Ashrae Journal - November 2008 - (Page B11)

Figure 1 (left): Clamping is the procedure of restricting the lighting level to a range of some speciἀed minimum to some speciἀed maximum as long as the light is on. Figure 2 (right): Fading is the procedure of changing from one level to another over a ἀxed period of time. the Present_Value property of the output object, blink warning would briefly turn the light off, then back on again and it would remain on for a short period of time. This short warning alerts occupants that the lights are due to turn off soon unless they take some action that extends the scheduled occupancy time. After this brief delay, the lights go off automatically unless this action is taken. To implement this feature, and to make it conἀgurable through BACnet, some additional proper- Figure 3: Ramping is the procedure of changing from one level to another at a ἀxed rate of change of level per second. ties are required that specify: • Whether blink warning should take place at all; In architectural lighting, it is common to require not only • If so, how long should the output blink off; control over the lighting level, but also control over the speed • How long should the light remain on again after blinking at which the level changes from one setting to another, for until automatic turn off; and example, dimming from 100% to 50% over a 10-second pe• Other specifications. riod. There are three common types of lighting operations for No standard properties for BO objects exist that represent level-based dimmers: these lighting-centric ideas. Some systems use additional Binary • Fading (Figure 2) is the procedure of changing from one Value (BV) and Analog Value (AV) objects to convey these palevel to another over a fixed period of time. rameters, and some systems use nonstandard properties of their • Ramping (Figure 3) is the procedure of changing from BO objects to accomplish the same goal. Obviously, it would one level to another at a fixed rate of change of level per be better if there was a standard BACnet object that already second. had properties to represent these concepts, and that was one • Stepping (Figure 4 ) is the procedure of incrementally of the motivations behind the creation of the Lighting Output increasing or decreasing lighting level in fixed steps (for (LO object). Dimmer outputs are even more complicated, but example when a wall panel  or  button is pressed). in their simplest form can be modeled using Analog Output These ideas are much more complex than the simple level(AO) or AV objects to represent the dimmer output, usually in units of 0% to 100% intensity. However, there are a number of oriented model in AO and AV objects. Again, the LO object provides additional properties that allow configuration of applications that again require more sophistication. Although the total range for dimmable lights is 0% to 100%, these types of behaviors for analog (dimmer) types of lighting often there is a need to distinguish between off (0%) and outputs. The LO object (Figure 5) packages all of these ideas into some minimum allowable level. This minimum level varies with application and may also vary based on scene presets. one lighting-centric object model and so is better suited to Similarly, the maximum level in a particular room under lighting applications that have these requirements. The LO particular conditions may need to be restricted to something object is similar superficially to an AO in the sense that its Present_Value property represents a 0-100% lighting intensity less than 100%. Clamping (Figure 1 ) is the procedure of restricting the output. The Present_Value is commandable, and has a range of lighting level to a range of some specified minimum to some 0% to 100% luminance where: • 0% = Off; specified maximum as long as the light is on. November 2008 B A C n e t ® To d a y | A S u p p l e m e n t t o A S H R A E J o u r n a l B11

Table of Contents Feed for the Digital Edition of Ashrae Journal - November 2008

Ashrae Journal - November 2008 Contents Commentary Industry News Meetings and Shows Feature Articles Reducing Data Center Energy Consumption Building Sciences Feature Articles Sustainable Nature Center Emerging Technologies Special Products Washington Report Special Supplement: BACnet® Today BACnet® Goes Green Seeing the Light with BACnet® BACnet® on Campus BACnet® for Net Zero Analyzing BACnet® BACnet® at Xerox Campus BACnet® in Europe BACnet® for Green Library BACnet® at the University of Minnesota: Flexibility, Interoperability InfoCenter Product Showplace Classified Advertising Advertising Index

Ashrae Journal - November 2008

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