Brushless DC motor control

Unlike brush-type DC motors, a brushless permanent magnet DC motor has an impedance which is resistive at low rotational speeds and becomes increasingly inductive at higher speeds. Consequently, a control for a brushless permanent magnet DC motor must control both supply voltage and commutation angle in order to maximize motor performance. The simultaneous control of both supply voltage and commutation angle is difficult to achieve and requires complex circuitry. The present invention overcomes this problem by providing first means for developing a motor performance command signal representing a desired motor operational characteristics, second means for developing a motor feedback signal representing an actual motor operational characteristic, and means coupled to the first and second developing means for deriving the commutation angle command signal and the voltage command signal from the motor performance command signal and the motor feedback signal so that the motor operates with the desired operational characteristic.

DESCRIPTION 
1. Field of the Invention 
This invention relates generally to motor controls, and more particularly, 
to a control for a brushless DC motor. 
2. Background of the Invention 
Conventional brush-type DC motors have inherent design difficulties which 
affect the life span and reliability of the motor. Among these 
difficulties are brush wear, brush arcing, acoustic noise due to brush 
contact and rotor heat dissipation. Because of these problems with 
brush-type DC motors, brushless DC motors have been finding wide 
acceptance in various applications such as tape or disk drives, and 
aircraft and missile electromechanical actuators. 
While eliminating the aforementioned problems relating to brush-type DC 
motors, the brushless DC motor presents problems of its own. In a 
conventional brush-type DC motor, the voltage-current operational 
characteristics are dominated by the resistance of the armature windings 
and the brushes at all practical rotational speeds. However, the impedance 
of a permanent magnet brushless DC motor is resistive at low rotational 
speeds and becomes increasingly inductive at higher speeds. This results 
in the requirement to control both supply voltage and commutation angle, 
or phase advance, in order to maximize motor performance. 
The simultaneous control of supply voltage and commutation angle is 
difficult to achieve and requires complex circuitry. 
The present invention is intended to overcome these and other problems 
associated with brushless DC motor controls. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a control for a brushless DC 
motor according to the present invention permits a brushless DC motor to 
be operated in a simple fashion according to a desired operational 
characteristic. 
Broadly, there is disclosed herein a control for a brushless DC motor that 
includes a permanent magnet rotor and a stator having stator coils which 
are energized in accordance with a commutation angle command signal and a 
voltage command signal for imparting rotation to the rotor. The control 
comprises first means for developing a motor performance command signal 
representing a desired operational characteristic of the motor, second 
means for developing a motor feedback signal representing an actual 
operational characterictis of the motor and means coupled to the first and 
second developing means for deriving the commutation angle command signal 
and the voltage command signal from the motor performance command signal 
and the motor feedback signal so that the motor exhibits the desired 
operational characteristic. 
Specifically, in the preferred embodiment of the invention, the motor 
control includes a motor system controller coupled to a motor electronic 
control circuit. The motor system controller develops a motor performance 
command signal in response to an input command signal and a feedback 
signal from the motor. The motor performance command signal may represent, 
for example, a command for constant torque or a command for torque as a 
function of motor speed. The function generator is responsive to the motor 
performance command signal and the motor feedback signal and develops both 
a commutation angle command signal and a voltage command signal therefrom. 
The commutation angle command signal and the voltage command signal are 
utilized by the motor electronic control circuit to drive the motor in the 
commanded fashion. 
The function generator is implemented either by software in a 
microprocessor or by hardware. In the peferred embodiment, the function 
generator utilizes mathematical models of the motor operation which 
correlate voltage and commutation angle commands with torque commands as a 
function of motor speed. 
In an alternative embodiment of the invention, means are included for 
developing a predicted DC motor current signal which is then compared to 
an actual DC motor current signal to develop an error signal. The error 
signal is used to modify the voltage and commutation angle command signals 
to minimize any such error and further improve motor performance. 
Further features and advantages of the invention will readily be apparent 
from the specification and from the drawings.

BEST MODE FOR CARRYING OUT THE INVENTION 
Referring first to FIG. 1, there is illustrated a block diagram of a 
control 10 according to the present invention for controlling a permanent 
magnet DC motor 12. The motor 12 includes a permanent magnet rotor (not 
shown) coupled to a shaft 13 for driving a load 14. 
