LINEAR MOTOR AND CONTROL METHOD THEREOF

Abstract: A linear motor is disclosed, which comprises a first magnetic pole (2a, 2b, 2c) having a plurality of alternatively protruded and recessed magnetic pole teeth of magnetic material arranged longitudinally at predetermined regular intervals of length, and a second magnetic pole (3a, 3b, 3c) having a plurality of alternatively protruded and recessed magnetic pole teeth of magnetic material arranged in opposition to the first magnetic pole with a very small gap with the first magnetic pole. The ratio of the numbers of the magnetic pole teeth providing the protruded opposed parts of the first and second magnetic poles is 2/3 or a multiple thereof.

Description: BACKGROUND OF THE INVENTION FIELD OF THE INVENTION The present invention relates to a linear motor, or more in particular, to a linear motor in which the ratio of teeth between the rotor and stator is fixed thereby to reduce the thrust pulsation for driving the rotor, thus making proper operation possible. DESCRIPTION OF THE PRIOR ART Conventional rotary-type motors, for example, have teeth of the magnetic poles thereof constructed as disclosed in the specification of French Patent No. 1452379. In such a construction, the protrusions and the recesses of the stator are arranged equidistantly, and each protrusion of the rotor teeth has the same length as the stator protrusion while the rotor recess is one half of the protrusion thereof in length. For moving the rotor, therefore, at least four rotor teeth (protrusions) are required. With an increase in the number of teeth, the frequency increases twice for the motors having the protrusions of the same pitch, resulting in an increased switching loss for control and increased noises. In the presence of a thrust pulsation of this type of linear motor having teeth in the stator and rotor, the controllability is greatly deteriorated for achieving a predetermined speed or positioning as well as for speed control. The thrust generated in the motor is determined by an induced voltage and the pattern and phase of the current supplied. The current pattern supplied has so far not been developed. In the linear induction motors, for example, the source voltage is applied directly, and there has been no cases describing the patterns of the current supplied. On the other hand, a linear motor having a position detecting device is disclosed in Japanese Patent Unexamined Publication No. 160874/82. This linear motor comprises a rotary pulse generator mounted on the rotor thereof in order to detect the position of the rotor or the speed thereof. As a result, the wiring from the rotor is connected to a control circuit, thus giving no consideration to the movement over a long distance. In the encoder of contact type, by contrast, any attention is not paid to the high-speed movement either. SUMMARY OF THE INVENTION It is an object of the present invention to provide a linear motor in which the movement of the moving member can be controlled smoothly whereby said linear motor provides a uniform thrust regardless of the position of its moving member. This object is achieved by the present invention by the features recited in claim 1. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a sectional view schematically showing the side of a linear motor parallel to the running direction thereof according to an embodiment of the present invention. Fig. 2 is a front view of the side thereof perpendicular to the running direction thereof. Fig.2A shows a sectional view of Fig. 2 Figs. 3 and 4 are sectional views schematically showing the sides parallel to the running direction thereof similar to the foregoing drawings. Figs. 5(A) and 5(B) are graphs showing the curves of magnetic fluxes, induced voltage and thrust thereof. Fig. 6 is a graph showing the curve of the induced voltage generated in the three-phase coil. Fig. 7 is a diagram showing the thrust generated in the motor. Fig. 8 is a sectional view schematically showing the side of the linear motor parallel to the running direction thereof according to another embodiment of the present invention. Fig. 9 is a sectional view schematically showing the side of a comparable linear motor parallel to the running direction thereof. Fig. 10 is a sectional view schematically showing the side of a linear motor parallel to the running direction thereof according to a further embodiment of the present invention. Fig. 11 is a graph showing curves for explaining the voltage induced in the three-phase coil. Fig. 12 is a diagram showing a general configuration of a control circuit used in the control method according to the present invention. Fig. 13 is a graph showing curves of magnetic fluxes, induced voltage and thrust generated therein. Fig. 14 shows waveforms of voltages induced in the three-phase coil. Fig. 15 is a diagram showing the thrust generated in the three-phase coil. Fig. 16 to Fig. 18 are graphs showing