Automationdirect.com DL05, User Manual

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PID Loop Operation
Maintenance
8–43
PID Loop Operation
Closed-Loop Auto Tuning – During a closed-loop auto tuning cycle, the loop
controller operates as shown in the diagram below.
Process Variable
Loop
Calculation
Manufacturing
Process
Setpoint Value
Control
Output
S
Error Term
+
–
PLC System
Closed Loop
Auto Tuning
Limit cycle wave
Process Variable
Response
When auto tuning, the loop controller imposes a square wave on the output. Each
transition of the output occurs when the PV value crosses over (or under) the SP
value. Therefore, the frequency of the limit cycle is roughly proportional to the mass
of the process. From the PV response, the auto tune function calculates the gains
and the sample time. It automatically places the results in the corresponding
registers in the loop table.
The following timing diagram shows the events which occur in the closed-loop auto
tuning cycle. The auto tune function examines the direction of the offset of the PV
from the SP. The auto tune function then takes control of the control output and
induces a full-span step change in the opposite direction. Each time the sign of the
error (SP – PV) changes, the output changes full-span in the opposite direction. This
procedes through three full cycles.
SP
PV
Output Value
PID Cycle
Auto Tune Start Auto Tune End
PID Cycle
Auto Tune Cycle
Closed Loop Auto Tune Cycle Wave: Limit Cycle Method
X
o
M
T
o
*
M
max
= Output Value upper limit setting M
min
= Output Value lower limit setting.
* This example is direct–acting. When set at reverse–acting, output is inverted.
Process Wave
Calculation of
PID parameter