Guide to Data Acquisition: About this Guide and Article Index



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In less than two decades, the PC has become the most widely used platform for data acquisition and control. The main reasons for the popularity of PC-based technology are low costs, flexibility and ease of use, and , last but not the least, performance. This solid and dependable trait is all thanks to the use of ‘off-the-shelf’ components. Data acquisition with a PC enables one to display, log and control a wide variety of real world signals such as pressure, flow, and temperature. This ability coupled with that of easy interface with various stand-alone instruments makes the systems ever more desirable. Until the advent of the PC, data acquisition and process monitoring were carried out by using dedicated data loggers, programmable logic controllers and or expensive proprietary computers. Today’s superb software-based operator interfaces make the PC an increasingly attractive option in these typical applications:

  • Laboratory data acquisition and control
  • Automatic test equipment (ATE) for inspection of components
  • Medical instrumentation and monitoring
  • Process control of plants and factories
  • Environmental monitoring and control
  • Machine vision and inspection




The key to the effective application of PC-based data acquisition is the careful matching of real world requirements with appropriate hardware and software. Depending on your needs, monitoring data can be as simple as connecting a few cables to a plug-in board and running a menu-driven software package. At the other end of the spectrum, you could design customized sensing and conversion hardware, or perhaps develop application software to optimize a system.





This guide gives both the novice and the experienced user a solid grasp of the principles and practical implementation of interfacing the PC and stand-alone instruments with real world signals. The main objective of this guide is to give you a thorough understanding of PC-based data acquisition systems and to enable you to design, specify, install, configure, and program data acquisition systems quickly and effectively. After reading this guide, we believe you will be able to:

  • Demonstrate a sound knowledge of the fundamentals of data acquisition (with a focus on PC-based work)
  • Competently install and configure a simple data acquisition system
  • Choose and configure the correct software
  • Avoid the common pitfalls in designing a data acquisition system

This guide is intended for engineers and technicians who are:

  • Electronic engineers
  • Instrumentation and control engineers
  • Electrical engineers
  • Electrical technicians
  • Systems engineers
  • Scientists working in the data acquisition area
  • Process control engineers
  • System integrators
  • Design engineers

A basic knowledge of electrical principles is useful in understanding the outlined concepts, but this guide also focuses on the fundamentals; hence, understanding key concepts should not be too onerous.

Table of Contents

Introduction

Definition of data acquisition and control

Fundamentals of data acquisition

2.1

Transducers and sensors

2.2

Field wiring and communications cabling

2.3

Signal conditioning

2.4

Data acquisition hardware

2.5

Data acquisition software

2.6

Host computer

Data acquisition and control system configuration

3.1

Computer plug-in I/O

3.2

Distributed I/O

3.3

Stand-alone or distributed loggers/controllers

3.4

IEEE 488 (GPIB) remote programmable instruments

Analog and digital signals

Classification of signals

1.1

Digital signals binary signals

1.2

Analog signals

Sensors and transducers

Transducer characteristics

Resistance temperature detectors (RTDs)

4.1

Characteristics of RTDs

4.2

Linearity of RTDs

4.3

Measurement circuits and considerations for RTDs

Thermistors

Thermocouples

6.1

Reference junction compensation

6.2

Isothermal block and compensation cables

6.3

Thermocouple linearization

6.4

Thermocouple types and standards

6.5

Thermocouple construction

6.6

Measurement errors

6.7

Wiring configurations

Strain gauges

Wheatstone bridges

8.1

General characteristics

8.2

Quarter bridge configuration

8.3

Half bridge configuration

8.4

Full bridge configuration

8.5

Wiring connections

8.6

Temperature considerations

8.7

Measurement errors

Signal conditioning

Introduction

Types of signal conditioning

2.1

Amplification

2.2

Isolation

2.3

Filtering

2.4

Linearization

Classes of signal conditioning

3.1

Plug-in board signal conditioning

3.2

Direct connect modular – two-wire transmitters

3.3

Distributed I/O – digital transmitters

Field wiring and signal measurement

4.1

Grounded signal sources

4.2

Floating signal sources

4.3

Single-ended measurement

4.4

Differential measurement

4.5

Common mode voltages and CMRR

4.6

Measuring grounded signal sources

4.7

Ground loops

4.8

Signal circuit isolation

4.9

Measuring ungrounded signal sources

4.10

System isolation

Noise and interference

5.1

Definition of noise and interference

5.2

Sources and types of noise

Minimizing noise

6.1

Cable shielding and shield earthing

6.2

Grounding cable shields

Shielded and twisted-pair cable

7.1

Twisted-pair cables

7.2

Coaxial cables

(D-E to be added soon)

