Author Topic: Ideas about connecting servo to threaded rod.  (Read 1857 times)

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Offline Inigo MontoyaTopic starter

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Ideas about connecting servo to threaded rod.
« on: June 21, 2013, 08:40:28 PM »
I have a Tower Pro SG90 micro servo modified for continuous rotation, and I need it to rotate a 1/4" threaded rod.  (Shown in photo.)  I need the servo to be removable, so it will have to be friction-fit to the (Futaba) servo arm, which is ok.  My first thought was to super-glue the servo arm to the threaded rod, and then trim the ends off.  Before I go about doing this, I was wondering if anyone had a more elegant way of solving this mechanical problem?  How do the pros usually connect servos/motors to rotating rods?

Offline jwatte

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Re: Ideas about connecting servo to threaded rod.
« Reply #1 on: June 21, 2013, 11:09:39 PM »
There are are all kinds of shaft couplers. You can find them anywhere from McMaster-Carr to SDP-SI to Amazon/SmallParts.
The main problem you might see would be that the output spline of the hobby servo is not a common type for commercially available couplings.

Typical linear actuators mount the motor parallel to the rod and use pinion gears instead; they do not mount the motor in-line.

Offline Pogertt

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Re: Ideas about connecting servo to threaded rod.
« Reply #2 on: June 22, 2013, 01:28:45 AM »
This link from one of the sites advertisers....................

http://www.servocity.com/html/set_screw_servo_shaft_coupler_.html
Mmmmmmmm....Cookies

Offline Inigo MontoyaTopic starter

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Re: Ideas about connecting servo to threaded rod.
« Reply #3 on: June 22, 2013, 10:21:23 AM »
Why thanks guys, I did find the perfect solution on one of the websites.  Sadly, the price of the coupler is 4x the price of the servo, and I'm going to need either 8 or ideally 16 of these...  The couplers will cost more than the rest of my robot combined.  If I have to I'll do it, but I'm going to have to hunt for a DIY solution first.  I'm going to try the epoxy method first before saving up for those.  I'll post the results of my epoxy experimentation here.  I wish I had a 3D printer, as plastic would probably do for these tiny servos.  Any suggestions on a DIY method other than epoxy and prayer?  :-)

Offline Inigo MontoyaTopic starter

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Re: Ideas about connecting servo to threaded rod.
« Reply #4 on: June 22, 2013, 10:33:10 AM »
I was thinking...  if I get a small flat piece of metal, such as a washer with a 1/4" inner diameter, and buy myself a tap & die set, I could thread the tip of the rod into the washer, and then braze the two together.  Then, I could drill tiny holes that would then fit the screw holes of the servo arm...  I'm beginning to think this could be a good semi-pro DIY solution.  What do you expert robo-dudes think?

Offline waltr

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Re: Ideas about connecting servo to threaded rod.
« Reply #5 on: June 22, 2013, 12:56:31 PM »
That is a good idea and a good DIY solution.
Lots of ways to attach a flange (disk) to the end of the rod. Brass is readily available, not too expensive and fairly easy to work with. Instead of drilling a tapping just braze or silver solder a piece of brass sheet (~0.025tk) to the end of the rod. Then file to a shape that matches the servo horn and drill holes to couple to the horn.
A flange on the end of the threaded rod would not be hard to couple to the servo. You may want to consider some rubber washer or grommet between the two to allow for slight misalignment between the servo and rod like a universal coupling.

Offline jwatte

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Re: Ideas about connecting servo to threaded rod.
« Reply #6 on: June 22, 2013, 05:27:40 PM »
That washer idea sounds reasonable!

If plastic is good enough, you can also just melt some plastic straight onto the output spline, and then flatten/drill it as appropriate. Jamming InstaMorph/ShapeLock against threads/pinions actually works pretty well ;^)

Using the existing horns and epoxying or screwing something to the shaft you're driving is also doable. There's nothing wrong with Epoxy, as long as the load is not too high.

I'd probably try the InstaMorph first, then the epoxy, and if that doesn't work, the washer.

How small is the pinion of the servo? Could you center drill a hole into the shaft, and then use a reamer or something to shape the mating part of the servo spline inside that hole, and get a direct coupler?

