NI LabVIEW Manual de usuario

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April 2004

323525B-01

Getting Started with the

LabVIEW Real-Time Module

This document provides steps to build a simple deterministic application

and references to programming examples and documentation for more

information as you build the application. Refer to the LabVIEW Real-Time

Module Release Notes for installation and configuration instructions. Refer

to the LabVIEW Real-Time Module User Manualfor information about

programming deterministic applications using LabVIEW.

Contents

Launching LabVIEW and Exploring Documentation ............................ 1

Creating and Running a Deterministic Application................................ 3

Creating the Control Loop ............................................................... 3

Adding Analog Input/Output and Data Processing

to the Control Loop....................................................................... 4

Setting Priority of the VI to Time Critical....................................... 6

Adding Timing to the Control Loop................................................ 6

Sending Data Out of the Time-Critical VI....................................... 7

Downloading and Running the Application .................................... 9

Optimizing the Control Application ....................................................... 11

Launching LabVIEW and Exploring Documentation

The LabVIEW Real-Time Module includes a comprehensive

documentation set designed to help you create deterministic applications.

The LabVIEW Real-Time Module Bookshelf, available by selecting

Help»Search the Real-Time Module Bookshelf from LabVIEW,

is a PDF that contains links to and descriptions of the available

Real-Time Module documentation.

1. Launch LabVIEW to open the LabVIEW dialog box shown in

Figure 1.

2. Open and explore the LabVIEW Real-Time Module Bookshelf.

™

ni.com

Getting Started with the LabVIEW Real-Time Module 2 ni.com

Figure 1. The LabVIEW Dialog Box

The LabVIEW Help contains VI reference information for all

Real-Time VIs.

3. Select Help»VI, Function, & How-To Help to open and explore the

LabVIEW Help.

The RT Module allows you to select the host computer or an RT target as

an execution target to run VIs built in LabVIEW. The LabVIEW dialog

box indicates the current execution target. The default execution target is

the development computer, or host computer. Refer to Chapter 2,

Connecting to RT Targets, of the LabVIEW Real-Time Module User

Manualfor information about selecting execution targets.

When you select an RT target—an RT Series plug-in device or networked

RT Series device—as the execution target, the LabVIEW development

environment continues to run on the host computer but VIs run on the RT

target. Refer to Chapter 1, Introduction to the LabVIEW Real-Time Module,

of the LabVIEW Real-Time Module User Manual for information about RT

targets.

©National Instruments Corporation 3 Getting Started with the LabVIEW Real-Time Module

Creating and Running a Deterministic Application

This section guides you through creating a deterministic PID control

application. You need to configure an NI RT Series PXI controller,

NI PCI-7041 plug-in device, or an RT Series FieldPoint module as an

RT target to complete this exercise. Refer to the Measurement &

Automation Explorer Help for information about configuring a remote

system as an RT target.

The application receives an analog input, processes the data using PID

control VIs, and outputs the resulting control data. Refer to the LabVIEW

PID Control Toolset User Manualfor information about the PID control

algorithm and the PID Control Toolset VIs. Refer to the LabVIEW

Real-Time Module User Manual for information about Real-Time Module

features.

Creating the Control Loop

Complete the following steps to create a control loop for the control VI.

1. Start LabVIEW and select the RT target from the Execution Target

pull-down list. Refer to the LabVIEW Real-Time ModuleUser Manual

for more information about connecting to RT targets.

2. Open a blank VI and display the block diagram.

3. Place a While Loop on the block diagram. Refer to the LabVIEW User

Manual for information about using While Loops.

4. Right-click the conditional terminal and select Create Control from

the shortcut menu to create and wire a Stop button control to the

conditional terminal.

The While Loop runs all code within its boundaries until the conditional

terminal receives a TRUE value. Use the conditional loop as the control

loop in the next section. When you press the Stop button on the front panel,

the button passes a TRUE value to the conditional terminal.

Getting Started with the LabVIEW Real-Time Module 4 ni.com

Adding Analog Input/Output and Data Processing to the Control Loop

Complete the following steps to add analog input/output and a PID control

VI to the control loop.

1. Place the DAQmx Read VI in the control loop on the block diagram.

(FieldPoint) Place the FP Read VI.

2. Right-click the task/channel in input of the DAQmx Read VI and

select Create»Control from the shortcut menu to create a control for

the task/channel in input on the front panel.

(FieldPoint) Create a control for the FieldPoint IO Point In input of the

FP Read VI.

