What voltage is read for SCXI-1520 use excitation for scaling

[RR LabVIEW]

We want to know what voltage is read when you select Use excitation for scaling on a DAQmx global channel. Is it the Excitation r the returned sense voltage?

[Ima U]

Hi,

Check Use Ex For Scaling if you want NI-DAQmx to divide the measurement by the excitation. You should typically select this option for ratiometric transducers. If you select Use Ex For Scaling, set Max and Min to reflect the scaling. For example, if you expect to acquire a voltage between -5 and 5, and you use an excitation of .10 volts to scale the measurement, set Min to -50 and set Max to 50. If you set Bridge Type to None, Use Ex For Scaling has no effect on the measurement. I hope this helps.

[RR LabVIEW]

Thanks for your response. Actually we were interested in which voltage within the SCXI-1520 module is read to be used in the calculation for scaling from the excitation. Is it the actual excitation voltage as produced by the 1520 module or the excitation value returned on the sense lines? I have suggested to my co-worker that it is the excitation for the channel. He is suggesting that it must be the excitation value returned on the sense lines as it will be the most accurate to use for the calculation. My understanding is that the sense line are the feed back to regulate the excitation and therefore should be ~equal to the excitation.

[Logan_K]

The "Use Excitation for Scaling" property doesn't actually read back the physical excitation for you. Like Ima mentioned, it is only used for software scaling purposes, and will use the excitation that you specify in your task for scaling - this isn't always desireable and depends on what type of sensor you are using. When you use remote sense on the SCXI-1520, the voltage output will be regulated automatically until the reading on the remote sense line is equal to the desired excitation value that you specified in software. For example, if you specify an excitation of 10V in software, and the remote sense line reads back 9.9V, then the output voltage will be regulated up to approximately 10.1V until the remote sense line reads 10V. This all happens automatically in hardware when you connect the remote sense lines - no additional software configuration is required to make this happen.

Regards,
Logan K

[RR LabVIEW]

Thanks Logan K for your response. So what your saying, if I understand your response correctly, is that the value specified is used for the mv/V calculation? No actual voltage is read? Doesn't that defeat the purpose for scaling your reading by the excitation? Our purpose for doing so was to eliminate any error in the excitation voltage. The regulation on the excitation is not perfect and some error will exist. When we measure the excitation voltage we can see a millivolt or two deviation from the requested excitation value. If this is the case we need to do something different because that method would be unacceptable.

Thanks,

RRLabVIEW 

[Ima U]

I believe there is some confusion between the questions and the answers we are providing you. Please take a look at the section called "Verifying Voltage Excitation" starting on page 7 of the SCXI-1520 Calibration Procedure. This section gives further clarification on the questions you are asking.

[RR LabVIEW]

Thanks for your response Ima U. Yes, I believe there is some confusion about the question I am asking. We are reading load cells and are selecting "Use Ex for Scaling" on our DAQmx task for the custom voltage channels. We are familiar with and have used the method of dividing our millivolt reading from the bridge by the applied excitation value to get a millivolt per volt value. This method is prefered because it eliminates excitation regulation error. On other systems that we have we read the voltage from the sense lines that are returned from the gauge as we feel that this is the most accurate representation of what the voltage is at the gauge. This value is divided into the read millivolt value from the bridge to obtain the millivolt per volt value. My question is  (and was) when you select "Use Ex for Scaling" what excitation voltage, if any, is read in order to perform the millivolt per volt value? Is it the excitation sent to the gauge or the voltage returned on the sense lines?

Thanks,

RRLabVIEW

Certified LabVIEW Developer

Rolls-Royce Corporation

[Ima U]

Hi,

When you check the "Use Excitation for Scaling" option in a Custom voltage task with excitation, you will also choose where the excitation voltage comes from and what the excitation voltage value is.

If you select "Internal" for "Ex Source" then the voltage amount that you specify in "Ex Value" is supplied by the SCXI 1520. The excitation value actually supplied by SCXI 1520 should be extremely close to what you specified as the excitation value. You have no actual way to measure this excitation value to confirm.

If you select "External" for "Ex Source" then you supply the excitation voltage using some sort of power supply. You specify in "Ex Value" how much excitation voltage you are supplying. The only way to test this is to hook a volt meter up to your external source to see how accurate it is.

The excitation used for scaling value and source are specified by you and sent to the gauge. It is not returned on the sense lines. I hope this helps to further explain this feature.

[Elizabeth W]

Hello,

After reading over this forum post, if I understand you correctly, your question is the following: When configuring a DAQmx global channel, if you place a check in the Use Ex for Scaling box, what is the actual excitation voltage value that is used to scale the voltage reading? You are wondering which of the following values is used to scale the measurement:

• The excitation value you entered when configuring the global channel, or
• The actual excitation value that is measured by the remote sense lines.

If this is indeed your question, the answer is, the excitation value you enter when configuring the global channel is used.

You mentioned that you need to use the measured excitation value to scale your voltage measurement, so you probably will not want to check the Use Ex for Scaling box. Instead, you could read the excitation voltage yourself and then scale the measurements based on the excitation voltage that you measure. Please see the KnowledgeBase document linked below for information on how to read the excitation value from the SCXI-1520:

KnowledgeBase 3R0G7CZA: Reading the Excitation Value of Each Channel on the SCXI-1520.

I hope this helps!

[RR LabVIEW]

Thank you Elizabeth for your reply. I have implemented the reading of the excitation voltage for scaling of our load cell data. The question that seems to persist here is what actual voltage is read when reading the excitation value for a channel? I guess on some of our legacy systems it is the sense lines that are read as opposed to the excitation lines going out to the transducer. Our thought is that the instantaneous voltage on the sense lines will be more accurate than the one being supplied on the excitation lines to the transducer since it is the feedback for the excitation regulation and is the most accurate representation of the excitation voltage after line loss to the transducer. Do you know where the reading is physically made internal to the SCXI-1520 module?

Thanks again,

RRLabview

Certified LabVIEW Developer

Rolls-Royce Corporation