WO2003028194A1

WO2003028194A1 - Linear motor with transducer arrangement

Info

Publication number: WO2003028194A1
Application number: PCT/GB2002/004335
Authority: WO
Inventors: Ian David Rose
Original assignee: Copley Motion Systems Llc
Priority date: 2001-09-25
Filing date: 2002-09-25
Publication date: 2003-04-03
Also published as: GB0123034D0

Abstract

An arrangement of magnetic field strength detectors useful for the communication and/or control of a linear electric motor is provided in which the arrangement either comprises a set of at least two detectors (12, 13, 14) separated from one another circumferentially in the same plane or a set of at least two detectors (18, 19) longitudinally separated by half a full cyclical pole pitch of the magnets of the stator to which the arrangement is to be applied. The first arrangement in which the detectors are circumferentially separated allows the detrimental effect of unstraightness of the stator (e.g. sagging due to gravity) and/or any misalignment of magnets in the stator to be reduced. The second arrangement in which the detectors are longitudinally separated allows two signals which are an anti-phase to be provided, these signals being capable of being combined so as to eliminate any signal components due to undesired parasitic magnetic fields. The two arrangements may be combined in a preferred embodiment.

Description

LINEAR MOTOR WITH TRANSDUCER ARRANGEMENT

The following invention relates to improved means for the commutation and position control of electric linear motors. The invention concerns in particular the application of this improvement to the commutation and control of a tubular linear electric motor, as described in granted UK patent no. 2,079,068B, the disclosure of which is hereby incorporated by reference.

Linear electric motors are now in widespread use in industrial applications requiring the rapid and accurate positioning of one component relative to another. An example of such an application is, for example, the rapid and precise positioning of a test probe over a printed circuit board to check the circuit pathways thereof, prior to loading it with expensive integrated circuits. A further example is one in which very smooth motion is required, with imperceptible variation in velocity of the component being moved. In either case, the achievement of tight and precise servo control is essential over the motion of the motor's armature relative to its stator.

For the purpose of commutating the coils of permanent magnet linear motors, and in some cases the positioning of the armatures thereof relative to their stators, it is well known to use one or more Hall effect transducers. ^• The transducers provide controlling signals to electrical circuits powering the coils of the motors, as well as to software used for the servo positioning of the armature of the motor. The transducers may be mounted, for example, on the armature of the motor, and as this traverses the stator, so cyclical signals are provided indicating the position of the armature relative to its stator. The signals vary in accordance with the strength of the repeating sequence of magnetic fields emanating from the stator. In many cases, the signal provided by the transducers approximates to a sinusoid, reflecting the sinusoidal variation in magnetic field strength along the length of the stator. An example of the use of such transducers is described in granted UK patent no. 2,235,783, the disclosure of which is hereby incorporated by reference.

Whatever the application and use of the signals provided by the Hall effect transducers, it will be appreciated that the signals must truly indicate the position of the stator of the motor relative to its armature. Were the signals to vary not as a result of relative motion, but through some other effect, a misrepresentation would be provided to the servo control hardware and software controlling the motion of the motor, as well affecting the predetermined regularity of commutation of the armature coil currents essential for a smooth and consistent motion. Such a misrepresentation compromises the degree of servo control possible, sometimes seriously.

When using Hall effect transducers to commutate and control the motion of the tubular linear electric motor mentioned above, a particular difficulty has been identified . To facilitate an appreciation of the subject matter of this invention as applied to this type of motor, a brief description follows of the construction of this type of motor. The stator of the motor comprises a hollow tube housing a sequence of axially magnetised permanent magnets, spaced one from the other, and each in repulsion to its neighbour. The armature of the motor comprises a cylinder for travelling coaxially along the tubular stator of the motor. The cylinder houses a sequence of contiguous annular coils. These, when energised in the correct sequence, provide fields for interacting with those provided by the permanent magnets of the stator, and thus the production of linear thrust. Appropriate commutation of the coils by Hall effect transducers mounted on the armature, ensures an even thrust is achieved in the desired direction of travel.

