TFC Rule Clarification: Extra Wheels

[eseaberg]

Can we mount a non powered wheel for speed sensing purposes?  Can this or any other modification extend past the dimensions of the car?  For instance, can the wheel trail in the rear of the vehicle?

[admin · Accepted solution]

You can add a wheel and you there is an imaginary box that constrains the dimensions.  See section 2 under Mechanical in the rules here: https://community.freescale.com/servlet/JiveServlet/download/93225-17-261358/TFC+Global+Championship...

or

No part of the car shall exceed dimensions of 250mm/9.85in (W) x 400mm/15.75in (L)x 305mm/12in.(H)

[david_dicarlo]

I bought some little magnets:

20 Pcs Neodymium Disc 1 6" x 1 16" Super Strong RARE Earth Magnet N35 D4X1 5mm | eBay

and some hall effect sensors:

Hall Switch Sensor Module for Arduino Smart Car Motor Speed Test New | eBay

but I'm still waiting for them to arrive on the slow boat...

I also forgot that I ordered some IR emitter/sensor pairs, too, as backup:

10 Pcs TCRT5000L TCRT5000 Infrared Reflective Optical Sensor Transistor Output | eBay

The plan was to glue the magnets to a disk and affix the disk to the inside of one of the wheels and use the hall effect sensor to detect the rotation. Alternatively, affix a disk with a black and white pattern on it to the inside of the wheel and use the IR sensor pair to read the rotation. I suspect because the hall effect sensor board has a comparator on it, it will be the easier sensor to interface digitally to the FRDM board.

[eseaberg]

I thought about making something attached to the drive wheel, but thought it might be easier to steal the scroll wheel parts out of an old mouse and just build a little bracket to trail behind the vehicle. 

[david_dicarlo]

The little mouse wheel can roll against the tires, too.  Either way should work.

My little magnets came last night...  But no hall effect sensor yet. :smileysad:

[danielhadad]

Would love to see it once it’s set up!

Has anyone tried speed sensing electrically using HBridge ?

[admin]

Ask craigs he had some ideas on the subject.  Not sure if he pulled it off or not!

[craigs]

I ended up buying optical reflective sensors from Digi-key. They need 5V.

I also created optical encoder wheels using PowerPoint that I plan to glue to a tire. So there is still a fair amount of

electrical, SW, and mechanical work to be done to have this up and running. But I do at least have all the parts now.

http://www.digikey.com/product-detail/en/GP2A25J0000F/425-2044-5-ND/720346

[david_dicarlo]

I have about 8 left over TCRT5000L sensors that I picked up for speed sensing.

Using the 3.3V fail, a current limiting resistor of 56 Ohms for the LED, and a 2.2K resistor between the emitter to ground, the photo transistor gives a swing from 0 to ~2.5V when using a mirror (scope plot below of moving the mirror in and out).  I have some left over shiny aluminum foil tape from another project; little squares of it should make a great speed sensing disk.

If anyone wants a sensor and some of the shiny foil tape ($1?), let me know.

[danielhadad]

I ordered 2 of these:

Photoelectric Speed Sensor Encoder Coded Disc Code Wheel for Freescale Smart Car | eBay

If it works out I can sell extra to someone who wants one.

Daniel

[danielhadad]

BTW, got accelerometer working -- mbed.org has the code already there which works swimmingly.  Just FYI...

[danielhadad]

All,

Found some parts on Ebay with 'freescale smart car' in description: freescale smart car | eBay

One of them is an optical sensor you could use for speed control.  Debating getting it--  not sure where you would hook it up to the wheel though.  Also no good specs attached so would be difficult to set up.

But in general interesting that things are up there specifically marketed for this competition!

Daniel

[david_dicarlo]

If you make the outboard wheel rotate against one of the drive wheels, you don't have to go beyond the footprint of the car.

[danielhadad]

I would use an extra gear connected to the gearbox.