Inertia mismatch

From Granite Devices Knowledge Wiki
Jump to: navigation, search
Motor vs load inertia.jpg
In servo motor system, inertia mismatch means the difference of physical inertia between the motor and the load. This is important because any flexibility of motor shaft coupling to load forms an oscillator that has resonance frequency that depends on the inertia mismatch. The higher the load inertia is compared to motor inertia, the lower the resonant frequency comes.

It is generally to avoid higher than 10:1 inertia mismatch, meaning that load inertia should be no more than 10 times the motor's inertia. However, for good servo stiffness it is recommended to keep mismatch below 5:1 or even 3:1. The lower the better. Some most demanding applications may require ratio of 1:1 or even 0.7:1 (motor inertia greater than load inertia). However, with very stiff machine construction combined with other countermeasures, mismatch of 1000:1 or more is possible. Achieving that however, might require deeper understanding of the physics behind this phenomenon.

High inertia mismatch typically causes difficulties achieving good servo stiffness without getting oscillations/overshooting or instability.

Caulculating inertias[edit | edit source]

Motor[edit | edit source]

To obtain motor inertia, refer to your motor data sheet.

Load[edit | edit source]

Load inertia depends on the size and shape of the load. Also in lead screw mechanics, the inertia will be the sum of rotary inertia and reflected linear inertia. For various calculation equations, see the links below:

Countermeasures[edit | edit source]

Several methods exist to counter the effects of high inertia load.

Reduction[edit | edit source]

Use reducer, such as low backlash timing belt to reduce the reflected inertia to motor. Reduction reduces load inertia by power of two compared to reduction ratio. I.e. if reduction is 1:2 then reflected load inertia reduces by factor of 4.

Stiffer coupling[edit | edit source]

As the coupling between the motor and load is part of the oscillating system, using higher stiffness coupler will make improvement.

Servo tuning[edit | edit source]

Try following tuning ideas:

  • Reduce torque control bandwidth Torque bandwidth limitTBW, note this also requires retuning of velocity and position control gains
  • Use notch filter that matches the resonance peak frequency (available in IONI Pro and similar drives)

Further reading[edit | edit source]


In no event the Product Information or parts hereof shall be regarded as guarantee of conditions or characteristics. The Product Information or any part thereof may also not be regarded as a warranty of any kind. No liability of any kind shall be assumed by Author with respect to Product Information or any use made by you thereof, nor shall Author indemnify you against or be liable for any third party claims with respect to such information or any use thereof.

As content of this Wiki may be edited by user community, Granite Devices Oy or it's affiliates do not take any responsibility of the contents of this Wiki. Use information at your own risk. However, Granite Devices staff attempts to review all changes made to this Wiki and keep information trustworthy.

Without written consent, Granite Devices' Products or Intellectual Property shall not be used in situations or installations where living beings, material property, or immaterial property could be harmed by the operation, features or failures of Product. Products may only be used in a way where hazards like moving parts, electric shock, laser radiation, or fire can't be realized even if the content of this Wiki would suggest otherwise.