Recorders. Recorders are primarily used to record data to provide a permanent
record of present and past conditions. Selection of recording signals should be based
on the following:
a) Federal and state requirements.
b) Equipment manufacturer recommendations.
c) Accounting purposes.
d) Safety requirements.
e) Operator requirements.
Recorders are available in different sizes, shapes, (roll, fold, circular)
and number of recording points. The selection of types should be based on suitability
to needs and, in particular, the number of points being recorded. Trend recording is
becoming more popular since it allows the selection of a large number of inputs to be
recorded and for time periods as desired by the operator. Trend recording also allows
for recording of critical points during startup. Most panel-mounted recorders are the
4-inch strip chart type because of space requirements. Most field-mounted recorders are
the 12-inch circular large case type because they usually meet NEMA Type 3 or better
protection, see NEMA, Standard Publication/No. 250, Enclosures for Electrical Equipment
(1000 volts Maximum). See Table 22 for selection of signals to be recorded. Recorders
are normally supplied with 115 V, 60 cycle, or 24 VDC for the chart drives, some can be
supplied with a pneumatic impulse or mechanical chart drives.
Controllers. Controllers can be used in either closed loop (feedback) or open
loop control configurations. In a closed loop control configuration, a measurement is
made of the variable to be controlled, and is compared to a reference or set point. If
a difference or offset exists between the measured variable and set point, the automatic
controller will change its output in the direction necessary for corrective action to
take place. See Figure 40a.
Open loop control simply does not have a measurement sensor to provide an
input to the controller for a comparison. See Figure 40b. Open loop control can also
occur when an automatic controller is placed in its manual position; saturation of the
controller output at zero or 100 percent of scale; or failure of the final operator,
when it can no longer be changed by its input signal.
Feed-forward control is relatively new and in most cases it is also used in
closed loop control configurations. While feedback control is reactive in nature and
responds to the effect of an upset which causes an offset between the measured variable
and set point of the controller, feed forward schemes respond directly to upsets and,
thus, offer improved control. See Figure 40c. There are also several types of control
units available such as: