Electromagnetic drive system

Abstract: A drive system is disclosed comprising a motor of which a first motor section (1) is movable relative to a second motor section (2) along a motion axis (3). The second motor section (2) comprises excitation coils (4,5,6). The first motor section (1) comprises a magnetic circuit (5,8,9,10) generating a magnetic field in the excitation coils (4,5,6). The first motor section further includes a winding to be denoted as a short-circuit winding (12) for only high-frequency indication currents. This short-circuit winding is magnetically coupled to the excitation coils (4,5,6). The extent of the electromagnetic coupling of at least one excitation coil (4,6) depends on the position of the first motor section (1) relative to the second motor section (2). By inserting the short-circuit winding (12) a position-dependent electromagnetic coupling between the excitation coil 5 on one hand and the excitation coils 4 and 6 on the other is achieved. When the motor is in operation this position-dependent coupling can be detected by superposing a high-frequency detection signal on the excitation systems in coil 5 and by detecting in the other coils (4,6) the currents induced in these coils by this high-frequency detection signal. Thus, a compact drive system with in-built position detector is obtained.

Description: The invention relates to an electromagnetic drive system comprising an electromotor of which a first motor section is disposed movably relative to a second motor section. In a prior-art drive system, as disclosed in US 4,616,153 ; the motor sections are constituted by two coaxial cylindrical bodies which are disposed slidably relative to each other. The internal cylindrical body constitutes the first motor section consisting of a tube around which an excitation coil and a short-circuit winding is disposed. The external cylindrical body comprises a magnetic circuit which generates a radially directed magnetic field in the excitation coil. The second motor section further includes two detection coils disposed coaxially relative to the short-circuit winding and having a position-dependent electromagnetic coupling to the short-circuit winding. When injecting a high-frequency detection signal into the detection coils, the position of the first motor section relative to the second motor section can be derived from the signals across the detection coils. In this manner a motor with in-built position detector is realised. A disadvantage of the prior-art drive system, however, is that the components necessary for position detection take up relatively much space, so that the drive system is less suitable for applications in which motors are desired to have very small dimensions, such as, for example, miniaturised portable recording systems such as used, for example, with electronic still picture. Further EP-A-0.183.277 discloses an electromagnetic drive system comprising a brushless electromotor with a stator and rotor disposed to permit relative movement therebetween. The stator comprises excitation coils and the rotor comprises a magnetic circuit for generating a magnetic field in the excitation coils. The rotor comprises sector-shaped conductors mounted on the pole faces of the magnets which oppose the coils of the stator. Each sector-shaped conductor has an electromagnetic coupling to at least two excitation coils, the electromagnetic coupling to at least one of the coils being dependent on the relative positions of the stator and rotor. It is an object of the invention to provide a drive system which is suitable for miniaturization. According to the invention this object is achieved by an electromagnetic drive system comprising an electromotor of which a first motor section is disposed movably relative to a second motor section, the second motor section comprising at least two excitation coils and the first motor section comprising a magnetic circuit for generating a magnetic field in the excitation coils, the first motor section further comprising a short-circuit winding having an electromagnetic coupling to at least two of the excitation coils, the electromagnetic coupling between the short-circuit winding and at least one of the latter coils being dependent on a position of the first motor section relative to the second motor section and the short-circuit winding being enclosed by the second motor section, the short-circuit winding being formed by a bus of non-magnetically conductive, electronically conductive material, which bus is enclosed by the second motor section and which bus encloses the magnetic circuit. The use of the bus achieves a twofold object, that is to say, the formation of the short-circuit winding and the inclusion of the magnetic circuit. An embodiment of the drive system is characterized in that the second motor section comprises three excitation coils electromagnetically coupled to the short-circuit winding, in that a first and a second coil of the three excitation coils are positioned symmetrically relative to the third coil of the three excitation coils, and in that a magnetic circuit is arranged for generating the magnetic field in the three excitation coils. This embodiment makes it possible to accurately detect the position by means of differential detection. An embodiment of the drive system in which such a differential detector is realised is characterized by an excitation circuit for generating excitation currents in the excitation coils, by a circuit for superposing a high-frequency detection signal on the excitation current of the third excitation coil, and by a detection circuit for deriving a position signal indicative of the position of the first motor section relative to the second motor section on the basis of the difference between the signals occurring in the first and second excitation coils and caused by the detection signal. An embodiment highly attractive for its simplicity and having differential position detection is characterized in that the excitation coils are connected in parallel, the drive system comprising a current source for feeding the parallel connection of excitation coils with an excitation current on which the high-frequency detection signal is superposed, a detection circuit for detecting the currents in the first and second excitation coils and a circuit for deriving the position signal from