a PID Controller
PID temperature controller is the most sophisticated controller
Three levels of tuning - Proportional, Integral, and Derivative -
exceptional performance at a surprisingly low price. But what is it,
Here is an article that will help you understand it!
Phil Johnson, McShane, Inc.
relatively new type
of temperature controller is called a "PID" controller which this
will attempt to describe in layman’s language.
and performance of many devices change with a change in temperature
them difficult to use in a particular operation. The change in
is caused by a change in environment. To hold characteristics constant
in a changing environment we must supply or remove heat to compensate
variations in ambient temperature. This is accomplished with
installations of temperature
controllers supply heat, or remove heat (chill), to hold the
at a constant point somewhat above or below the ambient temperature.
temperature controllers are most often used to vary the supply of an
current through a resistance heater to accomplish this when the
temperature is to be above ambient.
controlled device or
material can also be stabilized at some temperature below environment
controlling the flow of a refrigerant through a heat exchanger. Yet
type of low temperature control system (called buck and boost) supplies
cooling to drop the temperature below the desired set point and then
the temperature by supplying heat via a controller to get the exact
setting. This type of operation is needed when the desired set point is
close to the ambient temperature.
ideal world, once we
set the temperature of an area or device, the temperature would remain
the same over any length of time. Unfortunately we do not live in an
world. Thus, the need for temperature controllers.
one were to observe the
temperature of a controlled item over a period of time it would be rare
to always find that item at the exact target (set point) temperature.
would vary above and below the set point most of the time. What we are
concerned about, therefore, is the amount of variation.
of the newer temperature controller designs uses a sophisticated means
of reducing this variation. This controller is known as a PID
the operation of a PID (Proportional-Integral-Differential) controller
we should review a few basic definitions.
is a value which expresses the rate of change of another value. For
the derivative of distance is speed.
is the derivative of velocity with respect to time.
is the opposite of a derivative. The integral of acceleration is
and the integral of velocity is distance.
means a value varying relative to another value. The output of a
controller is relative to (or a function of) the difference between the
temperature being controlled and the set point. The controller will be
full on at some temperature which is well below the set point (or
temperature). It will be full off at some point above the set
a controller is the difference in degrees between the highest full-on
and the lowest full-off point. It is the band of temperature, or range
of temperature, over which the output of the controller is
The width and center point temperature of this band can be varied using
adjustments on the controller.
of the output
power versus temperature would be a level line at the full output level
from ambient up to the beginning of the proportional control band
is the temperature where the controller begins to provide less than
power). The graph of current output versus temperature then slopes
(to the right, in a typical chart) through the set point and on
to the full off temperature. It would then remain full-off (a level
now, at zero output on the vertical scale) as the controlled
increases. In this range, it is out of control unless cooling is
narrow bandwidth, the
slope of the line in the bandwidth temperature range gets steeper as
bandwidth is decreased. It is steep because the amplifier in the
has been cranked up to a higher gain or higher amplification setting.
this high gain condition, the slightest amount of error, or deviation
the controlled temperature and the set point, causes a large change in
the output of the controller. In this condition, it is very sensitive
will therefore result in a very closely regulated temperature. Because
it is more sensitive, it can also result in faster changes of power
... so much so that it can overshoot the mark and then overshoot the
again going back down. If the bandwidth is set too narrow, the
can violently oscillate around the set point causing undesired
in the controller output.
the other end of the bandwidth
adjustment, the bandwidth is quite wide. That is to say the temperature
points at which the controller turns full on and the temperature at
it turns full off are quite far apart. The graph now changes so that
slope of the power output of the controller in the proportional range
not steep. At this point, the gain of the controller amplifier is set
It takes quite a change in the signal to result in a change of the
This results in slow corrections of temperature. There is little or no
chance of overshooting and therefore the temperature is quite stable.
although the temperature is relatively stable it may be off of the set
point because of the gain not being high enough to keep up with changes
in the environment around the controlled temperature area. This
may occur when an unexpected blast of cold air sweeps across the
must be adjusted according to conditions in and around the controlled
area and other thermal dynamics such as lack of insulation and thermal
conductivity which results in loss of heat at a certain rate. In a well
insulated area, heat loss is low and the bandwidth can be set wide. In
a condition where there may be considerable heat loss, the bandwidth
be set narrower. What you want to adjust the controller for is an
point between stability and response time ... between holding a stable
temperature and making fast corrections.
we know about proportional
control. We also know about gain and stability. There is one more
and that is the ability to hold a given temperature set point.
given constant power
condition, heat loss through insulation will cause the actual
to be slightly less than it would be in a well insulated heated area.
difference is the "I" in PID. It can be manually corrected by changing
the position of the proportional band center point (called offset) so
result is the temperature you want to hold. The problem is that if the
heat loss conditions (insulation) change and the system begins to lose
heat faster, then that changes the offset and you may not be there to
correct it. To compensate for this, we monitor the change of that
point by watching the change in temperature of the sensor. We then take
the derivative of that change (get a value for the rate of change in
- the "D" in PID ) which is then added to the Integral value to make an