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### Topic: stepper motor resistance for soft based limits (Read 987 times)previous topic - next topic

#### RobertTheArchitect

##### Sep 12, 2016, 05:25 pm
is there a way to track a stepper motor resistance to know when the motor has reached it's min/max rotation limit.

basically my stepper motor is connected to a adjustable damper that has a can only be rotated (depending on model) between 5 and 7 rotations.

what I am trying to do, is via software drive the stepper counter clockwise until it hits this physical limit in the damper (set 0 step), drive clockwise until limit while counting the steps. finally supplying the number of damper settings/total steps I can calculate the damper position.

i just don't know how to find min/max travel

#### rpt007

#1
##### Sep 12, 2016, 05:50 pm
Try to measure the current when the motor stalls.
You could have a small resistor in series with one of the stepper coils and measure the voltage being produced by the current flow. When the motor stalls it should increase and you can use this rise for triggering actions.

Did you really read and understand How to use this forum ?
AND:
Do you have already some solution or is a part of the problem sitting in front of the screen?

#### jremington

#2
##### Sep 12, 2016, 05:52 pm
You need a limit switch to detect the end of physical travel.

#### Robin2

#3
##### Sep 12, 2016, 06:46 pm
Your description of what you want to do is not very clear.

A stepper motor applies its full torque all the time so there is no simple way of detecting a stall.

A complex solution would be to put a rotary encoder on the motor shaft to count steps. If the number of steps commanded is greater than the number counted by the encoder you would know that the motor had stalled.

If the thing the motor is pushing against is fixed and could have a limit switch attached to it that would be a much simpler solution.

...R
Stepper Motor Basics
Simple Stepper Code
Two or three hours spent thinking and reading documentation solves most programming problems.

#### RobertTheArchitect

#4
##### Sep 12, 2016, 07:34 pm
limit switch won't work, think of screwing a nut and bolt together, there is a unknown number of rotations in either direction before the bottoms out or removes out of the threads.

#### Robin2

#5
##### Sep 12, 2016, 08:15 pm
OK. But I still don't have a clear picture in my mind of what you are trying to do. Maybe you can post a diagram?

Are you trying to use the motor to keep turning until a specific torque is achieved - like using a torque wrench?

If so it may be that a simple DC motor would be more suitable as there is a relationship between current and torque.

...R
Two or three hours spent thinking and reading documentation solves most programming problems.

#### jremington

#6
##### Sep 12, 2016, 09:26 pmLast Edit: Sep 12, 2016, 09:26 pm by jremington
Quote
before the bottoms out or removes out of the threads.
Whatever that means, a limit switch can almost certainly be arranged to detect it.

#### RobertTheArchitect

#7
##### Sep 12, 2016, 09:37 pm
attached is a quick and dirty diagram

#### Robin2

#8
##### Sep 12, 2016, 10:52 pm

...R
Two or three hours spent thinking and reading documentation solves most programming problems.

#### Robin2

#9
##### Sep 12, 2016, 10:59 pm
Am I correct that you want to drive the adjustment screw on the damper to its limit without first knowing how many turns that might require?

If so I presume the stepper motor is intended to stall when that limit is reached.

And if my surmise is correct I think the only way to do it is with a rotary encoder on the motor shaft as I mentioned in Reply #3

If you have to go to the trouble of using a rotary encoder it may be simpler to use a DC motor.

...R
Two or three hours spent thinking and reading documentation solves most programming problems.

#### MorganS

#10
##### Sep 14, 2016, 05:08 am
A stepper is possibly the right choice for this, but you must be able to control the current that is driving the stepper. Using an A4988 or DRV8825 type of chip, you can set the current limit so that the stepper only has a specific torque available. Then just drive it a thousand steps or whatever.
"The problem is in the code you didn't post."

#### weldsmith

#11
##### Sep 15, 2016, 06:12 am

If you know which model of dampener you are using and the number of max turns, limit the torque and run it a full 600 pulses or what ever is required for a full stroke and work backwards from there. a stepper would work perfect in this application. by doing this in your setup it will reset the position every time it is powered up. I believe MorganS is saying the same thing.
David Smith

#### weldsmith

#12
##### Sep 15, 2016, 06:19 amLast Edit: Sep 15, 2016, 06:45 am by weldsmith

You may try to mount the stepper directly inline with the adjustment screw if you have the room. Maybe a coupler made from a deep socket?
David Smith

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