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March 2019
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The Anatomy of an Edge Controller

The reality is, none of these devices contain any parts that are truly proprietary or unavailable to the public.

Part 4 of 4 - The Software

Part 3 of 4 - The Hardware

Part 2 of 4 - The Processor

Part 1 of 4 - Introduction

Calvin Slater
Calvin Slater
Climatec

Contributing Editor

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The most important part of an Edge Controller is the software. It’s far more important to have high quality, reliable, maintainable, and extensible software than anything else. If given a choice between great software and only decent hardware, the emphasis should definitely be on the former.

Heterogenous System Architecture SoCs is moving forward at a rapid pace. Consider the recent announcement by silicon giant ST, who is now jumping into the apps processor market with the STM32MP1. This new hybrid Cortex-A7 & M4F chip offers the relatively high-throughput multitasking capability from its dual A7 cores, combined with the more deterministic 3-stage pipeline of an M4 microcontroller core, for performing critical real-time tasks. This device is very similar to NXP’s offering the i.MX7D which made its debut a year or two ago.

A decent edge controller only requires the bare minimum processor which has hardware support for multithreading/multitasking and memory virtualization. As long as we have controller hardware based on such a device, we have the platform to build a great system. Such a controller can possibly serve for decades without needing replacement. As mentioned previously, edge controllers use full operating systems. This gives our building automation applications space to breathe, as well as support via underlying software libraries to simplify and accelerate new development.

All of the new commercial controllers have operating systems of some kind. Some commonly encountered ones are Linux, Android, QNX, or even Windows. In many cases, these OSes are special compact embedded versions of their desktop counterparts. While others, such as those based the Linux Kernel, consist of the exact same code found on a full-blown PC. That is to say; there is no such thing as Embedded Linux, only Linux on an embedded device. This particular feature of Linux offers great advantages when it comes to applications development. The software can be developed conveniently on a desktop PC running a Linux distribution such as Ubuntu. An x86 machine running Linux can easily develop and build software for an embedded ARM target device using cross-compilation. For example, the native Unix code supplied with Sandstar and/or Sedona can be built and run on a Linux desktop, cross-compiled and run on the embedded target, or natively built on the target itself! This is because the native C/C++ code in these frameworks use the same standard libraries and system calls. There are only really two major differences between an embedded operating system and a desktop system:

1) Embedded operating systems are usually headless. This means that they do not have support for a screen, keyboard, or mouse. Because of this, they don’t need a desktop environment either. This helps to save a whole lot of space. If you want one, however, there are lightweight desktops such as LXQT.

2) Embedded operating systems have the necessary driver framework to interact with external sensors and I/O in real-time. Just think of it as a PC with relays, AOs, and AIs. Here is a fantastic write-up explaining how a user program would interact with I/O.

For the Edge Controller to be a successful replacement for legacy devices, we must avoid the pitfalls of using full operating systems. Products based on apps processors are significantly more complex than a microcontroller, which runs a single thread and executes code in place. One of the major issues with apps-based devices is boot time. With microcontrollers, even those using a lightweight RTOS, the main program starts running almost immediately when power is applied or restored. Contrast this to some production edge controllers which can take several minutes to start executing DDC code. This situation is completely unacceptable for a field device performing important tasks such as running a central plant.

A lot of the delay with DDC start time in these devices has to do with the control programs running within a virtualized environment such as Java.  Another problem with virtual environments is that the DDC applications lose the possibility of running in real-time. Remember that “runs fast” is not the same as “real-time.” Some manufacturers have already recently begun to address this issue by moving the time-critical portions of their control engines out of Java. Virtualization cannot be completely blamed, however. The operating system itself needs a bit of time to initialize and start running. A lot of time can be saved by not loading software which is not needed. Linux builds can be stripped down so they will boot to console very quickly. Take a look at this Linux build which boots in only one second.

The device must incorporate hardware security at boot-time and maintain a chain-of-trust spanning into user space. This usually involves the very first code run to be digitally signed and checked by the processor. Now that our applications have more space and resources to do stuff, so does malware. Apps and services will need to communicate to remote stations using secure and open networking protocols, which are mostly IP based such as HTTPS, and have the ability to directly connect to a building management database server located on site, in the fog, or in the cloud without the need for specialized intermediary network routers or gateways.

