Key factors for design verification of IIoT sensor nodes

How do you bring new technology to an already extant install base?

Director of Technology, The PTR Group

As the IoT continues to expand, we see a large interest in the use of distributed sensor nodes in the industrial space. However, many industrial applications fall into what is commonly referred to as “brownfield” applications, which require new hardware/software solutions to co-exist with existing solutions. In addition, many of these in situ solutions may be more than 20 to 30 years old. So how do you bring new technology to an already extant install base?

First, we need to understand the nature of the current problem set being addressed. In the industrial space, we are frequently talking about the online monitoring of existing physical processes. These could range from monitoring the temperature of the content in a crude oil pipeline to monitoring the amount of chlorine in water as it leaves a water treatment facility. In the industrial space, they tend to focus on simplicity of installation and reliability of the sensor technology. In some cases, the state of the art may not have progressed very far; that is, the good old way of doing it may be good enough. In other applications, there may be a significant advance in the technologies that could result in a lower cost of ownership and better reliability.

Next, we need to understand the connectivity involved with the current sensor system. This ranges from the type of physical connectivity (e.g., RS-232/422, RS-485, 20-mA current loop, Ethernet, etc.) to the protocols. Some of these protocols could be predicated on the use of synchronous serial communications à la HDLC, Bisync, DDCMP, X.25, and others. Or they may be using other industrial standards such as Profibus, Modbus, EtherCAT, DeviceNet, or even custom protocols.

Also, the nature of many industrial applications has a wide variety of harsh environments that they have to survive in, including extended temperature ranges, shock and vibration, high humidity, unreliable power, and intermittent connectivity. And a characteristic of the typical industrial operator is to run the equipment for as long as they can, even though there may be a “better” solution, to reduce costs by using already amortized equipment until it dies. Further complications that are often encountered in industrial infrastructure is that the solution may have been designed 30 years ago and all of the designers have moved on or retired. This leaves the current operators with a large number of unknowns when they start contemplating a phased rollout of a new sensor system.

Fortunately, the sensor marketplace now offers a broad selection of alternative approaches. Many aging analog components have now been replaced with microelectromechanical systems (MEMS) that are much more robust, smaller, and more power-efficient than their original counterparts. Often, these sensors are already designed to take advantage of new communications media such as LoRaWAN, NB-IoT cellular, Sigfox, or even Wi-Fi.

As much as the industrial operator may want to continue to use their existing cable plant, that may not be practical. Old industrial communications interfaces may not be practical for the updated sensor systems. Even if they are available, they may be cost-prohibitive. A set of trade studies will need to be undertaken to determine the most cost-effective communications approach. If the decision is to go with a wireless solution, then the designer will need to investigate the practicality of the solution given the physical characteristics of the system to be upgraded.

Before beginning any significant update of an existing system, the designers should conduct a simple prototype test that reproduces as many of the operational constraints as possible. This can be accomplished using one of the many development kits that are available on the market. These will often include a commercial-off-the-shelf (COTS) development board, an operating system such as Linux, and a COTS radio solution.

If we plan to replace hardline wiring with a wireless solution, there will need to be a site survey in the radio frequency (RF) spectrum to make sure that the frequency bands will work for the application. Oftentimes, industrial applications will require large motors or other sources of RF interference. Alternatively, just the presence of large metal structures, such as those found on an oil platform, will be enough to limit the effectiveness of a wireless solution. This sort of survey should be performed before the radio selection so that the printed-circuit-board (PCB) designers and software developers will have sufficient information to make informed decisions about the wireless approach.

The process of upgrading brownfield applications with new physical sensors will often begin with the verification of the new sensors at the silicon designers. Referred to as physical verification, this process involves a number of electronic design automation (EDA) tools, including a design rules check (DRC), layout versus schematic (LVS), electrical rule check (ERC), exclusive OR (XOR), and antenna checks.

Fortunately, there are many co-design suppliers that provide an automated suite of tools to perform these checks. The verified design will then be handed off to a silicon foundry and the circuits will be produced using an appropriate fabrication approach to meet the requirements (e.g., extended temperature or shock and vibration) of the original design. From here, the packaged chips go off to a PCB designer for board-level design.

For highly sensitive or mission-critical applications, the PCB designer must ensure that there exists a validated and audited supply chain. In the past, there have been documented cases of counterfeit chips, typically with lower quality and higher-than-normal chip mortality issues, which compel developers of board-level solutions to verify that the component suppliers are meeting the reliability requirements for the components. This is particularly true for applications that are part of the critical infrastructure such as power production and water processing.

Once the board-level solution is ready, it will need to be subjected to a series of environmental tests to validate the performance in the environments where the sensors will be deployed. This includes thermal cycling, shock and vibration testing, RF emissions and susceptibility testing (including FCC certification), and a host of other tests depending on the severity of the environment where the sensor will operate. For some applications, such as high humidity, the boards may also need to be conformal-coated or “potted” to protect the circuitry from the environment.

Assuming that the sensor application will use some sort of microcontroller or other processor (often needed for the encryption of communications or management of the wireless solution), the software design team will need to make a broad spectrum of decisions about whether or not an operating system will be needed, the software design philosophy, the communications protocols, and how to handle security. In addition, if this is a brownfield application, the software team will need to consider the compatibility with the existing system and the data-collection endpoints.

Security is a particularly tricky issue because we need to make allowances for software updates in the field, provisioning the sensors/radios, use of encryption keys, etc. No matter how many steps we take to thwart access to the device (special screws, potting, tamper-resistant cases, etc.), we must assume that the device has been compromised by the time that it reaches the field. This puts a special burden on the provisioning approach to make the device as secure from a software perspective as possible. This will require the generation of digital certificates and/or the use of security circuitry such as smartcard chips for secure key and parameter storage.

To summarize, the update of a brownfield industrial application will require that the designer takes into account a broad spectrum of design criteria. This ranges from the desired compatibility with existing systems to the rollout of the platform to even the supply chain if the application is part of the critical infrastructure.

If the plan is to replace existing cabled solutions with a wireless approach, the designer will need to perform a number of tests to ensure that the RF frequencies will propagate as expected at the site given the RF interference that may be present for certain frequencies. And the designer would do well to incorporate security mechanisms throughout the system to protect it from would-be attackers. The application upgrade will not be a simple task. But paying close attention to the details will result in a new solution that can both replace aging existing equipment and provide significant cost savings moving into the future.

Cover Image: Shutterstock

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