Servo - May 2011

The Devantech SRF05 sensor doesn’t require so much time between each reading. Its time delay is set at just one millisecond.

Note that readings from the ultrasonic and IR sensors are not taken while the turret servo is in motion. The servo is commanded to a new position and stops to take the reading. This is to help reduce measurement errors.

Likewise, the ultrasonic sensor is not read when the robot is turning. Lateral motion while taking an ultrasonic measurement can result in wild inaccuracies, though this greatly depends on the specific sensor you’re using. You get the best results when the robot is still or moving at a reasonable speed forward or backward.

As noted above, the ultrasonic sensor continually “pings” for objects straight ahead while the robot is traveling forward. If an object is detected to within three inches, the servos are slowed down (by adjusting their values close to center, which is a value of 90). If an object is detected to within an inch, the robot stops, backs up, spins for half a second to reorient itself, and then continues forward again.

Adjusting Timing Delays

The sketch relies heavily on delays to steer the robot, and to position the turret servo before taking a sensor reading. For the purpose of programming, the length of these delays is determined empirically. The delay values are based on the speed of the servos when operated from a fresh battery supply. Some servo motors are particularly fast; some are downright sluggish.

Bear in mind that servo motors slow down as the battery voltage decreases. A one second delay when the batteries are fresh might turn the robot one complete circle. But when the batteries are about dead, the same one second delay might turn the robot only half way.

In the ArdBot, most timing delays are set directly in code. An exception is the T_DELAY value, specified in the declaration area of the sketch. This value determines the delay used when rotating the sensor turret. The delay must be long enough to transit the servo from one extreme to the other, plus the time it takes to read the IR sensor (which is about 270 ms for all five samples).

You should strive to set delays based on average speeds. The ArdBot as presented lacks sensors for true navigation (compass, gyro, and wheel encoders for odometry), so the motion routines are intended only to demonstrate basic maneuverability. In a more elaborate ArdBot setup, you could use wheel encoders and/or a compass to more precisely determine how far the robot has turned. (It’s best to use both, if you can; wheels slip, resulting in errors in the wheel encoders. The compass is used to reorient the position of the robot to a known reference.) You may also wish to explore ways to avoid using delays, as the delay() statement literally stops the sketch until the specified time period has elapsed. The MsTimer2 library is a good example of using other techniques to time events. A number of ready-made libraries provide this kind of functionality, and allow you to easily create multiple timing objects.

As an example, try Metro, also available from the Arduino Libraries page. Metro allows you to create multiple timing objects, with each object set to “expire” after a certain length of time. With some crafty coding, you could replace most or all the delay() statements in the sketch with

Metro timers, allowing the program to be more multi-tasking and responsive to sensor events.

ArdBot Line Following Mode

When the Mode Select header is placed so that pin A2 is brought HIGH, upon startup the ArdBot is put into line following mode. As described in Part 6, I’ve provided two variations of line following sensors:

1. Basic two-sensor detector, using a pair of infrared emitters/detectors side by side. The two detectors are meant to straddle the 3/4” line of a length of black electrical tape. Use the mode_line_follow method with the two-sensor detector. The sketch assumes the detectors are wired as shown in the figures, and are connected to Arduino pins A3 and A4.

2. Enhanced multi-sensor array, using (in this case) a Pololu QTR-8RC eight-detector array. These units are very compact and cost less than when purchasing separate ready-made infrared emitter/detector modules. Use the mode_line_follow_array method with the QTR-8RC. The ArdBot uses five of the eight available emitters/detectors, and these are connected to digital pins D4, D5, D6, D11, and D12.

Listing 1 includes the code for both line following variations. When using an array — like the QTR-8RC — it is also possible to detect a “finish line” in the line following course. See the ArdBot Construction Notes site (check the Sources box) for more information.

For More Arduino Coverage

Though this is the final article in this series, there is plenty more to share about combining the Arduino microcontroller with robots. After all, there’s so much more to talk about. So, stay tuned! Be sure to check out the ArdBot Project updates, free code, and additional examples at www.robotoid.com/servomag/. SV infrared sensors for line following, and components:

If you’d like to build the ArdBot, be sure to start with the November ‘ 10 issue of SERVO Magazine for Part 1 of this series. Also check out the following sources for parts: