There are three basic types of controllers: on-off,
proportional and PID. Depending upon the system to be
controlled, the operator will be able to use one type or the
other to control the process.
On/Off
An on-off controller is the simplest form of temperature
control device. The output from the device is either on or
off, with no middle state. An on-off controller will switch the
output only when the temperature crosses the setpoint.
For heating control, the output is on when the temperature
is below the setpoint, and off above setpoint.
Since the temperature crosses the setpoint to change the
output state, the process temperature will be cycling
continually, going from below setpoint to above, and back
below. In cases where this cycling occurs rapidly, and to
prevent damage to contactors and valves, an on-off
differential, or “hysteresis,” is added to the controller
operations. This differential requires that the temperature
exceed setpoint by a certain amount before the output will
turn off or on again. On-off differential prevents the output
from “chattering” or making fast, continual switches if the
cycling above and below the setpoint occurs very rapidly.
On-off control is usually used where a precise control is
not necessary, in systems which cannot handle having the
energy turned on and off frequently, where the mass of the
system is so great that temperatures change extremely
slowly, or for a temperature alarm.
One special type of on-off control used for alarm is a limit
controller. This controller uses a latching relay, which must
be manually reset, and is used to shut down a process
when a certain temperature is reached.
Proportional
Proportional controls are designed to eliminate the cycling
associated with on-off control. A proportional controller
decreases the average power supplied to the heater as
the temperature approaches setpoint. This has the effect
of slowing down the heater so that it will not overshoot the
setpoint, but will approach the setpoint and maintain a
stable temperature. This proportioning action can be
accomplished by turning the output on and off for short
intervals. This “time proportioning” varies the ratio of “on”
time to “off” time to control the temperature. The
proportioning action occurs within a “proportional band”
around the setpoint temperature. Outside this band, the
controller functions as an on-off unit, with the output either
fully on (below the band) or fully off (above the band).
However, within the band, the output is turned on and off
in the ratio of the measurement difference from the
setpoint. At the setpoint (the midpoint of the proportional
band), the output on:off ratio is 1:1; that is, the on-time and
off-time are equal. if the temperature is further from the
setpoint, the on- and off-times vary in proportion to the
temperature difference. If the temperature is below
setpoint, the output will be on longer; if the temperature is
too high, the output will be off longer.
The proportional band is usually expressed as a
percentage of full scale, or degrees. It may also be
referred to as gain, which is the reciprocal of the band.
Note that in time proportioning control, full power is applied
to the heater, but cycled on and off, so the average time is
varied. In most units, the cycle time and/or proportional
band are adjustable, so that the controller may better
match a particular process.
In addition to electromechanical and solid state relay
outputs, proportional controllers are also available with
proportional analog outputs, such as 4 to 20 mA or 0 to
5 Vdc. With these outputs, the actual output level is varied,
rather than the on and off times, as with a relay output
controller.
One of the advantages of proportional control is the
simplicity of operation. It may require an operator to make
a small adjustment (manual reset) to bring the temperature
to setpoint on initial startup, or if the process conditions
change significantly.
Systems that are subject to wide temperature cycling will
also need proportional controllers. Depending upon the
process and the precision required, either a simple
proportional control or one with PID may be required.
Processes with long time lags and large maximum rates of
rise ( e.g., a heat exchanger), require wide proportional
bands to eliminate oscillation. The wide band can result in
large offsets with changes in the load. To eliminate these
offsets, automatic reset (integral) can be used. Derivative
(rate) action can be used on processes with long time
delays, to speed recovery after a process disturbance.
PID
The third controller type provides proportional with integral
and derivative control, or PID. This controller combines
proportional control with two additional adjustments, which
helps the unit automatically compensate for changes in the
system. These adjustments, integral and derivative, are
expressed in time-based units; they are also referred to by
their reciprocals, RESET and RATE, respectively.
The proportional, integral and derivative terms must be
individually adjusted or “tuned” to a particular system using
trial and error. It provides the most accurate and stable
control of the three controller types, and is best used in
systems which have a relatively small mass, those which
react quickly to changes in the energy added to the
process. It is recommended in systems where the load
changes often and the controller is expected to
compensate automatically due to frequent changes in
setpoint, the amount of energy available, or the mass to be
controlled.
There are also other features to consider when selecting a
controller. These include auto- or self-tuning, where the
instrument will automatically calculate the proper
proportional band, rate and reset values for precise
control; serial communications, where the unit can “talk” to
a host computer for data storage, analysis, and tuning;
alarms, that can be latching (manual reset) or non-latching
(automatic reset), set to trigger on high or low process
temperatures or if a deviation from setpoint is observed;
timers/event indicators which can mark elapsed time or the
end/beginning of an event. In addition, relay or triac output
units can be used with external switches, such as SSR
solid state relays or magnetic contactors, in order to switch
large loads up to 75 A.
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