Drive circuit for brushless DC motors

A drive circuit for a brushless DC motor capable of generating a torque in the acceleration and deceleration directions, respectively, and effective when used in a portable device such as an electronic still picture camera. In the drive circuit, a power amplifier having an output stage composed of a pair of complementarily connected tansistors is connected to each end of each of the stator coils. Thus, the drive circuit controls the input potentials of the power amplifiers so as to produce a potential difference across the terminals of each of the stator coils and thereby control the magnitude and direction of current flow through the stator coils. Also, all the transistors of the power amplifier output stages are cut off to prevent the flow of current through the stator coils.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a drive circuit for brushless DC motors. 
2. Description of the Prior Art 
Since no brush is employed, DC brushless motors are advantageous in many 
ways over brush-type DC motors in that durability is increased, and motor 
thickness is decreased by an amount corresponding to the use of brushes 
thus making it easier to form the motor into a flat shape. Brushless DC 
motors are widely used in such applications as floppy disc drives, VTR, 
etc., where the increased durability and the thinner shape are required. 
However, the brushless DC motor is greater in inertia than the brush-type 
DC motor and also the conventional brushless DC motor drive circuit 
employs switching elements such as transistors in place of the brushes 
with the result that the reverse rotation cannot be satisfactorily 
effected by simply reversing the polarity of the motor terminal voltage as 
in the case of the brush-type DC motor. 
In this case, the switching operation by the transistors is a 
unidirectional switching and therefore the timing of the switching must be 
changed to effect the reverse rotation, thus making the control circuit 
very complicated. While the desired bidirectional switching can of course 
be accomplished with the use of specially designed transistors or FETs, 
this also gives rise to various problems such as the increased cost, the 
difficulty in using ICs, the need to use a bipolar power source, etc. 
Also, where the brushless DC motor is subjected to servo control, if an 
external torque is applied in a direction to accelerate the motor, it is 
impossible to supply the current in the reverse direction with a simple 
construction and thus it is difficult to suppress the occurrence of 
irregular rotation due to the external torque in the acceleration 
direction. 
Furthermore, recently the marketing of small electronic still picture 
cameras incorporating recording means consisting of a magnetic disk has 
been investigated and also portable cassette tape players have been placed 
on the market. With these devices, it is desirable to use a brushless DC 
motor for the purpose of increasing the durability and decreasing the size 
of the device. However, due to the facts that the electronic still picture 
camera is held by one hand and used to make a follow shot and the like, 
that a large acceleration is imparted to the tape player causing a 
disturbance torque which is much greater than in the case of the floppy 
disk drive, VTR, etc., and that the disturbance torque also acts in the 
acceleration direction, the conventional drive circuit is not suitable 
since it is difficult to control the irregular rotation due to the 
disturbance torque in the acceleration direction. 
FIG. 1 is a circuit diagram of a conventional brushless DC motor drive 
circuit. The Figure shows by way of example the drive circuit for a 
6-pole, 2-phase brushless DC motor. 
The rotational angular position of a rotor ROT is detected by Hall effect 
elements H.sub.1 and H.sub.2 which are arranged to provide a phase 
difference of 90 degrees in terms of an electrical angle. Stator coils 
SC.sub.1 and SC.sub.2 are wound on the stator and terminals L.sub.1, 
L.sub.2, L.sub.3 and L.sub.4 of these coils are each connected through two 
of switching transistors T.sub.1 to T.sub.8 to a controlling circuit 1 and 
the switching of the transistors T.sub.1 to T.sub.8 is controlled in 
accordance with the outputs of the Hall effect elements H.sub.1 and 
H.sub.2 (the angular position of the rotor ROT). It is constructed so that 
the transistors T.sub.1 to T.sub.8 are supplied with current through a 
single power amplifier 2. 
Now noting the four transistors T.sub.1 to T.sub.4 associated with the 
stator coil SC.sub.1, a potential difference of +V.sub.cc is produced 
between the terminals L.sub.1 and L.sub.2 when the transistors T.sub.1 and 
T.sub.4 are turned on and the transistors T.sub.2 and T.sub.3 are turned 
off and a potential difference of -V.sub.cc is produced between the 
terminals L.sub.1 and L.sub.2 when the transistors T.sub.2 and T.sub.3 are 
turned on and the transistors T.sub.1 and T.sub.4 are turned off. Also, 
when the four transistors T.sub.1 to T.sub.4 are all turned off, an open 
condition is produced between the terminals L.sub.1 and L.sub.2. 
FIG. 2 shows an equivalent circuit of the drive circuit in the 
above-mentioned condition. As will be seen from the equivalent circuit, 
each of the transistors is equivalent to a switch connected in series with 
a diode. 
