GB2273166A - Linear motor with position and direction of motion sensor. - Google Patents

Abstract

A linear motor comprises a stator 1 producing a radial magnetic field which varies in the axial direction and an armature 2 which houses drive coils and at least two Hall effect magnetic field detectors 10, A, B,. The detectors are located so that, in any position of the armature, both directional and positional information can be derived from the detector outputs, and, at least one of the detectors can independently provide directional and positional information. The detectors are preferably positioned so that the outputs are phase shifted relative to one another. The outputs may be connected to a monitoring circuit which provides an output which is a series of cyclically repeating ramps during relative movement of stator and armature. At the start of operation, the motor may be driven to a known position so that subsequent ramp cycles can be counted to give a coarse position. <IMAGE>

Description

IMPROVEMENTS IN OR RELATING TO THE CONTROL OF LINEAR MOTORS The following invention relates to closed loop servo control techniques and control circuitry therefore for a linear motor as described by my granted patent GB-B-207906.8, the disclosure of which is incorporated herein by reference.

Linear motors generally comprise a stationary linear member (the stator) extending over the length to be travelled, and a moving member (the armature) for exerting 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 said axis, the generators of one sequence being mutually axially spaced to provide gaps therebeta7een and the generators of the other sequence comprising at least two independently energisarle 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 timebase. 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.

According to the present invention, there is provided a linear motor comprising first and second members moveable one relative to the other by the interaction of respective sets of magnetic flux generators, the flux generators of the second member being coils, a plurality of magnetic field strength detectors moveable with the second member, the disposition of the magnetic field strength detectors 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.

Two or more of the detectors may be 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.

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.

If the 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.

In a first embodiment of the invention, parts of the respective 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 linearily 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 cf 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).

In a second embodiment of the invention, 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 strength 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 la and ib show first and second versions of linear motor, which although not themselves embodying the invention, will be used in explaining it; Figures 2a and 2b show wave forms generated by the field coils in Figures la and lb; Figure 3 shows in detail, winding arrangements of the field coils; Figure 4 illustrates switching circuitry for obtaining a consistent velocity signal form the combination of field coils; Figure 5 shows a linear motor corresponding to Figures la and ib, modified so as to be an embodiment of the invention, and illustrates 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 llA, llB 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 llA, 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.

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, 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 the profile shown in c, and therefore 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 field coil (shown at 6) is wound around the field coils, but is connected to the main field 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, 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, which relates to an embodiment of the invention, 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.

Attention is directed to my copending application no. 9019235.2 (Publication no. GB 2235783) from which the present application was divided and which describes and claims a linear motor having field sensing coils as described above.

Claims (7)

1. A linear motor comprising first and second members moveable one relative to the other by the interaction of respective sets of magnetic flux generators, the flux generators of the second member being coils, a plurality of magnetic field strength detectors moveable with the second member, the disposition of the magnetic field strength detectors 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.

2. A linear motor according to claim 1 wherein two or more of the magnetic field strength detectors 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.

3. A linear motor according to claim 1 or 2 wherein said magnetic field strength detectors are "Hall Effect" detectors.

4. A linear motor according to claim 1, 2 or 3 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.

5. A linear motor according to claim 4 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.

6. A linear motor according to claims 1, 2 or 3 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 being such that no count signal from one detector can be generated simultaneously with a count signal of the remaining detector or detectors.

7. A linear motor according to any one of the preceding claims wherein the outputs of the field detectors when combined provide directly coarse positioning information and by means of further processing, fine positioning information.