Most engineers recognize that gearheads can be used to create a better inertia match between the object that is being moved and the motor being driven. Put simply, it is dictated by the gearhead ratio where the object reflected inertia to the motor is reduced by 1/ratio2.
So what should the correct inertia be?
Typically the rule of thumb says the object inertia should be no greater than five times the motor inertia (5:1 ratio) in a motion control application. This will deliver very stiff and controllable motion.
However, getting to this ratio may be difficult at times, mainly due to the fact that as you add more ratio to the gearhead, you will require more speed from the motor, which may not be available or may exceed the input speed limitations of the gearhead. Further, choosing a higher gearhead ratio will typically be costlier due to the extra gears that need to be incorporated into it.
So here are some useful insights from our application engineering experts on what you need to know about gearhead and servo motors:
In order to optimize motor control, the design engineering goal is to create a solution where the object inertia to motor rotor inertia ratio is as close to 1:1 as reasonable. This rule is not absolute and several factors impact it, including load distribution, acceleration/deceleration rates, settling time requirements, friction content, and motor coupling technology.
These general guidelines assume the motor can provide adequate torque for the load and move requirements. If the motor sizing is marginal you may experience stability and tuning issues regardless of the inertia ratio. This may also lead to issues later on when the mechanics wear and the loading characteristics change slowly over time.
5 things you need to know
- Under 10:1 inertia ratio provides good servo control in most applications assuming balanced loads and settling times > 10 msec allowed.
- When settling time of < 10 msec is required, a ratio closer to 1:1 will provide better stability and more likelihood of achieving the desired settling times.
- Non-balanced loads will impact stability and settling times so a ratio closer to 1:1 is important for better performance.
- Higher than 10:1 inertia ratios are acceptable if the application requirements allow lower acceleration/deceleration rates, longer settling times, or less positional accuracy.
- Direct drive and linear motors (or rotary with a direct couple) can tolerate higher inertia ratios as the load is typically mounted directly to the motor's rotor/forcer eliminating most of the coupler compliance issues. Some direct drive applications may be solved with inertia ratios as high as 300:1 (or higher) if lower acceleration/deceleration rates and slower speeds are acceptable while still achieving high positional accuracy.
Remember these additional factors
- Frictional content will impact settling time. Pure inertial loads (minimal friction from bearings only) will do better with closer inertia matching unless a direct drive or linear system is used.
- Rotary motor coupling choice will impact stability and settle time. Torsional stiffness in the coupler is important while still allowing for some axial misalignment.
- Higher resolution feedback will beneficially impact stability and tuning and thus overall servo control.
Special thanks to the Parker Hannifin Electromechanical and Drive application engineering team for sharing their established guidelines for inertia matching.
Article contributed by Jeff Nazzaro, gearhead and motor product manager, Electromechanical and Drives Division, Parker Hannifin Corporation.