I am sorry you are having trouble using the P*. However, the blog is not an appropriate place for troubleshooting your problem. Instead, you might post this on our forum with more details about your setup.
I am sorry you are having trouble controlling your servos; I think some people on the Pololu forum might be interested in taking a look. I recommend posting to the Software and Microcontrollers section, and including your code as well as oscilloscope traces.
It is possible to use more than one dual VNH5019 shield on an Arduino, but it would require a lot of changes to get it to work. You would have to remap the control pins, do some additional wiring, and write your own code to set up PWM on the correct pins for all of the shields.
You can read more about how to make some of those changes inside the "Remapping the Arduino Connections" section of the VNH5019 shield's user's guide, which can be found under the Resources tab of its product page:
By the way, we break out the connections to the motor drivers along a single side of the shield, so if you do not need to stack the shields, you could just make your logic connections there. This might make using both shields easier.
I had not thought of that. It sounds like a fun alternative configuration (the movement would be fun to watch!), but I suspect that there might not be enough time for the microcontroller to react to the readings from the QTR sensors if the array is placed directly between the drive wheels. In fact, I think it's better to have the sensor as far forward on your robot as it can go, though the length of your robot should probably be limited by the smallest turn radius in the line following course.
It's pretty quick, follows the line well, and has very far forward line sensors. So, I suspect that while moving the location of that sensor array back toward the drive wheels now might make it easier for my current code to follow the line, I should probably just change up my code to line follow better.
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