the thrust curves in the case where the phase difference is ▵x against the induced voltage. Fig. 19(A) is a side view schematically showing the side of a linear motor parallel to the running direction thereof according to an embodiment of the present invention. Fig. 19(B) is an enlarged view of the part designated by α of Fig. 19(A). Fig. 20 is a block diagram of the control circuit thereof. Fig. 21(A) shows a layout of the parts. Fig. 21(B) shows waveforms switched. Fig. 22 is a block diagram of a position correction circuit thereof. Fig. 23 is a signal timing chart for switching the encoder. DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the linear motor according to an embodiment of the present invention will be explained below with reference to the accompanying drawings. A three-phase linear motor is used in this embodiment. First, in Fig; 1 and Fig. 2, reference numeral 1 designates a yoke of a moving member, numeral 2 a moving member corresponding to a rotor, numerals 2a to 2c magnetic pole teeth, numeral 3 a stator, numerals 3a to 3c, 3m and 3n stator teeth, numeral 4 a coil assembly, numerals 4a to 4c coils, numeral 5 a truck, numeral 6 a wheel, numeral 7 a yoke, numeral 8 a magnet, and numeral 9 a base. Specifically, the moving member 2 has a plurality of protruded magnetic teeth 2a, 2b and recessed magnetic teeth 2c of magnetic material at predetermined regular intervals of length along the longitudinal direction on the yoke 1 of the moving member. With a very small gap with the magnetic pole teeth 2a, 2b mentioned above, a plurality of magnetic pole teeth of magnetic material including protruded stator teeth 3a to 3c, and recessed stator teeth 3m, 3n of the stator 3 are opposedly arranged thereto. Each of the stator teeth 3a to 3c has the length of P₁ as shown. In the shown embodiment, the length 2P₁ represents an electrical angle of 360°. The coils 4a, 4b and 4c of the coil assembly 4 are wound on the stator teeth 3a,3b and 3c respectively. The stator teeth 3m, 3n are arranged along the length of about P₁/3 between the stator teeth 3a and 3b and between 3b and 3c. The coils 4a, 4b and 4c are connected with each other in y - connection. The moving member 2 makes up a first magnetic pole and the stator 3 a second magnetic pole. Magnets 8 are arranged with the poles thereof of the same polarity opposed to each other with the teeth of the stator 3 on the side ends of the stator teeth 3a to 3c. The side of the magnets 8 of the other polarity is mounted on the yoke 7. The truck 5 has mounted thereon the moving member yoke 1 and the moving member 2 with the magnetic pole teeth 2a, 2b, 2c, and is spatially supported through the wheel 6 between the stator 3 and the yoke 7, the truck 5 being contacted only with the wheel 6. The wheel 6, on the other hand, is adapted to roll or slide on the base 9. This configuration enables the magnetic fluxes of the magnets 8 to pass through a magnetic path shown by a dashed line in Fig. 2. The operation of the system configured as above will be explained below with reference to Figs 3 to 7. Fig. 3 shows the magnetic pole tooth 2a and the stator tooth 3a laid one over the other, representing the case in which the magnetic reluctance shown by the dashed line in Fig. 2 is minimum. Fig. 4 shows the case in which the magnetic poles are positioned at the position of maximum magnetic reluctance, and the stator tooth 3a is located between the magnetic pole teeth 2a and 2b. Namely the magnetic pole tooth 2c and the stator tooth 3a are overlapping each other. As described above, since the magnetic reluctance of the magnetic circuit is changed by moving the magnetic pole teeth 2a and 2b arranged on the yoke one of the moving member along the stator tooth 3a, the magnetic fluxes 10a passing through the stator 3a is changed in the manner shown in Fig. 5(A). With the change in the magnetic fluxes 10a, an induced voltage 11a is generated in the coil 4a. The thrust of the motor is determined by the product of the induced voltage 11a and the current supplied upon energization of the coil 4a. Such a thrust is produced in positive direction when the current 12a is supplied in the same form and same phase as the waveform of the induced voltage 11a. The thrust in the opposite direction is produced, by contrast, when the phase is opposite. Numeral 13a in Fig. 5(B) represents a curve of the thrust line (the thrust curve generated in a phase) caused in the former case where the phase is the same. The thrust line 13a is generated in the coil 4a as shown in Fig. 3 and 4, while the induced voltages 11a, 11b, 11c are generated as shown in Fig. 6 in the coils 4b, 4c arranged P₁ + 1/3P₁ displaced in Fig. 1, or at points displaced by 240° electrical angle respectively. If the current of the same phase with respect to the induced voltage is supplied to the coil by detecting the position of the magnetic pole teeth and stator teeth in each phase of the three