Absolute Accuracy of a Load Cell measurement.

I see how to calculate absolute accuracy for the SCXI-1520 and SCXI-1600 combination. To call out the accuracy of a load cell measurement, do I have to add the load cell specification numbers for non-linearity, non-repeatability, and hysteresis to the absolute accuracy calculations?

Hi KPR,
If you want the entire system accuracy, including the load
cell, we definitely need to include the load cell accuracy in our calculations.
I am not sure which document you were looking at to get the accuracy thus far,
but KB 2X4HGEBG
gives a great explanation about accuracies. In order to include the load cell accuracy, first
get the absolute accuracy of the load cell, and then put this in the equation ‘System
Accuracy = ( (Absolute Accuracy 1)^2 + (Absolute Accuracy 2)^2 + (Absolute
Accuracy 3)^2 + ...) )^(1/2)’ as ‘Absolute
Accuracy 4’.
David L.
Systems Engineering
National Instruments

Similar Messages

Load cells and non contact strain measurement

I am designing a bi-axial machine which meant for a fatigue test.
Could someone recommend me where to get a dynamic load cells and as well non-contact strain measurement?
Thanks.

Hi Alvin788,
Do you presently use any hardware to measure these or are you enquiring as to what hardware/software you will need? Are you planning to use LabVIEW? The following three links talk about measuring Load Cells as well as Strain Gauges
Continuous Acquisition from a Smart TEDS Load Cell Sensor
How do I Configure a Load Cell in MAX?
Strain Measurement with a Strain Gauge
This post will also be highly helpful.

Question on Measuring Lift Force for MAV Ornithopter for Student Project using Load Cell

Hey everyone!
I am new to these forums so bare with me, please. I am part of a senior design student group working on making a micro aerial vehicle ornithopter (generates lift by flapping its wings) similar to this Design.
We are using this load cell connected to an AD620 Op-Amp circuit that basically has two input wires from the load cell coming into the op-amp with a gain of 100 (there is an Rg resistor with 500 ohms in the circuit). The load cell and op-amp circuit are both powered correctly with 10 V and +-15 V, respectively. There are two output wires coming out of the circuit. One comes out of the 6th terminal of the op-amp, and one is connected to ground. If we turn on everything and test the output of the circuit with a multimeter there is little if no fluctuation in the output voltage, which is about .25 V with zero load. If we add precise weights and form a linear graph of the output voltage vs. weight, we can find that there is about .0045 volts/gram relationship.
Here comes the part we we are having trouble. It seems as though the circuit it working correctly, but when we try and hook it up to our USB-6221 DAQ Board though the ai0 (or any analog input) and confirm a LabVIEW program to read the output, we get a ton of noise along with a weird looking graph.
Here is a picture of the front panel that shows the output we are getting http://img130.imageshack.us/i/frontpanelg.jpg/
Basically, I would have though we could get a straight line that continuely increases or decreases depending on the load on the load cell. Why would it graph a ton of vertical lines? I would think it would be just one data point per time value?
Here is the simpler block diagram just to test the output: http://img26.imageshack.us/i/blockdiagramsmall.jpg/
Here is the main block diagram that we were going to use to convert to force, etc. but its not working (no need for while loop?): http://img15.imageshack.us/i/blockdiagrambig.jpg/
The properties of the DAQ Assistant VI is as follows:
input range: -10 to 10 v
RSE terminal configuration
acquisition mode: n samples and not sure how many samples we want to read or how fast. we had 1k hz and 100 samples and the image above is basically what we got it the program runs continuously.
I don't know what else I can tell you, except that if anyone helps me you would be like a godsend to our group. It would be very much appreciated
Thanks!!!

