Miniature motor selection and the role of inertia
EDITOR ’ S CHOICE OPTIMISING PERFORMANCE
PORTESCAP
Miniature motor selection and the role of inertia
For design applications that depend on motion , whether that ’ s a robot , a surgical power tool , or a satellite control system , miniature DC motors are commonly used thanks to their performance and compact footprint . To accurately specify a miniature motion solution , understanding the role of inertia is crucial . While sizing tools can assist in this process , a thorough analysis of the application ’ s wider design , combined with targeted customisations , can help optimise performance .
When specifying a miniature motor , inertia is a key consideration . As a measure of a motor ’ s resistance to changes in rotational speed , its inertia value is based on a calculation involving the mass and radius of its rotor . High rotational inertia presents a greater challenge in accelerating the system , whereas a lower inertia indicates ease of acceleration . As less energy is required to accelerate or decelerate a motor with lower inertia , they are more energy-efficient , and this can make a significant difference in applications with frequent start-stop cycles . Motors with lower rotational inertia also usually offer improved control , which is a positive attribute for applications that require precise positioning .
While it may seem that lower inertia is optimal , matching the motor ’ s inertia with the load ’ s inertia is essential . In the most basic terms , if the load ’ s inertia is significantly greater than that of the motor , the motor will struggle to control the outmatched mass or size . The closer the inertia match , the more accurately the motor and drive system can control the load , especially in applications requiring precise movements .
As a measure of a motor ’ s resistance to changes in rotational speed , its inertia value is based on a calculation involving the mass and radius of its rotor .
Tackling inertia mismatch
Although a 1:1 inertia ratio is theoretically perfect , it ’ s neither practical or necessary to achieve . In real-world applications , striving for an inertia ratio close to 1:1 can
result in oversized components , higher system costs , and increased energy consumption . Instead , each use case has an acceptable range , although for applications that demand dynamic control and positioning , like high-speed assembly or textile yarn guides , a low load-tomotor inertia ratio is crucial .
The common challenge is an inertia mismatch caused by a high loadto-motor inertia ratio . This kind of imbalance can introduce stability issues that cause increased response times and lower system bandwidth . It can also result in wasted energy as the motor works harder to move the load . At its most serious level , it causes oscillations and resonance that could damage the motor as well as the load and connections .
Simplifying the transmission
Many motor manufacturers provide online tools and calculators to assist design engineers when selecting a miniature motor , including those such as Portescap ’ s MotionCompass™ . However , a comprehensive awareness of the contributing factors to inertia is useful in motion design and integration . While a motor catalogue and sizing tool can provide the inertia rating , holistic design tactics can improve overall application design and help close the inertial gap .
Gear reduction is a common step , and this technique reduces the load
34 PECM Issue 69