Building Management Systems: Aligning BMS hardware with operational intent – Enda Ruxton, Greentherm

From the evolution of hardwired controls to the complexities of modern network protocols, ENDA RUXTON, Managing Director, Greentherm, explores the technical architecture of Building Management Systems and explains why a strategic approach to functional design is essential for achieving true operational efficiency.

Prior to the advent of BMS control, building controls were hardwired and, as a result, were fixed in function and limited in intelligence. Any change to functionality required a change in wiring, resulting in considerable cost. The advent of electronic control revolutionised building controls, allowing enhanced intelligence and easy, low-cost engineering modifications.

There is a frequent misunderstanding about the differences between Building Management Systems (BMS), Energy Management Systems (EMS), and Supervisory Control and Data Acquisition Systems (SCADA).

BMS have become a standard feature of HVAC and lighting systems within modern buildings, providing operational control of electro-mechanical systems. By nature, they generally cater for single buildings.

The primary focus of EMS is for energy optimisation, generally, at building, campus or grid level. The main data it provides is energy consumption, and, at control level, it provides load optimisation. It can be local or distributed by nature.

SCADA is typically used for industrial process control of factories or infrastructure. At data level, it provides industrial process data and employs Programmable Logic Controllers (PLC) for field device control and operates over wide area networks.

BMS hardware

The central pillar of BMS hardware is the hardware controller. This contains the CPU chip that executes the program, contained within onboard memory. The memory is non-volatile, meaning it retains its contents in the event of power loss.

A Backplane is a data bus that connects the controller hardware to the Input/Output (I/O) modules that are mounted adjacent to it. This could be CAN-Bus or another protocol prescribed by the manufacturer.

I/O modules can be input-only, output-only, or a combination of both. Digital inputs accept binary signals from field devices, such as door contacts, pressure switches, and thermostats. Digital outputs are, in turn, used to switch on loads such as indicator lamps, acoustic alarms, and contactors for heaters, or to run signals to heat sources, including boilers and heat pumps.

Analogue signals work on scaling functions, such as voltage, 0-10vdc, current, 0-20ma or 4-20ma, or resistance, such as PT1000’s for temperature measurement. An example using a 0-10 V analogue input might be valve actuator position feedback, where 0 V is closed, and 10 V is fully open. The voltage measurement resolution would be used to interpret the position as a percentage of the total, eg, 5.2 V would represent 52% open. The advantage of using 4-20 mA over 0-20 mA scaling is to detect a cable break, which would show as a zero reading. This ensures no false reading, given that 4 mA is the lower end of the scale.

All BMS controllers will interpret analogue inputs in the form of voltage, current and resistance. However, as a general rule, analogue outputs are only 0-10 V, unlike Programmable Logic Controllers (PLCs), which frequently use 0-20/4-20 mA outputs. If required, this needs to be handled with an external signal conditioner. The key advantage of current outputs is that they are not susceptible to voltage drop, although this is generally not an issue within single buildings.

Visualisation

Visualisation provides the user interface for the operation and status of the controlled field devices. Today, this can be achieved in numerous ways. A Human Machine Interface (HMI) screen can be integrated into a control panel enclosure. These are typically IP65-rated devices.

BMS controllers contain onboard web servers, which allow any web server to remotely connect over Ethernet. In larger systems, it is typical to deploy a SCADA-like server-based visualisation and management platform on a computer server.

It is possible to set up remote access over mobile networks via a Virtual Private Network (VPN) to allow remote visualisation, program changes and alarm event notifications.

Software

Every hardware manufacturer provides its own proprietary software to program its respective controllers. The software application will contain a library of code blocks specific to the control of various field devices, such as mixing valves, AHUs, remote thermostats, and the configuration of network communications, as well as the visualisation of a user interface.

In general, where open network protocols are used, the only proprietary element is the central controller and its respective software application. In essence, it is technically possible to remove a given controller and replace it with a programmed controller from another manufacturer as a direct replacement. The only exception to this is where proprietary communication services are run on top of the open protocol. This could result in restrictions on certain diagnostic functions.

Network Communications

Digital communication protocols provide a reliable and efficient method to collect signals from, and control outlying field devices.

The key advantage of bus technology communications pathways is the ability to use a single communication cable to control outlying I/O modules wired locally to field devices. This eliminates the need to wire the same field devices back to the central controller individually, saving on cable and labour costs while optimising service shaft space.

Common protocols include Modbus, BACnet, M-bus, KNX, and Dali. Although there are some exceptions, most communication protocols use either Ethernet Internet Protocol (IP) or RS-485 communications serial ports.

The key network protocols in use today are as follows:

– BACnet: An object-oriented protocol where devices expose data as objects (eg, Analogue Input, Binary Output). Each object has properties such as value, status, units, etc. Communication is via BACnet/IP over Ethernet or BACnet MS/TP using RS-485.

– Modbus: Register-based open-protocol client-server communication. The protocol can have up to 247 clients. There are two varieties available, Modbus RTU utilising RS-485 and Modbus TCP using Ethernet.

– KNX: A decentralised system architecture where no central controller is required. Each device (sensor, actuator, controller) has its own embedded intelligence. Devices communicate directly via group addresses. This improves reliability, where the failure of one device doesn’t crash the system. It is typically utilised in lighting, HVAC, and blind automation applications.

– M-Bus: Can have up to 250 node devices and be up to 1km in network length, although the distance. It is a low-data network typically used to connect heat, water and gas meters. It is wired on a shielded twisted-pair network cable.

– DALI: A client-server protocol specific to the lighting industry. Each ballast driver has an address that is given a signal to turn on/off or dimming functions.

– MQTT: An Internet of Things (IoT) protocol that uses a publish/subscribe model via a broker. Devices publish data to topics within the broker, and other devices subscribe to topics of interest, making it an event-driven communication method. It’s used for IoT-enabled BMS systems, remote access and cloud integration to additional services.

Aligning BMS hardware with operational intent

BMS hardware can be standalone in a control-only enclosure panel or can be incorporated as part of a Motor Control Centre (MCC) panel. It is good practice to maintain a circa-25% free area within the panel to facilitate future expansion.

It can be argued that there are smaller construction projects where BMS is not appropriate, but equally larger projects where it should be incorporated into the overall system design.

When projects are small in scale and budget, it is possible to utilise IoT controllers that can be configured without specialised software or programming skills. These can provide lower cost solutions for smaller buildings, delivering energy efficiency for building stock that often has basic regulation.