HK1009565B - Electromechanical transducer comprising two rotors having permanent magnets - Google Patents

Description

Electromechanical transducer comprising two rotors with permanent magnets

Technical Field

The invention relates to an electromechanical transducer comprising two rotors with permanent magnets. In particular, the invention relates to an electric machine arrangement comprising two permanent magnet rotors, which electric machine arrangement has a planar structure. The electromechanical device according to the invention is particularly useful for applications in watches and clocks.

Disclosure of Invention

It is an object of the present invention to provide an electromechanical transducer comprising two permanent magnet rotors of identical construction which can be controlled independently of each other, either in step operation or in acceleration or continuous operation.

A particular object of the invention is to provide an electromechanical transducer of this type with a high level of reliability during operation of one or the other rotor in an accelerating or continuous mode.

It is a further object of the invention to provide an electromechanical transducer of the above-mentioned type in which one or other of the two rotors can be reliably rotated in both its directions of rotation.

Due to the operational reliability, it should be especially clear that the selectively controlled rotor causes the control circuit to determine the number of advances and the number of revolutions in a predetermined direction.

The present invention therefore relates to an electromechanical transducer comprising a stator and a first and a second rotor comprising a first and a second bipolar permanent magnet, respectively, having a radial magnetization, a first and a second stator bore being defined in the stator bore in which the stator is located, respectively, the first and the second bipolar permanent magnet. The stator includes a first stator portion defining first, second and third poles, the first and second poles completely defining a first stator bore and the second and third poles completely defining a second stator bore. The first, second and third magnetic poles are connected to first ends of first, second and third magnetic cores with first, second and third coils, respectively, and second ends of these first, second and third magnetic cores are connected to a second stator part of the stator for closing the magnetic circuit of the electromechanical transducer.

According to a first embodiment, the first and third coils are connected to a power supply device arranged to supply these first and third coils in order to drive the first and second rotor in rotation, respectively. The second coil is connected to means for detecting rotation of the first rotor or the second rotor. In a preferred alternative embodiment, the detection means is arranged to detect zero crossings of the induced voltage in the second coil when said first and second rotors are selectively driven in a fast or continuous mode of operation by power supply means arranged for that purpose. When the induced voltage in the second coil is zero, the detection means are electrically connected to the power supply, which provides an input signal. When they receive one of said input signals, the power supply is arranged to invert the polarity of the supply voltage.

In a second embodiment of the transducer of the invention, the second coil is selectively connectable to a power source for receiving current or to detection means for detecting rotation of the first or second rotor selectively driven.

In a preferred alternative second embodiment, the second coil is electrically connected in series or in parallel to the first or third coil, depending on whether the first or second rotor is driven in rotation in a step-wise mode of operation, and to the above-mentioned detection means when the fast or continuous mode of operation is activated.

Drawings

The invention will be described in detail below with reference to the accompanying drawings, given by way of non-limiting example, with the following description:

FIG. 1 is a schematic top view of an electromechanical transducer connected to two independent coaxial wheels in accordance with the present invention;

FIG. 2 is a cross-sectional view taken along line II-II of FIGS. 1 and 16;

FIG. 3 is a top view of a first portion of the stator of the transducer of FIG. 1, schematically illustrating the permanent magnets of the two rotors of the transducer;

figures 4, 5 and 6 show, respectively, the current in the supply coil of option 4, the voltage induced in the detection coil as a function of time, and the angular rotation speed of the driving rotor as a function of the rotation angle in the fast or continuous forward operating mode;

figures 7, 8 and 9 show, respectively, the current in the selected power supply coil, the voltage induced in the detection coil as a function of time, and the angular rotation speed of the driving rotor as a function of the rotation angle in the fast or continuous backward operating mode;

FIGS. 10, 11 and 12 show the same variations as in FIGS. 7, 8 and 9, respectively, but in the fast mode of operation two reverse step command cases result in additional rotational steps of the rotor;

fig. 13, 14 and 15 show the same variation as in fig. 10, 11 and 12, respectively, again for two opposite step commands in the fast operating mode, for which the desired number of steps of the rotor is correctly generated, and

fig. 16 is a schematic top view of a second embodiment of an electromechanical transducer in accordance with the invention.

Detailed Description

A first embodiment of the electromechanical transducer of the invention will be described below with reference to fig. 1 to 3. In this embodiment, the transducer operates as a motor.

The transducer comprises a stator 2 and first and second rotors 4 and 6, respectively, disposed in two rotor housings 8 and 10, respectively, mounted in two stator openings or bores 12 and 14, respectively, defined by a first stator portion 16.

