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A Python API for interacting with NI-DAQmx
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Constant and inconstant values using NI-DAQmx Python API although not issues with NI SignalExpress 2015 #543

Closed bjce closed 7 months ago

bjce commented 7 months ago

Introduction

I encountered issues with extracting data using NI-DAQmx Python algorithms (using Python 3.11 on Anaconda, with Spyder as the IDE). The data remains unchanged when I alter the settings, yielding a constant, incorrect value. This question outlines my troubleshooting process and the steps taken to resolve the issue.

Checking the Device Connected to the National Instruments (NI-DAQmx)

MAX

Initially, I utilized MAX (Measurement and Automation Explorer) to verify the device's functionality. The device appeared to operate correctly, and test panel readings matched expected theoretical values :

01_Max

NI SignalExpress 2015

Further testing with NI SignalExpress 2015 confirmed that the software could accurately retrieve the expected values, indicating no issues at this stage.

02_Signal_Express

Code Implementation in NI-DAQmx Python

Despite anticipating seamless integration with the NI-DAQmx Python API, problems emerged: The values returned by basic algorithms were static and inconsistent with both expected fluctuations and previous measurements, displaying values such as 5.3, which were incongruent with prior results. Below, I detailed the algorithms attempted and their respective outcomes.

First Algorithms and Results

03_Code_Python_1

04_Code_1_results

Second Algorithms and Results

05_Python_second_algorithm

Bug Resolution attempt

I verified the device name and ports, ensuring accuracy in their specification. Additionally, I regularly cleared the device of any residual data to prevent issues related to data overflow or corruption.

Conclusion

I am at a loss regarding the cause of these discrepancies between NI SignalExpress 2015 and NI-DAQmx or how to rectify them. Any assistance or guidance would be greatly appreciated.

bkeryan commented 7 months ago

Hi @bjce,

In MAX and SignalExpress, you are acquiring data with a +/-10 V range, but the Python add_ai_voltage_channel method sets min_val=-5.0 and max_val=5.0 by default:

    def add_ai_voltage_chan(
            self, physical_channel, name_to_assign_to_channel="",
            terminal_config=TerminalConfiguration.DEFAULT, min_val=-5.0,
            max_val=5.0, units=VoltageUnits.VOLTS, custom_scale_name=""):

This is clipping your data to approximately a +/-5 V range. The reason it reads 5.3 V rather than 5 V is that X Series devices have a small amount of overrange so that software calibration doesn't reduce the usable range.

Try specifying min_val=-10.0 and max_val=10.0.

bjce commented 7 months ago

Many many thanks @bkeryan for your time and your help. It worked perfefctly

Best regards