Motor quesion for a mod.

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SpazzeBlue

Weaksauce
Joined
Apr 30, 2005
Messages
84
Once again I return to ask people smarter than me questions.

Does a bipolar stepper motor need constant power to keep its location? I have a few applications, some rotation, some lifting, and I want it to be able to maintain the positioning until it’s told to retract.

For the lifting operation, there will be 2 motors to lift about 6 lbs vertically 4-6 inches and lower it later. While I saw through a lil search that such a topic was covered before, I didn’t see a finite answer on that thread. ( http://hardforum.com/showthread.php?t=888574 ) Linear actuators are pretty much out of the question since ones that move that far are quite costly.

The lifting one is the main one that I am having problems with, because I can’t think of a way to make sure that the platform stays up when it’s supposed to without constant power. I have thought of using another motor to form a locking mechanism. Having it spin out and slide 4 bolts into place to keep the platform from sliding down, and then reversing it to have it ‘unlock’, but I was curious as to if any of you had a better idea.

Just in case you want the answers to the questions asked on the other forum to that guy.
The platform will be, most likely, about 6”x6”, and the bottom will be made of acrylic, thickness undetermined. What I’m lifting is a secret, but it won’t weigh more than 8 lbs at MOST.I also don’t need there to be stops… all the way up and all the way down are all I need.

Any more questions feel free to PM/post and I’ll get back to you. And never fear, all of this is going to be in a mod that I will be starting soon. Just need to make sure I got all the hardware here first.
 
hm.. i never got to play with steper motors, but I do believe you WILL need power to maintain the platform elevated.

If you post a sketch of the mechanism, someone might be able to help you design a mechanical solution to keep it elevated.
 
Why not a latch-n-sprocket system?


motor turns the sprocket, which via some other method, lifts the platform, and with a pin that rides over the sprocket..it automatically locks, preventing it from retracting until you want it?
 
stepper motors will only keep their "step" locked while current is applied to the coils, as soon as you disconnect a current source it will turn into a generator. you can place resistance across the coil leads, but that will only marginally help.

unless you need exact 1 degree control, i don’t think that a stepper is the proper choice for this application. what you really need is (depending on precision required) a model sailing servo or a DC worm gear motor. the former is going to be more expensive, but will have relatively good position control built in, inside it has a gearbox and a POT that controls its position. it will try not to move, but once you kill power, its all up to the gear ratio, but you can turn them. most of these cost 150$ +, and use 5-10vdc and PWM control.

the latter is going to be cheaper: depending on where you are, you can use "van door motors", "window motors" or "windshield wiper motors" - all which run off 12v DC and will hold their position without power applied due to the worm gear. the van door can haul about 100 foot-pounds, window is about 25, and wipers are 10-15. head out to a junk yard or look online (ebay) to pick one up for $20. we use them in robotics all the time, the one drawback is that you pretty much have to stop them with sensors, photo gates, limit switches, which limits the accuracy to the number of sensors.
 
Hrm.. well I think that we have established that a stepper motor is out of the question for the lifting portion of the mod to be sure. The main reason that I was choosing the stepper motor was because I could program exactly how far for it to turn, thus eliminating any other form of control that would have to be integrated into my circuit design.

The worm gear idea. I was thinking of something like that, but I didn’t know what it was called… I will have to look into that idea further. I would go with an automotive part, but I fear that it would be large, and a little unwieldy to integrate into my mod idea. Trying to include form AND function gets a little interesting.


In any case, I appreciate the help, and the ideas from all of you.
 
if you want the smallest possible solution with at least semi-programmable position, you need a servo. for example, this one is on sale for $20. it has 5 inches/lbs torque when run at 6v. this one has a little bit more, about 8 inch/lbs, but its $40. for either, if you were moving anything more then 2 inches you would need some sort of pulley/cable system. you still need a little bit of control, such as a RS232-PWM card, but you can make it or buy pretty cheaply. like i said before, these are the more expensive solution, but they are smaller -- both around 1.5"x.75"x1.5" and if what your moving is very heavy or has a powerful spring, it can move the servo's gear train while it is off.

