Control circuit for stepping motor

A stepping motor-control circuit based on the micro-step control comprises an up-down-counter, a circuit for determining an excitation sequence for respective phases of the stepping motor in accordance with a count output of the counter, and a circuit for generating a stepped waveform excitation signal in accordance with the count output of the counter, so that the excitation state of each phase of the stepping motor is controlled in a stepped waveform consisting of a plurality of steps. The stepping motor is permitted to operate at high speed by raising the frequency of a reference clock signal applied thereto.

BACKGROUND OF THE INVENTION 
The present invention relates to a control circuit which can raise the 
frequency of the reference clock signal of a stepping motor to obtain a 
higher speed of operation for the motor. 
A stepping motor which receives a reference clock signal has been often 
employed in the driving portion of equipment such as X-Y plotters and 
printers in which a movable object needs to be precisely located in a 
predetermined place in response to an input signal. 
Accordingly, in order to raise the moving speed of the movable object 
without degrading the resolution (the amount of movement per pulse of the 
input reference clock signal) in such equipment, the frequency of the 
reference clock signal must be raised. In order to increase the speed of 
the movable object by raising the frequency of the reference clock signal, 
however, a stepping motor of excellent high-speed response characteristics 
must be used. This has led to the disadvantage that a sharp increase in 
cost is involved. 
Even in the case where a stepping motor of good high-speed response 
characteristics is employed by allowing for the increase in cost, the 
frequency of the reference clock signal needs to be raised from a low 
magnitude to a high magnitude during the starting operation when the 
frequency of the reference clock signal is made high. This has led to the 
disadvantage that the control circuit becomes complicated to further 
increase the cost. 
SUMMARY OF THE INVENTION 
An object of the present invention is to eliminate the disadvantages of the 
prior art described above, and to provide a control circuit for a stepping 
motor which can raise the frequency of a reference clock signal 
irrespective of the high-speed response characteristics of the stepping 
motor itself and which permits the high-speed movement of a movable object 
without degrading resolution. 
The present invention for accomplishing the object is characterized in that 
the excitation state of a stepping motor in each phase is controlled in a 
stepped waveform including a plurality of steps.

DESCRIPTION OF THE INVENTION 
First, the fundamental operating principle of the present invention will be 
described with reference to FIGS. 1 and 2. 
FIG. 1 shows the excitation sequence of a typical stepping motor 
(hereinbelow, simply termed "motor") of the prior art. It represents a 
case where a 4-phase motor is operated by a 2-phase simultaneous 
excitation system. 
As is apparent from the figure, the prior-art control method consists of an 
on-off control method wherein current flow through the winding of each 
phase of the motor is switched from 0 (zero) to a rated value. The "on" 
and "off" states of the respective phases change according to the 
reference clock signal. The rotor of the motor receives attractive forces 
(or repulsive forces) from the phases having turned "on" and rotates 
through predetermined angles in a stepped manner. Thus, the movable device 
driven by the motor can be located. 
Accordingly, when the frequency of the reference clock signal has become 
high and the interval between the pulses thereof narrows the rotation of 
the rotor over each predetermined angle cannot follow the state change 
exactly. In order to raise the frequency of the reference clock signal and 
to permit a high-speed movement, therefore, the characteristics of the 
motor itself must be so improved that the period of time required for the 
rotation of the predetermined angle per pulse of the input clock signal is 
short, in other words, that the high-speed response characteristics are 
excellent. Such an improved motor is inevitably high in cost. 
FIG. 2 shows an excitation sequence which employs the control according to 
the present invention. Current flow through the winding of each phase of 
the motor is controlled in a stepped waveform which changes according to 
the reference clock signal and which consists of n (for example, eight) 
levels differing in succession. 