induced voltages 11a to 11c, a fixed thrust line 14 as shown in Fig. 7 can be obtained. According to the embodiment under consideration, as explained above with reference to Fig. 1, two magnetic pole teeth including 2a and 2b relating to the protruded poles of the protruded parts opposed between the moving member 2. and the stator 3, and three stator teeth including 3a to 3c are involved. That is to say, the magnetic pole teeth arranged on the yoke 1 of the moving member are two. As a result, an induced voltage having three phases of 2 π/3 is capable of being generated thereby to produce a thrust with low ripples on the one hand, and a uniform thrust is produced at any place for an improved controllability. Now, a sectional view schematically showing the side of a linear motor parallel to the running direction thereof according to another embodiment is shown in Fig. 8. In Fig. 8, the same reference numerals as those in Fig. 1 designate the same component elements as in Fig. 1 respectively. Numerals 2d, 2e designate magnetic pole teeth relating to the recessed parts, and numeral 1-1 the yoke of the moving member. Specifically, in the same diagram, the magnetic pole teeth 2d, 2e and as long as P₁ are arranged on the right and left on the yoke 1 shown in Fig. 1, making up the yoke 1-1. Since the magnetic pole teeth thus arranged do not substantially affect the changes in magnetic fluxes, almost the same waveform of an induced voltage is obtained as in Fig. 6 even when the yoke 1-1 shown is moved, thus producing substantially the same characteristics as the linear motor shown in Fig. 1. In contrast with the embodiments mentioned above, if the number of the magnetic pole teeth 2a, 2b, 2c protruded on the yoke 1-1 is a multiple of three instead of two, the magnetic pole teeth 2a and 2m would be overload on the stators 3a and 3d of the same phase. Now, assume that the stator tooth 3a is U phase, 3b the V phase, and 3c the W phase. The stator tooth 3d is U phase, the stator tooth 3e the V phase, and the stator tooth 3f the W phase. In the case of Fig. 9 where the coils 4a and 4d corresponding to U phase cross the magnetic fluxes, a voltage about twice as large is induced in the phases of the coils 4a, 4d of the stator teeth 3a, 3d overlaid with the magnetic pole teeth 2a, 2m as shown in Fig. 11. In this way, in the case of a moving member having three magnetic pole teeth relating to protruded poles, the induced voltages 21a to 21c shown in Fig. 11 generated in the coils of respective phases represent a fixed magnitude, but undergo a change, and the resultant phase displacement greatly deteriorates the controllability. As explained above, if the number of the magnetic pole teeth providing protrusions is three as shown in Fig. 9, therefore, it is impossible to obtain a motor of high controllability. Fig. 10 is a sectional view schematically showing the side of a linear motor parallel to the running direction thereof according to a further embodiment of the invention. Fig. 10, the same reference numerals as in Fig. 1 denote the identical component parts to those in Fig. 1. Numerals 2m, 2n designate protruded magnetic pole teeth, 3d to 3f protruded stator teeth, numerals 4d to 4f coils, and numeral 1-2 a yoke of a moving member. Specifically, in the embodiment under consideration, the number of the magnetic pole teeth is twice of the number of the magnetic pole teeth shown in Fig. 1. In the case of the present embodiment, two yokes 1 as shown in Fig. 1 are connected in series with each other through a recessed portion. The induced voltage takes the form shown in Fig. 6. There is no magnetic saturation of the yoke 1-2, magnetic pole teeth 2a, 2b, 2m, 2n, the stator teeth 3a to 3f and the part equivalent to the yoke 1 in Fig. 1 making up the magnetic circuit. The magnitude of the induced voltage is about twice as that of the induced voltage shown in Fig. 6. As explained above, predetermined magnitude of induced voltages of three phases displaced with 2 π/3 can be obtained by making the number of the moving member magnetic teeth to be a multiple of two (2, 4, 8...) and the fixed thrust line 14 shown in Fig. 7 can be obtained. The induced voltage was illustrated in the form of a sinusoidal wave in Fig. 5. This may be replaced by a trapezoidal or stepped form depending on the size of the gap between the moving member and stator and the width of the magnetic pole teeth or the like, in which case it is also possible to obtain a fixed thrust line. In the embodiments explained above, the first magnetic pole was used as a moving member, and the second magnetic pole as a stator. Instead, the first magnetic pole may be used as a stator and the second magnetic pole as a moving member with the equal effect. In Fig. 1, the phases of adjacent coils are arranged to conform to the equation shown below. In equation (1) above, m is the number of phases of the motor. In the case of three phases, for