User manual

PID Loop Operation
(DL450 Only)
Maintenance
and Troubleshooting
8--26
PID Loop Operation (DL450 only)
DL405 User Manual, 4th Edition, Rev. A
Once the PID table is established in V--memory, configuring the PID loop continues
with the DirectSOFT PID setup configuration dialog. You will need to check and fill in
the data required to control the PID loop. Select Configure and the following dialog
will appear for this process.
Select the Algorithm Type
Chose either Position or Velocity. The default algorithm is Position. This is the choice
for most applications which include heating and cooling loops as well as most
position and level control loops. A typical velocity control will consist of a process
variable such as a flow totalizer in a flow control loop.
Enter the Sample Rate
The main tasks of the CPU fall into
categories as shown to the right. The list
represents the tasks done when the CPU
is in Run Mode, on each PLC scan. Note
that PID loop calculations occur after the
ladder logic task.
The sample rate of a control loop is simply
the frequency of the PID calculation. Each
calculation generates a new control output
value. With the DL450 CPU, you can set
the s ample rate of a loop from 50 mS to
99.99 seconds. Most loops do not require
a fresh PID calculation on every PLC
scan. Some loops may need calculating
only once in 1000 scans.
Enter 0.05 sec., or the sample rate of your
choice, for each loop, and the CPU
automatically schedules and executes
PID calculations on the appropriate scans.
Read
Inputs
Service
Peripherals
Ladder
Program
Calculate
PID Loops
Internal
Diagnostics
Write
Outputs
PLC
Scan
NOTE: If more than 4 loops are programmed, enter a minimum of 0.1 s econd.
Configure the PID
Loop