PID Parameters Tuning/Meaning in internal algorithm

Integral gain is the most important for speed control, while Proportional gain will be most important for position control. Other factors weigh into getting optimal speed control in closed loop as well. What are your PID gains set at now? What is the PPR of the encoder you are using? Is the encoder mounted to the shaft of the motor before the step down gear or on the other side of the gear? Does your PPR configuration match that of your encoder specs? Usually oscillations are indicative of to high of proportional gain. The algorithm is overshooting and causing too high of error every time thus having to compensate. Usually with our controllers in speed control if you need to add P gain it should only be in small increments such as .1 or .2.

Cool. I don't see any configs there for the encoder itself. Is your ppr configured to 500? (setconfig(_eppr, 1, 500)?

Just a tip it may be easier to tune your PIDs from the Config tab and then test and adjust as needed rather than trying to do it running a script. I don't know what encoder you are using but I have a motor here in my lab with a 500ppr encoder and am able to get quite accurate and stable speed control out of it so its likely you just need to play around with the gains a bit more.

PPR (Pulses Per Revolution), is an parameter related to the encoder itself. This means that for every full 360 degree revolution of the motor shaft your encoder will pulse the number of times as indicated by the ppr. So, if the spec sheet says that the encoder has 2000ppr then this is the number you want to configure to the encoder input of the motor controller. This is good by the way as 2000ppr is obviously a much higher resolution than 500ppr would, thus it will give you more accurate and precise speed data.

Your motor spins faster in closed loop than in open loop because when you are giving commands in closed loop speed mode they are relative to whatever you have configured as your max RPM. In open loop it is just sending commands proportional to your power supply. Please take some time to read through Section 8 (page 87) of our User Manual so that you can really understand how closed loop speed mode works, this will help you.