www.robowars.org

RoboWars Australia Forum Index -> Technical Chat

Linear actuators and Jaws
Goto page 1, 2  Next

Post new topic   Reply to topic
  Author    Thread
Nexus
Experienced Roboteer


Joined: 17 Jun 2004
Posts: 903


 Reply with quote  
Linear actuators and Jaws

Wanted to ask if anyone has some background knowledge on placing actuators in a way to expand the travel on the arm. So far have been mocking up the motion in a flash animation and are getting close but there must be some theory that lets you work this out.
THeres a relationship between the mounting point on the arm, the mounting point on the body and the pivot point.
Trying to get 6 cm travel on the actuator to give me about 180 degrees movement on the arm.
Sure others wanting to build any actuator systems would find this helpful, not sure if it is inverse kinematics but feels similar in that it is based around circular movements. Am sure can tinker till I get it but right but some theory on the matter might help me and others doing similar stuff so does this ring any bells with anyone?
_________________
Bots that do not destroy you, only make you stronger.

Post Mon Jul 12, 2004 4:30 pm 
 View user's profile Send private message
mytqik



Joined: 26 Jun 2004
Posts: 127


 Reply with quote  

The easiest way without using a computer would be to make up a simple profile of your jaws from cardboard. Then put a nail through the pivot point onto a block of wood, or the kitchen table Very Happy Then rig up a piece of string from where the ram is to mount on the jaws. On the piece of sting mark the closed centres & open centres of the actuator.

Open the jaws as far as you want then scribe an arc onto the block of wood with the sting at the closed centres distance. Then Close the jaws & scribe an arc at the open centres distance. Where the two arcs cross each other is the only point the the actuator can be mounted to get the degree of travel that you want.

Keep on changing jaw profiles, pivot locations until you get a jaw mechanism ithat is suitable for your bot. If this doesn't sense I can send you a drawing of how to do it.

Post Mon Jul 12, 2004 4:47 pm 
 View user's profile Send private message Send e-mail
Spockie-Tech
Site Admin


Joined: 31 May 2004
Posts: 3160
Location: Melbourne, Australia


 Reply with quote  

Wouldn't trying to get 180 degrees of rotational travel from a conventional linear actuator cause geometry problems when the linac is at 90 degrees to the desired direction of travel ?

If you imagine the arc that the arm travels along as a segment of a circle, then when the linear actuator is at 90 degrees, you are trying to produce a lot of angular travel for a very small amount of x-axis travel, effectively making the linac work hard through a big backwards lever as it goes through the "overcenter" section.

How about mounting a gear directly on the jaws pivot shaft and driving it with some sort of worm setup ? That would give you as much travel as you want with a constant torque output (I think).

The only problem with worms is you need to support the driving gear at both ends of they try and push away from the driven gear. (thats probably already known to you George, but maybe not everyone else).

Post Mon Jul 12, 2004 4:51 pm 
 View user's profile Send private message Send e-mail Visit poster's website MSN Messenger
Karmond



Joined: 16 Jun 2004
Posts: 97


 Reply with quote  

What design are you using?

Jaws mounted at two points and attached to the actuator at one point like this

or Jaws mounted at one point and attached to the actuator at two points like Kan Opener (I can't find a picture right now).

You might find that the first option would be more effective for small travel and the second would be stronger.

Post Mon Jul 12, 2004 5:14 pm 
 View user's profile Send private message
Nexus
Experienced Roboteer


Joined: 17 Jun 2004
Posts: 903


 Reply with quote  

WEll picked up brett. 180 degrees was an unrealistic coment. Want to end up with Jaws that open about 90 - 120 degrees., Has to be same top and bottom so jaws pass through each other. String idea is interesting. The ideal setup would let the jaws move with the actuators keeping parralel to them. THats what i am striving for. Parts are looking too good together to not set it up right.
Some tests have the actuators changing their angle on moveing so want to work out how to keep the actuator parralel with the arm at all times.
_________________
Bots that do not destroy you, only make you stronger.

Post Mon Jul 12, 2004 5:15 pm 
 View user's profile Send private message
Nexus
Experienced Roboteer


Joined: 17 Jun 2004
Posts: 903


 Reply with quote  

The design is a blatant ripoff of Diesector.
His design is a copy of a tree grapple arm for forrests. The mechanism has both jaws on the same pivot point. The bottom jaws are made of two arms and the top of one. When they close the top passes through the bottom two completely closeing like scissors. Each has its own actuator to close and lift and the whole system behaves the same when inverted apart that when inverted the top jaw would have the two arms and bottom would have one. Am almost their. Have animated it in flash and it looks very freakin cool. Am trying to set up the 2 mount points on the actuator to maximise travel and stop the actuator pushing itself off when at its extreme travel.
Some pics here of diesector
Most animated clamp bot around
http://www.mutantrobots.com/html/diesector.html
_________________
Bots that do not destroy you, only make you stronger.

