PROCESS CONTROLLERS

PROCESS CONTROLLERS
Controllers are the control devices in a process control system. Their functionistomaintainaprocessvariable(pressure,temperature,level, etc.)atsomedesiredvalue(Set-Point).Thisvaluemayormaynotbe constant.ControllerdevicecanbeeitherPneumaticorElectrical. Controllersareanintegralpartofanyprocesssystem.Controllersare designedtomaintainthecontrolledvariableatadesiredvaluethat ensuresthequalityoftheproduct.Adjustingthecontrollertoproducethe properresponsetoaprocessupsetiscalledtuning. Thefunctionisaccomplishedbycomparingaset-pointsignal(desired value)withtheactualvalue(controlledvariable).Ifthetwovaluesdiffer, an error signal is produced

DEFINITION OF PROCESS CONTROL TERMS
Instrumentation & Process Control System uses a lot of words which are explained/defined as follows:
A control system is a system of integrated elements whose function is to maintain a process variable at a desired value or within a desired range of values.
Control system input is the stimulus applied to a control system from an external source to produce a specified response from the control system.
Control system output is the actual response obtained from a control system.
An open-loop control system is one in which the control action is independent of the output.
A closed-loop control system is one in which control action is dependent on the output.
Feedback is information in a closed-loop control system about the condition of a process variable.
A controlled variable is the process variable that is maintained at a specified value or within a specified range.
A manipulated variable is the process variable that is acted on by the control system to maintain the controlled variable at the specified value or within the specified range.

OPEN LOOP (MANUAL CONTROL) SYSTEM
 Open Loop Control System is a control system with out a self(corrective)feedback mechanism. They often require an operator to (Manually)manipulate/control them. Manual control has its uses as it is cheap to install, maintain and simple to operate. However, it is very seldom used in industry because; The operator must remain in position at all times. It cannot be used if the operator is placed in a dangerous area. The process changes faster than the operator can react. A mistake by the operator can have dangerous results. These problems are avoided by using automatic control (closed loop).The job of theinstrumenttechnicianistomakesurethatthistypeofcontroloperates correctly. Modernhouseholdappliancesnowuseautomaticcontroltomakeworkeasier.For example:-Refrigerators and water heaters use automatic temperature control Washing machines use automatic heating and water control

AUTOMATIC (Closed Loop) CONTROL SYSTEM
An automatic control system is a preset closed-loop control system that requires no operator’s action. This assumes the process remains in the normal range for the control system. An automatic control system has two process variables associated with it: a controlled variable and a manipulated variable. Acontrolledvariableistheprocessvariablethatismaintainedataspecified value or within a specified range. example,thestoragetanklevelisthecontrolled variable. A manipulated variable is the process variable that is acted on by the control system to maintain the controlled variable at the specified value or with in the specified range. example, the flow rate of the water supplied to the tank is the manipulated variable.Inanyautomaticcontrolsystem,thefollowingarefour basic controller functions