A common arrangement enabling the commercial use of this form of linear motor, is for its load bearing armature to be guided along its length of travel, by a precision -and usually very straight- linear bearing. The tubular stator of the motor is supported at each of its ends, to permit travel of the cylindrical housing coaxially along its length. The inside diameter of the annular coils of the motor is designed to be sufficiently large to ensure a good air gap exists between the coils and the tube passing therethrough, so as to avoid any possibility of the two components scraping in use. However, a disadvantage arises from this arrangement. The stator sags under the influence of gravity. This is especially the case if the length of travel is long, for example, over a metre. This detrimentally affects the quality and usefulness of the signals provided by the Hall effect transducers. The reason for this is that -due to sag- a transducer will provide a different reading when positioned over, say, a peak in magnetic field at one end of the tube, compared to if it were to be reading the same peak at the centre of the tube. Were the transducer to be mounted on the armature such that it were to be above the tube, the physical increase in distance between the transducer and the tube at the centre would result in a diminished signal. Conversely, were the transducer to be mounted below the tube, the signal would be augmented. In either case, this arbitrary variation provides a false indication to the commutation circuitry as to the position of the armature relative to its stator, and thus compromises the accuracy of the commutation of the coil currents, leading to errors in servo control. It also compromises the possible use of the analogue signal provided by the transducer, as used to indicate the lateral position of the armature relative to its stator for the purposes of servo-controlled positioning control. Bearing in mind the earlier mentioned typical sinusoidal variation of such a signal, a moment's thought shows how any arbitrary variation is especially critical near its peak value.

According to the invention, there is provided a linear motor comprising: a stator; a first set of magnetic field strength detectors, said first set comprising at least two magnetic field strength detectors separated one from the other circumferentially around said stator so as to provide signals that can be combined to ameliorate the detrimental effect of any unstraightness of the stator and/or any misalignment of magnets in the stator.

A preferred embodiment comprises an arrangement of Hall effect transducers for the commutation and/or control of a linear electric motor is provided in which an array of three or more transducers are located and separated one from the other circumferentially around the stator of a linear motor, and in terms of their longitudinal spacing one from the other along the length of the stator, being sufficiently close as to ensure that each is affected to the same degree by any local sag in the stator of the motor, for each providing an electrically separate signal corresponding to the strength of flux fields emanating from the stator, the vector values of each of the separate signals being subsequently combined arithmetically to provide a single signal. The signal magnitudes from each individual detector of the set may be summed electronically by the use of e.g. operational amplifiers, or their respective signals may be converted to digital values and then summed by a software algorithm.

The arrangement of the invention thereby substantially eliminates any effect of unstraightness of the stator, e.g. sag, inasmuch that should the stator sag, the augmentation in signal received by the transducer or transducers nearest the stator, will be compensated for by the diminished signal contributed by the transducer or transducers furthest from the stator.

In a preferred embodiment, three circumferentially equi-spaced transducers are used, i.e. at 120 degrees from one another. This provides the most economical solution to cope with any direction of sag of the stator of the motor.

Practical experiments show a reduction is possible in variation of signal due to sag, from 20% without the above arrangement, to 0.2% with the above arrangement.

A further difficulty arising from use of the motor type mentioned above, is that for the purposes of easy assembly during production, the magnets inserted down the tube must necessarily be of a smaller diameter than the internal bore of the tube itself. In some cases, where due to tolerance variations this diametrical discrepancy is pronounced, it will be appreciated that one magnet may lie to one side of the tube, while the next may - for example- lie to the other side. Again this introduces a further detrimental arbitrary variation in terms of the spacing between the magnets and the transducers measuring their fields.

In a preferred arrangement, again three transducers are used, spaced circumferentially at 120 degrees to one another, but which are also in the same plane around the stator of the motor. By this means, this arrangement both compensates for sag, as well as any local diametrical variation in magnet position within the stator tube.

