5. Proportional Action
(P-Action)
A Proportional Band that is too
narrow causes hunting! The TC will
than behave like an ON/OFF
controller!
A correctly sized P-Band results in
an Overshoot, followed by an
Undershoot and than Stabilization,
with a small offset near the Set
Point.
With a (far) too large P-band the
Set point temperature will never be
reached!
6. Integral action
P+I -Action
When we add the “Integral Action” we can reduce the Offset to
0C, in a specified time.
The “Integral Action” will reduce the Offset completely, in the
“Integral time”
Because the “Integral Action” “Resets” the Offset caused by the
P-Action, the I action is in some books also called the “Reset
Action”.
7. Integral control is a second form of feedback control.
A negative error will cause the signal to the system to decrease,
while a positive error will cause the signal to increase.
I-only controllers are much slower in their response time than P
only controllers.
This slower response time can be reduced by combining I-only
control with another form, such as P or PD control.
I-only controls are often used when measured variables need to
remain within a very narrow range and require fine-tuning control.
I-control correlates the controller output to the integral of the
error.
8. Differential Action
P+ I+ D-Action
When we get a “Disturbance” in our process the PI
controller responds “rather slow”
The D-Action looks at the speed of change.
It INCREASES the Output capacity of the TC VERY
RAPIDLY
D-Action” we will have LESS Undershoot, LESS Overshoot
and win valuable Time.
9. D-control is a form of feed forward control.
It functions to minimize the change of error, thus keeping the
system at a consistent setting.
The primary benefit of D controllers is to resist change in the
system.
Unlike proportional and integral controllers, derivative controllers
do not guide the system to a steady state. Because of this
property, D controllers must be coupled with P, I or PI controllers to
properly control the system.
D-control correlates the controller output to the derivative of the
error.
10. •A proportional-integral-derivative controller (PID
controller) is a generic control loop feedback
mechanism (controller) widely used in industrial control
systems.
•A PID controller calculates an "error" value as the difference
between a measured process variable and a desired set
point
•The PID controller calculation algorithm involves three
separate constant parameters, and is accordingly sometimes
called three-term control: the proportional,
the integral and derivative values
11. Proportional
• P depends on the present error
P
Integral
I
Differential
D
• I on the accumulation
of past errors
• D is a prediction of future errors,
based on current rate of change
12. The Characteristics of P, I, and D
Controllers
A proportional controller (
) will have the effect of reducing
the rise time and will reduce but never eliminate the steadystate error.
An integral control (
) will have the effect of eliminating the
steady-state error for a constant or step input, but it may make
the transient response slower.
A derivative control (
) will have the effect of increasing the
stability of the system, reducing the overshoot, and improving the
transient response.
13. In fact, changing one of these variables can change the effect of the
other two.
With the PID controller we can set the P+I+D values so that we will
not have any Over or undershoot and reach set point directly.
PID controller has all the necessary dynamics: fast reaction on
change of the controller input (D mode), increase in control signal to
lead error towards zero (I mode) and suitable action inside control
error area to eliminate oscillations (P mode).
“This “combination”, of “Present + Past
+ Future”, makes it possible to control
the application very well”
14. The effects of each of controller parameters
Kp, Ki , Kd
CL
RESPONS
E
SPEED OF
RESPONSE
OVERSHOOT
S-S ERROR
Kp
Increase
Increase
Decrease
Ki
Decrease
Increase
Eliminate
Kd
Increase
Decrease
No Change
