Image Processing Reference
In-Depth Information
extend to less critical property safety alarms, such as water leakage detection. Similar considerations
apply to security systems.
These issues need to be addressed by carefully selecting the points of interaction between the dif-
ferent systems, with the general goal of making the flow of control traceable. Interfaces have to be
defined clearly to ensure that no unwanted influence is possible. Typically, systems stay standalone for
theircorepurpose.Onlyahighlylimitednumberofinteractionpointsareestablishedatthehighest
system level. However, such “high-level integration” can already yield considerable benefits. For
example, consider again the fire alarm example: the information that an alarm condition is present
is only a single bit.
29.3.2 Automation and Control
Building automation can be regarded as a special case of process automation, with the process being
the building indoor environment (and its closer surroundings). Although some applications, such as
shading, will actually involve outdoor sensors and actuators, environmental conditions will typically
only be controlled in the interior.
Providing a comfortable environment to humans is a very complex control target. Since some of
the factors influencing comfort will by their nature remain unknown to the system,
∗
continuous
manual intervention (by occupants and/or privileged operators) is part of routine system operation
in home and building automation.
Since HVAC processes involve large (thermal) capacities, system state only changes gradually.
Quick transients only have to be detected when optimizing system behavior, if at all. Since plant
response times are slow, requirements on controller response times are relaxed compared to indus-
trial control applications. Despite the general absence of high-speed control loops, HVAC control is
not without challenges. It has to deal with disturbances which change over time as a function of load,
weather conditions, and building occupancy. hese influences are of stochastic nature and therefore
not exactly predictable, although certain assumptions can be made. A comprehensive introduction
to HVAC control is, e.g., provided in [].
Closed-loop control is barely present in other building systems. Interestingly enough, timing
constraints are tightest in certain open-loop control relations (most notably simple light control func-
tions), where the response time is put in relation with the human perception time in the range of a
few hundreds of milliseconds.
Definitely, one key challenge in BAS is that large areas need to be covered especially in high-rise
buildings or larger building complexes. However, most feedback loops are highly local.
Regarding the required reliability (defined as the probability of a system to perform as designed)
and availability (defined as the degree to which a system is operable at any given point in time),
demands are moderate as well. The consequences for failing to meet them are merely annoying in
the vast majority of cases, although exceptions do exist (e.g., refrigerated warehouses or medical
applications).
In case of automation system failures, graceful degradation is expected. Basic functions have to be
provided with higher reliability and availability than more complex ones (e.g., examining trend data).
Also, control loops should be able to operate independently (i.e., loss of control within one zone or
building wing should not affect another).
Requirements in this respect are highly diferent for the security and safety domain. Here, very high
reliability is expected, although certain compromises can be made regarding availability (failures in
the system must be detected with high reliability).
∗
As examples, consider the clothing worn or lighting that is appropriate for an occupant's current mood.
