. In low power applications it is also known to use two or a multiple of two stator coils.

The rotor in such a synchronous motor has a magnetic field which is fixed in position with respect

to the rotor. In high power applications this is typically generated using a d.c. supply. In low power applications this may be produced by the use of a permanent magnet.

In operation the rotor magnetic field, and hence the rotor, rotates at the same speed and in the same direction as the stator field. Thus the angle between the two magnetic fields, conventionally labelled $ , is constant and the rotor is said to be rotating synchronously with the stator field. The torque generated by the motor theoretically varies with sin o , for cylindrical rotor motors and thus the greater the angle S between the two magnetic fields (up to a maximum of TT/2) the greater the torque that is generated by the motor. (If $ goes above TT/2 the motor loses synchronism. )

In large motors generating large amounts of torque it is assumed that all the torque is generated according to the angle a as described above and in these situations- the approximation is good. The motor therefore generates a constant torque according to the value of sin S , the value of S being constant when the motor is running in synchronism.

Because the above summarised theory is only applicable with complete accuracy to "ideal" theoretical machines the performance of real machines shows some departure from the theory. Thus other small torques are generated which act on the rotor. As stated above this does not affect the analysis of the torque produced by high power motors.

However these other torques which are generated affect the performance of motors, such as those with which the present invention is particularly concerned, which are required to generate very small torques. In such motors, S has only a very small value.

i.e. the angle between the rotor and stator fields is small.

These other torques are due to causes such as the slotting of the stator required to form the magnetic poles, and are of an order of magnitude large enough to affect the performance of low torque motors.

Although the description herein is of motors having a stator magnetic field which rotates with respect to the stator and a rotor magnetic field which is stationary with respect to the rotor, it is known to construct motors which have a stator magnetic field which is stationary with respect to the stator and a rotor magnetic field which rotates with respect to the rotor. The present invention is equally applicable, with necessary and obvious modifications, to such motors.

Thus the present invention provides a synchronous motor in which the voltage waveforms applied to the coils of the motor depart from the theoretically required waveforms so as to generate a stator magnetic field which takes account of the non-ideal nature of the motor so as to generate a substantially smooth rotation of the rotor.

A further problem encountered especially in low torque motors is that of "hunting". In such motors it is important that the ratio of the back e.m.f.s within the stator coils is close to unity. If this is not the case then motor positional error occurs because the motor torque constant is directly related to the emf constant.

As discussed above, the rotor field should lag behind the stator field by a constant angle S . However if there is imbalance between the stator coils the stator field does not have the constant magnitude and varying phase required by the theory and the rotor time

position vector "hunts" along the two differing load lines.

Thus the rotor torque angle varies as a function of any back emf imbalance and the resultant positional error follows a sinusoidal error pattern having a frequency of twice the basic frequency.

In these motors, the stator coils each have exactly the same number of turns and hence any imbalance of voltage results from a lack of magnetic symmetry, in particular from the axial position of the rotor .

Thus, from a further aspect, this invention also provides a synchronous motor as described above in which the axial position of the rotor may be fixed after the rotor has been allowed to locate the axial position nearest to magnetic equality for all of the stator coils.

Thus motors according to the present invention produce a smoother rotation of the rotor than prior art motors. This mean ^' s they may be advantageously used in instrumentation application for instance in car instrumentation.

The present invention may also be advantageously used in stepper motors which are a particular form of synchronous motor.

In order that the present invention be better understood preferred embodiments thereof will now be described by way of example with reference to the accompanying drawings in which:

Figure 1 shows a schematic diagram of a typical two-winding synchronous motor;

Figure 2 illustrates waveforms used in driving motors such as illustrated in Figure 1 ;

Figure 3 illustrates detent torque as experienced in motors such as illustrated in Figure 1 ;

Figure 4 illustrates preferred apparatus for

determining the waveforms to be used according to a preferred embodiment of the invention;

Figure 5 illustrates an alternative waveform which may be used in an embodiment of the invention;

Figure 6 is a schematic illustration of a motor drive circuit according to the present invention;

Figure 7 illustrates in more detail a preferred control circuit according to the invention;

Figure 8 illustrates apparatus implementing a further feature of the invention; and

Figure 9 shows, partially cut away, a motor according to the present invention.

