A sensor stirred up the World Cup and made a big surprise!

This technology makes Argentinean fans cry and the host host fans panic. It is a new technology applied in the Qatar World Cup - semi-automatic offside penalty technology, and the core of this technology is actually the IMU sensor embedded in the football.

On the evening of November 22, Argentina lost 2-1 to Saudi Arabia, becoming the biggest surprise of this World Cup.

In this game, Argentina only scored one penalty, three of which were ruled invalid due to offside. In addition, 10 offside decisions were made in this game, which also set a new high in the number of offside decisions in a single game since the application of VAR (Video Assisted Referee) technology in the 2018 World Cup. This World Cup has upgraded VAR and added semi-automatic offside penalty technology (SAOT).

In fact, this technology has already shown its edge in the opening battle.

In the opening game between Qatar and Ecuador, Ecuador scored a goal in the first three minutes. However, the referee considered the goal invalid after referring to the data given by VAR, and the shooter only offside less than half of the football when scoring.

Many media have introduced the role of the VAR system. The biggest change is that an IMU sensor is built into the official designated ball "Al Rihla", which can send signals at a frequency of 500Hz. Then, through the integration and analysis of the artificial intelligence system, it can accurately determine the football landing point and passing point. So the question is, what is the IMU sensor? How does it judge whether a player is offside?

Built in sensor of official ball Al Rihla

What is an IMU sensor?

The full name of IMU sensor is Inertial Measurement Unit (IMU), and its Chinese name is Inertial Measurement Unit (IMU). It mainly uses the principle of inertia to detect the rotating motion and acceleration of objects. IMU is usually composed of accelerometers in three directions and gyroscopes in all directions. It is the main structure for measuring inertia and motion. The 3 (accelerometer)+3 (gyroscope) structure is also a common 6-axis IMU sensor.

IMU accelerometer

Let's analyze the functions of the IMU in this sphere from the perspective of requirements. Whether offside or not requires accurate judgment of when the football is kicked away and the final destination of the football.

First, we need to measure when the football is kicked out. We can measure the instantaneous acceleration of the football to judge, which is inseparable from the three-way accelerometer in IMU. Accelerometers can be used to measure static gravitational acceleration and dynamic acceleration caused by impact, motion, collision or vibration (low frequency and ultra-low frequency vibration).

Common MEMS accelerometers are piezoelectric, piezoresistive and capacitive. Among them, MEMS capacitive accelerometer has many application scenarios. It detects acceleration based on capacitance change. MEMS capacitive accelerometer works by the inertia of the movable part of the structure.

The movable part is a cantilever structure. When the overall force of the sensor exceeds its supporting force, it will move, and the capacitance on both sides of the cantilever will also change. The change of capacitance is proportional to the acceleration. The change of capacitance is converted into digital signal and output after zero point and sensitivity correction. Capacitive accelerator has the advantages of simple manufacturing process, small temperature coefficient, good stability, and easy control of damping coefficient. It can measure acceleration in different ranges by changing the strength and elastic coefficient of the cantilever, so it has been widely used.

Cantilever capacitive accelerometer

MEMS capacitive accelerometer is mainly used on wearable devices and mobile devices, such as smart watches and mobile phones. Compared with piezoelectric and resistive accelerometers, its biggest advantage is that it can be installed on a PCB, but its measurement accuracy is low, and it is not suitable for measuring high-frequency changes. Therefore, it is not suitable for industrial measurement and aerospace fields, but it is more than enough for measuring acceleration on football.

In addition, because of Magnus effect, a spinning football in the air will draw an arc to bypass the defenders and shoot into the goal, which is also the principle of "banana ball". The rotation speed and direction of a football determine the bending angle of its flight path and ultimately its landing position. Therefore, the gyroscope that can detect the angular velocity of the football is very important. It is impossible to measure the complete rotation attitude of the IMU simply by the acceleration sensor.