the difference between these currents. In this embodiment the fact that with differential detection the high-frequency detection signal directly injected into the first and second coils by the current source has no effect on the result of the detection is used to advantage. Hence it is possible to feed all three excitation coils from a common current source. The invention can be applied to both rotary motors and linear motors. An embodiment attractive for its simple structure of a drive system comprising a linear motor is characterized in that the excitation coils are disposed behind each other and are essentially cylindrical with a common axis, the second motor section being cylindrical and disposed slidably within the excitation coils in a direction parallel to the common axis of the excitation coils, and the magnetic circuit being arranged for generating radially directed magnetic fields in the excitation coils. An extremely simple structure of the magnetic circuit is obtained in an embodiment characterized in that the magnetic circuit comprises cylindrical permanent magnets disposed behind one another which have a common axis parallel to the axis of the second motor section, the poles of which magnets being situated at the cylinder extremities and in which two opposite cylinder extremities of the magnets having the same polarity. If the distance between the poles of the cylinder magnets is chosen sufficiently small, a magnetoconductive path in the first motor section is not required to generate a sufficiently high magnetic field. Omitting the magnetoconductive path in the first motor section has the additional advantage that there are no adhesive forces between the first and second motor sections, so that a simple journalling of the motor sections can suffice. Although the position detector of the motor can be used for determining the commutation time of electronically commuted brushless d.c. motors, the invention is pre-eminently suitable for use in position servo systems. An embodiment in which this is realised is characterized in that the system comprises a control circuit for controlling the excitation currents in response to the position signal and a reference signal that indicates the desired position of the first motor section relative to the second motor section. The invention will be further explained with reference to the Figures 1 to 4 showing four embodiments of the drive system according to the invention. Fig. 1 shows a first embodiment of the drive system according to the invention with Fig. 1a showing a cross-sectional view of the drive system and Fig. 1b showing a perspective, partly cutaway view of the drive system. The embodiment shown relates to a linear motor comprising a first motor section 1 and a second motor section 2 which have the form of two cylindrical bodies disposed inside each other and movably disposed relative to each other along a motion axis 3. In the embodiments represented here the motor sections have a circle-cylindrical form. However, also cylinder forms having a non-cylindrical cross-section but, for example, a rectangular cross-section are suitable. The second motor section 2 comprises three cylindrical excitation coils disposed behind each other and disposed coaxially relative to the first motor section 1. The first motor section 1 comprises a magnetic circuit for generating a radially directed magnetic field in the coils 4, 5 and 6. Such a magnetic circuit may consist of two cylindrical permanent magnets 7 and 8 disposed behind each other and whose axes coincide with the motion axis 3. At the extremity of the cylinder, magneto-conductive plates 9, 10 and 11 are disposed. The magnetic poles of the magnets 7 and 8 are disposed at the cylinder extremities. The opposite poles of the magnets 7 and 8 have the same polarity so that a radially directed magnetic field is generated in the parts of the coils 4, 5 and 6 which are disposed opposite the plates 9, 10 and 11. For illustration, the field lines of the generated magnetic field are represented by means of dotted lines in Fig. 1a. The magnetic circuit is contained in a cylindrical bus 12 of an electronically conductive, magnetically non-conductive material, for example aluminium, copper or brass. When the excitation coils are excited and the direction of current in the coils 4 and 6 is opposite to the direction of current in coil 5, Lorenz forces will be exerted between the first and second motor sections so that they will move relative to each other. If, in addition, a high-frequency detection current is superposed on the excitation current of coil 5, an a.c. current will be generated in bus 12 because this bus operates as a short-circuit winding electromagnetically coupled to the coil 5. This a.c. current is equally distributed over the bus surface. Since the coils 4 and 6 are also electromagnetically coupled to the bus 12, an alternating voltage will be generated in the coils 4 and 6 as a result of the a.c. current in bus 12. The power of the a.c. voltage is almost directly proportional to the size of the bus surface opposite the coil concerned. This means that the power of the generated a.c. voltage is indicative of the position of the first motor section 1 relative to the second motor section 2. From the a.c. voltages generated in the coils, a position signal indicative of the position of the first motor section relative to the second motor section can then be derived in an extremely simple fashion. An embodiment of a drive system in which the position signal is derived in an extremely simple fashion is represented in Fig. 2. In this Figure the excitation coils 4, 5 and 6 are connected in parallel and are excited from a single voltage-to-current converter 20 with an excitation current on which the high-frequency detection signal is superposed. An adder circuit 21 by means of which a high-frequency voltage originating from an oscillator 22 is added to a control voltage Vs indicative of the desired excitation current is used for superposing the detection signal. The voltage signal of the adder circuit 21 is applied to an input of the voltage-to-current converter 20 that converts the voltage signal into the excitation current and applies this current to the parallel circuit of the excitation coils 4, 5 and 6. The currents through the coils 4 and 6 are detected by means of respective