National Instruments SCC-SG Series Manuale utente

SCC-SG Series Strain-Gauge

Modules User Guide

The SCC-SG Series strain-gauge input modules (SCC-SGXX) allow you to

take quarter-, half-, and full-bridge configuration strain measurements. Use

them as follows:

• SCC-SG01 for 120 Ωquarter-bridge configurations.

• SCC-SG02 for 350 Ωquarter-bridge configurations.

• SCC-SG03 for half-bridge configurations.

• SCC-SG04 for full-bridge configurations.

• SCC-SG24 for at least 350 Ωfull-bridge configurations.

Each module consists of two strain-gauge input channels, offset-nulling

circuitry for each channel, and an excitation circuit. Each input channel

includes an instrumentation amplifier with differential inputs and a fixed

gain of 100.

The SCC-SG11 is a shunt calibration module. It contains two pairs of

switches and resistors that you connect across strain-gauge elements where

you want to perform shunt calibration.

Contents

Strain Gauges .......................................................................................... 3

Conventions ............................................................................................ 4

What You Need to Get Started ............................................................... 5

Unpacking the Module............................................................................ 6

Installing the Module .............................................................................. 6

Connecting Strain-Gauge Input Signals.................................................. 7

Panelettes ......................................................................................... 8

SCC-SG0Xand SCC-SG24 .................................................................... 8

Variable Definitions......................................................................... 8

SCC-SG01/02 Quarter-Bridge Connection ..................................... 9

Quarter-Bridge Type I .............................................................. 10

SCC-SG03 Half-Bridge Connection................................................ 12

Half-Bridge Type I ................................................................... 12

SCC-SG Series Strain-Gauge Modules User Guide 2 ni.com

Half-Bridge Type II...................................................................13

Quarter-Bridge Type II..............................................................14

SCC-SG04/24 Full-Bridge Connection............................................16

Full-Bridge Type I.....................................................................16

Full-Bridge Type II ...................................................................17

Full-Bridge Type III ..................................................................18

Configuring the SCC System ..................................................................20

Traditional NI-DAQ .........................................................................20

NI-DAQmx.......................................................................................20

Virtual Channels, Physical Channels, and Tasks .............................21

Creating a Virtual Channel in Traditional NI-DAQ..................21

Creating an NI-DAQmx Task or Global Channel.....................22

Theory of Operation ................................................................................23

Wheatstone Bridges..........................................................................23

Quarter-Bridge Type I ......................................................................24

Quarter-bridge Type II .....................................................................24

Half-Bridge I ....................................................................................25

Half-Bridge II ...................................................................................26

Full-Bridge Type I............................................................................26

Full-Bridge Type II...........................................................................27

Full-Bridge Type III .........................................................................28

Signal Conditioning.................................................................................29

Filtering ............................................................................................29

Amplification....................................................................................29

Excitation..........................................................................................29

Bridge Completion ...........................................................................30

Offset Nulling Adjustment ...............................................................30

Nulling Range Adjustment........................................................31

Shunt Calibration Using the SCC-SG11 ..........................................32

Connecting the SCC-SG11 Shunt Resistors..............................32

Using Shunt Calibration to Correct for Gain Error ...................36

Replacing Resistors .................................................................................37

Replacing the Quarter-Bridge Completion Resistors .......................37

Replacing the Nulling Resistors .......................................................38

Replacing the Shunt Resistors..........................................................38

Using the SCC-SGXX ..............................................................................39

Converting Voltage Measurements to Units of Strain .....................39

Strain Equations................................................................................40

Formulas and Variable Definitions ...........................................41

Quarter-Bridge Type I...............................................................41

Quarter-Bridge Type II..............................................................42

Half-Bridge Type I ....................................................................42

Half-Bridge Type II...................................................................42

Full-Bridge Type I.....................................................................42

Full-Bridge Type II ...................................................................43

Full-Bridge Type III ..................................................................43

