ElmoMC SimplIQ Digital Servo Drives-Bell Getting Started - Manual (Page 94)

The SimplIQ for Steppers Getting Started & Tuning and Commissioning Guide
MAN-BELGS (Ver. 1.1)
94
For speed control, the derivative term represents acceleration. Acceleration is
hard to estimate with acceptable noise, so that the inclusion of the D term in
speed controllers is impractical. We therefore use the PI (PID without the D) form
for speed controllers. To summarize, for PI controllers we use
P
I K
s
K
s C
+ =
) (
(20)
that is
( ) ( )
( )
( ) ( )
( )
t t K d K ) t ( u
M P
t
M I
θ − θ ⋅ + τ τ θ − τ θ ⋅ = ∫
∞ −
& & & &
(21)
and for PID controllers we use (19).
B.4.3.2 The High Order Filter
The PI/PID controllers by themselves are fine for many applications, but they
have the following limitations:
They cannot notch out resonance.
They have poor high frequency measurement noise attenuation.
They have limited ability to add extra phase compensation, as required by
plants with a large mismatch between the inertias of the motor and the load.
The SimplIQ controller structure includes, in addition to the PI/PID controller, a
freely parameterized high-order (up to 8th order) linear filter to overcome the
PI/PID limitations. The resulting structure is:
KP[2]+KI[2]/s P(s)
s
-
Position
Speed
KP[3]
Ref
-
High
order
filter