PID Engine RPM Controller

Paul W. · 2004-06-13T20:12:51+00:00

Hi Guys,I'm building an engine speed controller using a 16 bit microcontrollerfor a gasoline engine. I have a 1.8 degree resolution stepper motorcoupled to the throttle plate which moves the throttle angle to thetarget position.The objective is to regulate the speed of the engine to one fixedsetpoint in terms of RPM. I'm measuring the engine speed with amagnetic pickup and calculating RPM in the microcontroller. Mystepper motor driver simply works by setting a direction and number ofsteps which is generatred from the micro.I'm trying to set up a good PID control loop. I have implemented somecode, but am a bit confused over some of the algorithm options. I'mconsidering using a velocity form (incremental). Basically, theengine speed is the feedback which is subtracted from a reference RPMand fed to the PID controller. The output of the controller would bethrottle angle (I think?). The throttle angle then results in acertain engine RPM for a given load (highly non-linear of course).The problem I'm having is a conceptual one as to how to treat thestepper motor and the associated input to the "plant". I understandthat a stepper motor is a "self integrating" device, but I don't havea good intuitive feel for what that means in real-life.Is the proper way to implement?:- Use an incremental algorithm to calculate my *change* in throttleangle for each sample period.- Take this delta, and translate it to stepper motor steps (anddirection) based on the change in throttle angle.- Update this calculation each sample periodThe unknowns to me are:- Can I use a positional algorithm instead of incremental by takingthe PID output and subtracting the last position to generate a deltaand send that to the stepper? I have tried implemented the algorithmas positional and simply send the PID output (after translating to #of steps) and send that directly to the stepper - this seems tofunction but do I wind up with an extra, unwanted integration? Inthis scenario I interpreted the...

Jun 13, 2004 41 Replies

Rate this thread:

Walter Driedger 22 years ago

But isn't that the problem with all integrating systems,: "Providing there are no missed steps." You need some way of re-syncing the position once in a while. Murphy's going to get you.

Walter.

Vote

Tim Wescott 22 years ago

First, the driver was inherently integrating (it was actually integrating in software because we had a strict limitation on the speed that we could ramp up the cooler, so it went PDD -> limit -> integrate). Second, the loop closure was at a Hertz or two, so the differentiators were leveled off at around 5-10Hz, which really kept the noise down. And finally, it sits in a proprietary product next to a bunch of other control loops and video acquisition, all of which need to be good to

14-16 bits, so the we _really_ knew our noise reducti> Wouldn't that have serious problems with noise? Or did you have an

Vote

Join the Discussion

Have something to add? Share your thoughts — no account required.

Didn't find your answer?

Ask the community — no account required