instance, P₂ may be 4/3 P₁. Fig. 12 shows a control circuit relating to the configuration described above for achieving the second object of the invention. Specifically, the coils 4a to 4c of the coil assembly 4 are connected to an inverter. The voltages induced in the coils are applied to an induced voltage decision circuit and the current flowing through the coils is applied to the induced voltage decision circuit. A current pattern switching signal produced from this circuit is applied to a current ROM relating to a selective current pattern supply curcuit, and then to the inverter in accordance with the result of the operation thereof. The above-mentioned current ROM is impressed with an output from the encoder relating to the position detector facing the moving member 2. The sign, trapezoidal and stepped wave forms are previously stored in the current ROM. Namely the signals from the encoder are pulses, which are proportional to the moved distance of the moving member and update the address of the current ROM, so that the current flows in the coil responsive to the magnetic pole position of the moving member. The control operation of the circuit having the above-mentioned configuration will be explained with reference to Figs. 12 to 18. Fig. 3 shows the case in which the magnetic pole tooth 2a and the stator tooth 3a are overlaid one on another and the magnetic reluctance of the magnetic circuit shown by the dashed line in Fig. 2 is minimum, and Fig. 4 the case in which a stator tooth 3a is located between the magnetic pole teeth 2a and 2b at the position of the magnetic poles associated with the maximum magnetic reluctance of the magnetic circuit. As explained above, while the magnetic pole teeth 2a, 2b arranged on the yoke 1 move on the stator tooth 3a, the magnetic fluxes 10a change in the manner shown in Fig. 13, thereby generating an induced voltage 11a in the coil 4a. The thrust of the motor is determined by the product of the induced voltage 11a and the current supplied to the coil 4a. In the case of Fig. 13, the induced voltage 11a takes a stepped protruded form. By the control circuit shown in Fig. 12, therefore, the current thus supplied is rendered like a supplied current 12a to reduce the phase difference to zero, thereby supplying a trapezoidal form of current, with the result that a positive trapezoidal waveform of thrust can be obtained. Also, the opposite thrust may be produced if the current of the opposite phase is supplied. Numeral 13a in Fig. 13 designates a thrust curve generated in one coil when the phase difference is zero. This thrust curve 13a is generated in the coil 4a shown in Figs. 3 and 4, while the induced voltages 11a, 11b, 11c shown in Fig. 14 are generated in the coils 4b, 4c arranged displaced by P₁ + 1/3 P₁ in Fig. 1, that is, by 240 degree of electrical angle. If the phases of the three induced voltages 11a to 11c are supplied with a trapezoidal current of the sage phase by detecting the positions of the magnetic pole teeth 2a, 2b and the stator teeth 3a to 3c, a thrust curve 14 with small ripples shown in Fig. 15 is obtained. According to this embodiment, as explained with reference to Fig. 1 , the number of the magnetic pole teeth arranged on the yoke 1 may be two or a multiple of two. According to this embodiment, a three-phase induced voltage having a phase difference of 2π/3 is supplied with a trapezoidal current of the same phase, thereby producing a thrust with small ripples on the one hand, and almost uniform thrust is obtained regardless of the place it is used, thereby producing a control effect for a motor high in controllability and low thrust ripples. The foregoing description concerns a waveform of the induced voltage of the motor having stepped protrusions and the supplied current having a trapezoidal waveform. In the opposite case where the induced voltage has a trapezoidal waveform and the supplied current stepped protrusions, a similar effect is obtained. The embodiments above have been explained with reference to the case in which the phase difference is zero between the induced voltage generated in the coils and the supplied current. In an actual control operation in combination with a control circuit, however, the delay time of the control circuit or the displacement of the position detector make it difficult to effect energization ideally with zero phase difference. Also, in the case of the stepped induced voltage, if the current 12a-1 of the same waveform as the induced voltage 11a is supplied with the phase difference ▵x as shown in Fig. 16, the thrust generated in the three-phase coils is indicated by the thrust curve 14A. As seen from this curve, a sag of thrust occurs by the width of the phase difference ▵x, thereby deteriorating the controllability. Fig. 18 shows the case in which the induced voltage of stepped waveform is supplied with a sinusoidal current 12a-2. When the phase difference ▵x occurs, the