Download The Proper Kind of DC Brushless Motor Controller

M.S. KENNEDY CORPORATION
4707 DEY ROAD LIVERPOOL, NY 13088
PHONE: (315) 701-6751 | FAX: (315) 701-6752
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Application Note 030
The Proper Kind of DC Brushless Motor Controller
By Dan Williams (ECN Article), MS Kennedy Corp.; Revised 10/4/2013
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Motors & Motion Control Subsystems
The Proper Kind of DC
Brushless Motor Controller
Edited by Jean Miller,
Managing Editor
supply. Modulating the on-off duty cycle of one or both of
so it was found to be
the switches will result in the proportional control of
unsuitable for the applipower to the motor, in one direction only. This is how onecation. The MSK 4362
quadrant operation is achieved.
was then used in place of
Adding a second pair of switches to the first pair, basithe other controller, and
cally making two totem pole half bridges, is how a twoit satisfied the requirequadrant controller is constructed. Modulating one or both
ments of the system.
of the second pair of switches will result in controlling the
The gimbals can be
by Dan Williams, M.S. Kennedy Corp.
Close-up view of the two
motor in the opposite direction. This is operation in quadoperated in either posiaxis motor controller card. tion mode or stabilization
hen making a DC brushless motor controller
rant three.
choice, there are several issues that have to be
The construction of a four-quadrant controller is
mode. In position mode,
addressed so that the proper device is selected
exactly the same as the two-quadrant controller. The difthe gimbal control loop holds the gimbal in a given posifor a system. If the application is a simple fan or blower, a
ference is in the modulation of the four switches. By modu- tion with respect to the vehicle. An angle-measuring
rather simple speed controller with just an over-current
lating the opposite pairs of switches together in a compleresolver is used as the loop feedback element in position
limit is all that may be necessary. If the direction of the
mentary fashion, there is modulation control occurring at
mode. In stabilization mode, the gimbal control loop holds
motor needs to be changed, this has to be taken into
all times. In the two-quadrant case, as the motor either
the gimbal in a given orientation in space. This allows the
account in the controller selection. If torque needs to be
stops or changes direction, the modulation decreases to
CIV optical viewing system to remain fixed on a distant
controlled, a controller with a current loop control needs to
zero and starts back up the opposite way. The control loop
target while the vehicle is “on the move.” An inertial angube specified. If the application calls for a high bandwidth
is out of control during the time the modulation is stopped. lar-rate measuring gyroscope is used as the loop feedback
servo control loop, a full four-quadWith a four-quadrant controller, modelement in stabilization mode. In either mode, the gimbal
rant controller must be chosen.
ulation is occurring at a 50 percent
controller sends a torque command signal to the motor
duty cycle when the motor is not turn- current loop closed by the MSK4362 motor controller.
Quadrants of
ing. The controller maintains control
In the CIV system, high slew rate is not of primary conOperation
as the motor speed passes through
cern, but high torque is. The maximum required slew rate
There are four possible modes
zero. The net result is tighter control
for the gimbal was 60 degrees per second, and the minior quadrants of operation using a
without any discontinuity at zero, and mum was 0.00278 degrees per second. In most stabilized
DC motor, brushless or otherwise.
the bandwidth capability of the congimbal systems, maximum slew rates are small compared
In an X-Y plot of speed versus
trol system is doubled because, in
to other types of servo systems. Peak currents during testtorque, Quadrant I is forward
effect, double the supply voltage is
ing were found to be in excess of 25 A. To be able to obtain
speed and forward torque. The
being utilized at all times.
the greatest amount of torque possible, pancake motors
torque is propelling the motor in
Using this concept in a three-phase were used instead of canned servo motors. Brushless DC
the forward direction. Conversely,
brushless DC motor controller, anothpancake torque motors with integrated Hall Effect sensors
Quadrant III is reverse speed and
er half bridge is added. The pairs of
were chosen to drive the two CIV system gimbals. The
reverse torque. Now the motor is
half bridges are controlled by the Hall Hall sensor outputs are fed to the MSK4362 controller to
The pancake motor used for driving
“motoring” in the reverse direcsensors, as they electrically commucommutate the internal three-phase bridge.
