28 n DRIVES January 2025 www.drivesncontrols.com Does servo inertia matching still matter? In the past, it was considered essential to match the inertia of a motor to the load when sizing servo systems. Matching the inertia – 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, but aiming for a 1:1 inertia match ratio can result in a much larger motor than might otherwise be needed, or can require a gearbox. Either approach creates a more expensive, less efficient outcome. With modern technologies using faster processors and advanced control algorithms, inertia mismatches can be corrected. Closed-loop servo systems which monitor and adjust themselves continuously based on feedback, enhance the control and stability of position, velocity and current/torque loops. The servodrive tunes the loops to operate with the required bandwidth, determining how fast the servo can adjust in response to commands. The drive also affects the level of stiffness, optimising precision and control by managing the response to deformation or displacement when a force is applied. As a result, the need for inertia matching is reduced significantly. Modelling and simulation Brushless motor technologies with low-mass, torque-dense NeFeB magnets have reduced inertia further still, but have extended the inertia mismatch. In response, increased processing power, as well as higher resolution feedback devices, allows servo controllers to create accurate mathematical models and simulation of system responses. These tools allow motion engineers and machine designers to create interactive analytics by showing the precise detail of mechanical systems. Crucially, this data indicates how to address performance limitations. To address compliance, it is necessary to look at the detail of interaction across a mechanical system. 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 bandwidths. If a large inertia mismatch is introduced into a system – such as when a small, hightorque motor is connected to a large load via a coupling device – the compliance problem is magnified. When the motor applies torque quickly, the load hesitates to respond due to its high inertia. This delay is a result of coupling compliance between the motor and the load that introduces windup before the load starts to move. As the load finally synchronises with the motor, the large inertia causes overshoot of the target speed, resulting in the motor slowing 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. The compliance challenge Most mechanical systems can be modelled and simulated mathematically using various excitation frequencies to identify the response point at which resonance occurs. However, the bandwidth of a system can never exceed the initial anti-resonance point. In fact, the higher the compliance, the lower the frequency of the initial resonance point, that reduces bandwidth accordingly. When the driven load is coupled directly to the motor to minimise compliance, the mismatch is mitigated, increasing the initial resonance frequency and creating a higher bandwidth system. Mathematical models show that the ultimate way of achieving a higher bandwidth and a cost-effective system is to increase the mechanical stiffness and reduce the total system inertia. Consider a directdrive system where the load is coupled directly to the motor with near-zero compliance. In cases like this, precisely controlling the system with a high bandwidth can be achieved with inertia mismatches as high as 30:1. As direct-drive systems are not suitable for all applications, compliant links will inevitably be introduced. However, advanced analytical tools can readily identify the compliant elements that reduce system performance. Applications characterised by a high inertia mismatch can include printing and labelling, as well as robots. Although this is no longer the main challenge, resolving the imbalance requires careful specification across aspects ranging from motor sizing, through to tuning and analytics of the control algorithms, and mechanical architecture. n Inertia-matching isn’t the requirement it once was when designing servo-based machines. However, when tuning a system to minimise the inertia mismatch, wider requirements remain to optimise performance. Gerard Bush, motion engineer at Inmoco, explains. A comprehensive approach to motion design will enhance system stability and precision.
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