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Techno Linear Motion Catalog 27 Technical Information Many advantages are realized with brushless DC servo motors.  The inside out construction allows for excellent heat dissipation.  This results in a higher continuous torque and a higher output rating than found in a comparably sized brushed motor.  Also, speeds up to 60000 rpm are not uncommon.  Speed torque ripple and cogging torque are both reduced, because there are no mechanical brushes, eliminating the brushed contact.  These motors also tend to be maintenance-free.  Disadvantages are primarily increased cost, increased control complexity, and increased weight. AC  servo  motors  are  another  variety  that  offers  high-end  performance.    Their  physical  construction  is similar to that of the brushless DC motor;  however, there are no magnets in the AC motor.  Instead, both the rotor and stator are constructed from coils.  Again, there are no brushes or contacts anywhere in the motor which means they are maintenance-free.  They are capable of delivering very high torque at very high  speeds;    they  are  very  light  and  there  is  no  possibility  of  demagnetization.    However,  due  to  the electronic commutation, they are extremely complex and expensive to control. Perhaps the largest advantage of using servo motors is that they are used in closed loop form, which allows for very accurate position information and also allows for high output torque to be realized at high speeds.  The motor will draw the required current to maintain the desired path, velocity, or torque, and is controlled  according  to  the  requirements  of  the  application  rather  than  by  the  limitations  of  the  motor. Servo motors put out enormous peak torque at or near stall conditions.  They provide smooth, quiet operation, and depending upon the resolution of the feedback mechanism, can have very small resolutions. Among the disadvantages of servo motors are the increased cost, the added feedback component, and the increased control complexity.  The closed loop feature can be a disadvantage for the case when there is a physical obstacle blocking the path of motion.  Rather than stalling, the servo motor will continue to draw current to overcome the obstacle.  As a result, the system hardware, control electronics, signal amplifier and motor may become damaged unless safety precautions are taken. Rotary Optical Encoders are the popular choice to supply signal feedback.  They are mounted directly on the shaft of a servo motor.  The basic principle of operation is as follows:  A disc or plate containing opaque and transparent segments passes between an LED and a detector to interrupt a light beam.  All rotary encoders consist of a light source, light detector, code wheel, and signal processor.  There are two basic encoder styles: absolute and incremental. Absolute encoders use multiple detectors and up to 20 tracks of segment patterns.  As the encoder disc turns, the binary output changes one bit at a time.  For each encoder position, there is a different binary output, therefore shaft position is absolutely determined.  The resolution of an absolute encoder is determined by the number of concentric pattern tracks on the wheel.  Absolute encoders may be necessary for accuracy critical applications, military applications, or applications requiring accurate position information after power up or power failure, but this level of position detection is not required for typical machining and positioning applications. Light Source Disk Grid Assembly Photo Detector Squaring Circuitry Square Sine