Servo - December 2014

distance measurement detectors used in robotics is the ultrasonic transducer; see Figure 2 for how they work.

These sensors not only can detect if objects are nearby, but with good accuracy, measure the distance between it and that object. This lets you use an ultrasound sensor to not only broadly determine if an object is nearby, but to check by how much.

Using the Parallax PING)))

Ultrasonic Sensor

Ultrasonic distance measurement is pretty much an old science. Polaroid used it for years as an automatic focusing aid on their instant cameras. To measure distance, a short burst of ultrasonic sound (usually at a frequency of around 40 kHz) is sent out through a transducer; in this case, the transducer is a specially built ultrasonic speaker. The sound bounces off an object and the echo is received by the same or another transducer. A circuit then computes the time it takes between the transmit pulse and the echo, and then comes up with distance.

At sea level, sound travels at a speed of about 13,560

inches per second. This time varies depending on atmospheric conditions including air pressure (which varies by altitude), temperature, and humidity.

The time it takes for the echo to be received is in microseconds if the object is within a few inches to a few feet of the robot.

The overall time between transmit pulse and echo is divided by two to compensate for the round-trip travel time between the robot and object. Given a travel time of For the PropBot, I’ve selected the Parallax PING))) — hereinafter referred to simply as Ping (to save on parentheses characters). This sensor is one of the most popular ultrasonic detectors used in hobby and amateur robotics. It’s not the cheapest such sensor you can buy, but it has great reliability. For sensors, it’s much better to spend a few more dollars for dependability. The Ping is easy to use, requiring just one signal line to the Propeller in addition to power and ground.

Figure 3 is a basic wiring diagram for the Ping showing the power, ground, and signal connection. Note the 1K Ω resistor in-line between the Ping and the Propeller I/O pin. Here’s why you need it: The Propeller is a 3.3V device; that is, it’s powered by 3. 3 volts, and its input/output pins are designed for 3.3V signals. The Ping is powered at five volts.

The good news is that the Propeller is designed to tolerate five volt inputs, as long as the current from the source is limited. So, even though the Ping is powered by five volts and its signal line outputs 5V, the 1K resistor safely limits the current reaching the Propeller.

You can use this same technique to interface most other types of five volt sensors to your Propeller (but check the datasheets just to be sure).

Figure 4 shows the hookup diagram between the Ping and Propeller Activity board (PAB). To make the wiring connection, I used a short three-wire servo extension which has three-pin female connectors on either end. On one end, simply slip the cable onto the Ping’s power and signal pins. On the other, use a three-pin double-long male header. This header is like the ordinary type of header with male pins, but on these the pins on both sides are long. This allows the pins to both plug into the female socket of the servo extension connector and into the solderless breadboard.