FEATURE Drives, controls & motors
Miniature motor selection and the role of inertia
Miniature motor selection and the role of inertia
BY Nicole Monaco, Global Marketing Manager at Portescap F
or designs that depend on motion, whether 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 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 the 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 with frequent start-stop cycles. Motors with
improved control, which is a positive attribute for applications that need 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 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.
Tackling inertia mismatch Although a 1:1 inertia ratio is theoretically perfect, it’s neither practical nor necessary to achieve. In real-world applications, an inertia ratio close to 1:1 can result in oversized components, higher system costs and greater 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-to-motor inertia ratio is crucial.
24 May 2024 | Automation MOTOR
The common challenge is an inertia mismatch caused by a high load-to-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 inertia in proportion to the square of the gear ratio. Moreover, modern control systems with advanced algorithms and high-resolution feedback devices can address inertia mismatch problems. However, the adverse to-motor inertia, can become worse by lack load compliance. To minimise these issues, we can look to wider components of the
DRIVE
application or machine to optimise rigidity. Load compliance challenges are more common in indirect drive systems. Here, the motor is not directly coupled with the load but is connected through one or more power transmission elements such as a gear mechanism, pulley belt systems, chain drives, or ballscrews. To simplify an indirect further potential causes of inertia, it is useful component beyond the motor shaft.
Direct drive systems
For demanding applications that would load inertia ratio, it is advisable to start with a direct drive system. This approach will keep the number of power transmission elements to a minimum whilst optimising compliance. With a direct drive, the motor is directly coupled to its load. This connection removes the inertia resulting from power transmission components, and the less inertia the motor needs to overcome, the less torque it requires to meet the desired acceleration rate. A direct drive system also minimises backlash, the play between mechanical components that can also impact power transfer and system reliability. Overall, it is best to do a comprehensive
review of all design aspects impacting the motion cycle. This means involving motion engineers early in the design process.
automationmagazine.co.uk
With a direct drive, the motor is directly coupled to its load. This connection removes the inertia resulting from power transmission components
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