Servo - November 2015

You’ve Always Wanted, that started with the January 2015 issue of SERVO) could be enhanced by adding a ranging laser to his turret. For a variety of reasons, I chose the SF02.

Figure 8 shows it mounted on Arlo’s turret which also supports an IR sensor, an ultrasonic sensor, a beacon detector, and a webcam. My goal was to replace the IR sensor with the SF02.

Arlo’s original IR sensor produced an analog output, so I was pleased that the SF02 also had an analog option.

Arlo’s original software obtained the distance data with a

10-bit ADC, but the number was scaled to eight bits. I was worried that eight bits would not be enough for the laser’s greater range and accuracy, but the SF02’s ability to be customized made it easy to solve the problem.

Parallax provides a program called the Lightware Terminal that lets you configure the SF02 in a variety of ways as detailed by Figure 9. The configuration options available on the SF02 proved to be exactly what I needed.

The SF02 outputs a voltage between 0 and 3. 3 volts (with 10 bits of precision) to indicate the distance measured. You can configure this output so that a full reading of 3. 3 volts indicates any valid distance. If the analog output is read with a 10-bit ADC, the resolution can be exceptional, but obviously it won’t fit in the eight bits needed to maintain compatibility with my original application.

I determined that a resolution of one inch was acceptable for my needs, which means that an eight-bit byte could hold distance measurements up to 255 inches. This distance (more than 21 feet) was also deemed acceptable for an indoor robot such as Arlo. Therefore, I set the laser’s maximum distance to 6. 22 meters (see Figure 9) which is 255 inches.

Most ADCs assume their input voltage has a range of 0V-5V. Remember, though, that the output from the laser is only 0V- 3.3V. Since 3.3/5 is essentially 2/3, we can effectively make the voltage a full range reading if we multiply it by 3/2. The reading will still be 10 bits, though, so we can shift it right twice (or divide by 4) to create the desired eight-bit number.

Both of these operations together simply multiple the acquired reading by 3 and divide that total by 8. This action proved to produce an extremely stable reading with reasonable distance and accuracy. It also shows the advantage of being able to configure the maximum range of the laser.

If your robot needs more accuracy or to be able to measure greater distances, there are easy modifications. Making the maximum range 12. 44 meters will let the laser measure more than 40 feet, but each unit in the eight-bit answer represents two inches.

Reducing the range to 3. 11

meters will improve the accuracy to 1/2 inch, but you will only be able to measure out about

10 feet. All things considered, the 6. 22 meter range and one inch accuracy seems to be a good tradeoff.

Of course, if you want to use a two-byte value for the laser reading, you can get both longer distances and improved accuracy.

The ability to configure Parallax’s laser sensors should not be taken lightly. As Figure 9 shows, you have many options. You can specify an offset to ensure, for example, that a zero distance is reported at your robot’s perimeter. You can even tell the sensor to smooth any jitter by providing you with an average of multiple readings. My experience with the SF02 showed no need for the averaging, but it is a nice feature when extreme stability is needed.

It is worth mentioning that the light emitted from a ranging laser can damage your eyesight if you look directly into the beam. For that reason, you should carefully evaluate the safety needs of any project that utilizes a laser.

The higher cost of adding a laser to your robot does not make sense for every application, but it is a viable option when your situation requires longer distances and improved reliability.