Control
Modes
On/off control operates with the plant
fully on or fully off, which results in the controlled temperature overshooting the
differential, due to the system and controller responses, especially at low
loads.
The
diagram below shows a Temperature / time graph for a heating system
demonstrating that the mean room temperature varies about the thermostat set
point according to the load.
A better form of control is to vary
or modulate the
plant output, so that as the temperature approaches the required value the output
can be reduced to stop any overshoot.
This can be done only if the plant
can be modulated,
for instance, by using a modulating
control valve or, in the case of a boiler, by having a modulating burner.
All of this adds to the expense of
the plant but produces better control.
The control system which can provide
this modulating or varying action is proportional plus integral plus differential (PID) control.
As there are three distinct parts to PID control, it is also referred to
as three-term control. It is possible to use only
proportional or proportional plus integral control but for computer controlled
systems like Building Energy Management Systems (BEMS) PID three-term direct digital control is used.
Proportional Control
Action
With this form of control the valve
or damper is positioned in intermediate
positions in proportion
to the response to slight changes in the controlled condition.
Therefore the controlled device does
not run through its complete
stroke as in the case of two-position control.
Also the controlled device does not
continue to move until it reaches a limit as in floating control.
Instead, with this form of control,
the controlled device immediately assumes a position in proportion to the system requirement.
A simple example of proportional control is the ball-cock valve in a
cistern.
As the water rises in the cistern,
so the ball is raised and gradually closes the valve.
When the cistern is empty, the valve
is fully open and when the cistern is full, the valve is shut.
All proportional controllers work
within limits of controlled variable to produce a full range of control action.
This is called proportional band. This can
be varied on most proportional controllers.
Another definition of proportional band is the range of variables through
which the actuator has gone from fully open to fully shut or visa versa.
The diagram below shows the result
of a system where temperature is being controlled by a valve.
The proportional
band is calculated as being 13 degrees C.
When the measured temperature is
lower than the set point temperature then
the actuator driving the valve will move.
As can be seen from the above diagram,
when the measured temperature is 15.5oC, the valve will be 50% open.
Proportional controllers have Proportional Band (PB) adjustment so that the valve
movement can be increased or decreased for a given range of temperatures.
In the diagram below the Proportional Band (PB) is only 2 degrees C.
This means that small changes in
temperature cause the valve to move more in one direction or the other.
A change from 20oC to 21oC
causes the valve to drive from 50% open to fully closed.
The diagram below shows the
temperature variable in a room being controlled in a proportional
mode.
Initially the controlled variable
will be driven towards the set-point at such a rate as to cause overshoot.
On the return cycle, the overshoot
will be less and this oscillation will continue until stable conditions
exist.
If the system is inherently unstable
then the hunting
will continue indefinitely.
This sometimes happens if too much
Integral action is added and the controlled variable never settles down
therefore causing hunting.
With proportional control there will
be nearly always be an off-set
from the desired point.
The temperature in above diagram will
never actually reach the desired point or set-point but may get close enough as
not to matter.
Offset can be reduced in Proportional Control by having smaller Proportional Bands. However, if the PB is set too small then hunting
will occur.
This is where the valve acts like
on-off control and hunts to the open or closed
position.
This may occur if the PB is set to as
little as 2 degrees C.
Also if the PB
is set too high there would be very large offset
and swings in temperature.
Usually a good PB for room temperature is 3oC.
Integral Control Action
Integral control is seldom used alone, it
usually is an important addition to other forms of control, particularly to the
proportional mode.
With Integral
action there is continuous movement of
the valve or damper when the sensor is giving a value above or below the
set-point.
While deviation form the set-point
exists, the controller will get the actuator to move at a speed which
corresponds with the amount of deviation from the set point.
In other words the rate of movement is a function of the amount of
deviation from the set-point.
In Integral control
the Integral Time (IA)
sets the time interval which allows the controller to give the same amount of valve
movement as that produced by the proportional action.
Derivative Control Action
Derivative control involves a
further development of integral action such that the controller output is a
function of the rate of change of the
controlled variable.
This form of control, like integral
mode, would not normally be used alone, but in combination with others.
P + I and P + I + D Control Action
Proportional plus Integral Control Action is sometimes referred to as
proportional with reset (or abbreviated to PI)
this combination gives stable control with zero off-set.
So long as there is deviation from
the set-point, the controller will continue to signal a change until zero error
exists.
This
approach would be applied, for example, to space temperature control in
circumstances where the load fluctuated widely
over relatively short periods of time.
This
could not be achieved by proportional control alone since the proportional band
would have to be too wide.
Also
PI control is used more generally for applications
where close control is required.
Proportional plus Integral plus Derivative Control Action is abbreviated PID. PID is sometimes called three
term control.
This
mode of control would be used where there are sudden
and significant load changes.
Also
it is used where zero off-set from the desired
set-point is required.
This
is seldom the case in heating and air-conditioning projects.
A
PID control system is shown below.
PI
control is normally suitable for hospital operating theatres or other areas
where close control is required.
Most
heating and air conditioning systems operate well with PI
control.
In
some fast moving air conditioning systems such
as VAV with high air speeds and fast damper movement requirements, PID control can be used.