
27-02-2015, 00:02
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Technical Director
AKA: Gus Michel II
 FRC #3946 (Tiger Robotics)
Team Role: Mentor
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Join Date: Jan 2014
Rookie Year: 2013
Location: Slidell, LA
Posts: 3,770
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Re: PID Tuning Help
Quote:
Originally Posted by SousVide
They really need to get into how PID works and why each of the variables do what they do..
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OK, time for an analogous case most people have seen to describe the terms: a coil spring and shock absorber suspension, as found on the front wheels of many cars. - P is the spring. The farther away you are from the set point, the harder P pushes towards it. Large magnitudes of P push even harder. P by itself causes oscillation unless there is physical damping (friction) in the system.
- D is the shock absorber. D pushes against motion. The faster the motion, the harder D pushes back. If D is too small, oscillation continues a long time (really old shock absorber). Moderate values of D dampen the oscillation (fewer cycles). A too-large value for D slows the stabilization down more than necessary; this is called over damping. The most familiar case of over damping is probably a door closer that seem to take forever to get through that last inch. OBTW, If you've got too much physical friction in your system, you may want to use a negative value for D to compensate.
- I doesn't have a good physical analog, though the closest might be those powered gas shocks like the L.A. low riders use. I helps compensate for "droop", which is when the system stabilizes either above or below the set point. Usually, you should leave I at zero unless you have a system that's pretty well tuned except for some droop.
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