National Instruments SCXI-1321 Manual de usuario

LabVIEW

™

, NI-DAQ

™,

and SCXI

™

are trademarks of National Instruments Corporation. Product and company names are trademarks or trade names

of their respective companies.

321924A-01

INSTALLATION GUIDE

SCXI-1321 OFFSET-NULL AND

SHUNT-CALIBRATION HIGH-VOLTAGE

TERMINAL BLOCK

This guide describes how to install and use the SCXI-1321 offset-null

and shunt-calibration terminal block with your SCXI-1121 module.

Introduction

You can only use the SCXI-1321 terminal block with SCXI-1121 revision

C and later revisions. The SCXI-1321 terminal block has a shielded board

with supports for connection to the SCXI-1121 input connector. In addition

to the 18 screw terminals, the SCXI-1321 has circuitry for offset-null

adjustment of Wheatstone bridges, and a shunt resistor for strain-gauge

shunt calibration. This terminal block was primarily designed for

bridge-type transducers such as strain gauges. The SCXI-1321 can also

easily accommodate thermocouples, RTDs, thermistors, millivolt sources,

volt sources, and current-loop receivers.

What You Need to Get Started

To set up and use your SCXI-1321, you will need the following items:

❑SCXI-1321 offset-null and shunt-calibration terminal block

❑SCXI-1321 Offset-Null and Shunt-Calibration Terminal Block

Installation Guide

❑SCXI chassis

❑SCXI-1121 module

❑No. 1 and No. 2 Phillips-head screwdrivers

❑1/8in. flathead screwdrivers

SCXI-1321 Installation Guide 2

National Instruments Corporation

❑Long-nose pliers

❑Wire cutter

❑Wire insulation stripper

Safety Information

Cautions DONOT OPERATE THE DEVICE IN AN EXPLOSIVE ATMOSPHERE OR WHERE THERE MAY BE

FLAMMABLE GASES OR FUMES.

KEEP AWAY FROM LIVE CIRCUITS. Do not remove equipment covers or shields unless

you are trained to do so. If signal wires are connected to the device, hazardous

voltagesmayexist evenwhentheequipment isturned off. Toavoida shockhazard,

do not perform procedures involving cover or shield removal unless you are

qualified to do so and disconnect all field power prior to removing covers or

shields.

Equipment described in this document must be used in an Installation Category

II environment per IEC 664. This category requires local level supply

mains-connected installation.

DONOT OPERATE DAMAGED EQUIPMENT. The safety protection features built into this

device can become impaired if the device becomes damaged in any way. If the

device is damaged, turn the device off and do not use until service-trained

personnel can check its safety. If necessary, return the device to National

Instruments for service and repair to ensure that its safety is not compromised.

Do not operate this equipment in a manner that contradicts the information

specified in this document. Misuse of this equipment could result in a shock

hazard.

Terminals are for use only with equipment that has no accessible live parts.

DONOT SUBSTITUTE PARTS OR MODIFY EQUIPMENT. Because of the danger of

introducing additional hazards, do not install unauthorized parts or modify the

device. Return the device to National Instruments for service and repair to ensure

that its safety features are not compromised.

When using the device with high common-mode voltages, you MUST insulate your

signal wires for the highest input voltage. National Instruments is NOT liable for

any damages or injuries resulting from inadequate signal wire insulation. Use

only 26-14 AWG wire with a voltage rating of 300 V and 60° C for measuring

250 to 300 V.

When connecting or disconnecting signal lines to the SCXI terminal block screw

terminals, make sure the lines are powered off. Potential differences between the

lines and the SCXI ground create a shock hazard while you connect the lines.

National Instruments Corporation 3 SCXI-1321 Installation Guide

Connections, including power signals to ground and vice versa, that exceed any of

the maximum signal ratings on the SCXI device can create a shock or fire hazard

or can damage any or all of the boards connected to the SCXI chassis, the host

computer, and the SCXI device. National Instruments is NOT LIABLE FOR ANY

DAMAGES OR INJURIES resulting from incorrect signal connections.

If high voltages (≥30 Vrms and 42.4 V peak or 60 VDC) are present, YOU MUST

CONNECT ASAFETY EARTH GROUND WIRE TO THE TERMINAL BLOCK SAFETY GROUND SOLDER

LUG, shown in Figure 1. This complies with safety agency requirements and

protects against electric shock when the terminal block is not connected to the

chassis. To connect the safety earth ground to the safety ground solder lug, run an

earth ground wire in the cable from the signal source to the terminal block.

