Motor control method

A signal representing the period between adjacent positive-going edges and the period between adjacent negative-going edges of a pulse train generated by a rotary encoder coupled coaxially to a motor is compared with a reference period signal for the motor to produce a deviation signal. The rotation of the motor is controlled by the deviation signal.

BACKGROUND OF THE INVENTION 
The present invention relates to a motor control method, and more 
particularly to a method of controlling the rotation of a motor with a 
deviation signal produced by comparing a reference period signal and a 
signal generated by a rotational speed detector coupled to the motor as it 
rotates at an ultra low speed, the generated signal representing the 
period between adjacent positive-going edges and the period between 
adjacent negative-going edges of a pulse train in each of a plurality of 
motor phases. 
For performing highly accurate control in servo systems, it has widely been 
practiced to attach a tachometer generator to the rotating shaft of a 
servomotor to provide a feedback loop, feed back an output signal 
indicative of the voltage from the tachometer generator through the 
feedback loop, and compare the signal with a control reference signal for 
controlling the rotation of the servomotor. 
The servo system for controlling the rotation of a servomotor at an ultra 
low speed would considerably be expensive if it incorporated a tachometer 
generator for producing a feedback signal. To avoid this drawback, there 
have been developed and used various devices in which the servomotor is 
associated with a relatively inexpensive rotational speed detector such as 
a rotary encoder, rather than the tachometer generator, to produce pulses 
which are counted for digital feedback control of the servomotor. 
In the devices in which the servomotor is required to be driven at an ultra 
low speed such as 0.3 through 0.5 revolution per minute, however, the 
pulses produced by the rotary encoder are quite few, and it would be 
difficult from the standpoint of accuracy to effect feedback control with 
the few pulses from the rotary encoder. One solution would be to use a 
high-frequency rotary encoder capable of producing many pulses when the 
motor rotates at an ultra low speed, but such a highfrequency rotary 
encoder would be very expensive to manufacture. 
SUMMARY OF THE INVENTION 
In view of the aforesaid shortcomings of the conventional servomotor 
control, it is an object of the present invention to provide a motor 
control method which is capable of increasing the controlling accuracy by 
measuring the period between adjacent positive-going edges and the period 
between adjacent negative-going edges of a pulse train from a rotary 
encoder coupled coaxially to a controlled motor, even when the controlled 
motor rotates at an extremely low speed and hence the rotary encoder 
produces only few pulses. 
According to the present invention, there is provided a method of 
controlling a motor coupled coaxially to a rotary encoder, comprising the 
steps of comparing a signal representing the period between adjacent 
positive-going edges and the period between adjacent negative-going edges 
of a pulse train generated by the rotary encoder with a reference period 
signal for the motor to produce a deviation signal, and controlling the 
rotation of the motor with the deviation signal. 
The above and other objects, features and advantages of the present 
invention will become more apparent from the following description when 
taken in conjunction with the accompanying drawings in which a preferred 
embodiment of the present invention is shown by way of illustrative 
example.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In FIG. 1, the reference numeral 10 designates a servomotor, and the 
reference numeral 12 a rotary encoder coupled coaxially to the rotating 
shaft of the servomotor 10. The rotary encoder 12 has an output terminal 
connected to a period measuring circuit 14 which is supplied with clock 
pulses as a reference clock signal for the servomotor 10 and a drive 
start/stop signal for the servomotor 10. 
The period measuring circuit 14 has an output terminal connected to a D/A 
converter 16 with its output terminal coupled to a compensation circuit 18 
which serves to compensate for a lead or a lag in phase. The compensation 
circuit 18 has an output terminal joined to a driver circuit 20 for the 
servomotor 10. 
The internal construction of the period measuring circuit 14 is illustrated 
in FIG. 2. 
The period measuring circuit 14 includes a circuit A for measuring the 
period between adjacent positive-going edges of pulses produced from a 
phase A of the rotary encoder 12, and a circuit A' for measuring the 
period between adjacent negative-going edges of pulses produced from the 
phase A of the rotary encoder 12. The period measuring circuit 14 may also 
optionally include a circuit B for measuring the period between adjacent 
positive-going edges of pulses produced from a phase B of the rotary 
encoder 12, and a circuit B' for measuring the period between adjacent 
negative-going edges of pulses produced from the phase B of the rotary 
encoder 12. 
The circuits A, A', B, and B' are essentially identical in construction to 
each other, the only structural difference being that the circuits A and B 
are actuated by the positive-going edge of the pulse produced by the 
rotary encoder 12, whereas the circuits A', B' are actuated by the 
negative-going edge of the pulse produced by the rotary encoder 12. For 
convenience only the circuit A will be described in detail. 
The circuit A is composed of a monostable multivibrator 22, an inverter 24 
connected to the Q output terminal thereof, an AND gate 26 which is 
openable in response to the output from the inverter 24, the reference 
clock signal, and the drive start/stop signal for the servomotor 10, and a 
8-bit counter 28 coupled to the output terminal of the AND gate 26. The 
counter 28 has an output terminal connected to a multiplexer 30 with its 
output terminal joined to a latch circuit 32. The monostable multivibrator 
22 has a Q output terminal connected to a decoder 34 having output 
terminals coupled to the multiplexer 30. 
The Q output terminals of the monostable multivibrators 22 in the other 
circuits A', B, B' are also connected to the decoder 34, and the output 
terminals of the counters 28 in the other circuits A', B, B' are also 
coupled to the multiplexer 30. 