Post Mon Jul 12, 2004 5:23 pm 
 View user's profile Send private message
kkeerroo
Experienced Roboteer


Joined: 17 Jun 2004
Posts: 1459
Location: Brisbane


 Reply with quote  

Well this is deffently my sort of subject.
It sounds like you are using the standard slider/cranck mechanism that I used in Adversary. You won't get a mechanisim like that to rotate past 180 degree unless there is a large amount of inertia in the system, which I know wont be the case here. In fact I would say that the effective range for these sorts of claws is about 120 dgrees. The main thing to remember is that the most amount of force at the tip of the jaw happens when the jaw link is perpendecular to the actuator. The more you push, the less force at the tip there is, until the jaw link and the actuator are inline, at which point all the force from the actuator is going into the bolts holding it together and nothing at the tip. Also you can't reverse out from that position as the force from the actuator will just go into the pivots like before, but in the oppisite direction. This point is called the dead center position. The only way is with inertia in the system. The best bet is to design it so the best crushing point is when the jaw and actuator are perpendicular.
But I'm not saying going up to 180 degrees can't be done. I have designed a mechanism that can go though the 180 degree mark with most of the force still at the tip. Just look at back hoes and escavators. I have also designed a claw mechanism that'll use the dead center position to increase the force at the jaw tip to close to infinity, and I wanted to put it onto Adversary, but the actuator was bigger then I planned on, so I went the boring way to start with.
_________________
Get Some!!!

Secretary of the Queensland Robotics Sports Club inc.

Post Mon Jul 12, 2004 6:03 pm 
 View user's profile Send private message Send e-mail Yahoo Messenger MSN Messenger
andrew



Joined: 16 Jun 2004
Posts: 3110
Location: Castle Hill, Sydney. N.S.W


 Reply with quote  

I am also planning a jaw type lifter/clamper (not a very good clamper but a good lifter/tipper and it will be cool looking) thats basically going to be a copy of their middleweight karcus. I just have to get plans done and parts prganised and such first off. I also have some issues as i need to maek it so mine will be supported around the jaw bit for the spinners and also it will have to self right the robot. THe mechanism rotates about 90 degrees so that should be failry easy.
_________________
Andrew Welch, Team Unconventional Robotics

Post Mon Jul 12, 2004 6:11 pm 
 View user's profile Send private message Send e-mail MSN Messenger
Totaly_Recycled
Experienced Roboteer


Joined: 15 Jun 2004
Posts: 1346


 Reply with quote  

Hi George the one main disavantage of getting more travel on the end of the jaws with a limited actuator travel will be a loss of presure on the ends of your jaws but iff you are only looking at clamping and not any crushing power then it should be ok.
A couple of the idea's already thrown up should work for you .
Getting 180 degress will let you fall into all the traps brett mentioned .
I've been down that track trying to get 180 degre's on our self righter arm on Vertex .
I tried gears withww motors and didn't have enough power ,then I steped up to winch motors and started stripping steel gears i tried an actiuator with a winch motor i broke a wire rope and bent the frame in then burnt out the motor .My last attempt was a two stage home made air ram with an over centre linkage but even it gets jambed occasionaly and it needs over 200 psi to roll the bot over .Good luck with it and keep us posted on your progres.

Post Mon Jul 12, 2004 6:11 pm 
 View user's profile Send private message
Nexus
Experienced Roboteer


Joined: 17 Jun 2004
Posts: 903


 Reply with quote  

THought there are others out their with this in mind.
@ Andrew, Karkass is actually the same jaws from a previous diesector so if thats what you want to make then you will face the same things as me.
Forget 180 degrees. It has to be 120 for each arm so you have an overlap of about 60 degress.
The parts i have fit like a glove. A pair of electric seat actuators and a scissor jack are about the same proportion to the Super Heavy weight but mine will be a lightweight instead.
Not wanting to crush, waste of time for this design. Its a control clamp bot.
Soon will have an animated plan of the system showing its movements, it will do the same as the original in that it will lift its front wheels off the ground when its mouth is fully open, super cool. The only difference in the actuators in Diesector have a long arm mounted further down each jaw. This extra travel is what I am tying to work around as Car seat actuators are not that loing. But it will be solved. Have it working in Flash but just need to tune it to reduce any angles on the actuator when it is being used. Dont know if that makes sense, TRy to put up a page tonite of the animation, looks sick as.
_________________
Bots that do not destroy you, only make you stronger.

