Inductive current switching system with resonance

An inductor is connected in parallel with a capacitor. A first and a second current source provide current to the inductor in opposite directions. Rapid current switching in the inductor is achieved by allowing the inductor and capacitor to resonate in a time period between the application of current from the first and second current source.

BACKGROUND OF THE INVENTION 
Field of the Invention 
This invention relates to electrical current switching and more 
particularly to rapid current switching for inductors. 
Description of the Prior Art 
Rapid reversal of electrical current in inductive loads is an engineering 
challenge that limits the performance of many electrical and 
electromechanical systems. The systems include brushless DC motors, 
stepper motors, voice coil motors, and magnetic recording heads. 
Brushless DC motors and stepper motors operate on the principle of 
repetitive force generation between electromagnets and permanent magnets. 
Typically, one set of magnets moves with the mechanical load while the 
other set remains stationary. The electromagnets start at one polarity 
such that attraction to the downstream permanent magnets occurs. As the 
motion brings the magnet sets together, the current in the electromagnets 
is abruptly reversed by reversing the winding current. Then, the 
electromagnet will attract the next downstream permanent magnet, producing 
motion in the same direction. The current reversal continues repetitively 
as long as motion is desired. From the electrical point of view, the 
motor's windings represent an inductive load to the circuit that drives 
it. 
U.S. Pat. No. 4,584,506 issued Apr. 22, 1986 to Kaszman; U.S. Pat. No. 
4,710,691 issued Dec. 1, 1987 to Bergstrom, et al.; and IBM Technical 
Disclosure Bulletin, Vol. 24, No. 1A, June 1981 by Nebgen illustrate 
inductor driver circuits for electrical motors of the prior art. 
The windings of a magnetic recording head during the writing of data also 
represent an inductive load to the driver circuit. Traditionally, the 
inductive characteristic limits the rise time of the current when confined 
to a typical power supply voltage. The magnetic field produced by the head 
is proportional to the current flow in the head's windings and the head's 
inductance limits the data rate of the overall recording system. 
The problem of inductance in recording heads is apparent in every segment 
of the disk drive industry. Small disk drives typically use ferrite heads, 
which have high inductance (several microhenries), and hence limit the 
data rates to about 1.25 megabytes per second. One solution is to use thin 
film heads which have lower inductance, however, these are much more 
expensive. 
It is well known to the art that faster switching can be achieved by 
increasing the supply voltage. High voltage supplies are costly, unsafe, 
and unavailable in most computer enclosures that house disk drives. Also, 
more power is consumed in the large voltage drop from the supply to 
ground. For small, battery powered portable disk drives, this extra power 
is not available. 
The limit on the data rate imposed by the inductance of magnetic recording 
heads is evident by calculating the time constant (inductance/resistance). 
For example, a typical ferrite head has an inductance of 2.5 uH, and needs 
a current flow of 50 ma to reach the writing field strength. If the supply 
voltage after the device voltage drop is 3 volts, then the resistance to 
ground is 60 ohms, making the rise time (L/R) equal to 40 ns. This rise 
time is sufficient for data rates of only 1.25 megabytes per second at a 
frequency of 3 Mhz. 
IBM Technical Disclosure Bulletin, Vol. 23, No. 11, April 1981, by Bailey, 
et al. shows a typical inductor driver circuit for a recording head of the 
prior art. 
There is a need for an inductor driver circuit which can achieve higher 
rates of current switching in order to improve the performance of 
electrical devices such as motors and recording heads. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a capacitor having a capacitance 
C is connected in parallel with an inductor having an inductance L. A 
first and a second switch are connected to the inductor. A first current 
source is connected to the first switch to provide current to the inductor 
in a first direction. A second current source is connected to the second 
switch to provide current in a second direction. A switch control is 
connected to both the first and second switches and controls the timing of 
the opening and closing of the switches. The switch control alternately 
opens and closes the first and second switches in turn and waits a period 
of .pi.(LC).sup.178 seconds between the time one switch is opened and the 
time the other switch is closed. The result is that the present invention 
uses the resonance between the inductor and the capacitor to rapidly 
switch the current direction in the inductor. 
