Method and apparatus for controlling a digital controlled brushless motor

There are provided a method of and an apparatus for controlling a pulse controlled brushless motor to immediately pull-in the motor to a predetermined rotational speed. The method has the steps of encoding rotational condition of a rotor into an electric signal, detecting the encoded signal to determine whether the rotor is in rotation or not, synchronizing a signal of a pull-in pulse rate with a clock pulse when the rotor is not in rotation and synchronizing the encoded signal with the clock pulse when the rotor is in rotation to form an input pulse train. The input pulse train is supplied to a peripheral circuit to excite the motor.

BACKGROUND OF THE INVENTION 
The present invention relates to a method of and apparatus for controlling 
a pulse controlled, constant speed brushless motor actuated by a 
predetermined frequency with less speed-irregularity or cogging, the motor 
being used particularly for, though not limited to, direct-driving a 
capstan for a tape recorder, drum drive mechanism for facsimiles, and disc 
rotation mechanism for floppy disc drives. More particularly, the present 
invention relates to a method of and apparatus for accelerating 
immediately and automatically a rotation of the motor up to a 
predetermined rotational speed when the motor is started to rotate, or 
otherwise when the motor is decelerated by some reasons. In the present 
application, the term "pulse controlled brushless motor" intends to mean a 
motor which has a structure similar to that of so-called a stepping motor 
and rotates continuously with a minimum speed irregularity in 
synchronization with continuous pulse signal inputs. 
The pulse controlled brushless motor of the type described will not start 
to rotate even when the motor is supplied with a pulse train of a 
predetermined operational pulse rate which is higher than the maximum 
pull-in pulse rate correspondent to a load of the motor. An attempt has 
been made in which pulse trains are provided, at the beginning, with a 
pulse rate lower than the maximum pull-in pulse rate and progressively 
increasing the pulse rate to a final, desired pulse rate, when an input 
pulse rate correspondent to a motor speed is higher than the maximum 
pull-in pulse rate. Accordingly, the input pulse rate should be swept into 
the slewing pulse rate range by utilizing a variable frequency oscillator 
every time when the motor is to be started to rotate or when the motor is 
overloaded by some reasons. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide an improved method of and 
apparatus for controlling the pulse controlled brushless motor. 
Another object of the present invention is to provide a new method of and 
apparatus for controlling the pulse controlled brushless motor, which 
permits to pull-in the motor immediately and automatically at the 
predetermined pulse rate which is higher than the maximum pull-in 
frequency. 
A further object of the present invention is to provide a new method of and 
apparatus for controlling the pulse controlled brushless motor, which 
permits a rerotation of the motor immediately and automatically when the 
motor decelerates or stops by an overload or the like. 
Another object of the present invention is to provide an apparatus for 
controlling the pulse controlled brushless motor, in which the apparatus 
can meet with an ecconomical requirement. 
Briefly, an apparatus according to the present invention incorporates a 
rotary encoder associated with a motor of the type described, detector for 
receiving output of the rotary encoder and detecting a rotation of the 
motor, clock pulse generator, frequency divider for dividing a clock pulse 
of the clock pulse generator to a pull-in pulse rate, and a device for 
combining the output from the rotary encoder, an output of the detector 
and the output from the frequency divider to synchronize the combined 
signal with the clock pulse, thereby forming an input pulse train. The 
pulse train is supplied to an excitation mode generator. 
In a method of controlling the motor, the encoder is associated with a 
motor of the type described to encode a rotational condition of the motor 
into an electric signal to detect the encoded electric signal. In case of 
no output from the encoder, a signal of pull-in pulse rate is synchronized 
with the clock pulse, while an encoded signal is synchronized with the 
clock pulse in case that an output is obtained from the encoder. The 
synchronized signal is supplied to a excitation mode generator. When 
rotational speed of the motor does not correspond to the frequency of the 
clock pulse rate, an input pulse train is formed corresponding to any one 
of the pull-in pulse and the encoded electric signal. 
Other objects and features of the present invention will become apparent 
from the detailed description of preferred embodiments thereof, which will 
be made with reference to the accompanying drawings. It is to be 
understood that accompanying drawings and description are for the purpose 
of illustration only and do not limit the invention and that the invention 
will be defined by the appended claims.

