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BACnet at the «field» level
The ISO 16484452 standard «Building automation and control systems — Hardware» (ISO — International Organization for Standardization) sets out an approach to constructing a three-level building automation system.
The lower («field») level includes terminal devices: drives, valves, switches, control buttons, sensors and other equipment.
The middle level, or «automation» level, is the controllers that receive signals from the lower level and transmit them to the upper level, as well as network equipment.
The upper level, or «control» level, includes operator workstations and, if necessary, can have a gateway to ERP systems (automated enterprise management systems).
If everything is more or less clear with the “management” and “automation” levels, then the approaches to organizing the lower (“field”) level have aroused particular interest.
Many Russian specialists have contacted the BIG-RU association with a request to summarize the world experience in building the lower automation level, and especially with the possibility of using the BACnet protocol on it.
Here is the opinion of leading foreign and domestic experts in building automation.
Bill Swan
Alerton company
Chairman of the BACnet Committee
SSPC-135ASHRAE)
BACnet is absolutely suitable for the «field» level in all respects!
About 98% of all BACnet devices sold by Alerton are «field» level devices.
Such equipment includes: VAV controllers with dedicated functions, programmable VLC controllers (all B-ASC *BACnet Application Specific Controller devices), as well as even more advanced programmable VLX controllers.
For nearly a decade, Alerton has been delivering BACnet systems from the rooftop to the basement, with the ability to use BACnet at all three automation levels.
Later, our major competitors (including Automated Logic and Delta Controls) also moved to solutions with BACnet devices at all automation levels.
Roland Hedded
Cairo
This is what I mean by the «field» level in the overall architecture of a building management system (BMS). The «field» level contains devices associated with the DDC (direct digital control) — the «automation» level controllers, such as temperature, humidity, level, pressure sensors and all sorts of switches.
The «field» level also includes control elements such as valves, dampers, actuators, control relays, etc.
Control and monitoring of signals between automation level controllers and field level components must be performed in standard ranges such as 0 to 5 V, 0 to 10 V, 4 to 20 mA, switchable 0 and 5 V, switchable 0 and 10 V, etc.
Based on the above definitions, field level components send simple low-current signals to DDC controllers.
A temperature sensor, for example, sends an analog signal proportional to the measured temperature (0 to 10 V, for example).
The received signal is interpreted by the logic of the automation level DDC controller as an analog input from the object.
This command or action is then converted into a BACnet object form.
There is no need for the instruments at the «field» level to «understand» or «interpret» the signals they send or receive.
Hans Krasch
HAK
Roland Hedded's answer is exactly what is written in the international standard ISO 1648452 «Building automation systems — Part 2: Hardware» in paragraphs 5.4 and 5.6.
This level of automation also allows «field» devices to communicate, but due to the high cost, there is little reason to use sensors or actuators with the ability to communicate.
Exceptions are «multisensors» or other BACnet devices with built-in control functions, which might include an «event log» display or room devices with sensors, display and control panels for end users.
Some of these devices use BACnet.
Michael Urbanshk
EBTRON
Both Roland and Hans are right about the «field» level sensors.
Our company is one of those that offers «field» level sensors on the market today that are not particularly expensive and can connect to networks (BACnet over Ethernet, BACnet/IP and MS/TP) to measure water flow, air temperature, etc.
There are many reasons to network sensors (electrical noise, d/a a/d conversion accuracy, etc.).
Hopefully, as time goes on and electrical components become significantly cheaper, we will see significantly more «field» devices networked.
I believe that implementing BACnet from the top of the building to the lowest control point is the best solution for the building owner.
Moreover, I am not advertising my company's products, but I am truly confident that even more equipment manufacturers should produce sensors capable of forming a network.
ISO 16484-3 Standard «Building automation and control systems. Functions»
Andrey Golovin
BIG-RU Association
As far as I know, in the US most companies use a two-tier architecture, where they refer to the network layer located closer to the sensors (the Fieldbus layer).
The Fieldbus layer is usually implemented on the RS485 interface with some internal/proprietary protocol, and BACnet MS/TP is on RS485. The «management» layer (Management level) is usually implemented on BACnet/IP or on a proprietary protocol using TCP/IP.
HansKrasch
HAK company
In the illustration of the ISO 1648452 standard, you can see that «field» devices must be connected to controllers directly or through some communication medium.
I know there is some difference between the terms «field» devices and «field» bus.
In industrial automation, «field» bus is mainly built as a peer-to-peer communication system (a communication model where each equal component has the same capabilities as any other and can initiate a communication session) between controllers.
In the field of building automation, we try not to use the term peer-to-peer. Therefore, the ISO standard, intended only for building automation and control, defines the following automation levels:
«field» level with input/output functions;
«automation» level with functional processes;
«control» level with databases and statistical functions.
And operator functions can be assigned to any level.
Of course, in practice you don't often see «field» devices connected to a network, in particular, actuators. Because this immediately makes them more expensive, but the ISO standard notes that «field» level devices must be able to integrate into a network.
John Erwin
ATS company
Most of the companies in the field of automation and control systems (BAS) that I have worked with monitor and control a large number of different «field» level devices — electronic actuators, relays, sensors, etc.
Many manufacturers of equipment for BA and BAS do not produce «field» level devices themselves, but either put their own label on other people's devices, or use them for free (for example, Belimo drives are very popular).
