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Unread 27-02-2015, 00:02
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Re: PID Tuning Help

Quote:
Originally Posted by SousVide View Post
They really need to get into how PID works and why each of the variables do what they do..
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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