# INERTIA MATCHING – THE MOTION CONTROL ENGINEER’S ‘HOLY GRAIL’

Dec 122011

My experience tells me that the single most critical factor in designing efficient and effective dynamic positioning systems is specifying the correct size of servomotor.

Why do I think this is so important?

Because getting it right, apart from being personally very satisfying, will make a real difference to return on investment.
It saves on capital cost – don’t buy more motor than you need
It saves on maintenance cost – buy enough motor to avoid overstress, burnout and downtime
It saves on running cost – use the smallest amount of energy making the product

A dynamic positioning system puts great demands on a motor and machine and calculating the match of inertia is critical to accurate dynamic performance.

Considerations

The inertia of the motor should be the same as the inertia of the load to deliver the maximum transfer of energy.
The textbook answer is a ratio of motor to load of 1:1. My experience in designing and implementing successful systems in a wide variety of locations and industry sectors worldwide is that a practical rule of thumb is 3:1.
It is key to understand precisely what the ‘motor’ and ‘load’ are. My recommendation is to see everything beyond the shaft of the motor as ‘load’ and the rest as ‘motor inertia’.
Torsional stiffness – In a torsionally rigid system turning a shaft turns a load but the less rigid the system the more important energy matching becomes. It is not sufficient just to have enough speed and enough torque to produce the desired result.

A servomotor within a positioning control design must also take account of the conditions within which it must operate. There may be heat, dust and humidity and, in addition, it may have to make thousands of moves a minute for years with infinitesimally small failure rates. Therefore a degree of tolerance must be factored in to maintain extremely high levels of reliability.

If you consider that a confectionary bar line will output in excess of1000 bars a minute, around a million bars per day then an acceptable failure rate will be less than one in a billion if customer satisfaction levels are to be sustained.

This is just one reason why ‘inertia matching’ is the ‘Holy Grail’

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