©National Instruments Corporation 3 SCC-SG Series Strain-Gauge Modules User Guide

Specifications .......................................................................................... 44

SCC-SG0XStrain-Gauge Input Modules ........................................ 44

Strain-Gauge Bridge Configuration.......................................... 44

Analog Input ............................................................................. 44

Transfer Characteristics............................................................ 44

Amplifier Characteristics.......................................................... 45

Excitation.................................................................................. 45

Strain-Gauge Bridge Completions............................................ 45

Completion Accuracy ............................................................... 45

Power Requirements................................................................. 46

SCC-SG24 Strain-Gauge Input Module .......................................... 46

Analog Input ............................................................................. 46

Transfer Characteristics............................................................ 46

Amplifier Characteristics.......................................................... 47

Dynamic.................................................................................... 47

Stability..................................................................................... 47

Null Compensation ................................................................... 47

Excitation.................................................................................. 48

Power Requirements................................................................. 48

SCC-SG11 Shunt Calibration Module............................................. 48

Digital I/O................................................................................. 48

Shunt Calibration ...................................................................... 48

Power Requirements................................................................. 49

SCC-SGXX Strain-Gauge Input Modules ....................................... 49

Maximum Working Voltage..................................................... 49

Physical..................................................................................... 49

Environmental .......................................................................... 50

Safety ............................................................................................... 50

Electromagnetic Compatibility ........................................................ 50

CE Compliance ................................................................................ 50

SCC-SGXX Module Pin Assignments .................................................... 51

Strain Gauges

Strain-gauge configurations are arranged as Wheatstone bridges. The

gauge is the collection of all of the active elements of the Wheatstone

bridge. There are three supported types of strain-gauge configurations:

quarter-, half-, and full-bridge. There are other strain-gauge configurations,

such as rosette and chevron, that National Instruments (NI) software does

not support. The number of active element legs in the Wheatstone bridge

determines the kind of bridge configuration. Refer to Table 1 to see how

many active elements are in each configuration.

SCC-SG Series Strain-Gauge Modules User Guide 4 ni.com

Each of these three configurations is subdivided into multiple configuration

types. The orientation of the active elements and the kind of strain

measured determines the configuration type. NI supports seven

configuration types in software. However, with custom software scaling

you can use all Wheatstone bridge configuration types with any NI

hardware product that supports the gauge configuration type.

The supported strain gauge configuration types measure axial strain,

bending strain, or both. You can use similar configuration types to measure

torsional strain; however, NI software scaling does not support these

configuration types. It is possible to use NI products to measure torsional

strain, but to properly scale these configuration types you must create a

custom scale in Measurement & Automation Explorer (MAX) or perform

scaling in your software application.

This document discusses all of the mechanical, electrical, and scaling

considerations of each strain-gauge configuration type supported by NI.

Conventions

The following conventions are used in this guide:

<> Angle brackets that contain numbers separated by an ellipsis represent a

range of values associated with a bit or signal name—for example,

DBIO<3..0>.

»The »symbol leads you through nested menu items and dialog box options

to a final action. The sequence File»Page Setup»Options directs you to

pull down the File menu, select the Page Setup item, and select Options

from the last dialog box.

This icon denotes a note, which alerts you to important information.

This icon denotes a caution, which advises you of precautions to take to

avoid injury, data loss, or a system crash. When this symbol is marked on

the product, refer to the Read Me First: Safety and Radio-Frequency

Interference document, shipped with the product, for precautions to take.

Table 1. Strain-Gauge Configurations

Configuration Number of Active Elements

Quarter-bridge 1

Half-bridge 2

Full-bridge 4

©National Instruments Corporation 5 SCC-SG Series Strain-Gauge Modules User Guide

bold Bold text denotes items that you must select in software, such as menu

items and dialog box options. Bold text also denotes parameter names.

italic Italic text denotes variables, emphasis, a cross reference, or an introduction

to a key concept. This font also denotes text that is a placeholder for a word

or value that you must supply.

monospace Text in this font denotes text or characters that you should enter from the

keyboard, sections of code, programming examples, and syntax examples.

