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Old January 26th, 2018, 05:35 AM   #1
PLC Pie Guy
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Help with linear servo.... I think.

Hey All.

I'm doing a little project at home where I have a need to automate something.
I have never played much with servo or stepper technology but this I think is well suited.

Its a small application where I need to move a lever a total throw of about 5 inches. No problem, but I need to stop in 6 pre-defined positions along the way.(One being home, full retraction) The lever is very easy to move and can be done with lightly applied finger type force. Speed is the name of the game. I need it to move between the positions quickly.

Iv been looking at a lot of the stuff available for this kind of thing and its endless. I am however having trouble finding a tiny linear servo, with position feedback that has any speed to it. One unit was .51"/sec. A little to slow.


As for driving it, I'm thinking of a small controller that may be part of a system to work with above mentioned linear actuator, or perhaps an Arduino solution, simply to see if my project is going to work anyway..

I have included a simple concept drawing to illustrate what I want to do. My drawing only shows 5 positions but I have a need for 6.

So, question is.. Can anybody recommend me some equipment that may help me. Has anybody done something like this before with fine positioning? Perhaps there are better methods than using a linear actuator????

Any thoughts??
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Old January 26th, 2018, 05:41 AM   #2
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After posting I started thinking,, what about a small position driven servo motor mounted on the shaft instead of using the lever. Lets just say that the shaft would rotate back and fourth about 90 degrees. Could I tell it to go to say 30 and have it stay there, until I tell it to go to 80 and stay there.

Its low demand as well. It may only move a few times a day.

Thanks

Just thinking out loud.
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Old January 26th, 2018, 06:06 AM   #3
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I have used these before. http://www.smac-mca.com/

Your issue with any solution is you are going to need absolute feedback so you don't have to do a homing routine. Not sure what your budget is, but this won't be cheap to do via linear or rotary servo motor.
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Old January 26th, 2018, 06:11 AM   #4
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Linear: https://uk.rs-online.com/web/p/elect...ators/8852256/

Rotary: https://www.smcworld.com/actuator/en...e.do?ca_id=679

used both with good success
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Old January 26th, 2018, 11:20 AM   #5
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You may want to look into IAI. They make various different kinds of servos that are pretty easy to setup, and integrate with your existing controls.

I have used a linear one and also a rotary table one, both with good results and reliability. Linear has been in service about 4-5 years, rotary about 2.
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Old January 26th, 2018, 11:32 AM   #6
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Iv been looking at stepper motors / controls. Iv only had a few minutes today to even think about it but.... although there is no feedback, they are remarkably fast and accurate to very small tolerances when using the step angle pulse method. Just a few youtube vids I watched closely demonstrated what I'm trying to do. Send it xx amount of pulses to move to xx position and apply staying torque. In my application, the load once position has been reached will be null if any. Again, I need to only turn a 3/8 shaft from 0 - 90 degrees, stopping at any 1 of 6 given points. Once in position it could be an hour or better until it gets moved again. Then it may go down in step angle or up depending on the move required.
Likely need a home switch though as the shaft is only able to turn 90 degrees mechanically. Making a full revolution of the motor impossible. Im wondering if the fact that the motor can never make a full turn going to be a problem?

I'm still researching. thanks for the tips!

Last edited by PLC Pie Guy; January 26th, 2018 at 11:34 AM.
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Old January 28th, 2018, 03:49 PM   #7
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yes arduino is cheap, and simple to program.
a stepper is very simple to program in arduino, have a look at arduino.cc
they also have a forum.
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Old January 28th, 2018, 07:56 PM   #8
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Quote:
Making a full revolution of the motor impossible. Im wondering if the fact that the motor can never make a full turn going to be a problem?
Most arduino stuff is going to have very low torque. If you gear it say 10:1 then the motor will make more than one rev. But, it shouldn't be an issue if you get one that can do the torque required without gears.

Another consideration is, with no feedback, what if there is power loss when the motor is at say 20 degrees? You may need to add a home sensor. The way the IAI and SMC's work is they drive towards a hard stop and the current is measured to detect this. Even the stepper SMC has an encoder but this hard stop method is used to provide a zero reference.
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Last edited by the_msp; January 28th, 2018 at 07:59 PM.
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Old January 28th, 2018, 08:51 PM   #9
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If you are considering going Arduino anyway, why not just use a hobby Servo?

Depending on torque required, of course.

Position is absolute via feedback potentiometer within the unit...

Something like: https://hobbyking.com/en_us/hobbykin...___store=en_us

Here's a decent article on what the specs mean: https://www.rchelicopterfun.com/rc-servos.html
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Old January 29th, 2018, 05:27 AM   #10
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Hey All.

Thank you so much for the pointers and resources.
Iv been reading a lot about the different types of servo and stepper technology. I feel like iv discovered whole new ball game. Its very interesting to see what they can do and how simple or complicated you can make it.
I do have some larger Kinetics and Sercos servo machines where I work but rarely do I have to nor do I want to touch them.

I did take an AB motion course but since have not had any opportunity or time to play with that equipment. That combined with the fact that other than a single L30ERM I just don't have any motion equipment lying around. Its a little expensive.

For this project I have picked out an Arduino UNO. An Arduino Pulse Stepper Drive module... I don't have the number in front of me... As well as a nice little high torque stepper motor. All for under $100.

I will have to use one direction full movement for the "hard stop" as feedback is a little expensive to get into for an experiment sake. First I want to test my system to see if my idea will work. Then, I will likely move to better more expensive equipment.

I'm sure Ill have some more questions going forward. I will shoot any Arduino questions to that forum though, don't worry!

Thanks again!
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Old January 29th, 2018, 05:43 AM   #11
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After playing with Arduino over the last few years, it makes you wonder why PLC's are so expensive ( l can hear the ex AB sales reps starting to wind up, hi to Ron and Ken) . Go ARDUINO.
The Arduino forum, i.e Google is limitless and the stuff people do with it is mind boggling, as l have said in previous posts, when was the last time you heard of a drone with a PLC in it? equivalent to PLC's from <$5.

Last edited by PLCnovice61; January 29th, 2018 at 05:52 AM. Reason: adding more dribble
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Old January 29th, 2018, 06:37 AM   #12
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Quote:
I will have to use one direction full movement for the "hard stop" as feedback is a little expensive to get into for an experiment sake. First I want to test my system to see if my idea will work. Then, I will likely move to better more expensive equipment.
Use a home sensor..
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