How do I determine the PID settings for electronic load control?
Tuning PID (Proportional, Integral, Derivative) controls can seem like more of an art than a science, but if you have a good understating of what each component calculates its actions from, things get easier. Each of the DYNOmite’s PID factor multipliers (Gain, Drift, and Rate settings) is explained individually below (in terms of an engine RPM based load control valve).
Tip: Doubling any of the Gain, Drift, or Rate factors (whether from 2 to 4 or from 8 to 16) doubles its influence. So, if your combination requires large numbers, you will also need to make proportionally larger changes (to see a noticeable change in behavior).
Gain – (proportional multiplier) adjusts how much the controller instantaneously moves in response to a change in either Engine RPM or Hold RPM (or vehicle speed on a chassis dynamometer). For example, with twice as large a Gain multiplier, the load control’s action (valve travel or current output) doubles – in response to any RPM increase (e.g. from suddenly opening the throttle).
Tip: Avoid increasing the Gain multiplier so much that it overcompensates. Ideally the load valve should initially under-compensate a bit, letting the Drift close any remaining RPM deviation within a few seconds – or less.
Important: When your torque vs. RPM slope is extremely steep, limits imposed by a water brake type absorber’s rate of filling or draining time (latency) can make it impossible for any automatic valve to hold a steady RPM. In such cases, you may get some small improvement by going to a dual servo valve combination (to then simultaneously control both the inlet and outlet flow). Additionally, adding rotating mass can stabilize an engine – but adding too much inertia reduces the dynamometer’s ability to simulate transient RPM conditions.
Drift – (integral multiplier) adjusts the load valve’s aggressiveness in removing longer term (1+ second) steady state errors between actual Engine RPM and the desired Hold RPM (or MPH). The larger the Drift multiplier the more quickly the controller works (gradually) to eliminate holding RPM deviation.
Tip: Increasing the Drift multiplier too much induces a slow oscillation! So, initially set the Drift entry fairly low (less than 50% of its likely optimum value) while you work out your best Gain and Rate numbers. Once the Gain and Rate #’s seem optimum, experiment with incrementally larger Drift numbers – to reduce the time the Drift algorithm takes to gradually eliminate any remaining deviations between Holding speed and Actual speed.
Rate – (derivative multiplier) helps the controller “fight” changes in actual engine RPM vs. the desired Hold RPM. The Rate multiplier scales the DYNOmite algorithm that compensates for control latency – deciding how far to momentarily “over travel” the load control to steady a rapidly accelerating engine.
Tip: If you increase the Rate multiplier too much you may actually cause a rapid oscillation. So, maintain your Gain, Drift, and Rate numbers in relative proportion until you understand how they interact.