Stepper motor drive circuit

A simplified drive circuit for applying bipolar drive current to a stepper motor includes a switch for alternately connecting a first end of the stator winding between supply potential and ground. The second end of the winding is coupled to supply potential by a resistor and coupled to ground by the parallel combination of a second resistor and a capacitor. The capacitor alternately operates as a short circuit when supply potential is applied to the first end of the winding and as a battery when the first end of the winding is grounded.

BACKGROUND OF THE INVENTION 
This invention relates to circuitry for driving the stator coil of a 
stepper motor and more particularly for providing two amplitudes of 
bipolar drive current to the coil depending on the instantaneous torque 
requirements of the motor. 
Typically, stator coils of bipolar stepper motors are driven by bridge 
amplifiers which effectively commutate the polarity of a supply potential 
across the coil. These circuits require at least four driver transistors 
to perform the requisite switching. In addition in order to change the 
coil current amplitude, for example, when the motor is in a hold mode as 
opposed to a drive mode, some means must be provided to alter the drive 
potential, alter the driving impedance, or to chop the drive signal to 
provide selectable "average" drive parameters, etc. Circuitry to permit 
such coil current adjustment usually complicates the system, adds to 
system cost, adds to system bulk and adds to the energy dissipated by the 
driver circuitry. 
Certain stepper motor applications require that the motor operate in the 
high torque mode for relatively short intervals with the major portion of 
its operation being that of stepping at a slow rate so that coil current 
demands are primarily holding currents. For example, in a video disc 
player a stepper motor may be used to drive a carriage which carries the 
recorded signal recovery transducer. At the beginning of play it is 
desired that the carriage move rapidly to seek the beginning of the 
recorded track on the record. The motor step rate may be at 300 steps per 
second, i.e., the motor must operate at high torque. However, during 
normal playback the carriage is required to move slowly in consonance with 
the transducer following the record track. In this mode the motor may step 
at 1-2 steps per second and will be operating in a low torque mode. 
Generally both the motor and the motor driving circuitry may be overdriven 
for short intervals without damage to either. Thus, for example, a 6 volt 
motor may be energized with 20 volts for periods of time too short to 
cause heating. The present inventor took advantage of this feature to 
design a bipolar stepper motor drive circuit which provides high torque 
for particular step rates, a low value of holding current and is less 
complicated and less costly than conventional bridge drive circuitry with 
similar drive features. 
SUMMARY OF THE INVENTION 
The present invention, for driving a stepper motor stator coil, includes a 
pair of switches connected to one terminal of the coil for alternately 
applying relatively positive or relatively negative supply potential. The 
second terminal of the coil is serially connected to first and second 
resistors having their respective other ends connected to relatively 
positive and relatively negative supply potentials respectively. The 
second end of the coil is also serially connected to a capacitor, the 
other end of which is tied to one of the supply potentials. 
The switches are selectively closed (stepped) to determine the direction of 
current flow in the coil. At high step rates the capacitor sinks current 
through the coil from the supply terminal to ground and on alternate 
switch phases sources (as a battery) reverse current through the coil to 
ground, the coil current being limited only by the driving impedance of 
the switches and the inherent coil resistance. Since the stepper motor is 
being overdriven it can generate relatively high average torque. At low 
step rates when a given switch is closed the capacitor initially acts as a 
short circuit passing a relatively high current pulse through the coil. 
The coil current ultimately charges the capacitor, this reduces the 
potential across the coil and thereby the current conducted in the coil. 
Once the capacitor is charged, all further coil current is provided 
through one of the two resistors and the coil current is limited to the 
desired holding current.

DETAILED DESCRIPTION OF THE INVENTION 
The circuit illustrated in FIG. 1 is a driving arrangement for one phase of 
a bipolar stepper motor. Typically this type of stepper motor will have two 
phases and therefore two such circuits would be required to energize the 
motor. The two phases would generally be energized with a 90 degree phase 
relation. 
