Applying Excitation Load to the SCXI-1520

We use the SCXI-1520 signal conditioner to test and calibrate bridge sensors that we manufacture and it works great. While the system is not used (i.e. when the operator connectes a sensor to the system) the SCXI-1520 is automatically isolated by using relays in a switching system. The primary intent for this is to avoid possibly damaging the signal conditioner. (Operators do regretful things.) This means that the excitation power is floating with no load while the signal conditioner is disconnected,. I was wondering if it would be appropriate to have the system automatically apply a load resistor to the excitation source to keep it "warmed up" so to speak. Most sensors are a 350 ohm bridge and may be as high as 5000 ohms. I was thinking of applying a common 330 ohm 1/2 watt resistor as a load. (We are currently only using two channels at a time.) Although I'm sure it would not make things worse, I just wonder if it's worth the bother. Please advise...

In my opinion, I don't think it is worth the trouble. The temperature
of the SCXI-1520 probably won't change much from connecting a load to
the exictation since there is plenty of heat generated from the
SCXI-1520 to keep it at a stable temperature with or without the
excitation source being connected. A bigger concern would be that
strain gages can have self-heating errors when current is applied,
which is usually accounted for by using a Quarter Bridge Type II
configuration with a dummy strain gage next to the active gage. These
self-heating errors occur when the strain gage is connected to the
excitation source - so if you wanted to keep anything "warmed up" it
should actually be the strain gage that you are disconnecting; not the
SCXI-1520.
-Logan K

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I Am Getting Inaccurate Readings When Some Channels of the SCXI-1520/SCXI-1314 Are Open Circuits

Am Getting Inaccurate Readings When Some Channels of the SCXI-1520/SCXI-1314 Are Open Circuits
My hardware is :-
PCI MIO 16E-1 - 1 no.
SCXI-1000 chassis - 1 no.
SCXI-1520 - 4 nos.
SCXI-1314 - 4 nos.
MAX is showing all hardware ( DAQ card + Chassis + SCXIs), set in Multiplexer mode.
I am using MAX task to configure channels ( as per requirement ) and then use the same task in Labview 7.1
When I am trying to generate task only for connected channels then reading are coming some times....... but when i generate task for all channels including (not connected channels) then the reading are either on - side or on + side (max or min).
I have tried to ground the +S but not work
tell me some solution
Thanks
Surender Kumar

No, DAQmx is superior and I strongly recommend you used DAQmx
Elijah, I guess I understood the problem that Sunder is facing, coz even I had experienced it 2 years back
What happens is if you scan only the channels to which sensors/strain gauges are connected, all is fine and you get good strain readings.
But if you happen to include an unconnected channel into the scan list, all the subsequent channels after the open channel will show saturated readings.
Right, Surender??
The problem we found, lies in the settling time of the E series DAQ card's ADC. Even we were using PXI 6071 E
Once the ADC saturates on reading an open channel, it cannot settle fast enough to read the next channel.Hence, the next channel also reads saturated value, instead of the correct value.
This is irrespective of the sampling rate because, this happened even when we did continuous sampling at low scan rates of 10 to 100 samples /sec
We found 2 solutions to this problem
Disable Simultaneous sampling and hold for the SCXI 1520 module ( using a DAQmx property node)
Use a M series DAQcard, which did not have this settling time problem
Hope this helps,
Regards,
Dev

Reading a thermocouple with the SCXI 1520

Does the SCXI 1520 have something to account for the interface connection heat, and is it needed? I've heard this is what the thermocouple modules do, and it is necessary.

The SCXI-1520 uses the SCXI-1314 Module which does not have cold-junction(CJ)compensation capability - meaning it can not compensate for either the temperature of the connection to the DAQ system, nor, the additional voltage effect due to the dis-similar metal contacts of the DAQ system. This correction is absolutly necessary for accurate temperature measurement. I suggest you use a "third-party" cold junction module from a vendor such as OMEGA Engineering (www.omega.com). The CJ sensors are battery powered and reference the signal to 32 Deg F while at the same time providing standard copper leads for the DAQ system.

I would like to programatically make settings on the SCXI 1520 Strain Gauge module. From MAX, I see that you can set the Gain, Fil

ter cutoff freq, Voltage Exc, and Bridge Config. And from the LabVIEW help for AI Parameter, that these correspond to parameters 13,14, 17 and ?. Bridge Configuration is not listed. Which parameter is used to set this? Is there an example VI available?

