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Slow settling occurs where the servo is sluggish when coming to rest. Systems with problems of slow settling may appear to respond reasonably well at speed, but will move into final position very slowly.
Slow settling can be caused by having small low-frequency gains. Examples of low-frequency gains are the velocity-loop integral and position-loop proportional gains. For PID position loops, the P and I gains are low-frequency gains. Normally, small low-frequency servo gains cause slow settling while low high-frequency servo gains cause Low Bandwidth. Examples of high-frequency gains are, for velocity loops, the proportional gain or, for PID position loops, the D gain.
Slow settling problems can be worsened by high stiction (static friction). When a motor is moving, the frictional forces are the lower "Coulomb" or sliding frictional forces. Just as the motor slows to pull into position, the frictional forces increase, sometimes by a factor of two. These forces can slow the motor coming into position.
You can verify this problem by raising low-frequency servo gains and observing the response. If you are using a PI or PDFF(PI+) velocity loop, you can experiment with the velocity-loop integral gain. If you are using a PID position loop, try raising the proportional gain. You can also use the position-loop integral gain. However, this gain can be difficult to tune because it can cause substantial overshoot.
The ideal command for this test is a low-amplitude square-wave velocity command. A square-wave velocity command is equivalent to commanding unlimited acceleration. Some positioners do not allow such a command. If you cannot command a square wave, command small incremental moves (say, 50 mSec moves), with the maximum acceleration rates allowable, and allow 500 mSec or 1 Sec in between move. Be sure to limit the amplitude of the command to avoid saturation of the current loop. Anytime you are observing servo response, you should keep the commanded current below the maximum. Saturating the current controller masks stability problems.
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