Electrical Drives
Motor-follower -- Electronic Gearing
The indexer receives position and velocity data from the reference encoder
every 10 msec or less. The indexer then provides step and direction data to the
secondary motor-drive system based on the incoming encoder steps and the programmed
move sequence. The secondary drive system translates the step-and-direction signals
into controlled shaft position.
Consider an encoder producing steps at a rate of 1,000 Hz in the positive direction.
If the ratio is 2:1, the indexer's output will be 2,000 Hz in the positive direction.
If the ratio is zero, the output will be 0 Hz. Likewise, if the ratio is -2:1,
the output will be 2,000 Hz in the negative or opposite direction. Without changing ratio,
if the reference speed drops to 500 Hz, the secondary axis follows at 500 Hz positive, 0 Hz,
and 500 Hz negative.
The ratio is changed by program statements unique to each segment of a profile.
A segment is defined by the number and ratio of motor steps to encoder steps it covers.
The indexer changes ratios instantly or gradually, over a user-defined number of encoder
steps. This allows the controlled motor to accelerate or decelerate to a new ratio without
stalling while the reference axis maintains a constant speed.
In many applications, move profiles are created by defining and piecing together
several move segments. For applications in which the total number of motor and encoder
steps covered is critical, defining segments allows a more precise profile specification
than simply ramping to new ratios.
A motion-control profile based upon encoder speed is illustrated by either a
profile as a function of time or as a function of encoder steps.
The first shows the X-axis as time in seconds, and the Y-axis in velocity or
motor steps per second. The velocity ramps from zero to 10,000 steps/sec over 10 sec,
stays at 10,000 steps/sec, and ramps back to zero over 10 sec. The ratio is velocity,
the change in ratio is acceleration, and the total distance covered over 30 sec is the
area under the profile, or 200,000 steps.
The second shows the X-axis as encoder steps and the Y-axis as the
ratio of motor steps to encoder steps. This ratio ramps from zero to 10 over 10,000
encoder steps, remains at 10 for 10,000 encoder steps, and ramps back to zero over
another 10,000 encoder steps. The total number of motor steps covered during 30,000
encoder steps, regardless of encoder speed variations, is the area under the curve, or
200,000 steps. If the encoder produced 1,000 steps/sec, the two profiles
would exactly match over the same 30-sec period.
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