User manual

PID Loop Operation
(DL450 Only)
MaintenancePID Loop Operation
(DL450 Only)
Maintenance
8--7
PID Loop Operation (DL450 only)
DL405 User Manual, 4th Edition, Rev. A
Standard DL405 analog input modules are used to interface to field transmitters to
obtain the PV. These transmitters normally provide a 4--20mA current or an analog
voltage of various ranges for the control loop.
For temperature control, thermocouple or RTD can be connected directly to the
appropriate module. The PID control algorithm, residing in the CPU memory,
receives information from the user program, primarily control parameters and
setpoints. Once the CPU makes the PID c alculation, the result may be used to
directly control an actuator connected to a 4--20mA c urrent output module to control
avalve.
With DirectSOFT, additional ladder logic programming, both time proportioning (eg.
heaters for temperature control) and position actuator (eg. reversible motor on a
valve) type of control schemes can be easily implemented. This chapter will explain
how to set up the PID control loop, how to implement the software and how to tune
the loop.
The following block diagram shows the key parts of a PID control loop. The path from
the PLC to the manufacturingprocess and back to the PLC is the closed loop control.
Process Variable
Loop
Calculation
Manufacturing
Process
Setpoint Value
Loop Configuring
and Monitoring
Control Output
External
Disturbances
Σ
Error Term
+
--
PLC System