This font is also used for the proper names of disk drives, paths, directories,

programs, subprograms, subroutines, device names, functions, operations,

variables, filenames and extensions, and code excerpts.

DAQ device Data acquisition (DAQ) device refers to all entry-level basic and

full-featured E Series multifunction DAQ devices.

SC-2345 SC-2345 refers to both the SC-2345 connector block and the SC-2345 with

configurable connectors.

SCC SCC refers to any SCC Series signal conditioning module.

SCC-SG0XSCC-SG0Xrefers to SCC-SG Series strain-gauge input modules SG01,

SG02, SG03, and SG04.

SCC-SGXX SCC-SGXX refers to any SCC-SG Series strain-gauge module.

What You Need to Get Started

To set up and use the SCC-SGXX, you need the following items:

❑SC-2345 with one of the following:

– SCC-PWR01

– SCC-PWR02 and the PS01 power supply

– SCC-PWR03 (requires a 7 to 42 VDC power supply, not included)

❑One or more SCC-SGXX

❑SCC-SG Series Strain-Gauge Modules User Guide

❑SC-2345 User Manual

❑Read Me First: Safety and Radio-Frequency Interference

❑SC-2345 Quick Reference Label

❑68-pin Basic or E Series DAQ device, documentation, and 68-pin

cable

SCC-SG Series Strain-Gauge Modules User Guide 6 ni.com

❑1/8 in. flathead screwdriver

❑Numbers 1 and 2 Phillips screwdrivers

❑Wire insulation strippers

❑NI-DAQ (current version) for Windows 2000/NT/XP/Me/9x

Note Software scaling of measurements is not supported on the Mac OS. Refer to the

Converting Voltage Measurements to Units of Strain section for more information.

Unpacking the Module

SCC modules are shipped in antistatic envelopes to prevent electrostatic

damage to the modules. Electrostatic discharge (ESD) can damage several

components on these products.

Caution Never touch the exposed pins of connectors.

To avoid damage from ESD when you handle the module, take the

following precautions:

• Ground yourself by using a grounding strap or by touching a grounded

object.

• Touch the antistatic envelope to a metal part of the computer chassis

before removing the module from the envelope.

Remove the module from the envelope and inspect the module for loose

components or any sign of damage. Notify NI if the module appears

damaged in any way. Do not install a damaged module.

Store the module in the antistatic envelope when it is not in use.

Installing the Module

Caution Refer to the Read Me First: Safety and Radio-Frequency Interference document

before removing equipment covers or connecting/disconnecting any signal wires.

This section explains how to install the SCC-SGXX in the SC-2345 carrier.

Note The SCC-SG0Xand SCC-SG24 act as single-stage modules only.

Plug the SCC-SG0Xor the SCC-SG24 into any SC-2345 socket J(X+1),

where Xis 0 to 7, and wire the bridge configuration signals to the module.

©National Instruments Corporation 7 SCC-SG Series Strain-Gauge Modules User Guide

The socket you choose determines which DAQ device channels receive the

SCC-SG0Xor SCC-SG24 signals, as explained in the Connecting

Strain-Gauge Input Signals section.

If you are using an SCC-SG11 for shunt calibration, plug it into any digital

input/output (DIO) socket J(X+9), where Xis 0 to 7. This is not a dual-stage

configuration. You must connect the SCC-SG11 to the bridge configuration

using additional wires. Refer to the Shunt Calibration Using the SCC-SG11

section.

Connecting Strain-Gauge Input Signals

The SCC-SGXX has a fixed screw-terminal receptacle and a removable

screw-terminal block, as shown in Figure 1.

Note The SCC-SG11 has a four-terminal screw-terminal receptacle. Refer to the Shunt

Calibration Using the SCC-SG11 for more information about using the SCC-SG11.

