# Selecting a dc motor for the position task

Hello to everyone!

At the moment i am trying to select a dc motor for my project. The motor has a task to make rotation in steps between 10° and 90° (it could be also 11° or 68° or another angle, which should be set). On the shaft should be mounted an encoder. This would made a closed loop system. (The motor should run till the measured angle is equal to the set angle).

The accuracy of the position should be <1°.

The dc motor, which i have been selected now, would rotate the system (shaft with encoder) with the speed of 8 rpm.

My question is, can the system with a speed of 8 rpm achieve angle for example 10° or 20° with an accuracy of < 1°?

If no, what rotation speed have to be?

The answer depends on "the system", which you forgot to describe in detail.

Tell us what sort of motor you will be using, the load parameters and if there is a gearbox, those details.

jremington:
The answer depends on "the system", which you forgot to describe in detail.

Tell us what sort of motor you will be using, the load parameters and if there is a gearbox, those details.

I cannot tell you the load parameters, because they will vary from 2.7 Nm to 20 Nm.

The system itself has a reduction of i=10 and eta=0.78

The motor would be 89 800 008 from here: DC Brush - Detailed Specifications & Drawings - Motors

Also i plan a gear box with a ratio of i = 58 with eta = 0.75

You are describing a servo motor with feedback for position control. Not a hobby type motor.

Paul

Unknown load moment of inertia, variable torque load, unknown gearbox backlash, unspecified motor driver, unspecified control algorithm: adds up to an excellent opportunity for learning!

load moment of inertia: not important, because 8 rpm (My opinion)

unknown gearbox backlash: I thought it is unimportant, because the encoder is on the shaft, which should be measured (output shaft of the system) and not at the dc motor.

unspecified motor driver: I have selected

motor driver: i wanted to ask which one you could recommend? the motor driver should have three input channels: enable, forward, reverse.

load moment of inertia: not important, because 8 rpm (My opinion)

Except for the EXTREMELY important periods when the motor starts rotating and accelerates to 8 RPM or decelerates from 8 RPM to a stop, as you make clear here:

The motor should run till the measured angle is equal to the set angle

Given that you are out of your depth, experimentation is your only option.

As for a motor driver, you need one that can easily handle the stall current of the motor. Pololu has by far the best selection. Pololu - Brushed DC Motor Drivers