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M.S. KENNEDY CORPORATION 4707 DEY ROAD LIVERPOOL, NY 13088 PHONE: (315) 701-6751 | FAX: (315) 701-6752 http://www.mskennedy.com/ MSK Web Site: Application Note 030 The Proper Kind of DC Brushless Motor Controller By Dan Williams (ECN Article), MS Kennedy Corp.; Revised 10/4/2013 Double click for full resolution version AN030 1 41 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 Useful Not Useful between the other end of the motor and the return to the fully around zero and with fast controlled torque reversal, xxx xxx xxx W Electromechanical/Mechanical Devices www.ecnmag.com • ECN • March 2001