However, if the working voltage of the motor is low, the diode is rendered 
in operable due to its dead zone and in fact there results the same 
condition as if the diode were not connected. Therefore, when the 
transistors T.sub.1 and T.sub.4 are on, for example, it is possible to 
effect the current flow only in a direction from the terminal L.sub.1 
toward the terminal L.sub.2, thus failing to produce a reverse torque in 
the rotor ROT. While it is of course possible to produce the desired 
reverse torque through the controlling circuit, this complicates the 
controlling circuit. As a result, it is in fact very difficult to overcome 
any irregular rotation due to the disturbance torque in the acceleration 
direction only with the use of the ordinary servo circuit. Thus, it is 
conceivable to provide, for example, a separate brake unit utilizing 
mechanical friction and in this case the drive mechanism on the whole is 
also complicated due to the use of the separate unit. 
SUMMARY OF THE INVENTION 
It is the primary object of the present invention to provide a drive 
circuit for brushless DC motors which is capable of preventing the 
occurrence of irregular rotation due to an acceleration torque caused by a 
disturbance, controlling the switching between the forward and reverse 
rotations simply through the control of the input voltage to the brushless 
DC motor, and operating at a low voltage. 
Thus, a drive circuit according to the invention is so constructed that two 
power amplifiers each having an output stage composed of a transistor 
complementary symmetry circuit are connected to each of the stator coils 
of a brushless DC motor whereby the potential difference between the power 
amplifier outputs is applied to each stator coil and the output stage 
transistors of the two power amplifiers are cut off to open each stator 
coil.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 3, there is illustrated a block diagram showing the 
construction of an embodiment of a drive circuit according to the 
invention. In the Figure, drivers PH.sub.1 and PH.sub.2 for stator coils 
SC.sub.1 and SC.sub.2, respectively, are identical in circuit 
construction. In the driver PH.sub.1, each of power amplifiers 11 and 12 
includes an output stage having a transistor buffer composed of a 
complementary symmetry circuit and the ends of the stator coil SC.sub.1 
are connected between the power amplifiers 11 and 12. Two input analog 
switches SW.sub.11 and SW.sub.12 are respectively connected to the inputs 
of the power amplifiers 11 and 12. While, as a matter of course, each of 
the switches SW.sub.11 and SW.sub.22 is in fact composed of a 
semiconductor switching circuit, in the Figure each switch is shown 
schematically as a mechanical switch. 
In the driver PH.sub.2, switches SW.sub.21 and SW.sub.22 respectively 
correspond to the switches SW.sub.11 and SW.sub.12 and also power 
amplifiers 21 and 22 respectively correspond to the power amplifiers 11 
and 12. Each of the switches SW.sub.11, SW.sub.12, SW.sub.21, SW.sub.22 is 
selectively connected to contacts a, b and c in response to the output 
signal of a controlling circuit 3. The contacts a of the switches 
SW.sub.11 and SW.sub.21 are each connected to a terminal supplied with a 
voltage V.sub.1 of a voltage source 5 and the contacts b of the switches 
SW.sub.11 and SW.sub.21 are each connected to a terminal supplied with a 
voltage V.sub.2 of the voltage source 5. A phase detector 4 includes Hall 
effect elements or the like for detecting the rotational angular position 
of the signal indicative of rotor and it supplies a rotor position to the 
controlling circuit 3. It is to be noted that when the driver PH.sub.1 is 
connected to the contacts a or b, thus supplying current to the stator 
coil SC.sub.1, the driver PH.sub.2 is connected to the contacts c and the 
stator coil SC.sub.1 is opened. When the driver PH.sub.1 is connected to 
the contacts a or b, the driver PH.sub.2 is connected to the contact c. 
FIG. 4 is a circuit diagram showing in greater detail the driver PH.sub.1. 
In this circuit, the power amplifier 11 includes an operational amplifier 
OA.sub.1 which receives the signal from the switch SW.sub.11 at one input 
terminal (+) and the signal from complementarily connected output 
transistors T.sub.11 and T.sub.12 at the other input terminal (-). 
Similarly, the power amplifier 12 includes an operational amplifier 
OA.sub.2 and transistors T.sub.13 and T.sub.14. 
Then, in the condition where the switches SW.sub.11 and SW.sub.12 are 
connected to the contacts b by the signal from the controlling circuit 3 
as shown in FIG. 4, the output of the power amplifier 11 becomes the 
voltage V.sub.2 and the output of the power amplifier 12 becomes the 
voltage V.sub.1, thereby producing the potential difference V.sub.2 
-V.sub.1 across the terminals L.sub.11 and L.sub.12 of the stator coil 
SC.sub.1. On the contrary, in the condition where the switches SW.sub.11 
and SW.sub.12 are both connected to the contacts a, the output of the 
power amplifier 11 becomes the voltage V.sub.1 and the output of the power 
amplifier 12 becomes the voltage V.sub.2, thus producing the potential 
difference V.sub.1 -V.sub.2 across the terminals L.sub.11 and L.sub.12 of 
the stator coil SC.sub.1. In this case, the complementarily connected 
transistors of the output stages of the power amplifiers 11 and 12 are 
connected as shown in FIG. 4 so that when the switches SW.sub.11 and 
SW.sub.12 are respectively connected to the contacts a and b, currents 
flow oppositely through the stator coil SC.sub.1 as shown by arrows e and 
f. 
