Simplifying sensors – an update on PICMG Industrial IoT standards
“Simple is hard,” goes that old axiom. Reducing the essence of a complex idea or system into something that is readily understood and easy to use takes time, effort, and creativity. Never has this been more true than in today’s push toward the Industrial Internet of Things (IIoT) and by extension its use of sensors. While much progress has been made on cloud analytics and back-end support, the sensor domain has remained largely impervious to change. With multitudes of different sensors, interfaces, and applications, standardization has been a necessary but missing ingredient. Simplifying this problem is hard.
In 2019 the PICMG standards organization, in collaboration with the DMTF standards organization, launched two new industry specifications targeted at bringing plug and play to the sensor domain of IIoT. The first of these specifications focuses on a small hardware module that today’s sensor vendors can use to create smart sensor nodes. The second specification defines a network architecture and data model that ensures uniformity at the software level. Our aim is to enable and accelerate industrial smart sensor market by making their creation and deployment simple.
Overall architecture
Figure 1 shows a decomposition of an IIoT installation as it relates to the sensor domain. Blue boxes show various functions, and circles show interfaces that must be accounted for. At the highest level, we find the IIoT installation, which is the facility or context into which the sensors will be deployed as well as the operations and backend software for control and management; at the other end, the lowest level, we have nonintelligent sensors and actuators. Sensors provide real-time measurement of physical quantities such as temperature or pressure. Actuators (sometimes referred to as effectors) are motors, solenoids, heating coils, and the like that allow the physical state of factory equipment to be manipulated. The sensor bridge and the sensor intelligence functions shown in Figure 1 provide interoperability between the IIoT installation and the nonintelligent sensors and actuators. These are the focus of PICMG standardization activity.
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Sensor bridge
The primary functions of the sensor bridge device are to aggregate data from multiple sensors into one collection point, and to present the sensor data to the IIoT installation in an IT-friendly format that plays well with the rest of the IT infrastructure of the facility. Though not prescribed by the PICMG specification work, it is envisioned that a sensor bridge aggregation point will exist for each major piece of factory equipment. This is a convenient physical location for sensor aggregation and provides an intuitive correspondence between aggregated sensors and their physical context. The sensor bridge and sensors with equipment-level aggregation are seen in Figure 2.
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Interface A (in Figure 1) of the sensor bridge is responsible for interacting with the upper layers of the IIoT installation and is expected to communicate over Ethernet using IT protocols. The PICMG IIoT network architecture technical subcommittee has selected the DMTF Redfish API as the primary method for presenting the equipment state to the factory-level controller. Redfish is a human-readable JSON-based API that has been readily accepted in a wide range of enterprise and cloud datacenter applications. It provides flexibility and extensibility expected by IoT operators and is a good fit for Industrial IoT with only minor adaptations.
Many existing computing solutions, including PICMG CompactPCI Serial and PICMG COM Express, are well-suited to implement the sensor bridge function. The PICMG IIoT architecture working group is focusing its efforts on the behavioral aspects of the sensor bridge IIoT function.
Sensor intelligence
A typical piece of factory equipment is expected to have multiple smart-sensor devices, each composed of two parts: the sensor (or actuator) element and the sensor intelligence function. The intelligence function is responsible for taking raw (non-smart-sensor) input and presenting it to the sensor bridge in a standard fashion. Alternately, the sensor intelligence might be connected to an actuator. In this case it would provide a standardized method for controlling the state of the actuator. Though not prescribed by the specification, PICMG is targeting the functionality of the sensor intelligence so that it can be implemented with a small 8-bit microcontroller.
Interface C connects the sensor intelligence function with a sensor or actuator. In a physical sense this is accomplished with a small connector. Due to the wide variety of sensor types, analog voltage, analog current, and quadrature inputs must all be supported. Actuator control is accomplished through either an analog or digital control output. Conversion of voltage levels for sensor inputs is also the role of the sensor intelligence hardware function.
From a software perspective, interface C also defines a method by which sensor vendors will configure the sensor intelligence module to work with their particular sensor. The PICMG IIoT Network Architecture and Data Model technical subcommittee is currently deliberating the requirements for this behavior; we can mention that a proof-of-concept of this method using tabular data was demonstrated at Sensor Expo 2019 in the PICMG Smart-Sensor Challenge.
Interface B defines the communications method between the sensor intelligence module and the sensor bridge. From a physical perspective, it is important that this interface support low-latency transfers and use an interface common on 8-bit microcontrollers. For this reason, a standard serial interface has been selected. From a data-representation perspective, binary coded data is more efficient on an 8-bit micro since binary data takes both less memory and less communications bandwidth. Since the sensor node communicates with the sensor bridge in binary fashion, it is the job of the sensor bridge to translate the binary coded data into Redfish and vice versa.
The final interface (EnvSI) related to the sensor intelligence function is the environmental and mechanical interface. The sensor intelligence module is required to function within the full industrial temperature range (-40 °C to +85 °C) and is expected to have a footprint of approximately 30 mm2.
Standardization summary
To date, PICMG has made great strides toward the standardization of the sensor domain for Industrial IoT. The hardware technical subcommittee has defined the physical signals, communication interface, and environmental conditions and begun to tackle the configuration and connector needs. The Network Architecture and Data Model technical subcommittee, on the other hand, has focused more on the overall network architecture, to enable sensor plug and play. We have created a framework for the architecture and behavior of the various elements and selected communications protocols. The work of both technical subcommittees is shown in Figure 3 (long bars show more completion than short bars).
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PICMG plans to make 2020 a year of significant IIoT advancement by releasing both the hardware and the network architecture IIoT specifications. The work done in 2019 lays a good groundwork for success. If you are interested in more information or want to participate in crafting the future of IIoT, please contact us at www.picmg.org. We are always looking for creative, motivated talent to help us specify interoperability and make the complex simple.
