PDA

View Full Version : What is DOF?



Raul
08-03-2008, 09:09 AM
hi.
i am a begginer in robotics, i want know what is 20DOF?
Because Robophilo offers 20DOF and robobuilder offers 16DOF.

cumpliments

Alex
08-03-2008, 10:14 AM
[Post moved into it's own thread]

DOF stands for Degrees of Freedom. This means how many points on the robot can actually move. Sometimes they're referred to as "joints".

LinuxGuy
08-03-2008, 12:00 PM
Each servo is 1DOF (degree of freedom, movable joint). :)

8-Dale

Adrenalynn
08-03-2008, 12:29 PM
[Post moved into it's own thread]


I moved it over to general - did I mess something up?

openmindedjjj
08-04-2008, 03:17 AM
ya dof is easier by counting joints. basiclly its anything that moves by a servo is a dof. for example the elbow is 1 dof and the knee is 1 and if the head turns left and right that is one but if it also moves up and down then that is also 1 makeing him have a 2 dof head. do you pretty much see where im goin.. so pretty much a robot who has more dof can move more.. and is usually move expensive. like my bioloid robot that was a thousand bucks has 17 dof but honda's million dollar asimo robot has like 45 dof.

Adrenalynn
08-04-2008, 11:37 AM
Not just servos. Steppers and encoded motors, "muscle wire", air muscles, pistons, rams, screws, ... Any actuator that adds another degree of freedom is literally adding DOF - Degrees Of Freedom.

Electricity
08-04-2008, 11:42 AM
ya dof is easier by counting joints. basiclly its anything that moves by a servo is a dof. for example the elbow is 1 dof and the knee is 1 and if the head turns left and right that is one but if it also moves up and down then that is also 1 makeing him have a 2 dof head. do you pretty much see where im goin.. so pretty much a robot who has more dof can move more.. and is usually move expensive. like my bioloid robot that was a thousand bucks has 17 dof but honda's million dollar asimo robot has like 45 dof.
Does it really?! That is freakin wild! I can't even imagine how you'd fit 45dof in a robot that size..

sam
08-04-2008, 11:55 AM
You can put 6-7 per leg, 7 per arm (26-28). 1 for the Waist rotate (27-29). You have the head (29-31).

Finaly you have the hands wich have a lot of DOF's. Maybe tehre are some extras in the toes too :veryhappy: (FYI I just checked, it has 34 DOf, so you can have 2 dof in the hands to contract and something else.

ScuD
08-04-2008, 12:10 PM
I'm actually surprised at the "low" amount of DOF it has..

Zenta
08-06-2008, 07:22 AM
Hi,

IMHO: The total amount of servos does not always reflect the correct amount of DOF for a robot. Ex. a hexapod with 18 servos has 18 DOF per joint/servo but the body does also have 3 DOF (Global rotation). So I would say that a 18 servos hexapod have a total of 21 DOF.

Do you agree?

ScuD
08-06-2008, 09:00 AM
Interesting thought, Zenta, and I must say I tend to agree with you on it.

It doesn't apply to, say a robotic arm fixed to a stationary table, but with a hex, quad, biped, ... I certainly agree that there's more DOF's than actuators.

Adrenalynn
08-06-2008, 10:03 AM
I thought it was generally expressed like "21 DOF + Translation" and the translation gets left out frequently because it's assumed?

ScuD
08-06-2008, 10:33 AM
Well, the "other" three DOF are a direct consequence of the "real" DOF's; if that sounds at all logical..

What I mean is, with the "real" DOF's at any given point in their range, be it rotational or translational motion, the "other" DOF's of the body can be calculated.
The "other" DOF's cannot be changed without altering the "real" DOF's, so maybe it really shouldn't count as DOF's.

Hmm, I hope at least one person can make sense of what I just typed.

Adrenalynn
08-06-2008, 10:37 AM
I think we're on the same page. Maybe. Or not. ;)

If we're going to count the translation of the base, then any 'bot I can lift has +6DOF. Nothing says I can't pick up this 6DOF arm and throw it through the air. +6 DOF = 12DOF arm because I can throw it? If I mount the arm on a rover did it just become a 10 DOF arm?

ScuD
08-06-2008, 11:00 AM
Even better, add a drop of uncompressible fluid, get it into space, and it has infinite DOF

Adrenalynn
08-06-2008, 11:10 AM
Well, I don't know about "infinite" - how about "finite, but so large as to appear infinite to the casual observer or careless mathematician"? :D

ScuD
08-06-2008, 11:15 AM
Oh, right, you're a mathematician.

Okay, finite, but so large as to appear infinite to the casual observer or careless mathematician it is! :veryhappy:

Zenta
08-06-2008, 12:51 PM
I thought it was generally expressed like "21 DOF + Translation" and the translation gets left out frequently because it's assumed?
Agree about leaving out the translation part. But I would claim that body rotation is a practical DOF.

I think we're on the same page. Maybe. Or not. ;)

If we're going to count the translation of the base, then any 'bot I can lift has +6DOF. Nothing says I can't pick up this 6DOF arm and throw it through the air. +6 DOF = 12DOF arm because I can throw it? If I mount the arm on a rover did it just become a 10 DOF arm?

Interesting, does your 6 DOF arm have a rotating base?, if so I don't think your adding any DOF when mounting it onto a rover. Just transaltion. But if your 6 DOF arm does not have a rotating base I would say that your adding one DOF with the rover (hopefully your 4x4 rover can do 360 deg rotate on the spot;) ).

Another example; mount a 4 DOF arm onto a hexapod (18x servos) and you have a 7 DOF arm.

metaform3d
08-06-2008, 01:38 PM
DOF is generally defined as the free dimensions of the state vector for the system. In other words, a robot's DOF is the number of independent numbers that you would need to write down to fully describe the pose, configuration, orientation and anything else interesting about the physical state of the robot. "Free" dimensions refers to the components of the state vector that can be set freely. You could write down the servo position and the current torque, but the one fully determines the other so that is only one DOF.

What's "interesting" depends on the problem you're trying to solve. If you're working on a biped gait then its position in space is irrelevant; if you are trying to navigate to a particular place, however, then translational degrees of freedom matter, as does orientation. When describing the general DOF for a robot I would say that number should include only what the robot can directly control itself. The interesting part of what robot arms do happens inside their envelope of operation -- the fact that they could be picked up and moved is incidental.

I'm not sure what something with near-infinite DOF would look like -- maybe a T-1000? Something freely moving in space requires only 6 DOF to describe.

ScuD
08-06-2008, 02:19 PM
I'm not sure what something with near-infinite DOF would look like -- maybe a T-1000? Something freely moving in space requires only 6 DOF to describe.

T-1000 would be a great example.
Think of every single molecule of the substance being able to move in 3 axes -X,Y,Z.
Because of the cohesion, the molecules stay together to a certain point, yet they can all move in every direction.

Hence, near-infinite DOF.

At least, that's my view of it. Lynn could probably smack my theory with a few astronomical figures and make a lot more sense of it :veryhappy:

Alex
08-06-2008, 04:14 PM
IMHO, I'd have to say that if the torso/body has 3 DOF in addition to the 18 DOF for the "appendages", then the hex would have a total of 21 DOF.

Personally, I'd really only count an extra DOF on a bot if the DOF serves an actual purpose. That's why I said 21 above. But, say you have something like the Futaba RBT-1, which I'd say has 19 DOF, even though according to specs, it has 20 DOF (http://www.trossenrobotics.com/Humanoid-Robot-Kit.aspx). The reason why I don't count the last DOF is because one servo is for the head to turn, which serves no purpose other than for aesthetics.