Post Mon Jul 12, 2004 7:09 pm 
 View user's profile Send private message
3Faze



Joined: 26 Jun 2004
Posts: 99
Location: Lincolnshire, UK


 Reply with quote  

Here's a couple of less-conventional ways to do a crusher (excuse the fag packet scribbles, couldn't be bothered with Flash.

The first one, I think, is how katnip works. Each jaw works a bit like a bell crank (such as an accelerator linkage in a flat front van/truck). A single ram/linac moves one end of each jaw, such that each jaw pivots part way along its length. The main problem with this is that the initial force has to be a pulling force, rather than a pushing force, so it doesn't work well with most pneumatic cylinders, which tend to be less efficient used this way.

The second one is how the Belgian bot TAN works, basically, it's a pair of pliers, the force pushes the "handles" apart, pushing the business end closed.



Just food for thought...

Post Mon Jul 12, 2004 9:31 pm 
 View user's profile Send private message
Nexus
Experienced Roboteer


Joined: 17 Jun 2004
Posts: 903


 Reply with quote  

Just uploaded a Flash animation showing what i am up to.
Address is
http://www.idi.com.au/robots/jaws.html
It shows my parts in scale to each other and placement of various motors yet to be confirmed. Wheel size is 6 inch so its not that big but should be tuff.
Animation shows atuators almost working as required, just some more tinkering before start building. THink it looks cool, any feedback suggestions feel free to comment.
_________________
Bots that do not destroy you, only make you stronger.

Post Mon Jul 12, 2004 11:05 pm 
 View user's profile Send private message
Spockie-Tech
Site Admin


Joined: 31 May 2004
Posts: 3160
Location: Melbourne, Australia


 Reply with quote  

By the way, for george, heres some links to Karcass in its latest incarnation..

http://4.47.235.131/botpics/mmer/npc/DSCN4275.JPG

*warning, non-dialup friendly pitcure - 1.5mb 2500pixel hi-res images below !*
http://www.team-moon.com/battleBots/NPC2004/111_1116.html
http://www.team-moon.com/battleBots/NPC2004/111_1117.html

and more hi-res photos in general from the recent NPC event in the US

http://www.team-moon.com/battleBots/NPC2004/NPC2004.html
_________________
Great minds discuss ideas. Average minds discuss events. Small minds discuss people

Post Thu Aug 05, 2004 7:10 pm 
 View user's profile Send private message Send e-mail Visit poster's website MSN Messenger
Nexus
Experienced Roboteer


Joined: 17 Jun 2004
Posts: 903


 Reply with quote  

Thanks for that Brett, was good to see how its been changed.

Not too unlike what i am building aprt from the camber and wheels are close together on Karkass, Both camber and wheel spacing are made easy if you connect gearhead motors directy to the wheels, like Tazbot.

With the scooter motors and chains it is hard to camber and close the wheel gap like Karkass but will try as much as possible.

One thing I noticed which was helpful was the mods done to the jaw to adapt it to this new frame, in particular where he had to grind down some of the top jaw so it would not hit the motor when fully extended upwards.

Thats where I am at and wasnt sure if I wanted to grind but feel better about it after seeing it done before.
Havent figure out if the bottom jaw is actuated, dont get it yet, cat see an actuator but in a shot the bottom jaw is lifted.

Havent worked out how the tail works, on both bots he has the same mounts so its a standard practise for something
_________________
Bots that do not destroy you, only make you stronger.

Post Fri Aug 06, 2004 9:13 pm 
 View user's profile Send private message
kkeerroo
Experienced Roboteer


Joined: 17 Jun 2004
Posts: 1459
Location: Brisbane


 Reply with quote  

Those linear actuators look very familure. I wonder if they are faster then mine.
_________________
Get Some!!!

Secretary of the Queensland Robotics Sports Club inc.

Post Fri Aug 06, 2004 9:43 pm 
 View user's profile Send private message Send e-mail Yahoo Messenger MSN Messenger
  Display posts from previous:      

Forum Jump:
Jump to:  

Post new topic   Reply to topic
Page 1 of 2

Goto page 1, 2  Next

Forum Rules:
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum

 

Last Thread | Next Thread  >
Powered by phpBB: © 2001 phpBB Group
millenniumFalcon Template By Vereor.