Poor Man's Encoder

How do you install a position encoder on a motorized cart so that the cart travels 12 inches and stops, resets its self, then moves another 12 inches and stops? The motor to drive this cart will be a Dewalt 14.4 volt or 18 volt cordless drill.

Thanks,
Joe

Normally an encoder mounts on the shaft that turns or the motors rear shaft.

No I havent done the test yet but I will try to this weekend. I am not positive now the motor you have is sufficient for the job but there are ways to do this.

Another idea is to use sensors/switches and timers with or without a plc, in your case I would look at a getting a plc. The encoder is a better idea tho but you need to determine a way to control the voltage/current to the motor to increase and decrease the speed. THere are many ways to do this, it takes time to look at what is needed and develop it.

I am curious, is this table similar to a pentagraph or plotter, the 2 table part has thrown me a little? I need to goto the other post and print it to look closer at the details/specs of your machine. I have to take my time doing those calculations but if Steve Bailey says you probably need a 1.5-2hp motor then I wouldnt debate the issue, I would check first.

BTW its possible I may have devices that would work in your application but "involve me and I understand".

I will do what I can.

I assume this is a continuation of your other thread.

In addition to the encoder, you'll need a totalizing counter. Check companies like Durant, Veeder-Root, Red Lion, Omron. You wire your encoder to the counter's input. Get a counter with a normally closed or Form C contact output so the contact is closed from an accumulated count of zero up to the preset value and opens when the accumulated count reaches the preset. Wire the NC contact in series with the switch on your drill. You'll also need a DC power supply for the encoder, usually 5 VDC, but you need to make sure that the encoder and the counter operate at the same voltage level.

You'll need to do a little arithmetic to figure out the preset for the counter. For example, say you get an encoder with 360 counts per revolution and couple it to a piece of 1/2-13 threaded rod. In 12 inches of travel the rod will rotate 12 (inches) times 13 (revs per inch) times 360 (counts per rev) = 56,160 counts.

Enter your preset value into the counter and reset the accumulated count to zero. When you press the trigger on the drill, you should turn the rod and the accumulated count should begin to increase. When you reach the preset value, the NC contact will open, stopping the drill.

The drawback to this approach is that the drill is turning at maximum speed when you shut it off, so it will coast to a stop. You'll find that your stopping point is beyond where you wanted to stop. A little experimentation will show you that your actual stopping point varies over some range. If the stopping range is within an acceptable tolerance, great. If not, the next step would be to find some method of slowing down the speed of the drill as you approach your target position. That way, when you reach the counter's preset and stop the drill, you're running slower so you don't coast as far.

I think by the time you toss together an 'encoder' (in whatever form it might take) and a free-standing counter you will probably find that one of the micro-PLCs and a true optical encoder will be just as cost-effective.

If you look at the Automation Direct DL05 or DL06 I think you get a couple of high speed inputs you could use for pulse counting. Most of the plc manufacturers make a similar product. The strength of the plc is you have much more flexibility in how your application works. The downside is it is a programmed device and someone needs to program it.

Keith

use a dynamic break resistor and contact. This will stop your cart with a minimum or zero drift, depending on the resistor value.
I have applied this to a cart I made for the kids to ride and I used no resistor, just the contact, and it has worked for a couple of years with no problems. It stops on a dime and has caused no damage to the motor armature. The breaking only takes place when no voltage is being applied, very effective.

Roger

I am going to combine a few things and see what everyone thinks. Steve came up with some hard numbers for the HP rating of the motor(s) so I will use them.

This may seem elaborate but I think it could be done and kept kind of cheap but cheap still may be 2k or so.

I was thinking of using ONE (1) motor to drive both tables with clutches and brakes to engage and/or stop the screws. That way you could use switches, sensors etc to start and start the tables with the motor running continuosly. This would be simple if it stops at the same place on each move every time. I have some conveyors that I have done in this fashion. I also have cutters that work in this fashion. The bad side to this is that Steve's calculations I think yielded 1.5HP maybe 2 HP per table so they either couldnt be driven at same time or you would need to double that to 3 HP. A 3 HP motor at 120 volts single phase would draw around 30 amps, not something you can plug into an ordinary wall receptacle. BTW they make electromagnetic clutches and brakes for applications like this. My first choice for control would be a plc (mainly because it fits the small footprint) but this could be done with relay logic. Let me rephrase that, I think this could be done this way.

If you need a more precise control we could elaborate on this concept in other fashions. One thing you could do is look at single phase to 3 phase inverters (VFD), again not something that can be plugged into wall outlet, will need 230v single phase I believe. There are single phase to 3 phase converters which could be combined with inverters and 3 phase motors. You may be able to get 3PH service to your home/shop at a reasonable cost, I have done it in the past.

I just got back from a call in and was thinking about this, there are many options you just have to look at the choices and determine which best suits your needs and fits your budget.

Your system

Joe, I live in brighton, and if your not in too big of a hurry(im going to a job site for 7 days tommorow), I can come over and look at your system. This is a little out of my area(im an oem developer for industrial heat treaters), but im willing to give it a look.

Matt

Joe, Rsdoran,

I want to clarify a couple of things. The motor size that I came up with is subject to the assumptions I made about acceleration time and the inertia of the drive screws. Joe really needs to nail down how much time he can allow the 12" move to take. I used an accel/decel time of 0.5 seconds and a maximum speed of 8 IPS. That gives 2" of travel during accel (0.5 seconds), 8" travel for 1 second at 8 IPS, and another 2" travel during the 0.5 seconds decel.