Offline Inigo MontoyaTopic starter

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Re: Ideas about connecting servo to threaded rod.
« Reply #7 on: June 22, 2013, 07:25:36 PM »
The hole in the centre of the servo spline is 1/16"  The horn comes with a screw that can mate the horn to the spline permanently.  I picked up some washers that fit, and will be trying the washer/screw trick, or washer/screw/epoxy method if that fails.  I will post the results.

Offline jwatte

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Re: Ideas about connecting servo to threaded rod.
« Reply #8 on: June 22, 2013, 10:11:42 PM »
When suggesting center drilling, I was suggesting doing it in the 1/4" shaft you're attaching, not the spline/servo shaft :-)

Offline Inigo MontoyaTopic starter

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Well here is my working solution.
« Reply #9 on: July 02, 2013, 11:16:19 PM »
First, I bought some washers wide enough to accommodate the futaba servo arm, with an inner diameter slightly smaller than the threaded rod.  I tapped the centre of the washer so the threaded rod could screw into it for strength.  I drilled 2 holes in the washer to mate with the servo spline.  I epoxied the washer to the end of the threaded rod, with a nut epoxied in place for added strength and stability.  Then, it was easy to screw on the servo arm!  For pocket change and some hard work, I was able to avoid buying eight $12 couplers.  Attached are some photos of my work.

Offline jwatte

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Re: Ideas about connecting servo to threaded rod.
« Reply #10 on: July 03, 2013, 10:12:09 AM »
Awesome!

Offline waltr

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Re: Ideas about connecting servo to threaded rod.
« Reply #11 on: July 03, 2013, 07:43:52 PM »
Great job. Also great pictures and description.
Please lets us know how it works out.

Offline Inigo MontoyaTopic starter

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Success!
« Reply #12 on: July 06, 2013, 09:49:25 PM »
Well this part of my robot project is done.  I will never again have to worry about how to connect a Futaba spline to a threaded rod.  Success!  To thank you, I've attached a picture of how it looks on my super-secret robot project.

Now like any other invention, solving one problem means now facing another.  The servos don't quite operate at the same speed, so the platform tilts.  Having a timing belt/chain is not possible, as it would interfere with what I am connecting to the platforms.  I am now looking into rotary encoders of various types, and perhaps an accelerometer instead to detect and correct the tilt on the machine.  Do any of you have any suggestions as to what would be best/easiest/cheapest?  Is there anywhere I should look first in my research?

Again, thank you guys very much for helping me along in my work.  You are my muse and inspiration.

Offline jwatte

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Re: Ideas about connecting servo to threaded rod.
« Reply #13 on: July 07, 2013, 10:54:43 AM »
I'd find a way to put a timing belt on there :-)

Or swap out the hobby servos for real servos with absolute control of position/velocity (Dynamixel MX-28 or whatever.) That will cost a bit more, though. Or use DC motors with H-bridges, PWM, and built-in encoders, to make them run at the same speed. Pololu has some good ones.

Offline VegaObscura

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Re: Ideas about connecting servo to threaded rod.
« Reply #14 on: July 08, 2013, 04:21:42 PM »
SparkFun offers a tilt sensor for just under $3 or a triple axis accelerometer for just under $15.

Offline waltr

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Re: Ideas about connecting servo to threaded rod.
« Reply #15 on: July 08, 2013, 05:28:30 PM »
The servo to threaded rod disk may make it easy to have a sensor to detect each revolution. Either a bump on the disk to trip a micro-switch on a 'tab' to block a photo-detector.
Then at least you can count to turns on each threaded rod. With some code (micro-controller) you could get the faster servos to wait for the slower servos on each revolution. This way the maximum tilt error can only be one thread pitch.
Of course you could have multiple 'tabs' to increase the resolution.

Offline jkerns

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Re: Ideas about connecting servo to threaded rod.
« Reply #16 on: July 09, 2013, 07:20:43 AM »
Wait - let me make sure I have this right.

You started with servos that have built in position sensors.

You disabled the position sensors.

Now you need to measure the position of the servos.

(Yes, I understand why you did it that way - but when you think about it, it is kind of funny - eh?)

You can get continuous rotation potentiometers - you would have to keep track of the number of turns and make sure you start out level, and you would have to deal with the glitch in the output every time the pots "roll over" but it would be a low cost solution with reasonable resolution that would work at reasonably slow speeds.

Or use stepper motors.

I get paid to play with robots - can't beat that with a stick.

http://www.ltu.edu/engineering/mechanical/bachelor-science-robotics-engineering.asp

 


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