current detectors 24 and 23. By means of a differential amplifier 25 the difference between the two detected currents is determined. Owing to the symmetrical positions of the coils 4 and 6 relative to coil 5 the difference between the two detected currents merely comprises an a.c. current component indicative of the position of the first motor section 1 relative to the second motor section 2. In this respect the amplitude of the differential current component indicates the motor position deviation from a central position which position corresponds with the position in which the bus surface situated opposite coil 4 is equal to the bus surface situated opposite coil 6. The phase of the differential current component indicates the direction of the deviation. A position signal indicating both the direction and magnitude of the position can therefore be easily obtained by means of a synchronous detector 26 to which a difference signal supplied by the differential amplifier 25 and a high-frequency signal supplied by the oscillator 22 via the phase shifter 29 can be applied. The position signal Vp can be tapped from the output of the synchronous detector 26. In the case where in the operating range of the motor the electromagnetic coupling between the short-circuit winding 12 and the central coil 5 is independent of the mutual position of the two rotor sections and the degree of electromagnetic coupling between the outermost motor coils 4 and 6, and the short-circuit winding has a linear relation to the mutual shift of the motor sections, the relation between the power of the position signal Vp and the position of the motor sections relative to each other is substantially linear, so that the position signal is readily suitable as a position detector signal in a position servo system for controlling the relative positions of the motor sections. In this respect in a differential amplifier 27 the position signal Vp can be compared to a signal Vg that indicates the desired position. An output signal of the differential amplifier 27 which is indicative of the difference found, is applied to a control circuit 28 deriving the signal Vs as a function of this signal so that the difference between the desired position and the actual position is kept almost equal to zero. In the embodiment described hereinbefore, the bus 12 becomes a short-circuit winding. Such a short-circuit winding may also be formed, however, by means of a cylindrical coil wound around the first motor section. Since the robustness of the motor construction is enhanced as a result of a short-circuit winding in the form of a bus, the latter is to be preferred. It should further be observed that the invention is not restricted to drive systems comprising three excitation coils. The invention is likewise applicable to embodiments in which two excitation coils or more than three excitation coils are electromagnetically coupled to the short-circuit winding. A motor comprising two coils in the second motor section which are electromagnetically coupled to a short-circuit winding in the first motor section may be constituted, for example, by the embodiment shown in Fig. 1, but from which for example coil 4 and magnet 7 are omitted. Such an embodiment can then comprise an excitation circuit for exciting the coils 5 and 6 and deriving the position signal as indicated in Fig. 3. In this embodiment coil 6 is excited by means of a voltage-to-current converter 30. The coil 5 is excited by means of a voltage-to-current converter 32 through a low-pass filter 31. An oscillator circuit 34 injects a high-frequency detection current into the coil 5 through a high-pass filter 33. The current in coil 6 is detected by means of a current detector 35. The current detector 35 is one of a customary kind producing a signal voltage proportional to the detected current. In a high-pass filter 36 this signal voltage is applied to a rectifier 37 At the output of the high-pass filter 36 an a.c. voltage is produced having an amplitude determined by the extent of electromagnetic coupling between coil 6 and bus 12. Since this coupling depends on the position of the first motor section relative to the second motor section, the amplitude of the a.c. voltage at the output of filter 36 will be indicative of this position. Once this a.c. voltage has been rectified by rectifier 37 a signal whose d.c. voltage level indicates the motor position can be tapped from the output of the rectifier 37. This signal may be used as the position signal Vp. When applied in a position servo system this signal Vp together with a signal Vg may be applied to a circuit 38 that derives the control voltage for the voltage-to-current converters 30 and 32 from the signals Vg and Vp. The embodiments of the drive system described hereinbefore are linear motors. However, the invention likewise relates to rotary motors. An embodiment of a drive system in the form of a rotary motor 62 is represented in Fig. 4. The rotary motor 62 comprises a cylindrical rotor 41 of a permanent-magnet material which rotor is disposed rotatably around a ferromagnetic stator 42. The angle between the rotor 41 and the stator 42 is referenced ϑ. The stator 42 comprises six stator teeth 50, ..., 55 angularly displaced by sixty degrees relative to each other and having shoe-like ends. Opposite the shoe-like ends of the stator teeth 50, ..., 55 there are four magnetic north poles 45, 47, 49, 51 and four magnetic south poles 44, 46, 48, 63 distributed evenly over the rotor circumference. Stator teeth 50 ...., 55 form the core of the respective stator coils 56, ..., 61. The coils of oppositely disposed stator teeth are connected in series. Opposite the shoe-like ends of the stator teeth 50, 51 and 55 within the stator 42 a short-circuit winding 64 is disposed which is electromagnetically coupled to one of the coils 56, 57 and 61 and in which within a certain range of angle ϑ the magnetic coupling between the short-circuit winding 64 and the coil 56 is independent of angle ϑ and the electromagnetic coupling between the short-circuit winding 64 and the coils 57 and 61 is substantially directly proportional to ϑ. The three sets of series-connected coils of the motor 62 can be excited by means of the circuit shown in Fig. 2. A position signal indicative of this angle can likewise be derived by means of the circuit shown in Fig. 2.

