Need help for explanation of transient response in motor speed control with PID

Currently, I am working with speed control for a motor and a payload for an inertially stabilized platform.

In each image below, there are 3 phases, in the first phase, the motor is held at angle 0, before changing into speed control mode and finally the stop mode.

As I was tuning the PID parameters, I encoutered a phenomenom that I could not explain thoroughly. When I use PI with very small values for Kp and Ki, the response is quite "strange" as you can see in the first image. However, as I increased the values of Kp and Ki, and added Kd as a damper, the response looked much better. I deduce the difference in transient response may stem from the static friction and spring-like characteristics in mechanical structures, but I cannot completely explain the reasons in detail.

I hope that members in the forum can take a look at my case and explain the phenomenom of these transient responses to me. Thank you a lot. Wish you a good day !

FIRST IMAGE
1788676389279_269831842443034276_2417900984454409364_a47219ada07b17db20dd539f65e5dc5f.jpg


SECOND IMAGE
1788676667739_269831842443034276_2417900984454409364_418aec3aeb60b68d7d0bd78147248d56.jpg
 
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