Sensors link up for smart building automation
15 December 2016
Smart-building design is moving into its third generation, in which different control systems exchange data to support a variety of building-management systems.
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The key to enabling this third generation lies in an increase in the number and variety of sensors that are distributed around the building.
The first generation of smart buildings treated each control system separately, with security and HVAC systems becoming the functions that larger buildings installed.
Second-generation smart buildings began to interconnect these systems using specialised networks to allow them to be controlled remotely and to enable a level of central scheduling, such as switching off lighting and heat to rooms marked unoccupied. By interconnecting systems more flexibly, third-generation smart building systems make it possible for control software to learn from the building and evolve functions as needs change. By combining multiple sensor types it is possible to increase overall system accuracy and responsiveness.
A key requirement for most building-automation systems is to understand building occupancy. With accurate information on occupancy on a room-by-room basis the management systems can ensure the appropriate settings for climate, security and lighting. For example, when no-one is in a room or thermal zone, the heating, ventilation and air conditioning (HVAC) services can remain idle and in a low-energy state until people arrive. Such a demand-driven operation of HVAC helps conserve energy.
Many will be familiar with the issues of a single sensor type being used to drive occupancy detection. Many existing systems use passive infrared (IR) motion detectors, such as those manufactured by Panasonic and ABB. These tend to work well in most cases in corridors and stairwells where people are expected to move through them quickly. However, in quiet areas people are often irritated by the system switching off lighting or heating because they have been apparently motionless according to the PIR sensor for a long period of time. Similarly, lack of motion may lead to the HVAC being shut off to a location leading to undesirably cold or hot working conditions.
Ideally, room-occupancy systems would detect not just the presence of people but their overall numbers and placement to better control HVAC. This can be achieved through the combination of more than one type of sensor, which may be used to drive other building-automation systems such as security. As a result, an integrated system is likely to not just increase efficiency and reduce costs, but also provide a level of comfort and control unmatched by non-integrated systems.
Wireless communication using protocols such as Thread or 6LowPAN makes it easier to deploy sensors where they are needed rather than picking sites based on accessibility to power or to network cabling. Protocols that support a producer-consumer data-distribution model make it relatively simple to relay data to a variety of control systems, letting those sensors feed lighting control, HVAC, access control and other building-management functions, with each one using sensor fusion on the multiple inputs. Sensor fusion helps ensure that individual sensors do not provide false-positive or false-negative results, such as the PIR sensor indicating that because there is no perceptible movement there is no-one in the room.
A useful additional source of data for room-occupancy detection is a temperature sensor. When people arrive in the room, they will not only trigger PIR sensors, but temperature sensors will detect a rise in temperature that will be proportional to the number of occupants. Such sensors can be augmented by near-IR image sensors, which can pinpoint sources of heat. Products are now available that provide a wide field of view and enough resolution to build up a coarse image of where heat sources are in a room. An example is the Panasonic Grid-EYE, which uses MEMS thermopile technology to provide a 64-pixel IR image. Based on the input from the image sensor, processing software can use long-term measurements to identify computers and coffee machines that are in place for long periods of time, subtracting those from the image to show the locations of occupants.
Carbon dioxide sensors, such as those made by Figaro, can be harnessed to provide data to the room-occupancy software as well as telling the HVAC system whether increased ventilation is needed when the air quality falls too far. Humidity sensors provide additional information that can be used to further optimise the operation of HVAC. A major concern for any employer is that the workforce stays healthy. Headaches, irritated eyes, sore throats, and dry skin are all symptoms of an indoor environment that is too dry.
Furthermore, dry air tends to lower the body’s own defences against airborne infections.
Factoring humidity into airflow control along with carbon dioxide levels not only helps maintain higher comfort levels but improved health. Humidity sensors, some of which include temperature sensing capability, are now being made by a variety of suppliers, including Honeywell, Sensirion and Texas Instruments. Humidity and carbon dioxide sensors can also feed into fire-detection systems, providing confirmation of an outbreak in addition to conventional smoke detectors.
A further possible source of room-occupancy data is a microphone. To ensure privacy of the occupants, the systems may not record speech but simply use low-level routines to detect voices and, if sensitive enough, breathing and small movements such as typing on a keyboard. A good place to mount such microphones is close to the windows. This lets them also supply data to security systems. A loud, sharp noise, for example, is an indicator of a window being broken. Software can often distinguish this type of break from other loud sounds: a shattered window typically produces a strong frequency peak between 10kHz and 15kHz.
Light-level detectors near the windows can help optimise the activity of the lighting and shading systems to ensure that the overall brightness of each room is consistent. Working in concert with the room-occupancy sensing system, the sensors determine whether the lights need to be activated as external light falls. The light levels from outside can also help control the HVAC system to ensure sufficient ventilation in strong sunlight to ensure the room remains comfortable.
For high-traffic areas of a building, such as lobbies and publicly accessible corridors and stairwells as well as in the retail environment, motion sensors based on PIR will not provide as much useful data, as they will be triggered much of the time. Strain gauges or pressure sensors in the floor can be used to measure foot traffic and used as low-overhead alternatives to video cameras or to augment their capabilities.
Pressure sensors and airflow sensors, such as Omron’s MEMS-based devices, will also help in the HVAC system itself, helping to detect problems in the pipework. Blockages caused by dirt can be detected by changes in pressure differentials particularly if they build up in areas ahead of those showing unusually low airflow. Vibration sensors in the motors used to drive the HVAC systems can provide another source of predictive maintenance — excessive or unusual vibration can point to potential issues in the equipment that signal-processing software can detect.
It is important to consider safe integration with other electronic and electrical systems within the building. The low-voltage smart-building controllers often need to interact with higher-voltage subsystems such as HVAC, AC lighting networks and the motors in automatic doors and shutters. Relays provide guaranteed isolation to the sensitive electronic control systems. Devices such as those in the PNOZ series made by Pilz deliver additional safety features, such as built-in redundancy, to prevent accidental operation in the event of internal component failure. High-quality connectors should be used to interconnect the systems, with passives such as resistors and capacitors employed to aid with filtering and other forms of signal conditioning.
Coupled to low-power wireless networks, the variety of sensors available for measuring environmental conditions will help enable a new generation of building-automation systems. The information they provide can be used across multiple control systems to support integrated building management. The systems will deliver higher efficiency, lower running costs and greater comfort for occupants thanks to the IoT.
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