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kkeerroo
Experienced Roboteer
Joined: 17 Jun 2004
Posts: 1459
Location: Brisbane
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quote:
Originally posted by MachineHead:
2 DOF requires 2 motors. one for each direction you want it to go in. In human terms the shoulder joint has 3 DOF, the elbow has 1 Dof and the wrist has 3 .
In bot terms it means a joint that can pivot two different ways. Lifting leg is one and swinging it back is another. each requireing its own motor or actuator. One works on the pitch whilst the other on the yaw. Adding a third would give you roll and making it a 3 DOF leg.
In order for some bots to lower their overall height they have another DOF on shoulder to enable this meaning another motor on each leg. Every DOF requires a motor and thats why shufflers are different. An Inverse Kinematic system, camms, chains and multiple linkages are 1 DOF only. You would have to duplicate your system, slant it on another angle and then it would be 2 DOF if you could get it to work. If you also had a movable knee joint and an ankle That would be another 2DOF your legs would have..
I think people are getting degrees of freedom and axis mixed up. I was just swinging my arms around and I can't get my sholder to move with more then 2 DOF.
In the world of 3D there are actually 6 axis and planes for things to move around. The classic planes of x,y x,z and y,z. But they can also rotate about x,x axis, y,y axis and the z,z axis. I know these are technically cirular movements on the planes, but this was how I was tuaght at uni to make things easier.
I have designed mechanisims with 2 DOF on just the x,y plane and the example consistantly mentioned here with up-down actuator and forward actuator would also be just on the one plane. DOF of freedom are the mechanical inputs needed to stop the system from flopping around. If you just had a cylinder from a car with the drive axial sitting between bearings the thing would just flop down. You connect it to the rest of the drive and it locks into position. One input needed to lock it down so one degree of freedom. My sholder moves forward and backward, as well as up and down, or a combination of this. No matter how hard I tried I couldn't get it to twist. Therefore I have the musles up the top of bottom of the sholder that lock the up and down motion as well as the ones on my back and my lovely tittys which lock the forward motion.
What I want to say is it all comes down to how many mechanical inputs you put into each leg. 1 motor, actuator, ect... into the leg's system is 1 DOF and therefore is a shuffler. 2 or more is a walker. And before Aaron keeps going on about his walker I remember him saying it had a gearbox that had 2 outputs which fed the legs. And he said it only had one motor per side therefore that could only be 1 degree of freedom or the the worst walker design ever (floppy legs). And Anarchy is not a walker, its a shuffler.
I also thought that an electro-mechanical way for timeing the legs would weigh more then a purely electrical one, so why is it being said that only the purely electrical ones get the weight bonus? _________________ Get Some!!!
Secretary of the Queensland Robotics Sports Club inc.
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Mon Jun 21, 2004 8:25 am |
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Nexus
Experienced Roboteer
Joined: 17 Jun 2004
Posts: 903
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Yes I aplogies again for my use of the word Axis. It was late I was tired blah blah, THanks Kheeroo for your words but have to disagree on your shouder not being able to twist. documents suggest there is 90 degrees of twist in your shoulder as in your wrist but being a ball joint would make it hard to notice slight movements, but if you twist your arm like a screwdriver you shoulder joint will twist as well, just not as much. Well said about the stabilising effects, it made me think of mechadons legs and how close the motor/actuators angles are to each other. THats a good example having about 14 motors. 2 on each leg i guess and a few on its body.
Have to clarify something I said. quote - (XYZ is not 3DOF). In one way you can have 3DOF on the XYZ but was trying to explain that in XYZ environment you can also have a robot that has 30 DOF as each degree of freedom is independant from the other and doesnt have to refer to the XYZ of the environment because its position is in relationship to the part its connected to.
It has a sort of a Parent/child relationship, apart from the example above of a shoulder joint i guess.
I think the key word here is freedom, degrees of FREEDOM. If your mechanism is not able to work on any other plane than the vertical then it can have only one degree, think about it. As soon as you put another motor on the same leg at a different angle, even something small like 20 degrees you have a second degree of freedom and as Kheeroo said you become more stable, not that you have necesassily increased your range of movement much but you have reinforced the whole leg.
As Mel once said "Freeeeeeeedooommmm"
Degrees of Freedom -
Wanted to thank everyone for keeping this subject on track and not being too hard on my errors along the way. _________________ Bots that do not destroy you, only make you stronger.
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Mon Jun 21, 2004 9:32 am |
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Spockie-Tech
Site Admin
Joined: 31 May 2004
Posts: 3160
Location: Melbourne, Australia
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or couldnt be bothered trying to see a clear consensus between all the emotional angst in the walker debate.. besides which, in most cases, most walker are allowed to compete regardless of the rules/weight bonuses because they're "cool" and will entertain the crowds.
So with those rules, Build something cool, and regardless of what it weighs, the Event Organiser (presumably inclduing the approval of the other competitors) can allow it to compete.
or build another SOW or other rule-exploiting bot that only vaguey looks any different to a rolling robot, or a wind-up toy and the EO can put you in with the bots in the same weight class.
seems like a reasonable compromise to me.. for now anyway, until someone builds something cool enough to make another exception for..
For a Robot that never won a fight, Mechadon sure created a lot of controversy..
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Wed Jun 23, 2004 9:22 am |
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