New Wind Sensor Takes Flight to Improve Drone Performance
Researchers have developed a more efficient wind sensor to improve the take-off and landing safety of drones, balloons, and other autonomous aircraft.
The demand for autonomous air vehicles is on the rise—with some forecasts predicting a leap in market value from $4.56 billion in 2019 to $16.23 billion by 2027. Within this market, wind sensors (or "anemometers") are critical for monitoring wind speed and direction during take-offs and landings. Wind sensors are essential to optimize the trajectory of aircraft and smaller aerial vehicles. They also forecast and optimize the performance of wind turbines.
An autonomous aircraft in the National Airspace System. Image (modified) used courtesy of NASA
Currently, aerial vehicles use pitot tubes to measure airspeed. These tubes contain moving fluid that is brought to rest. The stagnation pressure, along with the other parameters, is used to determine the fluid flow velocity or airspeed. However, these tubes require electric heaters to prevent fluid from freezing, which degrades the efficiency of small and micro-aerial vehicles.
To improve the safety of autonomous aircraft, researchers at Ohio State University have recently developed an airfoil-shaped anemometer that is lightweight, low-power, and more sensitive to changes in pressure. What design challenges of existing anemometers were the researchers up against?
Types of Anemometers
Many of the current wind sensors are expensive, consume high energy, or have a high atmospheric drag, making them ill-suited for small aircraft.
The most common anemometer is a mechanical anemometer. It contains a wheel with cups or propellers at the end of the spokes of the wheel, and one of them has a magnet attached. Each time the magnet passes a magneto-sensitive switch, it provides a reading of the wind speed. However, the size, slow response time, and high aerodynamic drag of mechanical anemometers make them unsuitable for aerial applications.
Hot-wire anemometers are small and highly accurate. Here, the temperature gradient is measured by a thermistor using a bridge circuit. The heating element needs to be constantly heated, resulting in high energy consumption.
Diagram of a revolving cup electronic anemometer. Image used courtesy of Encyclopaedia Brittanica
Another popular type of wind sensor is a drag anemometer, which measures wind speed based on deformations caused by airflow in active materials such as piezoelectric sheets. While they are small and consume less power, drag anemometers have limited sensing range and accuracy and are highly temperature dependent.
Sensor-based approaches like acoustic anemometers measure the transit time or phase difference between the transmitted and received acoustic waves, but they require complex signal processing for reliable results.
New Anemometer with Smart Materials
A team of researchers at Ohio State University has developed an airfoil anemometer for smart tether systems that stabilize aircraft. The sensor is an airfoil-shaped sleeve that fits over the tether system to reduce aerodynamic drag.
The team used a polymer called polyvinylidene fluoride (PVDF) for the airfoil, which is piezoelectric, to power the anemometer from energy generated from wind pressure. The sensor integrates data processing, wireless communication, energy harvesting, and other blocks.
Schematic of the airfoil-shaped anemometer. Image used courtesy of Frontiers
The airfoil acts as a wind vane, orienting itself in a stable "angle of attack." This is the angle between the chord (to which the foil is attached) and the airflow direction. The angular position can be used to determine wind direction, and the changes in voltage or capacitance of the PVDF film are related to wind speed. The angular position is determined by a magnetometer integrated into the airfoil. It provides wind direction relative to the Earth's magnetic field.
The New Airfoil Anemometer in Action
The researchers tested their device in a sealed chamber to determine its sensitivity. Then, they tested it in a wind tunnel. The sensor worked extremely well in both these tests.
Experimental setup for anemometer tests in a sealed chamber. Image used (modified) courtesy of Frontiers
The researchers noted that further improvements are needed to deploy this sensor technology in commercial applications. They continue to work on PVDF and other smart materials. Marcelo Dapino, the study's co-author and a professor in mechanical and aerospace engineering at Ohio State University, hopes their work will impact applications like wind turbines for generating clean and efficient energy.