there are many other places to find worm gear automotive motors - i didn’t know that size was such a concern (with 6lbs going up and down sounds huge to me ). a great and also cheap place to find smaller motors is from power seat assemblies. this shows a good pic of what they look like... its another 12v DC motor with a worm gear head, and this particular one has a screw already attached to it, you may find that useful in moving stuff up and down. there isn’t a good scale with the picture, but most of them are 2.5-3.5" long, about 1" wide, give or take a pinch. they will hold once power is removed no matter what (300lb people drive all the time without their seats sliding around), and have a ton (not really, but a lot) of torque. considering that this one is $15, IMHO its a better deal then the servo, even if you need to make your own simple hbridge to drive it.

another route you could go is a window motor with a built in linear actuator, like

this one. you could mount the motor some ways away from whatever you were lifting. again, i don’t know your design, so i couldn’t say for sure if it would be good. the other style of window motors look like this , and (cant see it in that pic) have a small output gear. i would buy an american made one, they generally can go through hell before they die.

if you are controling this with a computer, you can get a simple RS232 motor controller that will plug right into any 12v auto motor. this would then plug into the serial port of a computer. if you wanted to build your own controller it is relatively simple, get some transistors and it could plug right into the parallel port. if something sounds good, just let me know and ill see what i can dig up.
 
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With all the talk of worm drives, I got to thinking about things, and I wonder how just using bolts and a motor would work. I mean, mount 4 nuts to the bottom of the platform so they can’t turn, and have a motor at the bottom turn the 4 bolts either up and down. While this wouldn’t be the best programmable solution, I could just time it so that after so long it quits, or place some sort of stop switch at the end of the track. I know it’s ghetto, but, do you think it would work or would the friction be too much for a motor to really be able to do?

I’m sort of stuck with motors at the corners because the middle will have stuff on it, so I can’t lift directly from the middle. So that’s what got me thinking of the whole nut and bolt thing. I’m also thinking that, depending on the size of the nut/bolts, the coefficient of friction should be enough to keep them from coming down. Course, it might be too much for the motor too.

As for the space concerns, this is going in the middle of the top of my case, so I don’t have a WHOLE lot of room to work with, and I can’t really mount it from the sides of the case, cuz … I have plans for those too. Really… there’s a lot of stuff going into this mod, and it’s not even a totally custom job, so it’s going to be REALLY busy for a normal case. The one I have picked out is 2 feet tall, and I’m hoping to get everything stuffed in there and have room for the base components. (hehe least the case I found is cheap. $50 for a steel case that’s 2 ft tall didn’t seem that bad to me.)

Either way… once I get this thing done everyone will either REALLY love it or REALLY hate it... probably both, but most importantly I will have learned a real a$$load of stuff about electronics, and that’s the main goal.
 
SpazzeBlue said:
With all the talk of worm drives, I got to thinking about things, and I wonder how just using bolts and a motor would work. I mean, mount 4 nuts to the bottom of the platform so they can’t turn, and have a motor at the bottom turn the 4 bolts either up and down
Click to expand...

what you are describing is called a linear actuator, and it will work. industrial suppliers sell "real" versions of these, but as long as you aren’t doing CNC milling a threaded rod and a nut will work fine. if you have two sets (one on each side) you will need a chain or belt of some sort to link them together. you could use two motors, but there is no real way to ensure that they will turn the same speed, which could rip your thing apart (been there, done that).

the best way to make your own actuators is to get a 3' section of 3/8" (or whatever you have nuts for) threaded rod from Home Depot or Loews. get the biggest nuts you can find. pick up some copper or plastic tubing that fits perfectly (but not tightly) around the rod. you’ll also need some Teflon washers that fit inside of the tubing. a small (1/8") tap and a machine screw that can fit it helps too. to make a single actuator system, you’ll need 2 smooth rods and plastic tube that fits around them; but if your making a duel actuator system, you don’t need the extra supports (they cant hurt though).

once you figure out where you want it to go you can cut the threaded rod down to the length you want. smooth it out by taking some scotch-bright or steel wool in one hand and thread the rod through it with the other. after 20 minutes, wash it off with a degreaser and clean water, and apply a good coat of 3-n-1 oil (or wd40).

next, you have to make the motor linkage. a small 9v toy motor probably isn’t going to cut it, you’ll still need an automotive motor of some sort. i stand by my recommendation of a seat motor. there are several ways of linking the rod and the motor:

(method 1) grind away half of the top 1cm of the rod (with a Drimmel, angle grinder, or bench grinder), leaving a semi-circle. put it in a vice with the flat side facing up, and center punch a hole in the middle. drill out a 1/16 inch hole with a zirconium (or other metal) drill bit at a low speed. repeat the processes with the motor's output shaft. this should have 2 half circles that have holes that line up. when the rod and motor are in place, you drive a pin through the holes, and fold it over so it cant come out.

