PID Loop Operation
Maintenance
8–17
PID Loop Operation
Ten Steps to Successful Process Control
Modern electronic controllers such as the DL05 CPU provide sophisticated process
control features. Automated control systems can be 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 make 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 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 deg. 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 DL05 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, or use a analog I/O
combination module, among several control loops. The example shown sends the
PV and Control Output signals for two loops through the same set of modules.
Automationdirect offers DL05 analog input modules with 4 channels per module
that accept 0 – 20mA or 4 – 20mA signals. Also, analog input and output combination
modules are now available. Refer to the sales catalog for further information on
these modules, or find the modules on our website, www.automationdirect.com.
Step 1:
Know the Recipe
Step 2:
Plan Loop
Control Strategy
Step 3:
Size and Scale
Loop Components
Step 4:
Select I/O Modules
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