From IoT factories to operating rooms: How to design better communications systems

The foundation of Industry 4.0 is a reliable communications infrastructure. Decision makers use this infrastructure to extract data from machines, field devices, and factories. Ensuring the reliability of robots and human-machine interfaces begins with a deep understanding of the underlying technology options.

The factory floor and the operating room may be very different, but the equipment used must operate reliably and accurately, which is critical to the tasks they perform. As devices require smarter systems, more data, and higher fidelity, their demand for bandwidth continues to increase. At the same time, faster communication interfaces must provide the same reliability and safety while resisting environmental hazards and electromagnetic compatibility (EMC). EMC refers to the ability of a system to function as intended in its operating environment without generating or being unduly affected by electrical noise.

Robotics and Machine Vision

Vision-guided robotics can provide greater flexibility and increased production reliability in high-value manufacturing environments. Without vision guidance, robots can only perform the same tasks repeatedly until they are reprogrammed. With machine vision, robots can perform more intelligent tasks, such as scanning defective products on a conveyor belt in a production line and picking them up by a conditioned robot, as shown in Figure 1. In hazardous EMC environments such as factory automation, the reliability and effectiveness of the vision/robotics interface is determined by the selected wired transmission technology. There are many ways to implement machine vision camera interfaces, including USB 2.0, USB 3.0, Camera Link, or Gigabit Ethernet.

Table 1 compares several key specifications of the USB, Ethernet, and Camera Link standards. Industrial Ethernet offers several advantages, with cable lengths up to 100 meters for the 2-pair 100BASE-TX and 4-pair 1000BASE-T1 standards, and up to 1 km for a single twisted pair using the new 10BASE-T1L standard, with high EMC performance. Cable lengths of less than 5 meters are possible with USB 2.0 or USB 3.0, unless specialized active USB cables are used, and protection diodes and filters are required to improve EMC performance. However, the widespread adoption of USB ports in industrial controllers, with bandwidths up to 5 Gbps, offers some advantages to designers.

Figure 1. Camera machine vision and robotics—Ethernet, USB, or Camera Link interface

Camera Link requires dedicated frame grabber hardware in the industrial controller. USB or Ethernet do not require additional frame grabber cards in the industrial controller. Camera Link is a standard that was first introduced in late 2000 and is the most common interface for machine vision systems. Today, USB and Ethernet based machine vision cameras are more widely used, but applications that require pre-processing of multiple cameras still use Camera Link and frame grabbers to reduce the main CPU load. Compared to Gigabit Ethernet, even at basic speeds, the Camera Link standard outputs up to twice the amount of data and over shorter distances. The Camera Link physical layer is based on low voltage differential signaling (LVDS), which is inherently EMC robust because common mode noise coupled to each line is effectively cancelled at the receiver. The EMC robustness of the LVDS physical layer can be improved by electromagnetic isolation.

By using Ethernet on the camera and robot links, and industrial controllers with IEEE 802.1 Time Sensitive Networking (TSN) switches, the synchronization of industrial camera and robot operations can be maximized. TSN defines the first IEEE standard for time-controlled data routing in switched Ethernet networks. ADI offers a full range of Ethernet technologies, including physical layer transceivers and TSN switches, as well as system-level solutions, software, and security features.

Table 1. Communication interface standards for machine vision cameras

Human Machine Interface (HMI)

Human-machine interfaces (HMIs) are commonly used to display data from programmable logic controllers (PLCs) in a human-readable visual representation. Standard HMIs can be used to track production time while monitoring key performance indicators (KPIs) and machine output. Operators can use HMIs to perform a variety of tasks, including turning switches on or off and increasing or decreasing pressure or speed in a process. HMIs are typically equipped with an integrated display; however, HMIs with an external display option offer several advantages. HMI units with an external High-Definition Multimedia Interface (HDMI®) port are smaller and easier to mount into consoles that use standard DIN power rails and can also be used to monitor PLCs.

When using HDMI, cable lengths of up to 15 meters can be easily routed to touch displays and control rooms, as shown in Figure 2. Extending HDMI over longer cables is challenging in industrial environments because of EMC hazards that can affect the cabling. Indirect transient overvoltages can also occur on the HMI when motors and pumps are connected to DIN rail PLCs.

