Improvements in or relating to the control of linear motors

Abstract: A linear motor comprises a stator 1 and an armature 2 provided, respectively, with a set of spaced apart permanent magnets and a set of drive coils (not shown). The permanent magnets produce a radial magnetic field whose strength varies periodically along the length of the stator 1. The field strength is to derive speed and position signals by means of detectors constituted by field coils 4 and Hall Effect sensors 10. The periodic variation of the field strength is compensated for in the case of the field coils 4 by staggering their phases along the length of the armature and selectively combining their outputs; in the case of the Hall Effect sensors, the periodical variation is used to derive positional information.

Description: The following invention relates to closed loop servo control techniques and control circuit therefor for a linear motor as described in my granted patent GB-B-207906.8. Linear motors generally comprise a stationary linear member (the stator) extending over the length to be travelled, and a moving member (the armature) for extending force against the stationary member in the direction of movement along it. The moving member carries, or is coupled, to the payload to be moved by it. The linear motor incorporated by way of reference in this specification comprises an armature and a stator movable relative to one another along a lengthwise axis of the motor and each having a plurality of magnetic flux generators, which are each coaxial with said axis, each plurality providing a respective sequence of north and south poles along the length of the axis, the generators of one sequence being mutually axially spaced to provide gaps therebetween and the generators of the other sequence comprising at least two independently energisable coils as the flux generators thereof and means to enable the energisation of said coils, the pole pitches of the stator and the armature being different from one another so that, when one such coil overlies a flux generator of the first sequence, another, independently energisable, coil of the second sequence overlies the gap between an adjacent flux pair of generators of the first sequence and vice versa whereby with appropriate energisation of said coils by said means the difference in pole pitch of the armature and stator can result, in use, in a net thrust of the armature relative to the stator in a desired direction at any location within said range of relative movement. Preferably one of the sequences is relatively long and spanning the range of desired relative axial movement of the armature and stator and the other is relatively short. In general, to effect servo control over such a motor, it has previously been necessary to employ a form of feedback transducer, external to the linear motor, to provide positional information. Additionally, a velocity transducer may also be employed to provide velocity information, where such information cannot be adequately provided by the position transducer against a given time-base. A typical example of transducers used for these purposes comprises the combination of a travelling reading head passing a stationary linear "optical grating" extending over the length to be travelled. Such devices tend however to be vulnerable and can be difficult to accommodate within many potential applications for the linear motor, especially in confined spaces of hostile environments. It is known in the prior art to derive velocity information regarding the relative movement of the first and second members by using coils which move with the second member and which have emfs induced in them as they move through the fields established by the flux generators of the first member. An example of this, corresponding to the pre-characterising part of claim 1 is shown in Patents Abstracts of Japan, vol.8, no.243 which is an abstract of JP-A-59122359. A problem with such coils, which the present invention seeks to reduce or overcome, is that the profile of the emf/relative speed characteristic along the direction of relative movement of the first and second members is not flat, but exhibits ripples corresponding to the periodicity of the flux generators of the first member. According to the present invention, there is provided a linear motor comprising first and second members movable relative to one another, by interaction of respective sets of magnetic flux generators with which they are provided, the flux generators of the second member being coils, and a field-sensing coil arrangement moveable with the second member for providing an emf substantially proportional to the relative speed of the first and second members to control circuitry for regulating the speed of relative movement of the first and second members by control of the energisation of said flux generator coils, the field-sensing coil arrangement comprising a plurality of coils electrically separate from the flux generator coils of the second member distributed along the length of the second member such that in any arbitrary position of the two members within said range of relative movement at least one such field-sensing coil is capable of producing an emf substantially proportional to the relative speed of the first and second members, regardless of an output of the other field-sensing coil or coils characterised in that the number of turns per unit length of each field-sensing coil is varied along its length, by having fewer turns per unit length in its central region than at its ends, so as to flatten the profile, over said range of relative movement, of the emf/relative speed characteristic of each field-sensing coil. In particular, a number (two or more) of the field coils can be positioned lengthwise of the direction of travel of the armature and stator such that the waveforms which they produce are phase shifted relative to one another. The shapes of the waveforms will in general be similar to one another since the pattern and density of flux lines cut by each coil at a given relative position of armature and stator will be the same. Consideration of the shapes of the waveforms (which will vary depending upon the pole configuration of the flux generators producing them) enables a velocity-representing signal