Slip ring assembly matching degree is 100%, and the new 60 GHz wireless solution meets the stringent requirements of industry

Figure 1. Overview of the Industrial Revolution.

Smart factories integrate multiple cyber-physical systems that require faster and more reliable wireless solutions to handle the growing amount of data in harsh industrial environments. The main factors driving the development of these solutions to be deployed in demanding Industry 4.0 scenarios include: implementing mobile SCADA, replacing legacy systems, or enabling data transmission through mobile devices (which was previously not feasible or limited). This article focuses on wireless technologies driven by the last factor.

The first part of this article outlines the main requirements for communication interfaces between mechanical rotating subsystems in modern industrial applications. The second part attempts to classify the various data interface technologies used in these subsystems today based on the type of mechanism used to transmit data between the rotor and stator. This part briefly summarizes these technologies and discusses their advantages and disadvantages. The third part introduces a new 60 GHz wireless solution that supports high-speed, low-latency communication, which enables advanced data interface architectures in slip ring assemblies to meet the stringent requirements of new industrial scenarios.

Rotary joints, also often referred to as slip rings, are components that transmit data and power in rotating connections (see Figure 2). Modern industrial scenarios require faster and more reliable data transmission between rotating parts. As this demand grows, the bandwidth, crosstalk and EMI performance requirements of the data interfaces used in rotary joints are becoming increasingly stringent. Meeting these requirements is critical to ensuring real-time operation, continuous normal operation and maximum efficiency of the corresponding industrial equipment.

Figure 2. Rotary joint—high-level block diagram and requirements.

Industrial rotary data interface components must ensure high-quality continuous data transmission at very fast rotation speeds (5000rpm to 6000rpm) at a typical data rate of 100Mbps. In most cases, this data rate is sufficient, but some special applications require fast transmission at rates of 1Gbps or higher, which has become a benchmark indicator today. Industrial applications also require support for protocols based on IEEE802.3 (Ethernet), other industrial bus protocols, and deterministic real-time communication to enable time-sensitive applications and IIoT functions. Data interface solutions for these applications must be able to be immune to physical misalignment, electromagnetic interference, and crosstalk, and achieve error-free data transmission with a bit error rate (BER) equal to or lower than 1×10-12 ^. Contamination in industrial environments should not affect the operation of rotary joints, and ideally rotary joints are maintenance-free and wear-free. Finally, the data interface technology must be compatible with the rotary joint assembly power transmission subsystem to meet all functional requirements of the target application.

There are many different types of rotary joints, and their functional characteristics, form factors, rotation speed (rpm), maximum data rate, power range, supported interface types, number of channels, and many other design factors vary depending on the application requirements. Among these design considerations, some requirements regarding the data interface are very important, so it is critical to select the appropriate technology to properly implement the data interface in the slip ring assembly. The data communication technologies used to achieve this function can generally be divided into contact and contactless. There are some differences between these technologies, depending on the type of coupling they use to achieve the communication channel for data transmission.

Figure 3. Contact-type slip ring. Image courtesy: Servotectica/CC BY-SA 4.0.

Contactless rotary joints address these limitations by using radiating or non-radiating electromagnetic fields to transfer data between rotating parts. This technology offers several performance advantages over electrical signal transmission technologies. It has no mechanical contact points, which results in no contact wear, reduced maintenance requirements, and no data loss due to impedance at high rotation speeds.

The most common contactless solution is the fiber optic slip ring, also known as the fiber optic rotary joint (FORJ), whose schematic is shown in Figure 4. FORJ relies on optical radiation to transmit data, usually operating at infrared wavelengths from 850 nm to 1550 nm, and can transmit various types of analog or digital fiber optic signals at very high data rates of tens of Gbps, and is not affected by electromagnetic interference. However, fiber optic solutions are not without challenges. They suffer from strong non-intrinsic losses, which cause signal attenuation due to angular and axial misalignments. These misalignments are also the main factor causing rotation signal fluctuations, which is very critical for some applications. In addition, fiber optic rotary joints usually require a high level of protection in harsh industrial environments.

Figure 4. Fiber optic rotary joint. Image courtesy: Servotectica/CC BY-SA 4.0.

Another non-contact technology is based on a near-field coupling mechanism, which is achieved through the electric and magnetic fields generated by primary non-radiating inductive and capacitive circuit elements in the lower frequency band of the electromagnetic spectrum.

The inductive method uses the principle of electromagnetic induction to connect the moving parts in an assembly. Slip rings using this type of coupling (schematic diagram shown in Figure 5) are very useful for high-speed industrial applications, but they are more suitable for power transmission rather than high-speed data transmission. They are also widely used in wind turbine applications to provide electrical signals and power for pitch control systems, and in packaging applications where moving components operate at high speeds.

Figure 5. Inductive coupling.

Slip rings based on capacitive technology use electric fields to transfer data between the rotor and stator, as opposed to inductive slip rings that rely on magnetic fields. The capacitive coupling method shown in Figure 6 provides a relatively low-cost, lightweight solution with negligible eddy current losses and excellent offset performance. This technology can reliably transmit data at high speeds of several Gbps in harsh operating environments, regardless of rotational speed. Capacitive slip rings are typically designed for use in combination with Ethernet fieldbuses and are widely used in time-sensitive industrial applications.

Figure 6. Capacitive coupling.

