An ultrasonic distance sensor is a contactless sensing device that measures the distance between two points using high-frequency sound waves. These sensors typically generate ultrasonic signals via piezoelectric transducers operating in air and perform measurements based on the time it takes for these signals to reflect off an obstacle and return to the sensor. This method is called the "Time of Flight" (TOF) technique.

Working Principle

An ultrasonic sensor measures the time of the echo generated when the sound wave it sends hits an object and reflects back. This time is converted into distance using the speed of sound propagation in air. The distance calculation is based on the following formula:

$S= \frac{v*T}{2}$

Where:

• $S:$ distance between the sensor and the object [m]
• $v:$speed of sound in air (~343 m/s, varies with temperature) [m/s]
• $T:$total round-trip time of the echo [s]

Since ambient temperature directly affects the speed of sound propagation, temperature measurement is also performed in some applications. Each 1°C increase in temperature causes an approximate 0.6 m/s increase in sound speed.

Structural Components

Ultrasonic distance sensors consist of the following main components:

• Piezoelectric Transmitter: Generates sound waves in the air, operating at a frequency such as 40 kHz.
• Piezoelectric Receiver: Detects the sound wave reflected from the obstacle.
• Timing and Processing Circuit: Measures the time of flight and sends a signal to the microcontroller.
• Control Interface: Data is transferred to the microcontroller or processor.

In sensors like the HC-SR04, triggering and signal reception are performed via "Trig" and "Echo" pins. The microcontroller typically measures the duration of the echo signal using a timer.

In more advanced sensors, the parametric acoustic array principle is used to increase directionality. In this method, two separate high-frequency waves (e.g., 80 kHz and 120 kHz) are sent, and by leveraging the nonlinear properties of the medium, a directional secondary wave is formed at the difference frequency of these waves (e.g., 40 kHz). These systems include components such as a wave generation unit, piezoelectric discs, linear horn, stepped radiation plate, and countermass.

Factors Affecting Measurement Accuracy

• Ambient temperature: Sound speed is temperature-dependent. Therefore, temperature changes affect measurement accuracy.
• Structure and position of the obstacle: The hardness, inclination, and position of the surface affect the intensity and direction of the reflected signal.
• Sensor geometry: The distance between the transmitter and receiver determines the effective measurement angle of the sensor.
• Noise level: Acoustic noises in the environment can cause errors in signal detection.

In some applications, threshold detectors, automatic gain control, and analog/digital timing circuits are used for evaluating the echo signal.

Areas of Use

Ultrasonic distance sensors are used in various application areas:

• Automotive: Suspension height measurement, automatic parking systems, ground detection.
• Robotics: Obstacle detection, distance measurement, environmental mapping.
• Industrial Automation: Level control, object detection, production line control.
• Education and prototyping: In student projects and robotic applications with cost-effective modules.

^{Usage Example of Ultrasonic Distance Sensor in Robotics (Micromelon Robotics)}

Advantages and Disadvantages of Ultrasonic Distance Sensors

Ultrasonic distance sensors have advantages and disadvantages as explained in the table below.

Example Systems and Performance Characteristics

The HC-SR04 sensor can measure in the range of 2–400 cm, with a measuring angle of $\pm 15$° . A trigger pulse of $10\mu s$ is sent for signal generation, and the echo duration is measured by the microcontroller and converted into distance. In some sensors, a temperature sensor can be integrated for sound speed correction.

In a system based on the parametric array principle, two fundamental waves are sent at frequencies of 80 kHz and 120 kHz. A secondary wave with high directionality is created at their difference frequency of 40 kHz. The beam width obtained in this design is approximately $\pm 1.3$° .

Example Project: Microcontroller-Based Distance Measurement

^{How To Use Ultrasonic Sensors with Arduino (Robonyx)}

The reference video presents an application example that addresses the integration of an HC-SR04 ultrasonic sensor with the Arduino microcontroller platform. The study examines the hardware connection process of the sensor and the development of the necessary software for distance calculation. The application is based on the principle of sending an ultrasonic pulse via the sensor's "Trig" pin and measuring the Time of Flight of the reflected signal on the "Echo" pin to convert it into distance. As a derivative of this basic setup, two different system configurations are also demonstrated:

1. Sonar Scanning System: A system that graphically maps the position of detected objects on a computer interface.
2. Intrusion Detection System: A system that notifies objects violating a predefined distance threshold via audible and visual alerts.