National Instruments is NOT liable for any damages or injuries resulting from

inadequate safety earth ground connections.

Donot loosen orre-orient thesafetyground solderlug hardwarewhenconnecting

the safety ground wire; to do so reduces the safety isolation between the high

voltage and safety ground.

Temperature Sensor Output and Accuracy

The temperature sensor outputs 10 mV/°C and has an accuracy of ±0.9° C

over the 0° to 55° C temperature range. To determine the temperature, use

the following formulas:

T (°C) = 100 ×VTEMPOUT

T (°F) = T (°C) ×9/5 + 32

where VTEMPOUT is the temperature sensor output voltage, and T (°F) and

T (°C) are the temperature readings in degrees Fahrenheit and degrees

Celsius, respectively.

Note Use the average of a large number of samples to obtain the most accurate reading.

Noisy environments require averaging for greater accuracy.

Using the Nulling Circuitry and the Shunt Calibration

SCXI-1321 Nulling Circuitry

The nulling circuitry operates with full-bridge, half-bridge, quarter-bridge,

and strain-gauge configurations. Each channel has its own nulling circuitry

and its own trimming potentiometer as listed in Table 1.

SCXI-1321 Installation Guide 4

National Instruments Corporation

To null the static offset voltage of the bridge, use the following procedure:

1. Hook up your bridge configuration to the selected channel.

2. Select and read the channel output.

3. While monitoring the output, rotate the trimmer wiper with a flathead

screwdriver until you reach 0 V.

You have nulled your bridge and are ready for a measurement.

The nulling range that is provided with your terminal block is ±2.5 mV,

assuming that you have a 120 Ωstrain gauge quarter-bridge configuration

and 3.333 V excitation voltage. You can change this range by replacing the

nulling resistor with a resistor your choice of another value. Each channel

has an independent nulling resistor. You can therefore mix your ranges to

accommodate each channel requirement. Table 2 lists the nulling resistors

and their corresponding channels.

Thevalueofall the nullingresistors onyour terminalblock is 39 kΩ. Notice

that these resistors are socketed for easy replacement. These sockets best fit

a 1/4W resistor lead size.

To determine your nulling range, use the following formula (refer to

Figure 1 for visual help):

Table 1. Trimmer Potentiometers and Corresponding Channels

Channel Number Trimmer Potentiometer

0 R1

1 R2

2R14

3 R15

Table 2. Nulling Resistors and Corresponding Channels

Channel Number Nulling Resistor

0 R3

1 R5

2 R7

3 R9

Vnulling range Vexc

----------- VexcRdRnull Rg

()

nullRgRdRnull Rg

()

---------------------------------------------------------------–

National Instruments Corporation 5 SCXI-1321 Installation Guide

where

Rgis the nominal strain gauge resistance value.

Rdis either a completion resistor or a second strain gauge nominal

resistance.

Rnull is the nulling resistor value.

Vexc is the excitation voltage (3.333 V or 10 V).

For example, assuming:

Vexc = 3.333 V

Rg= 120 Ω

Rd= 120 Ω

Rnull = 39 kΩ

Vnulling range = ±2.56 mV

Assuming a strain gauge range with a gauge factor of GF = 2 and a

quarter-bridge configuration, this range corresponds to ±1,498 µε as given

by the strain formula for a quarter-bridge strain gauge configuration:

where

ε4Vr

–

GF 1 2Vr

()

--------------------------------=

Vrstrained voltage static unstrained voltage– Vexc

------------------------------------------------------------------------------------------------------=

SCXI-1321 Installation Guide 6

National Instruments Corporation

Figure 1. Nulling Circuit

Note Rcstands for the completion resistors.

Using the SCXI-1321 with RTDs and Thermistors

When using this terminal block with RTDs or thermistor-type transducers

and with the SCXI-1121 excitation set in the Current mode, you must

disable the nulling circuit of the channel of interest. You can do this in two

steps:

1. Place the enable/disable jumper in position D (disable) as shown in

Table 3.

2. Remove the nulling resistor from its sockets.

Table 3. Jumper Settings of the Nulling Circuits

Jumper Position Description

W1 Nulling circuit of Channel 0

is enabled; factory setting

Nulling circuit of Channel 0

is disabled

Trimmer

Potentionmeter

EX–

CH+

CH–

EX+

SCXI-1321

Rnull

RgRc

RdRc

National Instruments Corporation 7 SCXI-1321 Installation Guide

SCXI-1121 Shunt Calibration

Shunt calibration circuits are independent from each other but are

controlled together. In other words, when SCAL is set to 1 on the

SCXI-1121, all the shunt switches close; when SCAL is cleared to 0, all the

switches open. At startup or reset, all switches are open. This shunt

calibrationcircuitry configurationplaces a shuntingresistor in parallelwith

the strain gauge, as shown in Figure 2.

