Figure 2: Parts of a slipring unit
(Courtesy of B-COMMAND GmbH.)
A slipring unit is an assembly for transferring data, signals, or electrical power from a fixed component to a moving element.
In the case of a radar unit with a constantly rotating antenna, the antenna cannot be connected to a fixed power supply by cable because of its constant rotation. Not only the transmission energy has to be transmitted from the fixed part (stator) of the turntable (pedestal) to the rotating part (rotor) using an RF rotary transformer but also power supply voltages and a large number of switching and control signals as well as data lines. This transmission is made with slip rings often.
The traditional structure is as shown in Figs. 1 and 2. Ring-shaped contact surfaces are embedded in a carrier cast from insulating material in the form of a hollow cylinder, on which a spring contact allows current to flow during rotation. The conductive surfaces are usually hard gold-plated. To minimize wear, the contact spring presses against the rotating annular contact surface as weakly as possible but still sufficiently strong to ensure reliable current flow.
In the example shown in Fig. 2, the contact spring consists of thin, gold-plated steel wire cast into a so-called brush holder. If larger currents have to be transmitted, the contact spring consists of large-area carbon contacts. The abrasion of the carbon surfaces creates graphite dust which must be removed at regular intervals during maintenance cycles to prevent short circuits. Gold-plated contacts, on the other hand, are maintenance-free.
Figure 3: Pancake slip ring
(Courtesy of B-COMMAND GmbH.)
Pancake Slip Ring
Pancake slip rings consist of two flat discs facing each other. Concentric contact rings are arranged on one disc. On the opposite disc, spring contacts are fitted to match. These are often double or even quadruple. They can also be distributed over the entire surface of the disk to distribute the contact forces evenly.
This design has the advantage that it can be very flat. However, with a very large number of transmission channels, the diameter becomes very large and thus limits the maximum possible speed.
However, other connection types, such as fiber optic technology or encapsulated, radio hermetically sealed WLAN-like connections, are also possible. The functional unit of several such connections is then usually called a slipring unit anyway. With all these alternative transmission methods, either at least the power supply must be transmitted via two or three slip rings, or a rotary transformer is used for this purpose.
The rotary transformer is a special type of transformer in which the secondary side can rotate freely mechanically concerning the primary side and the power is transmitted exclusively by the alternatively oscillating magnetic field. To reduce the size of the rotary transformer, a higher frequency (up to 22 kHz) than the usual mains frequency is used for transmission, as in switching power supplies.