forearm design was closely modeled after the 2000 graduate student design, once
again incorporating the square aluminum tubing.
The tubing with a 1/8-inch wall thickness provides a compact,
lightweight supporting structure that completely encloses all forearm
components, protecting both the equipment as well as the user from potential
preliminary design employed a 2.5-inch square tube, similar to the preliminary
tube used in the lower arm. Uniform
tubes reduced the costs for obtaining raw materials and permitted a compact
rest position for the arm. The tubing
supported two mounting plates, which secured the motors driving the
differential gears at the wrist. Seen in
Figure 6.1, one motor was offset behind the other motor, permitting both to fit
within the enclosed tube. An aluminum
drive shaft, supported by a bearing, connected the offset motor to its
differential gear, while the pinion from the other motor was mated directly to
its respective gear.
the lower arm tubing was expanded for the final design to accommodate new
motors and because the 2.5-inch square tubing was expensive and difficult to
obtain, the forearm tubing was also enlarged to maintain uniformity. The 3.0-inch square tubing, which is readily
available, was chosen in place of the 2.5-inch square tube. The 3-inch tubing permits the two wrist
motors to be placed next to each other, eliminating the need for a second motor
mounting plate, bearing, and drive shaft.
Figure 6.2 illustrates the parallel position of the wrist motors. Fewer components within the forearm reduced
not only materials and machining costs but also the overall weight of the
Two oil-lubricated ball bearings are
press-fit into two wrist pillow blocks, machined from 0.25-inch aluminum plate,
and attached to the forearm tubing to support the wrist differential. The bearings and wrist pillow blocks can be
seen in Figure 6.3.
wrist twist and bend motions are controlled by a differential gear set
identical to the gearing on the 1999-2000 undergraduate students and the 2000
graduate student designs. The bevel-gear
configuration provides two degrees of freedom within a compact volume.
final design, placing the wrist motors side-by-side increased the distance
between the motor shafts. Spacers,
appearing as red in Figure 6.4, were fastened between the differential and the
outer bevel gears to accommodate the repositioning.
the differential gear set, purchased from Sterling Instruments, was one of the
most expensive components purchased for the arm, the mechanical advantages were
deemed to appropriately offset the high cost of the product. Providing two degrees of freedom within a
small space was essential to maximizing the movement and functionality of the
robotic arm design.
Table of Contents
5: Lower Arm