Digital controllers - Intelligent Buildings and Building Automation - page 22

specific tasks, a special sort of high-level language is often produced, provid-
ing a set of 'instructions' each of which comprises a small program even at
the normal high level. This set is automatically loaded into memory and each
instruction can be invoked by an operator typing the appropriate mnemonic
code together with any necessary data. A good example of this is the DOS
or Windows 'environment' supplied with personal computers.
For a highly specialised area of operation, such as the programming of BAS
control stations, these specialised but powerful instructions make it much
easier for the user to configure his or her own system compared with the case
if he or she had to write instructions in a generalised high-level language.
Such a 'software environment' or 'programming environment' is created for
the particular specialised systems.
A modern BAS is usually provided with some person-machine interface.
One function of the interface is to provide the services of compiling and
assembling. The interface allows the programmer to write the BAS appli-
cation software in a general high-level language or even by simpler means
specially designed. The BAS programming language and programming envi-
ronment will be discussed further in Chapter 3.
2.6 Sensors
Sensors form a vital component of any control system. Sophistication in the
computing and software functions cannot compensate for inaccurate infor-
mation provided by poor-quality or inappropriately mounted sensors. The
types of sensor available for use in building control systems are reviewed and
guidance on selection and installation is given below.
2.6.1 Basic categories
The word sensor is generally used rather loosely to cover all processes between
the measured variable and the input to the control module. According to
function, a sensor may be broken down into three elements:
￿ Sensing element: a component that undergoes a measurable change, such
as voltage or electrical resistance, in response to a change in the variable
to be measured.
￿ Transducer: an active device that produces an electrical signal which is a
function of the change in the sensing element.
￿ Transmitter: a device that produces an electrical signal which is a stand-
ardised function of the change in the physical variable and which can be
used as an input to the control module.
In practice, the functions of the transducer and transmitter are often com-
bined. Their combined function may be referred to as signal conditioning,
which may also include filtering to remove noise, averaging over time, or