Date: Tue, 17 Jan 2006 02:50:44 -0600 (CST)
Subject: battery tray and power output
X-UID: 123
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The battery tray is added. I'm not really happy with it. There's something 
wrong. The design doesn't look right. But...I don't have much more time to 
go through additional iterations.

The robot has evolved into a purely functional design. There's really 
nothing done to hide any of the mechanism. It's entirely exposed. This is 
convenient for maintenance but not very pretty. I'm not good at hiding 
mechanisms in design.

The white carry handle on the top is just about as low as it can be and 
still clear the router on top of the electronics box. I haven't tested the 
fit in the trunk of my car. But the opening of my trunk is about 18 inches 
high. The height of the robot is about 18 inches. The fit will be tight. I 
think that the front could go in first and then the front suspension 
pushed down into a squat before rotating in the rear. I'm going to have a 
shorter bar for the webcams so no disassembly or breakaway assembly is 
necessary. I want to be able to pull the robot in and out of the trunk and 
have it ready to go quickly.

Several readers challenged me on the top speed. I must admit that I don't 
know what it will do. One reader wondered if it will have enough power to 
move as the motors are very small and the robot somewhat large.

It's actually lighter than you might think. It's roughly the weight of a 
steel framed bicycle which isn't that much. As for power, the Panasonic 18 
volt packs are rated at 3.5 amp hours. In standard industry battery 
specification, C is then 3.5 amps. At a drain of 1 C, the power output is 
about

           2 packs x 18 volts x 3.5 amps = 126 watts

The average human can sustain 125 watts output. For a bicyclist, that is 
probably around 12 mph. Bicycle rolling resistance is lower than for the 
robot. But aerodynamic drag, constituting approximately 50% of resistance 
for a conventional bicycle, is proportionately less for the robot.

Electric RC cars have a typical run time of 15 to 20 minutes. The 
batteries are drained quickly. Assuming a current of 3C (10.5 amps), then 
the power level increases to 378 watts. I'd expect a runtime of maybe 10 
minutes at this current level. Available energy decreases with higher 
current given conventional battery technology.

So the robot is around 30 to 40 pounds and should have higher than human 
sustained power output (note that an athlete may have double or more the 
aerobic capacity of even fit normal humans - people like Lance Armstrong 
are genetic mutants). Rolling resistance is higher but aerodynamic drag 
significantly less. And mass is much less. So I'm hoping the performance 
is good. It won't have much endurance. But any electric vehicle will have 
this limitation without a power source like expensive military grade 
lithium batteries.

I'm starting to notice the numerous points of failure in the design. Any 
one of them will cause the entire system to fail. Everything must work or 
the robot can not function properly. It's too late now to do anything 
about this. I'll just have to be careful and check everything before each 
robot run.

Examples of critical failures

1. tension cable flies off of pitman arm resulting in loss of steering
2. bolt loosens on rear wheel hub resulting in sudden loss of drive power
    on one side
3. battery pack clip flies off resulting in sudden loss of drive power on
    one side