thrust as shown by the thrust curve 14B is generated in the three-phase coil, thus causing a pulsation in thrust during the section of the phase difference ▵x. Further, in the case where the induced voltage of this stepped waveform is supplied with a sinusoidal current zero in phase difference, the thrust curve obtained is not linear either, resulting in an inferior controllability. Fig. 17, on the other hand, shows the case in which the induced voltage 11a of stepped waveform is supplied with a trapezoidal current 12a with the phase difference ▵x. In this case, the thrust curve is indicated by 14-1, indicating that of all the cases of the supplied current including those of an induced voltage of sinusoidal wave and the supplied current of other waveforms, the thrust output is obtained with the least thrust pulsation. In summary, if the phase difference is zero between the stepwise protruded induced voltage and the current of the same waveform supplied with such an induced voltage, the thrust is represented by a predetermined linear curve, while if there is any phase difference between them, the thrust pulsation becomes considerable. Also, when a sinusoidal wave is used as a supplied current, the thrust pulsation deteriorates the controllability even when there is no phase difference. In contrast, as explained with reference to Fig. 17, according to the present invention, thrust is subjected to less variations even when there is a phase difference with the supplied current, and the thrust curve is linear also when the phase difference is zero, thereby leading to the best controllability. It will be understood from the foregoing description that a substantially uniform thrust output is obtained at all positions if a current of stepped waveform is supplied with an induced voltage of trapezoidal form, and a current of trapezoidal form with an induced voltage of stepped form generated in the coils. In the above-described embodiments, the first magnetic pole was used as a moving member and the second magnetic pole as a stator. As an alternative, the first magnetic pole may be used as a stator, and the second magnetic pole as a moving member. Now, explanation will be made about the method of position detection of a linear motor to achieve the third object of the invention with reference to Figs. 19A to 23. A general configuration is shown in Fig. 19A. A stator system comprises n sets of teeth 3U₁, 3V₁, 3W₁ to 3U n , 3V n , 3W n , coils 4U₁, 4V₁, 4W₁ to 4U n , 4V n , 4W n , magnetic pole position detectors 105U₁, 105V₁, 105W₁ to 105U n , 105V n , 105W n , and linear encoders LS₁ to LS n with each set including three phases (n being an integer not smaller than unity). Numerals 3, 4,105 in the drawing are a general designation of the teeth, coils and the magnetic pole position detector respectively. The linear encoder sensors LS₁, LS₂ correspond to the position sensors including the encoders for detecting the position of the moving member 2. The magnetic pole position detectors 105U₁ to 5W₁, 105U₂ to 5W₂, on the other hand, double as a switch for the sensors at the same time. The yoke 1 is supported by a support mechanism in such a manner as to maintain a predetermined size of gap in parallel with the teeth 3, and is freely movable. At the same time, the moving member 2 has arranged therewith a magnet 20, and a linear encoder magnetizer 106 is arranged in a place free of the effect of the leakage magnetic fluxes of the magnet 20. This linear encoder magnetizer 106 is arranged also opposedly with a predetermined gap to the linear encoder sensors LS₁ to LS n , so that all the detection elements of the magnetic pole detectors 105 are held with a predetermined gap in parallel to the magnet 20. Explanation will be made of the case where n = 1. The magnet 20 mounted on the moving member yoke 1 has N and S poles thereof arranged at pitches of π alternately, and are magnetized vertically upward of the teeth 3U₁, 3V₁, 3W₁ and the yoke 1. The magnetic pole position detectors 105V₁, 105U₁, 105W₁, on the other hand, are arranged at pitches of 2π/3 at the positions shown in the drawings. The teeth 3U₁ to 3W n are arranged at pitches of 4π/3 with the width of π. Fig. 20 shows a block diagram of the control circuit, with reference to which explanation will be made of the operation of the system having the configuration mentioned above. Coils 4 including 4U₁, 4V₁, 4W₁ for driving the moving member 1 are connected to an inverter 117, and a power supply for driving the inverter 117 is connected to a DC power supply 113. The inverter 117 is also connected with an amplifier 118 for controlling the duty cycle of the current supplied current for the inverter 117 in accordance with the difference between the signal of the current detector 114 and the current command signal of a processor unit 111. The processor unit 111 detects the position of the magnetic pole from the magnet 20 provided on the yoke 