# Thread: inertia example, love some feedback

1. ## inertia example, love some feedback

hello guys,

i did read a lot here but i need to ask this to be sure.

if the inertia off the servo motor is 0,003 lb-in-s

and the inertia for the leadscrew and coupling is
0,0004 lb-in-s

is this a good thing or not.

ratio is now 10 to 1 or is it 0,1 to 1 ?????????

is the ratio good for fast acceleration

regards

2. Hi,

What you need to compare is the inertia of the motor with the inertia seen by the motor. To calculate the inertia seen by the motor you need to add the inertia of all the mechanical transmission with the inertia of your moving load affected by the transmission ratio (lead screw, pulleys, etc.). You can use an application like Motion Analyzer from Rockwell Automation for proper calculations. If the inertia seen by the motor is more than 10 times larger than the inertia of the motor you will have to decrease the gains of your PID loop and you will come up with a less responsive system. Raise the transmission ratio of your system to balance the inertia ratio, but at top speed expenses. If you want acceleration choose a servo motor with a nominal torque value that satisfies your expectations.

Regards,

3. ## ok

but wath will the ratio of the numbers above be ?

i stil dont understand, the lanquage is difficult.

sorry

4. Also there is the Kollmorgen program, similar to Allen Bradley, you can plug the numbers in of your system and see the result for different Accel/decel rates. 10:1 is considered max, but less that 5:1 is good.
Also any reduction reduces the inertia ratio by the square of the reduction.
Al.

• The ratio of the two inertia values of your first post doesn't mean anything. Inertia is a measure of how a particular combination of mass and shape opposes to changes of speed. The application I told you about is free. You can download it and play with it. It will help you to understand these concepts. You can try there all your what if scenarios. Galil site has several tutorials and videos for beginners. It will help you to master the required knowledge.

• Originally Posted by Al_The_Man
Also there is the Kollmorgen program, similar to Allen Bradley, you can plug the numbers in of your system and see the result for different Accel/decel rates. 10:1 is considered max, but less that 5:1 is good.
Also any reduction reduces the inertia ratio by the square of the reduction.
Al.

i use the sureservo program designed by copperhill engineering, if i insert the data off the components i want to use the inertia off the system is 0,0004 lb-in-s2

the mcg brush servo has 0,003,
so is the ratio now 10 to 1

the inertia from the motor is bigger, or not ( the value is ?!?!)

help me imagine it ( like mister E`s oneliner)

regards

• Yes it's bigger. This is odd. Are you considering the load your machine is going to manipulate?

• ## figures

Originally Posted by interflexo
Yes it's bigger. This is odd. Are you considering the load your machine is going to manipulate?
this is wath i think, strange. aint it

its a small machine for milling alu with 6mm max mill

table weight around 60 pound
16mm ballscrew 17 inch long
calculated tangial milling force max 250 N

When i look at different motor (playing with the sizer software ),
the bigger the motor, the ratio goes to 1 to and bellow ( 1 to 0,3 )

in theorie this will be good right, or not

• ## Mariss

i see you are logged in to this thread, wath is youre opinion.

• Yes, no problems on the inertia issue...

• sounds nice

thanks forr the advice guys.

when using the sizer software, do you guys use a safety factor on torque requirments and so how much

• Make sure your sizer software allows you to define your cycle profiles. Calculate your possible worst case and allow at least a minimum of 20% torque headroom for good servo control.

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