Beach Cities Robotics

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Beach Cities Robotics


Introduction to
Mechanisms

June 4, 2007


Daniel Braley, ATP Mechanical Test Engineer


Northrop Grumman Corporation

1

Outline

Lift Concepts:


Scissor Lifts


Rotary Jointed Arms


Pneumatic Arms


4 Bar Mechanism


Telescoping Lifts


Cables & Pulleys


Combined Systems






2

Scissor Lifts


Overview


The scissor lift is unique in that it doesn't use a straight support to raise workers or
objects into the air. Rather, the scissor lift platform raises when the linked, folding
supports underneath it draw together, stretching it upward. It can be powered by
hydraulics or an electric motor, but it's a bumpy ride to the top. The scissor lift's
design keeps it from traveling with a constant velocity, instead traveling faster in the
middle of its journey and slower with more extension.


Pros


The max height of the platform is flexible


The height is determined by the number and
length of the linkages


Great for straight lifts


Can be used in a robotic arm to reach out straight


Cons


Requires great force to get the lift started


Synchronizing two scissors is difficult


Complex design

3

Rotary Jointed Arms


Overview


Rotary Jointed Arms work on the basic principle of the human arm. They have a wrist,
a shoulder, and an elbow. Depending on what the robotic arm is used for, a gripper,
like that of a hand can be added to the end of the arm, and ultimately give the user 3
axes of motion, generally referred to as pitch, yaw, and roll.


Pros


Allows for 3 axes of motion


Can emulate the human arm


Great idea when gripping objects is
required


Cons


Large moments can develop at the
shoulder and base plate, which can
ultimately lead to failure of the joint.


A motor is required at each joint, and
each has to be operated
independently


Programming a system to work on
several independent motors can be
quite tricky

4

Pneumatic Arms


Overview


The figure below is of a Lego Mindstorms Pneumatic Arm. The base of the robot
contains the control unit, ultimately made up of a battery pack, a compressor, and
command switches. The compressor is based on two small pumps, working in
opposition. The pumps are driven by two fast and powerful “monkey” motors. A
small piston, loaded by two rubber belts, moves an electric switch when pressure is
too high, and the compressor motors are then switched off.


Pros


Allows for 3 axes of motion


Can emulate the human arm


Great idea when gripping objects is
required


The pistons often counteract the large
moments of a heavy robotic arm and help
to support the weight of the arm.


The use of compressed air and pistons
allows for

a high degree of control and
precision

Cons


Very complicated system


Many moving parts


A single air leak can shut down the robot


A piston is required at each joint, and each
has to be operated independently


Programming a system to work on several
independent pistons can be quite tricky


Binary positioning

5

4 Bar Lifting Mechanism


Overview



The 4 bar mechanism is simple and effective. The opposite bars always remain
parallel retaining the orientation of the object. Many industrial robots use this
mechanism.


Pros


Simple


Object retains orientation


1 joint to power


Easily programmed


Provides reach


Cons


Large moments can develop at
the joint location


Required to lift “outside of the
box”


Can be vulnerable to side hits

6


Overview


Telescoping lifts are most commonly used within forklifts and cranes. They extend in
one direction and are usually powered by a chain or piston.


Telescoping Lift

Pros


Extends “within the box”


Mechanism protected by the base


Generally operates on a fairly
uncomplicated electrical system


Only requires one power source


Cons



Multiple segments translate up, resulting in
a higher center of gravity


Can become complex

7

Cables & Pulleys


Overview


In robotics, cables and pulleys are most often used as a hoisting mechanism, or for
lowering and raising an object from an arm. Cables and pulleys can also be used as a
simple alternative for the main drive mechanism of an arm, but tend to limit the range
of motion and control of the arm.


Pros


Very simple and easy to assemble


Great idea for when hoisting an object is
required


Lightweight


Great idea for lowering an object from a
robot


Cons


Tend to limit the range of motion


Control is limited


If the cable slips out of the pulley, the
mechanism fails


If the cable snaps, the mechanism fails

8

Combined Mechanisms


Overview



When one system doesn’t cut it, combining the mechanisms might help. You
may need reach, but the 4 bar doesn’t reach that high. Attaching a 4 bar to a
telescoping lift might be your answer.


Pros


Combines the pros of the various
mechanisms


Cons


Complex


9

References

1)
http://www.genieindustries.com/gs
-
series/gs
-
1930.asp

2)
Smart Robots: A Handbook of Intelligent Robotic
Systems
.

By V. Daniel. Hunt

3)
Handbook of Advanced Robotics
.

By Edward L.
Safford

4)
http://www.philohome.com/pneumarm/pneumarm.htm


5)
http://www.eng.fsu.edu/ciscor/research_highlights.htm

6)
http://www.packworld.com/view
-
21261

7)
http://teamster.usc.edu/~fixture/Robotics/Course.htm