�
The
previous Gateway teams invested much time into gripper design and some very
interesting ideas were developed.� Two
designs were mainly discussed: the 1999-2000 design, and Chris Fearon�s
graduate design.� Other previous designs
were quickly determined not to meet the goals of this year�s team.
Chris
Fearon designed a gripper that was very efficient at picking up large objects
(1.5 to 3 inches) but had some trouble with small objects.� He used three fingers (as opposed to the
previous Gateway teams two-finger wrench designs), which allowed for
more stability and a greater variety of objects that
could be handled.� Much of the work went
into the analysis of the fingers and the loads that they could handle.� Chris did extensive research and testing
using ANSYS to pick the best finger design that would limit finger deflection
and stress points.
Aluminum was chosen for the major components of the fingers and moving
pieces of the palm.� The motor was also
locked into place using aluminum plates.�
The side cover plates were made from Acrylic simply for aesthetics.� These two materials offered high strength to
weight ratios and were fairly low in cost.�
Actuation was handled through the use of a stepper motor/leadscrew
system.� The NEMA size 17 stepper motor
from Applied Motion and a lead screw and power nut assembly from Precision
Industrial Components Corp. were the products purchased. �The thrust bearing was used to eliminate the
thrust force that would be placed on the motor due to heavier objects.� The high torque size 17 stepper motor
provided 31.4 in-oz of torque at 300 rpm, which is more than enough to overcome
the joint friction and operate the gripper.�
The overall cost of material and machining for Chris� gripper was about
$620 for a prototype and about $430 for production of 50 or more
assemblies.� A solid edge model of Chris
Fearon�s gripper is shown in Figure 7.2.
�����������
The 1999-2000 Gateway team took a different approach to the design.� They took Chris Fearon�s design to the next
level by implementing underactuated fingers.�
A mechanism is underactuated if it has fewer actuators than degrees of
freedom.� This type of design allows the
fingers to wrap around an object as it closes and to pick up a wide variety of objects
(small and large).� The group then
decided to use a six bar, underactuated, two degree of freedom linkage.� This design took much kinematic research and
testing.� An example of a six bar linkage
closing around an object is shown in Figure 7.3.
This design could pick up cylindrical objects ranging in size from 1 to
4 inches.� Forces were studied and parts
were designed to reduce stress concentration and machining costs.� This meant rounding edges to create more
�dumbbell� shaped parts.
Most of the components were constructed of aluminum,
but the side plates and finger pieces were made of Lexan.� The gripper was again actuated by a stepper
motor/leadscrew system.� The Ht17-070
stepper motor from Applied Motion Products, and a �-inch leadscrew from PIC
design were the products purchased.� A
�-inch aluminum plate was moved by the leadscrew and slid in slots cut into the
sideplates.� This motion is what closed
the fingers and actuated the six bar linkage.�
����������� This turned out to be a
very cost effective design as well.� One
of the main reasons that Chris Fearon did not invest much research into the use
of an underactuated system is that he thought the cost would be too high.� After all of the kinematic analysis and
design was finished, the actual machining and material cost was only $547.� This was actually a lower cost than Chris
Fearon�s prototype gripper.�� A
production cost was not calculated for the 1999-2000 gripper but would have
been even lower due to the reduced cost of machining.
This year�s design team used most of the design from the 1999-2000 team
but made several modifications to increase durability and ease of control.� The first design change was that we
eliminated the use of Lexan.� We believed
that this material was used more for visual purposes than for the mechanical
benefits.� In examining the old gripper,
cracks had formed around the areas of high stress caused by the screws.� The Lexan is also much more flexible, which
allowed the side plates to bend and twist fairly easily.� Aluminum was chosen to replace these
parts.� Aluminum made the frame much more
stiff and durable without adding very much weight.� The cost of aluminum is also quite low, and
it is easier to machine than Lexan.�
Another design change came with the use of a cordless
screwdriver motor.� The motor chosen was
a 2.4 VDC Johnson Electric and was found in a Black and Decker Model #9072
screwdriver. The screwdriver itself came with a gearbox that was also used in
the new design.� The planetary gearing
used provided an output of 20 in-lbs of torque at 150 rpm.� This was very appropriate for our application
because it allowed for the gripper to move its full range of motion in about
4.8 seconds.� There was also a cost
benefit associated with the use of this product over the old stepper
motor.� The Ht17-070 cost $50, but the
entire cordless screwdriver only cost $15.
