You need to use:

>  braided wire;  since the strands are not in fixed relationship, the ability to slip increases lifetime.
>  high ductility copper;  this is hard to get and expensive, but some suppliers can provide cable with the higher ductility, which should reduce the damage.
>  large service loop to minimize the mechanical deflection of the cable

TTFN

Thanks for your info.

I have seen some cable used in other robot and it work. The cable consist of very fine conductor (about 0.05mm) and the size is AWG18 (outside dia. 2.5mm, the space is not enough for a large bundle of wire). The insulation layer is made of PVC. I am looking for this kind of wire. Where I can find this kind of cable? most supplier provide conductor only with 1.5mm or larger, it is easy to break in our case.

For stuff that skinny, you'll need to go to molded or flex cable, which tends to be custom.

Cicoil was one supplier of molded cables, but I don't know if they make them with that low a profile.  Kapton flex cables are used throughout the printer industry.

TTFN

OLFLEX is the best for motion and Robotic systems.

I forgot to mention that REAL robotic cable, you will find, has 'long radius protection' to protect against the major source of failure. You will find conductors running along a air filled 'open' vinyl tube, when a conductor length differential grows high as it will in tight bends, the outer radius wires 'crush' the air tube and relieve some stress. You will find this to be most flexible and pliant cable.

WHY ARE YOU MOVING STEPPER MOTORS????? BEST DESIGN FOR STEPPERS IS SOLID MOUNT AND MOTION TRANSFERD...IF NOT POSSABLE CHANGE ARC ON CABLE TO LARGER SIZE TO KEEP WIRE FROM WORK HARDENING...

Try cable made by SAB,have had excellent results and cost savings. see www.awpinc.com

If the movement is in a fixed location you may try using something other than wire for the connection.  I had a similar problem in one bot that kept breaking the wires in the "knee" joints.  I got around this by using a couple of the devices usually used to keep the phone cables from tangling.  Not the prettiest solution but it was cheap and (best of all) it worked! Hope this helps.

Monkey