DETAILED DESCRIPTION OF THE INVENTION 
Referring first to FIG. 1, an exemplified structure of a pulse controlled 
brushless motor will be described hereinafter. A motor which is indicated 
in its entirety at 10 has a stator part 11 and rotor part 15. The stator 
part 11 has a structure similar to that of a known stepper motor which has 
a permanent magnet rotor, and is provided with outer yokes 12, inner yokes 
13 and windings 14 interposed between the outer and inner yokes. The outer 
yokes 12 have a plurality of polar teeth extending towards their axes at 
constant interval from an inner circumference of the yokes. Similarly, the 
inner yokes 13 have a plurality of polar teeth. The winding 14 is disposed 
between the outer yoke and inner yoke. The outer yoke 12 and inner yoke 13 
containing therebetween the winding 14 is connected together such that the 
teeth of the outer yoke 12 are staggered relative to the teeth of the 
inner yoke 13. The inner yokes 13 are in an opposed relation with each 
other, and the teeth of one inner yoke are staggered or skewed by one 
fourth (1/4) of the angular degree of the teeth of the other inner yoke. 
The two pairs of the thus formed yoke structure are attached to a flange 
16, the latter having a bearing 17 at the center thereof. 
The rotor part 15 is provided with a flywheel 18 in an integral fashion. 
The flywheel 18 has a recess 19 at the central bottom thereof and a 
central hole communicated with the recess 19. A rotor shaft 21 is 
press-fitted into the hole 20 of the flywheel 18, in which the end of the 
rotor shaft 21 extends to a horizontal plane of the bottom of the 
flywheel. At the middle portion of the flywheel 18 is rigidly provided an 
annular permanent magnet 22 such as ferrite magnet which has an outer 
diameter substantially equal to the diameter of the middle portion. The 
annular permanent magnet 22 has multi-pole magnetization evenly spaced 
apart, the adjacent poles being of opposite direction, such that the 
number of poles of the rotor is equal to that of the stator. The flywheel 
18 is designed to have a maximum outer diameter substantially equal to the 
outer diameter of the combined structure of the yokes 12 and 13. Namely, 
the outer end of the lower portion of the flywheel 18 is substantially 
aligned, in a vertical relation, with an outer circumferential surface of 
the stator part 11. 
The rotor part 15 as described is snuggly inserted into the stator part 11 
such that the rotor shaft 21 is rotatably supported within the bearing 17, 
and thereafter a casing 23 is fixed to the flange 16. The rotor shaft 21 
is also supported at its end by a bearing 24 provided at a center of a 
casing 23. Thus the permanent magnet 22 is positioned in a symmetrical 
relation relative to the stator 11 or a dividing plane between the two 
inner yoke 13. 
In a case that the motor is applied for driving a magnetic tape recorder in 
which a leakage flux from of the motor must be shielded so as not to 
affect a magnetic head, it is desired that the flange 16 as well as the 
casing 23 be made of magnetic materials. 
The structure of the pulse controlled brushless motor is described in 
detail in a copending U.S. Application Ser. No. 198,235 filed Oct. 17, 
1980 and West German Application No. P 30 39 255.3 filed Oct. 17, 1980, 
assigned to the common assignee of the present application. The thus 
structured motor can provide a smooth rotation of the rotor at a constant 
speed in accordance with the pulse train of the predetermined frequency. 
In FIG. 2 which shows diagrammatically an electric circuit for operating 
the aforementioned motor, driving circuits of the motor 10 are similar to 
those of a known stepper motor. Namely, an excitation mode generator 32 
consists of an electrical circuit which distributes input pulse trains to 
each phase of the motor in accordance with the number of phase and 
excitation mode, and a driving circuit 33 receives the distributed signals 
and supplies to the coils 13 of the motor electric current pulses 
necessary to excite the motor 10. 
In accordance with the present invention, an apparatus for controlling the 
motor has a rotary encoder 34 which is designed to be associated with the 
motor 10, detector 35 for receiving an output from the rotary encoder 34 
to detect whether or not the motor is in operation, clock pulse generator 
36, and frequency divider 37 for dividing the clock pulse down to the 
level of a pull-in pulse rate. 
The rotary encoder 34 has a function to convert the information of rotor 
position and speed into an electric signal, and may be of any conventional 
type, such as an optical encoder in which one or more light emission 
elements and photo sensors are disposed at opposing sides of a rotary code 
disc mounted on an output shaft of the motor, or a magnetic encoder which 
incorporates a magnetized or magnet-attached rotary disc mounted on the 
output shaft and magnetic sensor such as Hall element. In the simplest way 
of forming the rotary encoder, a Hall element is disposed in the vicinity 
of the rotor of the motor which is designed to have multi-pole 
magnetization evenly spaced. 
