Servo - July 2010

to mate with the connector. That is why the SONAR board connections are called out as U2 pin numbers. See Figure 2 to locate U2 on the board.

Okay, that takes care of the hardware ... now for the code. The Arduino does a very good job at hiding the complexity of embedded programming. This means that what would normally be a complicated program using interrupts, hardware timers, Data Direction Registers, and the like turns out to be very simple.

Listing 1 is the entire SONAR program. This program gives the range in both cm and inches; you can choose your favorite unit of measure to use. I removed 200 µs from the time returned because I noticed that my ranges were about two inches ( 5 cm) too long (using my tape measure held in two hands while aiming). This problem occurs because I use C1 to delay the rise of the Init/Echo line which causes the pulse to stretch a little bit there. Also, the conversion that I use to calculate the range is based on the speed of sound at sea level at 15 degrees C. I live at 5,000 feet and my room was closer to 25 degrees C. Combining my inaccurate measurement technique, air pressure, temperature differences, and my delay capacitor together — along with some amount of code latency — means that my times are a little off. You’ll need to experiment to see what error the system introduces when you build this setup. I have typically found that the Polaroid boards hacked from cameras are a little off — some of this error may even be in the board itself. Regardless, after you apply a little fudge factor you’ll find that the SONAR boards are a handy addition to many projects.

I don’t think these cameras have been made in nearly 20 years, but you can still find them at thrift stores for a small cost and even eBay has them all frequently for between $4 and $10.

Figure 3. Using the serial monitor in Arduino.

The code in Listing 1 does a SONAR cycle every second to send the code to the serial port that you can configure and run in the Arduino IDE (see Figure 3). This is a convenient way to know what your program is doing. My layout uses a SparkFun Bluesmirf Gold Bluetooth connection, but just a serial cable for the typical Arduino board works fine.

I hope you’ve learned something this month or at least been entertained in the process. I enjoyed getting back to this old project and I hope that I may have re-invigorated you to look into your closet and pull out that dusty old Polaroid and look at it in a different light. Next year, I’m planning to enter the SparkFun AVC with a ground based vehicle. I’m not brave enough to risk a flying machine just yet! (See the sidebar on this year’s event.) As usual, if you have any robotics questions don’t hesitate to let me know at roboto@servo magazine.com. I’ll do my best to answer you! SV

Autonomous Vehicle Competition

In the Ides of April, SparkFun in Boulder, CO, held their second Autonomous Vehicle Competition. It was well attended and well played. The competitors were trying to get their autonomous robots to circumnavigate the SparkFun building in the fastest time. There were two classes: ground-based and flying. The competition included five aerial and 13 rolling robots. The audience, as well as the competitors, had fun. Everyone was treated very well by the SparkFun staff.

This competition was a blast to watch and listen to (more on that later) because there was such a wide variety of solutions to root for! All of the aerial entries were fixed wing (no helicopters) and made of foam.