Figure 1 shows a schematic diagram of a typical two-winding synchronous motor. The rotor 1 is represented as a permanent magnet having a north pole N and a south pole S. The two stator coils are represented at A and B. The stator positions diametrically opposite to A and B are designated A' and B' respectively.

This invention is described in relation to the illustrated two-winding motor but it is understood that it is equally applicable to motors having three or more windings.

Figure 2 illustrates in solid lines the theoretical voltage waveforms, V _a and Vj-,, which are applied to coils A and B respectively in order to generate a rotating stator magnetic field. As illustrated V _a = E sin wt and V]-, = E cos wt. As previously discussed rotor 1 and its associated magnetic field rotate in synchronism with the rotating stator field.

In motors used for instrumentation purposes the generated torque is very small and correspondingly S is small. For the purpose of this description o will be approximated to zero, although it is understood that

if this actually were the situation there would be zero torque produced. Under this approximation, the rotor field at all times coincides with the stator field.

Thus the rotation of the rotor may be understood as follows. It is assumed that the winding of the coils A and B is such- that when a positive voltage is applied to a coil that coil attracts the north pole N of the rotor. When a negative voltage is applied to a coil that coil attracts the south pole S of the rotor, which may be expressed as the north pole N being attracted to the diametrically opposite position (A ¹ or B')« Referring to Figure 2 it will be seen that when wt=0, coil B is energised positively and coil A is not energised. Thus pole N points to position B, and θ=0. When wt = τr/4 coils A and B are equally positively energised and each provide the same magnitude of attraction to pole N. Thus pole N points to a position halfway between A and B, designated A/B, and θ =tt/4.

Table 1 overleaf sets out the position taken by rotor pole N for given values of wt for a complete revolution.

In this case the mechanical angle θ is at all times equal to the electrical angle wt, but, as is known, this is not the case in motors which have multiple stator windings. This invention is equally applicable to such motors although they are not specifically discussed here.

The departure from the theoretical stator field in motors which have a slotted stator for instance as discussed previously, is such that, as well as the constant theoretical torque being generated by the waveforms applied to the coils, there is produced an additional detent torque which is due to factors such as the physical construction of the motor and may be understood as a torque which attempts to align the axis

Table 1

wt position of N θ

0 B T /4 A/B rr 12 A

3TT/4 A/B' π B'

5TT/4 A'/B' 37T/2 A* 77T 4 A'/B 2 TT B of the rotor which passes through poles N and S with either a line A-A ¹ or B-B' depending on to which of these two lines the rotor is closest.

That is, given that the rotor is rotating clockwise in Figure 1, i.e. θ increases with time, while θ is in the range O -τr/4, the detent torque is attempting to align the rotor with a line B-B ¹ , generating an anti-clockwise torque and acting to decelerate the rotor. While θ is in the range τr/4 - Tτ/2, the detent torque is attempting to align the rotor with a line A-A ¹ generating a clockwise torque and acting to accelerate the rotor. There is a similar pattern in each quadrant of the rotation, and this is illustrated in Figure 3, which only shows the detent torque T _β and shows it as positive when it is clockwise and hence accelerates the rotor and negative when it is anti-clockwise and hence decelerates the rotor.

This is also illustrated in Table 2 below which indicates, for each eighth of the rotation, the

lines (A-A* or B-B ¹ ) with which the detent torque is attempting to align the rotor and whether the detent torque is clockwise (+) or anti-clockwise (-).

Table 2

The rotor therefore does not rotate at a constant rate and this is known as cogging.

It is proposed to overcome this problem by varying the waveforms applied to the windings of the motor from the theoretically required waverforms. As will be understood from the above there may be considered to be two torques acting on the rotor: the constant torque independent of the angular position of the rotor generated by sin wt and cos wt being applied to the windings, and the detent torque which is dependent on angular position. The actual torque acting on the rotor is the nett sum of these two torques and is therefore dependent on angular position.