And when such companies give presentations, they try not to focus on the «field» level devices, but rather to promote their products.
It seems to me that «field» devices should be «universal»: 4520 mA, 2510 V, binary. And directly connected to the controller, they are immune to communication problems that arise in the network.
Imagine a large campus where all the «field» devices communicate using BACnet. Why do we need controllers at all then?
You only need one to coordinate all of these devices and route messages across the local area network (Ethernet).
Network traffic will increase significantly if there is a problem that prevents communication within the network, and you will lose control of everything.
And if the single controller fails, you will not be able to control everything either.
Other peer-to-peer solutions have end devices that in turn communicate with each other.
Winston Heatherington
BASS Consulting Services
Some equipment (smart sensors with displays or various measuring devices with built-in functions) has the ability to integrate into a network and uses digital methods of communication with end devices.
However, such equipment is not yet widely used.
Sensors for measuring temperature, humidity, pressure or status (open/closed contact monitoring) are used more often today, and voltage (V) signals must be received and interpreted by a controller in the network.
The term «field» bus is used mostly in industrial automation.
Reinhardt Seidl
Taylor Engineering
A reason why it would be beneficial to use sensors with IP addresses might be the following: say you have an old building made of asbestos.
All the controls in the building are pneumatic, and you want to install at least a few room sensors with remote control capability.
In this case, you can't lay a wire, and wireless sensors are not quite perfect today. In any case, as far as I know, they use closed protocols.
Also, we must not forget about the problem with replacing batteries for sensors.
So, what is an alternative solution?
If your building already has a local network (Ethernet), then a sensor with an IP address will solve the problems. Of course, this solution will be more expensive than with a regular sensor.
The example given is an exception, but I have just encountered such a case. And, unfortunately, I have not been able to find a worthy product.
Konstantin Trutko
Siemens
The technical policy of leading manufacturers of building automation systems, including Siemens, is to optimize the use of various communication protocols at different levels of the automation system.
We believe that such a powerful and multifunctional software level protocol as BACnet is advisable to use at the two upper levels.
At the field level, direct connection without any protocols is still relevant for connecting sensors and actuators.
And for integrating room controllers into a dispatching system, when only a few variables need to be transmitted, using BACnet is also inappropriate.
For room controllers, the LONMark and KNX protocols are used.
This is an international standard.
Another thing is that for the integration of room controllers «upstairs» we use programmable gateways PXR511(12), which really work on BACnet.
Nina Voevodenko
Sauter company
Sauter has a similar situation, i.e. «field» equipment (sensors, relays, actuators) is connected to the controllers using standard generally accepted signals through digital or analog inputs/outputs.
There is no point in using any protocol here, these signals are standard for everyone anyway.
It's a different matter if the «field» equipment includes more complex equipment, such as pumps, and you need to not only turn them on/off, but also have information about various parameters of this device. In this case, you need some protocol to communicate with this device. And it can be BACnet.
Sauter also has room LON controllers, but we prefer to implement room control using our Ecos controllers, which communicate with each other via our NovaNet protocol and can also be connected to BACnet via a gateway. And, by the way, this solution is much cheaper.
Roman Wroblewski
YORK International, A Johnson Controls company
Roland Heddad and Hans Kranz have provided comprehensive definitions of the «field» level in accordance with the ISO 1648452 standard.
When deciding on the use of the BACnet protocol at the «field» level, YORK takes into account the tasks facing a specific automation system.
In practice, there are two options for connecting «field» devices: direct connection to controllers and connection via a digital protocol.
Each option is good for solving certain problems.
Here are some examples.
If it is necessary to transmit one — two signals (for example, to measure the temperature and humidity of the air), we install the corresponding sensor and connect it directly to the controller using standard signals.
For local control of the climate, lighting and, for example, blinds in a separate room, it is necessary to measure the air parameters, as well as ensure interaction with the user.
If you provide separate measuring devices and controls for each function, you will get a cumbersome system, inconvenient for both installation and operation.
In this case, it is advisable to install a combined device connected to the «automation» level via the BACnet protocol and combining a temperature sensor, control panel and indicator. The indicator can be used to display the current system parameters and to select modes.
Another example: to control the air supply dampers in VAV (Variable Air Volume) systems — a system with variable air flow (widely used in hotels, offices and other premises where the presence of people in the premises is cyclical), a combination of an electric drive and a differential pressure sensor is used.
In order not to connect these signals to a remote controller, it is advisable to use a combined device with built-in communication capability via the BACnet protocol.
In short, the direct connection method is more suitable for connecting closely located «field» devices when it is necessary to transmit a small number of signals and usually in one direction.
On the contrary, if it is necessary to transmit several signals, receive feedback or there is a need to implement interaction with the user at the «field» level, it is advisable to use the connection of «field» devices via a digital protocol. In this case, the number of installation and commissioning works is significantly reduced and the network architecture is simplified.
In any case, the choice of a specific technical solution depends on the tasks at hand, and YORK has the necessary equipment and technologies to solve these problems in the best possible way.
Andrey Golovin
BIG-RU Association
I would like to note that thanks to the opinions of our colleagues from the USA, Europe and Russia, a general picture of the methods for building automation systems on different continents is emerging.
This observation is very valuable, since it allows you to compare the advantages and disadvantages of both options and choose the one that suits you best, especially since today both European and American equipment is present on the Russian market.