1 by means of the magnetic pole position detectors 105V₁, 105U₁, 105W₁, and prepares signals of each phase supplied to the coils 4 through a magnetic pole position distributor 119. The magnetic pole position detector 105 also produces an "H" signal in the absence of magnetic fluxes from the magnet 20 or when the magnetic pole is S in polarity, while it produces an "L" signal only when the magnetic pole is N in polarity. While the linear encoder magnetizer 106 mounted on the yoke 1 moves with the yoke, the output of the linear encoder sensor LS₁ takes a synchronous form such as sinusoidal wave according to the pattern magnetized on the linear encoder magnetizer 106. By shaping the waveform of this signal, output signals "H" and "L" are alternately produced in pulse form. In the case shown in Fig. 20, the output of the linear encoder sensor LS₂ is unestablished in the state of "H" or "L", and does not take the form of any pulse. When the yoke 1 is moved rightward in the drawing, however, the linear encoder sensors LS₁ and LS₂ are both covered by the linear encoder magnetizer 106 so that a pulse output signal is produced from both the linear encoder sensors LS₁ and LS₂. Since the N pole of the magnet 20 mounted on the yoke 1 comes to face the magnetic pole position detector 105V₂, however, the output of the magnetic pole position detector 105V₂ becomes "L", and the output of the linear encoder sensor LS₂ is selected by the linear encoder swtich 110 in accordance with the output of the magnetic pole position distributor 119 which output is applied to the processing unit 111. A command section 112 gives an instruction on the current supplied to the coils 4, and the speed of the moving member 2 is proportional to the current supplied. This instruction is applied as a speed command to the processing unit 111. The flow of operation is briefly explained above. More detailed explanation will be made with reference to Fig. 21A to Fig. 23. Fig. 21A shows the arrangement of the magnet 20 mounted on the yoke 1, the magnetic pole position detector 105, the linear encoder magnetizer 106 and the linear encoder sensor LS. Fig. 21B is a waveform diagram for switching operation, in which the signal waveforms of magnetic pole positions from U₁ to W₂ correspond to the output signals of the magnetic pole position detectors 105U₁ to 105W₂ respectively. On the other hand, the signal waveforms of S₁ and S₂ correspond to the output signals of the linear encoder sensors LS₁ and LS₂. This signal waveform S is prepared by the logic sum of S₁ and S₂, or from the gate circuit of a selector or like. If the yoke 1 of the moving member 2 located at the position shown in Fig. 21A, the magnetic pole position detector 105 becomes "L" at the N pole of the magnet 20 and "H" at the S pole thereof, so that V₁ and W₁ become "L" and "H" respectively at the positions where U₁ changes from "H" to "L". Also, the magnetic pole position detectors 105V₂, 105U₂, 105W₂ which are not affected by the magnetic field of the magnet 20 become "H", and with the rightward movement of the moving member 1, the magnetic pole position signals U₁ to W₂ undergo such changes as shown in the drawing. The encoder output signals S₁ of the linear encoder sensor LS₁, on the other hand, is selected when the logic product of three corresponding magnetic pole position signals U₁ to W₁ produced from the magnetic pole position detectors 105V₁ to 105W₁ is in "L" state, while the output signal S₂ of the encoder is selected when the logic product of the magnetic pole position signals U₂ to W₂ are "L". In the span SP indicated by a one-dot chain, however, since the logical multiplications of the magnetic pole position signals V₁ to W₁ and U₂ to W₂ are "L" levels, the encoder output signals S₁ and S₂ are prevented from being selected at the same time preferential selection of the output of the linear encoder sensor (the encoder output signal S₂ is previously selected this case) located in the way of movement of the moving member 1 or by giving priority depending on the magnitude of the value n. The foregoing description concerns the n value up to 2, and the same applies when n is larger than 2. In the foregoing description, a high accuracy is achieved when the encoder output signal S₁ of the linear encoder sensor LS₁ is synchronous with the encoder output signal S₂ of the linear encoder sensor LS₂. If the phases of the pulse signals of these two encoder output signals S₁ and S₂ are to be synchronized in satisfactory manner, the problem is how to mount the linear encoder sensors with high accuracy. Fig. 22 is a circuit diagram which takes this problem point into consideration, and provides a counter measure by employing the system mentioned below. The configuration of such a system will be explained. An analog output signal A₁ of the linear encoder sensor LS₁ is connected to a comparator CO. The output of this comparator CO is an "L" or "H" logic signal