to be derived from selected parts of the waveforms (or of combinations of them) which has a good degree of linearity (of velocity signal versus armature position). The appropriate points to switch between field coil waveforms (or combinations of them) can be determined empirically and the switching points can be established by monitoring the amplitudes and rates of change (including sign) of the signals. The field coils may comprise search coils surrounding the stator of the motor in such manner that as they pass through radial magnetic fields provided by the stator, emfs are generated directly within the coils. Alternatively, the search coils may be provided in the form of simple coils wound on for example a ferrous former, and located to be moved alongside the stator in such manner that the radial fields emanated therefrom generate emfs within the coils. In either case, the length of the coils used, and the disposition of their actual windings, is optimised to achieve an induced emf characteristic which is as linear as possible over their operative region, within the magnetic fields, provided by the stator. According to a subsidiary feature of the present invention, apparatus for providing primarily positional information on the movement of a linear motor of this invention comprises a plurality of magnetic field strength detectors disposed in the direction of travel, their disposition being such that in combination, both directional and positional information can be determined from the outputs from the detectors and at least one of the detectors is always capable of providing positional information, regardless of the output of its neighbour(s). The field detectors utilised may be the well known form of magnetic field detector known as the "Hall Effect" detector. These detectors, in conjunction with suitable circuitry, can provide voltages substantially proportional to field strength. Examination by control circuitry and software of the voltages provided by the detectors thereby provides directly positional information. Note, if compared against a time base, velocity information can also be derived, the resolution being dependent on the sampling rate. As with the field coils if the field strength detectors differ only in their positions lengthwise of the direction of relative movement of the armature and stator, their output waveforms will be similar in amplitude and shape, but will be phase-shifted relative to one another in the cyclically repeating flux pattern which the drive coils experience. Parts of the respective field strength detector output waveforms (or of algebraic combinations of them) can be selected, by use of suitable monitoring circuitry, to derive a digitisable ramp which cyclically repeats during relative movement of the armature and stator. The current value of this ramp may actually be a count held in a counter or a variable in a software procedure or function. The digitised ramp value indicates uniquely where in the current ramp cycle the relative position of the armature and stator falls. This does not, of course, in itself uniquely identify the relative armature/stator position, because the same ramp value will be obtained at the equivalent relative positions in other ramp cycles. However, by initialising the motor (by driving it to a known reference position) at the start of operation, the subsequent ramp cycles can be counted (as the relative armature/stator position traverses them) so that both the current ramp cycle (which corresponds to a "course" position value), and the position within the cycle (the "fine" position) can be determined and hence the relative position can be uniquely identified. The fact that the wave form used is a ramp also provides a means by which the direction of change of the relative position can be determined, by comparison of successive sample values. Within a single ramp cycle, of course, there is no ambiguity since an increase in the sample value (as compared with the immediately previous one) corresponds with a change in position in the direction "up" the ramp; likewise a decrease corresponds to a change in the direction "down " the ramp. The nature of a ramp, namely that successive points on it increase linearity from one linearity value until another linearity value, provides a means of identifying the direction of travel at a transition between one ramp cycle and an adjacent one. This derives simply from the fact that a ramp is not left/right symmetrical. Thus, at a transition between ramp cycles, if the new value is less than the preceding one, this signifies movement in one direction, whereas if the new value is greater than the previous one, this signifies movement in the other direction. It is important to note that it is not essential to this feature of the invention that the outputs of the detectors (or a combination of them) is, in itself, linear, provided that it can be mapped into a linear sequence of position values within each ramp cycle (eg, by means of a look-up table to correct for non-linearities). Rather than combining the detector waveforms to achieve a cyclically repeating ramp, the waveforms themselves can be analysed directly to derive positional and directional information. In this case, the waveforms are each digitised directly for comparison against a range of predetermined digital preferred values. When a digitised waveform reaches one of these values, a count signal is generated. However, the physical disposition of the detectors is so arranged, and the preferred values so chosen that no waveform can generate a count signal at the same time as count signals generated by at least one of the other waveforms. By this means, a succession of count signals are generated by the waveforms, to be added to or subtracted from a counter or variable in a software procedure or function. On account of the fact that successive count signals are derived from alternate waveforms, the direction of movement of the armature can be ascertained, to effect addition/subtraction from the counter as necessary. The processing of the field coil or detector outputs can be performed by hardware or in software, after digitising the outputs. The invention will now be described by way of reference to the accompanying diagrams in which: Figures 1a and b show