The methods for measuring wind speed and direction actually existed long ago. The famous story of Zhuge Liang using the east wind to set fire to the wall is a testament to his effective understanding of wind direction and speed, which led to a major military victory.
As a weather measurement device, wind speed sensors and wind direction sensors , which measure the direction and magnitude of wind, are widely used in various industries. Let's take a look at these two types of devices.
Wind direction sensor
A wind direction sensor is a physical device that detects and senses external wind direction information by rotating a wind direction arrow, transmits it to a coaxial encoder, and outputs corresponding wind direction values.
Wind direction sensors typically use a mechanical structure similar to a wind vane. When wind blows towards the tail fin of the wind vane, the arrow on the vane will point in the direction the wind is blowing. To maintain directional sensitivity, different internal mechanisms are used to help the wind speed sensor determine direction. These generally fall into three categories:
Electromagnetic wind direction sensor: Designed using electromagnetic principles, its structure varies due to the variety of principles. Currently, some of these sensors have begun to use gyroscope chips or electronic compasses as basic components, which has further improved their measurement accuracy.
Photoelectric wind direction sensor: This type of wind direction sensor uses an absolute Gray code disk as its basic component and employs a specially customized encoding method. Based on the principle of photoelectric signal conversion, it can accurately output the corresponding wind direction information.
Resistive wind direction sensor: This type of wind direction sensor uses a structure similar to a sliding rheostat, marking the maximum and minimum resistance values as 360° and 0° respectively. When the wind vane rotates, the slider of the sliding rheostat will rotate with the wind vane at the top, and the different voltage changes generated can be used to calculate the angle or direction of the wind.
Wind speed sensor
A wind speed sensor is a common sensor that can continuously measure wind speed and air volume (air volume = wind speed x cross-sectional area).
Wind speed sensors can be broadly classified into mechanical (mainly propeller-type and cup-type) wind speed sensors, hot air wind speed sensors, pitot tube wind speed sensors, and ultrasonic wind speed sensors based on acoustic principles.
Working principle of propeller-type wind speed sensor
We know that an electric fan uses an electric motor to drive the fan blades to rotate, creating a pressure difference in front of and behind the blades, which propels the airflow. The working principle of a propeller-type anemometer is exactly the opposite. The blade system aligned with the airflow is subjected to wind pressure, generating a torque that causes the blade system to rotate. Typically, propeller-type speed sensors measure wind speed by rotating a set of three- or four-bladed propellers around a horizontal axis. The propellers are usually mounted in front of a wind vane, ensuring their plane of rotation always faces the direction of the wind, and their rotational speed is proportional to the wind speed.
Working principle of cup-type anemometer
The cup-type anemometer is a very common type of wind speed sensor, first invented by Robert Robinson in England. The sensing element consists of three or four conical or hemispherical hollow cups. The hollow cup shells are fixed to a triangular support at 120° intervals or a cross-shaped support at 90° intervals. The concave surfaces of the cups are aligned in one direction, and the entire horizontal arm is fixed to a vertical rotating shaft.
When the wind blows from the left, wind cup 1 is parallel to the wind direction, and the component of the wind pressure on wind cup 1 in the direction most perpendicular to the wind cup axis is approximately zero. Wind cups 2 and 3 intersect at a 60-degree angle with the wind direction. For wind cup 2, its concave surface faces the wind and experiences the greatest wind pressure; wind cup 3, with its convex surface facing the wind, experiences less wind pressure than wind cup 2 due to the airflow around it. Because of the pressure difference between wind cups 2 and 3 in the direction perpendicular to the wind cup axis, the wind cups begin to rotate clockwise. The greater the wind speed, the greater the initial pressure difference, the greater the acceleration, and the faster the wind cups rotate.
After the wind cups start rotating, cup 2 rotates with the wind direction, so the wind pressure it experiences is relatively reduced. Meanwhile, cup 3 rotates against the wind at the same speed, so the wind pressure it experiences is relatively increased. The wind pressure difference continuously decreases. After a period of time (when the wind speed remains constant), when the pressure difference acting on the three wind cups reaches zero, the wind cups begin to rotate at a uniform speed. Thus, the wind speed can be determined based on the rotational speed of the wind cups (the number of revolutions per second).