In FIG. 1 a first end 20 of the stepper motor coil is connected to a 
relatively positive supply potential bus 30 by a 1.1 KOhm resistor R1 and 
connected to a relatively negative supply bus 40 by a 1.1 KOhm resistor 
R2. A 100 microfarad capacitor C1 is coupled between the first end 20 of 
the coil and the supply bus 40. The second end 10 of the coil is connected 
to the pole of the single pole-double throw switch S1, the contacts of 
which are connected to the positive (30) and negative (40) supply buses. 
Operation of the FIG. 1 circuit proceeds as follows. Assume the stepper 
coil has an internal resistance of 30 Ohms and an inductance of 40 
milliHenries. Further assume that the value of holding current for the 
particular motor application is low, e.g., approximately 20 milliAmperes 
and that at the start of the cycle terminal 10 is connected to ground (40) 
and capacitor C1 is discharged. At time To (FIG. 5) switch S1 connects 
terminal 10 to the 20 volt supply. Capacitor C1, being large, essentially 
represents a short circuit to ground with respect to the step function 
input voltage across the serial connection of coil and capacitor. The 
initial rate of increase in current di/dt is governed by the inductance L 
of the coil and is approximated by di/dt.apprxeq.V/L. The maximum possible 
current approaches V/R.sub.c where V is the applied voltage and R.sub.c is 
the coil resistance. Shortly after reaching peak value the coil current 
begins to decay with a time constant .tau.=R.sub.c C1 as capacitor C1 
charges toward the potential VR2/(R2+R.sub.c) which is the equilibrium 
potential at terminal 20, established by the resistor-divider of R.sub.c 
and R2 across V volts. (Note the resistor R1 is ignored in determining the 
equilibrium potential because it is much larger than R.sub.c). Once the 
equilibrium potential is reached at terminal 20, coil current is 
determined substantially by resistor R2. This value of current is the 
holding current and R2 is selected to provide sufficient current in the 
stator windings to hold the rotor fixed in the particular application. In 
the circuit shown the holding current is approximately 18 ma. 
FIG. 5 illustrates the instantaneous coil current as a function of applied 
coil voltage. Peaks of positive current occur coincidentally with rising 
edge transitions of applied potential at terminal 10, and peaks of 
negative current occur coincidentally with falling edge transitions of 
applied potential. The rate of change of the leading transitions of the 
current pulses are indicated by di/dt=V/L. The trailing current 
transitions of the current pulses are shown with a decay time constant of 
.tau.=R.sub.c C. Since power is proportional to current, and since peak 
current is conducted for a proportionately short portion of a stepping 
cycle, the circuit will dissipate relatively small average power per step 
(i.e., 20 V.times.18 ma or 360 milliwatts vs. approximately 13 watts for 
continuous peak current). 
For the negative step, switch S1 connects terminal 10 to ground and 
capacitor C1, which has been charged substantially to the positive rail 
potential V, initially functions as a battery to supply current in the 
opposite direction through the coil. The initial negative current pulse 
reaches an amplitude approximately equal in amplitude to the 
aforedescribed positive pulse and then decays with a time constant 
.tau.=R.sub.c C1 until the potential on the capacitor reaches the 
equilibrium value of VR.sub.c /(R1+R.sub.c). At this potential, holding 
current is sustained in the coil via resistor R1 at a value of 
approximately 18 ma in the illustrated circuit. From the foregoing 
analysis and the waveform in FIG. 5, it can be seen that the circuit 
produces a substantially symmetrical bipolar current drive to the stator 
coil. 
In the high torque or rapid step mode the holding current is required to be 
significantly larger than the slow step mode so that the inertia of the 
driven element does not rotate the rotor poles past the stator coils and 
into a state where rotor position is not precisely controlled by the 
stepping pulses. The high holding current is achieved by designing the 
circuit such that the coil is driven with overvoltage and the coil current 
is still increasing at the end of the step period. Despite the fact that 
the coil current has not reached its maximum value at the end of a step 
period, i.e., the maximum amplitude which may be attained in the low step 
rate mode, it will have attained a value greater than the current value 
recommended to drive the coil. The effect of switching before the current 
reaches its maximum is that the holding current is in fact greater than 
the driving current, tending to insure more precise control of rotor 
position. It must be remembered, however, when overvoltages are utilized 
to produce the requisite average holding current, operation at the fast 
rate cannot be sustained for significant periods of time or the motor will 
overheat. The coil current versus time waveform illustrated in FIG. 4 
exemplifies this mode of operation. 
