13 Sep 2003 dafyddwalters   » (Master)

Open Automaton Project Update

The Sonar Array Module is now complete. If you're looking for an inexpensive way of driving up to 16 SRF04 sonar sensors from an I2C host, check it out.

The custom hardware/firmware modules that have been completed to date are:

  • Input Module - emulates a PC keyboard, takes input from an IR or RF remote control, or on-board keypad
  • RF Remote Module - a handheld radio frequency remote keypad to use with the Input Module
  • Power Management Module - controls main battery charging and communicates with docking station by IR
  • Docking Station - ground-based charging dock which acts on IR instructions beamed from robot's PMM
  • Sonar Array Module - interfaces with up to 16 Devantech SRF04 sonar modules

The hardware/firmware modules still left to complete at this time are:

  • Motor Control Module - takes quadrature encoder signals from two wheels, outputs 20KHz PWM signals for two motor drivers. Firmware includes two PID control loops (one for translation, and one for rotation), as well as dead reckoning by odometry.
  • Head Control Module - drives two RC servos (PWM) of the Pan and Tilt Head, and interfaces with the Eltec 442-3 human body heat sensor

At this rate, it looks like I will be ready for the Robothon just in time. However, because I've taken a bottom-up approach to the project, and there won't be much time left when I've finished the hardware/firmware modules, the prototype is not going to be as "smart" as I'd hoped in time for the Robothon. It'll probably just have some basic behaviours. I'll be adding the real smarts after I land back in the UK.

DC to DC Converter Woes

I blew up my Morex DC to DC converter earlier in the week! This is the device that takes the 12V (nominal) voltage from the lead acid battery, and converts it to all the different voltage levels necessary to drive the motherboard and hard drive. I always had my doubts about that particular DC to DC converter because of the very limited input voltage range specification (11.4V - 12.6V). I knew that the battery voltage could go above the upper limit, so it was just wishful thinking to hope that this would not be a problem. This setback has turned out to be a blessing in disguise because it forced me to look around for a better DC to DC converter. The one I ended up choosing as a replacement is the PW-70 from Mini-Box.com. This can tolerate an input voltage in the range 10.5V to 15.5V - much better!

Another (obvious in hindsight) lesson I learned this week: Make sure that the wire gauge used for carrying the 12V power rails around the robot can handle the current draw of the various modules! While testing the Power Management Module and Docking Station, I was quite surprised to see a 0.7V drop between the 12V DC switched mode power supply inside the Docking Station, and the input of the DC to DC converter on board the robot. Quite a bit of this voltage drop was just due to wires that were not thick enough. The docking station coupling also accounted for some voltage drop. In fact, having already tried two different approaches to implementing the docking/charging contacts at the front of the robot (which mate with the Docking Station) it still seems that I'm going to have to come up with something better than I have now (which is using modified decorative brass hinges). I think this is going to have to wait until after the move, though.

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