For a fuller understanding of the nature and advantages of the present 
invention reference should be made to the following detailed description 
taken in conjunction with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows a generalized circuit of the present invention and is 
designated by the general reference number 10. Circuit 10 comprises an 
inductor 12, having inductance L connected in parallel with a capacitor 
14, having capacitance C. A current source 20 is connected to inductor 12 
via a switch 22. Source 20 provides current in a first direction to 
inductor 12. A current source 24 is connected to inductor 12 via a switch 
26. Source 24 provides current in a second direction to inductor 12. 
FIG. 2 is a graph showing the timing relationship for circuit 10. A line 50 
represents the position of switch 22 versus time. A line 52 represents the 
position of switch 26 versus time. A line 60 represents the voltage across 
inductor 12 versus time. A line 62 represents the current through inductor 
12 versus time. 
The operation of circuit 10 may now be understood. The present invention 
uses the resonance between the inductor 12 and capacitor 14 to achieve 
rapid current reversal in the inductor 12. The opening and closing of the 
switches 22 and 26 is timed to coincide with half of a cycle of the 
resonance between inductor 12 and capacitor 14. Let T be the transition 
time of the field (i.e. the time for the energy to flow from inductor 12 
to capacitor 14 and back again to inductor 12). Then T=.pi.(LC).sup.1/2. 
The time between the opening of one switch and the closing of the other 
switch (t2-t3 and t4-t5) is set to be approximately equal to T. 
FIG. 3 is a diagram of a preferred circuit of the present invention and is 
designated by the general reference 100. An inductor 102 having an 
inductance L, is connected in parallel with a capacitor 104 having 
capacitance C. A voltage source 110 is connected to a first side of 
inductor 102 via a P-channel FET transistor 112 and a diode 114. A 
P-channel FET transistor 116 and a diode 118 are connected between voltage 
source 110 and a second side of inductor 102. A N-channel FET transistor 
122 is connected to the first side of inductor 102. A resistor 124 is 
connected between transistor 122 and ground. A N-channel FET transistor 
126 is connected between the second side of inductor 102 and resistor 124. 
Transistors 112, 116, 122 and 126 are preferably MOS FETs. 
The operation of circuit 100 may now be understood. Initially, at time t1, 
a positive signal X is applied at a node 140 to the gates of transistors 
112 and 122, and a zero signal X is applied at a node 142 to the gates of 
transistors 116 and 126. Transistors 112 and 126 are energized, and 
transistors 116 and 122 are not energized. Current flows through 
transistor 112, diode 114, inductor 102, transistor 126 and resistor 124 
to ground. The current flows left to right across inductor 102. 
At time t2, the signals are reversed. The positive X signal is now applied 
to transistors 116 and 126, and the zero X signal is applied to 
transistors 112 and 122. Transistors 116 and 122 are energized and 
transistors 112 and 126 are not energized. However, diode 118 will block 
current flow from transistor 116. The electrical properties of the diode 
are such that it will block current flow when the voltage on the diode is 
negative. Between times t2 and t3 (see FIG. 2), diode 118 will experience 
a negative voltage and no current is allowed to flow into the inductor 
102. The inductor 102 and the capacitor 104 are isolated and will 
resonate. Energy is transferred from inductor 102 to capacitor 104 and 
back again, energizing inductor 102 in the reverse direction. The time it 
takes to achieve this is half a cycle or T=.pi.(LC).sup.1/2. At time t3 
the voltage across inductor 102 is again zero. Diode 118 will no longer 
experience a negative voltage and will now allow current from transistor 
116 to flow across inductor 104 from right to left through transistor 122 
and resistor 124 to ground. The current across inductor 102 has been 
reversed. 
When it is desired to again reverse the current, at time t4, the positive X 
signal is again applied to transistor 112 and 122 and the zero X signal is 
applied to transistors 116 and 126. Transistors 112 and 126 are energized, 
and transistors 116 and 122 are not energized. Diode 114 now experiences a 
negative voltage and will block the current flow between times t4 and t5. 
The inductor 102 and capacitor 104 are again isolated and will resonate 
for a time T=.pi.(LC).sup.1/2. At time t5 the diode 114 experiences zero 
voltage and will now allow current to flow from transistor 112 across 
inductor 102 left to right, through transistor 126 and resistor 124 to 
ground. 
