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### Author Topic: fast/accurate linear motion on a budget  (Read 1345 times)

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#### MrPrezident

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##### fast/accurate linear motion on a budget
« on: August 10, 2013, 07:30:17 PM »
I would like to build a juggling machine that can juggle at least 3 balls and mimic the 2d motion of human hands without worrying about the arms.  So I need 2d motion that is very fast (and accurate).  It would be kind of like this machine:
https://www.youtube.com/watch?v=9asDO_1A27U

I was thinking that something like the following linear motor configuration would be great (except I would want two 2d bots instead of one 3d bot):
https://www.youtube.com/watch?v=9asDO_1A27U

The problem with linear motors is that they are REALLY expensive.  I am willing to spend around \$2,000, but with quotes that I have gotten on linear motor prices, my budget would be off by an order of magnitude.  I was wondering if there is some other way that I could do this without spending more than \$2,000.

I did some calculations, and it looks like I will need something that will be able to move at 3m/s (or actually 8m/s preferable if I want it to throw at different heights).  The max acceleration would need to be 17m/s^2 (or 60m/s^2 preferable).  Does anyone have any ideas?  Linear actuators are generally slow, and the fast ones seems to be expensive.  Maybe there is some servo that I can use with belt or gears to get the speed/acceleration that I want?  The other piece of information is that the balls would be about 100g, so that would be the load.

thanks!
-Nathan

#### waltr

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##### Re: fast/accurate linear motion on a budget
« Reply #1 on: August 11, 2013, 03:52:30 PM »
Look into buying from surplus outputs and possibly ebay to get stuff at a fraction of their original cost.
You will need to make some design compromises since exactly what you need will not be available.

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#### MrPrezident

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##### Re: fast/accurate linear motion on a budget
« Reply #3 on: August 11, 2013, 08:48:43 PM »
Hmm... I've never heard of pneumatic slides, but it looks like they don't have accurate/consistent velocities, which I think would be a big problem for a juggling machine.  The most important thing for an accurate throw would be to have the correct velocity when throwing the ball.

#### jwatte

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##### Re: fast/accurate linear motion on a budget
« Reply #4 on: August 12, 2013, 10:14:01 AM »
A brushed DC motor with a good shaft encoder (for velocity) and PWM controller, driving a timing belt, and the pulley traveling along a set of rails with linear bearings, will probably give you the consistency/precision/power you need.

#### MrPrezident

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##### Re: fast/accurate linear motion on a budget
« Reply #5 on: August 12, 2013, 01:07:58 PM »
Why a brushed motor instead of brushless?  Is it because they are cheaper?  I'll take a look at shaft encoders.  I understand the concept, but am not familiar with how to use them.  Would that give me the ability to accurately control position, velocity, and acceleration?

#### jwatte

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##### Re: fast/accurate linear motion on a budget
« Reply #6 on: August 12, 2013, 02:39:54 PM »
Brushed motors are cheaper, and easier to control, than brushless. You can typically find a brushed motor with gearbox and encoder all built in. Pololu has some nice ones: http://www.pololu.com/catalog/product/1442
Put a small pinion on the motor shaft, and a timing belt, and drive it with a PWM controller and read the sensor. There are controllers that can do both of those functions for you; Pololu JRK; Roboclaw; and a bunch of others.
The one additional thing you need is an end stop, so you know where "zero" is.

#### Pogertt

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##### Re: fast/accurate linear motion on a budget
« Reply #7 on: August 14, 2013, 04:52:26 PM »
Here is how this was acomplished:

http://dce.fel.cvut.cz/juggler/

Mmmmmmmm....Cookies

#### EddieAutomated

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##### Re: fast/accurate linear motion on a budget
« Reply #8 on: September 11, 2013, 02:47:27 PM »
Try our discounted linear actuators section. You might find what you are looking for there. You'll certainly be able to find something with the precision and speed needed for a project like that.

Cheers,

Matt Edwards
ActuatorZone.com

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