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

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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

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Looks like Michael's post from 2001 is no longer current.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

Can I measure dynamic strain with a SCXI-1531 and how steady is the excitation current?

I am looking at the possibility of using an SCXI-1531 module to take measurements from a single strain gauge. I am only interested in the dynamic content of the signal and so am considering using the constant current excitation method which does not require bridge completion. Is there anyone out there that has tried this or could advise me as to whether this module is suitable?
Having read the spec of the module, it looks promising except for the data on the excitation current which states:
"Accelerometer input excitation current: 4mA +/-10%, 24V compliant."
Now, does that mean that the current will be steady at a value in the range 3.6 - 4.4mA or that it may fluctuate within this range during measurement? The latter would not be acceptable for my application as this would be indistinguishable from a variation in strain.
Thanks for any help.
CAS

Hi CAS,
Essentially the excitation provided by the module will not allow you to acquire as you would like.
However, the good news is that you are able to disable the excitation provided by the module, and provide your own external excitation. You should then be able to acquire the signal using the SCXI-1531.
To disable the excitation in LabVIEW using DAQm, place down a DAQmx Create Channel.vi and then configure it for Strain measurements. Then you can create a control/constantto wire to the Bridge Information Terminal by right-clicking on the terminal and selecting control/constant. Within this cluster control/constant will be an enum element where you can specify the voltage excitation source to be external.
I have attached an example VI demonstrating this.
Hope this helps.
Steve
Attachments:
Excitation Example.vi ‏18 KB

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

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

Scxi 1520 exitation of 11 -30 vdc

Hello
I want to know wether i can use a device in my strain (scxi 1520) gauge module which requires an exitaion of 11-30 vdc and gives an ouptput of 4-20 mA.if possible whats the configuration and if not ,if i provide an external exitation how do i use it with my scxi 1520 module..
plz its urgent
lalit

You can convert almost any current output to a voltage output by adding a resistor across the output terminals. Calculate the resistor such that the voltage across the resistor at max current does not exceed max input voltage of the acquisition system. Also, check the specs for max output voltage of the signal source. Don't expect the circuitry to supply more than the circuit supply voltage minus, say, 20 p.c.
Of course you will have to compensate the offset - at 4mA 'live zero' output you will of course get a reading different from zero. This should be possible by software.
However, if you need an excitation voltage higher than supplied by the board you can use an external power supply. Maybe you will have to use a grounding scheme different from what be used with an 'internal excitation' source (from the board).

How can I use LabVIEW to ground floating SCXI-1520 channels (in order not to lose scanning capabilities from the multiplexer)?

Currently I am reading quarter bridge strain with several SCXI-1520s. When in MAX I noticed my values were no longer reading after a certain gage and as I attempted to troubleshoot, found out that the multiplexer in this module will not scan past any open channel (or if it does, channel results are not consistent) for a very logical reason. If I lose a gage, I usually lose any data from any subsequent gages which we really cannot afford.
Are there any suggestions programming in LabVIEW (right now i'm using NI-DAQmx with an analog task set up)so that each time my loop executes, the program will check for any open channels and either not collect that data or change the order of acquisition (i.e. place the open channels last on the scan list?
I'm open to anything that might be of help

Yes, by "open channel" i mean that there is no signal returning from the gage because it has broken off the structure or the signal to the terminal block has been disconnected somehow.
When I lose data, I'm referring to the data that is acquired on following channels. The behavior appears to be fairly random (which was a troubleshooting nightmare ) but usually if a channel is open, then that and those behind it in the scan order do not return any data. It returns a number like 10.357E-6 and I do not pick up any noise or fluctuations. As far as I understand, the multiplexer stops scanning at this point (open channel) because it has nothing to ground to and subsequently "hangs" - not (or randomly) scanning the following channels. I have gone through the bridge configuration and as of yet, have not figured out how to fix this using any hard-wiring techniques. I emailed NI, and was sent a library file to "ground the channels" but it actually resets the Analog input Task I have defined each loop.
I am new to NI-DAQmx so I do not know if this will alter any configurations I already have in MAX for NI-DAQmx. Will it override? I am concerned it will slow my loop time, and also I cannot see the code for this sub.vi so am a little skiddish about using it if i do not know if it can be tailored to my app.
What do you think? Am I on the right path?

RTD excitation current

Hi,
I have a question regarding the SCC-RTD01 temperature module. I connected a RTD to the module, ran the LabView example, and set the excitation current (external) to the recommended 1mA. I was happy to see a value of 25 deg C. I figured since I hadn't really calibrated anything being off by 2 degrees was no big deal. At one point I decided to try reducing the excitation current to 0.9mA and got a temperature reading of 56 deg C. Is this normal for the excitation current to change the measurement so drastically?
Thanks,
Mark

Yes that is normal
As already recommended, try to use the internal current source, than you can expect the given spec.
U=R*I and the R of your RTD is only small but well known changing with temperature.
If you need precision with low thermal masses or low thermal conduction you might look at self heating of you RTD.
Greetings from Germany
Henrik
LV since v3.1
“ground” is a convenient fantasy
'˙˙˙˙uıɐƃɐ lɐıp puɐ °06 ǝuoɥd ɹnoʎ uɹnʇ ǝsɐǝld 'ʎɹɐuıƃɐɯı sı pǝlɐıp ǝʌɐɥ noʎ ɹǝqɯnu ǝɥʇ'

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.

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 for load measurement cable lengths of 500 meters

HI ,
I ned to measure lod from a 1000 ohm full bridge circuit. The distance between the sensor & my SCXI 1520 is 500 meters. Please let me know how can I compensate for the lead resistances both in Excitation & while reading back the actual sensor value.

grte,
For extremely long cable runs resulting in much higher than normal lead resistance to bridge resistance ratios, you must use remote sense, which is a six wire connection (four wires for excitation and two for signal). Refer to your SCXI-1520 manual for more information: http://www.ni.com/pdf/manuals/372583d.pdf, specifically page 2-9 and Chapter 4, Theory of Operation.

SCXI 1520 Starin Gauge Module

Hi....
I am using SCXI 1520 module for measuring strrain (Through Full Bridge strain Guage), I set the Input (Min ,Max)voltage to +\- 1 Volt then the value of starin comming out is 4.6mV but when I change Input Min ,Max to +\- 500mV the output of starin guage become 2.3mV please clarify my doubt.

When you reduce excitation,out put also reduces

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.
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Abhinav T.
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Connecting Omega PX309-10KG5V pressure transducer to SCXI 1520

I have an Omega PX309-10KG5V Pressure transducer that operates between 0-5V and requires a 9V excitation (http://www.omega.com/Pressure/pdf/PX309-5V.pdf).
I am having trouble getting the transducer running on my SCXI box with a SCXI 1520 card. I think I should be using custom voltage with excitation. I am not sure what I am missing.
Any help is appreciated.
Thanks!

floydie83 wrote:
I have an Omega PX309-10KG5V Pressure transducer that operates between 0-5V and requires a 9V excitation (http://www.omega.com/Pressure/pdf/PX309-5V.pdf).
I am having trouble getting the transducer running on my SCXI box with a SCXI 1520 card. I think I should be using custom voltage with excitation. I am not sure what I am missing.
Any help is appreciated.
Thanks!
What does this have to do with LabVIEW?
Post at http://forums.ni.com/t5/Signal-Conditioning/bd-p/300

Scxi 1520 excitation current specs?

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