Abstract:
A device for the precise positioning of a movable member includes means for generating signals permitting localization of successive stop positions of the moving member and means for generating a pulse whenever the member approaches any stop position within a distance that equals a given fraction of the distance between two successive positions; when the member approaches a prescribed stop position, the relevant pulse ensures the enablement of a circuit that controls the precise stopping of the member. The device is particularly useful for setting a magnetic head on a given track of a magnetic disc.

Description:
This application is a continuation-in-part of application Ser. No. 345,814, filed Mar. 28, 1973. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention is directed to an improvement in a device for the exact positioning of a movable part. It is particularly applicable to the exact positioning of a read and write head on a given track of a magnetic disc on which data is recorded. 
     Devices exist which serve to provide the displacement of movable parts according to a given rule of motion to bring them into a determined position of their trajectory. Such a device has been described in U.S. Pat. No. 3,729,668, granted Apr. 24, 1973. It particularly includes energizing media for the motor whose task is the displacement of the movable part. These media are controlled by a signal indicative of the difference between the real speed of the part, and the reference speed defined by the chosen rule of movement. This reference speed is given a constant low value as soon as the remaining distance for the part to travel becomes less than a given value, that is, when the part is close to its destination. By avoiding an abrupt deceleration of the part at the end of its course therefore the device permits a much greater rapid stabilization at the stopping point by serving to exactly position said part. In a copending United States application entitled &#34;Precisely Positioning a Movable Member&#34;, application Ser. No. 215,967, filed Dec. 30, 1971, and assigned to the assignee of the present invention, the means are described by which several signals of sinusoidal voltage are generated, whose zero passages represent the passages of a write and read head on the different tracks of a disc. Logical pulses, generated by the zero passages of these signals, are sent to a difference of address register indicating the difference between the track to be reached and the track arrived at by the served head. This register then makes it possible to send a signal to the servo circuit for exact positioning, when the head is on the recording track to be attained. Thus, when the previously selected track is reached, the servo circuit for exact positioning compares a signal representing the real speed of the head with a reference signal originating from a multiplexer. The error signal at the output of the servo circuit permits modification of the energizing of the motor, which displaces the head until its stoppage on the desired track. Though such a device for the exact positioning of a movable part reduces to a substantial extent the number of oscillations of the head in question, about its position of equilibrium, the time required for its stabilization is considerable. 
     SUMMARY OF THE INVENTION 
     It is the object of the present invention to reduce the exact positioning time of a movable part by rendering the stabilization of that part into the desired position negligible. 
     According to the invention the improvement of a device for exact positioning of a movable part comprises means for generating signals of sinusoidal voltage whose zero passages represent the different reference positions of the part in motion, a difference of address register indicating the difference between the stopping position to be reached and the reference position actually attained and control means for a servo circuit which receives these sinusoidal signals and which is enabled by means of a decodification circuit when the register signals a zero difference. 
     The servo circuit corrects the movement of the part until its stoppage in the arrival position. It is characterized in that the difference of address register is controlled by a logical pulse generator which receives, on the one hand, the said sinusoidal signals and, on the other hand, a threshold voltage, for generating pulses whose span is a function of a given fraction of the distance between the position to be reached and the preceding reference position, and it is characterized, furthermore, in that the output of the servo circuit is linked with a control circuit for the displacement of the part, validated by the decodification circuit of the difference of address register. 
    
    
     Other characteristics and advantages of the present invention will evolve in the following description, offered for non-limitating, exemplary, purposes with reference to the attached drawing, in which: 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 represents electric signals of the device, according to the invention, connected with certain reference positions of the movable part; 
     FIG. 2 is a schematic circuit diagram of the device, according to the invention; 
     FIG. 3 is a schematic circuit diagram showing details of a portion of the device of FIG. 2; and 
     FIG. 4 is a schematic circuit diagram showing details of a portion of the device of FIG. 2 and showing the manner in which the device is connected to the motor for moving the part. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the aforementioned application Ser. No. 215,967, means were described for generating the signals A, A, B, B, of FIG. 1; and the disclosure of that application is hereby incorporated herein by reference. By marking in P the respective positions 4N, 4N + 1, 4N + 2, 4N + 3 of the movable part, N being an integer and 0, 1, 2, and 3 the cyclic rank of each position, the zero passage of signal A corresponds with the position 4N, that of signal B with the position 4N + 1, that of Signal A with the position of 4N + 2 and that of signal B with the position 4N + 3. The comparison of the signals A, A, B, B, with the threshold voltages +S and -S permits the generation of the logic pulses J corresponding with the roughly linear sections a, a, b, b, of the signals A, A, B, B, encompassing, for placing, for instance 4N with an interval (4N -ε, 4N +ε) traversed by the part. 
     Thus the generator 1 of the signals A, A, B, B, of FIG. 2, is linked with the generator 2 of logic pulses J, in the same manner as a source 2b of threshold voltage S. 