The actual torque acting on the rotor is altered from that described above according to this

invention, such that it is independent of the angular position of the rotor or at least such that the variation in the nett torque with angular position is substantially reduced. This is achieved by altering the torque generated by the windings to be dependent on angular position in a manner inverse to the detent torque in order to compensate for the detent torque. Thus when the nett sum of the torques acting on the motor is calculated the total torque acting on the motor varies less with angular position.

The torque generated by the stator windings may be considered to be the sum of the torque generated by coil A and the torque generated by coil B. In the quadrant where θ ranges from 0 to ^~ τ/2 both coils are positively energised . Thus coil A is acting to turn the rotor clockwise in figure 1 , and coil B is acting to turn it anti-clockwise. In the sector where Θ ranges from 0 to r/4 the detent torque acts in an anti¬ clockwise direction, and thus to compensate for this, either the torque generated by coil A must be increased, the torque generated by coil B decreased, or a combination of these.

In the embodiment discussed in detail below the torque generated by coil A is increased in that sector of rotation. However, it is to be understood that in that and each other sector the essential feature is that the torque generated by the windings is altered to have the effect described above.

This may be achieved as follows: During the times at which the detent torque is acting to align the rotor with line B-B' the magnitude of the voltage waveform applied to coil A is increased above the theoretically required value. During the times at which the detent torque is acting to align the rotor with line A-A' the magnitude of the voltage waveform applied to

coil B is increased above the theoretically required value. Otherwise the waveforms applied to the coils follow the theoretical waveforms.

In the alternative mentioned above, during the times at which the detent torque is acting to align the rotor with line B-B' the magnitude of the voltage applied to coil B is decreased below the theoretically required value and during the times at which the detent torque is acting to align the rotor with line A-A ¹ the magnitude of the voltage applied to coil A is decreased below the theoretically required value.

The embodiment referred to above is illustrated in Figure 2 which shows the waveforms applied to the coils A and B as V _a * and V^' respectively. When V _a ' and V- _* , ' depart from the waveforms V _a and Vj-,, V _a ' and Vj-,' are shown in dotted lines. At other times V _a ' and V^' follow the solid lines of V _a and V^ respectively.

With regard to the quadrant described above where θ ranges from O to ττ/2, while θ is in the range O - TT 4 and the detent torque is anti-clockwise, the magnitude of the waveform applied to coil A is increased, thus providing an additional clockwise torque to cancel out the detent torque. While θ is in the range τr/4 - ττ/2 and the detent torque is clockwise the magnitude of the waveform applied to coil B is increased, thus providing an additional anti-clockwise torque to cancel out the detent torque. A corresponding analysis applies to each of the four quadrants.

It can be seen from the waveforms in Figure 2 that the modified waveforms are based on the two theoretical sinusoidal waveforms, out of phase from each other by τr/2 . At any time in this embodiment there is always one and only one of the theoretical waveforms which is modified. When a particular waveform is not

modified it follows the theoretical waveform. The modification is such that the waveform which, at a particular moment, would have the lower magnitude according to the theoretical model is the one that is modified, and is such that, during the periods a given waveform is modified, its magnitude is increased above that expected according to the theoretical model.

A conventional drive circuit for such a motor provides the two sinusoidal waveforms V _a and Vfc for application to the coils A and B of the motor. The waveforms may be generated digitally with, for instance, 2^ or 2 ^' O steps per electrical cycle, the values provided corresponding to sampled sinusoids.

The modified waveforms of the present invention may similarly be generated digitally, the values now corresponding to samples of V _a ' and Vj _j '. The actual modifications required depend on the design of each individual motor. Thus the modification required may be determined empirically for each motor and the effectively sampled value may be provided in ROM for sequential application to the coils.

A preferred method of determining the values for storing in ROM for application to a motor is described with reference to figure 4. Motor 100 has its two stator windings energised by respective variable d.c supplies 101a and 101b. The mechanical output of the motor is connected to a high precision, low friction encoder 102 which determines the angle of the motor shaft and displays the value by way of display 103.