When the wind cup rotates, it drives the coaxial multi-toothed disc or magnetic rod to rotate. A pulse signal proportional to the wind cup's rotational speed is generated by the circuit. This pulse signal is counted by a counter, and the actual wind speed value can be obtained after conversion. Currently, most new rotating anemometers use a three-cup design, with conical cups performing better than hemispherical ones. When the wind speed increases, the rotating cup can quickly increase its rotational speed to adapt to the airflow. When the wind speed decreases, due to inertia, the rotational speed cannot immediately decrease. In gusts, the wind speed indicated by a rotating anemometer is generally too high, known as the over-inflation effect (resulting in an average error of approximately 10%).
Working principle of thermal anemometer
Thermal anemometers use a hot wire (tungsten or platinum wire) or a hot film (a thin film made of platinum or chromium) as a probe, exposed to the air being measured. This probe is connected to a Wheatstone bridge, and the air velocity at the measured cross-section is detected by the balance of resistance or current in the bridge. The hot film anemometer has an extremely thin quartz insulating layer coated on top to insulate it from the fluid and prevent contamination. It can operate in airflows containing particles and has higher strength than metal hot wires.
When the air temperature remains constant, the power consumed by the heating wire is equal to the heat instantaneously dissipated by the wire in the air. The resistance of the heating wire changes with temperature, and within the normal temperature range (0–300℃), the resistance and temperature of the heating wire exhibit a linear relationship. The heat dissipation coefficient is related to the airflow velocity; the higher the velocity, the higher the corresponding heat dissipation coefficient, meaning faster heat dissipation; conversely, a lower velocity results in slower heat dissipation.
The airflow velocity measured by a thermal anemometer is a function of current and resistance. By keeping the current (or resistance) constant, the measured airflow velocity corresponds one-to-one with the resistance (or current).
Hot-wire anemometers come in two circuit designs: constant current and constant temperature. Thermostatic hot-wire anemometers are more commonly used. The principle of the constant temperature method is to maintain a constant temperature of the hot wire during measurement, keeping the bridge circuit balanced. At this point, the resistance of the hot wire remains constant, and the airflow velocity is only a single-valued function of the current. Based on the known relationship between airflow velocity and current, the airflow velocity passing through the end device can be calculated. Constant current hot-wire anemometers maintain a constant current value flowing through the hot wire during measurement. When the current value is constant, the airflow velocity depends only on the resistance of the hot wire. Based on the known relationship between airflow velocity and hot wire resistance, the airflow velocity passing through the anemometer can be calculated.
Hot-wire anemometers can measure fluctuating wind speeds. Constant-current anemometers have relatively high thermal inertia, while temperature-controlled anemometers have relatively low thermal inertia and higher speed response. The measurement accuracy of hot-wire anemometers is not very high, and temperature compensation should be considered during use.
Pitot tube anemometer working principle
Pitot tubes, also known as airspeed tubes or windspeed tubes, are tubular devices used to measure the total pressure and static pressure of airflow to determine its velocity. They were invented by the Frenchman H. Pitot and are named after him.
Directly measuring airflow velocity experimentally is difficult, but airflow pressure can be easily measured using a manometer. A Pitot tube is primarily used to measure aircraft speed, but it also has other functions. Therefore, the pressure can be measured using a Pitot tube, and the airflow velocity can be calculated using Bernoulli's theorem. A Pitot tube consists of a double-walled tube with a rounded head (see figure). The outer tube has a diameter of D, and a total pressure hole (0.3–0.6D) is opened at the center O of the rounded head, connecting to the inner tube. One end of the manometer is connected to this hole. A row of static pressure holes, perpendicular to the outer tube wall, is evenly opened circumferentially at point C, approximately 3–8D from O. The other end of the manometer is connected to this hole. The Pitot tube is placed in the steady airflow in which the velocity to be measured, with the tube axis aligned with the airflow direction and the leading edge of the tube facing the incoming flow. As the airflow approaches point O, its velocity gradually decreases, stagnating to zero at point O. Therefore, the total pressure P is measured at point O. Secondly, because the pipe is very thin and point C is sufficiently far from point O, the velocity and pressure at point C have essentially returned to the values equal to the incoming flow velocity V and pressure P. Therefore, the pressure measured at point C is static pressure. For low-velocity flow (where the fluid can be approximated as incompressible), the formula for determining the flow velocity using Bernoulli's theorem is:
Based on the total pressure and static pressure difference PP measured by the pressure gauge, and the density ρ of the fluid, the velocity of the airflow can be calculated according to equation (1).
