Ernie 8 Legged Walker
Posted: July 8th, 2006

Well here’s Ernie. Ernie is the first robot that I have built on my own with out the use of directions or anything of that nature.
Ernie is an eight legged walker and is five inches high, 17 inches long and nine inches wide, while walking Ernie will get a maximum height of about 6 inches.  Theirs four legs on each side each side has one gear reduction motor and a rotation sensor to monitor the position of the legs. When I first started this project I was using touch sensors to monitor to tell when the legs were fully extended. I ran into problems with the sensors always working correctly. So I then switched to rotation sensors. The rotation sensors worked great. When I first started with the rotation sensors I was keeping the legs synced by stopping and starting the motor that had the highest value from the rotation sensor. This worked ok except this slowed the robots walking down because of the motors starting and stopping. So now I am just letting the legs turn freely till the rotation sensor reaches a specific value then I stop the leg till the slower one catches up. This seams to be working pretty good. I am using two gear reduction motors one for each set of legs. When Ernie walks the motors are on a 25:1 ratio with the legs. The motors turn 25 times before Ernie can complete one step.
At the front of the robot there is a bumper that has two touch sensors one on each side of the bumper to determine which side the bumper was hit. Since I was already using two rotation sensors on ports one and three I was left with one port for two sensors. The solution for this problem came from mindsensors.com. I purchased a passive multiplexer from them. This thing is great. It has three inputs for touch sensors and returns a different value depending which sensor or sensors is pressed. Once a sensor is pressed a value less that 1023 is returned to the RCX from that its determined if the left, right or both sensors were pressed. If the left sensor is pressed the robot turns right and if the right is pressed it turns left. Now at this point I hit another snag Ernie would get stuck in corners. So what I did is if a touch sensor got hit with is 20 seconds of another one getting pressed Ernie would turn in the same direction that it did the first time. After doing this I didn’t have any issues with the robot getting stuck.


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