W2 Nulling circuit of Channel 1

is enabled; factory setting

Nulling circuit of Channel 1

is disabled

W3 Nulling circuit of Channel 2

is enabled; factory setting

Nulling circuit of Channel 2

is disabled

W4 Nulling circuit of Channel 3

is enabled; factory setting

Nulling circuit of Channel 3

is disabled

Table 3. Jumper Settings of the Nulling Circuits (Continued)

Jumper Position Description

SCXI-1321 Installation Guide 8

National Instruments Corporation

Figure 2. Shunt Circuit

The shunting resistors RSCAL are socketed so thatyou can replace themwith

a resistor of another value to achieve the required changes. The RSCAL

provided on your terminal block have a 301 kΩ±1% value.

Assuming a quarter-bridge strain-gauge configuration with a gauge factor

of GF = 2, the equivalent strain change introduced by the RSCAL shunting

resistor is –199 µε.

Use the following formula to determine the change due to this shunting

resistor:

Next, using the appropriate strain-gauge strain formula, and assuming that

you have no static voltage, determine the equivalent strain the RSCAL should

provide. For example, RSCAL = 301 kΩand a quarter-bridge 120 Ωstrain

gauge with a gauge factor of GF = 2 and Vexc = 3.333 V and R = 120Ω

produces the following:

Vchange = 0.3321 mV

Replacing the strained voltage by Vchange in the quarter-bridge strain

equation produces an equivalent –199 µε of change.

SCAL

EX–

CH+

CH–

EX+

SCXI-1321

RSCAL RgRc

RdRc

Vchange VexcRdRSCAL Rg

()

SCAL RdRSCAL Rg

()

----------------------------------------------------------------- Vexc

-----------–=

National Instruments Corporation 9 SCXI-1321 Installation Guide

Using Shunt Calibration with National Instruments

Software

If you are using LabVIEW for Windows 2.5.2 or LabVIEW 3.0, use

the channel string shunt

to enable shunt calibration. If you are

using NI-DAQ or LabVIEW 2.2.1 or earlier versions, use the

SCXI_Calibrate_Setup function or VI. If you are using register-level

code to program the SCXI-1121 and the SCXI-1321, you must write to the

SCAL register yourself.

Signal Connection

Note Refer to the Safety Information section before removing equipment covers or

connecting or disconnecting any signal wires.

When connecting your signals to the SCXI-1321, follow the labeling on the

SCXI-1321 for the appropriate module, as indicated in Figure 2.

To connect the signal to the terminal block, perform the following steps,

referring to Figures 1 and 2 as necessary:

1. Unscrew the top cover screws and remove the cover.

2. Loosen the strain-relief screws and remove the strain-relief bar.

3. Enable or bypass each of the nulling circuits, depending on the signal

you are measuring.

Note For correct operation of the SCXI-1121 excitation channels, disable the nulling

circuit on the excitation channels when you are using the SCXI-1321 with the

SCXI-1121.

4. Run the signal wires through the strain-relief opening. You can add

insulation or padding if necessary.

5. Prepare your signal wire by stripping the insulation no more than

7mm.

6. Connect the wires to the screw terminals by inserting the stripped end

of the wire fully into the terminal. No bare wire should extend past the

screw terminal. Exposed wire increases the risk of shorting and

causing a failure.

7. Tighten the screws to a torque of 5–7 in.-lb.

8. Connect safety earth ground to the safety ground solder lug. Refer to

the Safety Information section for connection information.

9. Reinstall the strain-relief bar and tighten the strain-relief screws.

SCXI-1321 Installation Guide 10

National Instruments Corporation

10. Reinstall the top cover and tighten the top cover screws.

11. Connect the terminal block to the module front connector as explained

in the Installation section later in this guide.

Figure 3 shows the SCXI-1321 terminal block parts locator diagram.

Figure 3. SCXI-1321 Parts Locator Diagram

1 Strain Relief Bar

2 Strain Relief Screws 3 Safety Ground

Solder Lug 4 Mating Connector

5 Thumbscrew 6 Top Cover Screws

7 Top Cover

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