In the illustrated embodiment, the motor is operated to supply motive power 
to a starting and stopping load, such as an actuator. It should be 
understood, however, that the motor may alternatively drive a continuously 
rotating load, such as a fan or blower, if desired. 
The motor control 10 includes a motor system controller 18 which receives 
an input command signal on a line 20 from a command signal generator 21. 
The command signal generator 21 may be any type of device for generating 
signals in response to operator input, such as a potentiometer. The motor 
system controller 18 also receives feedback signals from both the motor 12 
and the load 14 on lines 22 and 24, respectively. The motor feedback 
signal on the line 22 is developed by a motor sensor 26 and may represent, 
for example, motor speed. Similarly, the load feedback signal on the line 
24 is developed by a load sensor 28 which may represent load position. In 
the event the motor is used to drive a continuously rotating load, the 
feedback signal on the line 24 would be unnecessary. 
The motor system controller 18 is responsive to the command signal on the 
line 20, the motor feedback signal on the line 22 and the load feedback 
signal on a line 24 and develops a motor performance command signal on a 
line 30. The motor performance command signal on the line 30 may be, for 
example, a command for constant torque or a command for torque as a 
function of motor speed. 
The motor performance command signal on the line 30 is coupled to a motor 
performance function generator 32. Also coupled to the function generator 
32 is the motor feedback signal on the line 22. The function generator 32 
is responsive to the motor performance command signal on the line 30 and 
the motor feedback signal on the line 22, and derives both a commutation 
angle command signal on a line 34 and a voltage command signal on a line 
36 therefrom. 
The commutation angle command signal on the line 34 and the voltage command 
signal on the line 36 are coupled to a motor electronics control circuit 
38. Also coupled to the motor electronics control circuit 38 is a DC power 
supply, represented by a block 40 and a second motor feedback signal on a 
line 42 which is developed by a sensor 44 and which represents rotor 
position. 
The motor electronics control circuit 38 includes a voltage source inverter 
(not shown) and utilizes the commutation angle command signal on the line 
34 and the voltage command signal on the line 36 and develops AC power on 
lines 46a-46c to energize the stator windings of the motor 12 in 
appropriate fashion so that the motor 12 exhibits desired operational 
characteristics. 
The motor electronics control circuit 38 may be any known control for 
operating a brushless DC motor in accordance with commutation angle and 
voltage commands using a voltage source inverter. For example, the control 
may be that disclosed in Glennon, U.S. patent application Ser. No. 
451,642, filed Dec. 20, 1982 now U.S. Pat. No. 4,608,527, entitled "Phase 
Advance Waveform Generator for Brushless DC Actuator System Controller", 
assigned to the assignee of the instant application and the disclosure of 
which is hereby incorporated by reference. 
Referring now to FIG. 2, there is illustrated in greater detail the motor 
system controller 18 and the motor operation function generator 32 shown 
in block diagram form in FIG. 1. In the preferred embodiment, the function 
generator 32 is implemented by software in a microprocessor, although the 
function generator 32 could alternatively be implemented by means of 
analog electronic circuitry, if desired. 
In the illustrated embodiment, the motor system controller 18 receives a 
position command signal on the line 20 and develops the motor performance 
command signal on the line 30 for operating the motor 12. The motor 
performance command signal may comprise a torque command signal or another 
command signal. 
The position command signal on the line 20 and the actuator position 
feedback signal on the line 24 are both coupled to a first summing 
junction 60 which generates a first error signal V.sub.e1 on a line 62 
representing the difference between the two inputs. The first error signal 
V.sub.e1 is coupled to a control block 64 which modifies the error signal 
using a control algorithm represented by a transfer function H.sub.1 (s). 
The transfer function H.sub.1 (s) may be, for example, a gain and lead/lag 
compensation function. The output of the control block 64 is a speed 
command signal on a line 66 which is coupled to a second summing junction 
68. Also coupled to the summing junction 68 is the motor speed feedback 
signal on the line 22. The second summing junction 68 generates a second 
error signal V.sub.e2 on a line 72 representing the difference between the 
signals on the lines 66 and 22. The second error signal on the line 72 is 
coupled to a control block 74 which modifies the second error signal using 
a control algorithm represented by a transfer function H.sub.2 (s) which 
may be, for example, a further gain and lead/lag compensation function. 
The output of the control block 74 is the motor performance command signal 
on the line 30. 