Working principle of ultrasonic wind speed sensor
The working principle of an ultrasonic anemometer is to measure wind speed using the ultrasonic time-of-flight method. Since the speed of sound in air is superimposed on the airflow speed in the direction of the wind, if the ultrasonic wave propagates in the same direction as the wind, its speed will be faster; conversely, if the ultrasonic wave propagates in the opposite direction to the wind, its speed will be slower. Therefore, under fixed detection conditions, the speed of ultrasonic waves in air can be correlated with a function of wind speed. Accurate wind speed and direction can then be calculated. Because the speed of sound waves in air is greatly affected by temperature, and the anemometer detects two opposite directions on two channels, the effect of temperature on the speed of sound waves can be ignored.
The ultrasonic anemometer is lightweight, has no moving parts, is robust and durable, and requires no maintenance or on-site calibration. It can simultaneously output wind speed and direction. Customers can choose the wind speed unit, output frequency, and output format according to their needs. A heating device (recommended for use in cold environments) or analog output can also be selected. It can be used with computers, data acquisition units, or other data acquisition devices compatible with RS485 or analog output. Multiple units can also be networked for use if needed.
Ultrasonic anemometers are a relatively advanced instrument for measuring wind speed and direction. Because they effectively overcome the inherent limitations of mechanical anemometers, they can operate normally around the clock and for extended periods, leading to their increasingly widespread use. They will be a powerful alternative to mechanical anemometers.
Features of ultrasonic anemometers:
1. Employs acoustic phase compensation technology for higher accuracy;
2. By employing random error identification technology, low discrete error in measurement can be guaranteed even in strong winds, resulting in more stable output;
3. The measurement compensation technology for light rain and dense fog has stronger environmental adaptability;
4. Digital filtering technology provides stronger resistance to electromagnetic interference;
5. No starting wind speed limit, works at zero wind speed, suitable for measuring indoor breezes, no angle limit (360° all-around), and simultaneously obtains wind speed and wind direction data;
6. High measurement accuracy; stable performance; low power consumption and no calibration required;
7. The structure is robust and the instrument is highly corrosion-resistant, so there is no need to worry about damage during installation and use;
8. Flexible design, lightweight, portable, easy to install and disassemble;
9. Convenient signal input, providing both digital and analog signals;
10. No maintenance or on-site calibration required; truly 0–359° working angle (no blind spots).
Applications of wind direction and speed sensors
Although wind direction and wind speed sensors are two completely independent sensors, in most cases they are integrated into the same measuring device and work together by comprehensively processing the data.
Applications of wind direction and speed sensors in meteorology
In the field of meteorology, it is often necessary to observe many kinds of natural phenomena, such as changes in wind speed and weather, and of course, changes in wind direction. For wind direction measurement, the current solution is basically to use anemometers or wind direction sensor equipment.
Measurement of changes in surface wind direction: In the work of wind and sand control in desert and plateau areas, people usually need to pay attention to the speed of airflow and changes in wind direction in order to obtain more meteorological data and formulate more comprehensive control plans. Therefore, meteorological equipment such as wind direction sensors are used in the whole process.
Marine storm warning: It can be said that the marine meteorological warning system is one of the important applications of wind direction sensors in the meteorological field. The wind direction change data provided by the marine meteorological warning system is one of the important parameters for predicting the coverage area and "trajectory" of typhoons.
Application of wind direction and speed sensors in coal mining
Ventilation equipment installed in mines often comes in different models and has significantly different operating power. Therefore, it is necessary to use wind speed sensors to monitor the wind speed in each ventilation duct to prevent the occurrence of excessively high concentrations of harmful gases due to low ventilation rates in certain locations.
