Does inertia matching still matter when designing a servo system ?
EDITOR ’ S CHOICE STABILITY & PRECISION
INMOCO
Does inertia matching still matter when designing a servo system ?
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 .
Gerard Bush , motion engineer at Intelligent Motion Control ( INMOCO ), explains .
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
Motor sizing through to minimising mechanical compliance , a comprehensive approach to motion design will enhance system stability and precision .
An extended look at the detail of interaction across a mechanical system is also important to address compliance .
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 , torquedense 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
26 PECM Issue 71