Servo - September 2011

FIGURE 7. Front IR bumpers mounted on the RoboProp-Bot.

This allows you to write code to: • Travel a specific distance.

SirMorph provides an analog output; it can also be used as a virtual bumper, short range IR distance measurement device, or line following sensor, which makes it more versatile. As SirMorph was designed for the Solarbotics series of servo wheels, I am also replacing the standard Parallax wheels with white Solarbotics wheels to accomodate SirMorph. For the wheel encoders, I used: • Left wheel encoder on AN1.

The wheel encoders I am using can detect 1/10th of a rotation of the 2-5/8” wheel. Therefore, 1/10th of a wheel rotation travels (pi 2.625”)/10 inches — that is, approximately 0.825”. A full rotation travels 8. 25”.

You can calculate the RPMs achieved at a given servo speed by counting the number of edge transitions in one minute and dividing by 10; if lower precision is acceptable, just count the number of edges in six seconds for a direct RPM reading.

The RoboProp library “GetRPM(servo_pin, analog_input, speed)” function returns the rotations per minute for the servo, encoder, and speed as an integer.

You can use GetRPM to make a table of RPMs corresponding to different servo speed settings which will allow you to travel in straighter lines than before,

generating a calibration table simply by using the servo speed you need for each wheel to get the same RPM, then running both servos at the same speed for the same amount of time.

Replacing the Whiskers

For the simple Boe-Bot roaming demo program, I used the Parallax-provided whisker contact sensors to make Robbie turn to avoid obstacles, and to back up if he ran straight into something.