MicroStrain V-Link-LXRS Specification sheet
8401-0010
Wireless Sensor Networks Technical Note
LORD Corporation, MicroStrain Sensing Systems
Using a Load Cell with V-Link®-LXRS™
Overview
MicroStrain’s V-Link®-LXRS™8 Channel Wireless Analog
Sensor Node has 4 differential input channels (strain
channels) designed to support strain gauges, load cells, etc.
Futek’s LSB300 Series is an S Beam Load Cell designed for
use in platforms, silos and scales; for purposes of this
discussion, we are using the 200 lbs capacity, full bridge model
FSH02077.
This technical note presents a step-by-step approach to
connecting the load cell to the V-Link®-LXRS™, calibrating the load
cell, and operating the system. Familiarity with the V-Link®-
LXRS™, Node Commander®software and load cell operation is
assumed. This technical note is also applicable to SG®-Link-LXRS™
operations.
Wiring Connection
The Futek load cell presents a 5 foot, polyurethane sheathed, shielded, connection cable with 6
flying leads. Table 1 describes the individual leads’ purpose and nomenclature.
1 Red +EXC +Excitation
2 Black -EXC -Excitation
3 Green +SIG +Signal
4 White -SIG -Signal
5 Orange +SEN +Sense
6 Blue -SEN -Sense
Shield Silver Grounded Shield
Table 1: Futek Load Cell Cable Leads
•To connect to the V-Link®-LXRS™, we need only deal with the Red, Black, Green, and
White leads.
•Cut back the Orange and Blue leads and secure them to prevent shorting.
•Strip back each of the 4 remaining leads approximately ½”.
•Tin (with solder) each of the 4 leads to insure a good grip in the V-Link®-LXRS™terminal
block connector.
•It is also strongly recommended that you solder the shielding of the cable to the Black
lead as shown in Figure 1. This will reduce noise in the system.
•Connect the 4 leads of the load cell to the V-Link®-LXRS™as shown in Table 2 and
Figure 2.
•For this example we are connecting to differential input channel 1 of the V-Link®-LXRS™.
Load Cell Lead Load Cell Lead Function V-Link Function V-Link Pin
Red +Excitation SP+ 1
Black -Excitation GND 4
Green +Signal S1- 3
White -Signal S1+ 2
Table 2: Load Cell Cable Connection to V-Link®-LXRS™
Figure 1: Shielding and 4 Leads of Load Cell Figure 2: Leads Connected to V-Link®-LXRS™
Node Commander®software
•Power-up the V-Link®-LXRS™and launch Node Commander®software.
•Establish communication with the V-Link®-LXRS™as normal.
•Right-click the node and a drop-down menu will appear.
•Click Configure.
•Click Configure Node and the Configuration screen will appear.
•Click the Channels tab.
•Enable the channel to which you connected the load cell (Channel 1 in our example).
•Continue to the Channel 1 Configuration screen
by clicking the … button.
Select the Mid•scale bullet under the Auto-
•ill appear similar to the
example to the right.
l. Low-scale is used if you are
xpecting more positive and high-scale if more negative data.
V-Link -LXRS™ is likely not sound.
orrect the wiring and re-run the auto-balancing to verify.
Balance button.
•Click the Auto-Balance button.
A pop-up window w
The key information within the window is the 2048 bits and the 508 offset. We are looking for a
value of approximately 2048 (+/- 50) bits. This is “mid-scale” on the V-Link®-LXRS™0 – 4096
bit measurement range. Low-scale balancing will yield approximately 1024 bits; high-scale will
be approximately 3072 bits. You should use mid-scale balancing if you are expecting equal
amounts of positive and negative output from the load cel
e
The offset is used to get the measurement range to align with the expected load cell output. The
value for mid-scale should be approximately 512 (+/- 10) for a balanced bridge situation. If
these numbers are off, the wiring of the load cell to the ®
C
2
•Click OK to close the pop-up window.
3
lose the Channel 1 Configuration screen.
en.
node and a drop-down menu will appear.
•getting proper
•
•am to verify
proper operation.
We t
•. A limiting factor for this option is the number and accuracy of
•
t the load cell
•ne has a large expensive testing apparatus, but this will give the most
accurate results.
will support
the weights you apply to the load cell, as shown here.
•Click OK to c
•Click Apply.
•Click the Streaming tab.
•Uncheck the Continuous Streaming checkbox.
•Enter a value of 15000 Sweeps (~20 seconds).
•Click Apply and click OK to close the Configuration scre
•Right-click the
•Click Sample.
•Click Stream.
•Click Start and the node will start str
Exercise the load cell by hand to
check you are
eaming the load cell data on Channel 1.
output as shown.
If the data is opposite to what you
would expect, i.e., it goes negative
when you were expecting positive,
switch the Green and White leads
to the V-Link®-LXRS™.
Now re-run the auto-balance
procedure and stre
Calibrations
ac ually have several options to calibrate the load cell with the V-Link®-LXRS™:
The “poor man calibration” – Hang several weights off the load cell and read the output
of the V-Link®-LXRS™
the weights available.
Use the factory calibration of the load cell. Futek calibrates the load cell and provides a
sensitivity coefficient that allows us to calculate a slope and offset. A limiting factor for
this option is that the factory calibration does not take into accoun
connections to the V-Link®-LXRS™or the V-Link®-LXRS™electronics.
