This problem is overcome by using a term called Integral gain (KINT). This sums the error over time, so that the motor torque is gradually increased until the positional error is reduced to zero [ like a person gradually pushing harder and harder on your car until they’ve pushed it level with Demand].
However, if there is large load on the motor (it is supporting a heavy suspended weight for example), it is possible for the output to increase to 100% demand. This effect can be limited using the KINTLIMIT keyword which limits the effect of KINT to a given percentage of the demand output. Another keyword called KINTMODE can even turn off integral action when it’s not needed.
The remaining gain terms are Velocity Feed forward (KVELFF) and Acceleration Feed forward (KACCEL) described below.
In summary, the following rules can be used as a guide:
HKPROP: Increasing KPROP will speed up the response and reduce the effect of disturbances and load variations. The side effect of increasing KPROP is that it also increases the overshoot, and if set too high it will cause the system to become unstable. The aim is to set the Proportional gain as high as possible without getting overshoot, instability or hunting on an encoder edge when stationary (the motor will buzz).
HKVEL: This gain has a damping effect, and can be increased to reduce any overshoot. If KVEL becomes too large it will amplify any noise on the velocity measurement and introduce oscillations.
HKINT: This gain has a
HKINTLIMIT: The integration limit determines the maximum value of the effect of integral action. This is specified as a percentage of the full scale demand.
HKDERIV: This gain has a damping effect. The Derivative action has the same effect as the velocity feedback if the velocity feedback and feedforward terms are equal.
HKVELFF: This is a feed forward term and as such has a different effect on the servo system than the previous gains. KVELFF is outside the closed loop and therefore does not have an effect on system stability. This gain allows a faster response to demand speed changes with lower following errors, for example you would increase KVELFF to reduce the following error during the slew section of a trapezoidal move. The trapezoidal test move can be used to
HKACCEL: This term is designed to reduce velocity overshoots on high acceleration moves. Due to the quantization of the positional data and the speed of the servo loop, for the acceleration feed forward term to affect the servo loop the acceleration of the axis must exceed 1,000,000 encoder counts per second.
| MN1903 |