A slip ring is also called a rotary electrical interface or electrical rotary joint. It transfers power and data signals from a fixed structure to a rotating structure where continuous rotation is required. The transfer can use sliding or rolling contact, electrostatic coupling, or electromagnetic coupling. In an antenna mount, slip rings carry electrical signals and power between the rotating and stationary parts. Rotating parts often carry drive elements, measurement elements, and control elements. These need control signals and power from the equipment room, so slip ring assemblies are used. Some radar systems need more than one hundred circuits through a rotating joint.
By signal frequency, slip ring assemblies fall into low-frequency slip rings, medium-frequency slip rings, and high-frequency rotary hinges. The term slip ring usually refers to the first two.
Electrical performance parameters include insulation resistance, contact resistance, dielectric strength, and crosstalk.
For medium-frequency slip rings, higher frequency requires attention to shielding, impedance matching, and noise voltage. The mechanical design must first ensure reliable contact and continuous connection on all circuits. The brush material needs good conductivity. Brush pressure on the ring must be set correctly. The ring should have small eccentricity and runout, good wear resistance, low friction torque, and easy maintenance.
Low-frequency slip rings use sliding contact to transmit low-frequency signals and power. Common types are cylindrical slip rings and differential slip rings. The conductive rings in a cylindrical slip ring can be flat rings or V-shaped rings. Ring materials are usually copper, brass, coin silver, and gold.
Brushes include palladium, gold alloy, or gold-plated wire brushes, and copper graphite composite brushes. When many rings are needed, a cylindrical slip ring can use upper and lower brush groups with a differential transfer disc. This design has a large axial size. A differential slip ring reduces axial size, volume, and weight.
A differential slip ring has upper and lower brush groups and a differential transfer disc. The upper brushes rotate with the antenna azimuth. The lower brushes are fixed. The differential transfer disc has upper and lower contact pieces, and matching contact pieces are connected by wires. A differential mechanism makes the disc turn at one half the azimuth speed. When the antenna turns, current flowing into each lower brush passes through one or two contact piece circuits on the differential disc and leaves through the corresponding upper brush. This keeps the circuit between the fixed and rotating parts connected.
Wear powder from sliding contact can cause short circuits between rings. The structure should allow easy cleaning. Modular construction is common so parts can be repaired or replaced on site.
Medium-frequency slip rings transmit radar medium-frequency signals, in the tens of megahertz range, and power. These slip rings need shielding because of the higher frequency. Ordinary slip rings can transmit signals below 12 MHz. One ring connects the center conductor, and one ring connects the cable outer layer as a shield ring. For signals above 12 MHz, coaxial shielded slip rings are usually used. The cross section is groove-shaped, essentially a rectangular coaxial conductor.
There is also a capacitive medium-frequency slip ring. The center conductor is ring-shaped and supported by an insulating pad inside the shield layer. There is a gap between the rotating part and the fixed part, and they do not touch. The medium-frequency signal is coupled through capacitance. If the antenna rotation range is limited, a cable winding device can be used instead of a slip ring.
Hansheng Automation can machine custom slip ring housings, shafts, and contact components from customer drawings in single pieces or small batches.