As I am unable to look at youor images (the corporate filter here is blocking the url) I can't see what you are describing. I will say that a multimeter will not show fast transitions that will show up in a LabVIEW/DAQ application, so using a meter usually doesn't tell you that much about signal "cleanness". How long are the wires from the op amp/load cell to the DAQ? How long are the wires, and what type (shielded, twisted pairs, untwisted pairs, etc.) from the load cell to the op amp. The output levels of the load cell are usually quite low, so the possibility of also picking up "environmental" electrical noise is very high, and this will be amplified as well. Since the output of the op amp is higher, the noise introduced between it and the input to the DAQ is less of an issue, but there as well. So making sure that signal wires are properly shielded and grounded is important, and there are long papers describing this for low level analog signals. Since the load cells desired output will probably vary relatively slowly (a few Hz) it may also be helpful to put a simple low pass filter before the DAQ.
You should insert the images into your post, as jpg, or png (you have jpg images) Not bmp! (this is for others that might read your post, yours are jpg which should be ok). Generally most of us won't or can't go to links, corporate network filters and corporate policies frown on it, too much maleware lurking at the end of unknown URL's
Putnam
Certified LabVIEW Developer
Senior Test Engineer
Currently using LV 6.1-LabVIEW 2012, RT8.5
LabVIEW Champion

VI to measure Load Cell

Hello everyone,
So I'm extremely new at this and really need some help. I need to create a VI that is going to be able to read the force using a load cell.
if anyone can help me I am willing to pay for your time, I don't have much time to learn on my own so I need the help of someone.
I have a power supply load cell and DAQ
please email me if your willing to help me and make some money
Power Supply: http://www.testequipmentdepot.com/bk-precision/power-supplies/1786a.htm?gclid=COahppSjtbUCFQuqnQodmF...
Load cell: http://www.omegadyne.com/ppt/prod.html?ref=LC401&flag=1
DAQ: NI USB-6008
[email protected]

Hi,
If you need to acquire voltage from your load cell with a USB-6008, you can run the attached VI.
Regards,
Attachments:
Voltage.vi ‏34 KB

Is it possible to condition a load cell signal with a 1520 SCXI and then use it as the primary analog feedback for a 735X motion control board?

My system
PXI 1052 chassis
PXI 8196 RT controller
Flexmotion 7356 motion controller
Flexmotion 7354 motion controller
6259 M series DAQ
1520 SCXI
1520 SCXI
1314 SCXI frount mounting terminal block
DSM strain gage load cells
Exlar SR Brushless PM motors
Kollmorgen servo star CD series 5 drives
My objective:
I want to measure the force on the load cells with the 1520 SCXI (i.e. excite the load cell then amplify and condition its output) then I want to use this analog (force) signal as the primary feedback for my motion control system (7356 and 7354 motion controller connected to my drives and motors).
The problem:
The motion controller boards (7356 and 7354) can be configured for force feedback control via analog feedback from a load cell. However the boards implicitly expect the load cell signal to be amplified and conditioned with and external signal conditioner rather then using an NI SCXI module. Currently I’m am reading the SCXI signals via the 6259 M seriers DAQ and consequentially have no way to route the signal into the 735X boards for force feedback control. I can use softmotion to control the motors but then I loose the fast PID update rates possible with the 735X boards.
The question:
Is it possible to condition a load cell signal with a 1520 SCXI and then use it as the primary analog feedback for a 735X motion control board?
Thanks
Patrick Aubin
Ph.D Candidate
University of Washington &
VA MedicalCenter of Excellence for
Limb Loss Prevention and Prosthetic Engineering

Hi GG,
Thank you for your feedback.
>create your own external circuitry to amplify the signal.>
True a few op-amps could amplify and filter the signal externally but the performance of such an amateur built system will never match the accuracy and functionality of the 1520 SCXI. That’s why we spent the $2500 for the professional version.
>One thing that you could do is to use a UMI with your motion controller and then wire your signal from your SCXI to the UMI.>
As I understand it the UMI is basically a break out connectivity interface for third party drives. After conditioning and amplifying the analog force signal with the SCXI module the analog signal isn’t available to wire into a UMI. To my knowledge the conditioned signal of the SCXI resides only in the PXI backplane (i.e. there is no physical access to the conditioned analog signal). Can you explain how the UMI could be help here.
Again thanks for your input GG.
Patrick

Data acquisition from a 250N Load cell

Hi ,
I am very new at using the Labview.I am using Labview 6i(PCI 6052E).The load cell(250N) is connected to a pump.The pump has 2 bags.One of them is filled with water and the other is empty.I want to load the bag which is empty from the bag filled with water using the pump.The load cell is connected to the bag which is filled with water.I am trying to aquire a volatge signal continously from a load cell and display it on a chart.
My program has no errors and I am able to run the progam.But it is not aquiring any signal.Could u please help me out.Attached is my code for the same .
-Manasa

