ElmoMC SimplIQ Digital Servo Drives-Bell Getting Started - Manual (Page 62)

The SimplIQ for Steppers Getting Started & Tuning and Commissioning Guide
MAN-BELGS (Ver. 1.1)
62
A.4 Keep Margins
It is very tempting to increase the controller gains and enjoy the maximal
performance of your system. You must bear in mind that the price of maximum
performance is decreased robustness to system variations. The higher the gains, the
greater the chance that the system will become noisy or even unstable due to
changing work conditions, or due to ageing. The following tips will help you create
a stable, long-lasting motion system:
Tune for minimum inertia. If the inertia of the system varies, e.g. for a rotary
robot arm, tune for minimal inertia 3 . Positions or loading conditions with
higher inertia will have a slower response time, but are likely to remain stable.
The controller must remain stable (it does not have to maintain an optimum
response) when you double the selected gains, and also when the selected gains
are reduced by half4.
Acceleration limits. For a position controller, the maximum motor acceleration
(parameter GS[9]) must be set high enough so that it does not disturb normal
operation, but also low enough so that it prevents position disturbances from
creating large overshoots.
A.5 The Basic Concepts
This section concisely and informally explains the entities you will come across
when tuning.
A.5.1 Fixed- vs. Gain-scheduled Controllers
The drive can run either a fixed- or a gain-scheduled controller. A fixed controller
runs a fixed set of control parameters
5 .
Gain scheduling is the process of adapting the controller parameters "on the fly"
to a given situation. The Drive stores 16 sets of controller parameter sets. The
active controller parameters set is chosen by the gain scheduling process.
The drive supports two types of gain scheduling.
Automatic gain scheduling: the Drive adapts the controller to the speed
controller command, in real-time. The reasons for automatic gain scheduling
are:
o
When the speed becomes low, there is a large delay between consecutive
encoder position updates. This delay requires a decreased controller
bandwidth.
o
At low speeds friction becomes a dominant control problem. Increasing
the integrator gain at low speeds may improve low speed behavior.
3 Tuning for the least inertia may have a high price with high inertia postures or load conditions. You
can tune instead for several postures or loads and apply manual gain scheduling.
4 If you tune a speed controller you don't have to test with halved gains. Position controllers are,
however, conditionally stable. This means that a position controller will loose stability with gains that
are small enough.
5 The fixed/scheduled option refers to the proportional and the integral speed gains, position gain, and
some parameters of the Advanced filter.