User manual

PID Loop Operation
(DL450 Only)
Maintenance
and Troubleshooting
8--6
PID Loop Operation (DL450 only)
DL405 User Manual, 4th Edition, Rev. A
Introducing DL450 PID Control
The DL450 is capable of controlling a process variable such as those already
mentioned. As previously mentioned, the control of a variable, such as temperature,
at a given level (setpoint) as long as there are no disturbances (cold water) in the
process.
The DL450 CPU has the ability to directly accept signals from electronic sensors,
such as thermocouples, pressure, VFDs, etc. These signals may be used in
mathematically derived control systems.
In addition, the DL450 has built--in PID control algorithms that can be implemented.
The basic function of PID closed loop process control is to maintain certain process
characteristics at desired setpoints. As a rule, the process deviates from the desired
setpoint reference as a result of load material changes and interaction with other
processes. During this control, the actual condition of the process characteristics
(liquid level, temperature, motor control, etc.) is measured as a process variable
(PV) and compared with the target setpoint (SP). When deviations occur, an error is
generated by the difference between the process variable (actual value) and the
setpoint (desired value). Once an error is detected, the function of the control loop is
to modify the control output in order to force the error to zero.
The DL450 PID control provides feedback loops using the PID algorithm. The
control output is computed from the measured process variable as follows:
Let:
Kc = proportional gain
Ti = Reset or integral time
Td = Derivative time or rate
SP = Setpoint
PV(t) = Process Variable at time “t”
e(t) = SP--PV(t) = PV deviation from setpoint at time “t” or PV error.
Then:
M(t) = Control output at time “t”
M(t) = Kc [ e(t) + 1/Ti
∫ e(x) dx + Td d/dt e(t) ] + M0
t
0
The analog input module receives the process variable in analog form along with an
operator entered setpoint; the CPU computes the error. The error is used in the
algorithm computation to provide corrective action at the control output. The function
of the control action is based on an output control, which is proportional to the
instantaneous error value. The integral control action (reset action) provides
additional compensation to the control output, which causes a change in proportion
to the value of the change of error over a period of time. The derivative control action
(rate change) adds compensation to the control output, which causes a change in
proportion to the rate of change of error. These three modes are used to provide the
desired control action in Proportional (P), Proportional--Integral (PI), or
Proportional--Integral--Derivative (PID) c ontrol fashion.