Here's the problem;
Say you have a robot arm which has several joints along it's length,
each one is driven by a pair of steel cables. The other ends of the
cables are connected to gearmotors which are located off the arm, to
keep the weight down. So the cables which drive the farmost joints need
to run through the preceding moveable joints, but the path length of
each cable must not change. ie, if the cable were to stretch or become
slack 'round the bends' it would affect the positioning of joints
further along the arm.
It can be done using bowden-type cables of course, with an inner cable
and an outer sheath, but this is not a very elegant solution. You can
also try and route the cables through the centre of each joint, but
unless they will bend to a very tight radius, the length still changes.
Is there a simple geometric solution to this, whereby cables can be
routed around a joint, and as it flexes the path length remains
constant? The Barrett Technology WAM arm appears to work this way, but I
can't see how. I have thought up one solution using a pair of
counter-rotating rollers, but I don't think the WAM works this way, and
I'm sure there's a better way.
Any descriptions or links, especially with diagrams, much appreciated.
In message , steamer
Thanks, I'd actually already seen this site. His solution is to run the
cables straight through the centre of the joint and bend them round as
small a radius as possible when the joint moves. He admits this is not a
What I'm looking for is a mechanism that moves with the joint,
compensating for the path-length difference.
Disclaimer: I don't have much hands-on experience with these methods.
However, I have seen other systems using them.
Sprockets and chains can be used if you don't need many actuators. In
the trivial case, a single chain connects the motor to a sprocket on a
parallel joint axis; the chain length does not change as the motor and
joint rotate. In the next case, one chain connects the motor to a
free-wheeling sprocket on the first joint; another sprocket is fixed to
this one, and a second chain connects it to a sprocket on the second
joint. This easily iterates to cover N joints, but the number of
sprockets and chains quickly becomes excessive.
If the joints don't need to rotate more than 180 degrees, a cleaner
method can be implemented. A (vinyl?) coated cable is used with pulleys
instead of using sprockets and chains. As in a chain, the cable forms a
closed loop. One end of this loop is firmly attached to the motor. The
loop then criss-crosses between free-wheeling pullies collinear with the
joint axes. Finally, the other end is firmly attached to a pulley on
the joint to be actuated. The 180 degree restriction comes from the
need to attach the cable to pulleys on both ends; it could be alleviated
by attaching the cable to a chain/sprocket system on both ends. Chains
aren't used the whole distance since they do not handle sideways
In either method, the final joint angles move as intermediate joints
change. However, this is often desirable because the joint motion
preserves the absolute joint angle (e.g. a link stays horizontal as the
shoulder moves up and down).
Yet another approach is to use rotating shafts to transmit motion. At
each joint, these shafts terminate with a flexible joint (universal
joint or the like). This joint then connects to another shaft which
transmits power to the next joint, and so on. The flex-shaft which
comes with a Dremel tool can also be used for this purpose. Since its
intended for light torques, heavy gearing at the output might be
required. This class of approach generally cannot handle sharp joint
How about dual pullies?
Basically any cable only goes the length of a single joint where it
is wrapped around one of a pair of linked pullies (or sprockets).
These pullies would go through the center axle of the joint.
I don't think that this would be the most efficient way of doing
things, but you wouldn't have to worry about routing.
D. Jay Newman
You can take a look at my website aivision.com and see how
running spectra (fishing or kite string) lines through copper
platted spring coil can animate a robotic hand with 18 degrees
Yeh, like steel cabling but lubricated with conductive grease
"quietguy" wrote in
message news: snipped-for-privacy@REMOVE-TO-REPLYconfidential-counselling.com...
Read "Robot Evolution" by Marc Roscheim. His big picture
book of robot arm internals and designs is just what you need.
Roscheim makes a big point of getting all the cables inside
the arm, an important feature in industrial robots.