The stator 2 comprises three magnetic cores 18, 19 and 20 supporting first, second and third coils 22, 23 and 24, respectively, and a second stator part 26 for closing the magnetic circuit or passage of the transducer described herein. The first stator portion 16 defines first, second and third poles 28, 29 and 30. The pole 28 is electrically connected to the first end 18a of the core 18, while the poles 29 and 30 are connected to the first ends 19a and 20a of the cores 19 and 20, respectively. The second ends 18b, 19b and 20b of the respective cores 18, 19 and 20 are magnetically connected to a second stator portion 26.

The first and second poles 28 and 29 are magnetically isolated from each other by two necks 32 and 33 forming high reluctance regions, while the second and third poles 29 and 30 are magnetically isolated from each other by two necks 34 and 35 forming high reluctance regions. Two first arranged positioning recesses 36 and 37 and two second arranged positioning recesses 38 and 39 are arranged on the edge of the respective stator opening 12 and 14, respectively. The two necks 32 and 33 define a first zero-coupling direction between the coil 22 of the rotor 4 and the first permanent magnet 42, and the necks 34 and 35 define a second zero-coupling direction between the coil 24 of the rotor 6 and the second permanent magnet 46.

It should be noted that the first and second permanent bipolar magnets 42 and 46 are bipolar magnets having radial magnetization that are located in the first and second stator holes 12 and 14, respectively.

The rotor 4 is coupled to a first wheel 50 attached to a first pointer 52 and the second rotor 6 is attached to a second wheel 54 coaxial with the first wheel 50 and to a pointer 56. It should be noted that the first and second wheels 50 and 54 are disposed on either side of the first stator portion 16. However, the first and second wheels 50 and 54 are equal in diameter. The second wheel 54 is connected to the pointer 56 by a diametral shaft 58, free to rotate in a tube 60 attached to the stator 2. This particular arrangement then allows for the separate driving of coaxially rotating hands 52 and 56.

The two first disposed positioning notches 36 and 37 define a first minimum energy direction 64 of the first bipolar permanent magnet 42, while the two second disposed positioning notches 38 and 39 define a second minimum energy direction 66 of the second bipolar permanent magnet 46. The first zero coupling direction 40 and the first minimum energy direction 64, and the corresponding second zero coupling direction 44 and the second minimum energy direction 66 are interleaved at an angle alpha that is not zero. It should be noted that the angle β between the geometrical directions defined by the necks 32 and 33, 34 and 35, respectively, and by the two first and second set positioning notches 36 and 37 and 38 and 39, respectively, has a value substantially equal to the angle α, which in the presently described embodiment is slightly smaller than the value of the angle β.

The first and third coils 22 and 24 are connected to a power supply device 70 and the second coil 23 is connected to a detection device 72 for detecting rotation of the rotor 4 or the rotor 6 in dependence on one or the other of such rotors being driven. The detection device 72 is connected to the power source 70 by an electrical wire 74, shown in fig. 1, to allow the transmission of electrical signals between the detection device 72 and the power source 70.

Having described the operation of the transducer above, the method in which the transducer is controlled will now be described in detail. Basically four modes of operation are recorded. The first mode of operation is a step mode in which one or the other of the first and second rotors 4 and 6 rotates in the forward direction. The second mode of operation is a step mode in which one or the other of the first and second rotors 4 and 6 rotates in opposite directions. The third mode of operation is a rapid or continuous mode of operation (also referred to as an acceleration mode) of one or the other of the first and second rotors 4 and 6 in the forward rotational direction. The fourth mode of operation is a fast or continuous mode of operation of one or the other of the first and second rotors 4 and 6 in the reverse direction of rotation.

The forward and reverse rotational directions of the first and second rotors 4 and 6 are defined by the structure of the transducers described hereinabove. In the presently described embodiment, the forward direction corresponds to the forward rotation direction, i.e., the counterclockwise direction. Thus, the reverse rotational direction is the negative rotational direction, i.e., clockwise.

The construction of the transducer described hereinabove allows one or other of the two rotors to be moved forward in a stepped manner past a single pulse provided by each supply coil, which in turn necessitates the oscillation of the rotor in order to drive it in the reverse rotational direction, typically requiring the provision of three successive pulses, as will be explained more correctly in the remainder of this description.

In the forward stepping mode, the first coil 22 provides, in a known manner, successive pulses with alternating polarity to drive the rotor 4 in rotation. To drive the rotor 6 in rotation, the coils 24 are provided in the same way.

