MEMS Helps Keep Buildings Cool

More and more buildings are installing automation systems to reduce energy costs and shrink their carbon footprint. Demand for heating, ventilation and air conditioning control is a major element of this growth trend, driven by efficiency and the need to improve air quality in the workplace. Automation and intelligent control can switch off services in unused areas, offering substantial savings. Innovative new presence detection technologies can be integrated with temperature sensing and airflow measurement devices to create a complete demand management solution for both HVAC and lighting, ensuring good provision when needed and reducing or eliminating energy wasted in unoccupied areas. The newest sensors are so sophisticated that they can also be used in a whole host of secondary applications from detecting intruders to cancelling spurious lift calls.

These new infra-red sensors — based on measuring temperature — can detect stationary heat-emitting objects as well as those in motion, a major advantage over the PIR detectors traditionally used. Furthermore, they are able to do much more, such as measuring the overall room temperature, and even the temperature in specific zones of the space in which they are deployed. For example, it is possible to envisage a reception area where the temperature in the waiting area is monitored and kept at a comfortable level for visitors, whilst the temperature in the area near the exit is purposely kept lower to reduce heat emissions through the doors.

Absolute Or Relative

Conventional PIRs use the pyroelectric effect to sense people or animals in its field of view. As the pyroelectric phenomenon is a temporary change in a material’s structure during heating or cooling, the detection circuitry depends on measuring a difference in the heat pattern. They detect motion rather than presence.

New thermal sensors based on microelectromechanical systems (MEMS) do not suffer this disadvantage. MEMS thermopiles, essentially nano-scale arrays of thermocouples, measure the actual temperature of the sources rather than a differential value, so they detect people in a room even when they are not moving. In a typical building automation system, such sensors can also provide a primary input to such systems, sending analogue or digital signals direct to a controller.

MEMS sensors, like Omron’s new D6T, measure temperature across the whole field of view, contrasting with conventional thermal sensors that are restricted to measurements at a single point. Configured as an array (1x8 and 4x4 arrays are available currently), the sensor can assign temperature information to a specific cell, adding a positional dimension to its presence and temperature data. The result is higher accuracy, lower crosstalk and a wider field of view. Using several sensors in a room multiplies these benefits: a quad 4-x4 array sensor module can be used to obtain higher resolution or broader coverage, for example identifying a person’s location within 1m across a 16m² area.

The technology behind these new thermal sensors combines a high-performance silicon lens to focus the infrared rays onto its thermopiles with a MEMS micro-mirror structure to prevent radiation loss for efficient thermal detection. Proprietary application-specific integrated circuits then make the necessary computations and convert sensor signals into meaningful digital outputs. The result is high degC accuracy with low crosstalk and excellent noise immunity (measured as noise equivalent temperature difference) of 140mK. Larger 16x16 MEMS sensor arrays are currently being developed, which will provide still finer control.

With enhanced coverage and resolution, output data can be processed for example to distinguish between people standing and sitting in its range. The system could also identify hotspots, indicating fire hazards or faulty machinery such as a jammed conveyor or overheating motor.


Figure 1: Airflow measuring principle

Extending To Flow Measurement

A simple variant of this technology has been used to create airflow sensors that are more accurate than conventional devices, and operate on much lower airflows. By adding a micro-miniature heating element in between two thermopiles, the sensor measures flow by the shift in the resulting hot spot. Temperature difference between the two thermopiles is directly proportional to the mass flow across the sensor chip.

Such accurate maintenance of pressure enables OEMs to maximise the performance of HVAC systems, especially variable air volume (VAV) systems where the dumper can regulate the airflow in ducts and thus achieve noiseless and efficient control. Embodied in devices like the new Omron D6F-PH, this offers better accuracy than conventional differential pressure sensors, especially in low flow conditions.

The standard range features units capable of measuring flows from zero to over 200l/minute. The compact sensor element measures mass flow directly, and most models for air measurement are protected by a unique dust segregation system to avoid long-term performance degradation in contaminated environments. To help improve air quality further, these devices can also be used to detect clogged filters in ventilation and air-conditioning systems, thereby improving reliability and efficiency. Omron offers fully calibrated and temperature compensated sensors to measure flows of a range of non-corrosive gases including natural gas, propane, oxygen, nitrous oxide and air.

Pinpoint precision along with foolproof detection makes these sensors ideal in new applications. For example, CCTV cameras can be directed to investigate the presence of occupants in an area that should be unoccupied. They could be configured to detect the presence of people in front of a lift entrance; by processing the sensor outputs to sense the direction of motion it is also possible to use IR sensors to cancel a lift call if nearby people walk away. In busy offices, car parks and retail stores, the energy saved on wasted lift trips can be significant. These new sensors could also be used to reopen the doors if a lift is about to leave whilst passengers approach, and to monitor the number of people inside the lift to prevent overloads.

Whether it's heating, ventilation, lifts or lighting applications, these new devices are helping OEMs to achieve more efficient control, reduce costs and space in their products, making a substantial contribution to lower energy consumption overall.