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Wondering about using V slot rail (linear actuator)

Discussion in 'General Talk' started by richard3879, Jun 12, 2014.

  1. richard3879

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    Hi. I'm not much of an engineer, but I need a linear actuator (rail) for something. I found OpenBuilds through searching for linear actuator. It seems to me that perhaps I can save a lot of money building my own electric linear actuator with the V slot rail.

    Not sure what the actual specifications of V slot rail are, but it should do the job. I only need linear motion in one axis and need about 0.5mm accuracy in position.
     
  2. Mark Carew

    Mark Carew OpenBuilds Team
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    Hi @richard3879 welcome to OpenBuilds!
    I would check out some of the builds here of the linear actuators in the 'others' section.

    http://www.openbuilds.com/builds/mini-v-linear-actuator-build.48/
    http://www.openbuilds.com/builds/v-slot-belt-driven-linear-actuator-build.80/
    http://www.openbuilds.com/builds/v-slot-belt-pinion-example-build.97/
    http://www.openbuilds.com/builds/v-slot-lead-screw-acme-driven-actuators.634/
    http://www.openbuilds.com/builds/x-y-stage-hidden-acme-rod-example.66/
    http://www.openbuilds.com/builds/x-y-table-openrail-v-slot-build-examples.62/

    One of these styles should fit the bill of what you are after. Because of their modular nature, they are all easy and straight forward to assemble and adapt to your project.
    There are belt driven or screw driven of all sorts of varieties to choose from. You just need to see what would fit your application best.
    Have fun with your build :thumbsup:
    Mark
     
  3. richard3879

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    Hi. I'm a newbie to all this, so I'm still looking what the options are. I've been looking at the video "OpenBeam Linear Actuator using OpenRail System". I see I'm going to need something like what I see at 00:07 into the video.

    I'll likely make my own top-plate (whatever it's called, not sure of it's proper name) which will be about 500mm x 300mm. Anyway, I can see me using two V slots from 300mm to about 450mm between centers.

    The top-plate will likely (still designing) travel from "home" position to an end-stop about 350mm away from "home". Certainly that's the more straight-forward design, because all that is required is a switch to feedback when the plate reaches the end-stop and then to cut power to the motor.

    Press button once will send top-plate to end stop. Press again the plate will reverse the direction and go from end-stop to "home".

    Anyway, looks like the belt arrangement should work fine.
     
  4. richard3879

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    I wonder how easy it is to get the top-plate to stop at an end-stop and be within say 0.5mm of the end stop repeatedly.

    I might have to experiment with that. Maybe I'll need a "catch".

    Also, I need to figure out the control electronics.

    If anyone can point me towards what I need I'll be happy.

    To recap:

    Press button sends top-plate from "home" to end-stop.

    Press again sends top-plate to "home".

    Need PWM motor controller. I know that much. :)
     
    #4 richard3879, Jun 12, 2014
    Last edited: Jun 12, 2014
  5. richard3879

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    Actually, what I need to do is learn how to program and wire a Programmable Logic Controller. That knowledge along with what is being offered here in mechanical parts ought to see me right.
     
  6. Mark Carew

    Mark Carew OpenBuilds Team
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    Sorry Richard, once you get into the logic control side of things I am not schooled on that. Maybe one of the guys who are will jump in and help out. To me it sounds like you almost have it talked out on your own here. :)
     
  7. Brian Slee

    Brian Slee Veteran
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    Hi Richard,
    Here is an example by NYCNC for the Arduino that is pretty much exactly what you are talking about. If you wanted real repeatability you could throw in an end stop sensor at each end and use the signal to toggle the Dir signal for the stepper.



    Hope this helps
     
  8. bit53b

    bit53b Well-Known
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    I have done something kind of similar with an Arduino too - but any microcontroller would achieve the same thing. I've attached a video from a build I'm working on where I use a joystick controller to jog the motors using an Arduino pro micro and PWM to control the motors directly (but the finished solution will interface via GRBL). The joystick has an integrated pushbutton which I'm using to switch axis. Instead of using the manual control you could set your limits programatically
     
    #8 bit53b, Jun 14, 2014
    Last edited: Jun 16, 2014
  9. Robert Hummel

    Robert Hummel Custom Builder
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    Well done :thumbsup:
     
  10. richard3879

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    Hi. I've been posting somewhere else also, on a PLC site actually, because I was thinking I needed to use a PLC. I there posted a ladder logic circuit which involved only relays. The circuit would automate the gantry (top-plate) from home to end stop, then back to home (with no need for PLC involvement). I then said I may need slow start/stop and the last comment was to use something like Arduino.
     
