User manual

Barrett Technology, Inc.
BA4-310 System User Manual Version 1.1
31
5 BarrettArm Cable Maintenance
5.1 General Arm Cable Circuit Descriptions
This section describes the parts of various cable circuits and the elements that they have in
common. Subsequent sections explain how to replace worn drive cables as a part of standard
BarrettArm maintenance.
There are a total of 20 drive cables in the 4 degree-of-freedom BarrettArm. All four motor axes
have similar cable circuits, each of which reduces the motor shaft speed in two stages.
Additionally, the cabled differential uses four 2.38mm diameter cables to couple the output to
the inputs driven by motors 1 and 2 (motors are numbered beginning with motor 0).
Each cable circuit 1
st
-stage uses two 1.12mm diameter cables. Each 2
nd
-stage uses two 1.37mm
diameter cables. The 2
nd
-stage cables are each doubled-over to run in parallel and handle the
higher forces in the 2
nd
-stage. See Figure 14 for a schematic representation of a typical cable
circuit.
Each stage has a grooved pinion driving a larger-diameter ungrooved pulley. The first and 2
nd
-
stages are coupled through a matching pair of pulley/pinions that proportionately and
automatically propagate the pre-tension, applied in the first-stage pre-tensioner, to the 2
nd
-stage.
The pulley/pinions in the first three drives are identical (although mounted slightly differently).
In each case these identical pulley/pinions are mounted on parallel shafts in such a way that the
free cable lengths (and therefore the compliances) spanning between pinions and pulleys are
minimized. The two matching pulley/pinions in the fourth motor drive are mounted in the
elbow on a single shaft and share their pulley surfaces. A light preload allows relative slip
between these pulley/pinions during cable installation and cable pre-tensioning.