Kevin J. Miller 
Apparatus Repair & Engineering, Inc. 
Hagerstown, Maryland 
Technical Education Committee Member 

Motor sophistication, precise torque and speed control and accuracy are all common­place in the manufacturing environment today. They are not just reserved for the aerospace or machine tool industry. 

With modern processes in manufacturing we need to know and control the speed, position and velocity of electric motors. With this precision comes the need for feedback in these areas. 

Servo, closed loop vector, brushless DC, standard DC mo­tors and standard three-phase AC motors use feedback devices such as encoders and resolvers. Many AC and DC motors have the feedback device mounted on the exterior of the opposite drive endbell, mechanically attached to the rotating shaft. Servo, vector and brushless DC motors usually have an encoder or resolver mounted inside the endbell as an integral part of the equipment. 

Three basic forms of feedback devices are re­solvers, optical encoders and absolute encoders. 

Optical encoders 
Optical encoders for industrial applications typically consist of a shaft-mounted disk with concentric rings of alternate transparent and opaque segments. Early optical encoders used a glass disk and were susceptible to breakage from vibration or other mechanical means. Many manufacturers are now using a .0007”(.018mm) thick Mylar disk for flexibility and less breakage or longer life. 

The segments on the disk block or pass light from a light-emitting diode (LED) or incandes­cent light source to a group of photocells that convert the light pulses to an electrical output signal.

Condensation or contamination on the disk can also cause output errors. Their outputs can also limit acceptability in some applica­tions. 

While incremental encoders are reasonably in­expensive, their use sometimes results in a volatile system because they must be re-zeroed or reset after even a brief loss of power. They are found extensively in industry, although their reli­ability is somewhat marginal in harsh environments. Optical encoders are excellent lower cost devices provided they match the appli­cation consideration requirements. 

Absolute encoders 
An absolute encoder has each position of the revolution uniquely numbered. This means that instead of an output of pulses, you get an output that is a specific value in a binary numbering format. This is very useful when exact position­ing is required. Since each location of the absolute encoder’s revolution is a unique value, if the power should be lost, the actual position will be known when power is restored. The ex­act position will be known even if the controller loses power and the process is moved. Absolute encoders range from medium to high cost depending on the re­quired resolution. 

Resolvers 
Resolvers resemble small motors and are essentially rotary transformers. Fixed windings are placed on a laminated iron stack to form the ro­tor. Usually resolvers have a pair of windings on a rotor and a second pair on the stator, posi­tioned at right angles to each other for signal accuracy. When a rotor winding is excited with an AC reference signal, the stator windings pro­duce an AC voltage output that varies in amplitude according to the sine and cosine of the shaft position. Connection to the rotor is made by the brushes and slip rings, or by induc­tive coupling. 

Resolvers using the inductive method are re­ferred to as brushless types. Inductive (brushless) resolvers offer up to 10 times the life of brush types and are insensitive to vibration and dirt. This makes them ideal for use in many applica­tions. Typically resolvers have a higher initial cost than encoders but the cost factor may diminish based on the advantages of reliability, repeatabil­ity under power loss and harsh environmental considerations. 

The stator signals from a resolver are routed to a type of analog-to-digital (A/D) converter system known as a resolver-to digital (R/D) converter. Commercially available models include both track­ing and multiplexed types. Tracking and multiplex are adder signals that can be included for additional positions and references and will also accept multiple resolver inputs. 
type. Most encoders and resolvers attach electri­cally via a multi-pin connector. There are no standards for “pinout” connections for encoders and resolvers. Cables should always be the shielded type to minimize interference. 

Resolvers are one of the most reliable components available to monitor shaft position. They are well suited to hostile environments and their performance does not de­teriorate with time. They may be placed up to 500 feet (150m) from a system electronics package with a minimum of interconnect wiring. Resolvers offer infinite repeatability, provide outputs that are insensitive to electrical noise, and have a high degree of flexibility that simplifies matching them to converter electronics.

Other factors are couplings and cables. Couplings are special and almost always the spring type. Most encoders and resolvers attach electrically via a multi-pin connector. There are no standards for “pinout” connections for encoders and resolvers. Cables should always be the shielded
type to minimize interference.

List of manufacturers 
While there are many test equipment manufacturers, the following are some that benefit our industry with equipment that is useful for multiple manufacturers. Northstar Technologies, formally Lakeshore, sells an encoder tester.

Mitchell Electronics also makes an excellent tester with options for attachment cables for many different encoder types and manufacturers. Baldor Electric and Quality Solutions Co. offer courses at reasonable prices for basic technical training, proper identification, disassembly and assembly and testing.

Categories: Drives/controls

Tags: Motor controls Encoder Resolvers

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