Utilize a calibrated tensometer to apply at least ten loads over the entire range of the load
cell. Not everyo
Poor man calibration
Securely mount the load cell to a sturdy structure that
•Launch Node Commander®
software.
•Establish communication with the
V-Link®-LXRS™as normal.
•Proceed to the Configuration
screen.
•Continue to the Channel 1
Configuration screen.
•Select A/D Value under Class and
Bits under Units in the Conversion
Coefficients frame as shown.
•Click OK to close the Channel 1
Configuration screen.
•Proceed by streaming the node
and observing the bit output using
various weights. In our example shown in Figure 3, we are demonstrating four weights,
i.e., 0 (no weight), 5 lbs, 10 lbs and 25 lbs.
•After making weight measurements, calculate a slope from the data using the formula
y=mx+b. Please see our technical note Calculating a Linear Slope with Microsoft Excel
for a step-by-step instruction.
BitsLbs
20330
20695
210810
221325
Figure 3: Calculating a Slope
•In our example we see a slope of 0.1389 and an offset of -282.55.
•Return to the Channel 1 Configuration screen.
•Select Force under Class and Lbf under Units in the Conversion Coefficients frame.
•Click the Modify button and enter 0.1389 in
Slope and -282.55 in Offset.
•Click Lock to apply the Slope and Offset.
•Click OK to close the Channel 1 Configuration
screen.
•Proceed to again stream the node with no
weight on the load cell.
•Observe the value in the stream graph.
•If the stream is not at zero, return to the
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Channel 1 Configuration screen.
•Modify the Offset by increasing or decreasing the value to zero the output.
•Hang different weights on the load cell to verify calibration throughout its range.
Using Factory Calibration of Load Cell
•The load cell in this example has been calibrated at the factory in the tension direction.
•It has a calibration sheet indicating an output sensitivity of 1.7777 mV/V @ 200 Lbs.
•With this information we can calculate the slope as follows:
o1.7777 mV/V x 3 V(excitation voltage) = 5.3331 mV for full range of 200 Lbs
o5.3331 mV x 569 (+/-2.5 mV user selectable gain) (see note) = 3.0345339 V
o3.0345339 V / (3 V/4096 Bits) (see note) = 4145 Bits
o200 Lbs / 4145 Bits = 0.048250 (slope)
•Note: The V-Link®has a 12-bit A/D converter with a 3 volt excitation, resulting in 4096
Bits / 3 Volts = 0.000732 Bits/Volt. Select a gain that most closely matches the full scale
output of 4096 bits.
•Return to the Channel 1 Configuration screen.
•Select Force under Class and Lbf under Units in the Conversion Coefficients frame.
•Click the Modify button and enter 0.048250 in Slope and 0 in Offset.
•Click Lock to apply the Slope and Offset.
•Click OK to close the Channel 1 Configuration screen.
•Stream the node with no weight on the load cell.
•Observe the value in the stream graph.
•If the stream is not at zero, return to the Channel 1 Configuration screen.
•Modify the Offset by increasing or decreasing the value to zero the output.
•Hang different weights on the load cell to verify calibration throughout its range.
Using a Calibrated Tensometer
•The procedure is very similar to the “poor man
calibration”. Instead of hanging weights from the load
cell, we are mounting the load cell into a tensometer as
shown, applying precision loads, and noting the
corresponding bit values.
•Proceed by streaming the node and observing the bit
output using various weights. In our example shown in
Figure 4, we are demonstrating 11 loads, i.e., 0 (no
load), 21 lbs, 39 lbs, and so forth.
•After making weight measurements, calculate a slope
from the data using the formula y=mx+b. Please see
our technical note for a step-by-step using Microsoft
Excel.
5
BitsLbs
2036 0
2191 21
2324 39
2475 61
2617 80
2760 100
2908 120
3056 141
3201 161
3348 181
3495 200
Figure 4: Calculating a Slope
•In our example we see a slope of 0.1378 and an offset of -280.64.
•Return to the Channel 1 Configuration screen.
•Select Force under Class and Lbf under Units in the Conversion Coefficients frame.
•Click the Modify button and enter 0.1378 in Slope and -280.64 in Offset.
•Click Lock to apply the Slope and Offset.
•Click OK to close the Channel 1 Configuration screen.
•Proceed to again stream the node with no load on the load cell.
•Observe the value in the stream graph.
•If the stream is not at zero, return to the Channel 1 Configuration screen.
•Modify the Offset by increasing or decreasing the value to zero the output.
•Apply different loads on the load cell to verify calibration throughout its range.
•Note: You should apply at least ten loads (even more is better) over the entire range of
the load cell rating to increase the accuracy of the calibration.
Support
MicroStrain support engineers are always available to expand on this subject and support you in
any way we can.
LORD Corporation
MicroStrain Sensing Systems
459 Hurricane Lane, Unit 102 ph: 800-449-3878
8401-0010 Williston, VT 05495 USA fax: 802-863-4093
MicroStrain®, Node Commander®, LXRS™, MathEngine®, mXRS®, EH-Link®, SensorCloud®, TC-Link®, G-Link®, V-Link®, SG-Link®, Strain
Wizard®, HS-Link®, WSDA®, and Little Sensors, Big Ideas® are trademarks of LORD Corporation.
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