     Generator 2 includes a pair of double comparators C 11  and C 22  e.g., aμA 711 including cabled OR gates. Comparator C 11  receives inputs A and A and an input from the generator 2b of threshold voltages +S and -S, and comparator C 22  receives inputs B and B and an input from generator 2b of threshold voltages +S and -S. Comparators C 11  and C 22  determine when the value of a signal A, A, B, or B is equal to, or falls between the values of, threshold voltages +S and -S and, when this determination is made, provide a signal to OR gate 2a. This triggers OR gate 2a and causes it to act as a pulsing circuit providing the logic pulses J. Each pulse J sent to the difference of address register 3 enables this register to compute the difference between the position to be reached, and the arrival position, for instance 4N, from the moment in which the part arrives at the position 4N -ε  which precedes the instant of the first passage to the position 4N. Difference of address generator 3 is, for example, formed by a binary counter used to decount (by step decrementation in response to logic impulse J from the binary coded skip provided by skip generator 10 for indicating the number of steps to be made by the movable part) until binary value zero, corresponding to the passage of the movable part to the stopping position to be reached. At the outputs of difference of address register 3, successively encoded signals E are provided, each representing the differences between the positions reached by the part and the position to be reached. When the difference is equal to zero, a decodification circuit 4, which receives the output signals E from difference of address register 3 and which comprises a NAND gate, generates a logic signal representing the arrival of the part to the stopping position to be reached from the moment at which the part arrives at a position, preceding by a distance ε, the exact position to be reached. 
     This signal generated by circuit 4 is applied to a circuit 5 for the control of a servo circuit and enables, through four AND gates represented as a group at 5b, a M×5 multiplexer 5a. An address register 11 provides a signal on one of four outputs leading to the AND gates 5b corresponding to the cyclic rank 0, 1, 2, and 3 of the position to be reached by the part. Multiplexer 5a also receives the four input signals A, A, B, and B. 
     Turning to FIG. 3, it will be seen that multiplexer 5a includes four gates, 5c, 5d, 5e, and 5f, and a coupling circuit 5h, the four inputs of which are connected respectively to outputs of the gates. By a first input, gates 5c, 5d, 5e, and 5f are connected to generator 1 to receive signals A, B, A, B, respectively. The zero passages of signals A, B, A, B correspond respectively in FIG. 1 with successive positions 4N, 4N + 1, 4N + 2, 4N + 3 of the movable part. The four outputs of the address register 11 are respectively connected through AND gates 5b to a second input of said gates 5c, 5d, 5e, and 5f. Thus, when decodification circuit 4 enables AND gates 5b upon arrival of the part into the proximity of the position to be reached, a respective one of the four signals A, B, A, B is selected by the appropriate gate 5c, 5d, 5e, or 5f and gives at the output of coupling circuit 5h a signal f(d) which is a roughly linear function of the distance d between the part and the position to be reached. Thus, the passage of the position to be reached, the address of which is given, is indicated at the output of multiplexer 5a by one of the four signals A, A, B, and B as a function of the distance f(d). 
     One input of comparator C6 incorporated in a circuit 6 receives signal f(d), while the other input thereof is connected to a transducer 12 which supplies a signal f(Sp.) which is a function of the actual speed of the part. Circuit 6 thus generates a position correction signal C which depends on the actual speed of the part expressed by f(Sp.) for the position reached expressed by f(d). According to the actual speed, signal C has a value along distance ε are response to which the speed and displacement of the part D is controlled by means of a circuit 8 for smoothly slowing said part until it is precisely stopped at the prescribed position. 
     Circuit 8 controls the displacement of movable part D (see FIG. 4) and comprises a multiplexer M×8 having one input 9 connected to receive a signal from the servo circuit for coarse positioning of part D when it is at a distance with relation to the position to be reached |d|&gt;ε. At its other input, multiplexer 8 is connected to circuit 6 for controlling the exact positioning of the part when it is at a distance with relation to the position to be reached |d|≦ε. As indicated above, the fine control positioning of the part by means of circuit 6 is enabled by circuit 4 validating signal C applied to circuit 8. 
     Turning to FIG. 4, it will be seen that multiplexer M×8 of circuit 8 includes two gates 8b and 8a and an OR gate 8c the two inputs of which are connected respectively to outputs of these gates. By a first coarse input, control gate 8b is connected to input 9, and a first fine control input to gate 8a is connected to circuit 6. A signal generated by circuit 6 is selected by a second input of gate 8a, this second input receiving a signal from circuit 4 when |d|≦ε, d being the distance between the movable part and the position to be reached. A signal coming from input 9 is selected by the other gate 8b receiving by a second input a signal from an inverter 8d connected to circuit 4 when |d|&gt;ε. Either one of the signals selected in circuit 8 is sent by the output of coupling circuit 8c to a preamplifier 13 two outputs of which are connected respectively through a bridge arrangement of switches I1, I2, I3, and I4 to either end of the winding 16 of motor 15. It will be readily apparent that these switches will therefore control the direction of current from voltage source V through motor coil 16. As seen in FIG. 4, motor 15 is mechanically coupled to drive the movable part D. Preamplifier 13 is enabled by an enabling circuit 14 to provide ancillary control of the positioning motor. 
     The servo thus designed for the exact positioning of the part to the stopping position to be reached makes use of the circuits 2, 3, 4, and 8, well known to the expert. It is easy to comprehend that the device, which is the subject of the invention, makes possible the stability of the part in its stop position more rapidly by generating control signals before it reaches that position. The fraction is best selected as a function of the constant speed at which the part runs in the vicinity of its stop position, before the beginning of the exact positioning action. 
     The device, such as has been described especially permits the placement of a head on a given track of a magnetic disc, by starting to service it from the moment in which it is at a distance from this track equal to the fraction 4/100 of the distance between two sequential tracks. The thus obtained service time constitutes at the most 30% of the time which would be required if the stopping of the head were only controlled from its first passage on the track. 
     The present invention has especial application to exact and rapid positioning of the recording indexes.