Robust communication systems migrate into building intelligence

Industrial Embedded Systems — November 20, 2009

With so many networks to choose from, could a mature, tested, familiar industrial network be the answer for building intelligence? Here's the case for CAN in long-distance applications.

The world is becoming more automated in every respect. Our social surrounding has reached a level of intuitive compliance, as doors open automatically, lights turn off to save energy when occupancy is low, and heating systems adjust according to seasonal demand as well as interior temperature. All of these systems rely on a level of intelligence that is dependent on distributed control and information.

The way in which this control and intelligence is distributed around these networks is evolving to become more conformant. It is a natural progression that has been observed in many sectors, most notably computing. As the number of computers began to increase, so did the need to allow those computers to communicate, leading to the emergence of the Internet.

The same can be said for building automation. As the number of disparate systems for security, heating and ventilation, and ambient lighting control increases, these systems require access to more information in order to operate. Information such as occupancy, temperature, and level of access must be shared securely between systems. As that need increases, so too does the need to build a reliable network that allows those systems to communicate.

Fine-grained control

Today, people are accustomed to relatively simple automation that can open windows, dim lights, or adjust air conditioning. In the future, the level of automation will need to increase with much more fine-grained control.

An industrial or in-building environment has numerous opportunities for energy savings that can be achieved by automating based on demand. As buildings empty, lights can dim, photocopiers can hibernate, heating systems can cool down, and servers can enter into sleep mode. The entire industrial environment is geared around human occupancy and activity, which has far-reaching implications on the energy used within buildings during and after work hours. In the current economic climate, the benefits of reducing operating costs of this kind and in doing so, reducing carbon emissions, are clear.

Industrial demands for technological innovation will lead to a natural division. As new solutions are developed, they will battle for dominance in the same application areas. While this promotes competition, which is ultimately a positive process, it incurs a period of bespoke and proprietary development. To introduce confidence in the market, this development must eventually coalesce under an accepted standard.

The automotive industry has already passed through this process with respect to in-car communications. More than 20 years ago, the Controller Area Network (CAN, also referred to as Car Area Network) and Vehicle Area Network (VAN) protocols in Europe and the J1850 protocol in North America were defined to enable electronic module interconnection within automobiles. Communication protocols and physical interfaces were specified. Detailed validation and interoperability tests were also published to enable product development. Now, the same cycle is under way within the industrial control area, where numerous competing open standards and proprietary solutions currently exist.

For example, consider lighting control within larger buildings or workspaces. Possible solutions here include systems designed for that purpose, such as the Digital Addressable Lighting Interface (DALI), which is standardized under IEC 62386. It provides for a network of up to 64 lighting devices to be controlled centrally.

Consolidation has already taken place in other areas. The KNX standard has grown from the regionally successful BatiBUS and European Installation Bus (EIB) solutions. Under KNX, these were joined by the European Home Systems (EHS) protocol, which had already made inroads with white/brown goods manufacturers. Now recognized in alignment with international standards in Europe, China, and the United States, KNX incorporates lighting control, security, energy management, HVAC, metering white goods, and audio/video control.

CAN we network?

The connectivity used for these solutions currently spans power-line, wireless, and wired Ethernet variants. While this might suffice in closed systems, a higher level of interoperability is necessary to create real growth in this field. Data distribution is crucial in developing these systems; thus, industry must arrive at a standardized solution. But which technology will triumph?

To create a standardized network that can meet current and future needs for building automation, systems engineers are looking for technologies that have already proven themselves in other sectors. Here, CAN network topologies are being considered for industrial and in-building control based on their demonstrable robustness in automotive applications.

The similarities between existing application areas for CAN and industrial automation may not seem that obvious at first. However, in terms of their evolution, they are following comparable routes. Industrial automation is essentially at the same stage the automotive industry was when CAN was first conceived, and the protocol was contrived to solve similar problems to what industrial automation now faces.

CAN was an initiative to lower the amount of cables distributed around an automobile, which at the time had reached about 5 km in each vehicle. The amount of wire needed had become a serious issue in terms of weight and cost. In addition, it was negatively affecting fuel economy.

Using a standardized communications protocol enabled many systems from different suppliers to communicate reliably. This drove competition up and pricing down. Furthermore, CAN’s robustness ensured that reliable communication was maintained in an electrically harsh environment. These aspects are now present in the industrial and in-building automation paradigm, hence the need to tackle them in the same way.

In short, the goal of CAN was to reduce the number of wires needed in a system by using a standard communications infrastructure that could be distributed between many systems and, subsequently, open up the market to more module suppliers that could show compliance to the CAN standard. When considered under these terms, adopting CAN in building automation begins to make more sense.

A tried and tested protocol

One of the benefits CAN adopters have today that wasn’t present in the 1980s is its legacy. Its use is now so widespread that it has taken on a shared ownership akin to the open source movement currently spreading in other areas of electronic development.

