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Beach Cities RoboticsIntroduction to Mechanisms

June 4, 2007

Daniel Braley, ATP Mechanical Test Engineer

Northrop Grumman Corporation

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OutlineLift Concepts:• Scissor Lifts• Rotary Jointed Arms• Pneumatic Arms• 4 Bar Mechanism• Telescoping Lifts• Cables & Pulleys• Combined Systems

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Scissor LiftsOverviewThe 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

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Rotary Jointed ArmsOverviewRotary 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

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Pneumatic ArmsOverviewThe 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

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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

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OverviewTelescoping 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

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Cables & PulleysOverviewIn 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

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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

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References1) http://www.genieindustries.com/gs-series/gs-1930.asp2) Smart Robots: A Handbook of Intelligent Robotic

Systems. By V. Daniel. Hunt3) Handbook of Advanced Robotics. By Edward L.

Safford4) http://www.philohome.com/pneumarm/pneumarm.htm 5) http://www.eng.fsu.edu/ciscor/research_highlights.htm6) http://www.packworld.com/view-212617) http://teamster.usc.edu/~fixture/Robotics/Course.htm