Grounding Research: Synthesis of Vex Design Principles

As previously promised, here is a synthesis of the Vex Inventors Guide - a basic, clear and concise introduction to the relevant engineering principles for robotics. The forum is a wealth of information also...

Apologies for the point form and brevity... but there is a lot of information in those pages, and to expand on everything would take way too much space...

>> structural

•   rigidity
•   centre of gravity (low and over "support polygon" pp. 2-33!)
• weight of batteries + motors + chassis + tracks
• weight of payload (ie carton of beer+ice)
•  safety, bumpers etc to stop injury..
•  stress / torsional forces / mounting points / material data sheets
• bracing
• suspension
• exposure + vulnerability
• water proof
• withstand sitting, snags, twigs, 
• no exposed wires
•  (see 'plagiarism tank' http://letsmakerobots.com/node/25505

Subsystem interactions:  

    

structural - motion 

• needs to be codesigned, integrated for strength + support for integrals (motor, payload, batteries) 
• handle rough, uneven terrain

structural - power

• protect batteries, weight distribution  / centre of grave

structural - sensor

• mounting and stabilising role
• correct positioning for sensor tot work

structural - control

• mounting and protection
• antennae / rf shielding / range / reception

structural - logic

• mount and protect (esp impact and h20)
• wiring loom / connections robust and secure

               

>> motion:

• speed vs torque
• gearing / complexity / reliability / access to repair / maintain
• drive gear / idler / compound gear / ratios 
• materials for construction / durability
• non gear, belt is better when motor and wheel is far apart
• advantages and disadvantages of tracked systems
• see http://www.rctankcombat.com/articles/track-systems/
•  friction
• slip of belt / tensioning complexities
• durability
• side to side play
• aesthetics
• servo/pwm vs dc/h-bridge
• build vs buy h-bridge or integrated chip
• current draw (for batteries life and for motor controller integration
• lasercut parts from cad? http://www.viz.tamu.edu/courses/tutorials/hunter/presentation1.htm
• wheel size vs acceleration (smaller wheels = more torque)
• stop / start of natural human gait, esp at party
• safety, quick stop
• more revs for regen braking etc
• size for negotiation over obstacles (rocks, tussocks, hills)
• clutch / stall protection (natural slip of belt?)
• increase number of motors = cheap torque boost, but calibration..>?

motion - control - power subsystem interactions:

• integrated microcontroller / h - bridge (sabretooth or robot claw)
• regenerative braking / decelleration
• regulated +5v dc for microprocessor power / protection
• capacity / current handling capability of system matched

>> power

• capacity + 20% (heavy contraption, never want to have to carry this beast)
• discharge rates at full load / no load of motor / estimated Amp Hours (AH)
• good metering / indication of low current / overcorrect  / regulation
• possibilities of extending this with software control (similar to laptop power schemes, perhaps hack a laptop power monitor?)
• li/po or deep cycle or motorcycle or scooter
• shallow discharge = voltage drop
• 5-12v or 24v (ohms law, high current/low voltage needs expensive copper) 
• series / parallel benefits
• ecology / green benefits of regenerative decelleration far outweighs the step outside the 'budget' construction
• temperature 
• batteries location within chassis 
• heatsinking / fan for conroller

>> sensor

    eyes & ears, detect salient / important features of environment

• reliability across all conditions expected
• negative obstacles! cliffs, ditches
• distance of operation / pickup polar patterns
• analog / digital
• analog is difficult to maintain constant / specific signal
• digital is better for electrically 'noisy' conditions, but just hi/lo
• combination / backup systems with different types of sensors to alleviate specific weaknesses
• Tracking options...
• IR proximity (pros n cons)
• PWM for data coding (like IR remote control)
• ultrasonic prox (see MaxBotix)
• visibile light proximity http://letsmakerobots.com/node/1833
• gps
• accuracy? cheap options not hi res/fast enough updates...
• rfid
• signal strength (rough n ready!)
• line detector concepts (hi/low or pattern detect ilke MS surface)
• kinect depth mapping correlated to other sensor data (overkill, but interesting for future platforms? requires atx-mini-pc? robustness for doof?)
• kinect mic array / positional audio capabilities (great for vox commands, could even incorporate similar tech to blob tracking code to reduce noise / predict direction)

 structure subsystem needs to accommodate all sensors

• mount points
• optimal operating range / dispersion / position
• microcontroller / microprocessor handles regulated power supply (i.e BEC +5vDC supply from motor speed controllerr
• perhaps hardwired bumper switch disconnects motors without microprocessor control for safety backup?

