National-instruments PID Control Toolkit 371685C-01 Instrukcja Użytkownika Pobierz pdf (Strona 18)

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.

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National-instruments PID Control Toolkit 371685C-01 Instrukcja Użytkownika Strona 18