Hi Manasa,
Make sure you are handling your errors in your code. You can do this by wiring your error clusters through your VI and then terminate them in a General Error Handler VI. If you are doing this, make sure that your device works in Measurement & Automation Explorer (MAX). Do this by opening a test panel and verifying that you can get a known voltage (like a battery) to read correctly. If you can successfully get a voltage in MAX, try running one of the example programs. You can find them at Help » Find Examples..., then browse to hardware input and output. If you can get an example program that is similar to what you are trying to do to work, try comparing your code to the example.
Please post back if you have any questions. Have a great weekend!
Ryan D.
District Sales Manager for Boston & Northern New England
National Instruments

Not getting accurate readings off a calibrated load cell

I'm using a Cooper Instruments 200 lb load cell connected to a SCB-68 connector box hooked up to a 6036e DAQ card, and I'm having trouble calibrating it. (I've done this before, but it's been a while, and something just isn't working right.) So, I'm using the calibration wizard (the channels are set as Custom Voltage with Excitation), hanging a bunch of known weights from the cell, entering the values, and all that is going fine. However, when I go to use the load cells (in a very simple VI that just reads the value from the cell), it's reading consistently low by 2-3 pounds. I'm using a 10 V excitation signal, and I set the low end of the Signal Input Range to 0 (it kept wanting to set -5=-5 in the calibration when the low end was -5V, and that was even worse). I've tried modifying a custom scale, and that isn't fixing the problem either. Has anyone encountered this before or have any ideas on what else we can try?
Thanks,
Eric Martell

Hi Eric,
This is a curious problem. Is it every point that has a constant offset or does the offset vary from point to point?
Also what is the model number and sensitivity of your load cell? Have you gotten it to read correctly before? You mentioned that the values were stable out to the fourth decimal place, but haw significant is this decimal place? For instance, when I open the calibration wizard, the values can jump around depending on what I am measuring. If you load cell has a sensitivity of 1 mV/lb, then this could be large variations depending on what the decimal point was when you saved the value.
With regards to my mentioning averaging, when you first click the calibrate button in the DAQ Assistant, the first screen has a Number of Samples to Average control and a Rate control. If you leave the # of samples to average at 1000, but change the rate to 10,000, do the values begin to vary?
Regards, Mallori M.
Mallori M
National Instruments
Global Customer Education Program Mananger
Calculate the time you will save with NI Training

What is the "Random Noise" specification in the AI Absolute Accuracy Table

I have a PCI-6255 board that I'm planning to use for some sensitive measurements.
I'm trying to put together an end-to-end error budget and I have a couple questions.
My model is in terms of analog-to-digital converter counts (also known as LSB in your data sheet).
In the Absolute Accuracy table on page 5 of the NI 615x Specifications I see a column labeled Random Noise, (uVrms).
I think I understand the residual gain error, residual offset error, and associated Tempco, and the INL error but the Random Noise is puzzling me.
Here is why: on the +/- .2V range, the random noise is 16 uV. On this range, the ADC resolution is 6.1 uV so the random noise is about 2.6LSB
If I look at the +/-5V range I see a random noise of 140uV. On this range, the ADC resolution is 305uV so the random noise is well under one LSB.
Therefore, in my error budget I can use "1 LSB" for the random noise when on the 5V range but I have to use "2.6" or "3" LSB when on the .2V range.
I expect a signal of aproximately 150mV riding on a 2.5V DC level. I was concerned I might need to offset the signal (remove the 2.5V dc) in order to use the +/-.2 volt range to get better resolution. I also thought I would get better noise performance too. In looking at the numbers here though it looks like I would be better off using the +/-5V range (assuming it gives good enough resolution) because the readings have less noise counts (even though it is more noise volts).
I can account for the Gain Error and Offset Error by injecting known signals and doing a "software correction". Not sure, but I believe I can also correct for the INL if I use a polynomial fit of 2nd or 3rd order. It's the random noise I'm having problems understanding.
Here's the question: what is the Random Noise shown in the AI Accuracy table for the 625x boards? Can you give an example of how NI might have measured it?
Secondly, note 2 says "Sensitivity is the smallest voltage change that canbe detected. It is a fucntion of noise." Was this measured with a signal-to-noise ratio of "1" or "2" or some other standard, or is this a calculated value?
Thanks
MG

wrong forum. Sorry. I'll repost in proper forum

9237 with full-bridge load cell: load cell_null_off_shuntcal.vi throws error 200077

