Industrial control system makes light work at National Portrait Gallery
To provide effective control of natural lighting in the National Portrait Gallery in London, astronomical data is used by an industrial control system to calculate the position of the Sun and control louvres accordingly.
To meet the demanding environmental control requirements of the National Portrait Gallery in London a range of industrial control hardware and software is now used to manage building services. The hardware and software from Rockwell Automation is part of a £38 million investment programme. It includes Allen-Bradley logic controllers and motor-control gear, as well as a Rockwell Software Active Display Server supervisory control and data-acquisition system. The National Portrait Gallery houses over 10 000 works of art in its primary collection, and precise management of the environment in which they are shown is essential to avoid long-term damage. Displaying paintings and sculptures in a public building involves a delicate balancing between providing the right lighting for viewing while preventing ultra-violet and other harmful rays causing long-term damage. Accurate control of HVAC plant to maintain conditions close to the ideal of 20°C and 50% RH is also vital to prevent pictures deteriorating. Thousands of people pass through the building every day, and the HVAC system has to respond quickly to maintain constant humidity levels — especially when it is raining and large numbers of visitors arrive in wet clothes. David Coulthard has been NPG’s in-house manager of M&E engineering since 1994. He tells us that the gallery had already been using Allen-Bradley controllers for two years and that he saw the benefits of staying with them for the planned expansion projects. ‘The National Portrait Gallery isn’t like a factory or office building, and with paintings dating back to the fifteenth centre, the consequences of losing control of the building could be catastrophic. So we need a reliable system that offers precise control and functional flexibility.’ Kevin Dunn, managing directors of systems integrator Davenport Control & Instrumentation, says, ‘The gallery originally chose Allen-Bradley products for their reliability. We were also able to demonstrate that SLC500 controllers were powerful and flexible and could control conditions far more accurately than any building-management system. HVAC plant is controlled using PID loops to enable feedback from highly accurate room sensors to rapidly generate control signals. The control loops are so responsive that temperature and record charts shows little variation over a 24-hour period. Control of light levels is just as important and more difficult since daylight can fluctuate from dull and cloudy to 60 000 lx of bright sunlight in seconds. Current thinking among architects and curators is that as much natural daylight as possible should be allowed into the building so that works of art can be seen in conditions closed to those in which they were originally created. Daylight changes naturally with the weather and the movement of the Sun, creating a more dynamic environment than that possible with artificial light alone. Original oil and water-colour paintings are at great risk of fading if exposed to too much light. To prevent damage, it is essential that sunlight is never allowed to fall directly onto them. There are strict limits on overall lighting levels in the galleries and the total amount of light a painting can be exposed to in a year. Central to the brief for the refurbishment of the top-floor Regency Galleries was the integrated control of motorised solar roof blinds and gallery lighting to provide flexible and more accurate control of the mix of daylighting and electric control. To control the daylight entering various rooms through the roof lights, Davenport developed a sophisticated system of controlling motorised louvre blinds to replace the fixed louvres. Inputs from light sensors and positional feedback from the blind actuators are used to adjust the angle of each set of louvres every four minutes. Artificial light is adjusted to compensate for changes in daylight. A special routine uses astronomical data and spherical trigonometry to constantly calculate the position of the Sun relative to each set of blinds. Louvres are individually positioned to block direct sunlight, even in a cloudless sky. Blinds are fully opened early in the morning and late in the evening to maximise daylight. To avoid reducing the amount of daylight on dull, overcast days, a second routine progressively relaxes the calculated limited when the sky radiance falls below 10 000 lx. A sudden increase in sky radiance triggers the blinds back to their calculated positions. A third, over-riding routine monitors light reaching the pictures using special Brooks sensors. They are focused on a central area of a major portrait and measure the light falling on the painting. If this exceeds 200 lx for oil paintings and 50 lx for water colours, the blinds are immediately adjusted to bring light levels down to safe limits. The amount of light paintings are exposed to over a period of time is also logged. Following the installation of automated roof blinds on four rooms, the National Portrait Gallery intends to fit the system to a further 20 upper-level rooms.