Slip rings are electromechanical devices which continuously transmit power and electrical signals from a static component to a rotating component and vice-versa.
Such devices have many names, including rotary joints, electric rotary joints, sliding contacts, etc.
It is clear that this solution, with its capacity for continuous rotation, greatly simplifies assembly and does away with the need for complex articulated cabling systems and their associated risk of damage and downtime, thus drastically reducing maintenance requirements.
Depending on the transmission technology, slip rings can transmit power, data or both together; their power transmission capacity ranges from mW to MW, according to the needs of the application and configuration; data can be digital or analogue I/O, including fieldbus data up to 1 Gb/s.
Given the vast range of uses of slip rings, a variety of contact technologies have been developed which employ diverse configurations and materials. There are currently 4 principal types of contact.
Conductive block slip rings (also called carbon brush slip rings) are probably the most simple configuration. They are used above all for low technology applications, typically for transmitting power or simple signals. The brushes consist of blocks shaped to contact the rings. The block material may vary from classic graphite for power transmission, to more complex precious metal alloys for signal transmission. The brushes are usually spring-loaded to maintain contact with the rings in case of shock and vibration and to compensate for wear. This technology is subject to a high rate of wear, and requires regular maintenance to prevent conductive dust collecting in the slip ring.
This used to be the most commonly used type, due to its high performance in a small package. It does not use a brush/ring configuration. Instead, it uses a conductive liquid metal to transmit power and signals between the static and rotary components. Mercury is the most common liquid metal used for this purpose; nowadays, a gallium (Ga) alloy is sometimes used.
However, this technology is used only in a few markets at this time due to the RoHS restrictions on the use of heavy metals, and since the metal is also toxic if it leaks.
This is currently the most widely used type due to its great flexibility in use, making it suited to a vast range of applications, from high power transmission to high frequency signal transmission. The brushes may be monofiber for small capsule slip rings (with carcass), or multifiber for larger slip rings. Multifiber brush slip rings offer the best service life and contact quality: wear and dust are reduced due to the low friction between the contacts.
Multifiber brushes are composed of a bundle of thin metal filaments which ensure high contact quality and low wear due to their flexibility. In this case too, as for the conductive block technology, a variety of contact materials and substrates may be used to optimise performance. Alloys of steel and other common metals are usually employed for power transmission, while a tempered gold alloy is used for signal transmission. Gold is used for the latter application due to the fact that it does not oxidise and the quality of the contact is thus not degraded over time.
Wireless slip rings are a new type of product. They work on the principle of capacitative or inductive coupling for power and signal transmission by exciting the coils at high frequency without contact between the rotating and static components, which can also be separated by special materials if required. In general, they are less efficient than other solutions, and the loss of efficiency is directly proportional to the distance between the coupled components. In reality, this solution is not yet very wide spread. Its main benefits consist in the lack of wear and the possibility of configurations with a high seal/protection rating. On the other hand, they can transmit power only up to 150 W, have low efficiency and they require active electronics to excite the coil. Fieldbus data transmission is generally provided with a Wi-Fi or Bluetooth module integrated into the electronics. This means that each fieldbus connection requires its own module, and if no commercial solution is available it must be developed for the specific application. This type of slip ring may be used in niche applications characterised by high rotational speed and poor access for maintenance.
A fiber optic rotary joint is the optical equivalent of an electrical slip ring. These devices enable continuous rotation of one or more optic fibers without affecting the signals transmitted along them. FORJs are widely used in medical systems (OCTs) and in other applications where a twist-free fiber cable is essential.
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