Accordingly, the motor turns through predetermined angles corresponding to 
the n different levels in a stepped manner for every pulse of the 
reference clock signal, and it reaches the same rotational angle as 
attained every pulse in the prior-art example of FIG. 1 when the number of 
the pulses of the reference clock signal has reached n. That is, in the 
prior-art method illustrated in FIG. 1, one location is executed in each 
of four kinds of excitation modes involving the phase-A and phase-D, the 
phase-B and phase-A, the phase-C and phase-B, and the phase-D and phase-C, 
whereas in the method of the present invention illustrated in FIG. 2, n 
locations are permitted in each of the same four kinds of excitation 
modes. 
Therefore, when the levels of the respective steps in each phase are 
determined to appropriate values and the moving angles of the rotor to be 
attained at the respective steps are made equal, a resolution to be 
attained becomes n times higher than the intrinsic resolution of the motor 
(according to the method of FIG. 1), so that the drive is permitted at a 
speed enhanced n times. 
In addition, since the amount of movement of the rotor per pulse of the 
reference clock signal becomes 1/n, the same response characteristics are 
exhibited even when the frequency of the reference clock signal is raised 
n times, and equivalently the response characteristics are enhanced n 
times. In this way, the drive of the stepping motor by the reference clock 
signal of the higher frequency is permitted without the need to use an 
expensive motor of excellent response characteristics. 
Further, the rotational angle of the rotor per pulse of the reference clock 
signal is 1/n of that in the prior-art example, so that even when the 
reference clock signal of a predetermined frequency is supplied from the 
beginning of a starting operation, the rotation of the motor even in the 
case where a high-speed drive is carried out by raising the frequency of 
the reference clock signal, the frequency of the reference clock signal 
need not be lowered during starting. 
Now, an embodiment of a control circuit for a motor according to the 
present invention will be described with reference to FIGS. 3 to 5. 
FIG. 3 is a circuit diagram of the control circuit. Numeral 1 designates a 
4-bit up-down-counter which receives a reference clock signal CL as its 
count input and a rotating direction-indicative signal CCW/CW as its input 
for selecting countup or countdown. Numerals 2-7 designate exclusive OR 
gates (termed "EXOR"), numerals 8-14 inverters, numerals 15 and 16 4-bit 
decoders, numerals 17-26 amplifiers for interfacing, numerals 27 and 28 
4-input NAND gates, numerals 29-36 OR gates, numerals 37-40 NAND gates, 
numerals 41 and 42 flip-flops (termed "FFs"), numerals 43-46 NOR gates, 
and numerals 47-50 operational amplifiers. Numerals 51-54 indicate output 
terminals of phase-A-phase D, respectively, Q.sub.1 -Q.sub.4 represent 
transistors for driving switching circuits, Q.sub.5 -Q.sub.8 field-effect 
transistors constructing the analog switching circuits, and Q.sub.9 
-Q.sub.12 transistors for driving the motor. R.sub.1 -R.sub.8 and R.sub.1 
'-R.sub.8 ' denote resistors for setting stepped waveform voltages. Each 
resistor R.sub.f serves for feedback. 
Now, the operation of the embodiment will be described. 
When the reference clock signal CL and the rotating direction-indicative 
signal CCW or CW are supplied to the counter 1 from a drive control 
circuit (not shown) made up of a microcomputer etc., a hexadecimal countup 
operation is executed with the rotating direction-indicative signal CW, 
and a hexadecimal countdown operation with the signal CCW. Therefore, the 
outputs Q.sub.A -Q.sub.D of the counter 1 change with the supply of the 
reference clock signal CL as indicated in FIG. 4. 
These outputs Q.sub.A -Q.sub.D of the counter 1 are inverted every 8th bit 
by the EXOR gates 2-7 and then supplied to the inputs A-C of the decoders 
15 and 16. If necessary, the interface amplifiers 17-22 may well be used. 
At this time, the input data of the decoder 16 become the inverted forms 
of the input data of the decoder 15 as indicated in FIG. 4 because the 
inverter 8 is connected to one input of each of the EXOR gates 5-7. 
