What is rotary joints?

With the continuous advancement of modern science and technology, radar technology has higher and higher requirements for the transmission frequency of the fixed part and the receiving signal, and often requires the transmission frequency to reach medium and high frequency signal transmission in the range of tens of megahertz or hundreds of megahertz.

In the field of microwave communication technology, the rotary joint is a very important communication device, which is mainly used in various radar equipment, including air control of aircraft, airspace defense, mobile communication, satellite signal transmission, etc. A swivel joint differs from a conventional adapter in that it can rotate about an axis perpendicular to the direction of wave propagation. This type of rotary joint can freely rotate 360 degrees while maintaining the transmission characteristics.

These joints are essential for rotating radar applications in all applications where rotation is required. Without a rotary joint, it is impossible to connect the rotating part of the signal chain to the fixed part. The insertion of the swivel part introduces more mismatch problems than the high-precision adapter.

The rotary joint is an integral part of the radar and a key component of the radar antenna. It generally belongs to a part of the antenna feed subsystem, and its function is a device that can realize energy transmission in a rotating state. It can form electrical energy and reception between the rotating part of the radar and the fixed part. Even under the condition of non-contact structure of the transmission line, the rotation is a component that can realize microwave energy transmission.

During the working process of the radar, the rotating joint is used to realize the transmission of microwave signals between the dynamic antenna system and the static transmitting system and receiving system. At present, there are many types of rotary joints on the market, including microwave rotary joints, microwave coaxial radio frequency rotary joints, capacitive coupling coaxial joints, capacitive rotary joints, and column-wound coaxial joints.

The microwave rotary joint is a connection device used to realize the transmission of radio frequency signals between two relative rotating mechanisms. It is mainly used to keep the continuous transmission of radio frequency signals to the fixed platform during the 360° infinite continuous rotation of the continuous rotating platform.

For example, in the radar antenna system, the microwave rotary joint is one of the key components to ensure the normal operation of the antenna. When the antenna rotates 360°, it can transmit the microwave signal from the transmitter to the antenna, or from the antenna to the receiver. , to realize the search of azimuth and elevation.

Microwave rotary joints can be divided into contact type and non-contact type according to the contact form. The contact type is mainly coaxial rotary joint, which has many advantages such as wide transmission frequency, small size, and strong versatility. The disadvantage is the general life Shorter, fewer loops.

Non-contact rotary joints mainly include ring coaxial joints with internal conductor planing and coaxial non-contact joints. The non-contact rotary joint adopts an inner and outer two-layer coaxial coupling nesting structure. The machining accuracy of the inner and outer conductors is not high and the assembly concentricity is poor, which causes the change of the position of the moving ring excitation probe relative to the fixed ring microwave choke coupling groove, thus affecting its delay stability.

The coupling of electrical signals of coupled rotary joints is based on alternating current, and the inability to pass DC signals is a major feature of this type of rotary joint. When the radio frequency signal is transmitted through the coupling mechanism, its transmission characteristic is a function of frequency, which determines that this coupled rotary joint must be a frequency-limited device, and its operating frequency bandwidth is narrower than that of the contact type. much.

In addition, the size of the coupling mechanism in this rotary joint is usually related to the frequency of the radio frequency signal that needs to pass. Generally speaking, the lower the frequency of the signal that needs to pass, the larger the physical size of the corresponding coupling mechanism, resulting in the final size of the rotary joint. Therefore, the rotary joints with lower operating frequency that we see on the market usually have a larger volume, which is the reason.

At present, the non-contact type is mainly based on the waveguide rotary joint. Its main advantages are long life and simple structure. The disadvantages are generally large volume and narrow transmission frequency band. Coaxial rotary joints are serial components that must be used when connecting static RF systems with rotating RF lines. These swivel joints are often used with rotating antennas and radars.

Specifically, coaxial rotary joints are used in air traffic control, image transmission, medical/industrial and telecommunications control, and land-based, ship-based and air-based radar. Unlike other coaxial converters or series interconnects, coaxial rotary joints require additional mechanical considerations. These factors include an average rotational speed limit expressed in revolutions per minute (RPM). These limit values are usually determined by the bearings and mechanical assemblies of rotating systems.

Prolonged operation at speeds exceeding the average maximum speed limit may result in early failure of rotational capability and may degrade RF performance. Because coaxial rotary joints are series coaxial interconnects connecting rotary systems, their input and peak power requirements can be very stringent. When the power limit is reached or the operating range is exceeded, the rotational performance of the swivel joint may be reduced and the swivel joint may be more susceptible to damage.

Microwave coaxial radio frequency rotary joint is also called high frequency slip ring and radio frequency slip ring. It is a high frequency slip ring specially designed for radar, satellite antenna and other equipment. Devices for high-speed serial digital signals or analog signals such as microwave signals.

This series of devices can support single-channel or multi-channel high-frequency signal transmission alone, and can also support high-frequency signal and 24V control signal, communication signal, power supply, and fluid medium mixed transmission. The high-frequency signal uses a 50Ω characteristic impedance RF coaxial structure head. Other designated connectors can be transferred.

Radio frequency rotary joints include coaxial rotary joints, waveguide rotary joints, also called high frequency slip rings, radio frequency slip rings. One end of the high-frequency rotary joint is fixed and the other end rotates, which is used for continuous rotation to transmit high-frequency signals.

High-frequency slip rings can transmit high-speed serial digital signals and analog signals, and the maximum transmission rate can reach 40GHz. It supports single-channel or multi-channel high-frequency signal transmission alone, and can also support high-frequency signal and 24V control signal, communication signal, power supply, and fluid medium mixed transmission.

The role of high-frequency slip rings in radar is to connect the rotating part of the radar with the fixed part, and to transmit electricity and signals. Whether the slip ring works normally or not has a great impact on the radar. If the slip ring fails, false alarms such as normal elevation angle limit and dead angle limit will occur many times, and the radar will not work properly in severe cases.

With the further development of radar technology, the quality requirements for the data transmitted by the rotating part are getting higher and higher, and the amount of data transmission is increasing. In addition to the high-precision inner and outer conductors, the most important thing for the stability of signal transmission is stable coupling. gap. However, the brushes of traditional slip rings are easy to wear and other problems, which are not suitable for the use of radar.

The complex structure of the rotary joint lies in the joint limitation. Sometimes, some special cases may need to control the joint angle. Joint limits enforce joint angles to remain within a certain range. to limit the free rotation of the object on the revolute joint. In joint limits, include zero values for the joint angles, otherwise objects will be tilted when starting the simulation.

The capacitive rotary joint is the most widely used rotary joint. The structure of the capacitive rotary joint is a stator component and a rotor component, and the two components are directly assembled into a whole. According to the current manufacturing technology, commercially available capacitive rotary joints are generally dual-channel rotary joints, which are composed of internal and external channels, and each channel is composed of internal and external conductors. The transmission performance stability, delay stability and consistency are poor, and the performance is poor The cause of the difference is caused by the difference in concentricity of the mechanical assembly of the inner and outer conductors or the deformation of the inner and outer conductors caused by the temperature cycle.

This commercially available capacitive rotary joint adopts channel separation assembly design, and the assembly coaxial error is relatively large. Moreover, the dual-channel signal transmission conductor is thin (0.5mm), and the non-contact capacitive isolation gap is as small as 0.2mm. It is difficult to ensure the cylindricity and coaxiality of the rotary joint during the precision manufacturing and assembly process, in . Therefore, the existing capacitive dual-channel rotary joint has short service life, poor stability of delay transmission performance and difficult assembly.