WHAT IS A CONTROL SYSTEM?




TOPICS COVERED 

  • INTRODUCTION 
  • CONTROL SYSTEM IN POLYMER PROCESSING
  • CONTROL SYSTEM INPUT/OUTPUT
  • TYPES OF CONTROL SYSTEM 
  • AUTOMATIC CONTROL SYSTEM
  • PROCESS VARIABLES 
  • FUNCTION OF AUTOMATIC CONTROL SYSTEM
  • ELEMENTS OF AUTOMATIC CONTROL SYSTEM

INTRODUCTION

Control system  is a set of integerated elements whose fuction is to monitor the process variables to maintain within desired range of values.


CONTROL SYSTEM IN POLYMER PROCESSING

Control system is a device or set of devices used to manage the functioning of polymer moulding machine.
Control system of plastic processing machine is considered as its "brain" because the functioning of a machine depends upon efficiency of its control system.


CONTROL SYSTEM OUTPUT

Control system output is the actual response obtained from a control system
NOTE:- A Control system may have more than one input or output.

TYPES OF CONTROL SYSTEM 

  • Open loop control system
  • Closed loop control system

OPEN LOOP CONTROL SYSTEM

In an Open loop Control system the control action is independent of the output.
Example: A chemical (monomer) addition pump with variable speed contant.






In the above example the feed rate of monomer is determined by an operator to maintain proper chemical composition. If composition of the system changes the pump cannot respond itself by adjusting its feed rate (speed) without an operator (man).


CLOSED LOOP CONTROL SYSTEM

In a closed loop control system the control action is dependent of the output.





A closed loop control feed backs the output signal from signal devices and compares its value with the input set signal. A deviation from the set signal produce by interference is used to correct the controlled output.

Deviation = actual value - set value

A controller determines the differences between set point and actual and takes appropriate action so that the difference is eliminated. Eg: thermostat.


FEEDBACK

Feeback is an "information" about a process variable in case of a closed loop control system.
This information is continuously "feedback" to the control circuit, where it is compared with the set value to produce a proper control action.
example: In the chemical storage tank, the actual water level, sensed by the level transmitter, is feedback to the level controller.

AUTOMATIC CONTROL SYSTEM

  • Automatic control system is an error sensitive self-correcting device.
  • It is pre-set closed loop control system that requires no operators action.
  • Thus, automatic control is also referred as "feedback control".

PROCESS VARIABLES

An Automatic control system has two process variables.
  1. Controlled variable
  2. Manipulated variable

CONTROLLED VARIABLE

Controlled variable is the process variable which is maintained at a specified value or within a specified range of values.
example: chemical level, water tank level.


MANIPULATED VARIABLE

Manipulated variable is the process variable which is manipulated by the control action to maintain the controlled variable.
example: In the water storage tank level of water is the controlled variable and the flow rate of water into the tank, is the manipulated variable.


FUNCTION OF AUTOMATIC CONTROL SYSTEM

There are four basic function in any automatic control system
  1. Measurement
  2. Comparison
  3. Computation
  4. Correction

ELEMENTS OF AUTOMATIC CONTROL SYSTEM

The elements needed to perform the function of an automatic control system are:-
  1. Measurement element
  2. Error detection element
  3. Final control element

Relation between the elements and function

  • The measuring element perform the measuring function by sensing and evaluation the controlled variable.
  • The error detection element compares the value of the controlled variable with the set value, and then sends error signal if a deviation exists.
  • The final control element responds to the error signal by correcting the manipulated variable of the process.l


    ELEMENTS AND FUNCTIONS




TEMPERATURE DETECTOR

Combination of wire materials used for specialized applications are:-

  • Chromel-white gold
  • Molybdenum-Tungsten
  • Iridium-Rhodium
Control system input:- Control system input is the stimulus applied to a control system from an external source to produce a specified response from the control system.
In case of a central room heating unit control system input is the temperature of room as monitored by the thermostat.


Temperature measurement
  • Temperature is a measure of the kinetic energy of molecules which make up any material.
  • Most temperatures measuring device are the energy of the material or system they are monitoring to raise or lower the kinetic energy of the device.
  • Example: household thermometer- the mercury or other liquid  in the bulb of the thermometer expands as its kinetic energy is raised . By observing how far the liquid rises in the tube one can know the temperature of any object.












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