Hi,
I'm trying to use the example
load_cell_null_off_shuntcal.vi with a full-bridge load cell (Honeywell
Model 31, unamplified). I am using LabView 8.6, cDAQ-9172 and NI9237. The
load cell is connected to pins 2,3,6 and 7.
The inputs for the VI front panel
are: internal excitation10V; mV/V 2.1492 (calibration sheet); max weight 10
lbs; bridge resistance 350 ohms (Honeywell specs); 9237 internal shunt
resistance 100 kohms; shunt location R4 (default setting). I have selected
"Do offset null" and "Do shunt cal".
This is the error I receive:
Error -200077 occurred at DAQmx
Perform Shunt Calibration (Bridge).vi:1 Possible reason(s):
Measurements: Requested value is not
a supported value for this property.
Property:
AI.Bridge.ShuntCal.GainAdjust
You Have Requested: -61.980405e3
Valid Values Begin with: 500.0e-3
Valid Values End with: 1.500000
If the "Do shunt cal"
green button is not selected, there is no error. I understand that the Gain
adjust value should be approx 1, whereas the one I get is much larger. The subVI DAQmx PerformShuntCalibration
(bridge).vi contains a "Call library function node" which I don't
know how to interrogate.
Has anyone else had experience
with this error? Do you have any advice on:
1)
How to "see" the calculations being
perfomed inside the "call library function node"?
2)
What the correct shunt element
location for a full bridge load cell is? (although changing this location does
not eliminate the error, I can't find this info).
3)
Anything I may be doing wrong with
my inputs to cause this error?
Thanks,
Claire.
Solved!
Go to Solution.

Hi Claire,
You have to physically connect the SC terminals to one arm of the bridge (normally R3). The terminal is not provided for connecting external resistors.
See example
C:\Program Files\National Instruments\LabVIEW 8.6\examples\DAQmx\Analog In\Measure Strain.llb\Cont Acq Strain Samples (with Calibration) - NI 9237.vi
"A VI inside a Class is worth hundreds in the bush"
യവന്‍ പുലിയാണു കേട്ടാ!!!

9237 + full-bridge load cell: load cell_null_off_shuntcal.vi - error 200077

I'm trying to use
load_cell_null_off_shuntcal.vi with load cell (Honeywell Model 31,
unamplified). I am using LabView 8.6, cDAQ-9172 and NI9237. Inputs:
internal excitation10V; mV/V 2.1492 (calib. sheet); max weight 10 lbs; bridge
resistance 350 ohms (Honeywell specs); 9237 internal shunt resistance 100
kohms; shunt location R4 (default setting). Have selected "Do offset
null" and "Do shunt cal".
Error -200077 occurred at DAQmx
Perform Shunt Calibration
(Bridge).vi:1 Possible reason(s):
Measurements: Requested value is not
a supported value for
this property.
Property:
AI.Bridge.ShuntCal.GainAdjust
You Have Requested: -61.980405e3
Valid Values Begin with: 500.0e-3
Valid Values End with: 1.500000
If "Do shunt cal" green
button not selected, no error. Gain adjust should be approx 1. subVI
DAQmx PerformShuntCalibration (bridge).vi contains "Call library function
node" which is locked (?).
Any ideas?
What is the correct shunt element
location for a full bridge load cell? Changing this location does not eliminate
error.
Solved!
Go to Solution.

