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Choosing a Machine Control Architecture

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Overview

This document describes machine control applications and discusses some of the factors to consider when building a LabVIEW-based machine control system.

Table of Contents

  1. Machine Control Applications
  2. Control System Configurations
  3. Using LabVIEW in a Machine Control Application
  4. Considerations in Choosing Target Hardware
  5. Where to Go From Here

Machine Control Applications

Machine control systems typically combine a human-machine interface (HMI) and a real-time control system.  Real-time controllers provide reliable, predictable machine behavior, while HMIs provide the machine operator with a graphical user interface (GUI) for monitoring the machine's state and setting its operating parameters.

In a typical machine control system, the control system is implemented using a PLC- or PAC-based controller.  Baseline controller functionality includes:

  • Analog and digital I/O
  • A current value table for sharing variable (tag) values
  • A sequencing engine that defines the machine behavior
  • An output forcing option for debugging the machine hardware

In addition to these PLC-class capabilities, PACs support more sophisticated functionality such as

  • High-speed data acquisition and analysis
  • Motion control
  • Vision/inspection
  • Custom I/O behavior
HMIs can programmed on a PC running Windows or on a touch panel computer running an embedded OS such as WindowsCE.  HMI features typically include:
 
  • Touchscreen operation
  • A paged display system with navigation controls
  • Data entry objects (buttons, keypads, etc.)
  • Alarm/event displays and logs

Control System Configurations

The simplest machine control system consists of a single controller running in a "headless" configuration. 

Figure 1.  A headless controller

 

This configuration is used in applications where an HMI is not needed except for maintenance or diagnostic purposes.

The next level of system capability and complexity adds an HMI or additional controller nodes. 

 

Figure 2.  A local machine control system

This configuration is typical for machines that are controlled by a local operator.

Complex machine control applications may involve many controllers and HMIs.  They often involve a high-end server that acts as a data logging and forwarding engine.

Figure 3.  A distributed machine control system

This system configuration supports physically large or complex machines.  It can allow an operator to interact with the machine from various locations or it can be used to distribute specific monitoring and control responsibilities among a group of operators.

Using LabVIEW in a Machine Control Application

LabVIEW offers some key advantages for building machine control systems.
 
Many HMI and controller vendors do not use common development software, so machine builders must often learn multiple tools in order to build and integrate control systems.  In contrast, LabVIEW provides a single tool chain for programming all aspects of machine control.  LabVIEW features and extensions for machine control applications include:
 
  • LabVIEW Shared Variables:  Tag value sharing between processes and across the network
  • LabVIEW Touch Panel module: HMI support for Windows CE targets
  • LabVIEW Real-Time module: Deterministic control behavior
  • NI-DAQmx: Off-the-shelf I/O for PCs and PXI systems
  • NI-FP:  Off-the-shelf I/O for Compact Fieldpoint
  • LabVIEW FPGA module: Customizable I/O for PC-, PXI- or cRIO-based controllers
  • LabVIEW Datalogging and Supervisory Control (DSC) module: Data logging, alarming, security, and scaling for Shared Variables
LabVIEW also allows you to integrate third party devices (CAN I/O and PLCs for example) as well as intellectual property such as DLLs or VHDL code.
 

Considerations in Choosing Target Hardware

LabVIEW-based control systems can be assembled from almost any combination of touch panel computer, PC, PXI, cFP, or cRIO targets.  Here are some considerations and recommendations based on various system requirements:
 
Low Cost
  • cFP and cRIO embedded are the lowest cost options for machine controllers
  • WindowsCE-base touch panel computers are the lower cost option for HMIs
Ruggedness
  • cRIO embedded is the smallest and most rugged form factor for machine controllers.  cFP is a close second
  • Touch panel computers are designed for NEMA-style enclosures
Features/Performance
  • Multi-core/higher speed processors are available in the PC and PXI form factor
  • RIO and cRIO-based I/O provide more options for signal conditioning and customizable I/O behavior
Channel count
  • PXI- and PC-based controllers provide better processors and more I/O slots for higher channel count applications
Software
  • PC or PXI targets running MS Windows XP or VISTA support a broader variety of software options such as LabVIEW DSC and a fuller set of features for building HMIs
 

Where to Go From Here

To learn more about LabVIEW-based machine control, we suggest that you read the following document:
 
A Reference Architecture for Local Machine Control  - This is a reference architecture for building a local machine control system based on an NI Touchpanel Computer (TPC) HMI and a cRIO controller.
 
The following is a map of other documents that describe the machine control reference architecture.  You can click on the image to navigate directly to each document.

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