Manipulators and Mechanisms June 8 2008 Meredith Evans

Manipulators and Mechanisms June 8, 2008 Meredith Evans Andrew Aguinaldo Gabby Salo

Agenda • Physics Concepts • Arms and Lifts • Handling Objects • Mechanisms • Gear Ratio

Physics Concepts Forces, Vectors, Angles, and Torque

Force and Vectors • In mechanics, forces are seen as the causes of linear motion – Forces are vector quantities • A vector is a geometric object with magnitude and a direction • Magnitude and a direction must be specified – The SI unit force is the Newton • Newton = kg m/s 2

Torque • A torque is an influence which tends to change the rotational motion of an object. – Torque = Force applied x lever arm – The direction of the torque is given be the right hand rule – Note that the torque is maximum when the angle is 90 degrees. Point of application of the force Applied force r Lever arm is measured from the axis of rotation. θ Axis of rotation F Radius from axis of rotation to point of application of the force.

Torque • Example #1 - Lifting Θ=90° 10 m 5 kg

Torque • Example #2 – Lifting 5 kg – Same force, different angle, less torque Θ=50° Θ=90°

Power • Power is all about how fast you can move something OR

Power • Example – Lifting – Same torque, different speed 5 kg 0. 1 HP, 100 RPM Motor w/ 1” sprocket 5 kg 0. 2 HP, 200 RPM Motor w/ 1” sprocket OR 100 RPM w/ 2” sprocket

Power • In Summary: – Given the proper gear ratio and assuming 100% efficiency, any motor can lift any object. It’s the rate of lift that varies from motor to motor. • But no power transfer mechanisms are 100% efficient – If you do not account for these inefficiencies, your performance will not be what you expected

Arms and Lifts

Vertical Lifts - Scissors • 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.

Vertical Lifts - Scissors • 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 – – – – 13 Requires great force to get the lift started Synchronizing two scissors is difficult Complex design Needs to be heavy to be stable enough Doesn’t deal well with side loads Must be built very precisely Stability decreases as height increases Loads very high to raise at beginning of travel

Vertical Lifts - Extension • Overview – Two types • Continuous rigging • Cascade rigging • 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 • Cons – Needs to be heavy to be stable enough – Doesn’t deal well with side loads – Must be built very precisely – Stability decreases as height increases – Loads very high to raise at beginning of travel

Vertical Lifts - Extension • Tips – Power down and up • If not, make sure to add a device to take up the slack if it jams – – Segments need to move freely Need to be able to adjust cable length(s). Minimize slop / freeplay Maximize segment overlap • 20% minimum • more for bottom, less for top – Stiffness is as important as strength – Minimize weight, especially at the top – Keep the CG aft

Extension - Rigging Continuous Cascade

Extension - Rigging - Continuous • Speed of cable same for up and down • Intermediate sections may jam • Cable tension is low • Cable routing is more complex • The final stage moves up first and down last Slider (Stage 3) Stage 2 Stage 1 Base

Extension - Rigging - Continuous • All internal cabling • Cable routing more complex Slider (Stage 3) Stage 2 Stage 1 Base

Extension - Rigging - Cascade • Cables going up and cables going down have different speeds – Different cable speeds can be handled with different drum diameters or multiple pulleys • Intermediate sections do not jam • Lower stage cables have more tension – Needs lower gearing to deal with higher forces Slider (Stage 3) Stage 2 Stage 1 Base

Extension - Rigging (i. e. Telescope) • 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. • 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 20

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 21

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 22

Four 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.