The motor performance command signal on the line 30 and the motor speed 
feedback signal on the line 22 are coupled to the function generator 32 
comprising blocks 78 and 80. The function generator 32 implements 
mathematical models of the operational characteristics of the brushless DC 
motor. The mathematical models are represented by first and second 
bivariate functions f.sub.1 (X, N) and f.sub.2 (X, N), which correlate 
voltage and commutation angle with torque commands as a function of motor 
speed. 
More specifically, and with reference to FIG. 3, in the preferred 
embodiment, the first bivariate function f.sub.1 (X, N) is represented by 
a series of mathematical curves representing the relationship between 
torque command X and the commutation angle required to cause a particular 
motor to develop the desired torque at different rated speeds N. 
Similarly, and with reference to FIG. 4, the second bivariate function 
f.sub.2 (X, N) is represented by a series of mathematical curves 
representing the relationship between troque command X and the voltage 
command required to cause the motor to develop the desired torque at 
different rated speeds N. 
Data representing the curves shown in FIGS. 3 and 4 are stored in a memory 
and accessed by a microprocessor or the blocks 78 and 80 are implemented 
by analog circuits which provide the illustrated relationships, as noted 
previously. 
It should be noted that the series of curves shown in FIGS. 3 and 4 are 
exemplary only since they depend upon the particular brushless motor 
operated by the control 10 and the particular load application for the 
brushless DC motor. 
In the preferred embodiment, the function generator of the present 
invention can be utilized to operate a motor to develop torque as a 
function of motor speed, as shown, or alternately constant torque if so 
required. This results in an ability to emulate the operation of a 
brush-type DC motor, or to customize the operation of the brushless DC 
motor, as desired. 
It should be noted that the function generator of FIG. 2 utilizes an open 
loop form of control with regard to motor operating current. Referring now 
to FIG. 5 a second embodiment of the present invention is illustrated 
which comprises a closed-loop brushless DC motor control. 
In the second embodiment, a motor performance function generator 32' 
incorporates the elements of the function generator 32 of the preferred 
embodiment, and further includes a feedback loop 86. A motor modeling 
circuit or block 88 develops a signal on a line 90 representing the 
predicted DC motor stator current which should result in response to the 
commutation angle command signal and the voltage command signal on the 
lines 34 and 36 respectively. 
The predicted DC motor current signal on the line 90 is coupled to a third 
summing junction 92 along with an actual DC motor current signal on a line 
94 to develop an error signal V.sub.e3 on a line 96. The actual DC motor 
current signal on the line 94 is generated by any appropriate means, such 
as a Hall effect sensor. The error signal V.sub.e3 on the line 96 is 
coupled to a model tuning block 98. In general, the model tuning block 98 
develops a series of tuning signals on lines 99a, 99b, . . . 99n which are 
then coupled to the motor model block 88 to in turn modify the commutation 
angle command signal and the voltage command signal to compensate for and 
minimize the error in DC motor current to further improve motor 
performance. The modified commutation angle command signal is developed on 
a line 34' while a modified voltage command signal is developed on a line 
36'. These signals are coupled to the motor electronics control circuit 
38, previously discussed in connection with FIG. 1, to operate the motor 
with the desired operational characteristics. 
The present invention is effective to permit operation of a permanent 
magnet brushless DC motor in a commanded fashion so that the motor 
exhibits desired performance characteristics. 
More specifically, the motor model block 88 may comprise a function 
generator which is implemented in hardware or software. The block 88 
implements a mathematical model of the operational characteristics of the 
brushless DC motor as a function of, for example, the resistance of the 
windings of the motor. In this case, the signal on the line 90 represents 
the predicted DC current which should result in light of the assumed 
winding resistance and the commutation and voltage command signals. An 
error between the actual and predicted currents as represented by the 
error signal V.sub.e3 causes the model tuning circuit 98 to adjust the 
mathematical model implemented by the block 88. This adjustment in turn 
modifies the output signals 34', 36' so that the error in motor current is 
minimized. 
If necessary, a gain and compensation circuit 100 may be inserted between 
the summing junction 92 and the model tuning circuit 98. 
In effect, the embodiment of FIG. 5 introduces an additional variable, i.e. 
winding resistance, to the control of the instant invention so that the 
motor is operated in a more precise fashion.