3
types of controller are
•Discontinuous modes
•Multiposition modes
•Continuous modes
Discontinuous modes are controllers that
have only two modes or positions; on and off.
•Example
- hot water heater.
•When
the temperature of the water in the tank falls below setpoint, the burner turns
on. When the water temperature reaches setpoint,
the burner turns off. Because the water starts cooling again when the burner
turns off, it is only a matter of time before the cycle begins again. This type
of control doesn’t actually hold the variable at setpoint,
but keeps
the variable within proximity of setpoint in
what is known as a dead zone.
Multiposition modes
• controllers that have
at least one other possible position in addition to on and off.
•Therefore, the
oscillation around set point can be less dramatic when multistep controllers
are employed than when discrete controllers are used
•Continuous
modes
Controllers
automatically compare the value of the PV to the SP to determine if an error
exists. If there is an error,
the controller adjusts its output according to the parameters that have been
set in the controller.
The tuning parameters
essentially determine:
How much correction should be made?
• The magnitude of the
correction( change in controller output) is determined by the proportional mode
of the controller.
How long should the
correction be applied?
•The duration of the
adjustment to the controller output is determined by the integral mode of the
controller
How fast should the
correction be applied?
•The speed at which a
correction is made is determined by the derivative mode of the controller
Four modes of control commonly used for most
applications are:
Proportional (P)
Proportional plus Reset (PI)
Proportional plus Rate (PD)
Proportional plus Reset plus Rate (PID)
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