Electrical Overvies of Servo Motors

In older brushed servo motors, the phasing of the current supplied to the rotor was determined by the orientation of the commutator, a plate on the rotor which interated with the brushes to determine where the current should flow. In this manner, the motor’s operation could be controlled with a few potenionmeters. With this mechancial connection removed in brusheless motors, control became more complicated. All communtation became electrically controlled, requiring the use of motor controllers that employ some type of feedback sensor to determine the rotor’s position relative to the stator. The most popular sensors in use are Hall-effect sensors, optical rotary encoders and resolvers.

Hall-effect sensors operate by changing their volage in responese to a varying magnetic field. Thus, as the magnets on the rotor move, the voltage will change proportionally. Using sereral sensors spaced evenly around the rotor, one can determine the rotor position as well as its velocity. Hall-effect sensors have the added benefit of having no mechanical connection to the rotor, and are usually integrated into the armature windings. In most motors, they are used as the primary sensor to determine the require current phase to send to the motor, given their fast response and lack of additional control circuitry. A diagram of three Hall sensors in a servo motor is shown in figure 1.3, while a chart of the Hall sensor alignment and current phase for the Bodine E-Torq motor is shown in figure 1.4.

servo motor_1

In some cases, such as for stepping motors, velocity measurements are not required, but instead position. In this instance, the optical rotary encoder is the primary sensor, since no communitation control is needed. It uses a slotted disk attached to the rotor with either a binary or gray code cut into it. An IR signal is passed through the code wheel, and the resulting pulses aer processed in the controller to determine the rotor’s position. Of course, as the resolution goes up, so does the encoder’s complexity and cost Current encoders allow for milli-radian precision. The two main types are absolute and incremental encoders. The former measures the position based on an etched pattern in the wheel, while the latter measures interrupt pulses through a slotted disk. Absolute encoders aer the most popular, since they can also be used in an incremential fashion.

servo motor_02

An angular reslover uses a special rotary transformer to determine the rotor’s position. The theory behind these is relatively complicated, so the reader is reffered to for more detail. They can provide accuracy on par with an optical encoder, and their signals can be passed through a slip-ring, somethin useful for reducing cabling. Resovers tend to be complicated devices requiring sophisticated control circuitry as well as an adiitional rotary transformer to provide pwoer to the resolver itself, so they do not appear often in positioning applications, or situations with size and power constraints.

Servo systems must have feedback signals to close control loops. Often, these feedback devices are independent physical components mechanically coupled to the motor; for example, encoders and resolvers are commonly used in this role. However, the lack of a separate feedback device does not mean the system is not a servo. This is because the feedback device may be present but may not be easily identified. For example, head-positioning servos of a hard-disk drive use feedback signals built into the disk platter rather than a separate feedback sensor. Also, some applications use electrical signals from the motor itself to indicate speed. This technology is often called sensorless although the name is misleading; the position is still sensed but using intrinsic motor properties rather than a separate feedback device.


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