Robot Recipes: How to cook-up a robot using COTS items

Alan FedermanHomebrew Robotics Club

March, 2009

A short introduction to mobile platforms – mostly leaving controllers out.

Why present this talk?

• Hurdle (CTAG) between starter kits and a practical platform.• Lots of skills, techniques(tricks) and some ‘math’ needed.• Most of these skills acquired over a lifetime:

– Automotive bicycle mechanics/repair– Carpentry and cabinet making– Soldering and electronics fabrication (Heathkits)– Radio control models– HAM radio– Running scientific instruments/computer programming– Competition robotics (yes, Robogames counts)– Household appliance/plumbing repairs

What is a Robot?*(*not a trick question)

• A mechanism that performs a (useful)function normally performed by a biological organism (primate?).

‘Why’ robots - The four D’s:– Dirty– Dangerous– Dull– Distant Also FUN - Geeks off the Streets!

Primate vs. Robot

Primate RobotSkeleton/joints Chassis/running gearMuscles EffectorsSenses SensorsBrain/nerves Controller/wiresKnowledge/instinct ProgramsAir/Food/Water/ Power/Battery/wires

circulatory system

Building a Robot is like Baking a Cake

• We have to acquire our ingredients/components.• Heat up the oven/charge up or plug in the tools.• Get out our utensils/lay out our tools.• Make sure our hands are washed /work area is clean

and safe!• Creativity is great - but it helps to have a recipe.• Follow the recipe / adjust modify to finish.• Great cooks perfect recipes over time.• Next reality TV: Iron Robot Chef War Survivor!

Is it a Robot?

• IMHO a true robot needs to be able to both move and react to changes in its environment.

So:Traffic Lights, Vending machines, programmable

thermostats, and RC cars with attached webcams are not robots. An I-bot may be a robot if it can operate after ‘com’ is lost. The MERS are definitely robots. This lecture will concentrate on building an ‘almost’ robot, since it won’t be autonomous.

List of Ingredients

• 12 v rechargeable sealed lead/acid battery.• 2 automotive ‘geared’ motors• 2 wheels• 2 by 4’ sheet ½” birch ply, 8’1” by 1” • Casters• Axle/bearings• Fuse panel/fuses• Relays/pwm / H-brdge• Glue (wood, hot, epoxy, super) , screws, fasteners.• Aluminum/steel angle, sheet.• Salvage, adapt, improvise, overcome.

Tools – get/borrow/learn

• Wood saw (minimum hand, power jig – best table)• Electric drill (variable speed) drill press nice extra• Assorted drill bits, replacement blades• Hack saw (optional – pop riveter, files)• Crimper, wire stripper, Soldering iron(optional)• Measuring/marking tools (Square, tape measure, pencil)• Wood rasp, files sand paper• Dremel tool – nice optional equipment.• Wrenches/ screw drivers.• Adapt, improvise, overcome – Team-up, salvage - Techshop.

Mechanical Chassis (Skeleton)

• Plywood box beam construction very strong, lightweight• Glue/drywall screws make quick prototype possible.• Plumber’s tape good for motor mounts.• Wheels mounted on axle securely.• Off the shelf hardware available or you can modify what you

have - example – a threaded rod can be heated and bent into a U-bolt.

• Attaching motors to chassis, and the to wheels is non-trivial• Minimize weight/friction/inertia – maximize torque/power.• Platform needs to move/deliver payload.• Salvaged cable spool

Effectors

• DC motors, servos, steppers, linear actuators, pneumatics, etc.

• Wide variety – but for purposes of this platform – a couple of smaller automotive motors best - window or seat position motors are a good bet - starter motors a bad idea.

• Another benefit of these motors is they are already ‘geared’ down for acceptable speeds. A six inch wheel at 60 rpm means about a foot and ½ per second on the floor which is faster than a “Roomba.”

• ‘Easy’ to find 12v components for the drive-power chain.• Consider how to attach motors to chassis, wheels to motors.

Mechanics 101

F = MA

Chassis/Motor Mount/Wheel

Some 12v DC motors

Power and Control

• 12v sealed lead acid battery heavy, but easy to charge.• Components (wires connectors easy to obtain)• Relays are cheap, but PWM probably better (expensive)• Since an RC servo motor and PWM use the same signal easy

to control.• Relay(s) or H-bridge easier to implement.• Benefit of DC motor is to reverse rotation, reverse current.• In a perfect world Ohm’s Law rules: V= IR• The VOM is your friend.

Power/Control Distribution

Fuses and master CB/switch are always a good idea Fuse costs 50cents. PWM costs $100. New Globe motors also $100.

Practical Wiring

•6 AWG in battery-to-Power Distribution Board path•12 AWG in circuits connected to 40A breaker•14 AWG in circuits connected to 30A breaker•18 AWG in circuits connected to 20A breaker •20 AWG to power the cRIO, wireless adapter and camera from the PD•20 AWG for attaching Breakouts and Sidecars to individual 20A breakers on+ red white brown - black blue

Physics: Electric motors

• DC motors convert the V into F. • Current in wire – resistance/inductance• Force - friction/momentum• Reverse the current, reverse the direction• H-bridge uses switches* to control.

Sensors/Control

• Sensors can be divided into analog or digital• Most measurements are analog (continuous)• Digital – micro switch two states on/off – binary• Analog – temperature or light sensor – continuous• For ‘digital’ electronics need to convert analog to digital.• Converse for output – need to convert digital to analog.• Servos use the same control D to A as PWMs• Analog sensors typically convert a continuous 0 to 5v signal to

a digital measurement – one byte 0-255 integer.

Sensors/Controllers

Servos/ PWMs

Send one ‘byte’ to pwm:

0 - full reserse -90d115-130 – stop – 0d255 – full forward +90d

What’s cool same output for servo, PWM.

Robot Concepts in the Works

PiMP GoPHRPlanetary Mobility Platform General Purpose Household Robot