It should be appreciated that for situations where the motor is being 
overdriven circuit parameters may also be chosen which condition the coil 
circuit to achieve its maximum current value during a step period and in 
fact allow sufficient time for the current to begin to decay. Even a 
relatively large decay during the step period, for example 30%, may be 
tolerated provided the "average" current over the step period is 
sufficient to guarantee rotor control, i.e., rotor position or 
rotor-stator synchronism at the particular step rate. Note that there may 
be step rates between the 300 step per second rate and the slow rate where 
the average holding current will be insufficient to control the inertia of 
the driven element. 
FIG. 2 is an electronic switch which may be substituted for the switch S1 
in FIG. 1. The FIG. 2 circuit is an inverter comprising the series 
connection of complementary field effect transistors. In the circuit a 
p-type enhancement mode transistor P1 is connected between the positive 
supply potential and output terminal 10' to which the second coil terminal 
is connected. An n-type enhancement mode transistor is connected between 
the relatively negative supply potential and the output terminal 10'. The 
control electrodes of both transistors P1 and N1 are also connected to the 
input terminal at which step control pulses are applied. When a relatively 
positive control potential is applied to the input terminal, transistor N1 
conducts in the common source mode connecting terminal 10 to the relatively 
negative supply terminal via a relatively small drain-source impedance. 
Alternatively when a relatively low control potential is applied to the 
input terminal, transistor P1 conducts in the common source mode 
connecting terminal 10' to the relatively positive supply potential via a 
relatively small drain-source impedance. When the input control pulses 
swing between the relatively positive and relatively negative supply 
potentials the two transistors conduct in the alternative, the off or 
non-conducting transistor presenting a high (10.sup.5 ohm or greater) 
impedance to the output terminal. Thus the FIG. 2 circuit operates as an 
electronic single pole-double throw switch. 
The circuit of FIG. 3 is another electronic substitute for switch S1. In 
FIG. 3, a positive potential applied to the base electrode of transistor 
Q1 conditions transistor Q1 to conduct sinking current from output 
terminal 10" through the diode D1. The current conducted in diode D1 
develops a potential thereacross which is impressed against the 
base-emitter electrodes of npn transistor Q2, reverse biasing the 
base-emitter junction of transistor Q2 to hold it in a non-conducting 
state. The collector potential of transistor Q2 is held at the positive 
supply potential by resistor R4 biasing the pnp transistor Q3 off also. 
A relatively negative potential applied to the base of transistor Q1 places 
it in the non-conducting state. The collector potential of transistor Q1 is 
raised to supply potential by the collector resistor R3 connected to the 
positive supply. The high potential at the collector of transistor Q1 
biases transistor Q2 into conduction to provide emitter current to 
terminal 10. When transistor Q2 conducts it draws base current from pnp 
transistor Q3 biasing transistor Q3 into conduction in a common-emitter 
mode coupling terminal 10" to the relatively positive supply potential via 
the collector-emitter circuit of Q3. Thus, the circuit of FIG. 3 will 
simulate a single pole-double throw switch, controlled by potential pulses 
applied to the base electrode of transistor Q1. 
The transistors chosen for use in the FIG. 2 and FIG. 3 circuits may, in 
general, be rated (with respect to power dissipation) for the drive 
requirements in the slow step mode. Typically such devices may be 
overdriven for short periods (as for short periods of operation in the 
fast step mode) without damage to the devices. Since the power 
requirements in the slow step mode are small the circuit can be realized 
with inexpensive elements. When the switch is realized with a FIG. 2 type 
arrangement, bipolar current drive is achieved with only two transistors. 
This is a further economy in terms of reduced numbers of parts relative to 
the typical four transistor bridge type bipolar drive circuit.