4 Bar Lifting Mechanism • Pros – – – • Simple Object retains orientation 1 joint to power Easily programmed Provides reach Cons – Large moments can develop at the joint location – Pin Loadings can be high – Required to lift “outside of the box” – Can be vulnerable to side hits • Tips – Watch for buckling in lower member – If possible, counterbalance – Keep the center of gravity (CG) aft 24

Multi-Bar Mechanisms Crossed 8 bar

Multi-Bar Mechanisms Parallel 8 bar

Arms vs. Extension Lifts • Arms – Can reach over objects – Can help right a flipped robot – Can fold down to allow moving through barriers – Require complex controls and counter-balances – Harder to maintain CG over base – Need space to swing up – Need extra joints to reach higher which adds more complexity • Lifts – Limited reach – Can't not help right a flipped robot – Stay tall limiting movement through barriers – Simple controls – Maintain a better CG over the base – Can operaet in confined spaces – Lifts can reach higher with minimal added complexity

Handling Objects Manipulation Storage Acquisition Size Placement and Alignment Accumulators Conveyers

Ball Manipulation • Both Continuous intakes and single object grabbers are useful when manipulating small or medium sized balls. • Types of Manipulators for Balls – – – Two tank treads horizontally or vertically aligned. Three and Four Pronged Grabber Two Point Grabber/ Fork Bucket Intake Roller • Should be soft grip to be able to effectively control and contain the ball. 29

Ring Manipulation • Both Continuous intakes and single object grabbers are useful when manipulating small or medium sized rings. • Types of Manipulators for Rings – – – Two tank treads horizontally or vertically aligned. Three and Four Point Grabber Two Point Grabber/ Fork Bucket Intake Roller • Should be soft grip to be able to effectively control and contain the Ring. 30

Square Manipulation • Both Continuous and single object grabbers are useful when manipulating small or medium sized squares. • Types of Manipulators for Squares: – Three and Four Pronged Grabber with Grip – Two Point Grabber • Should be a strong grip to be able to effectively control and contain the square. 31

Triangle Manipulation • Single object grabbers are useful when manipulating triangles. • Types of Manipulators for Triangles: – Three and Four Point Grabber – Flat Bottom and X Shaped Intake Roller • Should be strong grip to be able to effectively control and contain the Triangle. 32

Storing Objects • All items of the same size can be similar • Storage Method – Stack – Tank Tread – Divide – Dump Tank – Or Simple Grab and Drop to Goal 33

Storing Manipulator Arm • This must be decided with the base so the arm and manipulator have a place to go when a) trying to fit the size and b) to effectively pick up the objects • Storage Method – U Shaped Base – 360` Pivot Joint (and Arm Extender) – Manipulator Attached to Base 34

Acquisition Size • Acquisition is the intake area of the object. • Large acquisition area is optimal when picking up • During match, need easiest and quickest pick up for driver • Able to get the most amount of objects at once • When deciding acquisition intake, keep in mind what object you’re manipulating. 35

Placement and Alignment • Along with the acquisition zone, the placement of the object on/into the goal must be able to have accuracy with ease. • Along with the storage of the arm, you must work with the drive train to be able to fit the robot against the goal for optimal stability (Unable to be pushed away from the goal when attempting to score) and accuracy. • Alignment is the drive train’s responsibility. Make sure they know exactly how your mechanism works and where it needs to be positioned to make a goal. • Placement must be easy to work from a drivers standpoint. In contrast to the large acquisition zone, the manipulator should be small enough to accurately place the object on or in. 36

Gripping Objects • Why is the grip important? – The manipulator cannot effectively hold on to the object if both the object and the manipulator have no grip. • You need friction. – Friction is the force that opposes the relative motion or tendency toward such motion of two surfaces in contact – In this case, the force that opposes is gravity and the two surfaces are the manipulator and the object. 37

Mechanisms Motors and Servos Limits

Motors and Servos • The Motors and Servos make the arm and manipulator move. • Motors can turn a shaft* clockwise and counterclockwise as many degrees as desired. • Motors are generally used for continuous intake. • Servos can only turn a shaft* 180 degrees in either direction (360 degrees in total) • Servos should be used on < 360 degree pivot joints for both the arm and the manipulator. • *The shaft allows the motor to connect to the arm for a powered pivot point. 39