A comparison of circuit 100 with the generalized circuit 10 is instructive. 
Transistors 112 and 126 are equivalent to switch 22. Transistors 116 and 
122 are equivalent to switch 26. The diodes 114 and 118 effectively 
provide the timing for the switches. A complicated timing circuit is not 
necessary. The result is a compact circuit having a minimum number of 
components which is able to achieve very rapid current reversal in an 
inductor. 
FIG. 4 shows an alternative circuit of the present invention and is 
designated by the general reference number 200. An inductor 202 having 
inductance L is connected in parallel with a capacitor 204, having 
capacitance C. The first end of inductor 204 is connected to a voltage 
source 210. Source 210 has a voltage of Vs/2. A voltage source 220 is 
connected to a second side of inductor 202 via a resistor 221, a P-channel 
FET transistor 222 and a diode 224. A diode 230 is connected to the second 
side of inductor 202. A N-channel FET transistor 232 is connected between 
diode 230 and ground. Transistors 222 and 232 are preferably MOS FETs. 
The operation of circuit 200 is similar to that of circuit 100. At time t1, 
a positive X signal is applied to the gates of transistors 222 and 232. 
Transistor 222 is energized and transistor 232 is not energized. Current 
flows from source 220 through resistor 221, transistor 222, diode 224, 
across inductor 202 from left to right to source 210. At time t2, a zero X 
signal is applied to transistors 222 and 232. Transistor 232 is energized 
and transistor 222 is not energized. Diode 230 blocks the flow of current 
between times t2 and t3 while inductor 202 and capacitor 204 resonate for 
.pi.(LC).sup.1/2 seconds. At time t3, current flows from source 210, 
across inductor 202 right to left, through diode 230, transistor 232 to 
ground. At time t4, the positive X signal is reapplied to transistors 222 
and 232. Transistor 222 is energized and transistor 232 is not energized. 
Diode 224 blocks current flow between times t4 and t5 while inductor 202 
and capacitor 204 resonate for .pi.(LC).sup.1/2 seconds. At time t5, 
current flows from transistor 222 through diode 224, across inductor 202 
left to right to source 210. 
The following analysis show the advantage of using the invention over the 
prior art. Pertinent quantities have the following definitions: 
L=winding inductance, 
R=total series resistance, 
I=current flow in winding, 
RT=rise time of current, 
Vs=supply voltage, 
C=resonant capacitor, and 
Vi=induced voltage amplitude. 
The following equations represent basic relationships between the 
quantities for the invention and the prior art: 
1/2CVi.sup.2 =1/2,LI.sup.2, 
RT=L/R for the prior art, and 
RT=.pi.(LC).sup.1/2 for the present invention. 
The time required to reverse the current in a head winding is derived from 
the basic equations to yield: 
RT=LI/Vs for the prior art, and 
RT=.pi.LI/Vi for the present invention. 
The present invention has an advantage in the rise time of .pi.Vs/Vi over 
the prior art. Consider an electrical stepper motor which has a 12 volt 
supply. Devices, including diodes and transistors, that can handle an 
induced voltage of 360 volts are available. Hence, for the present 
invention, the switching speed of this motor can be increased by a factor 
of 9.5. 
Next, consider a magnetic recording head which has a 5 volt supply. Heads 
and devices which can handle an induced voltage of 300 volts are readily 
available. The switching speed of this head can be increased by a factor 
of 15 by using the present invention. 
FIG. 5 shows a schematic diagram of a stepper motor system of the present 
invention and is designated by the general reference number 300. System 
300 comprises a mechanical stepper motor 310, which may be a STH-39D002 
motor made by Shimano Kenshi Corporation of Japan. Motor 310 has a 
permanent magnetic rotor 312 and a plurality of magnetic windings 314 and 
316. Only two windings 314 and 316 are shown for illustration purposes. 
However, as is known in the art, windings 314 and 316 are comprised of 
numerous alternating windings which are connected in series and arranged 
around the periphery of rotor 312. 