3. Select Browse from the pull-down list of the task/channel in control

on the front panel to open the Select name(s) dialog box. Click the

Create New button to open the DAQ Assistant and create a new

NI-DAQmx task. Refer to the NI-DAQmx Help for information about

using the DAQ Assistant to create an NI-DAQmx task. Select the

NI-DAQmx task that you create from the pull-down list of the

task/channel in control on the front panel.

(FieldPoint) Select the device in the FieldPoint IO Point In control.

Refer to the Measurement & Automation Explorer Help for FieldPoint

for information about configuring a device.

4. Place the PID VI in the control loop on the block diagram. You also

can create custom control VIs using LabVIEW.

5. Wire the data output of the DAQmx Read VI to the process variable

input of the PID VI.

(FieldPoint) Wire the values output of the FP Read VI to the process

variable input of the PID VI.

6. Create controls for the PID gains and setpoint inputs of the PID VI.

7. Place the DAQmx Write VI in the control loop on the block diagram.

(FieldPoint) Place the FP Write VI.

8. Wire the output output of the PID VI to the data input of the DAQmx

Write VI.

(FieldPoint) Wire the output output of the PID VI to the values input of

the FP Write VI.

9. Create a control for the task/channel in input of the DAQmx Write VI

and select an NI-DAQmx task. Refer to step 3 for information about

creating an NI-DAQmx task using the DAQ Assistant.

(FieldPoint) Create a control for the FieldPoint IO Point In input of the

FP Write VI.

The block diagram for an NI-DAQmx system should look similar to

Figure 2.

©National Instruments Corporation 5 Getting Started with the LabVIEW Real-Time Module

Figure 2. NI-DAQmx Control Loop

The block diagram for a FieldPoint I/O system should look similar to

Figure 3.

Figure 3. FieldPoint Control Loop

Getting Started with the LabVIEW Real-Time Module 6 ni.com

The VI reads analog data and sends the data to the PID VI. The PID VI

processes the data using a PID control algorithm. The VI returns the results

to the device. Refer to the LabVIEW PID Control Toolset User Manual for

information about using PID VIs.

(ETS) Refer to the examples\Real-Time\ETS\RT Control.llb for

examples of control VIs.

Setting Priority of the VI to Time Critical

To ensure determinism, you must assign time-critical priority to the control

loop VI. A VI set to time-critical priority receives enough processor

resources to complete time-critical tasks.

1. Right-click the connector pane icon at the top right corner of the

VI and select VI Properties from the shortcut menu to open the

VI Properties dialog box.

2. Select Execution from the Category pull-down menu.

3. Select time critical priority (highest) from the Priority

pull-down menu.

4. Click the OK button to set the priority.

Refer to Chapter 3, Building Deterministic Applications, of the LabVIEW

Real-Time Module User Manual for information about dividing and

prioritizing application tasks. Refer to the Priority Trouble VI in the

examples\Real-Time\RT Tutorial.llb for an example of priority

settings.

Adding Timing to the Control Loop

You must add a timing mechanism that pauses, or sleeps, the control VI to

ensure that other VIs in the application have enough processor resources to

complete. A time-critical VI that does not have any sleep time monopolizes

the embedded CPU resources. Refer to Chapter 3, Building Deterministic

Applications, of the LabVIEW Real-Time Module User Manual for

information about programming deterministic applications.

1. Place the Wait Until Next ms Multiple VI in the control loop on the

block diagram.

2. Right-click the millisecond multiple input of the Wait Until Next ms

Multiple VI and select Create»Constant from the shortcut menu.

3. Enter 10 into the millisecond multiple constant. The timing

mechanism assures you that the control loop code runs and then sleeps

until the time equals 10 ms from the start of the iteration. Other VIs in

the application can run during the sleep time.

4. Select File»Save As and save the VI as First PID Control.vi.

©National Instruments Corporation 7 Getting Started with the LabVIEW Real-Time Module

The block diagram should look similar to Figure 4.

Figure 4. NI-DAQmx Control Loop with Timing

Refer to Chapter 4, Timing Applications and Acquiring Data, of the

LabVIEW Real-Time Module User Manual for information about the

different methods for timing control loops. Refer to the examples\

Real-Time\RT Timing.llb for examples of different timing methods.

Sending Data Out of the Time-Critical VI

The First PID Control VI can run on the RT target at time-critical priority,

but you might need to monitor the status of the VI as it runs on the target.

Also, you might need to update the values of control loop parameters. Use

the Real-Time FIFO VIs to send data between the First PID Control VI and

a normal priority communication VI that also runs on the RT target. The

communication VI on the RT target can receive data from the First PID

Control VI using Real-Time FIFOs and then use network communication

methods to update the host computer VI. Refer to the LabVIEW Real-Time

Module User Manual for information about using the Real-Time FIFO VIs.