An established standard presents an attractive proposition for the industrial sector, which is more conservative when adopting new technology than the consumer, telecommunications, or computing sectors. The technology is tried and tested, so there aren’t any of the pitfalls associated with developing a new standard from scratch.

Aside from this, the most significant attribute an automotive-grade system offers is its intent for use in harsh environments. A bus system such as CAN is designed from the ground up to be electrically robust and uses a protocol that minimizes electromagnetic compatibility issues. CAN-compliant products are also designed to exacting resistance standards for temperature, shock and vibration, and water ingress.

These qualities make CAN systems functional in most in-building applications, without requiring the engineering team to compromise on location or proximity to other electrically noisy equipment. Their inherent high reliability also means they can be used in safety, security, and mission-critical applications.

Gaining distance

The only area where the automotive environment hasn’t imposed suitable requirements on CAN use for industrial control scenarios is transmission distances. In a vehicle, the distance between subsystems is typically much shorter than those in a building. While CAN is specified for a maximum cable length of 40 m, designers routinely require spans of 500 m in a building. For larger installations, distances of several kilometers might even be desirable.

This has several implications, as indicated in Figure 1. Achieving maximum transmission rates over long distances could be impossible, attenuation could cause errors, and, with CAN’s time-dependent nature, delays associated with long transmission paths could introduce false network activity. The CAN-in-Automation (CiA) group recommends that transmission should be restricted to 125 kbps at line lengths of 500 m and should drop to as low as 10 kbps for distances greater than 2 km.

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Figure 1: Bit rate levels for longer transmission paths

While these problems seem fundamental to CAN use within automation applications, the protocol’s legacy means that most of these issues have already been addressed by vendors. Designers can solve most of their problems by choosing an appropriate CAN transceiver IC as the basis of an industrial and in-building CAN system. Moreover, designers can save development cost and time by employing devices with integrated features that eliminate the need for external components.

Application-Specific Standard Products (ASSPs) such as ON Semiconductor’s AMIS-42xxx series (see Figure 2) include electrostatic discharge, overvoltage protection, and low electromagnetic emissions, eliminating the need for a common-mode choke. Wide common-mode signal range improves electromagnetic susceptibility performance.

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Figure 2: Schematic of the ON Semiconductor AMIS-42770 ASSP in a typical CAN-based network

Transceivers offered as repeater-type devices can be used to overcome the issues with signal attenuation found in longer transmission lines. They include both a CAN receiver and transmitter, along with the necessary signal-shaping circuits. The transceivers are typically designed to operate at all data rates, which seems simple, but actually creates an engineering challenge that requires an appreciation of the issues involved and the design expertise to overcome them.

For instance, at very slow transmission rates, a string of similar parity data bits can appear to the bus as a locked state, triggering a time-out error. This effectively limits the lower rate at which the bus can operate, in turn restricting the maximum reach of the transmission line. Transceivers that need to function over distances of more than 500 m (or in terms of data rate, below 60 kbaud) must therefore include the ability to disable or adapt this CAN time-out feature.

Adapting to speeds

One of the benefits of a standardized communication topology is that it allows any compliant device to be integrated easily. For harsh environments, however, conditions might require the device in question to adapt its behavior to remain compliant.

To achieve this, ON Semiconductor’s AMIS-42671 (shown in Figure 3) incorporates an automatic data rate detection function. By configuring the transceiver to listen on the bus and loop the local CAN controller’s error message back to the controller itself, the transceiver can adapt its data rate accordingly. This is achieved by forcing the controller to cycle through its available speeds until data is successfully received and no error message is generated. This technique produces very low overhead for building universal CAN connections, which automatically adapt to the speed of their network.

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Figure 3: The AMIS-42671 CAN transceiver IC from ON Semiconductor

Sophisticated automation

The use of more sophisticated building automation systems is creating demand for innovative control solutions. Implementing proven topologies such as CAN in new, but not dissimilar, environments brings significant advantages.

By overcoming the issues involved with creating a reliable and robust network for the automotive industry, CAN is perfectly positioned for use in this rapidly expanding application area, which will undoubtedly lead to more sophisticated, intelligent, and energy-efficient building automation systems in the near future.

Herve Branquart is ON Semiconductor’s director of business development for automotive and industrial products. Herve joined ON Semiconductor in 2008 with the acquisition of AMI Semiconductor, where he was director of worldwide strategic marketing for the automotive and industrial market. Prior to that, he held product management positions at Alcatel Microelectronics and Motorola. Herve earned a degree in Electronics, Microelectronics, and Software Engineering at the Institut Superieur d’Electronique de Nord (ISEN), with a focus on microelectronics, solid-state physics, image processing, microprocessors, and software.

ON Semiconductor
Herve.Branquart@onsemi.com
www.onsemi.com