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Data interconnection

14 July, 2020

The Fourth Industrial Revolution drives digital manufacturing forward by implementing new scenarios into the production process. These scenarios rely on fundamental design principles that include device interconnection, information transparency, technical assistance, and decentralised decisions.

The realisation of all these principles in modern smart factories would not be possible without advanced wireless communication technologies. They enable multifaceted applications for a broad range of areas including process automation, asset tracking, machinery control, intralogistics, and infrastructure networking. Smart Machines & Factories reports.

Smart factories incorporate various cyber-physical systems that require faster and more reliable wireless solutions to handle ever increasing amounts of data in the toughest industrial settings. The main drivers facilitating new developments of these solutions to be deployed in highly demanding Industry 4.0 scenarios include the implementation of mobile SCADA, replacement of legacy systems, and realisation of data transmission from moving equipment where it was not possible or was limited before. This article focuses on the wireless technologies driven by this latter aspect.

Industrial requirements

A rotary joint, also often interchanged with the term slip ring, is an assembly for transmitting data and power across a rotating connection. The growing need for faster and more reliable data transmission between rotating components in modern industrial scenarios imposes strict requirements on bandwidth, crosstalk, and EMI performance of the data interfaces used in rotary joints. Meeting these requirements is essential to guarantee real-time operation, continuous uptime, and maximum efficiency of the corresponding industrial equipment.

Industrial rotary data interface assemblies must ensure constant transmission quality at very fast rotational speeds of 5000 rpm to 6000 rpm at rates of typically 100 Mbps. In most cases these data rates are sufficient, but some specialised applications require faster transmission at 1 Gbps and higher, which is becoming a fairly standard benchmark nowadays. Industrial applications also call for support of IEEE802.3-based (Ethernet) and other industrial bus protocols, as well as deterministic real-time communication, to permit time sensitive applications and IIoT functionality. Data interface solutions designed for these applications must be immune to physical misalignments, electromagnetic interferences, and crosstalk to enable error-free data transmission with bit error rates (BER) of 1 × 10−12 or better. Contaminants present in the industrial environment should not affect the operation of a rotary joint that ideally must be maintenance-free and not suffer from wear. Finally, the data interface technology must be compatible with power transmission subsystem of a rotary joint assembly to meet all functional requirements of a target application.

Data interface technologies

There are different types of rotary joints that vary in terms of their functional features, form factor, rotational speeds (rpm), maximum data rate, power ranges, type of supported interfaces, channel count, and many other design aspects shaped by application requirements. Among these design considerations, the data interface has some of the most critical requirements and it is therefore crucial to make the right choice of technology for its implementation in a slip ring assembly. Data communication technologies used to realise this function can generally be classified into contacting and contactless. They abound with many variations depending on the type of coupling they utilise in order to realize a communication channel for data transmission.

Contact-type interfaces

Contact-type solutions typically rely on composite, monofilament, or polyfilament brushes on a stator that slide against conducting rings on a rotor, thereby creating an uninterrupted passage of electrical signals between moving and stationary components. The selection of brush type with regard to data communication depends on the signal bandwidth, data transfer rate, required transmission quality, operational currents, and rpm. Although this is a well-established technology that has been employed in slip rings since their invention, it has certain limitations. Reliability of contact-type slip rings suffers in harsh operating environments due to the presence of mechanical contacts requiring regular maintenance.

Contactless interfaces

Contactless rotary joints overcome those limitations by using radiating or non-radiating electromagnetic fields to transfer the data across rotating parts. This technology offers several performance advantages over electrical signal transmission. The lack of mechanical contacts avoids contact wear requiring less maintenance and does not suffer from data loss arising from resistance at high rotational speeds.

Fiber optic rotary joints: The most common example of a contactless solution is a fiber optic slip ring known as a fiber optic rotary joint (FORJ). FORJs rely on optical radiation to transfer data and operate typically at infrared wavelengths between 850 nm and 1550 nm allowing EMI-free transmission of every kind of analog or digital optical signal at very high data rates of several dozens of Gbps. However, fiber optic solutions are not without challenges. They experience strong extrinsic losses that result in signal attenuation caused by angular and axial misalignments. These misalignments are also the main contributor to rotational signal fluctuations that can be critical in some applications. Moreover, fiber optic rotary joints usually require high levels of protection in harsh industrial environments.

