Lego NXT Ball Picker

If you are impatient you can skip to the last page of this tutorial and watch the videos!

The NXT robot is a modified TriBot with the touch sensor removed and a wireless camera attached in its place. The wireless camera transmits the image to a base station PC that processes the image using RoboRealm. Using bluetooth the appropriate motor commands are then transmitted back to the NXT. This combination of a PC and NXT provides a great degree of processing power and flexibility but does limit the operational range of the NXT to within a couple feet of the base station.

We can build a more powerful TriBot by using a wireless camera and receiver. The wireless camera was attached using lego parts and can be purchased for around $40.00 from Computer Geeks.com. The camera's receiver requires a NTSC digitizer that can be purchased for around $50.00 from Hauppauge. The digitizer connects to the PC via a USB connector. For around $100.00 in COTS (Commercial Off The Shelf) parts you can provide your Lego NXT robot with machine vision capabilities.

      

Using this equipment the control loop is as follows: camera -> wireless transmission -> wireless receiver -> digitizer -> USB 2.0 -> PC -> RoboRealm -> Bluetooth -> NXT brick -> motor movements and then back to vision. This loop is someone longer than having an on-board vision processor BUT it does have a PC in the middle of the loop which is capable of a lot more processing than most onboard processors can provide.

In this tutorial we introduce a sequence that relies completely on vision without using any other sensors on the NXT (this is possible to do but that is for another tutorial). The sequence that we wish the TriBot to perform are:

1. Spin around in the current position looking for a blue ball
2. Approach the ball until sufficiently close enough to grab it
3. Grab the ball using the TriBot grippers
4. Spin around again looking for an orange cone
5. Approach the cone until sufficiently close
6. Drop the ball close to the cone

While this sequence is quite simplistic there are many problems that one encounters when using a vision system. So let's get started by understanding the camera setup first ....