SCXI Calibration Tutorial: External Calibration with DAQmx 8.0 and Later

Publish Date: Apr 02, 2012 | 4 Ratings | 1.00 out of 5 |  PDF

SCXI Calibration Tutorial: Using the DAQmx Calibration Tools for SCXI

Note: This is not a calibration procedure; this document is simply a guide for creating your own calibration procedure using the DAQmx calibration API.

Please refer to the NI-DAQmx help for additional information if you have a more recent version.

Setting up the hardware

1. Analog Input Modules

In order to calibrate an SCXI module, you must connect the DAQ board directly to the module which is being calibrated, and set it up in software for multiplexed mode communication. This is the default mode when SCXI equipment is originally installed. When configured in this way, the signals from the SCXI module can be accessed on CH0+ and CH0- (pins 3 and 4 respectively) from the 50 pin connector on the SCXI-1349 which is connected to the back of the SCXI module. For more information about configuring the SCXI module in multiplexed mode, refer to the SCXI Quick Start Guide.

For specific details on what type of calibrator and DMM to use, and how they should be connected to the module, refer to each module's calibration procedure on the Calibration Procedures page of ni.com/calibration . Here is an example setup for an analog input SCXI module:

 

Figure 1: Connecting a DMM to the back of a SCXI module

Note: The Calibrator must also be connected to the appropriate channels of the module’s terminal block.

2. Analog Output Modules

Some SCXI modules have analog output signals that can also be calibrated, such as the SCXI-1124 and the current output of the SCXI-1503. When calibrating the output signal of a module, only a DMM is required. Connect the DMM to the output of interest on the module in either Current or Voltage mode, depending on the type of signal being calibrated.

SCXI EEPROM:

There are 2 calibration areas on the EEPROM for each SCXI module:

1) Factory / External

2) Self / Internal

The DAQmx software loads the calibration data from the internal calibration area. When performing an external calibration of the device, the external calibration area and internal calibration area are both updated with the new calibration constants. Warning: the original factory calibration data will be permanently overwritten when an external calibration is performed. For SCXI devices that support self calibration, performing a self calibration only updates the internal calibration section of the module.  You can copy the external calibration area back to the self calibration area at any time by using the "DAQmx Restore Last External Calibration Constants" function on the device.

DAQmx Internal Calibration API:

DAQmx Self Calibrate Function:

You can use the DAQmx Self Calibrate function to perform an internal calibration of the SCXI module. National Instruments recommends performing self calibration the first time the module is installed in a system and when the module is first placed in its end location.

This function internally grounds each channel and performs a measurement. The result of that measurement is used to adjust the internal offset calibration constants to account for any errors caused by short-term fluctuations in the operating environment. When you self calibrate a device, no external signal connections are necessary. Not all SCXI modules support self calibration. Please consult the SCXI Module’s User Manual for more information.

DAQmx Restore Last Calibration Constants Function:

This function copies the data from the external calibration area to the internal calibration area. NI sets the external calibration constants at the factory, and those constants remain in effect until you perform an external calibration on the device.

If a self calibration has not been performed since the last external calibration of the device, then this VI has no effect.

Verifying the calibration

Verification of the SCXI modules should be done before and after calibration. This should be done before calibration to determine if an external calibration is necessary. You should refer to the module’s calibration procedure or user manual for the appropriate method and limits for verifying the module’s calibration.

Using the DAQmx External Calibration API for SCXI

SCXI calibration functions are a new feature of DAQmx 8.0. In general, external calibration should only be performed if you have the appropriate calibration equipment and experience, because the factory calibration constants on the SCXI modules will be permanently overwritten when using these functions.

The external calibration VIs are located in the LabVIEW functions palette under Measurement Hardware >> DAQmx >> Advanced >> Calibration. If you are using an API other than LabVIEW, there are equivalent functions which are documented in the DAQmx Help that corresponds to your API. The following is a list of the functional descriptions for each VI / Function needed to perform an external calibration:

SCXI Initialize External Calibration Function:

This function begins the external calibration session. It notifies the driver that an external calibration is about to take place.

SCXI Calibration Passwords:

The default password for SCXI modules that support password protection is “SCXI.” Currently, the SCXI 15xx modules are the only modules that support password protection. Use the DAQmx Change External Calibration Password function to change this password. All other SCXI modules will accept any password.

DAQmx Setup SCXI Calibration Function:

This function sets up the gain / range for each channel to be calibrated, and configures the channel to so that it’s output can be read by a DMM. Each SCXI module has a different set of gains or ranges that must be calibrated. For a complete list of the gain or range settings for your module, please see the SCXI Module’s User Manual.

After running this function you must set the calibrator, if applicable, to the appropriate value, and then measure the module's output using a DMM.

DAQmx Adjust SCXI Calibration Function:

For analog input modules, this function has a “reference voltage” input that will correspond to the voltage that is supplied to the module’s input from the calibrator. There is also a “measured voltage,” or “measured output,” input to the function, which corresponds to the voltage or current reading from the DMM. Each time this function is executed for a particular channel / gain setting, a data point is stored in memory. When the calibration session is “Committed,” the data points are used to determine the calibration constants that will be written to the module.