Figure 7.5 � Comparison of
old/new motors
This cost savings was offset by the addition of new components, but the
main reason for choosing this motor is for consistency and ease of
control.� One issue that we wanted to
solve this year was that we wanted to use a consistent type of motor.� The stepper motors were very difficult to
program using the OOPic controller.� A
stepper motor offers the ability to know the exact position of the motor at all
times.� However, we do not need to know
the exact position of the motor for this application.� The opening and closing of the gripper can be
controlled visually with sufficient accuracy.�
The screwdriver motor has the advantage of not being back drivable.� This eliminates the need for any braking and
allows the gripper to maintain a tight grasp on an object without drawing any
current.���
Several components were re-designed and several more were modified to
complete our final product.� One drawback
of the new design is that it is about 2.7 inches longer and slightly heavier
than the previous year�s gripper.� This
is a tradeoff that we were willing to accept to achieve our previously
discussed motor control goals.� The
length of the motor and gearbox was what caused the major design changes.� This required lengthening the side plates to
house the new equipment.� Since these
plates were now longer, they needed to be even more rigid (another reason for
choosing aluminum over Lexan).� Also, the
old plate that housed the motor had to be modified and moved in order to support
the new motor/gearbox assembly.� This
support plate fits around the Johnson Electric motor and rests against the
bottom of the plastic housing of the gearbox.�
This plate was then fixed into place using epoxy.� Fastening using epoxy was not the desired way
to attach the motor to the plate, but was used due to time constraints at the
end of the project.� The original idea
was to weld the support plate to the motor housing (A gas tungsten arc-weld, or
�Tig� weld, would have been used because of the thin housing of the motor and
the joining of aluminum parts).
The motor/gearbox assembly was not modified as it
came from the screwdriver.� The only
modification was that the fixture on the end of the gearbox output shaft was
reduced in length by � of an inch.�
Originally, a screwdriver was disected to see what parts could be
eliminated and how the output of the gearbox could be fastened to the
leadscrew.� These screwdrivers were
difficult to disassemble without damaging the components.� Therefore the decision was made to make use
of the hexagonal shaped slot that was already part of the screwdriver.� A special coupling was machined with a
hexagonal insert on one side and an identical match to the leadscrew on the
other end.� This allowed for the use of a
coupler to join the two pieces.�
����������� Because
the 1999-2000 gripper design was used as our baseline, the components discussed
above were the only parts that were fabricated from scratch.� The remaining parts of the gripper were
re-assembled and re-used, with the exception of the finger pieces.� These pieces were also re-machined from
aluminum instead of Lexan for consistency of appearance.� The mounting and dynamics of the gripper are
identical to those of last year�s design.�
The gripper is mounted to the wrist differential by fastening the bottom
plate by using six screws (the differential plate can be seen above in Figure 7.6).� This plate also serves as protection for the
motor itself.� Because of interference
between the gripper and the forearm housing, a spacer had to be added to create
a slight separation.
����������� The
gripper is again actuated through the use of a leadscrew that moves a plate
that slides in slots milled into the side plates.� The motion of this plate is what begins the
movement of the six-bar underactuated linkage.�
The upward motion of the plate forces the fingers of the gripper to
close until they make contact with an object.�
After contact has been made, the joints in the fingers will bend.� The bending of the joints is what allows for
the fingers to �wrap around� an object to create a firm hold.� This idea is shown in Figure 7.3 but can also
be seen in the closing sequence of the assembled gripper that is shown in
Figure 7.7.�
The final prototype cost of the gripper was determined to be $524.26 and
$414.88 for a production volume of 50+.�
Notice that the cost of the gripper is very similar to that of last
year�s design ($547.05 for 99-00 design prototype).� However, the lack of cost savings is made up
by the ease of control that was created by using the screwdriver motor.� The difference in price for a production
volume of 50 or more assemblies is mostly due to the reduced cost of machining
larger lots of parts.� This was estimated
in the calculations by reducing the machining time by 25% (from 17.5 hours to
13.125 hours).� The cost of the gripper
components, material, and machining is summarized in Appendix A (Bill of
Materials).�
A disadvantage of this year�s gripper design is the increase in length
(from 4.60-inch side plate length to 7.25-inch side plate length).� However, the arm design allowed for a maximum
gripper length of 12 inches, which we are still well below.� The final Solid Edge gripper assembly is
shown in Figure 7.8.