User manual

Barrett Technology, Inc.
BA4-310 System User Manual Version 1.1
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Figure 12 - Pinion to Pulley Transition Types
Pre-tension
– cable tension, internal to a cable circuit, which is present even when the drive has
no external loading. Pre-tension is added to eliminate backlash (in this case, cable slack) in the
transmission.
Tensioner
– the mechanism in a cabled transmission, which applies and maintains pre-tension
in the drive cables. Barrett’s patented single-point tensioner uses split motor pinions, which
counter-rotate via a simple setscrew adjustment to pre-tension entire cable circuits.
4.2 Pre-tensioning Mechanism
The BarrettArm and BarrettWrist cabled transmissions have no backlash while maintaining
near-zero friction. This is achieved through a constant cable pre-tension applied to all drive
cables. Since the drive cables experience only rolling contact (unlike gears) with the
transmission pulleys and pinions, pre-tension has a no effect on friction
*
.
Cable pre-tension is applied via Barrett’s patented single-point tensioner mechanism. One
adjustment point applies pre-tension to all stages of a cable circuit. Shown in Figure 13, the
mechanism uses a standard set screw, housed in the outer motor pinion, engaged with special
worm threads cut directly into the motor inner pinion, to force the motor pinion parts to
counter-rotate. When opposing cables are terminated at opposite ends of the motor pinion, that
counter-rotation acts like a winch to draw in cable from both sides of the circuit. As the
effective cable lengths get shorter, tension in the stiff cables increases rapidly. Propagation of
this pre-tension into the entire circuit requires only that the user push the backdrivable robot
through its normal range of motion.
*
Minute deformations in the cable cross-sections and friction in the pulley and pinion bearings do contribute
losses. However, measured efficiencies of cabled transmissions still exceed 98%.