Table 1 summarizes the various data interface technologies we have discussed, which offer many features and functions to meet the requirements of typical industrial slip ring applications. However, most of these traditional technologies only support short-distance data transmission, which requires the transceiver components on the rotor and stator to be very close to each other. In addition, the Fourth Industrial Revolution also puts forward strict requirements on the configurability, reliability and speed of the data interface for slip ring applications, which are often not met by existing traditional technologies.

Table 1. Classification of rotary joints based on data interface coupling technology

This article introduces a new solution based on contactless technology, which relies on electromagnetic millimeter waves to transmit data over long distances in the radiative near-field (Fresnel) and far-field regions, solving some key limitations of other methods. This solution not only provides a compact and cost-effective advanced microwave data interface for slip ring applications, but can also be combined with the coupling elements of traditional non-radiative rotary joints to achieve better performance at a lower cost.

The emergence of low-cost microwave component manufacturing technology has enabled it to be widely used in various commercial markets outside the military field. In particular, 60GHz millimeter wave technology is gaining increasing attention in the market due to its unique advantage of being located in the upper half of the microwave spectrum. This globally unlicensed and largely unoccupied frequency band can provide a wide bandwidth of up to 9GHz, support high data rates, provide short wavelengths that enable compact system design, and have a high attenuation ratio, resulting in low interference levels. These advantages make 60GHz technology attractive for applications such as multi-gigabit WiGig networks (IEEE802.11ad and next-generation IEEE802.11ay standards), wireless backhaul connections, and wireless transmission of high-definition video (proprietary WirelessHD/UltraGig standards).

In the industrial sector, 60 GHz technology is mainly used in millimeter wave radar sensors and telemetry links with lower data rates. However, with the rapid development of this field, 60 GHz technology is likely to enable high-speed, ultra-low latency data transmission in industrial subsystems.

This article introduces a new millimeter wave data interface solution for industrial slip ring applications using the 60 GHz frequency band. The key functional element of this solution is ADI's 60 GHz integrated chipset , which consists of the HMC6300 transmitter and the HMC6301 receiver , and their schematics are shown in Figure 7 and Figure 8, respectively. This complete silicon germanium (SiGe) transceiver solution was originally optimized for small cell backhaul applications and fully meets the data communication needs of industrial slip ring applications. The chipset operates in the 57 GHz to 64 GHz frequency range and can be tuned in discrete frequency steps of 250 MHz, 500 MHz, or 540 MHz using an integrated frequency synthesizer or can be tuned using an external LO signal to meet the modulation, coherence, and phase noise requirements specific to the target application.

Figure 7. Functional block diagram of the transmitter HMC6300.

Figure 8. Functional block diagram of the HMC6301 receiver.

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

OOK is a modulation method commonly used in control applications because it does not require the use of expensive and power-hungry high-speed data converters, thus enabling a simple, low-cost communication solution. In addition, since the OOK system architecture does not include complex modulation and demodulation stages, it can provide low latency performance, which is very important for industrial real-time applications.

ADI transmitter HMC6300 and receiver HMC6301 integrated solutions are both packaged in a small 4mm×6mm BGA package, combining features and performance advantages in a unique way to meet the stringent requirements of modern high-speed slip ring applications. In addition to the core transceiver components, the complete concept design of the full-duplex slip ring data interface also includes antennas, power management, I/O modules, and auxiliary signal conditioning components, which can be selected according to the needs of the target application. See Figure 9 for a detailed block diagram of the entire 60GHz full-duplex data interface solution concept. This solution enables high-speed, ultra-low latency data transmission at speeds greater than 1Gbps with negligible bit error rates. Reliable communication can be achieved over a distance of tens of centimeters using appropriate antenna design and gain settings, which opens up opportunities for the widespread use of slip ring solutions in specific industrial scenarios.

Figure 9. Block diagram of a 60 GHz full-duplex data interface.

An example of a complete signal chain solution for a 60 GHz data interface supporting data rates above 5 Gbps is shown in Figures 10 and 11. This OOK solution is implemented using ADI’s standard RF components and basic custom blocks, including passive components, matching circuits, branch filters, bias tees, attenuators, etc. (not all components are shown in the figures).

Figure 10. Complete signal chain solution for a 60 GHz transmitter (OOK modulator).

Figure 11. Complete signal chain solution for a 60 GHz receiver (OOK demodulator).

This discrete solution is based on a single detection system architecture. However, depending on performance requirements, the RF signal can also be down-converted before the video detection stage, which helps to implement a super-heterodyne architecture.

Industry 4.0 is driving changes in many technologies, one of which is industrial communications. New application scenarios driven by the fourth industrial revolution require faster, more reliable and more accurate ultra-low latency data transmission between rotating components of automation equipment running in real time.

ADI offers a wide range of high-performance integrated and discrete RF and microwave components covering the entire frequency spectrum to support specific application designs for contactless Gbps-level data transmission through rotary joints. This article introduces an integrated and discrete data interface solution that uses millimeter-wave electromagnetic waves to achieve data transmission between the rotor and the stator. The solution introduced in this article not only provides high-speed data transmission, ultra-low latency, negligible bit error rate, strong interference attenuation, and maintenance-free operation, but also can withstand higher levels of misalignment, support data transmission over longer distances, and support a wider range of slip ring components to meet the growing needs of modern industrial applications.

ADI provides Industry 4.0 partners with deep industrial domain expertise and experience in next-generation capabilities to help develop faster, more cost-effective and advanced solutions for today’s factory infrastructure, ready for the future.