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6dof (Six-Axis hexapod) with complete 3D motion

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Posted: 2011-07-21 14:18 
Thanks for the pix, I haven't seen it before.

That could work!

For our medical robot, the master manipulators (7 axis) are weakly driven (real DC) servos commanded to a floating (Gravity Compensation) or neutral position. Then the operator moves the left and right hand manipulators which easily overdrives them. Their positions are all reported via dual encoders per axis to the system which then does a mass of calculations and transforms to position the end effectors (medical instruments) on up to four arms on a remote patient cart.

I'm wondering if you could similarly use modified servos in your masters. You would have to connect to the pots anyway; so you could split off the logic voltage in the R/C servos and give them 5 V, which would allow you to run the motors in the R/C servos down at perhaps 2-3 V, and thus give them a very weak, override-able torque. You'd then command them to a "neutral" balance point (platform up and level?). then you'd be able to move your masters, read the pots, record and command the Slave manipulators (Stewart Platform).

Supplying a regulated logic and sensor (pot) voltage would also improve the "jitters".

Alan KM6VV

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Posted: 2011-07-21 18:18 
some interesting stuff there Alan. thanks for sharing

Slave manipulators (Stewart Platform) is built as you know but i have just started constructing the Master controller or Telemanipulator. i had 4 modified pots sitting about already but ill have to order another 2 soon. i have tested the code. the platform moves as told to and looks very cool. i will post images when iv ordered, modified and installed the other servos.

for more info on the process of making a servo into an output here is how:
viewtopic.php?f=6&t=4362&p=70743#p70743

now to work on the Puppet!

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Posted: 2011-07-21 19:05 
Oh yeah, I remember when that topic came up.

an R/C servo is a terrible thing to waste...

I'd like to see how well the gravity comp would work with a "weak" R/C servo as a master.

I wonder if one could attach a 3-Axis accelerometer as the end effector of a master, and "drive" it around to determine the amount of GC needed in the work envelope?

You want the master to stay where put, and provide minimal resistance to movement. Although incorporating Haptic feedback would really enhance the "Waldo" experience!

Alan KM6VV


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Posted: 2011-07-22 09:43 
i haven't soldered the motor wires together as the resistance would be too much and id end up with an aching wrist. The servo gears have enough resistance to allow for the Master to hold position. in fact it does still require a bit of force but im getting a nice flow of movement.

im using the Cheapo Hitec HS-311 servos for the pots. and i had a few 475's laying about too.

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Posted: 2011-07-22 13:37 
Maybe that's why no one has bothered to do it before. Too much resistance (drag).

It's really cool to see the masters dance around when they build and test the Surgeons Consoles, and the slaves on the Patient Carts as well!

Alan KM6VV

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Posted: 2011-07-22 14:41 
On the Left is the SLAVE and on the Right is the MASTER



Just some images of the mod: (Using a 475 servo)
Remove the back of the servo. i tend to keep one of the screws in as it help to keep the casing together.



Removed the board and resoldered RED-RED / YELLOW-YELLOW / BLACK-GREEN



I think im going to remount the Slave and Master on a better more ridged base.

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Last edited by innerbreed on 2012-12-24 19:40, edited 3 times in total.

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Posted: 2011-07-22 15:06 
Ha! That's brilliant Jonny! You've made a 6 DOF joystick! :lol:

Very simple, yet clever way to solve the control part. You could build a DIY remote around the master and have a receiver on the slave. Should be very easy to make. :D

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Posted: 2011-07-22 15:35 
yes, just a bit of fun.

the project only really starts once i can get the puppet made. working on little bits at a time so i can try and get it looking right.

every time i do something like this i always get other ideas.. lol

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Posted: 2011-07-22 17:19 
Yes, I like your joystick! Wish I could get the feel of it.

You could also add a PB (and/or a pot) into the grip.

Nice job!

Alan KM6VV

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Posted: 2011-07-22 18:09 
innerbreed wrote:
yes, just a bit of fun.


