Question about swinging/regulated rotation

[xgarmothx]

Hello!

I am a beginner - scouring the web, this site, and some books for a month or two now. I am going to build the $50 tutorial robot within the next few weeks.

Throughout this learning process, I have been familiarizing myself with CAD programs, and creating rough ideas for robot projects I'd like to undertake down the road, when I have more experience.

That said, I have a mechanical question. Imagine I'd like a component that swings a jointless "arm" (say, a 3' piece of PVC piping) 180 degrees. Think of it like a baseball bat. I'd like it to:

- Swing exactly 180 degrees and return to its original position.
- Be able to do so with programmable/variable speed...from a 2 second journey to a very fast, almost immediate journey.
- Be able to withstand conflicting force without damage. For example, if it swings around 160 degrees and is stopped.

What would be the best bet for this? I am somewhat new to this mechanical world; Seems like a motor wouldn't have the control - are servos that size commercially available? Can they withstand the opposition I mentioned?

Any insight into where to begin looking into this would be very helpful. Thank you!

[pomprocker]

Would this swinging arm come against any force traveling the opposite direction?

Or is it just going to be cutting through the air?

[xgarmothx]

Thanks for the reply!

It would just be swinging through the air (horizontally, for 180 degrees), usually coming in contact with a stationary, immovable object somewhere in the second half of the journey, and returning.

The very short version of a very long story, is this would be part of an experiment that tests human reaction speed (and the improvement thereof) Think of the motion like that of a very padded, very light baseball bat swinging towards a stationary human. He or she would ideally be bringing his or her hand up to block the swinging motion before it makes contact with the torso. The speed of the swinging arc would have to be programmable.

Thanks.

[pomprocker]

For this I would look at some sort of pneumatic setup.


You could try a high speed servo with a servo saver, but i dunno!!

[xgarmothx]

I'll look into servo savers, since I've never heard of them.

As for a pneumatic setup - I'll do some research there too. Any other direction would be greatly appreciated.

[xgarmothx]

Upon initial research...pneumatics seems like it would be more complex and expensive than would be necessary.

The idea of the servo saver intrigues me...even the very concept of a spring (or spring like) action associated with the swinging action that would allow for a predetermined amount of leeway.

[pomprocker]

solenoids maybe, but I don't know how you could get positional feedback, except maybe aim a sharp IR rangefinder at it to measure the distance it travels.


EDIT: these look like they may work:

http://www.servocity.com/~servo/html/linear_servos.html

[fuzzyt]

The very short version of a very long story, is this would be part of an experiment that tests human reaction speed (and the improvement thereof) Think of the motion like that of a very padded, very light baseball bat swinging towards a stationary human. He or she would ideally be bringing his or her hand up to block the swinging motion before it makes contact with the torso. The speed of the swinging arc would have to be programmable.

Hmm, I've been pondering the same ideas for Kempo practice.    Haven't gotten very on it yet.  I look forward to seeing what you come up with. 

[Soeren]

Hi,

Arm turning in bearings, large spring to load the force into (by motor if need be), strong solenoid to release (use something like the lock on a cross bow to get wear and a tell tale sound down), use friction plates with silicone grease of the kind used in broadcasting size tripods and make the interdistance adjustable within a short span to go from 180° in a very long time to 180° in a blink of an eye.

Just make sure that the innermost section is made somewhat flexible and the "bat" is padded, or you might be liable in case of injury.

What's wrong with the steady "clothes rack" that Brucie used for his practice?

[xgarmothx]

Thank you all very much for the information. I definitely have some homework ahead of me.

Don't worry about padding - this will be safe as a cotton candy factory...uh...without hot vats of melted sugar. Actually, that doesn't sound very safe at all.

I will look into a lot of the methods and setups suggested and be sure to keep you informed!

[xgarmothx]

Arm turning in bearings, large spring to load the force into (by motor if need be), strong solenoid to release (use something like the lock on a cross bow to get wear and a tell tale sound down), use friction plates with silicone grease of the kind used in broadcasting size tripods and make the interdistance adjustable within a short span to go from 180° in a very long time to 180° in a blink of an eye.

Soeren,

I looked into a lot of what you described last night - and I thought I'd ask...if you happen to be bored, do you think you'd mind going into a little more detail? I have a pretty good picture of your overall suggestion, but am having a hard time understanding some of your reasoning.

Here's what I'm picturing from this description: Springs fire the arm. In the event that the arm doesn't come in contact with a (human) blockage, it reaches a certain point at which it's stopped by a blockage close to the base. The base would have a gear on it, and a bike-like chain around the gear would begin to reel the arm back towards the springs after a half second or so (enough time for even a low speed setting to reach the farthest rotation). Once the chain has brought the arm back to the springs, an electromagnet would pull the arm the rest of the way toward the springs, creating tension. By using an array of electromagnets of differing strengths, the tension on the springs could be affected. Once the electromagnets are turned on, the chain would stop turning.

Am I wayyyy off?

Again, if you have the time, I'd appreciate the help. If you simply don't, I totally understand and appreciate your input thus far. I will continue to research.

- Matt

[colorclocks]

Hello!

.. Imagine I'd like a component that swings a jointless "arm" (say, a 3' piece of PVC piping) 180 degrees. Think of it like a baseball bat. I'd like it to:

- Swing exactly 180 degrees and return to its original position.
- Be able to do so with programmable/variable speed...from a 2 second journey to a very fast, almost immediate journey.
- Be able to withstand conflicting force without damage. For example, if it swings around 160 degrees and is stopped.

I guess I'd try a belt drive for this, so the belt could just slip if the arm hits an obstruction.  Attach a pulley to the motor shaft, and one to a bearing mounted on the chassis.  Attach the arm to the second pulley.  something like that.

[Admin]

Just some stuff for you to google up for more ideas . . .

limit switch
four-bar linkage
homemade encoder