(method 2) get a small metal tube that fits over the motor shaft. chances are that the motor shaft will be smaller then the screw shaft. if this is the case, youll have to grind down the screw shaft until it fits in the tube. this is fairly easy if you have a access to a lathe, drill press, or even a hand drill -- chuck the threaded rod in, and rotate it while you grind. don’t turn on the grinding tool until after the rod is turning to ensure that it is centered properly. once both parts fit inside of the metal tube, you can drill 2 holes in the tube and pin both parts. alternatively, you can use epoxy to hold it together, but make sure to roughen and clean up both surfaces and keep it spinning while the epoxy dries so that it doesn’t dry off-center.

(method 3) if the motor has a gear on the end: obtain a small metal gear, and drill a hole slightly smaller then its axle into the end of the threaded rod. tap the hole, and use loctite to keep the screw in. once you know it works, you can solder/sweat the gear to the rod.

once you have the motor attached to the rod, you can mount both. now is a good time to thread on the follower - the nut that the screw moves up and down, as well as some ~1/2" pieces of that plastic tubing. in order from (bottom to top), you should have two tubes, the nut, and two more tubes, then the motor.

each screw needs two bearings. the bottom bearing is the simplest: a piece of tubing attached perpendicular to the threaded rod. using epoxy (or hot glue, lol) attach it directly to the bottom of the case. only apply the epoxy to the outside of the tube, leave the inside empty (so it stays flat). then take the Teflon washer and put it in the bottom of the tube (no need to glue it). the top bearing is a little trickier, and you’ll have to find an edge to glue the tube on (the upper bearing is also attached with epoxy) you can use a piece of wood or plastic as a spacer, because you’ll have to attach the motor to the same edge. the motor is best held down with metal straps that are screwed on. if you cant do that, you can use epoxy, but make sure to keep the thing running while it sets.

now you should have 2 free moving pieces of tubing and 1 nut. these get epoxyd to whatever you are moving up and down. keep the tubing as far away from the nut as possible, to give the whole thing more support, but not so far as that it limits motion.

when that epoxy has cured, your are done! you can use limit switches to cut power when it hits the bottom or top - each is a NO and NC switch with a common C terminal (i.e. SPDT). you can either use the switches to provide feedback, or have them actually control the flow of 12v through some diodes and maybe a relay.

ii hope this is helpful, this is basically everything i learned over a couple years of robotics.
 
Man... I really need to borrow your brain for awhile… could you send it to me FedEx? I’ll even spring for insurance. Hehe.

That is a great walk through on how to make a linear actuator. And with some cosmetics, I bet it won’t even look ghetto. ;) Even better is the fact that we have hundreds of those limiting switches at work, so for once my job actually pays off, even if it is only in the really cheap parts area.

I have come across some linear actuators on the net, and that was what got me looking into the whole nut and bolt idea. The only thing that I was worried about was the fact that there was a lot on lubrication of the ball screw, so I wasn’t sure if my ghetto idea would work without the lube, but thanks to your info I’m going to have to try it out and see what sort of results I come up with.
 
as long as you are lifting < 10% of rated weight (for a 3/8" threaded rod should be ~5,813Lbs, so less then 500Lbs) you shouldent need any lube at all. smothing out the threads helps alot (to stop jerks) and 1 coat of WD40 will last a looooong time.
 
Because of the length/weight that I need to move, a servo wouldn't really accomplish my goal. Most servo's that are not ungodly expensive are short stroke, and since I need to move a lot of weight, for hobby motors, it really doesn't work for a servo.

For the most part, I have the idea firmly set up in my head, just waiting for the money to get the req'd motors and see if it can lift what I need it to. Specially since I just upped my weight to nearly 15 lb's... ;)

Thanks for the input tho, as it is always well recieved and appreciated.


SpZ
 
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