2 ms·

Everyone will say "depends on the system" but generally the D term is not only not needed, but typically considered counter-productive or even against best prac

Everyone will say "depends on the system" but generally the D term is not only not needed, but typically considered counter-productive or even against best practices. PI-loops tend to be much more stable than PID-loops and far easier to successfully tune for an acceptable response. You do get overshoot with a decently tuned PI-loop, but it's not enough to cause problems. A bit of overshoot doesn't tend to hurt anything. Of course, a "perfectly" tuned PID loop will be "better" - more efficient, faster to reach setpoint, and minimize overshoot. But the effort/engineering time/operation time involved in tuning it, combined with the risks presented by a subtly poorly tuned loop that goes haywire one day when the system paradigm (characteristic) changes...just isn't worth it. Obviously there are times when you really do need a D term, but I can confidently say that they're not commonly used in HVAC or petrochemical plants. I'm sure there are other domains where the D term is needed, but in my time as a controls engineer I didn't run across them.

GabeIsko 3y ago
Especially in a video game context, where you can always go in and change how the system works. A really good way to tell is to consider the relationship between your input and output. PID get's thrown around all the time because a lot of the time you are talking about controlling an accerlation (gas pedal, throttle) to effect a position, so you theoretically need a PID to stand in for a 2nd order differential equation since position and acceleration are two derivatives apart with respect to time. But as you pointed out, you usually can generalize your input and not worry about the D term. In the video game I'm making, I don't really worry about derivative control, but PID control is mostly there to give the illusion that someone is driving a vehicle rather than it be controlled perfectly by a computer program. I don't want to model out how "skilled" the driver is - all I have to do to give that impression is just give the vehicle better handling.
polishdude20 3y ago
Yep, D terms are needed in systems where there isn't a natural dampening or reversing force that will naturally bring the value away from what your output tries to do. With a heater,once you get to zero error, any overshoot will reduce back the opposite way because heat is always lost.

GabeIsko 3y ago

Especially in a video game context, where you can always go in and change how the system works. A really good way to tell is to consider the relationship between your input and output. PID get's thrown around all the time because a lot of the time you are talking about controlling an accerlation (gas pedal, throttle) to effect a position, so you theoretically need a PID to stand in for a 2nd order differential equation since position and acceleration are two derivatives apart with respect to time. But as you pointed out, you usually can generalize your input and not worry about the D term. In the video game I'm making, I don't really worry about derivative control, but PID control is mostly there to give the illusion that someone is driving a vehicle rather than it be controlled perfectly by a computer program. I don't want to model out how "skilled" the driver is - all I have to do to give that impression is just give the vehicle better handling.

polishdude20 3y ago

Yep, D terms are needed in systems where there isn't a natural dampening or reversing force that will naturally bring the value away from what your output tries to do. With a heater,once you get to zero error, any overshoot will reduce back the opposite way because heat is always lost.