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Properly Size Servo Motors in 6 Easy Steps

Properly Size Servo Motors in 6 Easy Steps

Sizing and choosing a servo motor for your application is no simple matter, but new technology is making it easier by the day. In the early days, we sized servo motors by hand, using Mathcad, and using spreadsheets, and it was often hard to iterate over and over as the design evolved. Today, thanks to new tools made by our motion control partners, we can size and select the appropriate drives and motors more quickly and with greater precision. We have sized motors, drives, and mechanical systems for hundreds of OEM rotary and linear servo motor applications through the years, and these new tools allow for quick iteration and accurate results, so long as the input data is good! 

Why do you need the right size servo motor? It’s not only about performance. While it’s true having small a motor will decrease the performance of your application, that doesn’t mean you should buy the largest servo you can find. Having too large a servo motor increases the cost of building your application, and you don’t want all that power to go to waste. 

Using Yaskawa’s SigmaSelect

Our solutions partner, Yaskawa has a software design and selection tool that we at AMMC have high confidence in using to select appropriate motor and drives  SigmaSelect. 

Download Yaskawa SigmaSelect Sizing Software 

This guide will give you a quick six-step sizing process that uses the SigmaSelect advanced sizing software tool. 

Motion System Design Step 1: Mechanical selection

Most machine sections can be modeled as basic motion mechanisms. Some sizing software is pre-programmed to ask for the machine-axis mechanism type and then present a set of fields to define its parameters. 

Yaskawa’s SigmaSelect software is an efficient tool for selecting specific series of motors for an array of motion system designs. It’s faster and more precise than performing various design-engineering tasks using a trial-and-error approach.  

One very common power-transmission device for linear motion is the ballscrew. Most ballscrew-based designs employ a rotary servomotor that is coupled to the ballscrew by a coupling. A rotary motor is translated into linear motion moving a load on linear rails. We’ll use this example moving forward. 

mechanism selection yaskawa sigmaselect

Motion system design Step 2: Gathering and Entering Data 

Getting mechanical and motion information can sometimes be challenging, but it is vital. The sizing software gives engineers the ability to visually see and enter data, and iterate with changes to tweak performance. The software then automatically identifies which servomotors can serve as the motion input for the application. 

showing lead of ballscrew in yaskawa sigmaselect

After entering the load and lead information, you must factor the inertia of the ballscrew and the coupling into the design. If this isn’t found from manufacturer data, use the software’s ballscrew inertia calculator to determine the inertia of the ballscrew (and coupling). 

After inertia and lead values are set, define the application’s move profile. Here, enter the distance, time and acceleration of the move. Include dwells or pauses  these are key for determining continuous torque required in highly cyclical applications. While it is good practice to enter both forward and reverse moves (return move) in a horizontal application, it is an absolute necessity if the motion is vertical to appropriately calculate torque and regen requirements (typically higher torque while ascending, higher regen while descending).

move profile yaskawa sigmaselect

Motion System Design Step 3: Selecting a Motor

With all the information entered, the design software will often return a sizable list of servomotors that are suitable for the application. 

In this case, if cost is an issue (and it usually is!) sort the suggested motor results by the cost factor to find the lowest (initial) cost servomotor that will satisfy the application requirements. 

Something else to leverage in the software is the ability to correlate the selection’s cost factor to the factor of safety. In many cases, spending just 10% more on the initial servo system purchase can give an additional 20% factor of safety. Marginally up-sizing allows for future scope creep — a common occurrence for motion systems as they are being developed. 

sigmaselect motion control software

Next, verify that the inertia ratio is at or below the allowable inertia ratio number. Refer to the software screenshot labeled “Allowable inertia” for an example of how these results appear — in this case, highlighted in green in the allowable inertia-ratio column. For our ballscrew-based design example and motor selected in the screenshot, the design’s load inertia ratio divided by the servomotor’s motor inertia is 44% — which is ok for the application. If the inertia ratio is high, consider sizing up the motor or adding a gearbox (below). 

Third, verify the speed and torque of the application sizing is at or under the motor’s maximum allowable values (motor manufacturers offer different series of servomotors for different applications to make this step easier). Note in the screenshot labeled “Speed and torque” that our chosen motor’s speed and torque ratings exceed what the application requires by some “safety” margin. 

gearbox values

Motion System Design Step 4: Account for Any Power-Transmission Additions to the Assembly

Remember, our sizing was done as a servomotor directly coupled to the ballscrew. What if the application can accept the use of a gearbox with a 5:1 ratio? Here is where we can iterate to select the best motor size, gear ratio, and inertia ratio – go back and enter new gear ratio data.  

This step may result in a selection of a smaller servomotor because gearboxes multiply the torque by the ratio number and reduce the inertia ratio by the square of the ratio. Be sure the sizing stays within the speed constraints of the motor! 

Motion System Design Step 5: Regeneration and Drive Requirements

A regenerative resistor may be needed, especially in vertical or high deceleration applications. Again, be sure to define the appropriate motion profile acceleration/deceleration, and as noted earlier define both directions of moves (a must in vertical applications).

regen yaskawa sigmaselect linear motion 

Motion System Design Step 6: Generating Reports for Sharing

Report generation with SigmaSelect software includes a simple PDF icon that prompts the output of a six-page report. Design summaries are easy to share with OEMs and end users so they can verify all data. 

sizing report yaskawa sigmaselect

Sizing motors is in our DNA. We’d be happy to help you navigate through the complexities of selecting the appropriate electro-mechanical system!

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