Encoder feedback can provide improved system accuracy, stall detection and peace of mind. Closed-loop positioning is most frequently used in critical applications to compensate for the mechanical elements of the system.
Position Tracking provides circuitry to interface an incremental optical encoder with a Compumotor Indexer. Dual-channel incremental encoder with quadrature and with single-ended or differential-TTL outputs may be used. The encoder may be used as part of a closed-loop system or as an independent means to verify position. The encoder may also be used to detect whether a stall has occurred during a move. The Compumotor Indexers listed below support Position Tracking, using virtually any incremental encoder.
Model Number:
6K, ZETA61XX, AT6400, AT6200, OEM750/750X, 6200, 6201, SX, PC21, 4000, 500
The number of encoder steps seen by the Indexer is equal to four times the number of encoder “lines”. For example, a 1,000-line encoder mounted directly on the motor will generate 4,000 encoder steps for each revolution of the motor shaft. A minimum of three motor steps per encoder step is required for successful operation of the Position Maintenance function. In the example above, a motor resolution of better than 12,000 steps/rev would be required.
If a reducer is introduced between the motor shaft and the encoder, the number of encoder steps/rev must be divided by the equivalent gear ratio to define the number of encoder steps received by the indexer.
Motor Stall Detection
When the motor is being moved, the indexer repeatedly compares the number of encoder steps coming in against the number of motor steps being sent out. If the encoder position deviates excessively from the desired position in the course of a move, the indexer assumes the motor has stalled.
This feature has several optional configurations. These include setting a “window” of allowable deviation to account for mechanical backlash and motor wind-up, terminating the move and/or sending an output signal is all stall is detected.
Applications where stall detection could be useful include: positioning glass vials in blood analysis, chemical mixing, X-ray head/table positioning, laser or water jet cutting, polishing mirrors for telescopes or drawing fiber optics.
Encoder Position Maintenance
The indexer may be configured to resist any spontaneous change in position when at rest. In Position Maintenance mode, the indexer repeatedly compares actual encoder step reference position with desired position while the motor is stopped. If external forces cause the encoder position to change, an error will be detected. The magnitude of this error determines the magnitude of the indexer's correction response. The correction response calculation yields a correction velocity, the stepper will then drive at the correction velocity until the error is reduced to zero.
Encoder position maintenance is often used in machining and metal removal applications. During a metal removal operation, forces can be developed and transmitted that cause the motor shaft to deflect. The indexer will sense the movement by counting the subsequent encoder steps. To maintain or compensate for the undesirable movement, the indexer will command the motor to return to the desired stop location.
Encoder Referenced Positioning
Indexers typically operate using position parameters in units of motor steps. With Position Tracking, the parameters can be in units of encoder steps. In this mode, when making a preset move, the indexer sends out step pulses to the motor until the correct number of encoder steps is returned.
Position information provided by the indexer may be in units of either encoder steps or motor steps.
Additional References