Electrical Drives
Motor-follower -- Electronic Gearing
Encoder tracking is a technique that monitors a reference motor with an
encoder and controls a secondary motor. The method provides programmable motor-to-encoder
ratios that determine relative speed between two axes. A dynamic change in ratio produces
acceleration or deceleration.
Absolute motion-control profiles are normally set for a predetermined number of
encoder and motor steps. But indexers can delay a secondary profile from a reference
by a certain number of encoder steps, in either a fixed or dynamically changing manner.
Compared to the reference motion, the secondary motion profile can be made as simple or
as complex as needed. Applications for encoder tracking include electronic gearboxes,
conveyor belts, electronic cams, and web processors.
Many control applications require the coordination or synchronization of two or
more axes of motion in a specific pattern. The most familiar example is found
in X-Y plotters, but simultaneous motion in two axes need not always describe
a plane figure. Control of constant speed ratio between two pumps or the motion of a
welding head with respect to a moving conveyor belt are other examples. However, these
examples all require that the motion profile of a second axis be based on the position,
speed, or acceleration of a reference axis.
Before the introduction of tightly coupled indexers, primary and secondary motion
profiles were specified individually. As a result, small deviations or errors from an
ideal profile described for the primary reference axis often resulted in a gradual
drift between secondary and primary motion. But in programming the indexer, the absolute
motion profile of the secondary axis is not specified. Instead, the indexer is programmed
in terms of ratio, offset, or more complex relationships between the measured (reference)
motion and the controlled (secondary axis) motion. This eliminates the need for precise
absolute motion specifications for both the reference and secondary motion profiles.
A typical control system consists of an incremental, quadrature output encoder, an indexer,
and a secondary motor-drive system. The incremental encoder is connected to the primary
reference motion with a direct shaft coupler, belt and pulley, or rack and pinion.
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