tion, spinning backwards with the the gimbal.
tate the motor with the three half
In the process of selecting a brushless DC motor conreverse torque. Quadrant II is
bridges. At any given time, only two of
troller, understanding the electrical requirements of the
where the motor is spinning in the forward direction, but
the half bridges are being used, but they are modulated
controller plus understanding the physical and perfortorque is being applied in reverse. Torque is being used to
exactly as discussed previously. This method of control is
mance requirements placed on the motor are paramount
“brake” the motor, and the motor is now generating power
the basis for the MSK4362 three-phase
to achieving a successful system
as a result. Finally, Quadrant IV is exactly the opposite.
brushless DC motor controller.
design. Studying the quadrants
The motor is spinning in the reverse direction, but the
The MSK4362 is a full four-quadin which the motor has to opertorque is being applied in the forward direction. Again,
rant DC brushless motor control
ate will help tremendously in
torque is being applied to attempt to slow the motor and
“torque amplifier.” It is designed to propicking out the correct PWM
change its direction to forward again. Once again, power is vide closed loop current control of a
modulation for the controller.
being generated by the motor.
brushless motor by sensing the current
Finally, selecting the input
A one-quadrant motor controller will drive the motor in through the motor, thereby controlling
requirements to the controller
one direction only. An example of this would be a small fan the torque output of the motor. In a DC
so that the correct control loops
or blower, such as the brushless fans used on some PC
motor, torque is proportional to curcan be established will ensure
power supplies. A small pump that only needs to run in
rent. Enough torque produces speed,
success in implementing the
one direction can also use such a controller. A two-quadand the MSK 4362 is used as the inner
correct controller in a system.
rant controller has the capability of reversing the direction loop of a servo speed control system. By
I would like to thank Jim
of the motor. If the pump needs to be backed up, this
controlling torque directly instead of
Clemenger, James Debruin and
would be the controller to use. A four-quadrant controller
speed, better control of a motor in a
Dan Sparks of Raytheon Systems
can control the motor torque both in the forward and the
servo system is realized. In other con- CIV (Commander’s Independent Viewer)
for their input and support durinfrared gimbal for the Bradley Fighting
reverse direction regardless of the direction of the motor. A trollers, the loop control is lost as the
ing the writing of this article.
servo control system needs just this kind of control.
controller passes through zero torque. Vehicle from United Defense L.P.
In order to have complete control of torque, the feedThis is not acceptable in most servo
back loop has to allow the amplifier to maintain control of
control systems. This discontinuity will disrupt the control
the torque at all times. A missile fin actuator or antenna
system in many cases.
pointing system needs to have complete control of motor
Several years ago, Raytheon Systems was experimenttorque at all times in order to satisfy the system requireing with a controller for running the stabilization system
Dan Williams is a Design Engineer at M.S. Kennedy
ments. Examining what happens during the PWM
for the Commander’s Independent Viewer (CIV), an
Corp., and is responsible for their motor control prodsequence will reveal the difference in controllers.
infrared observing aid for the Bradley Fighting Vehicle,
uct design and development. He can be reached at 4707
developed and manufactured for the U.S. Army by United
Dey Rd., Liverpool, N.Y. 13088; (315) 701-6751 ext. 211;
PWM Proportional Control
Defense L.P. In image stabilization, most of the controlling
[email protected]; www.mskennedy.com.
Pulse width modulation, or PWM is the method by
is done right around a given point in space. This means
which all class D amplifiers operate. By turning the supthat the system is constantly moving to continue pointing
ply voltage on and off at a high rate to a load and letting
at a target but has to be able to respond quickly to
the characteristics of the load smooth out the current
changes in commanded torque as the vehicle is bouncing
EDITORIAL EVALUATION
spikes, a much more efficient means of varying the power
up and down. There are large torque disturbances, but not
Write in Number or Reply Online
to the load will be achieved. Put a switch between one end
necessarily a lot of motor movement. The controller in
I found this article:
of a DC brushed motor and the supply and another switch
question did not have the capability of controlling successVery Useful
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between the other end of the motor and the return to the
fully around zero and with fast controlled torque reversal,
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Electromechanical/Mechanical Devices
www.ecnmag.com • ECN • March 2001