Controllers as name implies controls particular system. There are many controllers which are working on different tasks with different working mechanism for example temperature controllers, Humidity controllers & PH controllers.
All these controllers are working as a Closed loop system.
CLOSED LOOP SYSTEM
A closed loop or control loop system monitors the input & according to the set parameters; It produces an output. Further according to the feedback it regulates the output.
For better understanding, A closed loop system of heating is been discussed next:
A temperature is required to be set on the controller while a temperature sensor is connected to it as an input which provides the present temperature of the system. An error or difference between set value & present value been calculated by the controller, Than according to the control method controller produces an output which further controls the heater element.
When heater get ON; temperature starts increasing, Meanwhile controller continuously monitors the present temperature & calculates the error value. By controlling the output; Controller try to achieve the set temperature.
Diagram of Control loop or Closed loop System
TYPES OF CONTROL METHODS
Control Method defines how a output been utilized to achieve the set parameter. For example lets again talk about the heating control system. Suppose system’s set temperature is 50°C & present temperature is 20 °C.
Simple method of controlling (ON-OFF CONTROL)
At present, temperature is 20°C. Controller hold the heater element ON until temperature 50°C is achieved after that even heaters get disconnected from controller side; temperature still raising which leads to overshoot in present temperature.
Due to the overshoot, Present temperature get stop near around some greater value than the set value. Heaters do not get connected until temperature falls below the set 50°C.
In this simple control method present temperature fluctuates around the set value but do not able to get the stable around the set value. BECAUSE this control do not care about the ‘magnitude’ & ‘time span’ of error value, In along it also do not care about the ‘rate of change’ of present value.
Advance method of controlling (PID CONTROL)
At start, present value is 20°C. Up to 40°C, It directly connects the heaters ON after that heaters get disconnected but even after temperature remains increasing which leads to temperature reaches to 45°C. After 45 °C, Controller connects the heaters in discrete pulse form like ON OFF ON ON ON OFF OFF ON. This discrete behavior continue until A set temperature been achieved.
At set 50°C, controllers monitors the rate of change of present value & controls the heaters output in discrete pulse form; accordingly. By doing so, controller try to achieve the set value at steady state.
This PID control monitors the ‘magnitude’ & ‘time span’ of error value. In along it also monitors the ‘rate of change’ of present value.
UNDERSTANDING BOTH THE CONTROL METHODS WITH DEEP ANALYSIS
ON-OFF CONTROL
In ON-OFF control, Controller produces an output until it reaches to the set point, On reaching the set point; controller stops the output, Further output get on when present value falls below the set value.
In such control, Hysteresis range can also be set around the set point. In this; For a particular set point; A range been set around the set point. Controller tries to stay inside the set range.
In this control process, value remains oscillating around the set value; It will not get steady state around its set valve. ON-OFF control is not suitable for highly sensitive processes; as present value do not get stable around the set value.
For better understanding, Heating control through ON-OFF control with having a hysteresis range been discussed through a Graph:
A Graph shows ON-OFF Control for heating element
In This, Square shape x-axis graph represents the heater ON condition while a y-axis line graph represents the position of temperature.
Horizontal axis represents the time axis in which equal time units been divided while vertical axis represents the temperature in °C. 50°C is the set temperature. A hysteresis range of ±10 is set around the set temperature.
It is clearly seen in the graph that at the time of starting temperature raises proportionally for time units up to T4 until it reaches to 40°C. On reaching 40°C heater get off but temperature still increasing gradually which lead to temperature reaches to the peak value which is 60°C. After that temperature starts falling & get reaches to 40°C.
At 40°C, heater again get ON for fixed interval & temperature starts increasing & reaches to 60°C. From 60°C,Temperature again starts to fall. At finally 40°C, heater again get ON. Like this, circle remains continue.
A clear oscillation been seen in the graph where present value oscillates around the set temperature.
PID CONTROL
PID is the algorithm of three techniques Proportional, integral & derivative. PID provides stable control on the system & makes the system equal to the set point in a steady state If PID parameters been set according to the characteristics of the system.
PID control output depend upon the ‘magnitude‘ & ‘time span‘ of error value, It also monitors the rate of change of present value.
Next, We are going to discuss all the three techniques one by one !
PROPORTINAL BAND
Proportional control as name suggests control the output proportional to the size of error signal. It works in a band around the set value & its been controlled with the units °C.
The width of band determines the magnitude of the responses to the error. If it is much wider than; Overall output response will be sluggish & if it is too narrow than overall output response fluctuates around the set value. This width should be consider according to the characteristics of the system.
Below down the set value, Proportional band makes the output fully ON 100% & above the proportional band it makes the system output completely off; clearly shown in an graph below:
A graph(Output VS Temperature) shows the working of Proportional Control
Near around the set point; it is difficult for proportional band to achieve the stable target parameter as it follows the error proportionally. Near to the set value, Error value get low.
A graph(Time VS Temperature) shows the working of Proportional Control
“In easy language-We can say a proportional controls creates a range or a band around the set value. Below the set value, it makes the output completely ON while above the set value; it makes the output completely OFF.
But in the range around the set value, It precisely follows the error value in a direct proportion to the error.”
INTEGRAL TIME
In proportional control, Between the present value & set value; An error is must to produces an output. It does not care about the rate of change in error value. On its own, Proportional control fluctuates around the set point.
Integral mechanism is used to achieve the zero steady state error Which control the magnitude of the output according to the magnitude of error value. If present value is below the set value, it makes the magnitude of output higher to reach the set value. If present value is above the set value it makes the magnitude of output lower.
The units of integral control is time (1 to 9999seconds). Larger the time units makes the system slower to achieve the set point while a low time units makes the system fluctuating. Time units of integral control is very important which should be set according to the characteristics of the system.
A graph(Time VS Temperature) shows the working of Integral Control
“In a simple language- in ON-OFF control, there are only two things ON & OFF. This control do not care about how much ON & how much OFF; for how much time interval. But Integral control care about all these things”
DERIVATIVE TIME
Derivative control provides sudden change in output, depending upon the rate of change of error. Higher the rate of change of error; more the magnitude of output power. Lower the rate of change of error leads to lower magnitude of output power.
A graph(Time VS Temperature) shows the working of Derivative Control
“In an easy language- On reaching the set value; Derivative control enters into the play. Derivative control actually monitors the rate of change of error value & then it produces the output accordingly.”
PID Controller P104 by EUROTHERM
SUMMARY
A Closed loop or control loop system monitors the input & according to the set parameters; It produces an output. Further according to the feedback it regulates the output.
Closed loop has a control method according to which it controls the output.
Control Method defines how a output been utilized to achieve the set parameter.
Basically there are two control methods; One is ‘ON-OFF‘ control & another one is ‘PID‘ control.
In ON-OFF control, Controller produces an output until it reaches to the set point, On reaching the set point; controller stops the output, Further output get on when present value falls below the set value.
PID is the algorithm of three techniques Proportional, integral & derivative. PID provides stable control around the set value.
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