Diagram of the principle of ultrasonic technology, what are the advantages of applying it to UAV landing?

UAV landing assist is a function of the UAV, which can detect the distance between the bottom of the UAV and the landing area, determine whether the landing point is safe, and then slowly descend to the landing area. While GPS monitoring, barometric pressure sensing, and other sensing technologies aid in the landing process, ultrasonic sensing is the primary and most accurate judgement for the drone during this process. There are also hover and ground tracking modes in most drones, mostly for capturing footage and land navigation, where ultrasonic sensors help keep the drone at a constant altitude above the ground.

Principle of Ultrasound

Ultrasound is defined as the use of sound waves above the upper limit of human hearing – see Figure 1.

Diagram of the principle of ultrasonic technology, what are the advantages of applying it to UAV landing?

Figure 1: Ultrasonic Range

Ultrasonic waves can pass through various media (gas, liquid, solid) to detect objects with mismatched acoustic impedance. The speed of sound is the distance per unit time that a sound wave travels in an elastic medium. For example, in dry air at 20°C (68°F), the speed of sound is 343 meters per second (1,125 feet per second). Ultrasonic attenuation in air increases with frequency and humidity. Therefore, air-coupled ultrasound is generally limited to frequencies below 500 kHz due to excessive path loss/absorption.

Ultrasonic ToF

Like many ultrasonic sensing applications, drone landing assistance systems use the time-of-flight (ToF) principle. ToF is an estimate of the round-trip time of the ultrasonic waves emitted from the sensor to the target object and then reflected from the object back to the sensor, as shown in Figure 2.

Diagram of the principle of ultrasonic technology, what are the advantages of applying it to UAV landing?

Figure 2: Schematic diagram of ultrasonic ToF for drone landing

At point 1 in Figures 2 and 3, the drone’s ultrasonic sensor emits a sound wave, which is represented as saturated data on the return signal processing path. After sending, the signal processing path is muted (point 2) until the echo bounces off the object (point 3).

Diagram of the principle of ultrasonic technology, what are the advantages of applying it to UAV landing?

Figure 3: Phase of Ultrasonic ToF

Equation 1 calculates the distance from the drone to the ground or from the drone to another object:

Diagram of the principle of ultrasonic technology, what are the advantages of applying it to UAV landing?

 

Distance (d) is the distance from the ultrasonic sensor on the drone to the ground/object, ToF

Why use ultrasonic sensing for drone landings?

While numerous sensing techniques can detect the proximity of objects, ultrasonic sensing works well in terms of detection distance, cost of the solution, and reliability on different surfaces when the drone lands.

A common requirement for ground tracking and landing of drones is the ability to reliably detect distances up to 5 meters above the ground. Ultrasonic sensors in the 40-60kHz range can usually meet this range, assuming proper signal conditioning and processing.

Texas Instruments’ PGA460 is an ultrasonic signal processor and sensor driver for ultrasonic sensing in air-coupled applications such as drones that meet or exceed the 5-meter requirement. However, the coordination of ultrasonic sensing is a limitation in the near-field detection of objects. All ultrasonic sensors used in air-coupled applications have an excitation period, called the decay time or oscillation time, during which the piezoelectric film vibrates and emits ultrasonic energy, making it difficult to detect any incoming echoes.

To effectively measure objects during ringing, many drone designers install separate sensors for the transmitter and receiver. By separating the receiver, the drone can detect objects during excitation from the transmitter. So the PGA460 has superior near field detection performance – down to 5cm or less.

Ultrasonic sensing technology is also a cost-competitive technology, especially when using integrated solutions such as the PGA460, which already includes most of the required chips. The PGA460 can drive sensors either directly using a half- or H-bridge, or using a transformer; the latter is primarily used for hermetically sealed “hermetic” sensors. The PGA460 also includes a complete analog front end for receiving and conditioning ultrasound echoes. In addition, the device can calculate ToF through digital signal processing (see Figure 4).

Figure 4: PGA460 functional block diagram

Ultrasonic sensing can detect surfaces that are difficult to solve with other techniques. For example, drones often encounter glazing and other glass surfaces on buildings. Light-sensing technology sometimes passes through glass and other transparent materials, making it difficult for drones to hover over glass buildings. Ultrasonic waves are reliably reflected off the glass surface.

While ultrasonic sensing is primarily used for drone landing assistance and hovering, its robust price/performance ratio is prompting drone designers to explore other applications for the technology. The rapidly growing field of drones has enormous potential.

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