logic system requires cleanest data possible, with minimal processing time to make decisions quickly and accurately (necessary for following human)

audio recognition will require perhaps noise reduction / telstra technology ;)

control system will need backup (perhaps ability of manual operation in case of sensor subsystem failure) 

feedback (visual) from sensors and actuators integrated into aesthetics, but very useful for debug / troubleshooting, esp when 'back to nature' or out bush...

*phew*

-&c

Grounding Research: Delving into the archives..

Hmm, so i'm going to try collate the various images ideas and research (with regards to the tracked-base) i have so far over the next few posts... Bear with me as i shake the bookmarks and text files into a shape fit for public consumption ;)

The basic idea is (as previously mentioned) to create a high-payload tracked mobility platform that can (for the moment) be controlled via remote or wii nunchuck...

[you can read that last paragraph as: tank base]...

Initial ideas of lasercutting a slick chassis and parts out of aluminium were dashed when i discovered at school we don't lasercut much but ultrathin perspex and timber laminates :(

There goes the hopes of upscaling my little Tamiya track kit in 3d CAD and printing parts....



So, in keeping with my passion for recycled and low budget, we are going to hack one up at home.  First port of call was downloading the VEX inventors guide and reading cover to cover the engineering notes - especially the really valuable theory on structure, motion, control, power and sensors... i really learnt a lot from this, so will save the discourse for another post...
Another great site for beginners mechanics and engineering is Rob Ives...

From here i managed to find a number of really interesting (and not-so-interesting) war and tank-specific documentaries (probably copyright, so i won't link them here), which i devoured in a sunny afternoon... One of the main points i took from this was the different approaches and pros and cons of the historical decisions made. For example, the beautifully engineered German Tiger tanks cost so much and took so long to build - rapidly got destroyed by the cheaper, faster and much more plentiful Russian T34's due to sheer numbers!  Just goes to show there's nothing wrong with a functional backyard job !

the over-engineered German Tiger

The quick-and-lethal Russian

 vs


Seeing these documentaries and the historical development of the tank over the years, I started to delve into the various track systems, and even contemplated making a 3d CAD model to perhaps try lasercut in rubber...

One design stood out amongst the crowd of DIY... the 'plagiarism tank' ... This is exactly the chassis design i had in mind, simplicity plus.  Here is something i can build at home, with minimal tools and time...It will be made to the dimensions of the esky...

The track system and power train is still under research, however i got lucky the other day and scored 2 Ford Laser wiper motors for $10 out west.  These beauties have a low rpm (~40 by my rough count)...and options for a few different wiring schemes affording different voltage/torque/rpm's depending on which of the 5 pins are used.

Another entirely monstrous score from the west (thanks for the hint, Andy ;) was the purchase of a 12v/240v portable fridge! and better than the crappy peltier element that i was thinking about, this Rig has the old-skool, much more efficient compressor... only issue i can see with it is it 's slightly larger than i hoped, and slightly more top heavy (due to the compressor and circuitry mounted in the lid...... Upside is, it's nice and wide, so the centre of gravity should still sit quite low, especially when loaded with cold beers :)

 

Grounding Research: Technical considerations...

So, thinking about the imminent project for the Device Lab, due in three weeks:

• needs to be practical: a solution to a clear statement of brief and intentions
• guided by a necessity for a stable development platform for the conceptual explorations (Device Studio)

This leads me to the thoughts of creating a high payload, rugged mobility platform with all-terrain capabilities.  In the case of the mobile esky idea, this would mean all mechanics, power, drive train *and* a case of beer / ice...

It needs to have the capacity to integrate various (future) sensors and actuators, but perhaps for now could be remote controlled (to allow me to focus on the important mechanical / engineering concepts).  

If time allows, various refinements could be made to efficiency, speed and mobility.  A rudimentary safety mechanism (based on proximity sensor) could be developed as part of the software.

If the idea should change, the focus shall remain the same: efficiency, expandability/modularity and stability.