Reasonable value of servo motor load inertia ratio
The Moment of Inertia is a measure of the inertia (the characteristic that the rotating object maintains its uniform circular motion or rest) when the rigid body rotates around the axis, and is represented by the letter I or J. The role of moment of inertia in rotational dynamics is equivalent to the mass in linear dynamics, which can be visually understood as the inertia of an object for rotational motion. The moment of inertia has no significant influence on the accuracy, stability and dynamic response of the servo system. In the servo system application, the ratio of the load inertia converted to the motor shaft to the inertia of the motor cannot be too large, and the value must be reasonable. Otherwise, the system generally There will be oscillations and even loss of control. But why do you need a proper inertia ratio, and this recommended inertia ratio is reasonable in practice? These are the problems that engineers often find confused.
1. Suitability analysis of servo motor load inertia ratio
1.1 Inertia matching - - optimal power transfer and maximum acceleration
All mechanical systems have a degree of resilience (ie, rigidity is infinitely infinite), while some mechanical systems have backlash. When either of these two types reaches a certain level, the system response performance will be extremely poor. Therefore, the problem that the so-called inertia mismatch may cause is actually due to insufficient mechanical rigidity, and there may be a problem of motion instability that may occur due to large elasticity or backlash. In the servo system, the amount of motion we need to control is the position or speed of the load end, but in reality, the position or speed signal detected by the feedback device mounted on the motor is used instead of the target load control amount, and due to the limited rigidity, this Under certain conditions, especially when the inertia ratio is too large, the probability of instability will occur with a large probability.
To improve the rapid response of the system, it is first necessary to increase the resonant frequency of the mechanical transmission components, ie to increase the rigidity of the mechanical transmission components and to reduce the inertia of the mechanical transmission components. Secondly, by increasing the damping voltage and lowering the resonance peak, it is also possible to create conditions for improving the fast response. In many equipment applications, it is common for mechanical transmission components to be insufficiently rigid and excessively inert. Therefore, the inertia of the moving parts should be reduced as much as possible while satisfying the strength and rigidity of the components.
For a particular motor, if a speed reduction mechanism is used, the load inertia that is reduced to the motor shaft matches the inertia of the motor (the load inertia is equal to the motor inertia, ie, the inertia ratio is 1), ignoring the increased inertia of the reducer and In the case of loss of efficiency, the system can achieve the best power transmission and get the maximum load acceleration, which is the meaning of inertia matching. A similar interpretation is also available in [5].
However, it is customary in China to replace the concept of “suitable inertia ratio” with the concept of “inertia matching”. In the foreign inertia ratio study, the concept of "inertia matching" is basically not mentioned, but the concept of "Inertia Mismatch" is mentioned. For example, in the literature [2], [3], [4], based on the fact that most servo system applications are "inertia mismatch", how to achieve fast response of the servo system without instability problem is studied.
