Image Processing Reference
In-Depth Information
f
FIGURE .
Data flow between function blocks.
This is when point
meta-data
come into play. These additional attributes are associated with data
points besides their main characteristic (the present value) to further qualify the data point value.
A unit attribute adds a semantic meaning to the present value by describing the engineering unit
ofthevalue.Aprecisionattributespeciiesthesmallestincrementthatcanberepresented.Attributes
such as minimum value, maximum value, and resolution may describe the observable value range of
the data point more precisely. he resolution can be the actual resolution of the physical sensor and
may be less than the precision.
The location of a point certainly is a key attribute. It is often correlated to a name. Building planners
may design the point name space according to geographical aspects, such as building, floor or room
and/or according to functional domain aspects, such as air conditioning or heating. An example
pattern is Facility/System/Point, e.g., “Depot/Chiller/FlowTemperature.”
If the system allows limit monitoring for a data point, certain bounds can be set on its value. Alarm
indicator attributes then show if these bounds have been exceeded. For example, when a temperature
limit has been exceeded, the data point can switch from normal mode to alarm mode. his attribute
can be persistent so that it can be used to detect alarms also after the value has returned to be in
bounds again. Often, a distinction is made between events (any exceptional condition, e.g., that an
air filter needs to be replaced) and alarms, which require immediate action. Notifications can there-
fore be associated a priority. An additional, important concept for data points are point priorities.
In building automation applications it is common that output points are associated with multiple
functions (control strategies) in parallel, whose outputs can be in conflict. For example, a damper
may be closed by the standard control loop. This may be overridden manually to allow ventilation
for a meeting after hours. his command will again be defeated when a fire alarm is raised and the
damper needs to be closed for fire protection. he priority of the function that is currently controlling
the state of a point can be represented as one of its attributes.
29.4.1 Data Communication Services
As will be discussed in following sections, a distributed implementation approach is well suited to
BAS. Where system functions require multiple devices to cooperate over the network, suitable com-
munication services need to be present. his relates to semantics as well as quality of service aspects.
