Hi, new to forum, hello all 
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I'm sorry, this must have been asked before, but I'm still struggling to find the answer I want and I've been looking for hours. I'm buying this robot chassis (it's one of those cheapo ones with the yellow DC motors):
http://www.ebay.co.uk/itm/WST-Motor-Smart-Robot-Car-Chassis-Kit-Speed-Encoder-Battery-Box-For-Arduino-WK-/181564206296?
No tutorials seem to discuss this:
My question is, being a complete noob at this, does this chassis come with everything I need for speed encoding? I see the small wheel things that have something to do with it, but do I need some kind of sensor to go with them for speed encoding to work? Is that hardware already built into the DC motors?
I already have an UNO board, been messing with lighting up LEDs. Now I want to start using motors. I shall be buying a motor shield. I have no problem with coding for this stuff, I'm just too ignorant of the required hardware.
Do I even need speed encoding?
Thx very much for replies .
Well let's start with the encoders. Motor encoders allow you to perform odometry, or using some sort of sensor to incrementally compute the position of your device. So if you're planning on doing some room mapping or maze following, odometry would be required.
If your application does not require knowing the speed or position of the platform, then you do not need encoders.
Now for the platform, I do not see sensors for the encoder disks. You would have to purchase reflective IR or slot IR sensors for counting the holes in the disks as they rotate. For determining direction, you need two sensors per disk. I would recommend spending more money and making life easier, and getting a platform with sensors.
Good answer, thank you . The reason I was looking at encoders is because I read that two DC motors may not spin at the same precisely teh same speed even if they are the same type, causing wheeled projects to not run in straight lines. I guess it can't be much of an issue else you would mention it?
The actual effect on the speed differences is mostly too small to have an effect at the speeds this platform will go. If this turns out to be a problem, simple re-mapping of one of the motors PWM values will solve it, as in software calibration.