At first, one winding is energised at full stator voltage and the other winding is not energised. The position the motor takes under these conditions is taken to be zero. The currents supplied by the d.c supplies 101a, 101b, are changed to the values theoretically required for the first mechanical step of

the motor and the position taken up by the motor is displayed by display 103. The current supplied by one or both of the d.c supplies is adjusted if necessary until the motor takes the correct mechanical position for the first step, and the values of the currents from the d.c supplied are recorded, or at least the ratio therebetween. These values can then be stored in ROM as required.

Alternatively in some cases it is possible to modify the sinusoidal waveforms by an exact algebraic expression. Generally this will not provide the exact correction for the detent torque described above but it may have some compensating effect. One example of this is to modify the waveforms to be sin wt + k sin 3wt and cos wt - k cos 3wt where k is a constant. This is illustrated in figure 5 which shows the modification of Va for wt between 0 and f/2. Line Va indicates sin wt, W represents sin 3wt and Va' indicates sin wt + k sin 3wt where k = 0.1 -for the purposes of illustration.

^■ This modification therefore increases the torque generated by coil A while wt is near to zero and decreases the torque as wt approaches 7f/2, and there is a corresponding alteration to the torque generated by coil B. This therefore compensates for the detent torque although it may be that the effect of the detent torque is not completely cancelled in this case. It is found that for a typical motor, k = 0.1 - 0.3 provides good compensation for the detent torque.

In other motors, having a different pattern of detent torque, it is found that using waveforms of the form sin wt - k sin 3wt and cos wt + k cos 3wt produces good compensation for the detent torque.

The waveforms described above may be sampled and stored in ROM as previously described or may be generated as required by the control circuitry.

A schematic illustration of a drive circuit which may be used in this invention is illustrated in Figure 6. Input interface 10 may be adapted to accept analog or digital drive inputs. Digital inputs may be in serial binary form or in time dependent (wherein the required motor shaft angle is proportional to the input frequency) form. Interface 10 generates corresponding addresses which are applied to ROMs 12, 14 by way of bus 11. This causes ROMs 12,14 to output appropriate values along the modified waveforms V _a ' , V]- respectively. These outputs are input to digital-to-analog converters 13,15 respectively which have outputs connected to coils A and B respectively.

Thus the modified waveforms V _a ' and V]-, ¹ are applied to coils A and B respectively. The form of modification depends on the information stored in ROMs 12 and 14 and this information is selected to be appropriate to the motor being driven as described above.

Figure 7 illustrates in more detail the control and drive circuitry according to a preferred embodiment of this invention, in which the motor is used for car instrumentation. A signal f is provided and it is required that the angle taken by the rotor is proportional to the frequency of signal f. This is the case for instance in a car speedometer drive.

Signal f is input into frequency/voltage converter 20 which gives an output, the magnitude of which is proportional to the frequency of signal f. This is then compared with a feedback signal on line L1 to give an error signal e. This signal is input to converter 21 which give as an output a signal having a value equal to the magnitude of signal e and to converter 23 which gives as an output a binary signal according to the polarity of signal e. The output from

converter 21 is input to voltage controlled oscillator (VCO) 22 which outputs onto line L2 an oscillating signal the frequency of which is proportional to the magnitude of signal e. The output of converter 23 is applied to line L3 via a latch 24 which is clocked by the output from VCO 22.

Lines L2 and L3 are connected to two up/down (U/D) counters '25, 27, L2 as the clock input and L3 as the direction input. The output of U/D counter 25 is input to digital to analog (D/A) converter 26, the output which is provided as the feedback signal on line L1. It will be appreciated that this feedback loop causes U/D counter 25 to count, up or down as appropriate, until the output of D/A converter 26 is equal to the output of frequency/voltage converter 20. Also, due to the presence of VCO 22 the larger the discrepancy between these two outputs, the faster the counter will count to reduce this to zero.

