US 3700985 A
The relative current in a pair of inductive loads is rapidly varied by maintaining a current in each load, so that when current is removed from one the rapid decay of current in that coil permits a substantial relative current differential since the other load need not rise from zero current due to the current pre-established therein. To further enhance the rapid current differential a choke in series with a low voltage supply provides a constant current source so that a large voltage spike is generated when the current goes to zero in one load thus enhancing the current buildup in the other load.
Description (OCR text may contain errors)
United States Patent Applequist  METHOD AND CIRCUIT FOR DRIVING INDUCTIVE LOADS  Inventor: James E. Applequist,
 Assignee: Memorex Corporation, Santa Clara,
22 Filed: Dec. 17,1970
[21 Appl.No.: 99,129
 US. Cl. ..317/ 148.5 R, 317/DIG. 4  Int. Cl. ..H0lh 47/32  Field of Search ..3l7/DlG. 4, 148,5 R
 References Cited UNITED STATES PATENTS 3,315,092 4/1967 Wiley ..3l7/DIG. 4
[4 1 Oct. 24, 1972.
3,388,300 6/1968 Allmark et'al ..'..3l7/DIG.4
Primary Examiner-L. T. Hix Attorney-Limbach, Limbach & Sutton I  ABSTRACT The relative current in a pair of inductive loads is rapidly varied by maintaining a current in each load, so that when current is removed from one the rapid decay of current in that coil permits a substantial relative current differential since the other load need not rise from zero current due to the current preestablished therein. To further enhance the rapid current differential a choke in series with a low voltage I supply provides a constant current source so that a large voltage spike is generated when the current goes to zero in one load thus enhancing the current buildup in the other load.
15 Claims, 11 Drawing Figures PATENTEDum 24 1912 A 3 7 00.98 5
sum 2 or 2 m FILM MOTION (b) 1 I A BRAKE BIAS SIGNAL, S
( BRAKE SIGNAL,S2
(d) I CLUTCH SIGNAL, 8,
'" CLUTCH CURRENT, I
(f) L .l l BRAKE BIAS CURRENT, 1
(g) BRAKE CURRENT, 1
CHOKE JUNCTION VOLTAGE J CLUTCH AND BRAKE COIL CURRENTS NTOR F 2 JAMES E. A SQ EEQUIST ATTORNEYS CROSS-REFERENCES TO RELATED APPLICATIONS The present invention is particularly adapted for use in the microfilm printer described in U. S. Pat. application Ser. No. 864,036, filed Oct. 9, 1969.
BACKGROUND OF THE INVENTION The invention relates to a circuit and method for driving a pair of inductive loads and more particularly to a circuit and method for driving the stator coil of a pair of magnetic particle clutches used to control a drive shaft. I
A magnetic particle clutch is a device forclutching and/or braking a rotatably shaft. The control medium in the device is adry magnetic particle compound contained within the annular gap between the continuously rotating input member and the low inertia output member. When the stator coil is energized, a magnetic field traverses the gap, binding the particles together in the manner of bar magnets, along the lines of flux.
,These chains of magnetic particles provide the coupling between the input and output members. Under a slip condition, these chains shear at their centers, leaving an immobile layer next to the input and output members. The strength of the coupling is proportional to the stator excitation current.
Particle clutches exhibit a relatively slow torque buildup, typically greater than 2 milliseconds. However, it has been observed thatthese same clutches lose torque rapidly when the steady state current is disrupted, generally being an order of magnitude faster than torque buildup.
r 2. torque level but less than the torque level of thebraking device. In order .to initiate starting, the braking torque is. allowed to decay to zero, and does so rapidly due to the characteristics ofthe magnetic particle clutch. Thus the drive clutch torque is left to effect rapid start. After a brief starting period a brake bias torque, less than the drive torque in the steady-state stop condition, is applied so that the transition from running to stop may be made in the same manner. Thus to stop, the driving torque is'allowed to rapidly decay leaving the brake bias torque to which further braking torque is added, the sum of the established brake bias torque and the rising further brake torque causing a rapid stopping of the shaft. After a brief stopping period, the drive. torque, less than the total brake torque, is re-applied so that the cycle may be repeated with the same starting conditions.
