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Author Topic: Using Mach to drive an automated scissorlift  (Read 2300 times)

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Offline dbren

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Using Mach to drive an automated scissorlift
« on: July 19, 2014, 02:09:10 PM »
Hello,
We are thinking of using a scissor lift for a parts picking operation.  We would like to load the locations using XZ coordinates, then the lift would go to that X location on the isle and raise to the Z rack height.  We would need to use a controller that would link to the electric drive of the lift and incorporate acceleration/deceleration.  M00 codes with safety devices would make sure the lift doesn't head for the next location unexpectedly.  The safety devices would likely include mechanical limit switches as well as a two-handed 'resume motion' command input.  Position feedback would probably be encoders on the drive wheels for X position and maybe just a timer on the hydraulic drive for Z motion to determine height.  Each axis would reset zero when X hits its home position when returning to the end of the isle and Z when it lowers to the bottom, and hopefully this would be adequate for any error correction.

Any thoughts on the best controller type to use for this operation?  I don't know if something like the Arduino could be linked to Mach and do everything we need.  We will likely need to use relays with more amps to control the scissor lift, but that's an easy thing.


Re: Using Mach to drive an automated scissorlift
« Reply #1 on: July 19, 2014, 11:14:07 PM »
This is the kind of task handled very well by a PLC.  PLCs are way more robust than a PC, already have real world I/O designed for this kind of task. AutomationDirect.com has very inexpensive PLCs, limit switches, encoders etc.  I used to build watering/spraying robots for large commercial greenhouses. They could be programmed to go anywhere in a 100,000 sq/ft greenhouse and apply water or chemicals from 8 different sets of nozzles and move at different rates while doing it. If you have an ethernet connection you can have a local control panel or direct the machine from any PC or even your Iphone!

Offline dbren

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Re: Using Mach to drive an automated scissorlift
« Reply #2 on: July 20, 2014, 09:58:08 AM »
Hey Gary, thanks for the reply!  I just recently started through a course on PLCs and have software to simulate the projects.  I'll check out the boards at Automation Direct.

So, do the PLCs have plenty of memory to download a few dozen locations? 

Would the encoders input directly to the PLC for location?

Thanks
Re: Using Mach to drive an automated scissorlift
« Reply #3 on: July 20, 2014, 06:06:38 PM »
Encoders are connected via timer counter modules that take quadrature inputs, so they count up and down as you move.  I have used an inexpensive Dart 10 pulse per revolution counter to locate the greenhouse bays along a 400 foot path.  The encoder should never be on a driven wheel, only an idler.  This gets me within a couple of inches, then I drop into creep speed and stop by a photoeye looking for a retro reflector. This gets close positioning even when things have shifted due to expansion and contraction of the building, rails etc.  Approaches for alignment always come from the same direction too.  So when going back to an earlier location when the counter says I have arrived I brake to a stop and go forward again until the photoeye aligns.  At both ends of the path is a long metal bar that is detected by an inductive proximity sensor.  This warns the machine that it is near the end and forces it to slow down to avoid crashing into the end.  If it is anywhere along the bar it is only allowed to creep.  The counter always resets at the home position.  On power up the machine always goes home first to reset the counter.

The newer PLCs like the PAC 3000 and DoMore series have lots of memory now.  That said, my machines from 20 years ago had enough memory to store 47 positions in each of 60 bays!  Those machines are still running today, with only occasional problems, mostly mechanical.

If you use a C-More 6" or larger HMI locally on the machine it has a web server that lets you access it over Ethernet.  A 5 dollar app for an Iphone gets you access from you phone too.

Offline dbren

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Re: Using Mach to drive an automated scissorlift
« Reply #4 on: July 21, 2014, 07:35:29 AM »
You make it sound so simple!  :)  Thanks for all the good info.  It may take me awhile to go through the project, but you have given me enough direction to keep me busy for awhile.  I may have some more questions as I go through it.

Thanks again!
Re: Using Mach to drive an automated scissorlift
« Reply #5 on: July 21, 2014, 08:03:17 AM »
I've done lots of projects like this. You can contact me directly at gary.lucas@verizon.net

Offline dbren

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Re: Using Mach to drive an automated scissorlift
« Reply #6 on: July 24, 2014, 07:10:57 AM »
Once they commit to going forward, I'll contact you.  Thanks!