The output of the rotary encoder 34 is fed to the detector 35 to detect 
whether the motor is in rotation (Y signal) or in a stopping state (N 
signal). The detection output from the detector 35, the output from the 
encoder 34 and a signal f.sub.L obtained by dividing the clock pulse from 
the oscillator 36 by means of the divider 37 are combined together, and 
then synchronized with the clock pulse to form an input pulse train T. The 
obtained input pulse train is fed to the excitation mode generator 32. The 
divider 37 can be replaced by an oscillator which can generate the signal 
f.sub.L. 
In a pulse train former circuit 38, a logical product of the encoder output 
and the aforementioned "Y" signal is obtained by a first AND circuit 41 
while a logical product of the "N" signal and the divided signal is 
obtained by a second AND circuit 42, and outputs of those logical products 
are fed to an OR circuit 43. An output from the OR circuit 43 is fed to a 
terminal D of a D-type flip-flop 44 while the clock pulse from the 
generator 36 is fed to a clock terminal CK to produce an output Q, which 
is an input pulse train T. In other words, when the motor 10 is not in 
rotation, the "N" signal becomes of HIGH level and a divided signal of a 
pull-in pulse rate is fed through the AND circuit 42 and OR circuit 43 to 
the D-type flip-flop 44 and synchronized with the clock pulse CLK.uparw. 
to produce the output Q. When the rotor starts to rotate, the "Y" signal 
becomes of HIGH level, and the encoder output is fed through the AND 
circuit 41 and OR circuit 43 to the D-type flip-flop 44 and synchronized 
with the clock pulse to form the aforementioned input pulse train T. 
The input pulse train T is fed to the excitation mode generator 32 to drive 
the driving circuit 33. When rotational speed of the rotor 15 does not 
correspond to the frequency of the clock pulse, the motor 10 is driven by 
either the divided signal or a signal which has a frequency correspondent 
to the encoder output. Accordingly, the rotor can start to rotate and the 
predetermined constant rotational speed can be obtained even though the 
rotor is slow down in a condition of overload by some reasons. 
In FIGS. 3 and 4 in which the former shows more specific circuit 
construction, the encoder output detector 35 is formed with a 
retriggerable monostable multivibrator and utilizes, as the "Y" signal and 
"N" signal, the output Q and inverted signal Q of the output, 
respectively, the output Q being of HIGH level for a predetermined time 
after triggered. The clock pulse generator 36 is formed with a simple 
circuit having a known "555" timer IC, of which oscillating frequency can 
be adjusted to a necessary motor driving frequency by selecting values of 
C and R. If a high stability of frequency is required, a signal of a 
desired frequency can be obtained by utilizing a quartz crystal osillator 
and a frequency divider circuit. The frequency divider 37 is formed with a 
monostable multivibrator and a part of the D-type flip-flop 44. The 
monostable multivibrators of the detector 35 and of the frequency divider 
37 can be formed with a single DIP (dual-in-line-package) IC, while the 
flip-flops of the pulse train former 38 and of the frequency divider 37 
can be formed with another DIP IC, so as to minimize the number of ICs. 
The excitation mode generator 32 is formed with two J-K flip-flops. 
An operational mode will be described with reference to FIGS. 3 and 4. When 
the motor is operated in the first place, the motor 10 is driven by the 
divided signal f.sub.L which is fed in such a manner as f.sub.L 
.fwdarw.t1.fwdarw.t3.fwdarw.T (input pulse train) because an encoded 
signal does not exist. When the rotor is in an initial stage of rotation, 
the divided signal f.sub.L is inhibited and the encoder output 
ENCODER.uparw. drives the motor, the encoder output being fed in such a 
manner as ENCODER.uparw..fwdarw.t2.fwdarw.t3.fwdarw.T, until a clock pulse 
CLK.uparw. coincides in frequency with the encoder output ENCODER.uparw.. 
The ENCODER signal of FIG. 4 shows a relationship among the widths of 
successive pulses emanating from encoder 34 as the rotor speed increases. 
As shown, the width of the pulses decreases as rotor speed increases with 
time. When the ENCODER signal from the AND gate 41 is applied to the "D" 
input of flip-flop 44, the CLK signal applied to the CK input of the 
flip-flop 44 produces a driving pulse train T in such a manner so as to 
effect a smooth, continuous increase in the speed of the rotor until the 
encoder feedback signal synchronizes with the CLK signal. By this circuit 
arrangement, the invention avoids the rather abrupt changes in applied 
driving signal T which may cause correspondingly abrupt changes in the 
speed of a tape or disk device in which the invention might be used. 
Though the present invention has been described with reference to the 
preferred embodiment, many modification and alterations can be made within 
the spirit of the invention.