Hello CFJ,
The problem is most likely in your external connections of the NI 9237 and the load cell. As referenced in the NI 9237 Operating Instructions and Specifications, page 9, the SC+ and SC- pins should be connected across the resistor specified in the DAQmx Perform Shunt Calibration (Bridge).vi (in the case of a full bridge it would be R3).
Let me know if you are still having issues with your calibration.
Message Edited by Dan_K on 01-22-2009 04:31 PM
Regards,
Dan King

PASCO Load Cell and NI DAQ Interfacing

Hi everyone,
I have a PASCO Load Cell PS-2200 and I am trying to interface it with an NI-9201 AI (in conjunction with a NI-cDAQ 9172). Instead of attaching wires to the cell's pinout, I stripped a PS2 connector and rearranged the wires to match the pinout of the sensor (which are the strain gauge input, strain gauge output, the excitation voltage of 4.2 and the ground). I connected the strain gauge signal to a channel of the NI 9201 and I am using an external power source to provide voltage to the cell. My VI is trivial and consists of an input DAQ and Lowpass filter.
The problem is that the input signal is obviously not right, it looks periodic for one. The filtered signal appears stable at first but slowly increases over time. And the signal does not respond to applied stresses.
I would really appreciate any help on this. I am a ME sophomore and have little experience with LabVIEW.
Thank you.

Thank you Jignesh and Dave,
Jignesh, I do not have a scope I can measure the ouput with. And no, the cell does not respond to variable loads. Also, please find attached the snapshots you had asked for. The front panel snapshot shows the cell's reponse when under no load
Dave, the module I am using has a 16 bit resolution and within a +/- 10 full inut range, don't we have enough resolution to acquire the smallest change?
Also, from the pinout of the load sensor, the pin for the excitation terminal and the positive strain gaugw output are adjacent. They are not the same terminal. We took every possible precaution to make sure that the terminals of the PS2 connector did not touch during soldering.
Thank you for your help again.
Attachments:
block diagram.jpg ‏159 KB
front panel.jpg ‏390 KB

Closed loop velocity control based on load cell force

Hello,
My application is for a drill, that drills into rocks of various densities for the purpose of collecting rock core samples.
My setup has 2 motors which get controlled, one spins the drill bit at a constant velocity, the other moves the drill mechanism along a Z axis.
For efficient cutting, it is desired to apply a constant force between the drill bit and the rock. I have a load cell which measures the force being exerted by the drill bit on the rock surface, and this force can be adjusted by changing the velocity of the Z axis. So I would like to employ closed loop control to adjust the Z axis velocity to maintain a constant force on the rock.
Platform: cRIO 9073, with NI 9505 & 9215 Modules, Labview 2010 Full with RT and FPGA modules.
The load cell is by Transducer Techniques, and I use their TMO-1 module to condition the signal, the output of which is attached to an input of the 9215 module, where 0-100 lbs equates to 0-8VDC.
The motors and encoders for the Drill and Z axis are connected directly to the 9505 modules.
Right now I am using a modified version the example found in ...\examples\CompactRIO\Module Specific\NI 9505\Velocity Control (closed loop)\Velocity Control (closed loop) - NI 9505.lvproj to accomplish velocity control of the motors.
My questions are:
1) Do I have the appropriate NI hardware/software for this task?
2) With my current hardware setup, what would be an appropriate way to control my Z axis velocity rate based on Analog feedback from the load cell?
3) Development time is a critical factor, so are there any toolkits etc that are easy to get started with that would drastically decrease my development time, or do I already have everything I need?
Thank you for your time
-MK Hokie

1. Your hardware and software look appropriate assuming the motors are compatible with the 9505s. You didn't mention the NI SoftMotion module in your software list which is something you will need.
2. There are a few ways of doing this. One method would be to have a force PID loop that would attempt to maintain a force setpoint by directly outputting values to your torque loop. In this case, the drill would essentially move as fast as it could while maintaining the force setpoint. Another option is to have the force loop output a velocity setpoint. You would then have a velocity PID loop that outputs torque values to the torque loop. By adding this additional velocity loop you could have control over your maximum and minimum velocities. There are likely other alternatives as well, but these are the first two that come to mind.
3. Unfortunately there are no shipping examples that close the loop on force feedback. My advice would be to start with the NI 9505 shipping example and adapt it to your needs. There are quite a few things you will want to change though. Do you know if you will need to use the trajectory generator to move the drill into position before starting the force control?
Assuming you don't need any trajectory generation, you can scrap the entire RT portion of the NI 9505 example and just create the necessary FPGA code. On the FPGA, you won't need the Spline or Synchronization code either because this information would no longer be coming from RT. You could take these out and replace the position loop with a force loop and possibly a velocity loop and your FPGA program would basically be finished. In fact the only real motion IP that you will need is for the encoders (assuming you want velocity control) and PID. Then you would need to create an RT VI that allows you to send down enable, disable, PID gains, and setpoints.
If you do need trajectory generation, you would want to keep most of the example code the way that it is, but then program in a 'Force Mode' that utilizes the force and velocity controly as described above. You could think of it as having two different routines programmed side by side.
Regards,
Burt S

