What Is A Slip Ring?
A slip ring is used where it is necessary to send power or data or other electrical signals across a rotating interface. The interface may be rotating incomplete revolutions in one direction continuously, or it may rotate incomplete revolutions in either direction, or it may oscillate through parts of a revolution, depending on the application.
A slip ring assembly (SRA) has three main parts:
- A rotating part with conductive rings that are separated by insulation material. The majority of SRA’s have the conductive rings arranged with their planes parallel to each other to form a cylinder. A minority of SRA’s have their conductive rings arranged concentrically, in the same plane; this is a “pancake” SRA. Which layout is chosen is usually determined by the shape of the space envelope that is available for the SRA design in the system into which it fits. In either design type, the conductive rings can be made from solid gold or gold-plated brass or solid silver or silver-plated brass, depending on the application’s requirements. We always “diamond turn” our rings to produce a mirror-class finish. Pandect gold rings usually have extremely precise v-grooves cut into them, also using “diamond turning”.
- The second part is made up of stationary brushes that are the main part of the stator. In Pandect these are usually made from a gold alloy or from silver-graphite. Our brush blocks have a minimum of two brushes per ring and these will have different lengths so that they will have different resonant frequencies.
- Terminations to get to power, data and signals in and out of the SRA. Sometimes customers require these to be flying leads and the cabling depends on the particular specification of the customer’s needs. Sometimes the solution that is required is to have connectors mounted on to the stator body and on to the body of the rotor; in this, the choice of the connector type and their specs may be critical to the application and so may their orientation and spacing. A third solution is to have the cables that emerge from the solution being terminated by connectors after some cable length. A termination solution that includes connectors makes testing of the SRA more efficient because it is easier to use automatic test equipment (ATE).
Pandect sometimes designs and provides slip rings solutions with the rotary rings part supplied separately from the brushes. This allows the customer to have more flexibility in the solution’s integration into the customer’s system. However, most designs that Pandect provides to our customers are integrated SRA’s combining the rotary rings and the stationary brushes in a rigid body which includes bearings and perhaps seals (depending on the environment in which the SRA has to operate).
The variety of voltages and currents in different Pandect SRA solutions is enormous. An SRA for the guidance system of a man-portable surface-to-air missile (SAM) may have a few rings with small voltages and small currents. It will be tiny and very light. An SRA for a land-based long-range radar may have so many rings (hundreds) that two or three groups of them may be arranged concentrically and some of them may carry many hundreds of amps and volts.
Pandect may supply an SRA solution that includes other rotary joints: An example is a satellite-on-the-move application where some sort of land/marine/airborne vehicle has to communicate continuously with a satellite and this solution from Pandect may be an SRA with a microwave joint integrated inside it. Another example is a high definition camera in a targeting system for an armoured vehicle. The data rate of the camera may be so high as to need a fibre-optic rotary joint to be part of the Pandect SRA solution. A third example is a large radar where Pandect needs to integrate into the SRA solution a rotary joint to provide cooling to the radar’s electronics either by water/glycol or air.