According to an alternative embodiment, the second coil 23 and the detection means 72 are not used in this mode of operation. In another alternative embodiment, the detection means 72 is used to analyse the induced voltage signal generated in the second coil 23 when the rotor 4 or 6 is selectively driven in rotation. The above analysis of the induced voltage signal is effective to determine the performance of the permanent magnets driving the rotor at each step to detect whether rotation has occurred or whether no rotation has occurred. It is well known to those skilled in the art how to perform such an analysis using detection means associated with the power supply coil. In the present case, the analysis of the induced voltage (or induced current) generated in the auxiliary coil, i.e. the second coil 23, is effective. However, the control device 72 is arranged and electrically designed such that the above analysis is similar to that known to those skilled in the art as described above.

Figures 4 to 6 illustrate schematically the operation of the transducer in a forward fast or continuous mode. As mentioned above, the rotor 4 is driven in the forward direction of rotation by a sequence of pulses of alternating polarity, each such pulse passing through a step, in the present case through an angle of 180 ° to drive the rotor into rotation. In the following, the operation of the transducer in a forward fast or continuous mode will be described for the rotor 4, in which mode the drive of the rotor 6 is the same, but the supply coil 24 is used instead of the supply first coil 22.

Fig. 4 shows that the first coil 22 is supplied with a supply current I22 by the power supply 70. Due to the supply current, a current flows into the coil as a result of the voltage applied to such a coil. This current has a sequence of pulses 76 of alternating polarity and substantially the same shape. Fig. 5 shows the induced voltage U23 (or a similar induced current I23) as a function of time in the second coil 23. According to the invention, whenever the sense voltage U23 (or sense current I23) has a zero value, the polarity of the supply voltage is reversed, resulting in a reversal of the polarity of the supply current I22. Thus, it is ensured that the sequence of pulses 76 of the supply current I22 is relatively optimal to ensure rapid or continuous operation in a reliable and efficient manner. The zero crossings of the induced voltage U23 correspond substantially to the axis of magnetization 78 of the first bipolar permanent magnet 42 coinciding with the zero coupling direction 40. As seen in fig. 3, such instructions may ensure that rotor 4 rotates more than 90 ° and passes through the geometric axis defined by notches 36 and 37. In this way it is ensured that the rotor 4 is driven through one step per pulse, i.e. through an angle of 180. Then, after the magnetic axis 78 of the first bipolar permanent magnet 42 is aligned with the minimum energy direction 64, the polarity change of the power supply voltage can be substantially achieved. Thus, a continuous or quasi-continuous rotation of the rotor 4 is ensured in a reliable and uniform manner, and when operating in a fast or continuous manner, the current consumption can be reduced and the transducer yield can be increased according to the invention.

The same analysis applies to the second bipolar permanent magnet 46 whose magnetic axis is represented by arrow 80.

Fig. 6 shows the angular rotation speed of the rotor 4 and the first bipolar permanent magnet 42 attached thereto as a function of the angular position θ of the rotor 4 during rapid or continuous forward rotation. Fig. 6 shows the curves measured by the inventor, showing that rotation is achieved continuously at quasi-constant speed, which has the advantage of making products equipped with the motor arrangement according to the invention attractive for the purpose of saving energy and rapidity, in particular in the case of driving the hands.

In the reverse jump mode, it is known to those skilled in the art how, in the case of a simple single-phase motor with one rotor, to provide three successive pulses to obtain a reverse rotation, via a technical requirement called oscillation. The first pulse is for driving the motor in a positive direction with an angular travel of less than 90. The second pulse then provides a reverse polarity, so that the drive can be made in a reverse rotational direction. Finally, a third pulse of the same polarity as the first pulse is supplied to the power supply coil to ensure the execution of the reverse stepping.

In the reverse stepping mode, the second coil 23 is connected to the detection means 72, which can advantageously be used to determine the end of the second pulse and the start of the third pulse. It should be noted that for the purpose of driving reliability in the reverse rotation direction it is advantageous, because the second pulse has a sufficient duration so that the magnetic axis of the permanent magnet of the selected motor passes through the above-mentioned zero-coupling direction, but it is not too long, that is to say, the duration of the second pulse is not long enough to allow the magnetic axis of the permanent magnet of the selected motor to align itself in a quasi-static manner with the above-mentioned zero-coupling direction. To meet these conditions it is proposed according to the invention that zero crossings of the induced voltage in the second coil 23 are detected during the time period in which the second pulse is generated by the power supply 70, in an alternative embodiment. During the above-mentioned period, when the induced voltage in the second coil 23 crosses zero, the supply voltage is reversed, thereby ending the second pulse to generate the above-mentioned third pulse. Thus, stepwise rotation of one or the other of the rotors 4 and 6 in the reverse rotational direction is ensured.