U/D counter 27 receives the same inputs as U/D counter 25.and hence its output is identical, or at least counts up and down correspondingly. The output of U/D counter 27 is input to ROM 28 which has stored therein the values for the waveforms required to be applied to the windings of the motor. The appropriate values are addressed by the input taken from U/D counter

27 and are output to respective latches 30, 32. The ROM

28 and latches 30, 32 are controlled by timing controller 29 which receives the signals on lines L2 and L3 as inputs. The digital values are then output via D/A converters 31 , 33 to windings A and B respectively of the motor.

A further problem which is encountered in low torque motors such as those used form driving a needle on the dial of a car instrument is that of magnetic stiction and mechanical hysteresis effects. In general,

because of the materials used and the pressed method of construction typical of motors of this type, substantial work hardening of the stator poles occurs. This results in a considered hysteresis effect in the motor and the positional error when the direction of rotor rotation changes may be some 3-4°. This problem may be overcome by causing the motor to oscillate a very small amount around its correct position. Such an oscillation may be so small as to be imperceptible to a human eye and hence have no effect on the motor's applicability to car instrumentation. The effect of the oscillation signal is to AC demagnetise the pole tips and to provide some mechanical de-stiction.

This is illustrated in figure 8 with reference to the embodiment described above and illustrated in figure 7. Corresponding reference numerals are used in figure 8 for parts which are identical to those in figure 7. Thus signal f is input to voltage / frequency converter 20, the output of which is input to block 40. Block 40 represents the feedback loop of figure 6 and comprises items 21-27 of that embodiment. The output is input to ROM 28 which has outputs fed to latches 30, 32 and D/A converters 31 , 33 as described above.

The circuitry of figure 8 further comprises ac current sources 34, 35. These superimpose, on the outputs.of D/A converters 31, 35 respectively a oscillating signal (eg. 16-20 Hz) with a low magnitude. This generally low frequency signal has a frequency which is relatively high compared to the movement of the motor in applications such as car instrumentation.

As will be appreciated this causes the rotor of the motor to oscillate about its position determined above. The magnitude of the oscillation provided by the sources 34, 35 is equivalent to approximately 3-5 bits in the output of ROM 28 or an oscillation of the rotor

of 0.1 -0 .1 5°.

The oscillation of the motor rotor may alternatively be achieved by the design of the digital control circuit such as illustrated in figure 7. It is usual when designing servo-control systems to design the deadband to be as small as possible, ie the amount of overshoot when the deadband approaches its "null" position, when the error signal reaches zero, is minimised. This reduces the oscillation and provides the best positional accuracy.

However, in the present invention, if a fairly wide deadband is allowed in the design of the control system such that an overshoot of, for example, 5 bits of data at the input of the U/D counters is produced, this causes the rotor of the motor continually to oscillate as described above. The 5 bits of data mentioned above are equivalent to approximately 0.15° of rotor angle movement. Thus the inherently oscillatory nature of the DC servo system is used to provide an advantageous effect.

As stated previously the problem of hunting is overcome by allowing the rotor of a motor to self- locate, in its axial direction, according to the magnetic field of the stator.

Because, in a practical motor it is necessary to have the rotor in a fixed axial position some means of fixing is required. However, the conventional approach of designing the fixings, typically spring pre¬ load washers within the motor, does not allow for variations in the manufacturing tolerance and axial location of the rotor magnet and stator coils which cause variations in the axial position of the stator field.

A preferred example of a motor overcoming this problem is shown, partially cut away, in Figure 9.

Figure 9 shows the rotor 1 mounted on rotor axle 3 and stator coils 2 enclosed in casing 4. As can be seen the rotor 1 is manufactured shorter in the axial direction then the stator assembly. Initially the rotor axle 3 and hence rotor 1 is allowed to move freely in the axial direction.

When the stator 2 is energised the rotor locates itself magnetically in the position of magnetic stability with respect to the stator field. Once this has occurred clips 5, preferably press fit self locking retaining rings, are clipped on to axle 3 abutting the external casing 4, thus preventing any further axial movement of the rotor during use.

Both the positioning of the rotor in this manner and the use of modified waveforms to energise the stator as described above cause the rotation of the rotor to be smoother than it would otherwise be. A motor showing both these features runs particularly smoothly and has particular application to instrumentation devices, especially car instrumentation devices.