On 31 October 2017 at 01:22, Chris Albertson <[email protected]> wrote: > I think the difference between position and velocity is how the error > signal is computed. After you have that. I don't think a PID algorithm > cares if it is controlling temperature, heading, airspeed or position. > Originally the algorithm was invented for steering ships at sea on a > constant heading.
It makes a big difference. PID is basically three different orders of differentiation of the error signal with respect to time. We have the first order differential (D), the zeroth order differential (P) and the -1th order (Integral, I). The output of the PID controller affects the process variable on a similar spectrum. For example a torque-mode position controller is actually controlling the _second_ order derivative of the process variable. So, a Torque mode drive will tend to use a fair bit of D. A velocity mode drive will tend to mainly use P. A position-mode drive with feedback would tend to mainly use I. The velocity-mode servo controlling to velocity is close to the position-position mode above, and will be mainly I (and FF0). Look at this another way, consider a velocity-mode drive controlling spindle speed with only a P-term. The PID output is P x (commanded_speed - measured_speed). If the spindle is exactly at the setpoint the output of such a P-only controller is zero. ie, a zero-velocity request Such a system will _always_ stabilise at a level below the setpoint that is a simple factor of the P-gain. In such a system the P-term is the equivalent of the D-term in a velocity-position loop. -- atp "A motorcycle is a bicycle with a pandemonium attachment and is designed for the especial use of mechanical geniuses, daredevils and lunatics." — George Fitch, Atlanta Constitution Newspaper, 1916 ------------------------------------------------------------------------------ Check out the vibrant tech community on one of the world's most engaging tech sites, Slashdot.org! http://sdm.link/slashdot _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
