Servo Driver Working Mode
Motion servo drive are generally three-loop control system,from inside to outside are current loop,speed loop and position loop.
1.First of all,current loop:current loop input is the output of the speed loop PID regulator,we call it "current loop given",then it is the "current loop given" and "current loop feedback" comparing value difference which make PID regulator in current loop and output to motor, "current loop output" is the phase current of motor per phase, "current loop feedback" is not encoder feedback but Hall element that install inside the drive at each Phase(magnetic induction into current and voltage signals) send feedback to the current loop.
2.Speed loop:The input of speed loop is the output of position loop PID adjustment and the feedforward value of position setting,
which we call "speed setting".This "speed setting" and "speed loop feedback" comparing values difference that make PID adjustment
in speed loop(mainly proportional gain and integral processing),then output.This is the above-mentioned "current loop given." The feedback of the speed loop comes from the feedback of the encoder through the "speed calculator".
3.Position loop:Position loop input is external pulse(usually,except the servo drive which write data directly to the drive address),the external pulse after smoothing filter deal and electronic gear calculate as "position loop setting" .The above menthioned speed loop setting is constituted by the combined value which are position given feedforward signal and the setting with encoder feedback pulse signal output value that calculate by deviation counter and PID adjust by position loop(proportional gain adjustment, integral-less differentiation).Position loop feedback also comes from the encoder.
The PID constants value of the servo current loop are generally set internally in the servo driver,so operation user does not need to change.
The speed loop is mainly PI (Proportional and Integral),and the proportional is the gain,so we have to make the appropriate
adjustments to the speed gain and speed integral time constants to achieve the desired result.Position loop mainly make P
(proportional)adjustment.For this we just set the proportional gain of the position loop.
There is no fixed value for parameter adjustment of position loop and speed loop.According to the mechanical transmission connection mode of external load,load movement mode,load inertia,speed and acceleration requirements,as well as the servo motor's rotor inertia and output inertia,and many other conditions to determine the adjustment.Adjustment simple method is based on the experience of the external load within the context of the gain parameters from small to big,the integral time constant from big to small,with no overshoot of the steady-state value of vibration as the best value setting.
When the position mode needs to adjust the position loop,it is best to adjust the speed loop first(in this case,the proportional gain of the position loop is set at the minimum value of the empirical value).After adjust the speed loop,the gain of the position loop increases gradually.The position loop response is better to slower than the speed loop,otherwise it is prone to speed shock.