Building competitive manipulators Greg Needel DEKA RD Rochester

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

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

Types of Manipulators • • • Articulating Arms Telescoping Lifts Grippers Latches Ball Systems

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

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 10 lbs 0. 1 HP, 100 RPM Motor w/ 1” sprocket 10 lbs 0. 2 HP, 200 RPM Motor w/ 1” sprocket OR 100 RPM w/ 2” sprocket

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 : 229 2005

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 (Stage 3) Stage 2 Stage 1 Base

Extension: Continuous Internal Rigging • Even More complex cable routing • Cleaner and protected cables Slider (Stage 3) Stage 2 Stage 1 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 Slider (Stage 3) Stage 2 Stage 1 – Needs lower gearing to deal with higher forces Base • I do not prefer this one!

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 Feature Arm Lift Reach over object Yes No Fall over, get back up Yes, if strong enough No Go under barriers Yes, fold down No, limits lift potential Center of gravity (Cg) Can move it out from over robot Centralized mass over robot small space operation No, needs swing room Yes How high? More articulations, more height (difficult) More lift sections, more height (easier) Complexity Moderate High Accumulation 1 or 2 at a time Many objects Combination Insert 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 (2 x, 4 x)

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 -4 x) – 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 • Springloaded latch • Motorized release • Smart Mechanism • 2003

Latch example: 118 • Springloaded 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) www. chiefdelphi. com www. robotphotos. org www. firstrobotics. net www. firstrobotics. uwaterloo. ca

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” 1. 0” Hollow w/ 0. 1” walls 0. 8” Solid 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: Bending Tensile Shear Where: = Bending Stress M = Moment (calculated earlier) I = Moment of Inertia of Section c = distance from Central Axis Where: = Tensile Stress Ftens = Tensile Force A = Area of Section Where: = Shear Stress Fshear = Shear Force A = Area of Section

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