One application requiring the rapid starting and stopping of a shaft rotation is the capstan of a highspeed microfilm printer. In that application a film is moved to successive line locations and rapid start/stop times are necessary for the film movementas described in the referenced application. Other applications may readily occur to those of ordinary skill in the art including the control of tape deck capstans and machine control in industrialprocesses.
Rapid starting and stopping of an incremental drive capstan, as in a high speed microfilm printer, is provided by a pair of matched magnetic particle clutches. One is connected to operate as a clutch to drive the capstan while the other is connected as a brake to stop it.
Normally high voltage power supplies are required to drive high inductance circuits when fast current rise time is required. Thus in order to achieve a rapid start/stop time using a pair of magnetic particle clutches, one would expect to use a high voltage supply to drive the stator coils.
SUMMARY OF THE INVENTION The present invention overcomes the problems in achieving rapid start/stop times by providing an electronic circuit and method wherein acceleration or deceleration of the drive shaft is accomplished with torque previously established by decaying the torque of the opposing member. Thus in the steady-state stop condition the drive device is maintained at a given To further enhance the start and stop times of the arrangement, a choke is placed in series with the low voltage power supply to provide a constant current source. The characteristics of a constant current source and the two inductive loads is used to provide high voltage spikes at the start and stop times to enhance the current flow in the active clutch member to provide greater starting and stopping torque,
' These andother advantages of the present invention will become apparent as the following description is read and understood.
- BRIEF DESCRIPTION OF THE DRAWINGS DESCRIPTION OF THE PREFERRED I EMBODIMENTS 3 Referring now to FIG. 1 of the drawings, wherein a schematic diagram of an embodiment of the invention is shown. A choke I isconnected between a voltage source +V and a junction point 1,. A pair of parallel circuits is connected between junction point .1 and ground. The first circuit includes a series connection of an inductor L aresistor R and the collector and emitter of an NPN transistor 0,. The base e of transistor Q, receives a signal 8,. The second circuit includes the series connection of an inductor L and two further sub-circuits in parallel. The first circuit includes a series connection of a resistor R, and the collector and. emitter of an NPN transistor Q The base of Q, receives a signal S The second sub-circuit includes a resistor R and the collector and emitter of an NPN transistor 0;. The base of Q receives a signal S A clamping diode zener diode arrangement is provided as a practical expediency to protect transistors Q: and Q, from voltages at their collectors'that may exceed their ratings and thus damage the transistors. The cathodes of diodes D D and D are connected together to the cathode of zener diode Z The anode of I 3 Z is grounded, and the anodes of diodes D,, D and D are connected to the respective collectors of 0,, Q and Qa- Inductors L, and L, are the stator windings .of magnetic particle clutches. Both'clutches are mounted on a common shaft for the film drive. The clutch of inductor L, is mounted to provide clutch action and the clutch of inductor L, is mounted to provide braking action.
Referring now also .to FIG. 2, wherein various waveforms are shown to assist in the understanding of the operation of the circuit of FIG. 1. In the steadystate condition prior to time t, all three transistor 0,,
*Q, and Q, are in saturation and the clutch coil current I, (through clutch-inductor L,) isfless than the brake less than R,. In an exemplary embodiment, R, and R are 12.5 ohms and R .is 25 ohms. The resistance of inductors L, and L, is small and may be ignored. The
value of +V is, for example, 24'volts. This voltage is v also present at 1, prior to t The diode clamping protection circuit D,, D D and Z, may also be ignored prior to t In fact, it may be ignored throughout the operation for the purposes of understanding the inventive concept.
At time t the time at which the start of film motion is desired, the brake bias signal 8;, and the brake signal S, are pulsed negative sufficiently to cutoff Q and 0,, respectively. I, rapidly becomes zero, but the current I from choke L cannot change instantaneously, i.e.: L acts as a constant current source. I, must therefore increase accordingly, but since I, is flowing through an inductive load L, that resists rapid current changes the voltage at J, necessarily increases substantially to drive the additional current through L,. Diodes D and D conduct to limit the collectors of Q and Q, to the zener voltage V in order to protect the collectors from the high voltage spike at J FIG. 2 (e, f & g) show the currents I,, I and I for a perfect constant current source. I, is, of course, equal to I plus I and FIG. 2(i) shows the currents in an actual circuit. The high voltage available at J, at t causes the current I, in L, to rise rapidly thus effecting fast clutch action in combination with the rapid cessation of current I, in the brake. Also, since I, was already at a level above zero prior to t the high energization of L, was further facilitated.