Read from both load cell and accelerometer

Hi,
I'm trying to collect data from both load cell (which doesn't need excitation) and accelerometer (which needs 4 mA excitation). As you can see below, I wrote a simple program to collect data from both devices. However, my program provides 4 mA of excitation to all channels, one of which shouldn't receive any current. How can I selectively provide the excitation to specific channels?
Any suggestions?
Solved!
Go to Solution.
Attachments:
readfromtwodevices.vi ‏47 KB

Good morning jeongho20000,
Both aeastat and falkpl have provided excellent solutions for this. I also wanted to mention a good KnowledgeBase article found at ni.com by searching for "daqmx channel same task" in the internal search bar. This helps to outline both solutions. After searching, you can then select multiple entries, but the best that I found was the 3rd choice, titled: Using Different Types of DAQmx Global Channels in the Same Task.
You can download from the article the attached example VI titled Multiple Channels Created.vi, and modify the block diagram to fit your needs. Wiring the Task Out to the Task In of two DAQmxCreateChannel VIs is the programmatic equivalent of adding multiple Global Virtual Channels created in Measurement and Automation Explorer (MAX) to a common Task in MAX. Either method works, it is only a user preference. Feel free to reply if you need further clarification/explanation.
Best,
Message Edited by cyclone2k9 on 04-06-2010 11:08 AM
Message Edited by cyclone2k9 on 04-06-2010 11:09 AM
Adam
Academic Product Manager
National Intruments

NI 9219 with load cell

Hello,
I am using the NI 9219 with a Futek Load Cell whose specifications are as follows:
Name
Min
Typ
Max
Unit
Channel: 1
Compensated Temperature
60
160
F
Excitation
1
20
Vdc
Hysteresis
-.25
.25
% of R.O.
Input Resistance
744
Ohms nom.
Insulation Resistance
500
Mohms @ 50 Vdc
Nonlinearity
-.25
.25
% of R.O.
Nonrepeatability
-.05
.05
% of R.O.
Operating Temperature
-60
200
F
Output Resistance
700
Ohms nom.
Safe Overload
150
% of R.O.
Temperature Shift Span
-.01
.01
% of Load/F
Temperature Shift Zero
-.01
.01
% of R.O./F
Zero Balance
-1
1
% of R.O.
Capacity
500
lbs
Rated Output
2
mV/V nom.
Calibration Excitation
10
Vdc
I am getting a signal from my load cell which is great, but I was wondering how you can adjust the signal output to have minimal interference from noise, vibrations, etc. and how to program the load cell that it can be calibrated to zero (0) before any data is aquired?
Any help is greatly appreciated!
Thanks,
Yatsco

Hello Yatsco,
Thank you for using NI forums. One thing you will probably want to look into is using the filtering VI's in LabVIEW. A low-pass filter can help to eliminate any high frequency noise from your measurement. Also, take a look at the Field Wiring and Noise Considerations article for some more information on eliminating noise. Lastly, this community example shows how you could calculate the DC offset and compensate for it before measuring your signal. Please let me know if you have any additional questions concerning this application.
Regards,

Connection for load cell to NI9237

Hi,
I have this omega load cell part LCKD-10 which is a half bridge load cell that I need to connect to the NI9237. Please advice on the connection.
Thanks
Joelle

Thanks. I need to write out a program which checks the centrality of force. I will be using 5nos of the omega load cell, 1 in the centre(Isolated) and the other 4 at all 4 corners. I'm suppose to firstly calibrate the load cell and then take measurement of the force applied to the 5 load cell and display the result in a form of a radar graph (Just a dot in the centre of the graph to show the centrality of the force). Appreciate if you can provide some procedures or program that I can use or try out.

Absolute Accuracy of a Load Cell measurement.

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