Hi Frank -
Looks like Michael's post from 2001 is no longer current. Let's update the list of resources available for this module.
You can find the user manual for anything at ni.com/manuals. The manual for the SCXI-1520 is currently here.
If you're programming in LabVIEW, you can find example programs for this device in the Example Finder. (Click Help » Find Examples to open it.) I recommending filtering by your hardware. To do so, click the Hardware list in the bottom-left of the window and select "Add device". After adding the SCXI-1520, select it from the list and check the Limit results to hardware box. This will filter our examples down to the ones that work with your device. They'll probably all be in the "Hardware Input and Output » Analog Measurements » Strain" folder.
You can also search Support for the 1520 to see KnowledgeBase articles on it.
And you can search the Developer Zone for additional example programs or whitepapers/app notes on this device.
David Staab, CLA
Staff Systems Engineer
National Instruments

What is the combined effect of the SCXI-1520 Gain and NI-Max Gain?

LabVIEW 6.1
SCXI-1000
SCXI-1520
SCXI-1314
PCI-6031E
Windows 2000
Greetings-
I'm trying to determine how the gain that is settable on a given channel of an SCXI-1520 module is combined with the gain that NI Max assigns to a Virtual Channel.
For example, if you set a gain of 100 on ch0 of an SCXI-1520, and you then create a Virtual Channel to read the mV output of the Ch0 sensor, (in this case a full bridge load cell), should the AI limits (range) that you specify in the Virtual Channel creation reflect the gain of 100 that the 1520 module is already applying to the signal? And after the 1520 module applies it's gain, does NI Max attempt to apply another gain if there is still capacity to amplify the signal within the AI range of the daq device?
Also, when measuring a full bridge sensor as an analog input voltage signal, and after performing a Null on the sensor, there is usually some residual offset. Does one then programmatically (in LabVIEW) tare, or zero this residual offset by subtracting its value from all subsequent measurements?
Thanks for your help.

Hello Steve,
How are you applying gain if not through a virtual channel?
When you set the input signal ranges in MAX for your virtual channel, SCXI and DAQ board gains are applied. For example, if you create a voltage channel to read the mV output of a sensor, and set the range to +/- 10mV, the SCXI Module would apply a gain of 1000 to the signal and send the resulting +/-10V to the MIO DAQ device. The DAQ device will then attenuate this signal to +/-5V (the range of the ADC) by applying a gain of 0.5. Now, if you set the input signal range to +/-1mV, the SCXI Module will apply its maximum gain of 1000 and send the resulting +/-1V to the MIO device. The DAQ device will then amplify this signal to +/-5V by applying a gain of 5.
If you are still seeing a residual offset after null calibration, you should use the Auto Zero property. This property measures ground before an acquisition and then subtracts that value from every subsequent measurement. You can set this in a strain task, or with a Channel Property Node in LabVIEW.
Let me know if you have any further questions.
Sean C.

Does NI offer anything that can handle an AC Excitation similar to the SCXI 1121 which has DC Excitation?

I have a LeBow strain gauge Torque Sensor that requires 10 VAC Excitation does NI have anything to accomodate this? I know the SCXI 1121 handles 10 VDC Excitation.

Joey,
We only offer one SCXI module with VAC excitation, but the highest voltage it can give is 3 Vrms (1-3 Vrms) and in frequency from 2.5 to 10 kHz. You can take a look to it by going to the main page at www.ni.com, and then click on the link for "Product Catalog", then select "Products and Services" and select "Signal Conditioning"... then select "SCXI", then "SCXI Analog Input", and then on the select box that says "Signal Conditioning Feature" select "Excitation"...and then click on the "SCXI 1540" link... or follow this link as well...
SCXI 1540 Catalog Page
In your case if you really need the 10VAC excitation, you most probably are going to need an external excitation source.
Good luck!...
Nestor Sanchez
Applications Engineer
National Instruments
Nestor
National Instruments

SCXI-1520 excition measurement

After setting the excition voltages of the SCXI-1520 either with MAX or the AI Parameter.vi, I can read back the value for a given channel with AI Parameter.vi's "Get" function or with the SCXI channel strings "ob0!sc1!md1!p_posX" & "ob0!sc1!md1!p_negX" (where X is the channel number).
The values reported are slightly different than the commanded value. Which of these is the most accurate value of the actual voltage?
Rich

Greetings Rich,
Using the SCXI channel strings will actually read back the excitation voltages using the A/D of the DAQ device cabled to the SCXI chassis. Using the AI Paramenter VI to get the excitation value will just read the NI-DAQ configuration file and return what you set the excitation value to within MAX or LabVIEW. So the AI Parameter VI should just be used to ensure that the excitation was set (this isn't actually reading the excitation voltages using the A/D of the DAQ device).
So, reading with the SCXI channel strings will allow you to confirm the voltage levels of excitation. Using the AI Parameter VI will ensure that the setting for excitation has taken affect.
I hope this helps. Let me know if you have additional questions. Have a great day.
Regards,
Todd D.
NI Applications Engineer