In response to the clock signal (CLK) supplied via a frequency divider (not 
shown) from a non-illustrated quartz oscillator, the servomotor 10 rotates 
at an ultra low speed such as 0.3 through 0.5 revolution per minute. Upon 
rotation of the servomotor 10, the rotary encoder 12 generates a pulse 
signal composed of about 60 pulses per second. 
The output pulses from the rotary encoder 12 are compared with the 
reference period of a reference signal in the period measuring circuit 14. 
More specifically, the period measuring circuit 14 measures the pulses 
from the rotary encoder 12 for the period al (FIG. 3) from a 
positive-going edge to a following positive-going edge and the period a2 
from a negative-going edge to a following negative-going edge, and 
compares the measured periods al, a2 with the reference period to produce 
a deviation signal. Then, the deviation signal is supplied to the D/A 
converter 16 (FIG. 1) which converts the supplied signal to an analog 
signal that is applied through the compensation circuit 18 to the driver 
circuit 20 as a motor drive control signal. 
The foregoing operation will be described in greater detail with reference 
to FIGS. 2 and 4. 
A pulse train fed from the phase A of the rotary encoder 12 is supplied to 
the monostable multivibrator 22 which is energized by positive-going edges 
of the supplied pulse train to deliver an output signal to the inverter 
24. The AND gate 26 is arranged to be opened in response to the output 
signal from the inverter 24, the reference clock signal CLK, and the drive 
start signal for the servomotor 10. Therefore, when the output signal is 
supplied from the Q output terminal of the monostable multivibrator 22, it 
is inverted by the inverter 24, and hence the AND gate 26 is closed. Upon 
elapse of a reference period time T determined by a constant CR, the 
monostable multivibrator 22 stops supplying the output signal from the Q 
output terminal thereof. Then, the inverted output signal from the 
inverter 24 opens the AND gate 26 to allow the counter 28 to start 
counting the clock pulses. The counter 28 keeps counting the clock pulses 
for a time .alpha. until a next positive-going edge of the pulse train 
from the phase A of the rotary encoder 12. The counter 28 issues the 
number of pulses counted in the time .alpha. as a deviation signal to the 
multiplexer 30. The output signal from the Q output terminal of the 
monostable multivibrator 22 is delivered to the decoder 34 which decodes 
the supplied signal to enable the multiplexer 30 to supply a pulse output 
corresponding to the time .alpha. to the latch circuit 32. The latch 
circuit 32 temporarily holds the pulse output until a next output signal 
is fed from the multiplexer 30, and then issues the pulse output to the 
D/A converter 16. The servomotor 10 is controlled by the analog output 
signal from the D/A converter 16. 
The circuit A' is triggered by a positive-going edge of the pulse train 
from the rotary encoder 12. More specifically, the monostable 
multivibrator 22 issues an output signal from its Q output terminal to the 
inverter 24, which issues an inverted output signal to close the gate 26. 
Upon elapse of the reference period time T, the monostable multivibrator 
22 enables the inverter 24 to open the AND gate 26, whereupon the counter 
28 starts counting the clock pulses for a time .beta. until a next 
negative-going edge of the pulse train from the rotary encoder 12 is 
reached. The number of the counted clock pulses is converted to an 8-bit 
signal which is fed to the multiplexer 30. The 8-bit signal is issued from 
the multiplexer 30 to the latch circuit 32 by a signal which is supplied 
from the decoder 34 in response to the output signal from the Q output 
terminal of the monostable multivibrator 22. The latch circuit 32 supplies 
the latched signal to the D/A converter 16 for controlling the rotation of 
the servomotor 10. 
While the method of the invention has been described as being combined with 
the rotary encoder which issues a single train of pulses, the method of 
the invention can be used with a rotary encoder which issues at least two 
pulse trains for an increased control accuracy. 
Where there is employed a rotary encoder capable of producing two pulse 
trains from phases A and B, for example, the circuits B, B' as shown in 
FIG. 2 may additionally be utilized. The pulse train from the phase A is 
compared with the reference period in the same manner as described above. 
For a pulse train from the phase B, a period b1 from a positive-going edge 
to a following positive-going edge and a period b2 from a negative-going 
edge to a following negative-going edge are compared with the reference 
period. More specifically, the pulses after a carry output is issued are 
counted for a period .gamma. between adjacent positive-going edges of the 
pulse train from the phase B, and the pulses after a carry output is 
issued are counted for a period .epsilon. between adjacent negative-going 
edges of the pulse train from the phase B'. The counted pulses which are 
out of synchronism with each other are utilized as control signals for the 
servomotor 10. Since the four periods are compared with the reference 
period in this embodiment, a higher control accuracy can be achieved. 
With the present invention, as described above, the conventional problem of 
insufficient servomotor control due to few pulses available at the time 
the servomotor rotates at an ultra low speed can be solved by observing 
the period between adjacent positive-going edges and also the period 
between adjacent negative-going edges of the pulse train, and comparing 
the periods with the reference signal. Therefore, the control resolution 
can be improved for various control modes with a high accuracy in 
controlled systems which rotate at ultra low speeds, i.e., produce few 
pulses. 
Although a certain preferred embodiment has been shown and described, it 
should be understood that many changes and modifications may be made 
therein without departing from the scope of the appended claims.