Although the above described arrangement is highly efficacious in terms of reducing any arbitrary variation due to sag, or magnet diametrical variation, a further independent factor may influence the performance of the (preferably Hall effect) transducers. This is the influence of external magnetic fields. Clearly, any external field acting on the transducers will distort their true outputs. The external variation may occur due to other magnetic fields present adjacent to the travel of the armature, or, due to the physical presence of the transducers on the armature, in which stray fields arising from the very energisation of the armature coils can be created.

Accordingly, there is provided a linear motor comprising: a stator; a first detector means; a second detector means separated longitudinally along said stator from said first detector means, said longitudinal separation being substantially equal to, or a multiple of, half a full cyclical pole pitch of the magnets of the stator, such that the signals produced by the first and second detector means in use are in anti-phase.

In a preferred embodiment, each detector means comprises one of the said detector sets.

According to this preferred feature of the invention, a further set of transducers is located circumferentially around the stator of the motor, but spaced from the first set axially along the length of the stator by half a full cyclical pole pitch of the magnets, thereby providing a signal which is in anti-phase to the signal provided by the first set, the signal from the first set being combined with that of the second set by differential addition, the overall signal thus representing the combination of the first set signal and the second set signal, the arrangement providing as a result of the differential addition, a means of substantially eliminating the effect of any parasitic magnetic fields acting together upon the two sets of detectors.

By way of explanation, the elimination of the effect of a parasitic field disadvantageously acting upon the transducers occurs inasmuch that such a field, acting on the two sets together in the same sense, is eliminated by the differential addition of the two Hall effect signals. However, the signals emanating from the detector sets are in anti-phase, and therefore combine to provide an overall signal which is substantially immune from the effect of parasitic fields. In a preferred embodiment of this feature of the invention, four sets of detectors are used. The first set is used to detect the magnetic signal from -by way of reference-say a south pole, and the second set is positioned a full magnet's pitch -or integral multiple thereof- away from the first, in order to provide the anti-phase signal -according to the above described feature of the invention, for combining with the first detector set signal to substantially eliminate the effects of parasitic fields acting thereon. The third set is displaced along the length of the stator by half a full magnet pitch from the first set so providing (together with the first set) two varying signals displaced in phase by 90 degrees. (The 90 degrees offset arrangement is commonly used by servo controllers and the like for deducing, from the phase displaced signals, the physical position of the armature relative to the stator, as well as the direction of travel. It is also used advantageously to commutate the motor coils.) The fourth set is then positioned a full magnets pitch (half a full cyclical pole pitch) away (or multiple as just described) from the third set, in order to provide the antiphase signal for combining with the third detector set signal.

It will be appreciated that the detector sets described above can extend for some length along the length of the stator of the motor. For example, for a motor having an inter- magnet pitch of 20.8mm, in order to achieve the correct phase relationships, the total array length will be 62.4mm. In a first configuration, the whole array may be positioned at one end of the armature of the linear motor. However, a limitation of this arrangement is that parasitic fields arising from the end armature coil (housed in effect at the furthest extent of the armature and therefore nearest the first detector set), will affect this first set considerably more than fourth set, positioned, e.g. as mentioned above, some 62.4 mm away. This may result in the parasitic field balancing circuit working more effectively for the first and second sets, than for the third and fourth sets.

According to an aspect of the invention, the first and second sets are located at one end of the armature, while the third and fourth sets are located symmetrically at the other end of the armature. By this means, the distance which spaces the parasitic fields emanating from the armature acting on each of the third and fourth sets, is the same as that of the first and second sets. It must be borne in mind that in the case of this arrangement, where the two sets of detectors are spaced apart by the length of the armature, other external parasitic fields local to one array will not affect the other, and therefore an imbalance may arise.

In order for the fields detected by the detector sets to be as close as possible to sinusoidal, according to a further preferred feature of the invention, careful selection is made both of the radial spacing of each of the Hall effect sensors from the stator of the motor as well as the physical size and or spacing of the permanent magnets one from the other.