At time 2,, a relatively non-critical time after t chosen to be at least long enough for 1 to reach zero cannot .change instantaneously. I, therefore rises rapidly to its maximum value and 1, falls to zero resulting in maximum braking and minimum clutch action. The drive shaft rotation is therefore rapidly stopped.
At time I also a non-critical time, sufficiently long after t to permit I, to reach its maximum value and I, to drop to zero, the circuit is returned to its steady state motion stopped condition. Q and 0:, are already in saturation and Q, is allowed to saturate by removal of the negative signal at its base. 1 must therefore drop as I, increases from zero to its steady state value.
Referring now to FIG. 3, a drive mechanism for a capstan 50 employing a pair of magnetic particle clutches is shown. The clutches may be driven using the circuit of FIG. 1. The drive mechanism for the incremental drive capstan 50 includes a shaft 120 rotatably mounted by means of a bearing 122 in a rigid housing 124 which is attached to the frame 14 by screws 126. The other end of shaft 120 is rotatably mounted in a bearing 128 which is supported inside of a sleeve 130 which is rigidly attached to the frame 14. The sleeve 130 forms part of a brake assembly 132 which is sold by the Power Equipment Division of Lear Siegler Industries under the trademark FASTEP magnetic particle clutch Model 97015-012.
This brake 132 includes a casing 134 rigidly attached to the sleeve 130 and a central sleeve 136 which is rotatably attached to the frame 134 by bearings 128 and I, to reach maximum, the brake bias current is turned on by removal of the negative signal from the base of Q causing O to saturate. Since R is about twice the resistance of R,, 1 (equal to 1 will be less than 1,. The voltage J, drops'somewhat since some current is now flowing into L I also drops the amount that I, increases, since I must remain substantially constant. 0 remains cutoff. At time t, the film motion is to be stopped; Q controlling brake bias is allowed to remain in saturation and the negative signal from the base of O is removed switching it from cutoff to saturation. Simultaneously the base of Q, is pulsed negatively driving it to cutoff.
Thus the clutch current I, must fall to zero and I is al-.
and 138. A chamber 140 is provided between the casing 134 and sleeve 136, and a cylindrical blade 142 rigidly attahcecl to the sleeve 136 extends into the center of this chamber adjacent to sleeve 130 and casing 134. The remainder of the chamber 140 is filled with ferromagnetic particles which, in the absence of a magnetic field are in a fluid state. An electromagnetic coil 144 (corresponding to L of FIG. 1) surrounds the chamber 140, and upon the application of electric current v to the coil 144, the ferromagnetic particles in chamber 140 solidify to rigidly connect the sleeve 130 to the sleeve 136. The sleeve 136 is keyed to shaft by set screws in apertures 146 so that the application of an electrical current to the coil 144 applies a brake between the shaft 120 and the frame 14.
A second identical FASTED clutch 148 is mounted with its center sleeve keyed to the shaft 120 by a set screw in aperture 150, and the outer sleeve a of the clutch 148 carries a pulley 152 over which the belt 116 is'entrained. The application of electrical current to the coil 149 (corresponding to L, of FIG. 1) of clutch assembly 148 establishes a rigid connection between the drive belt 116 and shaft 120 so that incremental drive capstan 50 is driven by motor 112.
It will be apparent to those of ordinary skill in the art that many modifications of the disclosed embodiments are possible without departing from the spirit and scope of the invention. For example, switching devices other than transistors may be employed and protective schemes other than the diode-zener clamping circuit disclosed may be used. And the invention is not limited to use in controlling capstan drives. The invention is therefore to be limited only by the scope of the appended claims.