shaped in waveform and is connected as an input to a selector SELECTOR on the one hand and is connected as a clock input of latch circuits LC₁ and LC₂ on the other hand. The analog output signal A₂ of the linear encoder sensor LS₂ is connected to the comparator CO₂, the comparator CO₁ and the A/D converter AD. The output signal of the A/D converter AD is connected with latch circuits LC₁ and LC₂ for converting the analog output signal A₂ of the linear encoder sensor LS₂ into a digital signal. The outputs of the latch circuis LC₁ and LC₂ are connected through the D/A converters DA₁ and DA₂ to the comparators CO₁ and CO₂ respectively. The outputs of the comparators CO₁ and CO₂ connected to the input of the EOR, the output of which is in turn connected as an input to a flip-flop circuit like a J/K flip-flop. The output of the flip-flop circuit FF is connected as the other input to the selector SELEC. The select signal SS for selecting an input of the selector SELEC is produced by switching the pull-up resistor pulled up to the power supply V by a switch T. The function of this select signal SS is equal to the logic product of the magnetic pole position signals U₁ to W₁ and U₂ to W₂ produced from the magnetic position detector 105 mentioned above. The signal S₁ or S₂ is selected by this select signal SS and the selected signal is produced as an output signal SO of the selector SELEC. The operation of this circuit will be explained with reference to Fig. 23. The analog output signals A₁ and A₂ produced from the linear encoder sensors LS₁ and LS₂ take sinusoidal waveforms as shown. In the example shown, the analog output signal A₂ is delayed behind the signal A₁ by π/5, indicating that a mechanical position of the linear encoder sensor LS₂ is displaced from that of LS₁ and also the time when the linear encoder sensors are to be switched. As mentioned above, S₁ and S₂ designate waveform-shaped output signals obtained from the shaping of the analog output of the linear encoder at the zero cross point. The signal S₁ corresponds to the output of the linear encoder sensor LS₁, and the signal S₂ to the output of the linear encoder sensor LS₂. Actually, the signal output necessary when switching from the linear encoder sensor LS₁ to the linear encoder sensor LS₂ requires the correction of the phase displacement from the analog output signals A₁ and A₂ produced from the linear encoder sensor by the switching point P in the presence of a mechanical position displacement of the linear encoder sensor LS₂ against LS₂ as shown by the output signal SO. The signal S₂" shows the absence of phase correction. In this case, the amount of phase displacement is added at time "L" at the switching point P, thus causing an error with the actual position. The phase correction will be explained below. In the case where the analog output signal A₂ is generated in the linear encoder sensor LS₂, the voltage value of the analog output signal A₂ of the linear encoder sensor LS₂ is converted into a digital signal by use of the A/D converter AD shown in Fig. 22 at the leading and trailing edges of the waveform-shaped output signal S₁, so that the value of the trailing edge of the waveform-shaped output signal S₁ is stored in the latch circuit LC₁ and the value of the leading edge thereof in the latch circuit LC₂. At the same time, these values are used as reference values for the comparators (negative inputs in this case) connected with the analog output signal A₂ produced from the linear encoders through the D/A converters DA₁ and DA₂, thereby producing the output signals V₁ and V₂ shown in Fig, 23 at the output terminals of the comparators CO₁ and CO₂ respectively. The outputs of the comparators CO₁ and CO₂ shown in Fig. 22 are applied through the EOR circuit EOR to produce an output signal from EO as shown in Fig. 23. The leading and trailing edges of this signal EO may be synchronized with the phase of the zero cross position of the analog output signal A₁ of the linear encoders. Further, the output of the EOR circuit EOR is applied to the flip-flop circuit FF, and therefore the signal S₂' shown in Fig.23 is obtained by reversing the output of the flip-flop FF at the leading edge of the output EO of the EOR circuit. By the above-mentioned operation, the input signals S₁ and S₂" of the selector circuit SELEC are switched by the select signal SS, and a signal with the position thereof corrected is obtained even when there is a mechanical position displacement of the linear encoder sensor LS₂ at the switching point P. According to the present invention, there is provided a linear motor with high and low thrust with a simplified construction of the magnetic poles making up a moving member and with a reduced weight of the moving member. Since the optimum current pattern is supplied in accordance with an induced voltage, the optimum thrust is generated for an improved efficiency and improved controllability.