Inductive and Capacitive Interfaces: Another type of contactless technology is based on near-field coupling mechanisms realised through electric and magnetic fields generated by primarily non-radiating inductive and capacitive circuit elements in lower frequency bands of the electromagnetic spectrum.

Inductive method applies the principle of electromagnetic induction to interface moving parts of an assembly. Slip rings using this type of coupling, are useful for industrial applications with high rotational speeds but they are more suitable for transmission of power rather than for transmission of high speed data. They are widely used in wind turbine applications to provide electrical signals and power for blade pitch control systems, and in packaging applications where moving parts run at high rpm.

In contrast to inductive slip rings relying on a magnetic field, the slip rings based on capacitive technology use electric fields to transfer data between a rotor and a stator.

The capacitive coupling method offers the realisation of a relatively low cost and light- weight solution with negligible eddy current losses and excellent misalignment performance.

Other types of interfaces

Apart from contactless slip ring technologies using primarily inductive or capacitive coupling mechanisms, solutions relying on a combination of both types of mechanisms can be realised by using proper coupling structures such as waveguides or transmission line elements. There are also special types of slip rings: for instance, those relying on mercury as a conduction media. However, mercury-wetted slip rings have strict demands for operational environment settings and cannot be used at high temperatures, which makes them not suitable for industrial use cases.

Millimeter wave data interface solution

60 GHz frequency band

The emergence of low cost microwave component fabrication technologies has recently made them a commercial reality for broad market applications beyond the military space. In particular, millimeter wave 60 GHz technologies are receiving increased attention from today’s broad market due to the advantages of this frequency band located in the upper region of the microwave spectrum. This global license-free and largely uncongested band offers a wide bandwidth of up to 9 GHz, which permits high data rates, provides short wavelengths that allow for a compact system design, and that has a high ratio of attenuation, which results in a low interference level. These benefits made 60 GHz technologies attractive for such applications as multigigabit WiGig networks (IEEE 802.11ad and the next-generation IEEE 802.11ay standards), wireless backhaul connectivity, and wireless transmission of high definition video (a proprietary WirelessHD/UltraGig standard).

Integrated data interface architecture

Anton Patyuchenko, Analog Devices field applications engineer, proposes a millimeter wave data interface solution using 60 GHz frequency band for industrial slip ring applications. The key functional element of this solution is the Analog Devices 60 GHz integrated chipset comprised of the HMC6300 transmitter the HMC6301 receiver. Patyuchenko explains that this complete silicon-germanium (SiGe) transceiver solution was originally optimised for the small cell backhaul market and it meets the data communication requirements of industrial slip ring applications. The chipset operates in the frequency range of 57 GHz to 64 GHz, and it can be tuned using either an integrated synthesiser in discrete frequency steps of 250 MHz, 500 MHz, or 540 MHz, or with an external LO signal to meet the specific modulation, coherency, and phase noise requirements of the target application.

The transceiver chipset supports a wide variety of modulation formats including on-off keying (OOK), FSK, MSK, and QAM with the maximum modulation bandwidth of 1.8 GHz. It offers a maximum output power of 15 dBm that can be monitored using an integrated detector. The chipset features flexible digital or analog IF/RF gain control, a low noise figure, and adjustable low-pass and high-pass baseband filters. One of the unique features making this solution ideal for ultralow latency industrial slip ring applications is an integrated AM detector in the receiver signal chain that can be used for demodulation of amplitude modulations such as OOK.

OOK is a very popular modulation method for control applications because it does not require the use of expensive and power-hungry high-speed data converters, thus enabling implementation of simple and low-cost communication solutions. Moreover, since OOK system architecture does not include complex modulation and demodulation stages, it offers low latency performance, which is important for industrial real-time applications.

Discrete data interface architecture

Performance capabilities of the proposed integrated solution are sufficient for most industrial slip ring applications, but the general trend toward customisation of industrial components may require an implementation of even faster data interfaces supporting multigigabit speeds. In this case, it is possible to configure a customised solution to meet specific requirements using discrete components.

This discrete proposal is based on a single detection system architecture. However, depending on the performance requirements can also help realise a superheterodyne architecture by down converting the RF signal before the video detection stage.

Industry 4.0 is driving change in many technologies and one such change is industrial communication. New application scenarios shaped by the Fourth Industrial Revolution demand faster, more reliable, and more accurate ultralow latency data transmission between rotating parts of automation equipment operating in real time.

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