# How to select the correct DC servo motor

I have to buy DC servo motors for a project. The problem is, I don't know how to pick the correct one. I'm going through the datasheets from different companies, and the models they sell are based on peak stall current, continuous torque at stall, rated speed rpm, rotor inertia and so on.

The information I have is the weight of my load, the power supply, the resolution I want, and the diameter and pitch of the leadscrew. Using this information, how do I select the appropriate motor?

Before looking at motors, you need to define a few parameters, apart from the load mass and leadscrew pitch. First, what is the motion profile? I assume the device won't be in constant motion. Is there a defined sequence of movements? What distance and elapsed time for each move? What elapsed time between moves?

Next, the load. Is it affected by gravity? Are there other external forces acting on the load? What about friction? You should also know the diameter and material of the leadscrew (for inertia calculation).

With all of that, you can proceed to calculate peak torque, RMS torque, and speed required at the drive end of the lead screw. That will indicate what sort of motor you need.

Then you need to consider the ratio of the reflected load inertia to the motor's rotor inertia. A high ratio will necessitate de-tuning the position loop in order to get stability. Ideally, the ratio is 1:1 (hard to achieve), but under 3:1 is generally very good. Under 6:1 is generally acceptable. If the coupling is "stiff", 10:1 can be OK.

If the inertia ratio needs reduction, first look at a bigger motor. Otherwise, a gearbox may be required. Reflected inertia reduces as the square of the gear ratio. But speed requirements increase.

Start with calculating the mechanical power you need to raise your load at a given speed. Each metre you raise 1kg requires an input energy of m.g.h where m is mass, g is grav constant and h is 1m. If it's 1kg then you need approximately 10 joules to raise it a metre.

If this is required to be done in 1 second, then that is a power of 10 watts.

Work out what power output your motor needs to generate and then choose a motor that is 1.5 x (or more) the power output for the "just-in-case" situations.

Now you have your motor power and for any motor it will probably produce max power at a certain range of rpms. The data sheet should tell you what current your power supply needs to be to achieve this rpm under the load conditions.

• This ignores the lead screw, wihich functions like a large gear. Jan 5, 2014 at 20:29
• @ScottSeidman I've not made any assumptions about any form of speed reduction device (gear box) however, for any gearbox, power in = power out + losses so, calculating power is the right thing to do. The power rating of the motor is 1st thing in my book. Jan 5, 2014 at 21:01