Controllers
A controller is a tool in the control system engineering branch containing a mathematical model that handles all processes' responses.
What is the function of the controller?
The Controller controls its output parameters based on the difference
between the required set point value (SP) and the measured feedback’s actual
values (PVs). to achieve the designed target of the process which is the
(SP). It is used in industrial manufacturing processes, robotics, and
communication systems to drive the output equipment with the electrical
signal called manipulated value to achieve the (SP).
Besides the main function of the control, the controller has another very
important function which is improving the controller’s behavior performance of
the process by applying special formulas and equations that affect the
process’s performance.
The controller could increase or decrease the process’s speed, dealing with
the error between the reference set point and the actual value to decrease
the maximum overshot of the system, and at the steady state (when the
parameters are not changed and the difference between the set point and the
actual value is constant), it minimizes the gap between the set point and
actual value which called (steady-state error).
Where does the controller exist in the process control system?
The controller exists in the automation pyramid at the control level between
the field level and the supervision level. It reads the process values (PVs)
from the field level and delivers the manipulated variable again to the
process through the field level. The supervision level reads all the
parameters of the controller as well as the process parameters to confirm
the process performance.
How is the controller working?
The controller collects the process variables (PV) data from the sensor or a
transducer as feedback. The controller analyzes and compares these signals
with the reference set point values (SP) that are stored in the controller.
according to the controller’s program or algorithm, the difference between
the reference set-point (SP) and the actual feedback (PV) is calculated. It
is denoted as the error value (E).
Based on the error value, the controller drives a control signal called the
Manipulated variable (MV) to the output equipment which acts according to
the (MV). At the point that the value of the error (E) is equal to zero, the
controller will take no action. If the (PV) changes for any reason, the
controller will recalculate the new error and drive the new (MV) again. The
combination of the reference input signal (SP), the actual feedback signal
or the process value (PV), the controller, and the manipulated variable (MV)
is called the control loop.
Temperature Control loop.
Let us take the temperature control loop as an example to explain the
controller. The process is a burner working with fuel to generate
temperature. Let us say that the requested (SP) is 1000 o C. At the
beginning of the process, the temperature (PV) is equal to the room
temperature which means that the (PV) is 20 oC. The error value (E) in this case is equal to 1000 – 20 = 980 oC.
At this point, the controller will drive a manipulated variable (MV)
to the fuel valve to open (based on the controller program and algorithm).
The temperature starts to increase. As long as the (PV) is lower than the
(SP) which means that the error (E) is positive, the controller will send an
open control signal to the fuel valve (check the
Instrumentation system
article).
If the (PV) temperature exceeds the reference set point (SP) for
example it reaches 1010 oC, the controller starts driving a close control signal to the fuel valve.
As long as the (PV) is more than the (SP) which means that (E) is negative,
the controller will send a close control signal to the fuel valve. At the
point of the (PV) is equal to the (SP) where (E) is zero, the controller
will stop driving any signal to the fuel valve. As long as the (E) value is
zero, the output from the controller will be zero. If the value (E) changes
positively or negatively, the action of the control valve will change open
or close accordingly.
What are the types of controllers in industrial control systems?
The word controller types could be identified as control types based on
their function, based on their parameters, or based on their location’s
configuration in the control system’s pyramid. Let us first speak about the
types related to the function. The main three types of controllers’ types
are:
1- Start/Stop Controller.
The on/off controller is the basic type of control and from its name, it
starts or stops the equipment by activating its output which is a relay. A
simple example of that controller is the one being in your home in the
kitchen’s oven or the fridge. (Note that what we are talking about at home
is the one in the industrial control with complicated techniques and large
scale. Even if it could be used as it is). In those controllers, the SP is
adjusted. The controller will start the oven’s heater or the fridge’s
compressor as long as the actual value is below the SP. When the PV exceeds
the SP it will stop the heater or the fridge’s compressor.
2- Proportional controller.
It has the same function as the on/off controller except that the time of
the on and off of the controller are considered. This type of controller
could decrease the power consumption by adapting the on-time and off-time
(this will decrease the maximum level of temperature reached over the SP
which is called overshot). Although the power consumption decreases, it has
a disadvantage which is making the process slower than the normal on/off
controller.
3- PID controller.
This type of controller like other controllers reads the PV from the system
and continuously calculates the error between the SP and PV then applies the
corrective action based on a specific mathematical model containing the
proportional part (P), Integration part (I), and derivative part (D) forming
the well-known PID controllers. The PID deals with the process speed, the
overshot of the system, and the steady state error. All these conditions
should be adjusted, adapted, and tuned to achieve the perfect performance of
the process to achieve its. Targets.
More articles about control and control systems will be explained in detail later.
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