Linear Actuator Control

[kamenges]

Joe-

You're welcome. In the time I have been visiting this forum I've noticed that as long as you are willing to put in the effort on your side everyone will be happy to help you along. And it certainly sounds like you are putting in some effort. And, as I'm sure you've seen by the comments, successful automation projects have as much to do you the mechanical details as they do with the control details.

The company I work for used as rack and pinion design with the pinion gear riding on the rack. If you are serious about this idea you really shouldn't run the pinion directly on the rack. They really aren't designed to go peak to valley; there should be a small amount of backlash. You will see all sorts of fine metal shavings in a very short time if there is no backlash. Also, rack and pinion components jump in price pretty quick as the load values go up. Your load level is pretty low so you should be able to use a light rack and pinion if all it is doing is maintaining alignment and transferring motion force. Take up the weight load with a set of wheels.

I realize you are cost sensitive and you want the system to be simple. But be careful of being penny wise and pound foolish. Your time is valuable too. If it takes you two weeks to implement a solution that saves you $300 did you really save anything? Take it from a guy who has burned several thousand dollars just to save a few hundred.

The reed switch idea isn't bad. However, you will give yourself much more flexibility if you mount the reed switch to the table and mount several magnets to the travel rail. Don't permanently affix the magnets either. This will allow you to adjust magnet positions to fine-tune each stop point. Once all the moves work the way you want them to you can lock the magnets down permanently.

If you want to use dynamic braking you will need to make sure the motor you use will allow it. You will either need a motor with permanent magnets in it's construction or a motor with some way of maintaining a field current. I think a series wound DC motor will do that assuming you can count on a little residual magnetism to carry you through switching. Can anyone else comment on that?

Good luck and get some sleep!!!
Keith

 

[joethepro]

And, as I'm sure you've seen by the comments, successful automation projects have as much to do you the mechanical details as they do with the control details.

I'm finding that out more and more. I read somewhere on the web that if you want to build something that has a motor you should start at the motor and work back. I'm finding out how true that really is.

Here's a design I'm working on...

http://hartlandgolfrange.com/breaking_system.htm

What do you think?

 

[Steve Bailey]

The biggest problem with using a brake will be consistency. The pads will wear and possibly glaze. The action of the brake will vary with temperature and humidity, as will the reaction time of the solenoid. If you can work out a method to incorporate a positive stop like a detent, you'll get more consistent stops.

Consider this. If the carriage is travelling at 10 IPS, and the reaction time of the brake averages 100 milliseconds, but ranges between 95 and 105 milliseconds, that 10 milliseconds uncertainty represents 0.1 inches of travel, approximately 3/32". That's why it will be better if you can slow the motor down as the carriage approaches the stopping point.

 

[joethepro]

If you can work out a method to incorporate a positive stop like a detent, you'll get more consistent stops.

Ok, how about this.... Have two motors, one for the main power and the second would be a smaller motor. Have two reed switches w/magnets set up on the wheel. Set up the 12" magnet at the top of the wheel and set up the second magnet one inch before and one inch lower than the 12" magnet. When the wheel turns it will hit the first magnet send a signal to the main motor to disengage and at the same time will engage the second motor which is smaller or is the same size as the main motor but has a 50:1 gear box on it, that would slow it down before it stops... correct?

I think I have to abandon the idea of 10" to 12" p/s, it's too fast for this weight and is causing too many problems, but then again I don't want this thing creeping along at 1" p/s either.

Idea's?

 

[rookie1]

Joe,

Welcome to the world of custom equipment!Things that often seem simple at a glance are in fact not.I believe your idea with the drill motor would be the most effective at this point, as it sounds like you are attempting to build a fully operational prototype.The drill has a built in brake, this is not as durable as its industrial counter parts, but would probably work for a good while as long as it were not used continusly.As for myself I would try using timing belts and pulleys as they are inexpensive and would help keep your dynamic load down. I own a fab shop in Adrian,MI about an hour outside of Detroit, my email is [email protected] if you decide you need some help. Aso for your control options go to automationdirect.com you can get inexpensive encoders,plcs,VFD's,panels and the big plus is the programming software is dirt cheap.Good luck.

 

[kamenges]

Joe-

I'm going to have to go with Steve on this one. I don't think that using a brake to perform a full-speed stop is going to work for you. Like Steve said, the variability in actuation time will probably cause you unacceptable variation. Yes, there are brakes out there that would be consistent enough to do the job for you. But then you bring your cost back up.