In order to ensure the safe operation of coal mines of all sizes, relevant regulations require the installation of wind speed sensors in coal mines. Wind speed sensors should be installed in every mining area, wing return airway, and main return airway. Since the tunneling face is part of the mining area, it is necessary to install wind speed sensors on the tunneling face.
In fact, there is another major reason why wind speed sensors need to be installed at the tunnel face: harmful gases such as methane, carbon monoxide, and gas are most likely to appear at the tunnel face in coal mines. In some cases, the gases in the underground "gas chambers" are directly harmful gases. Therefore, wind speed sensors need to be installed at every location in coal mines and connected to ventilation equipment.
Applications of wind direction and speed sensors in wind power generation
In order to make better use of wind resources, modern large wind turbines usually control the rotor direction not by the tail fin, but by the wind direction sensor. The wind direction sensor is usually installed on the top of the wind turbine, but it is necessary to prevent the rotor from obstructing the sensor's measurement. If the height of the sensor reaches a certain level, people also need to pay attention to lightning protection and leakage protection for the generator and the sensor.
Wind direction sensors typically installed near wind fields serve two main purposes:
1. Ensure that the wind turbine blades are aligned with the wind direction in real time to ensure that they are in normal working condition.
2. Wind direction measuring instruments on meteorological stations near wind farms can ensure that strong winds do not pose a threat to wind turbines.
Application of wind direction and speed sensors in tower cranes
Typically, to ensure the progress of construction projects, most tower cranes are equipped with wind speed sensors. These sensors can trigger an alarm when strong winds affect the crane's operation. However, once strong winds begin to affect the crane's operation, it is often necessary to pay attention to changes in wind direction in order to take appropriate measures for different wind directions. Therefore, some cranes have already adopted wind direction sensors.
Applications of wind direction and speed sensors in air conditioning and ventilation equipment
Variable air volume (VAV) terminal units are one of the main components of VAV air conditioning systems. Anemometers are a critical component of VAV terminal units; therefore, the type and performance of the anemometer directly affect the quality of airflow detection and control within the system. Currently, VAV terminal units manufactured in my country and Europe/America primarily use Pitot tube anemometers, while Japanese manufacturers generally do not.
Applications of wind direction and speed sensors in the aviation field
The pitot tube on an aircraft is a typical pitot tube anemometer and a crucial measuring tool. It must be installed in an area outside the aircraft where airflow is minimally affected by the aircraft itself, typically directly in front of the nose, the vertical stabilizer, or the wingtip. As the aircraft flies forward, airflow rushes into the pitot tube, and the sensor at the end of the tube senses the impact force of this airflow, known as dynamic pressure. The faster the aircraft flies, the greater the dynamic pressure. By comparing the static pressure (pressure of still air) with the dynamic pressure, we can determine the speed of the incoming air, and thus the aircraft's speed. The instrument used to compare these two pressures is a hollow, circular box with a corrugated surface, called a diaphragm, made of two thin metal sheets. This box is sealed but connected to the pitot tube by a tube. If the aircraft travels at a high speed, the dynamic pressure increases, causing the pressure inside the diaphragm to increase, and the diaphragm to bulge. A device consisting of small levers and gears measures this deformation of the diaphragm and displays it with a pointer; this is the simplest form of an aircraft airspeed indicator.
The static pressure measured by the pitot tube can also be used as a parameter for calculating the altimeter. If the diaphragm is completely sealed, the pressure inside remains equivalent to the air pressure at ground level. Thus, when the aircraft flies into the air and its altitude increases, the static pressure measured by the pitot tube decreases, causing the diaphragm to bulge. Measuring the deformation of the diaphragm allows the determination of the aircraft's altitude. This type of altimeter is called a barometric altimeter.
The speed measured by the pitot tube is not the aircraft's actual speed relative to the ground, but only its speed relative to the atmosphere, hence the name airspeed. If there is wind, the aircraft's speed relative to the ground (called ground speed) should be added to the wind speed (for tailwinds) or subtracted from the wind speed (for headwinds).
With the development of modern science and technology, new types of wind speed sensors, such as lasers, have begun to be used in wind speed detection. It is believed that in the near future, various new wind direction and speed sensors will be increasingly used in various fields such as construction machinery, railways, ports, docks, power plants, meteorology, cableways, environment, greenhouses, and aquaculture.