first and second versions of linear motor with detector field coils disposed on the armature, which may be adapted, as described below, so as to embody the invention; Figures 2a and 2b show wave forms generated by the field coils; Figure 3 shows in detail, winding arrangements of the field coils in an embodiment of the invention; Figure 4 illustrates switching circuitry for obtaining a consistent velocity signal form the combination of field coils; Figure 5 shows the disposition of Hall Effect detectors for obtaining positional information; Figures 6a to 6c show waveforms provided by the Hall Effect detectors, and successive waveforms obtained therefrom for providing positional information. Referring to Figure 1, a linear motor comprises a stator 1 extending over the length to be travelled, incorporating permanent magnets for providing radial fields. An armature, 2, houses drive coils 11A, 11B for providing magnetic fields to interact with the radial fields provided of the stator, so producing thrust in the direction of travel. Extending from the armature housing 1 is a servo component detection pod 3. Incorporated within the pod are detector field coils 4 surrounding the stator; unlike the coils 11A, 11B, the coils 4 are not energised, but instead are used to produce velocity-indicating emfs, as follows: As the armature travels along the stator, currents are generated within the field coils by the radial magnetic fields emanating from the stator. The pattern of these fields is shown in figure 2a. (The faster the travel, the greater the induced emf). It will be noted however that at various points, the emf provided by any one coil crosses zero and cannot therefore be used to provide velocity information. However, the coils are so disposed relative to one another that when one coil crosses zero, the other is producing emf at its maxima. By suitable processing of the waveforms a continuous profile can be obtained as shown by the dark outline in figure 2b. The mean of the profile indicates the speed of the armature and can thus be used by servo circuitry to control the same. An alternative form of search coil is shown in figure lb. These are shown at 5 within the pod 3, and comprise coils wound on a ferromagnetic former. In this case, emfs are generated as the radial fields cut the coils on the side of the coils nearest the stator, producing substantially the same effect the field coils show at 4. Note that, as illustrated, the motors of Figures 1a and 1b do not embody the invention, though as will become apparent from the following, they may be adapted so that they do. In practice, it is desirable to obtain a flat velocity profile. The approximately sinusoidal waveforms of figure 2b can result in "ripple" around the mean. To overcome this, in accordance with the invention, the actual number of turns per unit length along the field coils is so varied (see figure 3) as to obtain as near a flat profile as possible in the operative region, resulting in a virtually linear velocity signal. Note, more than two coils can be employed to optimise this process. An important aspect of the invention concerns mutual inductive coupling interference form the main coils of the actual armature. "Switching spikes" and other noise may interfere with the low-signal "velocity emfs" being induced in the field coils. To overcome this, a secondary coil (shown at 6) is wound around the field coils, but is connected to the main flux-generating coil circuitry. The connection and direction of winding is such as to induce a current to exactly counteract that produced by the parasitic mutual inductance coupling from the main coils, thereby leaving a substantially unaffected emf. Figure 4 illustrates the waveforms A, B produced by two sensing coils 4 phase shifted 90° relative to one another and, in its lower part, circuitry for selectively combining them to produce the final, velocity-representing signal E. The waveform C is derived by summing A and B, while waveform D is derived by reversing the polarity of B and summing the result with A. These waveforms can thus be derived with simple, well known operational amplifier circuits. The resulting waveforms are then squared by squaring circuits to derive the control input signals to two electronic change over switches 20A, 20B, which receive at their respective pairs of inputs the signal A and a signal A which is A reversed in polarity and signals B and B similarly. The outputs of the switches 20A, B are added by means of op-amp 21. Referring now to figure 5, two magnetic field detectors are shown at 10. These detectors which may be of the "Hall" effect type, produce in combination with suitable circuitry, voltage waveforms as shown in figure 6(a), proportional to the field strength. By suitable electronic adding and switching of these waveforms, shown schematically in figure 6a, a series of near straight line waveforms may be achieved, as shown in figure 6b. These waveforms repeat sequentially at the same known pitch of the fields emanating from the stator of the motor. By feeding the voltages to an analogue to digital converter, a series of digital values can be obtained which so indicates the physical position of the armature relative to the stator. It will be noted that the values increase travelling from left to right up each waveform. From this "direction" information can be obtained. Additive counting of the values therefore provides, from left to right, a count representing the absolute position of the motor. From right to left, the values can be seen to decrease. Subtractive counting is then used to decrement the count accordingly. A positional count is thereby achieved for servo control of the motor. Referring to figure 6c, in an alternative method of obtaining count information, the waveforms themselves are digitised to provide a number of count signals. The digitised values are so chosen to ensure no count signal from one waveform can arrive at the same time as that generated by its neighbour. The signals are accumulated in a count register, addition or subtraction being determined by a network examining the sequence of arrival of the count signals and therefore the direction of travel of the armature. Numerous variations of the above within the scope of the invention will be apparent.

