Making sensors make sense

In making better use of energy in our buildings, which is certainly something we need to do, it’s important that we don’t just look at the energy consuming plant and equipment but also to how it is controlled. That seems an obvious statement, and is reinforced in Part L. Consequently, controls are selected and installed in the majority of projects now – but it is surprising how little thought is sometimes given to ensuring they are the right type of controls.

Occupancy detection – using presence or absence detection – is a case in point. Clearly there are huge advantages to turning energy-consuming services down or off when there is nobody in the space to benefit from them. But there is a tendency to lump all occupancy detectors together, rather than recognising that these sensors can vary greatly from one to the other and ensuring the best sensor is chosen for each job.

For instance, there are significant differences between passive infra-red (PIR) and microwave detectors. PIR detectors work by passively detecting a rapid change in the infra-red spectrum within their field of view. In contrast, microwave detectors actively transmit energy into the space and measure the reflected energy from the background. When a person enters the space, the pattern of the reflected energy changes and this is detected as a change in the Doppler pattern.

So there’s a great deal more to achieving significant savings from this type of control than simply specifying that occupancy detection be included. Specifications need to be more precise about what type of technology is used (microwave or PIR) and the levels of functionality and controllability.

To a very great extent, the choice of detector should be determined by the space it is controlling – a decision that should be based on performance rather than price. In particular, range and direction make a significant difference to the performance of the system and making the right choices can also minimise the number of sensors required for effective control.

Corridors are a common example of situations where the chosen sensor often doesn’t deliver maximum performance. People will often install a basic sensor with 360 degree coverage in a corridor and, while this will work in most cases, it’s far from being the best solution. A radial detector with a range of, say, 8m in each direction will provide more appropriate coverage and reduce the number of detectors needed to cover the space. In fact, with the right choice of detector, many corridors can be served perfectly adequately by a single radial detector.

Similarly, in open plan spaces there are considerable benefits from being able to control lighting in zones to respond to changing occupancy patterns. Here, a sensor with only limited adjustment capabilities will fail to provide the necessary zoning. A close cell sensor with very precise adjustment capabilities is the only way to achieve this.

Open plan spaces may also benefit from combining the occupancy detection with a light sensor to ensure that lighting isn’t activated when there is sufficient natural daylight entering the space. In many such cases, to ensure illuminance levels are maintained, it may be preferable to dim the lighting, rather than turning it off completely. So it will be necessary to use a detector that provides a variable output to achieve dimming. Manual override through a remote controller may also be a requirement in some projects, giving users some personal control of the lighting in their space.

Where sensors are being retrofitted to an existing lighting system they are most likely to be operating on a stand alone basis rather than simply sending signals to a lighting management system. In these cases, the sensors need to have the capabilities to deliver the required control functions themselves so a basic detector that just switches lighting on and off will not fit the bill.

Clearly, then, the whole field of occupancy detection is more complex than many people realise – and it’s important to introduce another level of complexity – the ability to distinguish between human movement and other movements and eliminate false responses. Similarly, the detectors need to be able to differentiate between people and other heat sources- typically by not just detecting a presence but also analysing its size and pattern of movement.

This is one area where presence detectors vary greatly because their ability to do so depends on the quality of the lenses and the programming in the sensor that recognises the characteristics of human movement.

For all of these reasons I would urge specifiers and installers to give deeper consideration to the occupancy sensors they choose and ensure they opt for the solution that will give the end user maximum benefits. Not doing so fails the customer and fails to demonstrate what our industry can achieve for the environment.