User manual

PID Loop Operation
(DL450 Only)
Maintenance
and Troubleshooting
8--16
PID Loop Operation (DL450 only)
DL405 User Manual, 4th Edition, Rev. A
Ten Steps to Successful Process Control
Modern electronic controllers such as the DL450 CPU provide sophisticated
process control features. Automated control systems can be very difficult to debug,
because a given symptom can have many possible causes. We recommend a
careful, step-by-step approach to bringing new control loops online:
The most important knowledge is -- how to produce your product. This knowledge is
the foundation for designing an effective control system. A good process recipe will
do the following:
S Identify all relevant Process Variables, such as temperature, pressure,
or flow rates, etc. which need precise control.
S Plot the desired Setpoint values for each of the process variables for the
duration of one process cycle.
This simply means choosing the method the machine will use to maintain control
over the Process Variables to follow their Setpoints. This involves many issues and
trade-offs, such as, energy efficiency, equipment costs, ability to service the
machine during production, and more. You must also determine how to generate the
Setpoint value during the process, and whether a machine operator can change the
SP.
Assuming the control strategy is sound, it is still crucial to properly size the actuators
and properly scale the sensors.
S Choose an actuator (heater, pump. etc.) which matches the size of the
load. An oversized actuator will have an overwhelming effect on your
process after a SP change. However, an undersized actuator will allow
the PV to lag or drift away from the SP after a SP change or process
disturbance.
S Choose a PV sensor which matches the range of interest (and control)
for our process. Decide the resolution of control you need for the PV
(such as within 2°C), and make sure the sensor input value provides the
loop with at least 5 times that resolution (at LSB level). However, an
over-sensitive sensor can cause control oscillations, etc. The DL450
provides 12-bit and 15-bit, unipolar and bipolar data format options, and
a 16--bit unipolar option. This selection affects SP, PV, Control Output,
and Integrator sum.
After deciding the number of loops, PV variables to measure, and SP values, you
can choose the appropriate I/O modules. Refer to the figure on the next page. In
many cases, you will be able to share input or output modules among several control
loops. The example shown sends the PV and Control Output signals for two loops
through the same set of modules. Up to four loops could be handled by the modules
shown.
AutomationDirect offers DL405 analog modules with 4, 8, and 16 channels per
module in various signal types and ranges. Also available are thermocouple and
RTD modules which can be used to maintain temperatures to within a 10
th
of a
degree. Refer to our sales catalog for further information on these modules, or find
the modules on our website, www.automationdirect.com.
Step 1:
K
now the Recipe
Step 2:
Plan Loop
Control Strategy
Step 3:
Size and Scale
Loop Components
Step 4:
Select I
/
O Modules