SCXI-1520 Zero Offset Changes with Variable Loads

We are using the SCXI-1520 to calibrate sensors based on strain gauge bridges. The problem I'm having is that the offset changes when changing the bridge resistance. For example, the output reads 0.0000 mV/V for 350 ohm bridges. It reads 0.0112 mV/V for 700 ohm bridges. It reads 0.0135 mV/V for 1000 ohm bridges. The signal input is a constant zero for all three of these examples. The span (gain) is very consistent. In other words, changing the bridge resistance does not affect the gain. I know the excitation sense lines are working and they are applied very near the bridge. Enabling or disabling the autozero mode makes no difference. What can be done to eliminate the variable offset when changing the bridge resistance?

Dear Dj143,
Are you calibrating your strain gauge lines in the Measurement and Automation Explorer (MAX) or in your LabVIEW program?
In MAX you can create a task and use the calibrate tab in the task to calibrate your line. You would then call the task in LabVIEW. If you are using this method, make sure that you are specifying the correct Gage Resistance (gauges in an unstrained position). The default is 350 so it would make sense that you are getting 0 V with this resistance.
In LabVIEW you can use DAQmx Perform Bridge Offset Nulling Calibration.vi to calibrate your lines. There is an example in the Example Finder that shows how to use this VI. (LabVIEW Help menu>>Find Examples. Search calibration. Cont Acq Strain Samples (with Calibration).vi). In this case make sure that you are specifying the correct Nominal Gage Resistance (gauges in an unstrained position). Once again the default is 350.
If you still get incorrect voltages when specifying the correct resistance, let me know how you are calibrating your lines and I will see what I can think of. Have a great day!
Sincerely,
Marni S.

SCXI-1520 AND 700 OHM LOAD CELL?

Will the SCXI-1520 work with a 700 ohm full bridge load cell? I'm a load cell mfg proposing a 700 ohm load cell solution to a customer. He says he doesn't think the 700 ohm cell will work with his 1520. The load cell will can take a maximum 20 VDC excitation and outputs 2 mV/V.

Hi,
Can you verify that the voltage coming out of the thermocouple is correct? You can do this by measuring across the thermocouple using a hand held multimeter. You may also want to input a known voltage into the module (a simple battery would suffice) and see if this produces the correct voltage in Measurement and Automation Explorer.
Abhinav T.
Abhinav T.
Applications Engineering
National Instruments India
LabVIEW Introduction Course - Six Hours
Getting Started with NI-DAQmx
Measurement Fundamentals

Scxi 1520 excitation current specs?

In p. A-3 of the scxi 1520 manual, for the excitation specs there, it says:
"Max. operating current in all ranges - 29 mA"
Is this for the whole SCXI 1520 or just for each channel? Because I need for each channel 5V with 1.6 mA supply. That means for all channels, about 12.8 mA.
Can the SCXI 1520 support this?
Please help me on this. Thanks!

Dear cb34,
Thank you for choosing National Instruments Support!
The SCXI 1520 Module has the operating current range of 29 mA per CHANNEL and not for the whole board. So, You are in good shape to use the system that you have because 1.6 mA per channel is well below the limits that you would need.
Thanks and Regards
Shekhar S
National Instruments

SCXI 1520 quarter bridge setup will not output excitation voltage, but full bridge will

My SCXI system will not output an excitation voltage to the quarter bridge(verified with voltmeter). It works for a full bridge setup. When connected to a 'quarter bridge I' configuration, I can use MAX to null and shunt calibrate. The quarter bridge circuit appears to be balanced after the calibration, but the calibration results post a software gain factor close to zero, as well as a close to zero excitation voltage (eventhough I have the voltage configured for 5 volts). I have the SCXI 1520 module, the virtual channels, and the wiring properly configured (several years experience with this device). I'm using an old E series DAQ card AI-16XE-50, with the 50 pin cable that plugs in the back of the SCXI chassis. I noticed I had two bent pins with the cable connector module that plugs into the SCXI backplane. I was able to straighten them, so the cable connects properly, but I suspect the bent pin incident caused electrical circuit damage. Any suggestions on trouble shooting? None of the three 1520 modules I have will output an excitation voltage for a quarter bridge.