Limits • There are two types of stops: – Hard stop • The hard stop is a sturdy metal part of the robot’s structure. • This does not allow the manipulator / arm to go any farther. • However, the hard stop does not tell the motors to stop working, thus, the motors will break. • This is why we have soft stops. – Soft stop • Soft stops are limit switches. • When programmed correctly, the limit switch tells the motor or servo to stop. • Limits should be placed anywhere that contains a hard stop. – i. e. Arm pivot joints, two+ prong gabber, fork lifts, etc. 40

Gear Ratio

Gear Ratio • “There is no way that this motor can pull the arm and manipulator up against gravity. It’s just too heavy. ” • If the motor is expected to work against a lot of weight, gear the ratio down. – i. e. 1: 2 meaning for every one shaft revolution (360 degrees), the motor does two. • If the motor is not expected to work against a lot of weight and work quickly, gear the ratio up. – i. e. 2: 1 meaning for every two shaft revolutions (two * 360 degrees), the motor does one. • However, you must have enough torque to pull the arm and manipulator up and down. • You must also have enough torque to keep hold on the object the robot is manipulating. It is not just friction holding it there. 42

Gear Ratio Calculations • The gear ratio is the relationship between the number of teeth on two gears that are meshed or two sprockets connected with a common roller chain, or the circumferences of two pulleys connected with a drive belt. d D

Appendix

Force • In mechanics, forces are seen as the causes of linear motion • Forces are vector quantities, – They require vector math – Magnitude and a direction must be specified for a vector quantity • The SI unit force is the Newton – Newton = kg m/s 2

Vectors • Geometric object with magnitude and a direction • Represented by a line segment connecting the initial point A with the terminal point B • Vector Math C+D D – Addition • Tip to toe method C – C+D – Subtraction • Flip the direction of vector E -E A AB B

Angles • Measuring angles • Trigonometry Hyp oten Opposite use θ Adjacent • Note: Units in Radians. • Radians to Degrees 180° = π rad

Introduction – Center of Mass • Unique point in an object or system which can be used to describe the system's response to external forces and torques. • The concept of the center of mass is that of an average of the masses factored by their distances from a reference point. x 2 x 1 xcm Center of mass

Friction • Friction is the force resisting the relative motion of two surfaces in contact – The heavier an object is the larger the frictional force – The material on which the object is sliding also affects the frictional force.

Power • Example #1 - Lifting 10 lbs Power = 100 W Motor w/ 1” sprocket

Accumulators • Accumulator = rotational device that pulls objects in • Types: – Horizontal tubes - best for gathering balls from floor or platforms – Vertical tubes - best for sucking or pushing balls between vertical goal pipes – Wheels - best for big objects where alignment is predetermined • Most efficient in gathering balls – If set up in the proper orientation, will not knock the ball away, just suck it in

Conveyors • Conveyor - device for moving multiple objects, typically within your robot • Types: – Continuous Belts • Best to use 2 running at same speed to avoid jamming – Individual Rollers • best for sticky balls that will usually jam on belts and each other

Conveyors Why do balls jam on belts? - Sticky and rub against each other as they try to rotate along the conveyor Solution #1 - Use individual rollers - Adds weight and complexity Solution #2 - Use pairs of belts - Increases size and complexity Solution #3 - Use a slippery material for the nonmoving surface (Teflon sheet works great)

Passive Assistance • What is passive assistance? – – – SPRINGS or BRAKES! Use Elastic Energy to your advantage. Surgical Tubing Constant Force Springs Gas Springs Torsion Springs

Braking - to Prevent Back-driving • Ratchet Device - completely lock in one direction in discrete increments – ie. winches • Clutch Bearing - completely lock in one direction • Brake pads - simple device that squeezes on a rotating device to stop motion - can lock in both directions – Disc brakes - like those on your car – Gear brakes - applied to lowest torque gear in gearbox • High ratio worm gear (window, van-door motors) – Note : any gearbox that cannot be back-driven is probably very inefficient

Torque • Example #3 - Pulling on object – One angle helps secure object – The other does not This one wants to rotate clockwise and let go This one wants to rotate counter- clockwise and grab even harder