How do you spec a motor+drive combo?

PreLC

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Hello PLCs.net!

I've worked with servos and robotics pretty often(guess not deep enough), but I've never really specced a drive+motor combo.

For this discussion, I would just like to specify a standard motor+drive combo, which, let's say, is trying to move a gantry coupled to the motor via a ball screw. How does one go about doing this?

Regards,
-PreLC
 
Size the motor based on the load, the acceleration/deceleration you need to move it at the speed you need, and how often you need to move it. Then select the proper drive for the motor. Many servo motor manufacturers also have motor sizing software. Or the people you buy the system from may be willing to run the calculations for you.
 
This is good info.
So let's say I need a standard 2000mm/s speed, with an acceleration of 1000mm/s^2.
Lets for this example, just assume that our ball screw's ratio is 360*=100mm, therefore desired omega=40pi rads/s and accel=20pi rads/s^2.

Lets say our mass to be moved is 100kgs, how do I calculate the torque requirements based on the data above?
 
The relation between torque and axial force on a screw is:
Torque = Force x Lead / (2 x PI x gear reduction ratio x %eff)

Efficiency of a ballscrew is usually given as 85-90% (that is, 0.85-0.90). If you are only moving a mass with no external forces applied, then the bulk of the force will be due to accelerating the mass, which can be figured with F=ma. Be sure to use consistent units.

For a thorough analysis, you should also calculate the inertia ratio. This is a bit more involved but basically you divide the inertia of the moving load (calculated) by the inertia of the motor (published). This value should not exceed 10 but many consider 2-5 to be better. If the ratio is too high, you won't get good dynamic performance even though your torque may be sufficient. This is where gearing can really help you because the load inertia seen at the motor is reduced by the square of the gear ratio. In other words:
J[load at motor] = J[load] / n^2
inertia ratio = J[load at motor] / J[motor]
 
The ballscrew is an inertial load that needs to be accelerated and decelerated. You need to include its inertia in your calculation along with the inertia of the motor. Servo motors have both a continuous and a peak torque rating. During acceleration and deceleration you can operate within the peak torque range as long as the RMS torque over the entire duty cycle falls within the continuous torque range.
 
The best way is to put your information of parts and motion cycle into a sizing program.

Bosch Rexroth has IndraSize. I believe Rockwell and Kollmorgen also have sizing software.

Given your inputs they will generally spit out a range of solutions that will work.
 

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