Step motor drive circuit

A drive circuit for a step motor having a multi-phase winding arrangement comprises means for generating a control signal which represents a fixed value during acceleration and deceleration periods of the step motor and functions as repetition pulses during a constant speed period thereof, switch means responsive to the control signal from the control signal generator means, power supply means responsive to an on-off operation of the switch means to deliver a drive current to respective phase windings of the step motor, and comparator means for comparing an actual current flowing through each of said phase windings with a reference value to deliver an output indicative of a compared result to the switch means.

BACKGROUND OF THE INVENTION 
The present invention relates to a step motor drive circuit. 
Ordinarily, carriage feed step motors for use in serial printers have three 
modes of acceleration operation, constant speed operation and deceleration 
speed operation which are cyclically repeated wherein printing operation 
is carried out in the mode of constant speed operation. 
In general, the modes of acceleration and deceleration operations require a 
large torque (current) proportional to inertia of load (carriage) as 
compared to the mode of constant speed operation. Further, it is required 
for realizing stable start and stop to control the step motor so that the 
value of a current is kept constant. In addition, even in the mode of the 
constant speed operation, various kinds of speeds are required in 
conformity with printing systems or methods employed and it is desirable 
for providing an optimum printed result to vary a winding current of the 
step motor in accordance with respective speeds so that its response is 
substantially equal to the critical response. 
In the prior art, drive and control for such step motors has been carried 
out on the basis of a constant current self-excitation chopper system to 
detect a winding current of the motor to control an actual winding current 
by the feedback of the winding current detected, or a separate-excitation 
chopper system wherein the above-mentioned feedback control is not 
conducted and a method is instead employed to create a chopper signal to 
effect a chopper control using the chopper signal. 
However, drawback with these conventional systems is as follows. With the 
self-excitation chopper system, a threshold level for chopping is set in 
dependence upon a winding current. Accordingly, in the modes of 
acceleration and deceleration operations and in the mode of constant speed 
operation, when an attempt is made to vary chopping threshold level to 
change the winding current, the circuit configuration becomes complicated, 
with the result that such a system is not practically acceptable. To 
eliminate this inconvenience, when a control is effected with the chopping 
threshold level being set at a constant value (e.g. torque at the time of 
acceleration or deceleration), this torque becomes an excessive drive 
torque in the mode of the constant speed operation, giving rise to the 
problem that vibration of the motor occurs, the motor is unnecessarily 
heated or the like. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a step motor drive circuit 
which has eliminated the drawbacks with the prior arts, and which can 
ensure a stabilized and smooth revolution free from vibration or heating 
etc. over an entire speed range with a simplified circuit configuration. 
To achieve this object, the present invention provides a drive circuit for 
a step motor having a multi-phase winding arrangement comprising: means 
for generating a control signal which has a constant voltage level during 
acceleration and deceleration periods of the step motor and a series of 
pulses during a constant speed period thereof; switch means responsive to 
the control signal from the control signal generator means; power supply 
means responsive to an on-off operation of the switch means to deliver a 
drive current to respective phase windings of the step motor; and 
comparator means for comparing an actual current flowing through each 
phase winding with a reference value to deliver an output indicative of a 
compared result to the switch means.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
The present invention will be described in detail with reference to 
attached drawings. 
FIG. 1 is a circuit diagram illustrating an embodiment of a step motor 
drive circuit according to the present invention. As seen from this 
figure, one ends of phase windings 10, 20, 30 and 40 are connected to 
collectors of corresponding phase transistors 12, 22, 32 and 42 through 
diodes 11, 21, 31 and 41, respectively. The other ends of the respective 
phase windings are connected to collectors of voltage control transistors 
101. The phase transistors 12, 22, 32 and 42 are turned on or off in 
response to control signals delivered to their bases 13, 23, 33 and 43, 
respectively. These phase transistors effect switching operation in a 
manner that curents flow through either of the phase windings 10 and 20 
through either of the phase windings 30 and 40. 
A control signal generator 107 delivers a control signal S107 to the step 
motor drive circuit according to this embodiment. In FIG. 1, circuits on 
the right and left sides connected to the control signal generator 107 
have the same construction in regard to the control of the phase windings 
10 and 20 and the phase windings 30 and 40. Accordingly, only the circuit 
(on the left side) related to the phase windings 10 and 20 will be 
described. 
