I am developing a laser scan head for a laser show using BLDC motors. I am facing a problem with regards to the tuning of the motors.

I have obtained the transfer function for the BLDC motor and plotted its open loop step response. Using the Zeigler NIchols open loop method i have calculated the coefficients - Kp = 5, ki=1800, Kd=0.004.

However on feeding this values to the BLDC SERVO Drive the mirrors start oscillating. Why is this so?

Moreover on using the autotune software, kp=155, ki=7,kd=1000 are the values obtained. The system does not oscillate and works fine with this order of values. Why is there such a huge difference in the values obtained and which one is correct? what are kp ki kd magnitude order generally in use?