Ensuring system robustness requires careful selection of interface technologies. As Industrial Ethernet has grown rapidly, fieldbus technologies such as CAN or RS-485 have become popular. According to industry sources, there are more than 61 million RS-485 (PROFIBUS®) nodes installed worldwide, with PROFIBUS process automation (PA) devices growing 7% year-over-year. The PROFINET (an implementation of Industrial Ethernet) installed base is 26 million nodes, with 5.1 million devices installed in 2018 alone. 1 As mentioned earlier, high EMC performance can be achieved with Ethernet-based technologies because electromagnetics are written into the IEEE 802.3 Ethernet standard and must be used at every node. RS-485 devices can include electromagnetic isolation for improved noise immunity, and protection diodes can be integrated on-chip or placed on the communication PCB to improve immunity to electrostatic discharge and transient overvoltages.

HMIs often require immunity to electrostatic discharge and utilize ESD protection diodes to improve signal robustness. For industrial HMIs, integrated reinforced isolation protects operators from electrical hazards. While reasonable isolation solutions are available for Ethernet and RS-485, today, video transmissions are primarily isolated using expensive optical fibers that support Gigabit transmission speeds. Recent advances in electromagnetic isolation technology from Analog Devices, such as the ADN4654/ADN4655/ADN4656 family, which can exceed 1 Gbps data rates, provide designers with competitive, lower-cost alternative solutions.

Figure 2. Human-machine interface (HMI) with Ethernet and RS-485 inputs, and HDMI output

Endoscope

Surgical imaging, including endoscopy, is a unique application that must provide high-fidelity images while ensuring patient safety. Previous generation endoscopy devices, known as video endoscopes, used a series of glass lenses and a light guide to transmit the image from the imaging head to the charge-coupled device (CCD) sensor. Using visible light as the medium to transmit the image from the patient to the endoscope was an approach that isolated harmful currents, but it was not ideal in terms of manufacturing cost and image quality.

Recent surgical imaging devices have overcome these challenges by going digital and moving from CCD to CMOS sensors, which are easily scalable and can be embedded in the camera head. Using CMOS cameras eliminates the need for serially connecting multiple lenses and improves overall image quality. Lower production costs have enabled the use of disposable surgical endoscopes, which do not require sterilization concerns. Cameras have further shrunk, making minimally invasive surgery possible.

With the move to digital endoscopy, a high-speed electronic interface must be provided between the CMOS image sensor (in contact with the patient) and the camera controller unit (CCU). LVDS and Scalable Low Voltage Signaling (SLVS) layers are becoming the common physical layers for implementing this interconnect, providing high bandwidth and relatively low power. 4 This interface, unlike the one in video endoscopy, is currently electronic and may be capable of transmitting hazardous currents. Because it does not have the isolation of optical media, the system must be designed to isolate the patient from potentially harmful currents.

Figure 3. Electronic interface of a digital endoscope with a CMOS image sensor

Patient safety is paramount for any medical system connected to a mains power source. The IEC 60601 standard for medical electrical equipment places stringent requirements on components that protect the patient (MOPP) from harmful voltages. Transmitting image data using high bandwidth solutions while meeting these stringent safety requirements presents a significant challenge to system designers. An example of this is the transmission of electronic video from a CMOS image sensor to an endoscope CCU, which requires a safety-compliant, high-speed connection. ADI’s unique solution performs high bandwidth transmission within a trusted safety barrier to meet the requirements of the IEC 60601-1 standard.

Medical Displays

Other medical devices, such as ventilators and electrocardiograms (ECGs), are connected directly to the patient for breathing assistance and monitoring. Information about the patient is displayed on the medical device’s own graphical display for easy viewing by the operator. The display in this medical device is known, trusted, and certified for use as a medical device according to the IEC 60101 standard. This cannot be guaranteed for any off-the-shelf external TVs and monitors. To ensure patient safety, isolation should be added to the external connections between the medical device and peripherals to protect the patient. For traditional low-speed interfaces such as RS-232, RS-485, and CAN, this isolation may not be critical and can be implemented using standard digital isolators.