DOES INERTIA MATCHING STILL MATTER WHEN DESIGNING A SERVO SYSTEM ?
GERARD BUSH , MOTION ENGINEER AT INTELLIGENT MOTION CONTROL ( INMOCO ), EXPLAINS .
With today ’ s technology , inertia matching isn ’ t the requirement it once was to design a servo-based machine .
However , when tuning a system to minimise the inertia mismatch , wider requirements remain in order optimise an application ’ s motion control performance .
From motor sizing through to minimising mechanical compliance , a comprehensive approach to motion design will enhance system stability and precision .
To size a servo system , previously , it was always considered essential to match the inertia of the motor with the load .
Matching the inertia , that is the resistance to changes in rotational motion , ensures the necessary torque to achieve the required acceleration and deceleration . It also enables the right dynamism and stability , and optimises efficient power transfer .
These rules remain true , yet the problem of aiming for a 1:1 inertia match ratio would often result in a much larger motor than might otherwise be required , or it would require a gearbox .
Either approach would create a more expensive , and less efficient outcome . Instead , today ’ s technologies involving faster processors and advanced control algorithms enables correction of the inertia mismatch .
Thanks to a closed loop servo system , which continuously monitors and adjusts based on feedback , the control and stability of position , velocity , and current / torque loops are enhanced . The servo drive tunes the control loops to operate with the required
bandwidth , determining how fast the servo can adjust in response to commands . The servo drive also impacts the level of stiffness , optimising precision and control of the system by managing the response to deformation or displacement when force is applied . As a result of these improvements , the development of the servo drive significantly reduced the need for inertia matching .
Modelling and simulation
The introduction of brushless motor technology and low mass , torque-dense NeFeB magnets , reduced motor inertia further still , however this extended the inertia mismatch .
In response , the development of increased processing power , as well as higher resolution feedback devices , allowed the servo controller to create accurate mathematical modelling and simulation of system responses .
Today , these tools enable motion engineers and machine designers to create interactive analytics by showing the precise detail of the mechanical system . Crucially , this data indicates how to address performance limitations .
An extended look at the detail of interaction across a mechanical system is also important to address compliance .
This challenge represents the natural springiness of the mechanisms between the driven load and the motor that creates delayed response times , leading to reduced system bandwidth .
When a large inertia mismatch is introduced to the system , such as a small , high torque motor connected to an exceptionally large load via a coupling device , the compliance problem is magnified .
When the motor quickly applies torque , the large load hesitates to respond due to its high inertia ; the delay is a result of coupling compliance between the motor and load that introduces windup before the load begins to move .
As the load finally synchronises with the motor , the large inertia causes overshoot of the target speed , resulting in the motor adjusting to slow down .
When the system adjusts the overspeed of the inertia , the target speed is again passed , triggering the motor to adjust once more . This causes a continued cycle of repeated adjustment that creates resonance and an unstable system .
34 AUTOMATION , CONTROL & ENGINEERING