Figure 1. SCC-SGXX Two-Part Screw-Terminal System

After you install the SCC-SGXX, complete the following steps to connect

the signal lines to the module:

1. Remove power from the signal lines.

2. Strip 7 mm (0.25 in.) of insulation from the ends of the signal wires.

1 SCC Screw-Terminal Receptacle 2 Removable Screw-Terminal Block

SCC-SG

SCC-SG Series Strain-Gauge Modules User Guide 8 ni.com

3. Insert the wires into the screw terminals.

4. Tighten the screws to 0.5–0.6 N · m (4.4–5.3 lb · in.) of torque with a

flathead screwdriver.

Each screw terminal on the SCC-SG0Xand SCC-SG24 is labeled by pin

number <1..6>. The input stage of the SCC-SG0Xand SCC-SG24 provides

two differential analog input channels for measuring strain. The input pins

to the SCC-SG0Xand SCC-SG24 differ for each module. Pin 4 is a

single-ended channel routed to DAQ device channel X+8, and pin 6 is a

single-ended channel routed to DAQ device channel X. The value of Xis

0 to 7 depending on the socket where you plug in the SCC-SG0Xand

SCC-SG24, J(X+1) or J(X+9).

Panelettes

If you have an SCC carrier with configurable connectors, you can install

connectivity and interface panelettes to customize the SCC system.

Panelettes simplify signal connection to the SCC modules and eliminate the

need to rewire signals to the screw terminals when you change the signal

source.

The most common panelettes used with the SCC-SG0X and SCC-SG24

strain applications are the 9-pin D-Subminiature (DSUB), MIL-Spec, and

LEMO (B-Series).

SCC-SG0Xand SCC-SG24

The various bridge configurations discussed in the Strain Gauges section

require different SCC modules. The following sections discuss the bridge

configuration types supported by each module, and provide connection and

electrical diagrams for the different bridge configurations.

Variable Definitions

In the following figures, the variables are defined as:

εis the measured strain (+εis tensile strain and -εis compressive strain).

νis the Poisson’s ratio, defined as the negative ratio of transverse strain to

axial (longitudinal) strain.

VCH is the measured voltage of the signal.

VEX is the excitation voltage.

©National Instruments Corporation 9 SCC-SG Series Strain-Gauge Modules User Guide

SCC-SG01/02 Quarter-Bridge Connection

In a quarter-bridge configuration, you use only one active strain-gauge

element. The following section describes how to connect a quarter-bridge

strain-gauge configuration type I.

Note NI software supports two quarter-bridge configuration types. However, to measure

the quarter-bridge configuration type II you must use the SCC-SG03. Refer to the

SCC-SG03 Half-Bridge Connection section for more information.

The SCC-SG01 module label displays the icon shown in Figure 2 and the

SCC-SG02 module label displays the icon shown in Figure 3. These icon

labels also represent the bridge configurations of the modules. On the

SCC-SGXX module labels the boxed resistors represent external active

strain-gauge elements.

Figure 2. SCC-SG01 Icon

Figure 3. SCC-SG02 Icon

120

350

SCC-SG Series Strain-Gauge Modules User Guide 10 ni.com

Quarter-Bridge Type I

This section provides information for the quarter-bridge strain-gauge

configuration type I. The quarter-bridge type I measures either axial or

bending strain.

Figure 4 shows a general quarter-bridge type I circuit diagram.

Figure 4. Quarter-Bridge Type I Circuit Diagram

The following symbols apply to the circuit diagram and equations:

•R

1and R2are half-bridge completion resistors.

•R

3is the socketed quarter-bridge completion resistor

(dummy resistor).

•R

4is the active element measuring tensile strain (+ε).

Figure 5 shows how to set up two quarter-bridge type I configurations with

the SCC-SG01/02.

–

–+

VEX

R2R3

VCH

R4(+ )

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