Building competitive manipulators Greg Needel DEKA R&D, Rochester Institute of technology Owner, Mentor teams: 131, 1511

Read the rules Outline the game objectives Look for the “gimmie” robot design Try small simulators Whatever you choose STICK WITH IT! Strategy, Strategy, Strategy!

Types of Manipulators Articulating Arms Telescoping Lifts Grippers Latches Ball Systems

Example #1 - Lifting –Same force, different angle, less torque 10 lbs < D D Arm: Forces, Angles & Torque

Power Power = Force x Distance / Time OR Power = Torque x Rotational Velocity Power (FIRST def.) – how fast you can move something

Arm: Power Example –Same torque, different speed 0.1 HP, 100 RPM Motor w/ 1” sprocket 0.2 HP, 200 RPM Motor w/ 1” sprocket OR 100 RPM w/ 2” sprocket 10 lbs

Arm Design “Arm”: device for grabbing & moving objects using members that rotate about their ends Think of your materials (thin wall is good) Every Pivot has to be engineered (less is more) Linkages help control long arms. Use mechanical advantage (it is your friend) Think of the drivers (pivots on pivots are hard) Operator Interface (keep this in mind)

Arm Advice K.I.S.S. doesn’t mean bad Feedback Control is HUGE –Potentiometers, encoders, limits –Automatically Take Action Based on Error –Design-in sensors from the start of design Think outside the box. Off the shelf components are good (andymark.biz, banebots.com )

Four Bar Linkage Pin Loadings can be very high Watch for buckling in lower member Counterbalance if you can Keep CG aft

4 bar linkage example :

Arm Example: 234 in 2001

Arm Example: 330 in 2005

Arm Example: 1114 in 2004

Telescoping Lifts Extension Lift Scissor Lift

Extension

Extension Lift Considerations Should be powered 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 / free-play Maximize segment overlap –20% minimum –more for bottom, less for top Stiffness is as important as strength Minimize weight, especially at the top

Extension - Rigging Continuous Cascade

Extension: Continuous Rigging Cable Goes Same Speed for Up and Down Intermediate Sections sometimes Jam Low Cable Tension More complex cable routing The final stage moves up first and down last Slider (Stage3) Stage2 Stage1 Base

Extension: Continuous Internal Rigging Even More complex cable routing Cleaner and protected cables Slider (Stage3) Stage2 Stage1 Base

Extension: Cascade Rigging Up-going and Down-going Cables Have Different Speeds Different Cable Speeds Can be Handled with Different Drum Diameters or Multiple Pulleys Intermediate Sections Don’t Jam Much More Tension on the lower stage cables –Needs lower gearing to deal with higher forces I do not prefer this one! Slider (Stage3) Stage2 Stage1 Base

Team 73 in 2005 elevator

Scissor Lift

Scissor Lift Considerations Advantages –Minimum retracted height - can go under field barriers Disadvantages –Tends 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 I recommend you stay away from this!

Team 158 in 2004

Arm vs. Lift FeatureArmLift Reach over objectYesNo Fall over, get back upYes, if strong enoughNo Go under barriersYes, fold downNo, limits lift potential Center of gravity (Cg)Can move it out from over robot Centralized mass over robot small space operation No, needs swing roomYes How high? More articulations, more height (difficult) More lift sections, more height (easier) ComplexityModerateHigh Accumulation1 or 2 at a timeMany objects CombinationInsert 1-stage lift at bottom of arm <-

Braking: Prevent Back-driving Ratchet Device - completely lock in one direction in discrete increments - such as used in many 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 Note : any gearbox that cannot be back-driven alone is probably very inefficient

Power Summary –All motors can lift the same amount (assuming 100% power transfer efficiencies) - they just do it at different rates BUT, no power transfer mechanisms are 100% efficient –Inefficiencies (friction losses, binding, etc.) –Design in a Safety Factor (2x, 4x)

Grippers Gripper (FIRST def) grabbing game object How to grip How to hang on Speed Control

How to grip Pneumatic linkage grip –1 axis –2 axis Motorized grip Roller grip Hoop grip Pneumatic grip

Pneumatic linear grip Pneumatic Cylinder extends & retracts linkage to open and close gripper 254 robot: 2004, 1-axis 968 robot: 2004, 1-axis Recommended