Claims: Electromagnetic drive system comprising an electromotor of which a first motor section (1,41) is disposed movably relative to a second motor section (2,42), the second motor section (2,42) comprising at least two excitation coils (4-6; 56-61) and the first motor section (1,41) comprising a magnetic circuit (7-11; 44-49; 51, 63, 50-55) for generating a magnetic field in the excitation coils, the first motor section (1,41) further comprising a short-circuit winding (12,64) having an electromagnetic coupling to at least two of the excitation coils, the electromagnetic coupling between the short-circuit winding (12,64) and at least one of the latter coils (4,6; 57, 61) being dependent on a position of the first motor section (1,41) relative to the second motor section (2,42), and the short-circuit winding (12, 64) being enclosed by the second motor section (2,42), the short-circuit winding being formed by a bus (12) of non-magnetically conductive, electronically conductive material, which bus is enclosed by the second motor section and which bus encloses the magnetic circuit (7-11). Drive system as claimed in Claim 1, characterized in that the second motor section (2; 42) comprises three excitation coils (4-6 ; 56-61) electromagnetically coupled to the short-circuit winding (12, 64), in that a first (4) and a second coil (6) of the three excitation coils are positioned symmetrically relative to the third coil (5) of the three excitation coils, and in that the magnetic circuit (7-11; 44-49; 51, 63, 50-55) is arranged for generating the magnetic field in the three excitation coils (4-6 ; 56-61). Drive system as claimed in Claim 1 or 2, characterized in that the excitation coils (4-6) are disposed behind each other and are essentially cylindrical with a common axis, the first motor section (1) being cylindrical and disposed slidably within the excitation coils (4-6) in a direction parallel to the common axis of the excitation coils (4-6), and the magnetic circuit (7-11) being arranged for generating radially directed magnetic fields in the excitation coils. Drive system as claimed in Claim 3, characterized in that the magnetic circuit (7-11) comprises cylindrical permanent magnets (7, 8) disposed behind one another which have a common axis parallel to the axis of the first motor section (1), the poles of which magnets being situated at the cylinder extremities and in which two opposite cylinder extremities of the magnets (7, 8) having the same polarity. Drive system as claimed in Claim 3, or 4, characterized in that in the second motor section (2) a magnetoconductive path for conducting the magnetic field generated by two opposite poles of the magnetic circuit (7-11) is omitted. Drive system as claimed in Claim 2, 3, or 4, characterized by an excitation circuit (20) for generating excitation currents in the excitation coils (4, 5, 6), a circuit (21, 22) for superposing a high-frequency detection signal on the excitation current of the third excitation coil (5) and a detection circuit (23-26) for deriving a position signal (Vp) which signal is indicative of the position of the first motor section (1) relative to the second motor section (2) on the basis of the difference between the signals occurring in the first (4) and second (6) excitation coils and caused by the detection signal. Drive system as claimed in Claim 6, characterized in that the excitation coils (4, 5, 6) are connected in parallel, in that the drive system comprises a current source (20) for feeding the parallel circuit of excitation coils with an excitation current on which the high-frequency detection signal is superposed, a detection circuit (23, 24) for detecting the currents in the first (4) and second (6) excitation coils, and a circuit (25, 26) for deriving the position signal (Vp) from the difference between these currents. Drive system as claimed in Claim 6, characterized in that the system comprises a control circuit (27, R) for controlling the excitation currents in response to the position signal (Vp) and a reference signal (Vg) that indicates the desired position of the first motor section (1) relative to the second motor section (2).

Inventors: DE LA RAMBELJE PETER HUMPREY, VAN ALEM ANTONIUS ADRIANUS MAR, DE LA RAMBELJE, PETER HUMPREY, VAN ALEM, ANTONIUS ADRIANUS MARINUS
Applicants: PHILIPS ELECTRONICS NV, PHILIPS ELECTRONICS N.V.
Application year: 1991
Main classification: H02K41
IPC codes: H02K29, H02K41