  11. richard3879

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    I've had in my mind using a dc motor. For some reason. Anyway, let's imagine I use a stepper motor. What happens at the end-stop? Lets say it takes 5000 steps to reach the end stop. I can program 5000 steps. Then it has to go back to home, that's 5000 steps in the opposite direction. If there is slippage/lost steps, then there will be a problem. So, I guess we must have a limit switch that stops the step pulses when the gantry hit's the limit switch on the right. Then, the same again as the gantry goes back to home. I guess if the stepper was not physically restrained at the end stops, the gantry might go past the position corresponding to the last step. So, I need to rely on a physical arrest of the stepper motor - all said and done?
     
    #11 richard3879, Jun 20, 2014
    Last edited: Jun 20, 2014
  12. richard3879

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    Accurate: As long as there is little inertia in the system? As long as speeds are moderate?
     
  13. richard3879

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    I wonder if anyone has experience of the stopping and can comment. If you have a 1.8 degree stepper motor and a 2cm diameter pulley on the drive shaft, each step will produce 0.314mm of linear motion. If you needed a stopping accuracy of about 0.5mm you want the gantry to stop within 2 steps. How realistic is it that a gantry traveling at 1m/s with a mass of say 3kg will stop within 2 steps of the last step pulse? If this is not realistic how do you solve?

    I think there are two main ways:

    a) you drive the gantry at full speed up to very near the end stop and apply braking (which is probably shorting the windings to the stepper).

    b) you arrange for a soft stop, so when the last pulse is sent, the gantry can stop within 2 steps.

    c) you arrange for a soft stop and braking.

    Crucially, we cannot have the gantry bumping into the end stop just to have it bounce back 5 steps or whatever.

    Anyone have any practical experience of the stopping issue?

    Thanks.
     
  14. Brian Slee

    Brian Slee Veteran
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    Ok I probably should have been more specific. The example provided was for doing the control part using an Arduino board. Now if you take one of Marks linear actuators and a properly sized motor depending on your app you should have what you need to make you what you described ;)

    Someone posted a great link to a site with info on calculating motor size based on load and speed requirements. Maybe they can repost...
     
    #14 Brian Slee, Jun 21, 2014
    Last edited: Jun 21, 2014
  15. Tweakie

    Tweakie OpenBuilds Team
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    Hi Richard,

    Almost all CNC control software has, as part of it's configuration, motor Acceleration, travel Velocity and Deceleration settings. These settings are adjusted so that the driven mass can be accurately moved to any position (without any loss of position). Once set they seldom need re-adjustment and exact position is guaranteed (but we always fit limit switches just in case :) )

    Tweakie.
     
  16. richard3879

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    Hi all. Like I say, I'm a noobie and know little. :)

    Okay, so it's very regular thing to set/tune the acceleration, the velocity and deceleration. Makes sense. :)

    I think it also makes sense (I surmise) that you would/should in many cases, not be trying stop or brake a mass of 3Kg going at 1m/s in two 1.8 degree steps.

    So, soft start/stop is required I think in my build. Certainly from the right end stop back to home.

    I'm trying to avoid using an MCU or programming. All I want to do, if I can, is set switches or potentiometers. I've asked elsewhere if ICs exist that allow for that.
     
  17. Rick 2.0

    Rick 2.0 OpenBuilds Team
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    You should have given us these parameters at the very beginning.

    DC motor with dynamic braking. Add a soft start circuit to the motor (schematics online). Dynamic braking will create the soft stop. End switches at the side of the track so they are tripped as the slide passes. Relay setup for reversing. Motor should be direct drive with no gearing.
     
    #17 Rick 2.0, Jun 21, 2014
    Last edited: Jun 21, 2014

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