Rick Armstrong
Development Manager
Danaher Motion
Wood Dale, Ill.

These rotary encoders are typical of the wide range of styles, accuracy, and resolution available today.

Servomotor-powered motioncontrol systems are expected to be fast, accurate, and reliable. Yet even the most expensive servomotors can be victims of external factors degrading their performance. Leading the list is the feedback device sending position and other information back to the motor controller. So a fundamental understanding of the different types and uses of feedback devices is a prerequisite for speed and accuracy.

Servoapplications may use one or more feedback schemes. These schemes can be based on their location in the system, whether they measure incrementally or absolutely, or the sensing mechanism for detecting motion.

FEEDBACK LOCATION

Many servomotor manufacturers now integrate a feedback device directly into the motor housing. This AKM motor from Danaher Motion is designed to accommodate most types of feedback devices without requiring extensive modifications in the housing or on the feedback device.

For position control the best location for the feedback device is at the load. Position errors created by mechanical play and slippage place loads in locations where they shouldn't be. Loadmounted position sensors eliminate the effect of this end play. These sensors are in addition to any feedback device already mounted inside the motor. Brushless motors require rotor-position feedback for electronic commutation and speed control. Some systems use these internal feedback sensors to monitor position by determining the direction and distance the motor shaft moves. When motor-mounted feedback devices can't be avoided it is important to keep the total error between sensed and actual true position of the load within acceptable limits.

There is one scenario when using internal sensors is okay. External sensors aren't needed when motors drive the load without any intermediate mechanism. Known as direct drive, the effect is the same as connecting external sensors directly to the load. There are rotary and linear motors with enough resolution that include feedback from their internal sensors for direct-drive applications. Direct-drive motors also minimize maintenance and eliminate compliance, or lost motion, that reduces the responsiveness or bandwidth of the system.

ABSOLUTE OR INCREMENTAL

Feedback sensors report position in either absolute or incremental formats. An absolute position sensor reports a pattern or code that signifies its exact position within one electrical cycle of system power-up. By contrast, the incremental position sensor typically provides output pulses for each increment of motion. By means of counting the pulses, the system knows how far and in what direction it has moved.

Most encoders (both absolute and incremental) use a light source which projects a beam through a narrow slit in a code wheel and through a precision aperture onto a light sensor. The wheel revolves in synchronism with the servomotor rotor. Light falls on the sensor when the slit and aperture align.

Systems equipped with incremental encoders must be "homed" or set to a known starting position at power-up. From this home position incremental movements track how far the load has moved and in what direction. Combined with a position memory, the system reports an absolute position as long as power remains on. On power loss the system "forgets" its current location and must be reset to the home position before position data is again usable. Connecting the controller to an uninterruptible power supply prevents such problems for critical applications.

Some sensors are extremely rugged and target the industrial machine-control industry. Others are relatively fragile, intended more for precision laboratory equipment. And then there are applications where the requirements overlap. Semiconductor manufacturing calls for high accuracy in a clean environment with high-speed throughput to meet demanding production schedules.