Sunday, June 9, 2013

4WD Rover

Flying a model plane in the desert is difficult.  There is sand laden wind during the day, so one can only fly at night and then one cannot see the plane properly.  So I decided to make a little runabout.

A runabout robot uses much the same parts as a plane.  An aircraft RC system is used for backup and test and the autopilot is the same, just with a different ArduRover software load. Details here http://rover.ardupilot.com/

Where to get the parts

Pololu.com sells a range of motor speed controllers with various control inputs.  The Easy series has linear, digital and RC control inputs.  It can also blend two RC channels for differential control.  This way, one can make a 4WD robot chassis, using the Throttle channel for speed control and the Rudder channel for direction.  The blending will add or subtract the two channels such that the left/right will speed up or slow down the one side slightly, to make the model turn.

I picked the 18V 15A speed controller http://www.pololu.com/catalog/product/1376

Pololu also has geared motors and wheels to match http://www.pololu.com/catalog/category/51

So, I picked up a wood bread board at Carrefour, epoxy glued four geared motors with 50 by 120mm wheels to it and la voila, a 4WD Rover!


The advantage of using a wood bread board as the base, is that one can easily screw and glue things to it.  A metal chassis is much more difficult to work with and costs a whole lot more.

Here is the Schematic of the RC motor control - to test and configure the speed controllers before the autopilot is added to the mix.


Ultimately, I want to add a short range proximity sensor on each corner to protect the wheels and avoid banging into things, as well as a camera and range finder on a pan/tilt swivel for distant obstacle/object detection.  Jameco.com sells a range of pan/tilt swivels and robot grippers, that will be handy for this project http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_2144518_-1

Video

To process video/stills, one would need OpenCV, which is beyond the capability of an Arduino auto pilot.  A Texas Instruments Beaglebone Black running at 1 GHz (or an old netbook computer) may be a better platform for that, so ultimately I can see the Arduino going out the window http://beagleboard.org/Products/BeagleBone%20Black

Motor Control

The Pololu motor controller user guide is here http://www.pololu.com/docs/pdf/0J44/simple_motor_controllers.pdf

In the guide on pages 15 and 16 it explains where to get the software required to configure the motor controllers.  The default configuration is for analogue inputs, which means that the speed controllers won't work by default - you have to configure them.

The software is available from here http://www.pololu.com/file/download/smc-windows-121204.zip

I tried the Linux version first, but it requires some non-existent libraries, so then I tried Windows on Virtualbox on my Mac and then found that I cannot yield control of the USB port to Windows since something in the Mac grabs control of it.  Therefore, I dug out my old Linux netbook and installed the Windows software all over again on XP.  This time it worked.

Clicking around a bit, I found the RC settings and configured the one controller as Mix Left and the other as Mix Right.  I also disabled the 'motor safe start' since I am not sure how it will work in practise and I don't want to ever run this program again just to reset the controllers.

When testing motors, put the machine on a couple of Sparkfun component boxes, to keep it from running away.

Test run

Once I got the controllers configured for RC, I gave things a try and the magic smoke didn't escape, but the controls were 90 degrees out, so I had to swap channels 3 and 4 around.

Overall, this seems to be a good rover platform that can zoom around at a decent walking speed.


No comments:

Post a Comment

On topic comments are welcome. Junk will be deleted.