A circuit 320 comprises a voltage source 322, transistors 324, 326, 328, 
330, a pair of diodes 332 and 334, resistor 336, capacitor 338, and 
winding 314. Circuit 320 is similar to circuit 100 of FIG. 3 with the 
winding 316 substituted for inductor 102. A circuit 350 comprises a 
voltage source 352, transistors 354, 356, 358, 360, a pair of diodes 362 
and 364, a resistor 366, a capacitor 368 and winding 314. The circuit 350 
is similar to circuit 100 of FIG. 3 with the winding 314 substituted for 
inductor 102. 
A bipolar transistor 360 is connected between a voltage source 326 and 
ground. Transistor 360 is connected to transistors 324 and 326. A bipolar 
transistor 364 is connected between a voltage source 366 and ground. 
Transistor 364 is connected to transistors 328 and 330. A bipolar 
transistor 370 is connected between a voltage source 372 and ground. 
Transistor 370 is connected to transistors 354 and 356. A bipolar 
transistor 374 is connected between a voltage source 376 and ground. 
Transistor 374 is connected to transistors 358 and 360. 
A flip-flop 380 is connected to transistors 360 and 364. A flip-flop 382 is 
connected to transistors 370 and 374. An inverter 386 is connected to the 
clock input of flip-flop 382. A squarewave generator 390 is connected to 
the clock input of flip-flop 380 and inverter 386. 
The operation of system 300 may now be understood. Generator 390 outputs a 
squarewave signal at a frequency 2F. This signal provides the clocking for 
flip-flop 380. The signal is inverted by inverter 386 and provides 
clocking for flip-flop 382. Flip-flop 380 outputs a phase A signal to 
transistor 360 and a phase A signal to transistor 364. Phase A and A are 
squarewaves at frequency F which have a phase difference of 180 degrees. 
Flip-flop 382 outputs a phase B signal to transistor 370 and a phase B 
signal to transistor 374. Phase B and B signals are squarewaves at 
frequency F which have a phase difference of 180 degrees. The phase B 
signal is phase shifted 90 degrees behind the phase A signal. Phase A and 
A signals cause transistors 360 and 364 to alternately energize the 
transistors of circuit 320 similar to the operation as described for 
circuit 100 of FIG. 3. The phase B and B signals cause transistors 370 and 
374 to alternately energize the transistors of circuit 350 to similar to 
the operation as described for circuit 100 of FIG. 3. The result is that 
the current in windings 314 and 316 is rapidly reversed with the reversal 
of current in winding 314 lagging the reversal of current in winding 316 
by 90 degrees in phase. This current switching drives the rotor. Current 
switching to drive electrical motors is well known in the art. However, 
circuits 320 and 350 of the present invention allow the windings 314 and 
316 to be driven at a much higher frequency. This greatly improves the 
maximum speed of motor 310. 
In another embodiment, the circuit 200 of FIG. 4 may be substituted for 
circuits 320 and 350 of system 300. In such a case, only the phase A and 
phase B signals would be required. 
FIGS. 6A and 6B show the voltage and current versus time for an inductor 
winding of a typical prior art stepper motor. The inductor winding is 
driven by the standard H driver type of circuit as is known in the art. 
The motor was operating at a maximum speed of 600 steps per second, with 
the inductor being driven at 600 hertz. The inductor could not be driven 
at a faster speed. Note the relatively gradual current reversals and the 
skip in the current signal at around 7.0 milliseconds. 
FIGS. 7A and 7B show the voltage and current versus time for inductor 316 
of system 300. The motor was also being operated at 600 steps per second 
(600 hertz for inductor 316). Generator 390 was being driven at 1200 
hertz. Note the sharp current reversals. 
FIGS. 8A and 8B show the voltage and current versus time for inductor 316 
of system 300. Here the motor is being run at a speed of 1500 steps per 
second (1500 hertz for inductor 316). Generator 390 was being driven at 
3000 hertz. Note the current reversals are still very sharp. The motor 
system 300 of the present invention was able to increase the speed 
performance of the motor by 2.5 times. 
FIG. 9 shows a schematic diagram of a data recording system of the present 
invention and is designated by the general reference number 400. System 
400 comprises a recording disk 402 rotatably mounted on a spindle 404. 