As shown, a junction of the phase winding 10 and the diode 11 and a 
junction of the phase winding 20 and the diode 21 are connected to one end 
of a Zener diode 121 through respective flyback diodes 119 and 120. The 
other end of the Zener diode 121 is connected to a power supply +V.sub.1. 
Emitters of the phase transistors 12 and 22 are grounded through a resistor 
114 for current detection and also connected to an inverting input 
terminal (-) of a comparator 111. To a non-inverting input terminal (+) of 
the comparator 111, a voltage obtained by voltage-dividing a power supply 
voltage V.sub.0 by resistors 112 and 113 is delivered as a reference 
voltage. 
The control signal generator 107 is provided with an output terminal 
connected to an input terminal of an IC 106 of the open collector type. 
Output terminals of the open collector IC 106 and the above-mentioned 
comparator 111 are connected by a wired OR 108. To the wired OR 108, the 
power supply voltage V.sub.0 is connected through a pull-up resistor 105. 
In addition, an output signal of the wired OR 108 is delivered to a base 
of a transistor 104. The transistor 104 has a collector connected to a 
base of the voltage control transistor 101 through a resistor 103, and an 
emitter connected to ground and to the collector of the voltage control 
transistor 101 i.e. respective one ends of the phase windings 10 and 20 
through a flyback diode 122. 
The voltage control transistor 101 has an emitter connected to its base 
through a resistor 102 and also connected to the power supply +V.sub.1 and 
to the other end of the Zener diode 121. 
The circuit on the right side in FIG. 1 i.e. the circuit related to the 
phase windings 30 and 40 has the same circuit configuration as stated 
above and circuit components represented by the same reference numerals as 
those in the circuit on the left hand have the same functions, 
respectively. 
The operation of the step motor drive circuit shown in FIG. 1 will be 
described. FIG. 2 is a view showing the relationship of a rotation speed 
of the step motor versus time wherein T.sub.A, T.sub.S and T.sub.D 
represent acceleration, constant speed and deceleration periods, 
respectively. 
FIGS. 3A, 3B, 3C and 3D are time charts showing on-off operations of the 
transistors 12, 22, 32 and 42, respectively. FIG. 4 shows a waveform of 
the control signal S107. FIGS. 5 and 6 show waveforms of output signals 
S108 and S108' of the comparators 111 on the left and right sides in FIG. 
1, respectively. FIGS. 7A and 7B show waveforms of currents I10 and I30 
flowing through the phase windings 10 and 20, respectively. The 
above-mentioned FIGS. 2, 3A to 3D, 4, 5, 6, 7A and 7B are all represented 
by using the same time axes, respectively. 
In high speed serial printers in which a step motor is used, a method is 
hardly employed to use the motor within a self-starting frequency range, 
because this method allows the motor to be large. Ordinarily, a method is 
employed to set three periods of acceleration, constant speed and 
deceleration to gradually increase a speed from the stopped state during 
the acceleration period to shift to the constant speed operation when a 
required speed is reached to gradually decrease a speed when the operation 
enters the deceleration period to stop the motor. During the acceleration 
and deceleration periods, because a large load torque is required, it is 
necessary to drive the step motor using a large current. On the contrary, 
during the constant speed period, it is possible to efficiently drive the 
step motor by using a small current. 
Since the phase windings 10 and 20 and 30 and 40 of the step motor are 
driven with antiphase relationship, only the control of the phase windings 
10 and 30 will be described. 
During the acceleration and deceleration periods, the control signal S107 
is fixed at high level. At this time, the transistor 104 is turned on and 
the voltage control transistor 101 is also turned on. Thus, the power 
supply voltage +V.sub.1 is applied to the phase windings 10, 20, 30 and 
40. When the phase transistor 12 is turned on, a current labeled a of the 
current I10 shown in FIG. 7A flows through the phase winding 10. By the 
current a, a terminal voltage V114 of the current detection resistor 114 
linearly rises. This terminal voltage V114 is compared with the reference 
voltage of the comparator 111 of the open collector type. When the 
terminal voltage is above the reference voltage, the output signal S108 of 
the comparator 111 shifts to low (L) level (the comparator 111 is turned 
on). As a result, the transistor 104 is cut off, thus allowing the voltage 
control transistor 101 to be cut off. When the voltage control transistor 
101 is turned off, a current based on an energy of the phase windings 10 
and 30 which are energized flows through the phase windings 10 and 30 and 
the current detection resistor 114 via the flyback diode 122. This current 
appears as a current labeled b in the waveforms of the phase winding 
currents I10 and I30 shown in FIGS. 7A and 7B. As a result, the terminal 
voltage V114 across the current detection resistor 114 drops. When this 
terminal voltage V114 is lower than the reference voltage of the 
comparator 111, the output S108 of the comparator 111 shifts to H level 
(off). As a result, as previously described, the voltage transistor 101 is 
turned on, thus allowing a current labeled c to flow through the phase 
windings 10 and 30 for a second time. Thus, for a time period during which 
the control signal 107 is maintained at high (H) level, the currents I10 
and I30 flowing the respective phase windings 10 and 30 are subjected to 
chopper control by a self-excitation constant current chopper circuit 
including the comparator 111 and the respective phase winding currents 
have waveforms as indicated by A. The same control as that in regard to 
the phase windings 10 and 30 is applied to other phase windings 20 and 40 
after phase is switched in the rotational control of the step motor. 