Magnus effect

The gyroscope is composed of gyro rotor, inner and outer frames, driving motor and signal sensor. Its working principle is to use the conservation principle of angular momentum and the Corio effect to measure the angular velocity of moving objects. Similar to the working principle of the accelerometer, the upper active metal and the lower metal of the gyroscope form a capacitance. When the gyroscope rotates, the distance between it and the lower capacitance plate will also change, and the upper and lower capacitance will also change accordingly. The change of capacitance is proportional to the angular velocity, so we can measure the current angular velocity.

As a whole, IMU sensors can be divided into stable platform system and trapdown system. Literally, the first type is mainly used for application scenarios that need to remain stationary, such as PTZ and camera stabilizer, while the second type of sensor needs to be fixed on the object to be detected. The sensor platform will integrate the angle information in real time through the gyroscope, and use the obtained angle information to transform the coordinate axis of the acceleration information. After coordinate axis conversion, gravity effect correction is carried out, and finally, position information is obtained by two integrations.

How to transmit the flight parameters of football to the outside

After obtaining the precise parameters of football flight, the IMU sensor also needs to transmit the data to the host outside the field for judgment by the referee. There are many ways of data transmission, such as using Bluetooth, WIFI, ISM and other common communication methods. However, we must first solve the interconnection between IMU and communication chips ² C and SPI are the current common choices.

I ² C generally refers to I ² C bus. I ² C bus is a simple, two-way two-wire synchronous serial bus developed by Philips Company. It only needs two lines to transfer information between devices connected to the bus. With this mode, the relationship between the master and the slave, transmitter and receiver on the bus is not constant, and the transmission depends on the data transmission direction at this time.

If it is only necessary to transmit signals unilaterally from the sensor in the football, that is, the host will receive the data sent from the device. The process is that the host addresses the slave device first, then the host receives the data sent from the device, and finally the host terminates the receiving process. In addition, I ² C is a multi host bus. If two or more hosts initiate data transmission at the same time, data destruction can be prevented through conflict detection and arbitration. Any device can act as a host or slave, but only one host can work at the same time.

SPI is the abbreviation of Serial Peripheral Interface. It is a high-speed, full duplex, synchronous communication bus ² C, SPI only allows one host. SPI's hardware performance is stronger, and only four lines are occupied on the chip pins, saving the chip pins, so the function saves PCB space. Whether I ² C or SPI, both of which are used for short distance communication in the board. To achieve wireless transmission to the off-site host, a wireless communication module is also required.

A Possible Wireless IMU Scheme

There are also many ways to choose for wireless IMU communication. From the actual situation of the stadium, if conventional Bluetooth communication is used, it will be very unstable in an environment with many people and equipment. If the signal transmission causes delay or packet loss, it may seriously affect the referee's judgment of the match results. Although the transmission speed of conventional WIFI is faster, but the transmission range is not large, it will still be interfered by off-site signals. In addition, the WIFI module usually consumes a large amount of power and is not suitable for carrying in a small space. Therefore, the wireless transparent transmission technology different from conventional Bluetooth and WIFI is a better choice.

Transparent transmission is also called serial port transparent transmission. Transparent transmission is transparent transmission. The received data can be transparently transmitted to the target device through the connection between the serial port and the target device. Transparent transmission is a working mode. It can be transmitted based on Bluetooth or WIFI technology. It does not need to pay attention to the implementation of various protocols when the underlying signal is transmitted. Therefore, it is simple, stable and reliable. It can be thought of as a connected line. When there is data transmission, data will be sent from one end of the connection to the other, and no other signal processing will be carried out. This also reduces the energy consumption of information at the transmitting end. Therefore, using the transparent transmission mode of WIFI for communication can not only take advantage of the long distance transmission advantages of WIFI, but also avoid the disadvantages of high energy consumption and instability.

Summary

When judging whether a player is offside or scoring effectively, the referee needs to know the position and final landing position of the football at the moment before it is kicked out. The IMU sensor integrating accelerometer and gyroscope can accurately measure each parameter. These data will also pass the I ² C or SPI bus is transmitted to the integrated wireless transmission module, and then transmitted to the off-site host through WIFI transmission mode. When a goal is scored, the VAR technology combined with the off court multi angle high-speed camera and the football internal sensor can help the referee to make accurate judgments and realize various "millimeter offside" judgments.