Disk 402 may be a magnetic disk. Spindle 404 is attached to a spindle 
motor 406 which rotates the spindle 404. Motor 406 is attached to a disk 
drive body 410. A voice coil motor 412 is attached to body 410. Voice coil 
motor 412 is attached to an actuator arm 414. Voice coil motor 412 moves 
arm 414 in a radial direction over disk 402. A magnetic recording head 416 
is located at the end of arm 414. Head 416 is illustrated electrically as 
an inductor winding. 
A circuit 430 is comprised of a voltage source 432, transistors 434, 436, 
438, 440, a pair of diodes 442 and 444, a resistor 446, a capacitor 448, 
and head 416. Circuit 430 is similar to circuit 100 of FIG. 3 with the 
head 416 substituted for inductor 102. A bipolar transistor 460 is 
connected between a voltage source 462 and ground. Transistor 460 is 
connected to transistors 434 and 436. A bipolar transistor 470 is 
connected between a voltage source 472 and ground. Transistor 470 is 
connected to transistors 438 and 440. An inverter 480 is connected to 
transistor 470. A data line 482 is connected to transistor 460 and 
inverter 480. 
The operation of system 400 may now be understood. Digital data is applied 
to line 482. Bipolar transistors 460 and 470 alternately energize 
transistors 434, 436, 438 and 440, respectively similar to the operation 
as described for circuit 100 of FIG. 3. The current is rapidly reversed in 
the inductor head 416 and data is written on the disk 402. 
In another embodiment, circuit 200 of FIG. 4 may be substituted for circuit 
430 of system 400. 
FIGS. 10A and 10B show oscilloscope tracings of the current and voltage of 
a magnetic head used in a conventional recording system. The magnetic head 
has a inductance L=2.5 microhenries and a resistance R=4.3 ohms. The head 
was being driven at 4 Mhz. Note the relatively gradual current reversals. 
FIGS. 11A, 11B, 12A and 12B show the oscilloscope tracings of the current 
and voltage of head 416 in system 400. The same inductor head as was used 
in the measurements for FIGS. 10A and 10B above was used as head 416. The 
head 416 was driven at 4 Mhz in FIGS. 11A and 11B and 8 Mhz in FIGS. 12A 
and 12B. The quality of the waveform in FIGS. 12A and 12B is sufficient to 
support the recording frequency and represents a doubling over the 
frequency of the conventional circuit. 
FIG. 13 shows a circuit diagram of an alternative embodiment of the present 
invention and is designated by the general reference number 500. Elements 
of circuit 500 which are similar to elements of circuit 100 of FIG. 3 are 
designated by a prime reference number. Circuit 500 has a delay line 510 
connected between node 140' and transistor 112' and a delay line 512 
connected between node 142' and transistor 116'. Other types of signal 
delay devices may also be used. 
Circuit 500 may be used when extremely high frequency current switching is 
required. At frequencies below 1 Mhz, the elements of circuit 100 perform 
as nearly ideal components. However, as the switching frequency goes above 
1 Mhz, these elements may not operate as well due to nonlinear behavior of 
the transistors and diodes. This nonlinear behavior is due to stored 
charges and resistive effects which may cause a time dependent phaseshift 
in the current and voltage waveforms. 
Circuit 500 solves this problem by providing a slight delay to the signals 
to the transistors 112' and 116'. This delay is a small percentage of the 
cycle time of the circuit. In a preferred embodiment the delay may be 10 
to 20 nanoseconds. This slight delay precharges the LC portion of the 
circuit before it goes into resonance. This precharge effectively shifts 
the phase of the voltage and current waveforms of the circuit to 
compensate for the phaseshift caused by nonlinear effects. 
Circuit 200 of FIG. 4 may also be adapted to work at higher frequencies in 
a similar manner. In this case, a delay device would be inserted between 
the X-signal source and transistor 222. The resulting circuit would also 
achieve the precharging effect as shown in circuit 500. 
FIG. 14 shows such an alternative circuit embodiment and is designated by 
the general reference number 600. Elements of circuit 600 which are 
similar to elements of circuit 200 are designated by a prime reference 
number. Circuit 600 has a delay line 610. 
While the preferred embodiments of the present invention have been 
illustrated in detail, it should be apparent that modifications and 
adaptations to those embodiments may occur to one skilled in the art 
without departing from the scope of the present invention as set forth in 
the following claims.