DAQmx Close External Calibration Function:

This function has an “action” input. When this input is set to “commit,” the DAQmx driver will use the data points from the DAQmx Adjust SCXI Calibration Function to determine the new calibration constants that will be written to the module’s EEPROM. The calibration constants that are written to the module will depend on the data points that were stored in memory. In general at least 2 different reference voltages per gain setting per channel are required to calibrate the analog input modules. Any exceptions to this rule are documented in the NI-DAQmx help for the DAQmx Adjust SCXI Calibration Function.

Calibrating Analog Input Modules:

After your SCXI system is configured in Measurement & Automation Explorer (MAX) and the calibrator and DMM are connected, then refer to the following flow chart, and follow these steps for the calibrating any analog input SCXI module:




Figure 2: Flow Chart for Calibrating Analog Input Modules


1. Configure the module for external calibration using the DAQmx Initialize External Calibration function. This function starts an external calibration session for the specific SCXI module.
2. Setup the SCXI channel for calibration using the DAQmx Setup SCXI Calibration function. The specific channel being calibrated and the gain are configured with this function.
3. Configure the external calibrator to output a reference voltage. Any time the signal on a particular channel is changed, you must wait for the reading to settle before measuring the output value. On the SCXI-1102, NI recommends waiting 45 seconds for the signal to stabilize. On the SCXI-1125, you will only need to wait 1 second.
4. After the signal has settled, record the voltage output from back of the SCXI Module with the Digital Multimeter (DMM). Refer to Figure 1 for connection details.

Note: Steps 3 and 4 are performed externally and require no programming in the ADE.

5. Use the DAQmx Adjust SCXI Calibration to record the voltage reading. “Reference voltage” is the calibrator’s output voltage from step 3, and “measured voltage” is the DMM reading from step 4.
6. Go to step 3 and repeat steps 3-6 for all reference voltages at the current channel / gain setting.

A minimum of one reading must be recorded in order for a new offset constant to be generated and a minimum of 2 points must be recorded in order for the DAQmx driver to configure a new gain constant for the specific channel.

National Instruments recommends taking 7 voltage readings per gain per channel, which allows for the DAQmx driver to create more accurate gain and offset constants. Here are the recommended reference voltages:

1. 0V

2-3. +/- 90% of the channel voltage range

4-5. +/- 60% of the channel voltage range

6-7. +/- 30% of the channel voltage range

7. Go to step 2. Repeat for all channels and gain settings.

Note: The SCXI 1520 stores offset constants at 4 different gains when performing calibration unlike the SCXI 1102 and 1125 which store an offset constant for each gain setting. This module requires a reference signal of 0.0 V at gains of 1, 15, 20, and 310 on per channel in order to apply an offset calibration constant.

8. Run the DAQmx Close External Calibration function inputting “Commit” for the action.

Note: The factory calibration constants will be permanently overwritten when you commit an external calibration.

Verifying the calibration

After adjusting the calibration constants on the modules, verify that the channels meet the accuracy limits for the module. You should refer to the module’s calibration procedure or user manual for the appropriate method and limits for verifying the module’s calibration.

Calibrating Analog Output Modules:

After your SCXI system is configured in MAX and DMM is connected, then refer to the following flow chart, and follow these steps for the calibrating your analog output SCXI module:


Figure 3: Flow Chart for Calibrating Output Channels of a Module


1. Configure the module for external calibration using the DAQmx Initialize External Calibration function. This function starts an external calibration session for the specific SCXI module.
2. Connect the Digital Multimeter (DMM) to the appropriate output channel and configure for voltage or current measurements, depending on the output signal being calibrated.
3. Setup that SCXI channel for calibration using the DAQmx Setup SCXI Calibration function. For the current output channel, set the output range and the output setting with this function. Refer to the help for this function regarding which outputs to use. If you are calibrating the SCXI-1503 current output, it does not require any additional setup so you can skip this step.
4. After the signal has settled, record the DMM reading for that channel.
5. Use the DAQmx Adjust SCXI Calibration to record the DMM reading. Input the DMM reading you measured in step 4 to the “measured reading” input of the function.
6. Go to step 2 and repeat steps 3-6 for each output, range, and channel.
7. Run the DAQmx Close External Calibration function inputting “Commit” for the action.

Note: The factory calibration constants will be permanently overwritten when you commit an external calibration.

Verifying the calibration

The appropriate verification methods will vary between modules. You should refer to your module’s calibration procedure or user manual for the appropriate method and limits for verifying your module.

Other Calibration Tools / Methods for SCXI

Calibration Executive:

Calibration Executive is software specifically for calibrating National Instruments Hardware. This software supports automatic calibration where the software will programmatically go through all of the required data points to perform offset and gain calibrations as opposed to individually calibrating each point as described above. Please refer to the Software for Digital Multimeters product page for more information.

System Calibration:

System calibration is a method of calibrating the entire signal path of a DAQ system. It can compensate for offset, gain, nonlinearity and other unspecified errors. A precision external source is required for system calibration.

The system calibration quantifies all of the measurement errors in the system and compensates for them in software. It does not change any of the circuitry or write to the EEPROM of any of the hardware and it should not replace device calibration. Instead, each device in a system should be calibrated individually, and then a system calibration would increase the overall measurement accuracy.

Once a system calibration has been performed, you cannot replace any device in the system without performing another system calibration. Temperature and time will still affect the accuracy of a calibrated system, so you should calibrate once every six to twelve months.

 



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