National Instruments PID Control Toolkit 371685C-01 - Manual (Page 18)

© National Instruments Corporation 3-1 LabWindows/CVI PID Control Toolkit User Manual
3
Using the PID Control Toolkit
This chapter contains the basic information you need to design a control strategy using the
PID Control Toolkit functions.
Designing a Control Strategy
When you design a control strategy, sketch a flowchart that includes the physical process and
control elements such as valves and measurements. Add feedback from the process and any
required computations. Then use the PID Control Toolkit functions to translate the flowchart
into an application.
You can handle the inputs and outputs using DAQ devices, FieldPoint I/O modules, GPIB
boards, or serial I/O ports. You can adjust polling rates in real time. Potential polling rates are
limited only by your hardware.
Setting Timing
According to control theory, a control system must sample a physical process at a rate that is
approximately 10 times faster than the fastest time constant in the physical process. For
example, a time constant of 60 s is typical for a temperature control loop in a small system.
In this case, a cycle time of 6 s is sufficient. Faster cycling offers no improvement in
performance (Corripio 1990).
The PID control feature, lead-lag feature, and setpoint profile feature in the PID Control
Toolkit are time-dependent. A component can acquire the timing information either from a
value you supply to the
pidAttrDeltaT attribute or from the built-in internal timer. By
default, the pidAttrUseInternalTimer attribute is set to 1, so the component uses the
internal timer. Call PidSetAttribute and PidGetAttribute to set and get PID controller
attributes.
The internal timer calculates new timing information each time PidNextOutput is called.
When the function is called, the timer determines the time since the last call to
PidNextOutput and uses that time difference in its calculations.
You can set the component to use the value you have supplied to the pidAttrDeltaT
attribute by setting pidAttrUseInternalTimer to 0. Use the pidAttrDeltaT attribute
for fast loops, including instances in which you use acquisition hardware to time the
controller input.