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 Post subject: Two 4WD Chassis for Robot Use
PostPosted: Sun Apr 11, 2010 11:29 pm 
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Master

Joined: Mon Aug 18, 2008 1:26 am
Posts: 799
Location: CA bay Area
Check out this robot chassis contender on Ebay:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&Item=160422549752&Category=163866&_trkparms=algo%3DLVI%26its%3DI%26otn%3D2
At $85.50, which includes shipping and handling, this looked like a good starting point to build a robot up from if a 4-wheel drive is what interests you. It comes out of the box with geared-down motors, power switch, DC barrel jack for power-in, wheels and wheel-motion sensors installed. These sensors allow the user to monitor wheel position and speed, albeit with a limited 10 pulses/rotation. The tires suggest this will do well on hard, flat surfaces, but should have more than enough power to handle ramps and uneven terrain. The chassis is peppered with mounting holes for add-ons, like a pan-and-tilt assembly, electronics boards, and sensors, to mention a few possbilities.
If this is a first-robot project for you, this is a big step up over what you can typically find offered at robot and hobby sites. Take my word for it, I spent a couple hours Googling while using different search criteria. But, hey, if you have a better offering, feel free to show us!
Attachment:
Ebay Chassis.JPG
Ebay Chassis.JPG [ 31.96 KiB | Viewed 8470 times ]

The other possibility was this rugged contender from LynxMotion, seen at:
http://www.lynxmotion.com/Product.aspx?productID=603&CategoryID=110
At $220 before shipping and handling, this is a bit pricey, but the quality is obviously superior. The all-terrain, outdoor-handling wheels mean when you want to go in the dirt, this baby will oblige you. This comes out of the box with all the same goodies as the Ebay item, but nothing is said about built-in sensors or a DC-In jack.
Attachment:
LynxMotion Chassis.JPG
LynxMotion Chassis.JPG [ 13.77 KiB | Viewed 8470 times ]

The Ebay item is totally unwired as far as the motors, power switch and DC-In jack go, so break out the solder gun and solder and have at it. The wheel-motion sensors have a short cable harness supplied for both sensors.
The LMotion item has a battery harness provided. I can't say about the power switch.

As for add-ons, both companies offer numerous sensors and pan-tilt kits. LMotion has scads of other chassis enhancement choices.
The Ebay store has a pretty interesting PING-like ultrasonic sensor. This has an AVR chip controlling it. It can communicate back to any microcontroller of your choice using PWM, TTL serial, or RS232 serial. It even uses thermal sensing to correct for the effects of ambient temperature. Most interestingly, it has a single servo motor controller output, so you can literally control a wheel motor from this! Check it out at:
http://cgi.ebay.com/Ultrasonic-Sensor-with-temperature-sensor-servo-control_W0QQitemZ160418365958QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item2559ae0206
Quite the interesting toy for only $35.50 after S&H.

Neither has any controller electronics. I have a Nano Driver Board to try out. You can check the data sheet for this at:
http://www.basicmicro.com/downloads/datasheets/nano_microcontroller_driver_board_B0114.pdf
For $25, you can't beat this thing with a stick. You can control servos, steppers (to 24V), and DC motors. It can use the Nano's outputs for servos, or the common ULN2803 8-channel small-power chip, the L293 Dual H-Bridge chip, or four honking FETs for big steppers or DC motors. Each FET can handle 9 Amps! Make your own Mars Rover or wheel chair!

So I sprang for the Ebay chassis. It seemed the ideal launching point for me and robotics. My main interest is in starting a running-notes project to be done here in the forums as I progress from raw chassis to limited motion control, add sensors, and maybe go autonomous. Fortunately BasicMicro has provided plenty of demo code and app notes to refer to.
Later!

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 Post subject: UPDATE Two 4WD Chassis for Robot Use IS HERE
PostPosted: Fri Apr 23, 2010 8:30 am 
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Master

Joined: Mon Aug 18, 2008 1:26 am
Posts: 799
Location: CA bay Area
The package arrived Monday. The package is a padded bag just small enough to still fit in a mailbox. And all I can say is: Oops! :? It doesn't come assembled like I thought. But as the pictures show, everything was quite intact and usable. The contents are:
1. a few flat, stamped and punched aluminum panels.
2. a bag with the four motors.
3. a bag with miscellaneous hardware, like screws, power switch, the power jack, and lengths of red and white 22 GA wire.
4. the wheel sensors with cable assemblies.
5. the four wheels.
6. a 4-battery (AA) battery holder, wired

I supply pictures of the bag and its contents.
I have already pulled the motors, wired them and given them voltage tests. I also charted the current draws at 3V, 5V, 6V and 9V DC. But you'll have to wait to see that, and the fix for horrendous brush-induced noise, in my new post that tracks the progress of Project 10, from introduction to autonomous navigation and operation... I'd like to think. ;)
I hope. :?
Look for it.
Attachment:
Bag and Contents.jpg
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Attachment:
Chassis parts and sensors.jpg
Chassis parts and sensors.jpg [ 143.17 KiB | Viewed 8439 times ]

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http://blog.basicmicro.com/
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 Post subject: Re: Two 4WD Chassis for Robot Use
PostPosted: Fri Nov 04, 2011 12:27 pm 
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Joined: Fri Nov 04, 2011 12:23 pm
Posts: 1
The problem with using wheel rotation for odometry on a 4wd skid steering device would be that the wheels will slide sideways when turning. different wheels might rotate differently depending on type of turn etc.

-or am I too pesimistic?


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 Post subject: Re: Two 4WD Chassis for Robot Use
PostPosted: Fri Nov 04, 2011 5:14 pm 
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Joined: Thu Mar 01, 2001 7:00 pm
Posts: 1316
Location: Temecula, CA
You aren't going to control precise positioning perfectly but what you are going to do is make sure your robot moves in straight lines when you want it to. Also smooth curving motions can be produced with good control. The closer to actually full skid turning you get the worse your encoder error will be. But I tested with a Lynxmotion 4wd robot with those big monster truck tires and it actually didn't do bad at all even with full skid turns. A robot with narrower tires will work better. The monster tires tended to catch and edge randomly during a turn which would cause the robot to lose position slightly

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