Single axis position control of carriage

[mnshonk]

Working on a pretty simple PLC controlled machine with a carriage that needs to travel horizontally to various user selected positions (up to 36"), move freely while the machine does its thing, and return to a zero position after the process is over. Speed doesn't matter, and the carriage would take maybe 30-40lbs to move. I need to select a motor (DC, stepper, servo?), power transmission (rack and pinion, ball screw, etc?), and controls (ideally as simple as possible).

I'm the only engineer in the building, it's my first engineering job, and I worked for a non-profit for four years after getting my BS in ME; so I've definitely forgotten a great deal and have zero real life experience in industry. I just need some nudging in the right direction.

 

[tygerdawg]

There are an infinity of solutions available. You start with educating yourself about motion control.

Define your parameters of payload mass, working element mass, velocity of motion, how fast you achieve that velocity (accel) and slow down (decel).
Then make a decision on means of converting rotary motor torque into linear motion. All you have mentioned are valid, but all have advantages & disadvantages.
Once you decide the mechanical method, then this dictates how much torque must be supplied, and at what speed. This sizes your prime mover (motor).
Websearch & locate the Smart Motion Cheat Sheet (freely downloaded in PDF from many sites). One of the best summaries of the necessary equations of motion I've ever seen.

Then the fun starts with designing all of the mechanical bits & pieces, no small task.

The fastest solution is to purchase a ready-made unit from somebody.
Find vendors for this type of equipment (search strings: linear motion, linear actuator, positioner, etc.). Then get your hands on the catalogs and Engineering Data Sheets and Engineering Guides offered by most of those companies to start learning how to size and configure a unit that will meet your needs.

Ultimately you will have to send I/O signals to/from the PLC to make it go/stop/recover. There's a half-infinity of solutions available on how to drive motors.

Or be a project manager for your company and hire a systems integrator to do everything for you.

TygerDawg
Blue Technik LLC
Virtuoso Robotics Engineering

http://www.bluetechnik.com

 

[BrianPetersen]

The phrase "move freely while the machine does its thing" phrase has me concerned.

Do you mean:
a. The positioner can stay put in any suitable location (parked) while the machine does its thing? If so, that's easy. Or...
b. The positioner can follow a prescribed arbitrary motion profile while the machine does its thing (e.g. pre-position itself ready for the next operation)? If so, that's also easy. Or ...
c. The positioner has to be "in neutral" so that the carriage can move freely "in neutral" while the machine does its thing? If so, that's hard. Small motors with large gear reductions on them, or lead-screws or ball-screws that tend to be typical of this type of apparatus, don't like to be "back-driven" and may not even be possible to back-drive. A mechanism for mechanically disconnecting the carriage from its drive mechanism - aside from adding a whole bunch of complexity and cost and failure points! - also means that as soon as that carriage disconnects, you now no longer know where you are. You've lost the positional relationship between the carriage and what it's moving. While suitably imaginative design can probably find a way to regain position (more or less) ... it's hard. Let's just say that this is not something you would ever want to do in a CNC machine.
d. If the above (c.) is conceptually what is needed, could you instead follow the load "while the machine does its thing" by following a prescribed motion profile, thus reverting to (b.)

 

[rb1957]

if "does it's own thing" is as loose as it sounds, you could put sensors on it; so it'd retain "situational awareness".

another day in paradise, or is paradise one day closer ?

 

[handleman]

If the "Move freely" thing is as described by BrianPetersen above, sounds like you need two actuators: One that actually does the pushing, and then one that engages/disengages from the carriage.

The zero position of the carriage (set by some hard stop at limit of travel) should line up with the zero position of the actuator. When both are in zero position, the second actuator engages the carriage (pin in hole, something).

The motor pushes the carriage out to the desired position, then disengages while "the machine does its thing". Meanwhile, the motor is extending out to its furthest position, clear of the limit of travel of the carriage.

When the machine is done, the second actuator extends the pin and the motor pulls back, with its engagement pin catching a hard stop on the side of the carriage. Once it drags the carriage home, it retracts the pin and moves the last little bit to its zero position.

 

[GregLocock]

That's exactly like a lathe's carriage moving along the ways. You have a lever to engage and disengage the carriage from the the lead screw. However until the OP clarifies what he needs idle speculation is the best we can do.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[mnshonk]

Wow, thanks for all the feedback everyone! This is great. We're modifying and automating an existing machine and my boss is annoyingly secretive about it or I'd be less vague. BrianPeterson hit it on the head with his concern. Essentially after the carriage is positioned with the set up we're discussing, the process will start and the carriage will get pulled back a few inches.

My coworkers (field electricians) were thinking there were motors that would revolve freely as long as the PLC wasn't sending them power, and we could just use a laser sensor to track its position. It'd be super simple programming, i'm just not sure how precise it would be or how well it would hold up over time.

I think you're on to something with the actuator disengaging from the carriage. I think I'm going to run with that!

Any other thoughts or feedback?

Thanks so much!

 

[kjoiner]

Hello,

We are working with Teknic and are using their Clearpath servo motors for a product that involves linear motion. These motors have an integrated encoder and controller with various models to perform different functions without a lot of difficulty. The one we are using is the software control version (SC SK model). Using this motor, you can drive to a position and then disable the motor which effectively allows it to move freely. During the "freewheel" period, the encoder is still tracking position so the motor can later be enabled and returned to a home position.

https://www.teknic.com/products/cle...ervo-motors/clearpath-sd-stepper-replacement/

You still need to evaluate and account for the loads and inertia in the system but as others have mentioned, there are tools out there to assist with this process.

If you keep your accelerations within reasonable limits, you might be able to use a timing belt. There are pre-packaged linear stages that use timing belt drives and you provide the motor.


Kyle