Claims: A linear motor comprising: a first magnetic pole including a yoke (1) and a plurality of first magnetic pole teeth (2a, 2c, 2b) of magnetic material having protruded (2a, 2b) and recessed parts (2c) extending in a longitudinal direction, each first magnetic pole tooth having a predetermined longitudinal length; and a second magnetic pole (3) including a yoke (7), a plurality of second magnetic pole teeth (3a, 3b, 3c; 3m, 3n) of magnetic material having protruded (3a, 3b, 3c) and recessed parts (3m, 3n), a coil assembly (4) having a plurality of coils (4a, 4b, 4c), a respective coil being wound on a respective one of said second magnetic pole teeth, and permanent magnets (8), said yoke and said second magnetic pole teeth being opposed to said first magnetic pole teeth with slight gaps, a relative position of said first and sais second magnetic pole teeth in a longitudinal direction being changeable, characterized in that a longitudinal length of said protruded (2a, 2b) and recessed parts (2c) of said first magnetic pole and said protruded parts (3a, 3b) of said second magnetic pole being substantially equal to each other, a longitudinal length of said recessed parts (3m, 3n) of said second magnetic pole being substantially one-third of the longitudinal length (P₁) of said protruded parts (2a, 2b) of said first magnetic pole, and a number of said protruded parts of said first magnetic pole being a multiple of two, and wherein a trapezoidal current having a same phase of a voltage induced in each of said coils (4a, 4b, 4c) flows through each of said coils when a waveform of said induced voltage is stepwise and one of said first and second magnetic poles is driven. A linear motor according to claim 1, wherein a stepwise current having a same phase of voltage induced in each of said coils flows through each of said coils when a waveform of said induced voltage is trapezoidal and one of said first and second magnetic poles is driven. A linear motor according to claim 1, wherein said first magnetic pole comprises a moving member (2) and said second magnetic pole (3) is a stator. A linear motor according to claim 2, wherein said moving member (2) is constructed only of a magnetic material such as an iron core. A linear motor according to claim 3, wherein a first air gap is provided between the lower surface of said moving member yoke (1) and the second magnetic pole (3), and a second air gap is provided at the upper surface of said moving member (2). A linear motor according to claim 5, wherein the yoke (7) of the stator is disposed in the vicinity of said moving member yoke (1) whereby said second air gap is provided between said moving member yoke (1) and said stator yoke (7). A linear motor according to claim 2, wherein the position of the magnetic pole of the moving member (2) is detected by: a plurality of position sensors (LS₁, LS₂) divided and arranged on a stator side in a longitudinal direction, a plurality of magnetic pole position sensors (105U₁, 105V₁, 105W₁... 105U n , 105V n , 105W n ) divided and arranged on said stator side in a longitudinal direction, a magnet (20) provided on the yoke (1) of the moving member (2), a linear encoder magnetizer (106), a magnetic pole position distributer (119), a linear encoder switch (110), a processor unit (111), and a command section (112).

Inventors: KANAZAWA, HIROSHI, MIYASHITA, KUNIO, YAMASHITA, SEIZI
Applicants: HITACHI, LTD.
Application year: 1986
Main classification: H02K41
IPC codes: H02K41