Getting Started with the LabVIEW Real-Time Module 8 ni.com

Complete the following steps to control the Stop button from the host

computer.

1. Open the First PID Control VI in LabVIEW.

2. Remove the Stop button control wired to the conditional terminal of

the control loop on the block diagram.

3. Place the RTFIFOCreate VI to the left of the control loop on the block

diagram.

4. Right-click the name input of the RTFIFOCreate VI and select

Create»Constant from the shortcut menu. Enter Stop Control into

the name constant.

5. Wire a numeric constant to the type input of the RTFIFOCreate VI.

The type input represents the type of data that you want the RT FIFO

to contain.

6. Place the RTFIFORead VI in the control loop on the block diagram.

7. Right-click the While Loop and select Add Shift Register from the

shortcut menu. Use While Loop shift registers to pass data from one

iteration to the next. Refer to the LabVIEW User Manual for

information about using shift registers.

8. Wire the rt fifo output of the RTFIFOCreate VI to the left terminal

of the shift register and then wire the terminal to the rt fifo input of

RTFIFORead VI.

9. Add another shift register and repeat step 8 to wire the error out

output of the RTFIFOCreate VI to the error in input of the

RTFIFORead VI.

10. Wire the rt fifo and error out outputs of the RTFIFORead VI to the

right terminal of the shift registers.

11. Place the Equal? function in the control loop to the left of the

conditional terminal. Wire the element out output of the RTFIFORead

VI to the xinput of the Equal? function. Right-click the yinput of the

Equal? function and select Create»Constant from the shortcut menu.

Enter 1into the constant.

12. Wire the x = y? output of the Equal? function to the conditional

terminal of the control loop.

13. Select File»Save As and save the VI as Second PID Control.vi.

©National Instruments Corporation 9 Getting Started with the LabVIEW Real-Time Module

The block diagram should look similar to Figure 5.

Figure 5. DAQ Control Loop with RT FIFOs

Downloading and Running the Application

You have created the time-critical control loop that runs on the RT target.

You can use the RT Engine -Normal Priority Loop (TCP Comm).vi

and the Host Machine (TCP Comm).vi examples located in the

examples\Real-Time\RT Tutorial.llb for the normal priority and

host computer VI.

Complete the following steps to download the time-critical VI and the

network communication VI and test the functionality.

1. Without closing the Second PID Control VI, open the RT Engine -

Normal Priority Loop (TCP Comm) VI and view the block diagram.

2. In the upper-right corner of the Second PID Control VI front panel

there is a connector pane image of the VI. Click and drag the image

to an area outside the While Loop on the block diagram of the

RT Engine - Normal Priority Loop (TCP Comm) VI to use the Second

PID Control VI as a subVI in the normal priority communication VI.

3. From the RT Engine - Normal Priority Loop (TCP Comm) VI block

diagram, select Browse»Show VI Hierarchy to open the Hierarchy

window. Notice that the thumbtack for RT Engine - Normal Priority

Loop (TCP Comm) VI is in the horizontal position, shown at left,

indicating that you have not downloaded the VI to the RT target.

Getting Started with the LabVIEW Real-Time Module 10 ni.com

4. Close the Hierarchy window.

5. Select Operate»Download Application to download the VI.

LabVIEW prompts you to save the VI. Save the VI as Top Level

Application.vi.

The block diagram should look similar to Figure 6.

Figure 6. Communication VI

6. Select View»Redraw in the Hierarchy window to refresh the screen.

Notice in the Hierarchy window that now the thumbtack for RT

Engine - Normal Priority Loop (TCP Comm) VI is in the vertical

position, shown at left, which indicates that you have downloaded

the VI to the RT target.

7. Click the Run button. LabVIEW runs the VI on the RT target.

8. Select File»Exit without closing RT Engine VIs to close LabVIEW

on the host computer while continuing to run the VIs on the RT target.

9. Start LabVIEW again and open the Host Machine (TCP Comm) VI.

The Host Machine (TCP Comm) VI publishes and controls the Stop

button in the time-critical VI on the RT target. Click the Stop button.

10. Select Operate»Switch Execution Target and select the RT target to

reconnect to the target and verify if the RT Engine - Normal Priority

Loop (TCP Comm) VI stopped running.

11. Exit LabVIEW by selecting File»Exit and, when prompted, close all

VIs running on the RT target.

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