A further refinement, according to yet a further preferred feature of the invention, is the accommodation of one or more temperature sensing transducers, for sensing the local temperature of the stator adjacent to each detector set or detector means, and thus enabling compensation for any change in field emanating from the permanent magnets resulting from their warming during use of the linear motor.

The invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:-

Fig 1 shows a schematic representation of a three Hall effect transducer set, according to the invention, placed relative to a stator of a tubular permanent magnet linear motor;

Fig la shows a stator of extended length, and bowing under the effect of gravity; Fig 2 shows a circuit schematic used for combining the signals; Fig 3 a shows two detector sets for enabling signal compensation due to the effect of parasitic fields;

Fig 3b shows a circuit for combining the signal provided by the two sets of Fig 3 a;

Fig 4 shows an array of four detector sets, providing a full set of signals for position control and/or commutation of the motor's armature coils;

Fig 5 shows a full circuit schematic for receiving and processing the signals provided by the array of Fig 3; and Fig 6 shows a linear motor armature with the detector sets mounted at each extremity thereof.

Referring to Fig 1, the magnetic stator of a tubular linear motor is depicted at 10. It will be seen that this houses a series of permanent magnets 11, spaced one from the other, and alternating in magnetic polarity. For the purpose of detecting the position of the stator, as well as for commutating armature coils of the linear motor (not shown), a set of three Hall effect transducers, 12, 13 and 14, are located at 120 degree spacing circumferentially -in the same plane- around the stator. Each of these produces a signal, 15, according to the strength of the magnetic field emanating radially from the stator. By the careful selection of the radial spacing of the transducers from the surface of the stator, as well as the internal design of the magnets and spacers within the stator, the detected fields vary substantially sinusoidally as the stator passes though the set of detectors.

In use, the stator of the motor sags due to gravity, especially over extended lengths, see Fig la, where this is shown in exaggerated form for the purpose of clarity. It will be appreciated that in this case, the top sensor of the set, as shown at 16, is closer to the tube at one end, than at the middle, 17. This results in a false indication of position, due to the fact that the detected signal varies according to the combination of the absolute strength of the emanated magnetic field, as well as its spacing from the stator. For example, at one end, the signal, after due processing by electronic circuitry, may have a peak value of 5.0 volts. A displacement to either side by a few millimetres might reduce this to 4.5 volts. At the centre, the peak value might only achieve a value of 4.5 volts. This both compromises use of the signal for position detection, as well as its use for . commutating smoothly the coils of the motor.

The detector set of the preferred embodiment of the invention substantially eliminates this effect as follows. The signals produced by the three transducers are summed arithmetically in a first combiner. This can be achieved electronically, or by software algorithms. A combiner circuit for summing the signals arithmetically is shown at Fig 2. Resulting from this, any arbitrary decrease in the signal received by the top sensor, as just described, is compensated for by the augmented signal received by the lower sensors. Because the sensors are placed at 120 degrees, the direction of sag is unimportant, so enabling the motor to be used in any plane. A highly effective, and simple solution is thereby provided, for furnishing a consistent signal both for the commutation of the motor, as well as for position control. It should be note that as well as compensating for sag, the arrangement of the detector set also compensates for any diametrical displacement variation of the magnets within their surrounding tube, and thus radial distance variation between them and the transducers, arising for example due to manufacturing tolerance variations,

In practical use, a linear motor may be exposed to external parasitic fields, due to proximity with other magnetic equipment. In addition the armatures of linear motors create their own parasitic fields, arising from their very operation. These fields may act detrimentally upon the detection set described, inasmuch that the transducer outputs are affected not only by the stator fields, but also by any parasitic fields present.