As mentioned above, for the purpose of improving reliability, when the zero-crossing period of the induced voltage is equal to about twice the duration of the first pulse, as counted from the start of the first pulse, it remains without influence under the control provided by the first coil 22 or the third coil 24, and the deceleration or acceleration of the permanent magnet driving the rotor, especially the change of the direction of rotation following the end of the first pulse, may generate a zero-crossing of the induced voltage in the second coil 23. To remedy such contingencies, an initial transient pulse is then provided during the time that the means for detecting the zero crossing are inactive.

Figures 7 to 9 schematically illustrate two reverse fast or continuous modes of operation for selectively driving one or the other of the rotors 4 and 6. For the sake of simplicity, the rotor 4 will be driven again in the reverse fast or continuous mode of operation, the rotor 6 being driven the same as the coil 24 instead of the first coil 22.

Fig. 7 shows the supply current I22 as a function of time and fig. 8 shows the induced voltage U23 (or similar induced current I23) in the second coil 23 as a function of time. The current I22 has a pulse train of alternating polarity. The first pulse 78 and the last pulse 80 are different from the middle pulse 82. In a manner similar to reverse step operation, the first pulse 78 is an oscillating pulse that drives the rotor in a forward direction. The duration of the pulse 78 is fixed and deterministic, so that a forward drive does not result in rotation of the rotor by one step in the forward direction, which would drive the rotor in the desired reverse direction instead.

Those skilled in the art are aware of this problem and know how to determine the duration of the pulse 78 and its application, particularly with sampling type pulses, to ensure the required oscillatory and reverse operation of the drive rotor. A sequence of pulses 82 of alternating polarity following the first pulse 78 is applied to the first coil 22. The duration of each intermediate pulse 82 is determined by the zero crossings of the induced voltage in the second coil 23, as shown in fig. 7 and 8. When the induced voltage in the second coil 23 crosses zero, the polarity of the voltage is reversed according to the invention for the same reasons as described above for the forward fast or continuous mode of operation.

In the previously described forward fast or continuous mode of operation, the inventors have noted that this does not occur in the reverse fast or continuous mode of operation if the required number of steps and the last pulse of substantially the same length as the previous pulse are correctly obtained. In the forward direction of rotation, it is not allowed to pass the final minimum energy direction at the end of the last pulse in order not to generate additional rotational steps, due to the fact that the permanent magnet must undergo a rotation following the last pulse of more than 90 °. Then, in the reverse rotation direction, the duration of the substantially identical last pulse to the preceding drive permanent magnet exceeds the desired last minimum energy direction, so that the stroke of the angular hold produces an additional step of less than 90 °. The last pulse 80 provided by the power supply to the coil is then greater in duration than the previous pulse. As will be described in more detail below with reference to fig. 10 to 15.

Fig. 9 shows the angular rotation speed Vr of the driving rotor as a function of the angular position θ of such a rotor. Let us remember the opposite rotation direction, which corresponds to the negative rotation direction. Thus, in fig. 9, the driven rotor starts from angular position θ 1. As previously explained, fig. 9 shows that the rotor rotation first exceeds the first portion 84 of the curve in the positive direction and then exceeds the second portion 86 of the curve in the negative direction of rotation, at the end of which the rotor undergoes oscillation, corresponding to the last portion 88 of the curve shown in fig. 9. As shown in the portion 88 of the graph of fig. 9, where the last pulse 80 is relatively long, the rotor first stops at a first position θ 2 corresponding to the zero-coupling direction described above, and then at the end of the last pulse 80, the rotor reaches a last position θ 3 corresponding to the minimum-energy direction described above.

With reference to fig. 10 to 15, the problem of adding a final step in the reverse fast or continuous mode of operation and the solution proposed by the present invention, as already explained above in this text, will be explained.

Fig. 10-12 illustrate the same variables as fig. 7-9, respectively. For the sake of simplicity, only two step instructions in the reverse fast mode are described herein. As is clear from fig. 10 and 11, the last pulse 80' is also terminated by the zero crossing of the induced voltage U23. Thus, the duration of pulse 80' is substantially equal to the duration of the preceding pulse 82. It should be noted that the inductive braking current 84 is not important in current considerations. It is clear from an analysis of fig. 12 that the rotation by an unexpected additional step is caused according to the manner provided by fig. 10 and 11. In fact, the end of the portion 88' of the curve shown does not represent an oscillation of the rotor back to the expected final position θ 3, whereas the final position of the rotor is θ 4 corresponding to an additional rotation of 180 °.