As long as all these potential pitfalls are properly addressed and avoided, the advantages of an OS equipped edge controller outweigh its drawbacks. Devices with operating systems are easier to maintain than ones with only firmware. One ongoing problem that the edge controller can eliminate is hardware obsolescence due to unsupported firmware. Often the controls are the first thing replaced in a new building due to the original vendor’s inability to support them. This shouldn't be the case with controllers that are only a few years old. We should be able to maintain these devices for decades if possible. The underlying system can be updated and maintained by a qualified system administrator without significantly affecting the functionality of installed applications and services running on top.

The question of who should maintain the underlying OS is still up in the air. In one model we have the device manufacturer or a third party maintaining the operating system. This was the proposition for the now-defunct Android Things platform. Google was to provide Android support with system updates for a set period of time and would push these updates to the devices over the air while installing in the field. Possibly a more viable solution would be for owners to maintain their own systems. The facility’s OT administrators would be responsible for maintaining and updating their own platforms. This would not require so much specialized HVAC and lighting system knowledge, as these specialists would only be applying changes to the underlying operating system. They would easily be able to accomplish this as it is an environment they are familiar with. There are many community supported Linux distros out there that would be well suited for such use, such as Debian.

Once we have a platform with a general purpose operating system rather than proprietary firmware images, we are free to install and uninstall user space building automation software as needed and can continue using the device for many years. An owner would only have to replace and update the software, rather than completely remove and replace the entire controller.  Installed applications can be paid, or free such as Sandstar.

Figure 2

Project Sandstar provides a good example of what a complete edge controller system would look like. At the foundation, we have a hardware board based on the AM335x processor. The entire hardware support for booting the Linux Kernel on this particular device has been available in open-source for about ten years now. The demo available on the Sandstar repository uses Debian as an operating system. However, this demo will run on any ARMv7-A target, that uses any Linux distro without having to be re-compiled.

At the very lowest level, we have the Sandstar engine which interacts with the controller’s I/O and makes updates to a Haystack grid. Controller points are abstracted to other processes in the form of channels. Sandstar uses the device driver framework that is already built into the Kernel. For example, if the engine wishes to read an Analog Input, it would do so by reading the appropriate ADC input voltage via the Kernel’s IIO Subsystem. The engine will be able to identify the available channels and point types by reading the controller’s device template. The template, which is a .csv file, would be present in every type of controller and abstracts available I/O points in hardware. Similarly, the grid is a two-dimensional matrix of tagged entities that is used to model the controllers physical I/O points in software. This is why it’s possible to have portable Sedona apps with Sandstar. On most other Sedona devices, the I/O kits use native code specifically tailored and created for that device only, which is why apps are not portable to other devices. Sandstar’s model does not dictate what physical inputs or outputs should exist on any platform. Instead, common types of controller points are given in a generic kit only and abstracted as channels. For example, you can create an instance of an Analog Input in your Sedona app using the AI component in the Sandstar kit if it exists. If the point exists on the controller, then the channel will exist.

PlantPROCOREThe Haystack grid serves a second purpose other than providing scalar control values to the Sedona application. The built-in Haystack server supplies meta-data directly from the device. In many cases we see Haystack data being presented not by the actual equipment controller, but by a proxy device. There is no longer a good reason to do this. The edge controller, with its increased capability, offers the ability to present data about the equipment it’s attached to and controlling directly. This offers two distinct advantages. For one, there is no need to commission two separate devices relating to the same piece of equipment. The second much more powerful aspect is the ability to use tagging not just for information and analytics, but to actually control the equipment. Consider the use case where a collection of VAV boxes are served by a particular Air handler, which in turn is being served by a Chiller. In many proprietary systems, some type of external data structure must be generated to describe how individual pieces of equipment should request resources such as “cooling” from one another. Tags could be used for this purpose. Tags attached to these pieces of equipment can be used to automatically describe their purposes as well as their relationship to each other. Currently, existing tags have individual definitions that are consumable by software. But as of yet, there is no formal definition of how these standardized tags are related to each other. Not yet….

Take a look at working group 551 on the Project Haystack site.  There is an effort underway there to formalize the relationship between existing tags that can be presented in machine-readable formats. Someday we won't even need a central BMS server. All management, control, histories, and analytics will occur in the fog, greatly reducing bandwidth external to the enterprise.  Each controller will contain its own part of the BMS system, as well as backups of other parts. Relationships to other equipment on and off-site will be encoded in the form of tags which will be used for M2M control.


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