An arrangement for compensating for this is shown at 18 in Fig 3. A second detector set 19 is positioned along the direction of the travel of the stator, away from the first set 20, by half a full magnetic cyclical pitch, 1. In other words, when one detector set is situated over a north pole, the next detector set is situated over a south pole, and so on. The signals so generated by the second set are in anti-phase to those generated by the first set. These are combined, for example, by a second combiner circuit such as the differential electronic amplifier shown at Fig 3b. Because they are in anti-phase, but are combined differentially, as shown at 21, the resultant output is equal to their signed mathematical sum. However, any parasitic magnetic field present acts of course upon each detector set in the same sense. Any additional signal arising from this is cancelled out by the differential addition carried out by the operational amplifier. The signals carrying parasitic noise are shown, schematically, at the input to the operational amplifier of Fig 3b. The result is a clean signal, substantially independent of the effect of both any parasitic field effect, as well as independent of any tube sag, or magnet diametrical irregularity.

The resultant signal can be used both for smooth commutation of the coils of the motor, as well as for position control. In practice, commercial position controllers used for servo positioning of electric motors normally require the supply of position indicating signals which are displaced one from the other by a 90 degree phase shift. This provides sufficient information both for deriving the absolute position of the motor, as well as its direction of travel.

Referring now to Fig 4, an arrangement of detector sets is shown for achieving this objective. Detector sets 22 and 23, each comprising Hall effect transducers A,B and C, together provide, by way of reference, a first sinusoidal signal. (The two detector sets 22 and 23 function exactly as described above with reference to Figs 1 to 3.) The second pair of detector sets 24 and 25 is displaced from the first two sets by a 90 degree magnetic phase shift. In other words, when for example the first set 22 is sited over magnets, the other two sets 24 and 25 are sited over the gaps between them, as shown in Figure 4. By this means, overall signals 26 and 27, displaced in magnetic phase by 90 degrees, are provided for supply to a suitable servo controller or the like for position control, and as appropriate, to other circuitry for commutating the coils of the motor. Fig 5 shows an overall schematic circuit diagram for processing the signal provided by arrays 22to 25, and the supply of the two phase shifted output signals. It can be seen that the individual detector outputs of a set are combined in a first signal combiner and that the resulting signal is combined with the signal which it is in anti-phase in a second combiner, to provide a pair of signals which can be used to control the motor and which are free of the effects of an unstraight stator or parasitic magnetic fields.

Referring now to Fig 6, a linear motor armature 28 is shown with a pair of detector sets mounted at each end, as shown at 29 and 30. The spacing between the detector sets at each end of the armature is a multiple of magnetic pole pitches, plus one half, such that their outputs are again displaced by 90 degrees, as in Fig 4. The purpose of this arrangement is as follows. Due to the fact that each detector set is equi-spaced from the end armature coil adjacent to it, the degree of parasitic magnetic interference acting on each array at each end is equal, given correct phasing of the armature coils. This is in contrast to the arrangement shown in Fig 4, which, were it to be mounted at one end of the motor, a different degree of interference would be experienced on the array further from the motor, compared to that nearest, thereby leading to an imbalance in the signals provided.

Numerous variations will be apparent to those skilled in the art. In particular, it will be appreciated that the use of three magnetic field strength detectors in a detector set is merely preferred. In situations where the direction of bowing of the stator can be predicted, such as for example when the bowing is due to gravity, two detectors may be used in the set circumferentially separated by 180 degrees and vertically aligned so that any augmentation of one signal caused by bowing is accompanied by a depletion of the other signal. Other circumferential separations may be used if different signal combining algorithms (i.e. other than simply summing the signals) are used.

As will be appreciated, the invention is applicable to magnetic field strength detectors in general although Hall effect transducers are preferred.

Claims

1. A linear motor comprising: a stator; a first set of magnetic field strength detectors, said first set comprising at least two magnetic field strength detectors separated one from the other circumferentially around said stator so as to provide signals that can be combined to ameliorate the detrimental effect of any unstraightness of the stator and/or any misalignment of magnets in the stator.

2. A linear motor according to claim 1, wherein said magnetic field strength detectors are circumferentially separated in substantially the same plane, so that the detectors each measure fields emanating from substantially the same portion of the motor stator.

3. A linear motor according to claim 1 or 2, wherein said first set comprises at least three magnetic field strength detectors.

4. A linear motor according to claim 3, wherein said first set consists of three magnetic field strength detectors circumferentially separated from one another by 120 degrees.