Since the resistors R.sub.1 -R.sub.8 and R.sub.1 '-R.sub.8 ' are 
respectively connected to the decode outputs 0-7 of the decoders 15 and 
16, one of R.sub.1 -R.sub.8 and one of R.sub.1 '-R.sub.8 ' turn "on" with 
the supply of the reference clock CL as indicated in FIG. 4. 
As apparent from FIG. 3, these resistors R.sub.1 -R.sub.8 and R.sub.1 
'-R.sub.8 ' serve as the input resistors of the operational amplifiers 
47-50 when the switching transistors Q.sub.5 -Q.sub.8 turn "on." 
On the other hand, the output voltage V.sub.OUT of each of the operational 
amplifiers 47-50 is determined by the magnitudes of the input voltage 
V.sub.IN thereof, the input resistor R.sub.n and the feedback resistor 
R.sub.f, as follows: 
##EQU1## 
In this case, the input resistors R.sub.n are the resistors R.sub.1 
-R.sub.8 and R.sub.1 '-R.sub.8 '. After all, the output voltages V.sub.OUT 
of the operational amplifiers 47-50 are determined by the resistors turned 
"on" by the decode outputs of the decoders 15 and 16. 
Therefore, the resistance values of the resistors R.sub.1 -R.sub.8 and 
R.sub.1 '-R.sub.8 ' are set at predetermined values with respect to the 
resistance value of the feedback resistors R.sub.f in advance so as to 
obtain the stepped waveform output voltage V.sub.OUT shown in FIG. 5. 
The transistors Q.sub.5 -Q.sub.8 which construct the analog switch circuits 
turn "on" and "off" the inputs of the operational amplifiers 47-50 so as 
to switch the excitation modes based on the 2-phase simultaneous 
excitation system. The outputs Q.sub.A -Q.sub.D of the up-down-counter 1 
are processed by the logic circuits which consist of the inverters 9-14, 
NAND gates 27 and 28, OR gates 29-36, NAND gates 37-40, FFs 41 and 42, and 
NOR gates 43-46. Switching signals of predetermined levels are produced 
through the interface amplifiers 23-26 and the transistors Q.sub.1 
-Q.sub.4, and are applied to the gates of the transistors Q.sub.5 
-Q.sub.8. 
Accordingly, as understood by referring to the waveform diagram of FIG. 2 
and the outputs Q.sub.A -Q.sub.D of the counter 1 in FIG. 4, the following 
operations are conducted: 
1. When the rotating direction-indicative signal is CW; 
(1) The transistor Q.sub.5 is "on" in its initial state. After detecting 
pulse No. 15 of the reference clock signal CL, it has its "on" and "off" 
successively inverted by pulse No. 0. 
(2) The transistor Q.sub.6 is "off" in its initial state. After detecting 
pulse No. 15 of the reference clock signal CL, it has its "on" and "off" 
successively inverted by pulse No. 0. 
(3) The transistor Q.sub.7 is "on" in its initial state. After detecting 
pulse No. 7 of the reference clock signal CL, it has its "on" and "off" 
successively inverted by pulse No. 8. 
(4) After detecting pulse No. 7 of the reference clock signal CL, the 
transistor Q.sub.8 has its "on" and "off" successively inverted by pulse 
No. 8. 
2. When the rotating direction-indicative signal is CCW (direction of No. 
15.fwdarw.No. 0 in FIG. 4, the initial condition is No. 0); 
(1) The transistor Q.sub.5 is "on" in its initial state. After detecting 
pulse No. 0, it has its "on" and "off" successively inverted pulse No. 
15. 
(2) The transistor Q.sub.6 is "off" in its initial state. After detecting 
pulse No. 0, it has its "on" and "off" successively inverted by pulse No. 
15. 
(3) The transistor Q.sub.7 is "on" in its initial state. After detecting 
pulse No. 8, it has its "on" and "off" successively inverted by pulse No. 