While overcoming the above-mentioned main drawbacks, the present invention proposes to increase the duration of the last pulse, so as to ensure the proper performance of the rotor in order to correctly produce the required rotational steps. Again, the variables shown in fig. 7-9 are the same as the variables shown in fig. 7-9, respectively. The labels of which have already been described and will not be described again here.

As schematically represented in fig. 13, the last supplied pulse 80 has a duration substantially twice that of the preceding pulse 82, the duration of the last pulse 80 being fixed in the transducer control circuitry included in the power supply 70. Analysis of the curve of fig. 15 from the results of the feeding pattern according to fig. 13 and 14, only two steps of 180 ° are achieved, the rotor being at the angular position θ 3 required for its rotation. Note that, contrary to fig. 9, the rotor is not stopped when the magnetic axis is aligned with the zero-coupling direction corresponding to θ 2 in fig. 9.

According to the analysis of the results obtained by the inventors, when the duration of the last pulse 80 is equal to 1.5 times the duration of the preceding pulse, it is sufficient to allow the rotor to operate properly at the end of the rotation. However, for reliability purposes, it is therefore proposed that in a preferred alternative embodiment, the duration of the last pulse 80 is at least twice as long as the duration of the preceding pulse 82.

Referring to fig. 16, a second embodiment of the transducer of the present invention will be described. The reference numerals associated with fig. 1-3 have been described and will not be described in detail herein. A transducer according to a second embodiment, different from the first embodiment previously described in the second coil 23 of the magnetic pole 29 associated with the common first and second rotors 4 and 6, can be selectively connected to the power source 70 or the detection means 72 by means of a switch 90 schematically indicated in fig. 16.

In an alternative embodiment, it is advisable to connect the second coil to the power supply 70 when a forward stepping mode of operation is required. In the first alternative embodiment, the second coil 23 is connected in parallel to the first coil 22 or the third coil 24 depending on whether the rotor 4 or 6 is selectively driven, while the non-provided third coil 24 or the first coil 22 is short-circuited. In the second alternative embodiment, the second coil 23 is connected in series to the first coil 22 or to the third coil 24 depending on whether the rotor 4 or 6 is selected to be driven, while the non-provided third coil 24 or first coil 22 is short-circuited. It is then possible to drive the rotor 4 or the rotor 6 to rotate using the second coil 23 as a power source.

In the reverse rotational direction step mode of operation it is possible to connect the second coil 23 to the power source 70 and in another embodiment to connect the second coil 23 to the detection means 72 so that the second coil 23 and these detection means 72 can be used in a similar manner to that described for the first embodiment in the backward step mode of operation.

In the fast or continuous mode of operation, the second coil 23 is connected to the detection means 72 and functions equally as described for the forward or reverse fast or continuous mode of operation of the first embodiment.

Finally, it will of course also be mentioned that the detection means 72 are arranged to provide an input signal to the power supply 70, in particular when the induced voltage in the second coil 23 has crossed zero. The detection means 72 are not limited to the detection of zero crossings of the induced voltage (or induced current) in the second coil 23 but also allow the detection of other information related to the induced voltage or induced current, especially in the case of erroneous stepping.

Claims (14)

1. An electromechanical transducer comprising a stator (2) and first and second rotors (4 and 6) including first and second bipolar permanent magnets (42 and 46) having radial magnetization, said stator having first and second stator holes (12 and 14) therein, said first and second bipolar permanent magnets being located in said first and second stator holes (12 and 14), the stator including a first stator section (16) defining first, second and third poles (28, 29, 30), characterized in that:

-said first and second poles completely defining said first stator bore and said second and third poles completely defining said second stator bore, said first, second and third poles being connected to first ends (18a, 19a, 20a) of first, second and third magnetic cores (18, 19, 20) with first, second and third coils (22, 23, 24), respectively, the second ends (18b, 19b, 20b) of these first, second and third magnetic cores being connected to a second stator part (26) for closing the stator of the electromechanical transducer;

magnetically connecting first and second stator portions of said stator by said three magnetic cores, said coils being connected to a power supply, said power supply being configured to control said first and second rotors in an independent manner in either a step mode or a fast or continuous mode of operation;

wherein the first and second magnetic poles (28 and 29) are magnetically insulated from each other by two first necks (32, 33), the second and third magnetic poles (29 and 30) are magnetically insulated from each other by two second necks (34, 35), two first arranged notches (36, 37) are arranged at the edge of the first stator hole (12) so as to define a first minimum energy direction (64) of a magnetic axis (78) of the first bipolar permanent magnet (42), and two second arranged notches (38, 39) are also arranged at the edge of the second stator hole (14) so as to define a second minimum energy direction (66) of a magnetic axis (80) of the second bipolar permanent magnet (46);

wherein the first neck portion (32, 33) defines a first zero-coupling direction (40) between the first coil (22) and the first bipolar permanent magnet (42) when the magnetic axis (78) of the first bipolar permanent magnet is aligned with the first zero-coupling direction, and the second neck portion (34, 35) also defines a second zero-coupling direction (44) between the third coil (24) and the second bipolar permanent magnet (46) when the magnetic axis (80) of the second bipolar permanent magnet is aligned with the second zero-coupling direction, the first and second zero-coupling directions being interleaved at an angle other than zero in the first and second minimum energy directions, respectively.

2. A transducer according to claim 1, wherein the first and second rotors (4, 6) are mechanically coupled to first and second wheels (50, 54) arranged coaxially on either side of the first stator portion (16) of the stator.

3. A transducer according to claim 1 or 2, wherein the first and third coils (22 and 24) are connected to a power supply (70) arranged to supply the first and third coils to drive the first and second rotors (4, 6), respectively.

4. A transducer according to claim 3, wherein the second coil (23) is connected to detection means (72) for detecting rotation of the first and second rotors.

5. A transducer according to claim 4, wherein said detecting means (72) is arranged for detecting each erroneous step of said first rotor (4) or said second rotor (6) selectively driven by said power supply (70) in a step-wise operation.

6. A transducer according to claim 4, wherein the detecting means (72) is arranged to detect a zero crossing of an induced voltage U23 in the second coil (23) when the first and second rotors (4 and 6) are selectively driven in a fast or continuous operation by the power supply (70) arranged to allow fast or continuous operation of the first and second rotors, the detecting means being electrically connected to the power supply providing an input signal when the induced voltage crosses zero, the power supply arrangement being capable of inverting the polarity of the supply voltage when receiving one of the input signals.

7. The transducer according to claim 6, wherein the power supply (70) is arranged in both rotational directions of each of the first and second rotors allowing for a fast or continuous mode of operation.

8. A transducer according to claim 7, wherein the power supply (70) is arranged such that the polarity of the supply voltage is inverted at each instant of receipt of said input signal when an induced voltage zero crossing is received, when the fast or continuous mode of operation is driven in the positive direction.

9. A transducer according to claim 3, wherein said second coil (23) is selectively connectable to said power supply (70) to receive a supplied current or to detection means for detecting rotation of said selectively driven first (4) and second (6) rotors.

10. The transducer according to claim 9, wherein when the second coil (23) is connected to the power supply, when the first rotor (4) is rotationally driven, the second coil is electrically connected in series to the first coil (22) and then the third coil (24) is short-circuited, and when the second rotor (6) is rotationally driven, the second coil (22) is electrically connected in series to the third coil and then the first coil is short-circuited.

11. The transducer according to claim 7, wherein when a second coil (23) is connected to the power supply, the second coil is electrically connected in parallel to the first coil (22) and then the third coil (24) is short-circuited when the first rotor (4) is rotationally driven, and when the second rotor (6) is rotationally driven, the second coil is electrically connected in parallel to the third coil and then the first coil is short-circuited.

12. A transducer according to claim 9, wherein the second coil (23) is connected to the power supply when driven in a step-wise operation; when driven in a fast or continuous mode of operation, the second coil (23) is connected to the detection means (72).

13. A transducer according to claim 12, wherein, when driven in fast or continuous mode of operation, the detecting means (72) is arranged to detect zero crossings of an induced voltage U23 in the second coil (23) as the first and second rotors rotate, the detecting means providing an input signal to the power supply (70) each time a zero crossing of the induced voltage is detected; when said power supply receives one of said input signals, it is arranged to invert the polarity of the supply voltage.

14. A transducer as claimed in claim 13, wherein the power supply (70) is arranged, when driven in the forward direction in a fast or continuous mode of operation, to invert the polarity of the supply voltage each time said input signal is received at a zero crossing of the induced voltage.