Date: Mon, 26 Dec 2005 22:25:34 -0600 (CST)
Subject: GPS works and electronics grade silicone
X-UID: 114
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The GPS receiver works. I've only tried it inside the apartment so haven't 
been able to get a satellite lock. The serial communications between the 
GPS receiver and either the single board computer on the robot or the 
laptop are visible. The GPS applications seem to work. Of course, I won't 
really know until I'm outdoors with a clear view of the sky and a visible 
satellite constellation.

Now that the radio and GPS modules work with both the robot computers and 
the laptop, I felt comfortable closing up the electronics enclosure. At 
this point, it all looks good, everything working and nothing broken.

I found a tube of electronics grade silicone at Altex. It's different from 
the Loctite silicone. Here are my four rules for distinguishing between 
types of single part moisture curing silicones as regards electronics.

1. If it does not specifically say "electronics grade", then it is not and
    will corrode copper, etc.

2. Electronics grade silicone doesn't necessarily cost much more than
    conventional silicones.

3. Electronics grade silicone does not change appearance when it cures.
    Specifically, it does not cure to become clear. It comes out of the
    tube in a milky translucent goo and remains this way after it has
    hardened.

4. Electronics grade silicone emits no detectable smell. Non-electronics
    grade silicones do smell even if the aroma is very mild.

So there is some corrosion on the copper leads of the RS-232 to 5 volt TTL 
conversion boards. This probably doesn't matter as the copper leads are 
thick enough that a bit of surface corrosion won't matter. In the photo, 
the greenish tinge about some leads indicates attack by ammonia released 
during the silicone curing. Were this kind of corrosive silicone to be 
applied to a printed circuit board, it might eat through the circuit 
tracings and substantially increase resistance as the copper corrodes. The 
underside of my boards are wired with 30 gauge wire which is thin enough 
that corrosion might eat through them. The electronics grade silicone is 
used on the underside to avoid this.

Flite is a lightweight, embeddable and open source version of the Festival 
speech synthesis system from Carnegie Mellon - very impressive. If you 
recall speech synthesis from the late 1970s and early 1980s, this is far 
more advanced. Attached is a sample generated by:

flite "My name is Marvin. No one can help me." marvin.wav

The default voice heard here can be changed. There are other languages and 
accents available. It's some work to do this so I don't want to mess with 
it now. But it did make me think that very soon now someone will make a 
RoboSapien type toy that looks like Marvin the robot and will speak in the 
paranoid robot voice.

The roadmap GPS application uses flite to announce locations. When the 
robot is within WiFi range, it's location can be displayed by connecting 
to the gpsd server running on the robot. However, when out of WiFi range, 
then the Maxstream radios will have to be used. Out of the box, the GPS 18 
LVC receivers operate at 4800 baud. During normal operation, position 
updates occur once per second. As the radios operate at 9600 baud, there's 
enough capacity for both GPS position updates and a command channel.

The remote kill switch command has already been mentioned. One reader 
suggested the need for a heartbeat between the robot and base station. I 
agree. So the robot will periodically send a challenge to the base 
station. If the correct response isn't received, then the robot may decide 
to enter a safe mode. I expect to use a LFSR (linear feedback shift 
register) which isn't that secure but as most militaries used them 
(Schneier, 1994) should be o.k. for this application.