Date: Wed, 8 Mar 2006 02:04:54 -0600 (CST)
Subject: ackermann skid steering
X-UID: 144

Tonight at the Dallas robot club, we saw a screener's copy of a PBS NOVA
episode about the DARPA Grand Challenge. It should be on TV later this
month. You do see some details that aren't commonly communicated. The CMU
Red Team might have won if they hadn't rolled their primary vehicle ten
days before the race or chosen a different race management strategy. What
was really wild was seeing the CMU team trailer. It was like mission
control inside. It was packed with workstations and students optimizing
the robot's route. Red Whittaker had made the statement that his team
would be like the "Red Army". The atmosphere did seem like something
military. Of course, this is reality television so you don't really know
the lens through which reality is viewed.

I realized that all steering is skid steering. My car skids when turning
up a hill. The rubber wheels do not track on the ground unless it is
perfectly flat and uniform. If the lane my car is in slants to the side
for drainage, then the wheels must be turned slightly uphill to maintain
position.

So the robot is the same. While Ackermann front wheel steering minimizes
skid, it happens nonetheless. Turning the front wheels provides lateral
thrust that causes the vehicle to yaw. The robot also has differential
drive to the rear wheels. That's another way it can steer. A combination
of front wheel lateral and rear wheel differential thrust controls vehicle
yaw about the normal to the ground.

The rate gyro is the key. It gives me a "true" (within reason) measure of
vehicle yaw rate. Yaw rate determines how the vehicle is turning. This is
the output from the plant to be controlled. And depending on vehicle speed
and ground conditions, a different combination of differential rear wheel
thrust and front wheel steering might be required. It's just how computer
controlled automobile systems sometimes steer differently at high speed
rather than at low speed.

There's a main robot club meeting at Fair Park this Saturday. I've been
talked into going so must hustle to get the robot in a state where it can
demo. I haven't slept in two days. (time to sleep now)