Claims: A linear motor comprising first (1) and second (2) members movable relative to one another, by interaction of respective sets of magnetic flux generators with which they are provided, the flux generators of the second member being coils (11A,11B), and a field-sensing coil (4;5) arrangement moveable with the second member (2) for providing an emf substantially proportional to the relative speed of the first and second members to control circuitry for regulating the speed of relative movement of the first and second members by control of the energisation of said flux generator coils, the field-sensing coil arrangement comprising a plurality of coils (4;5) electrically separate from the flux generator coils (11A,11B) of the second member (2) distributed along the length of the second member such that in any arbitrary position of the two members (1,2) within their range of relative movement at least one such field-sensing coil (4;5) is capable of producing an emf substantially proportional to the relative speed of the first and second members, regardless of the output of the other field-sensing coil or coils (4;5) characterised in that the number of turns per unit length of each field-sensing coil (4;5) is varied along its length, by having fewer turns per unit length in its central region than at its ends, so as to flatten the profile, over said range of relative movement, of the emf/relative speed characteristic of each field-sensing coil. A linear motor according to claim 1, wherein the magnetic field strength of the field generated by the flux generators of the first member as sensed by each field-sensing coil (4;5) varies periodically with displacement along said first member and two or more of the field-sensing coils are positioned lengthwise of the direction of said relative movement of the first and second members such that the magnetic field strengths to which they are subject and thus the emfs which they produce are phase shifted relative to one another and at any arbitrary relative position the emf of at least one of the field-sensing coils (4;5) is within a prescribed range of a turning point of its magnitude. A linear motor according to claim 2 wherein there is associated with the field-sensing coils a commutator circuit (20A,20B) for selectively combining said emfs such that at any arbitrary relative position of the first and second members, the resulting signal varies substantially linearly with relative speed and is substantially independent of the relative positions of the first and second members. A linear motor according to claim 1, 2, or 3, wherein the field-sensing coils (4;5) surround the first member in such a manner that as they pass through radial magnetic fields provided by the first member, emfs are generated directly within the field-sensing coils. A linear motor according to any one of claim 1 to 4 wherein a secondary coil (6), connected to one of the flux generator coils (11A,11B), is wound around each of the field-sensing coils (4;5) in such a direction as to induce a current to compensate for that produced by any parasitic mutual inductance coupling from the magnetic flux generators of the second member. A linear motor according to any one of the preceding claims and including a plurality of magnetic field strength detectors (10A, 10B) movable with the second member (2), the disposition of the magnetic field strength detectors (10A,10B) being such that in any arbitrary position of the two members within their range of relative movement, both directional and positional information can be derived from the outputs from the detectors and at least one of the detectors is always capable of providing directional and positional information, regardless of the output of the remaining detector or detectors. A linear motor according to claim 6, wherein two or more of the magnetic field strength detectors (10A,10B) are positioned lengthwise of the direction of said relative movement of the first and second members such that the emfs which they produce are phase shifted relative to one another. A linear motor according to claim 6 or 7 wherein said magnetic field strength detectors are "Hall Effect" detectors. A linear motor according to claim 6, 7 or 8 further comprising a monitoring circuit and a counter and wherein parts of the respective detector outputs may be selected to derive a digitisable ramp which cyclically repeats during said relative movement so that the ramp value indicates uniquely the relative position of the first and second members within one ramp cycle and wherein at the start of operation the motor may be driven to a known relative position such that upon said relative movement, subsequent ramp cycles may be counted by said counter to give a coarse position value. A linear motor according to claim 9 further comprising means to compare successive values of said ramp whereby the direction of said relative movement is indicated by the result of said comparison. A linear motor according to claims 6, 7 or 8 further comprising a circuit wherein said respective detector outputs are digitised such that when one of said outputs reaches a predetermined value a count signal is generated, the digitisation and the disposition of the detectors (10A,10B) being such that no count signal from one detector (10A,10B) can be generated simultaneously with a count signal of the remaining detector or detectors (10A,10B).

Inventors: KELLY HUGH-PETER GRANVILLE, KELLY, HUGH-PETER GRANVILLE
Applicants: LINEAR DRIVES LTD, LINEAR DRIVES LIMITED
Application year: 1990
Main classification: H02K41
IPC codes: H02K11, H02P25, G01P3, H02K7, H02K41