Pneumatic linear grip Pneumatic Cylinder, pulling 3 fingers for a 2-axis grip 60 in 2004 Recommended

Motorized Linear Grip Slow More complex (gearing) Heavier Doesn’t use pneumatics 49 in 2001 Not recommended

Roller Grip Slow Allows for misalignment when grabbing Won’t let go Extends object as releasing Simple mechanism 45 in 98 and 2004 Recommended

Hoop grip Slow Needs aligned Can’t hold on well 5 in 2000 Not recommended

Pneumatic Grip Vacuum: –generator & cups to grab Slow Not secure Not easy to control Simple Problematic Not recommended

Hang on! Friction: High is needed (over 1.0 mu) –Rubber, neoprene, silicone, sandpaper Force: Highest at grip point –Force = multiple x object weight (2-4x) –Linkage, toggle: mechanical advantage Extra axis of grip = More control

Speed Quickness covers mistakes –Quick to grab –Drop & re-grab Fast –Pneumatic gripper Not fast –Roller, motor gripper, vacuum

Grip control Holy grail of gripping: –Get object fast –Hang on –Let go quickly This must be done under excellent control –Limit switches –Auto-functions –Ease of operation

Latches Spring latches Hooks / spears Speed & Control

Latch example: 267 Pneumatic Latch 2001 game Grabs pipe No “smart mechanism”

Latch example: 469 Spring- loaded latch Motorized release Smart Mechanism 2003

Latch example: 118 Spring- loaded latch Pneumatic release Smart mechanism 2002

Latching advice Don’t depend on operator to latch, use a smart mechanism –Spring loaded (preferred) –Sensor met and automatic command given Have a secure latch Use an operated mechanism to let go Be able to let go quickly –Pneumatic lever –Motorized winch, pulling a string

Ball Systems 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 pre-determined

Conveying & Gathering 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 non-moving surface (Teflon sheet works great)

Roller example: 188

Accumulator example: 173 & 254

Questions? Thanks to: Andy Baker (45)

Extra Stuff Pneumatics vs. Motors Materials Shapes / Weights Fabrication processes Environment

Pneumatics vs. Motors Some, but not all important differences Cylinders use up their power source rather quickly the 2 air tanks we are allowed do not hold much Motors use up very little of the total capacity of the battery Cylinders are great for quick actuations that transition to large forces Motors have to be geared for the largest forces Our ability to control the position of mechanisms actuated by cylinders is very limited We are not given dynamic airflow or pressure controls We are given much more versatile electronic controls for motors Since air is compressible, cylinders have built-in shock absorption Cylinders used with 1-way valves are great for Armageddon devices - stuff happens when power is shut off This could be good or bad - use wisely

Materials Aluminum, thin-wall tubing Polycarbonate sheet, PVC tubing Fiberglass (used rarely, but advantages) Spectra Cable –Stronger than steel for the same diameter –Very slippery Easy to route Needs special knots to tie –Can only get it from Small Parts and select other suppliers Pop Rivets –Lighter than screws but slightly weaker - just use more –Steel and Aluminum available –Great for blind assemblies and quick repairs

Shapes Take a look at these two extrusions - both made from same Aluminum alloy: –Which one is stronger? –Which one weighs more? 1.0” 0.8” Hollow w/ 0.1” wallsSolid bar

Shapes, cont. The solid bar is 78% stronger in tension The solid bar weighs 78% more But, the hollow bar is 44% stronger in bending –And is similarly stronger in torsion

Stress Calculations It all boils down to 3 equations: Where:  = Bending Stress M = Moment (calculated earlier) I = Moment of Inertia of Section c = distance from Central Axis Where:  = Tensile Stress F tens = Tensile Force A = Area of Section Where: = Shear Stress F shear = Shear Force A = Area of Section BendingTensileShear

Structural Shapes I am willing to bet that none of our robots are optimized with respect to strength to weight ratios –We all have more material than we need in some areas and less than we need in others. –It would take a thorough finite element analysis of our entire robot with all possible loading to figure it all out –We only get 6 weeks!! But, this does not mean we cannot improve

Fabrication Processes Laser cutting causes localized hardening of some metals –Use this to your benefit when laser cutting steel sprockets Cold forming causes some changes in strength properties –Some materials get significantly weaker –Be aware of Aluminum grades and hardness's Welding - should not be a problem if an experienced welder does it