On the other hand, where the switches SW.sub.11 and SW.sub.12 are connected 
to the contacts c, the noninverting inputs of the operational amplifiers 
OA.sub.1 and OA.sub.2 are respectively reduced to the ground level through 
resistors R.sub.1 and R.sub.2 so that the transistors of the output 
stages are cut off and no current flows through the stator coil SC.sub.1. 
Thus, in this condition the stator coil SC.sub.1 is open. 
By thus controlling the connections of the switches SW.sub.11 and SW.sub.12 
in accordance with the output of the controlling circuit 3, the direction 
of current flowing through the stator coil SC.sub.1 is changed or the 
stator coil SC.sub.1 is made open. Also, instead of controlling the 
connections of the switches SW.sub.11 and SW.sub.12, at least one of the 
voltages V.sub.1 and V.sub.2 of the voltage source 5 may be changed so as 
to control the value of current flowing through the stator coil SC.sub.1. 
Thus, by for example maintaining one of the voltages V.sub.1 and V.sub.2 
constant at such a value which would not cut off the transistors and using 
the other voltage as a servo control voltage, it is possible to control 
the rotational speed of the motor. In this case, the input impedance to 
the servo control voltage is so high that there is no need to include any 
power amplifier in the output stage of the servo control circuit. 
Also, instead of switching the switches SW.sub.11 and SW.sub.12 between the 
contacts a and b, it is possible to change the direction of rotation of 
the motor by permanently connecting each of the switches to either of the 
two contacts and changing the polarity of the difference between the 
voltages V.sub.1 and V.sub.2. For example, this can be achieved by setting 
V.sub.2 =V.sub.cc -V.sub.1 so that the motor is rotated in the forward 
direction when V.sub.1 &gt;V.sub.cc /2, then the motor can be stopped when 
V.sub.1 =V.sub.cc /2 and rotated in the reverse direction when V.sub.1 
&lt;V.sub.cc /2. Here, V.sub.cc represents the power supply voltage for the 
power amplifiers 11 and 12. It is to be noted that the transistors forming 
the output stage of each power amplifier may be replaced with a field 
effect transistor. Also the operational amplifiers used in this embodiment 
are of the type in which if the potential of the noninverting input 
terminal (+) is set to the ground level, the output potential is also held 
at the ground level, thus preventing the device from operating as an 
amplifier. 
Where operational amplifiers of any other type are used, it is only 
necessary to directly control the base potential of the output stage 
transistors at a cut-off potential. For example, the circuit shown in FIG. 
5 may be used. FIG. 5 shows only a power amplifier 11 and the other power 
amplifier 12 is omitted. In FIG. 5, the same reference numerals as used in 
FIG. 4 designate the equivalent components and will not be described. In 
the present embodiment, an analog switch SW.sub.3 connected to the output 
of the operational amplifier OA.sub.1 is switched between its contacts by 
a controlling circuit 31 so that the base potential of the transistors 
T.sub.11 and T.sub.12 is held at the ground level and the power amplifier 
11 is effectively disconnected from the stator coil SC.sub.1. Of course, 
the power amplifier 12 is the same in construction as the power amplifier 
11 so that the stator coil SC.sub.1 is also effectively disconnected from 
the power amplifier 12 and thus no current flows through the stator coil 
SC.sub.1. 
FIG. 6 is a modification of the circuit shown in FIG. 5 in which the dead 
zone of the transistors T.sub.11 and T.sub.12 is increased and the power 
amplifier 12 is omitted as in the case of FIG. 5. In the present 
embodiment the dead zone is increased to V.sub.cc +1.4 V as compared with 
the dead zone of 1.4 V in the circuit of FIG. 5. The same reference 
numerals as used in FIG. 5 designate the equivalent components and they 
will not be described. This embodiment is constructed so that in order to 
disconnect the stator coil SC.sub.1 from the power amplifier 11, the 
positions of analog switches SW.sub.4 and SW.sub.5 connected to the output 
of the operational amplifier OA.sub.1 are changed by a controlling circuit 
32 so that the ground potential is applied to the base of the transistor 
T.sub.11 and the power supply voltage V.sub.cc is applied to the base of 
the transistor T.sub.12. This is the same with the power amplifier 12, 
thus also disconnecting the stator coil SC.sub.1 from the power amplifier 
12, thereby supplying no current to the stator coil SC.sub.1. It is to be 
noted that in all of the above-described embodiments the input voltages of 
the four power amplifiers can be controlled at the same voltage within a 
range of voltages at which the power amplifiers are properly operable, 
thereby rapidly stopping the motor.