Okay! I have tried as hard as I can to follow along, but you guys with your "bits of fun" are giving me a serious headache now. :? :oops:

This forum really needs a Genius Free Zone so guys like me can still come out to play safely. :P

If you fellows don't mind indulging me, please let me restate and summarize what I think is happening, and anyone please tell me where I'm right and explain to me where I'm wrong. Bear in mind that I don't know the full capabilities/purposes of all the goodies that Lynxmotion and BasicMicro provide:

1. The Master provides control signals to position the Slave in a manner somewhat analogous to the way an indoor control rotor used to control an outdoor TV antenna rotator. (Little history lesson there for you young guys. :lol: )

2. There are many ways matched motor-like combinations can control each other: stepper motors, motor-dynamo, etc. In this case, continuous rotation is ruled out, and reversability is required. So the matched combinations are pairs of servomotors in which one of the servos is modified to act like a potentiometer. This pot servo is in the master.

3. The pot servo modifies an output voltage which becomes the input signal that tells the servo in the slave what to do.

Here's where I start getting shakey, so watch me carefully. :wink:

4. The master and the slave can have completely separate power supplies of differing voltages. So to avoid power conflicts, the signal wire between them should probably be isolated somehow, perhaps through an optoisolator.

And now I am completely lost. :? :oops: :(

5. How does the signal wire connect? Can it go directly to the signal pin on the slave servo? Or can it go to an input pin on an SSC-32? Or must it go first to an input pin on a BotBoard II before going to the servo or to an SSC-32?

6. Where does isolation occur? Do the BBII and/or SSC-32 provide it, or must there be an additional component?

Thanks to all who help. And remember, the fact that some of us don't understand all this immediately does not diminish the lustre of those of you who do. :D

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Posted: 2011-07-22 18:11 
I don't want to be a spoilsports, the project looks very interesting, but did you really need to use 6 hitec servos (cheap ones, but still) to get potentiometers ?
This seems to be incredibly wasteful, considering that big potentiometers with enough drag to stay in place by themselves cost less than 50c a piece...

Anyway, I see that you are going for an interesting non-IK route, I'm looking forward to see it move with it's puppet mounted on it :D

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Posted: 2011-07-22 18:29 
Yeah, a good Hitec servo is a shame to waste..

However consider: Same mechanical connections, duplicate master/slave hardware. Better pot (perhaps) then in a really cheap R/C servo). Range and scale of pot would be the same as in the slave servo. (oh yeah, they're both linear pots, so they HAVE to be equal).

Simple interface to the required BB II. Which answers the previous question, we typically don't need the isolation with the master and slave in proximity, we're using TTL levels into (OK analog 0-5 V) and out of the BB II. Six axis, not enough inputs just on an SSC-32, and you'd need a uP (BB II) anyway.

Never used motor-generators to transmit position, maybe you mean Selsyns or Synchros?

Alan KM6VV

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Last edited by KM6VV on 2011-07-22 18:30, edited 1 time in total.

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Posted: 2011-07-22 18:29 
@RoboTed:
Check this thread: http://www.lynxmotion.net/viewtopic.php?t=4362 to learn more about the master/slave setup.

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Posted: 2011-07-22 19:16 
Xevel:, yes you are totally right and it just seem a bit of a waste. its a shame to mod the servos but i do keep the boards so if i need them at a later date i can "fix them." :wink:

and trust me, if my programming skill's weren't as bad as they are, then right now we would have a fully operational Stewart Platform working with Full Inverse Kinematics.

Ted:
Quote:
This forum really needs a Genius Free Zone so guys like me can still come out to play safely.

Love your sense of thought.

When i first joined, i knew nothing of robotics, except that i wanted to make one. Robots are cool!
There is still lots for all of us to learn, and the good thing is, is that we can do it at our own pace.

I certainly need to learn more. From what you see here on these pages, and other posts iv made 'i have only learned these things through everyone else, and i have them to thank.
It empowers me to think i can inspire others or even teach others, but its great to see projects grow from nothing.

Create, Innovate, .... Inspire! :wink:

Alan:
i did think about putting one on the top but id have no use for it. after all, iv already been greedy with a 6AxisJ/Stick anyway. :lol:

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Posted: 2011-07-22 19:38 
Ha!

OK, we'll leave that (the extra pot) as an exercise for the reader...

Alan KM6VV

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