1. A circuit for providing a rapid change in the relative current between a pair of inductors comprising means for supplying a substantially constant current,
a first circuit including a first of said inductors, first resistance means, and first switch means in series with said means for supplying a substantially constant current,
' a second circuit including a second of said inductors and a parallel circuit in series with said means for supplying a substantially constant current, said parallel circuit including a first a nd second series circuit, said first series circuit including second resistance means and second switch means and said second series circuit including third resistance and third switch means, said first and second resistance means having substantially the same resistance and said third resistance means having a resistance substantially equal to the sum of said first and second resistance means, whereby closing said first, second and third switch means maintains the current in said second inductor greater than the current in said first inductor, opening only said second and third switch means provides a rapid increased in said first inductor current relative to said second inductor current, and opening only said first switch means provides a rapid increase in said second inductor current relative to said first inductor current.
2. A circuit according to claim 1 wherein said first, second, and third, switch means are transistors, connected to provide a closed circuit between .the respective emitters and collectors when said transistors are in saturation and to provide an open circuit between the respective emitters and collectors when said transistors are cutoff.
3. A circuit according to claim 1 wherein said first and second inductors are the stator coils of first and second magnetic particle devices, respectively.
4. A circuit connectible to a DC voltage source supplying a substantially constant current for rapidly switching the relative current in inductiveloads in response to control signals comprising first inductive load means, second inductive load means, first inductive load control means connected to said first inductive load means and to said DC voltage source for controlling the current in said first inductive load, second inductive load control means connected to said second inductive load means and to said DC voltage source for controlling the current in said second inductive load, said first and second inductive load control means responding to control signals in a steady state stop mode of operation to provide a predetermined first inductive load current and a predetermined second inductive load current and greater than said first inductive load current, and responding to control signals in a start mode of operation to reduce said second inductive load current to substantially zero, and responding to control signals in a steady state start mode of operation to reestablish said second inductive load current at a level less than said first inductive load current, and responding to control signals in a stop mode of operation to reduce said first inductive load current to substantially zero. 5. A current according to claim 4 wherein said first inductive load control means comprises first resistance means and first switch means responsive to a first control signal, said first inductive I load, said first resistance means and said first switch means connected in series. I
6 A circuit according to claim 5 wherein said second inductive loadcontrol means comprises a first branch circuit comprising second resistance means and second switch means responsive to a second control signal, said second inductive load, .said second resistance 'means and said second switch means connected in series, and
a second branch circuit comprising a third resistance means and third switch means responsive to a third control signal, said second inductive load, said third resistance means and said third switch means connected in series.
7. A circuit according to claim 6 wherein said switch means comprises solid-state switching device means for providing open or closed circuits in response to control signals.
8.. A circuit according to claim 7 wherein said first resistance means has a resistance close to the resistance of said second resistance means and said third resistance means has a resistance close to the sum of. the resistance of said first and second resistance means.
9. A circuit according to claim 7 wherein said solidstate switching device means comprise transistors each having an emitter, base and collector, said emitters and bases providing switchingaction as the base voltage is controlled to vary said transistors between cutoff and saturation.
10. A circuit according to claim 9 wherein said DC voltage source is connectable between the junction of said load inductors and said switching transistors.
11. A circuit according to claim -10 wherein said inductive loads are the stator coils of first and second magnetic particledevices.
12.'A circuit for'providing a rapid change in the relative current between a pair'of inductors comprising a first load inductor,
a second load inductor,
a choke, I 1
means for connecting one end of said load inductors together through said choke to a first polarity of a source of DC voltage,
first resistance means,
means for connecting the second end of said first load inductor to said first resistance means,
first switch means connected between said first resistance means and the second polarity of said source of DC voltage, second resistance means, means for connecting the second end of said second load inductor to said second resistance means,
second switch means connected between said second resistance means and the second polarity of said source of DC voltage, third resistance means, means for connecting-the second end of said second load inductor to said third resistance means,
third switch means connected between said third resistance means and the second polarity of said source of DC voltage.
13. A circuit according to claim 12 wherein said switch means comprise first, second and third and the remaining side of said diodes connected to each of said transistors, respectively.
15. A circuit according to claim 13 wherein said first and second load inductors are the stator coils of first and second magnetic particle devices, respectively.