If you increase the accel/decel time to 1 second, you get 4" travel during accel, 4" travel for 0.5 seconds at 8 IPS, and another 4" travel during the 1.0 second decel for a total of 2.5 seconds. With that profile, the peak torque drops to 26.9. The friction torque stays at 1.8, the motor speed is the same, but 28.6 Lb-In at 1920 RPM is 0.87 HP.

Also remember that I assumed a 1.5 inch diameter ball screw. I don't think that a 0.5 inch diameter threaded rod will work for this application. Joe says his table has to move over a ten foot range. That means that in the worst case, he could have that 1/2 inch diameter rod unsupported over a ten foot span. I'd be worried about that skinny rod starting to whip when you try to rotate it at 1900 RPM.

That being said, I reran the calculation using a 3/4-10 threaded rod as the leadscrew, and I reduced the traverse speed to 6 IPS for a motor speed of 3600 RPM. With that, I came up with a 1/4 HP, 3600 RPM motor.

The point is that the performance parameters and the mechanical design have a huge influence on the size of the motor required to do the job.

Pretty nice offer!!!

Joe-
You might want to consider taking Matthias up on that offer. That's pretty friendly so make sure you have beer on hand. It would also give you another set of eyes to catch details you may have overlooked. And anyone who tells you they don't miss details somewhere along the line is most likely fibbing.

So if you have the time take the offer.

Keith

less than $1500

Check ebay you can pick up a micro logix processor for about $100 to 150$. With that two prox switches, two mechanical limit switches, two relays, two pieces of metal 1.91" long (the radius of a circle with the circumference of 12") that can be mounted to your drive shaft or motor shaft. You should be able to go 12" stop 12" stop and reverse the process. If Ebay doesn’t have what you are looking for there are a couple of industrial clearing houses on the net that might have an inexpensive alternative. It shouldn't cost you more than $1500.
http://www.automationtechies.com/sitepages/pid412.phphttp://www.automationtechies.com/sitepages/pid412.php

Man this is a GREAT site with GREAT people!! I really want to thank all the people for helping out, you guy are the BEST!!

For the last 48 hours I've gotten about 2 hours of sleep. And since last October (when I started working on this thing) I've gotten about 4 hours (on average) of sleep trying to figure out how to do this. I have been working on this project for almost a year now. I get an idea then I redesign, idea, redesign and so on. The other day I counted up about 200 various designs.

Though I can not tell anyone what this thing is, I can say there is nothing like it in the world. The plan is to eventually patent this thing, which of course is a whole different ballgame. There are many other parts to this system that I have not elaborated on, so all the advice I'm receiving from this forum has to be weighed with the other mechanical parts as a whole. As for Matthias offer, I have to tell you Mattias that I agree with kamenges it is extremely kind of you and I really thank you for it, but I really don't want to show this invention to anyone at this point.... can I take a rain-check on the offer? Once I get a working prototype and a patent pending application I'll give you a shout. Send me your email address, ok?

UPDATE:
All these great idea's from all these electrical guru's out there makes me feel like Elmer Fudd. I really don't know what you guys are talking about but again I will not allow my ignorance in this area stand in the way of progress. So that being said... After working all night searching the web I came up with an idea that is more in my ballpark, but before I get into that I would like to say something to kamenges in the "Linear Actuator Control" post.... kamenges, that is what I was thinking too. Maybe I should sideline the actuator idea and hook up a drive line directly to the cross member that connects the two tables. Use gears as wheels and set those wheels onto a linear geared track (rack and pinion), that would give me more room for a motor and would eliminate all the problems with the lead screw actuator. But again the stopping problem that remains..... Which brings me to an idea I had last night....

If I took the cross member (that connected the two tables) and put wheels on them, then glued a small magnet to the outside of one of the wheels and hooked up a reed switch, every time the magnet crossed the reed switch the switch would activate sending a signal to another switch which would shut off the motors. To solve the drift problem I could hook up a what rogerhollingsworth mentioned, a "dynamic break resistor" or even an electrical brake might work. This would keep the system very simple, easy to design and build.

Thanks!
Joe

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.

Click to expand...

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?

joethepro said:

But again the stopping problem that remains..... Which brings me to an idea I had last night....

If I took the cross member (that connected the two tables) and put wheels on them, then glued a small magnet to the outside of one of the wheels and hooked up a reed switch, every time the magnet crossed the reed switch the switch would activate sending a signal to another switch which would shut off the motors. To solve the drift problem I could hook up a what rogerhollingsworth mentioned, a "dynamic break resistor" or even an electrical brake might work. This would keep the system very simple, easy to design and build.

Click to expand...

The method of stopping will all depend on how accurate and repeatable you need the final stop position to be. Simply put, more accurate/repeatable = more $$$. Also, the faster your top speed is, the more critical it becomes.

Using a dynamic braking resistor is a cheap and simple method of braking. It will definitely be worthwhile to at least give it a go. I think you'll be amazed at how fast you can stop a DC motor with one!

I noticed the site you're using to host your drawings. Hmmm, might this project involve golf balls in some way?...

I'm picturing something like Jesse James' Golf Ball Collector/Launcher from Monster Garage...

beerchug

-Eric

joethepro
National Manufacturing Week would be highly recommended.
March 3 - 6 2003 Mc Cormick Place, Chicago
www.manufacturingweek.com
Free admission if registered in advance

You will see all the parts, products that have been
posted above.
Others would also encourage you to attend.

Don

Breaking or Braking?

Just a reminder, dynamic braking will stop you where you want to be but a mechanical brake will be needed to hold you there (typically).

Point of interest;
At my previous work site we had a motor that had an armature that measured more than eight feet in diameter and rotated at 1500 RPM, we would routinely bring this motor from top speed to a stand still in less than 1.5 seconds. Without any undue stress on the motor, you can't do that with a mechanical brake.

Roger