what is Servo loop circuit?

What is servo current loop gain?

The inner control setting that shapes motor current response

Servo current loop gain determines how quickly and strongly the drive responds to the difference between commanded and measured motor current. In servo systems, this function normally belongs to the innermost current control loop. “Loop circuit” describes a feedback circuit in general; current loop gain is the more precise term for the setting discussed here.

Current, velocity and position loops have different jobs

In a common cascaded arrangement, the position controller generates a velocity reference, and the velocity controller generates a torque or current reference. The current controller calculates the voltage command applied to the motor windings. Thus, the current loop governs electrical response, the velocity loop regulates speed, and the position loop regulates the target position. Changing position loop gain is a different adjustment. The current loop normally has a higher bandwidth than the outer loops.

Current feedback comes from the drive's motor current measurement. An encoder or resolver supplies rotor position and commutation information; it does not measure winding current on its own. Current sensing may use shunt resistors or Hall effect current sensors. The power stage applies the controller's voltage command through PWM.

The torque connection: q-axis current

Field-oriented control represents current in the d and q axes. For common surface-mounted permanent-magnet servo motors operating with constant flux, torque can be approximated by T = Kt × Iq. Accurate current tracking therefore underpins servo torque control. IPM motors can also produce reluctance torque, and field weakening or saturation can make this simple relationship insufficient. The RMS or peak conventions for Kt and Iq must match.

What proportional and integral settings do

A common current-loop solution is a PI controller, part of the PID control family. Proportional gain Kp responds to the instantaneous current error; integral gain Ki corrects accumulated error. A simplified parallel expression is u = Kp × e + Ki × ∫e dt, where e is current error and u is the voltage correction command. An actual drive may also include feedforward, axis decoupling and output limits.

Some drives expose an integral time Ti or Tn instead of Ki. In the standard PI form, Ki = Kp / Ti, so reducing Ti strengthens integral action when Kp remains fixed. “Increase the integral setting” is therefore ambiguous without knowing the parameter definition. Manufacturers use different scaling and controller structures; numerical gains cannot simply be copied between brands.

What happens when gain is too low or too aggressive?

Insufficient gain may cause delayed current tracking and a sluggish torque response. Excessively aggressive tuning can introduce overshoot, oscillation and sensitivity to measurement noise. Audible noise alone is not a diagnosis: mechanical resonance, encoder problems and outer-loop settings may produce similar symptoms. Current-loop bandwidth, PWM carrier frequency and controller sampling frequency describe different quantities.

Gain is not a current limit. Raising gain does not increase the motor's continuous or peak current rating. At higher speeds, back EMF and available DC bus voltage constrain how quickly current can change. Increasing gain alone cannot correct voltage or current saturation; anti-windup, which limits integral accumulation during saturation, also matters.

Where commissioning should begin

First verify the motor model, winding resistance and inductance, feedback type and commutation setup. Many matched motor–drive systems calculate current-loop settings from motor data and need no manual current-loop adjustment. Follow the manufacturer's motor identification and automatic tuning procedure during drive configuration. Gain changes cannot compensate for an incorrect motor–drive match or inadequate load and transmission sizing.

If manual intervention is required, back up the existing parameters and follow the manufacturer's test conditions. Record commanded and actual current together, then assess rise time, overshoot and settling. For example, if a permitted test changes the reference from 1 A to 2 A and current peaks at 2.6 A, the overshoot is 60% of that 1 A step. This is a calculation example, not a recommended test current. Verify the inner loop before tuning the outer loops.

If measurements are noisy, inspect wiring, shield connections and grounding; an EMC filter does not replace correct loop tuning. Increasing inner-loop gain also cannot fix a slow supervisory PLC command or communication update. The objective is a stable, sufficiently fast response, rather than the largest gain value.

Editor's note: Current-loop gain should not be the first adjustment for every vibrating servo. When the motor–drive pairing and manufacturer's current settings are correct, the cause may lie in velocity/position gain, mechanical backlash or resonance filtering. Identify which loop a parameter belongs to before changing it.