5-14 Operation MN1903
Thispr oblemis overcome by using a term called Integral gain (KINT). This sums the error
overtime, sothat the motor t orque is gradually increased until the positional error is reduc ed to
zero[ like a person gradually pus hing harder and harder on your car until they’ve pushed it
levelwith D emand].
However,if there is largeload on the motor (it is supporting a heavy suspended weight for
example),i t is possible for the output to increase to 100% demand. This effect can be limit ed
usingthe KINTLIMIT k eyword which limits the effect of KINT to a given percent age of the
demandoutput. Another keyword called KINTMODE can even turn off integral action when it’s
notneeded.
Theremaining gain terms areVelocity Feed forw ard (KVELFF) and Acceleration Feed
forward(KACCEL) described below.
Insummar y,the followi ng rules can be used as a guide:
HKPROP: Increasing KPROP will speed up the respons e and reduce the effect of
disturbances and load variations. The side effect of increasing KPROP is that italso
increases the overshoot, and if set too high it will cause the sy stem to become unstable.
Theaim is to s et 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
KVELbecomes too large it will amplify any noise on the velocity measurement and
introduceos cillations.
HKINT:This gain has a de-stabiliz ing effect, but a small amount can be used to reduce any
steady state errors. By default, KINTMODE is set so that the KINT term is either ignored,
oris only applied during periods of constant veloc ity.
HKINTLIMIT:The integration limit determines the max imum value of the effect of integral
action.T his 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
theveloc ity feedback if the velocity feedback and feedfor ward terms are equal.
HKVELFF:This is a feed forward term and as such has a different effect on the servo
systemthan the previousgains. KVELFFis outside the closed loop and therefore does
nothave an effecton system stability. This gain allows a faster response to demand
speedc hanges with lower following errors, for example you would inc rease KVELFF to
reducethe f ollowing error during the slew section of a trapezoidal move. The trapezoidal
testmov e can be used to fine-tune this gain. This term is especially usefulwith v elocity
controlleds ervos
HKACCEL: This term is designed to reduce velocity over shoots on high acceleration
moves. Dueto the quantiz ation of the positional data and the speed of the servo loop, for
theacc eleration feed forward term to affect the servo loop the acceleration of the ax is
mustex ceed 1,000,000 encoder counts per second.