Ultrasonic Sound Waves

Abstract

Sonar Sensors have been widely used for distance bearing in underwater navigation and robotic navigation applications. Sonar Ranging is one of the most common forms of distance measurement used in Mobile Robotics. A speaker or Transducer is used to emit a short burst of sound or a Ping. The sound wave travels through the air and reflects off a target to create an Echo, which goes back to the Transducer. By measuring the time between when the Ping is emitted and the Echo was detected, one can calculate the distance between the target and Transducer.

Sonar Sensors send pings and receive echoes through the use of Piezo Electric Crystals, which are minerals such as Quartz, Rochelle salt, and PZT ceramic (PZT is lead-zirconate-titanate). These minerals form six-sided (hexagonal) crystals or masses of crystals. When a voltage is applied to these are masses or crystals they expand and contract. Then they emmit a signal or ping at their natural frequency, 40 kHz. 40 kHz is an ultra-sonic frequency which puts it beyond the human hearing ability.

Introduction

By using the characteristics of Piezoelectric Crystals we hope to be able to get two separate sonar sensors to interact with one another. Through the use of their pings and echoes we want the receiver of one sensor to recognize a Ping sent out by another Transducer at the same frequency as an echo and to register a signal.

Methods: Set-up

Step One: Assemble the station from which the Sonar Sensors were going to be working off of. Hardware used included two Devantech SRF04 Ultra-Sonic Rangers, One Object-Oriented Programmable integrated circuit (OOPic) chip, one Bread Board, 4 Light Emitting Diodes, a +9 volt and +5 volt power supply, miscellaneous wires, and power adaptors.

Step Two: Get one Sonar Sensor to send, receive, and recognize it’s own signal. This was done by modifying and downloading template computer program written in PBASIC to the OOPic chip.

Step Three: Get the Sensor to recognize distances by entering desired values into the computer program. Light Emitting Diodes monitor this process.

Step Four: of the process was to find a way of attaching another Sonar Sensor to the same OOPic chip that operated the first sensor. The program for the OOPic Chip was then modified so that two Sonar Sensors were able to work off of the same OOPic chip, but they were still working independently (sending and receiving their own signals).

Methods: Technique

When the entire setup was complete the ST170 Air Leak Detector was used to determine which component of the Sonar Sensor was the receiver and which was the sender. With the receiver and sender identified we were able to experiment with techniques and strategies that would allow the two Sonar Sensors to communicate with one another. These included blocking (muffling) signals and echoes. The receiver of the sending Sonar Sensor was muffled and the transducer of the receiving Sonar Sensor was muffled. The Air Leak Detector was once again used to determine if there was any noise escaping from any of the muffled areas. With all the sound leaks properly muffled we were able to get the two Sensors communicating. With this achieved we started experimenting with their acoustic properties and abilities.

Results

The two Sonar Sensors were able to communicate due to the properties of the Piezo Electric

Crystals. The frequencies of the two sensors were similar enough so that the receiver was able to process

the sound wave of the sent signal.

It was also discovered that when the sound only had to travel from a transducer to a receiver the range could be increased 2 or more times under certain conditions. When the Sonar Sensor had to send a signal and receive the echo the distance could not be increased more than its’ recommended range.

Light Emitting Diodes were able to monitor the activity of the Sonar Sensors. The program used for the OOPic used high (1) and low (0) values as ways of activating the LEDS. When the receiver was receiving a signal the value became high (1) and the Light Emitting Diode was lit. When there was no signal being read the value dropped to low (0) and the Light Emitting Diode was turned off. Two sets of LED’s were used with one set to monitor the execution of the program and another to monitor the activities of the sensors.

Conclusion

When a voltage is applied to the Piezoelectric Crystal of the transducer it emits an ultra-sonic wave or ping that is sent to the receiver. The ultra-sonic wave strikes the Piezoelectric Crystal of the receiver, which in turn creates a voltage that activates the receiver and registers that it is receiving a signal. The Piezoelectric Crystals emit a ping at the same frequency of 40 kHz, which means the receiver of a Sonar Sensor is not able to distinguish if the echo that it is receiving is its own or the echo from another transducer. Therefore Sonar Sensors are able to communicate between one another just as they would with themselves.