During the constant speed period, the control signal S107 is delivered as a 
chopper signal having fixed pulse width and frequency. By this chopper 
signal, currents flowing through the phase windings 10, 20, 30 and 40 
undergo an on-off control. Accordingly, current as indicated by B 
determined by the pulse width and the frequency of the chopper flow 
through the respective phase windings. At this time, the terminal voltage 
V114 across the current detection resistor 114 is sufficiently lower than 
the reference voltage. Accordingly, the output of the comparator 111 has a 
H state and the control signal S107 is applied to the base of the 
transistor 104 as the signal S108. As stated above, during the constant 
speed period, the phase winding currents are controlled by the constant 
pulse chopper signal of the control signal S107. The control signal S107 
is delivered as a signal of H level during the acceleration and 
deceleration periods and as a signal having fixed pulse width and 
frequency corresponding to a drive speed of the step motor during the 
constant speed period. 
FIG. 8 shows an example of the control signal generator 107. The control 
signal generator 107 comprises a programmable timer 301, a microcomputer 
302, a read only memory (ROM) 303, and a random access memory (RAM) 304. 
In accordance with an instruction from the microcomputer 302, data 
indicative of an arbitrary constant pulse value (pulse width and 
frequency) written into the ROM 303 is set to the timer 301 to create the 
control signal S107 corresponding to a speed during to constant speed 
period. In this instance, the RAM 304 operates as a work area for carrying 
out these processings. The timer 301 is controlled by the microcomputer 
302 so that its output voltage is fixed at H level during the acceleration 
and deceleration periods. At the time of phase switching labeled t.sub.c 
of the step motor shown in FIGS. 3A to 3D, an electromagnetic energy 
accumulated or stored in the phase windings 10, 20, 30 and 40 and an 
counter-electromotive force produced due to the revolution of motor are 
discharged to the power supply +V.sub.1 through the flyback diodes 119 and 
120. The Zener diode 121 functions to rapidly take out the energy 
accumulated in the phase winding from the phase windings to improve a 
response speed at the time of the revolution of the step motor. Namely, 
the Zener diode 121 prescribes sharpness of falling d of a current at the 
time of phase switching. 
The resistor 102 connected between the base and the emitter of the 
transistor 101 functions to increase the turn off speed of the voltage 
control transistor 101. The diodes connected in series with the phase 
transistors 12, 22, 32 and 42 functions to prevent an electromagnetic 
induction current produced between the phase windings 10 and 20 and the 
phase windings 30 and 40 from flowing from the emitter to the collector of 
each of phase transistors 12, 22, 32 and 42 in an opposite direction to 
disturb response speed of the step motor. 
As described in detail, the step motor drive circuit according to the 
present invention is operated on the basis of both the constant current 
self-excitation chopper system and the separate-excitation chopper system 
having a programmable pulse width. Accordingly, during the acceleration 
and deceleration periods, the drive circuit is operated as the 
self-excitation chopper to effect the constant current chopper drive 
corresponding to a set value, thus making it possible to realize a stable 
and surge-free drive. On the other hand, during the constant speed period, 
since the pulse width can be changed in accordance with the drive speed, 
the setting of the optimum load response can be made in accordance with 
various operation speeds. Thus, by making use of the both drive systems, a 
stable and high efficient driving of the step motor is possible. In 
addition, even when there occurs an abnormal condition in the 
separate-excitation chopper circuit, i.e., an increase of the pulse width 
or a continuous on-state, the self-excitation chopper prevents an 
overcurrent from continuously flowing through the windings, thus making it 
possible to protect the windings.