Sorry that I don't have an answer for you tyea, but I'm interested in your thread because I have a similar issue with a 1520. My excitation votltage, as measured with a DVM, is always 1V. I've tried 0V, 1V, 5V --- it's always 1V, on Full or Quarter bridge. I'm using a similar DAQ to yours - a PCI 16XE-10 (and a PCI-6030E on antother system).
I don't have a lot of experience with these modules. This could be a wiring or programmatic thing I'm doing wrong.
Richard

Intermittent channel in SCXI-1520

I have been using an SCXI-1520 and recently channel 0 has become intermittent in operation. It drifts off to either the plus or negative rail and stays there. This problem is noted with and without connections made to it, including the use of the Test Panel function in MAX. Resetting the module sometimes helps, but not for long. It appears to me there is a malfunction in the module, but I wanted to get an additional opinion before I send it in. (An additional uncertainty is that the SCXI-1520 seems a little touchy, or unstable in it's setup. It's just an impression I have.)
Also, is it possible to programmatically reset the module in LabVIEW?

Part of the reason I don't want to send it in for evaluation is we are using it in production. (We make strain gauge based load cells.) We are expanding operations and so I've ordered an additional module and chassis. However, I will not be able to get to it till January.
I reassigned channels to keep production going until I can sort things out.
Thanks for your response. It gives confidence that I'm not missing something in the programming.

How do I deal with a cascaded system, First Stage SCXI-1520's and Second Stage SCXI-1143's?

I am aware of the following link:
http://digital.ni.com/public.nsf/allkb/653505DF63AD04E686256DD7005D42F6
I'm kind of stuck here. The system is already in place. 1520's in first stage and 1143's in second. I am supposed to configure the 1520's and scan the 1143's.
What problems do you see with this?
How do I go about doing an end to end calibration?
Should I just rip out the 1143's and hope nobody notices?

Hello Frank,
Let's start from the beginning. I understand that you have some SCXI-1520s and 1143s that you are using in your measurement system. What type of sensors are you acquiring data from? It seems that you intend to use the 1143s to filter the data between the 1520s and the software. Is this correct? Another question that I have is what do you mean by "end to end calibration?" The more specific that you are about the application you are developing, the easier it will be for others to help you with any questions that you might have.
Looking at the knowledge base article that you linked in your original post, it seems that the point was to filter the data from the SCXI-1520 in software, using the LabVIEW Filter Express VI. Would this be a viable option for you? If so, then you could just rip out the 1143s.
Regards,
Wallace F.
National Instruments
Applications Engineer

Using SCXI-1520 in Parallel Mode.

Hi all,
I am using SCXI-1000 chassis and PCI-1024E DAQ card.
I want to use my SCXI-1520 in Parallel mode.
Is there any other way to configure my 1520 in parallel mode other than directly connecting to DAQ board using SCXI-1180 feed through panel and SCXi-1302?
Some of my friends are telling that there are some Patch files, through which we can use the scxi-1520 in parallel mode. Is it true? If it is true please give me how use the Patch files( or alternative solutions).
Thanq
Regards
Preetam

Preetam,
As explained in the SCXI-1520 user manual, Traditional NI-DAQ does not support true parallel mode operation of the SCXI-1520. However, NI-DAQmx does. I would recommend installing NI-DAQ 7.2, which includes the latest version of both Traditional NI-DAQ and NI-DAQmx. Then you can use the NI-DAQmx driver to control your SCXI-1520 in parallel mode. To configure the SCXI-1520 in parallel mode with NI-DAQmx, simply select the Parallel Mode check box when setting up your chassis and modules in MAX. I hope this information helps.
SCXI-1520 User Manual
NI-DAQ 7.2 Download

How do I wire my SCXI-1520 quarter bridge completion in parallel?

I have a quarter bridge set-up using the qtr completion in the system. In parallel with my strain gauge I'd like to have another resistor. I'm using the strain gauge as a dummy, and am interested in the current traveling in a parallel path through a film resistor as I change the film thickness. However, I'm not convinced that what I've wired has the strain gauge and other resistor in parallel. I have the strain gauge wired to: P+, QTR, S+ and the other gauge wired to QTR, S+.
Thanks!

Greetings,
You will want to wire the other gauge between P+ and QTR or between P+ and S+. To make this decision, you will need to determine how the lead resistances will affect your measurement. Page 2-4 of the SCXI-1520 User Manual (link available below) contains the wiring diagram for Quarter-Bridge Configuration I.
SCXI-1520 User Manual
Good luck with your application.
Spencer S.

Applying Excitation Load to the SCXI-1520

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