5. A linear motor according to any one of claims 1 to 4, further comprising a first signal combiner for combining the signals produced by each magnetic field strength detector of the first set.

6. A linear motor according to claim 5, wherein said first combiner is constructed and arranged to add the signals together, thereby substantially removing any signal components due to unstraightness of the stator and/or misalignment of stator magnets.

7. A linear motor according to any one of claims 1 to 6, wherein said unstraightness is caused by sagging of the stator due to gravity.

8. A linear motor according to any one of claims 1 to 7, further comprising a second set of detectors comprising at least two magnetic field strength detectors separated one from the other circumferentially around said stator, said second set being separated longitudinally along said stator from said first set.

9. A linear motor according to claim 8, wherein said longitudinal separation is substantially equal to, or is a multiple of, half a full cyclical pole pitch of the magnets of the stator, such that the signal produced by the second set is in anti-phase with the signal produced by the first set.

10. A linear motor according to claim 8 or 9 when appendant to claim 5, further comprising a second signal combiner for combining the signal produced by the first signal combiner with a signal produced by combining the signals of the detectors of the second set.

11. A linear motor according to claim 10, wherein said second combiner is constructed and arranged to differentially combine the signals so as to produce a signal that is substantially free from the effect of any parasitic magnetic fields acting on the first and second set of detectors.

12. A linear motor according to any one of claims 1 to 11, further comprising a third and fourth set of detectors each comprising at least two magnetic field strength detectors separated one from the other circumferentially around said stator, said third set being separated longitudinally from said first set and said fourth set being separated longitudinally from said third set.

13. A linear motor according to claim 12, wherein said third set is separated longitudinally along said stator from said first set by a quarter of a full cyclical pole pitch of the magnets of the stator, or multiples thereof, such that the signal produced by the third set is 90 degrees out of phase with the signal produced by the first set.

14. A linear motor according to claim 12 or 13, wherein said fourth set is separated longitudinally along said stator from said third set by a half of a full cyclical pole pitch of the magnets of the stator, or multiples thereof, such that the signal produced by the fourth set is in anti-phase which the signal produced by the third set.

15. A linear motor comprising : a stator; a first detector means; a second detector means separated longitudinally along said stator from said first detector means, said longitudinal separation being substantially equal to, or a multiple of, half a full cyclical pole pitch of the magnets of the stator, such that the signals produced by the first and second detector means in use are in anti-phase.

16. A linear motor according to claim 15, further comprising a signal combiner for combining the signal produced by the first detector means with the signal produced by the second detector means so as to produce a signal that is substantially free from the effect of any parasitic magnetic fields acting on the first and second detector means.

17. A linear motor according to claim 15 or 16, further comprising a third detector means separated longitudinally along said stator from said first detector means by a quarter of a full cyclical pole pitch of the magnets of the stator, or multiples thereof, such that the signal produced by the third detector means is 90 degrees out of phase with the signal produced by the first detector means.

18. A linear motor according to claim 17, further comprising a fourth detector means separated longitudinally along said stator from said third detector means by a half of a full cyclical pole pitch of the magnets of the stator, or multiples thereof, such that the signal produced by the fourth detector means is in anti-phase with the signal produced by the third detector means.

19. A linear motor according to claim 18, further comprising a further signal combiner for combining the signal produced by the third detector means with the signal produced by the fourth detector means so as to produce a signal that is substantially free from the effect of any parasitic magnetic fields acting on the third and fourth detector means.

20. A linear motor according to any one of claims 15 to 19, wherein each said detector means comprises a detector set.

21. A linear motor according to any one of claims 1 to 14 or,20, further comprising a temperature sensor adjacent each said detector set for sensing the local temperature adjacent each detector set.

22. A linear motor according to any one of claims 1 to 21, wherein said magnetic field strength detectors are Hall effect transducers.

23. A linear motor according to any one of claims 1 to 22, wherein said stator comprises permanent magnets longitudinally arranged in a .. -NS-SN-NS-... configuration.