7. 
(4) The transistor Q.sub.8 is "off" in its initial state. After detecting 
pulse No. 8, it has its "on" and "off" successively inverted by pulse No. 
7. 
Thus, likewise to the case illustrated in FIG. 1, the exciting currents are 
distributed to the respective windings of the motor, the phase-A, phase-B, 
phase-C and phase-D. 
As a result, during the period during which the transistor Q.sub.5 is "on," 
the voltage changing in the stepped waveform as shown in FIG. 5 appears at 
the output of the operational amplifier 47 owing to the resistors R.sub.1 
-R.sub.8 sequentially selected by the decoder 15. It is amplified by the 
transistor Q.sub.9 and then taken out at the output terminal 51, from 
which it is supplied to the phase-A winding of the motor. Likewise, when 
the other transistors Q.sub.6, Q.sub.7 and Q.sub.8 turn "on," the stepped 
waveform voltages are supplied to the windings of the phase-B, phase-C and 
phase-D, respectively. 
Accordingly, the stepped waveform excitation currents shown in FIG. 2 are 
supplied to the windings of the respective phases of the motor in response 
to the reference clock signal, and the micro-step control is executed. 
As already explained, regarding the voltage values of the respective steps 
of the output voltage V.sub.OUT (refer to FIG. 5) of each of the 
operational amplifiers 47-50, the resistance values of the resistors 
R.sub.1 -R.sub.8 and R.sub.1 '-R.sub.8 ' must be set so that the 
rotational angles over which the rotor of the motor rotate at the 
respective steps may become equal. 
FIG. 6 shows an embodiment of an X-Y plotter to which the micro-step 
control according to the present invention has been applied. Numeral 60 
designates a controlling microcomputer, numerals 61 and 62 stepped 
waveform generator circuits, numerals 63 and 64 distributor circuits for 
switching excitation modes, numerals 65 and 66 motor driver circuits, 
numerals 70 and 71 4-phase (stepping) motors, and numerals 72 and 73 
speed-up transmissions made up of speed-up gears etc. 
Here, the portion at 61, 63 and 65 or at 62, 64 and 66 is the control 
circuit shown in FIG. 3. 
The speed-up transmissions 72 and 73 increase the rotating speeds of the 
motors 70 and 71 n times (eight times in the embodiment of FIG. 3) and 
transmit the increased rotations to an X-direction transmission system and 
Y-direction transmission system, respectively. Thus, the amount of 
movement per pulse of the reference clock signal can be made the same as 
in the case of the conventional excitation control illustrated in FIG. 1. 
In case of the micro-step control having n divided steps, the response 
rates of the motors 70 and 71 to the reference clock signal become n times 
equivalently as already explained. After all, in this embodiment, the 
motors 70 and 71 can be driven by a reference clock signal at a frequency 
which is n times higher than the reference clock signal frequency 
determined by the intrinsic clock response characteristics of the motors, 
and an X-Y plotter capable of high-speed operation can be provided without 
employing any expensive motor. 
The control circuit according to the present invention is not restricted to 
the X-Y plotter described above, but is applicable to any equipment 
employing a stepping motor such as a printer, X-Y table, X-Y recorder and 
electronic copying machine. The equipment capable of high-speed operation 
can be fabricated at low cost. 
As set forth above, according to the present invention, the drive of a 
stepping motor is subjected to the so-called micro-step control, so that 
the response characteristics of the motor to a reference clock signal are 
sharply improved and that the drive employing a reference clock signal at 
a higher frequency is permitted at the same resolution by using a speed-up 
rotation transmission system. Therefore, a control circuit for the 
stepping motor can be provided which can eliminate the disadvantages of 
the prior art and realize a high-speed drive without employing any 
expensive motor and which can construct an X-Y plotter, a printer etc. 
capable of high-speed response at low cost.