The problem with the dual motor solution is the speed transition. It's going to be tough to go from one speed to the next without a nasty jolt.

Don't give up on speed control too soon. Like rookie said automationdirect.com is a real good place to look for low cost new stuff. I took a quick look at this link:
http://www2.automationdirect.com/store/Shopping/Catalog/Drives/AC_Micro_Drives_(GS_Series)/GS1_Drives/GS1-10P5
Its a brand new 120VAC supply 3-phase drive, about 1/2 HP. All for $120.

These links are from eBay:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=26209&item=3104967631
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=26210&item=3105815543
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=26210&item=3105868054
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=26209&item=3104990750
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=1267&item=3106539507

They are for drives and motors for sale that would most likely work for your application. Again, the biggest power draw in your original design was spinning up the leadscrews. If you can come up with a way to dirve without them the total power tends to go down. Rookie's idea of a timing belt setup is a good one to look at too. It has the same advantage as the rack an pinion in that there is not much load added to spin the driveshaft up to speed and the belt adds almost nothing to the load.

Actually, your drill would provide more than enough torque and speed for a rack and pinion or belt system. The tough part would be the variable speed issue. You would want to be able to control it's speed somehow. If you had the torque capacity you could constantly brake the assembly. Just make sure you had more brake than was required to decel the table and load and leave it on all the time you are indexing. Then just ramping the speed down would keep you in control. The motor would always be driving forward.

Keith

 

[rsdoran]

MY last offer, use rails to support the table and drive with a screw, the rails will support more of the load and the screw just has to do the movement. I work with machines that on a daily basis work in this fashion with speeds that exceed what you refer to.

I have other machines which at this time I can not state the specific speed in reference to your concerns. I can tell you they can run 36" wide paper at a rate of 144" a second..ie 12 feet of paper passes thru a second. Of course there is a weight difference with paper but my point is that these machines run 144" of paper and cut it using brakes and clutches, with a plc and encoders involved.

I have worked on bottling lines which could fill and cap 100's or more units per second. I think one place did 6000 bottles a minute...you couldnt look at them without getting dizzy.

These guys are great and can offer some excellent advice but you are approaching this endeavor wrong. You must look at the overall mechanics first...ie load, motor etc to match your speed compared to load. Once that has been established then work on the control system but expect to pay more ( or learn more ) to develop it.

I have no idea what you are attempting to build but IMHO you do not have the skills to do whatever it is. At this point I think your idea has probably been done before. This place is loaded with pickle slicers

I wish you luck.

 

[ndzied1]

joethepro:

Not sure what it is about this thread but it definately has everyone hopping

Here's a link I found you might find interesting:
a new idea?

Now, my wacky thought for driving this thing is to use a rotary actuator (you must have a air compressor if you're a real back yard mechanic ). Set up the rotary actuator w/ gearing or belting so that one complete stroking of the actuator corresponds to your 12" travel. Hook it up to the system through a brake/clutch so you can disengage the actuator to reverse it. To get the slides back home, you could index back or clutch in a different motor for the return???

Maybe you haven't heard of a rotary actuator before, click here

Also, maybe there is a more mechanical way to do what you want. Here's a real simple stop that might be useable if the gearing works out to an even fraction of the index distance
:

There are also Geneva Mechanisms, Scotch Yolks, Walking Beams which can do indexing.

Just a few more thoughts.

 

[joethepro]

Here's a real simple stop that might be useable if the gearing works out to an even fraction of the index distance

ndzied1..... Now yer talkin! I tip my hat to you ndzied1, very nice. Very simple design, easy to make and very effective. I checked out your recommendations of the Geneva Mechanisms, Scotch Yolks, Walking Beams. I couldn't find anything on the web about "Scotch Yolks" so I don't know what that is, as for the others I did find so info on them but I think your picture of the mechanical stop work best. Figuring out the travel distance would be a piece of cake. I know the speed going into the stopper will have to be low, I don't think that will be much of a problem.

In my search of ndziedl's Geneva Mechanisms I came across this web site... http://216.239.37.100/search?q=cach...p?BookID=10+Geneva+Mechanisms&hl=en&ie=UTF-8I ..... read some great reviews on Amazon about this book so I think I'll order a set.

Got to go work on designing this stopper now..... I can see its going to be another sleepless night!!

Thanks again everyone!!

 

[Tom Jenkins]

I haven't done motion control for eons, but we used to use spring wrap brakes from Warner electric or Hilliard for printing press indexing. Very reliable, repeatable, and a less harsh deceleration than just banging a pin into a cam!