Plug-in data acquisition boards

Serial data communications

Definitions and basic principles

  • 1.1 Transmission modes – simplex and duplex
  • 1.2 Coding of messages
  • 1.3 Format of data communications messages
  • 1.4 Data transmission speed

RS-232-C interface standard

  • 2.1 Electrical signal characteristics
  • 2.2 Interface mechanical characteristics
  • 2.3 Functional description of the interchange circuits
  • 2.4 The sequence of operation of the EIA-232 interface
  • 2.5 Examples of RS-232 interfaces
  • 2.6 Main features of the RS-232 Interface Standard

RS-485 interface standard

  • 3.1 RS-485 repeaters

Comparison of the RS-232 and RS-485 standards

The 20 mA current loop

Serial interface converters

Protocols

  • 7.1 Flow control protocols
  • 7.2 ASCII-based protocols

Error detection

  • 8.1 Character redundancy checks
  • 8.2 Block redundancy checks
  • 8.3 Cyclic redundancy Checks

Troubleshooting & testing serial data communication circuits

  • 9.1 The breakout box
  • 9.2 Null modem
  • 9.3 Loop back plug
  • 9.4 Protocol analyzer
  • 9.5 The PC as a protocol analyze

Distributed and stand-alone loggers/controllers

  • 1 Introduction
  • 2 Methods of operation
    • 2.1 Programming and logging data using PCMCIA cards
    • 2.2 Stand-alone operation
    • 2.3 Direct connection to the host PC
    • 2.4 Remote connection to the host PC
  • 3 Stand-alone logger/controller hardware
    • 3.1 Microprocessors
    • 3.2 Memory
    • 3.3 Real time clock
    • 3.4 Universal asynchronous receiver/transmitter (UART)
    • 3.5 Power supply
    • 3.6 Power management circuitry
    • 3.7 Analog inputs and digital I/O
    • 3.8 Expansion modules
  • 4 Communications hardware interface
    • 4.1 RS-232 interface
    • 4.2 RS-485 standard
    • 4.3 Communication bottlenecks and system performance
    • 4.4 Using Ethernet to connect data loggers
  • 5 Stand-alone logger/controller firmware
  • 6 Stand-alone logger/controller software design
    • 6.1 ASCII based command formats
    • 6.2 ASCII based data formats
    • 6.3 Error reporting
    • 6.4 System commands
    • 6.5 Channel commands
    • 6.6 Schedules
    • 6.7 Alarms
    • 6.8 Data logging and retrieval
  • 7 Host software
  • 8 Considerations in using standalone logger/controllers
  • 9 Stand-alone logger/controllers vs internal systems
    • 9.1 Advantages
    • 9.2 Disadvantages

The universal serial bus (USB)

1 Introduction

2 USB overall structure

  • Topology
  • Host hubs
  • The connectors (Type A and B)
  • Low-speed cables and high-speed cables
  • External hubs
  • USB devices
  • Host hub controller hardware and driver
  • USB software driver
  • Device drivers
  • Communication flow

3 The physical layer

  • Connectors
  • Cables
  • Signaling
  • NRZI and bit stuffing
  • Power distribution

4 Datalink layer

  • Transfer types
  • Packets and frames

5 Application layer (user layer)

6 Conclusion

  • Acknowledgements

Specific techniques

  • 1 Open and closed loop control
  • 1.1 Definitions
  • 1.2 Fluid level closed loop control system
  • 1.3 PID control algorithms
  • 1.4 Transient performance – step response
  • 1.5 Deadband
  • 1.6 Output limiting
  • 1.7 Manual control – bumpless transfer
  • 2 Capturing high speed transient data
  • 2.1 A/D board operation and memory requirements
  • 2.2 Trigger modes (pre- and post-triggering)
  • 2.3 Trigger source and level

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Updated: Monday, March 17, 2014 23:16 PST