Lesson 4: Data acquisition

Introduction: Electric drive systems are multi-physics devices that require capturing data across different domains for evaluating performance. This lesson will guide you through the intricacies of data acquisition in these systems.

Understanding Data Acquisition:

  • Signals to Consider:
    • Directly from the Unit Under Test (UUT).
    • From testing environment’s devices.
    • These signals help in evaluating results or tracing back problems.
  • Commonly Measured Quantities:
    • Voltage, current, torque, force, acceleration, acoustic noise.
    • Speed, position, temperature, flow rate, pressure.
    • Digital messages on the communication bus (e.g., CAN).

Figure Many types of signals from different sources are present during a measurement and must be measured simultaneously

Challenges in Data Acquisition:

  1. Collecting Concurrent Signals:
    • The sheer number and variety of signals make simultaneous collection challenging.
    • Signals can be analog low voltage or already digitalized by the sensor, requiring varied interfaces on the acquisition device.
    • Often, recording is distributed across different modules or measuring cards.
  2. Time Synchronization:
    • Ensuring signals share the same time-base is crucial for obtaining meaningful data.
    • Without this, it’s tough to form a consecutive chain of events for analysis.
    • Each signal or set of signals comes with a timestamp synchronized to a master device.

Achieving Precision in Data Acquisition:

  • Accuracy: The system’s accuracy should be 10x better than the smallest deviation to be evaluated.
    • Example: If the maximum allowed torque deviation is 2 [Nm], the system should resolve the signal in 0.2 [Nm] levels.
  • Sampling Frequency:
    • The Nyquist-Shannon theorem dictates that the sampling frequency should be at least double the frequency of the measured signal: π‘“π‘ π‘Žπ‘šπ‘π‘™π‘–π‘›π‘”>2β‹…π‘“π‘ π‘–π‘”π‘›π‘Žπ‘™
    • In practical terms, often a frequency 10 times higher is used for sampling.
    • If sampling frequency is less than double the signal’s frequency, aliasing occurs.
  • Avoiding Aliasing:
    • Measured signals go through a low-pass filter before digital sampling.
    • This filter cuts off high frequency parts to prevent aliasing.

Conclusion: Data acquisition in electric drive systems is intricate. Understanding the challenges and the science behind accurate data capture is vital to obtain meaningful results and insights from tests.

Quiz:

  1. Why is time synchronization crucial in data acquisition?
  2. Explain the relationship between the sampling frequency and the signal’s frequency as per the Nyquist-Shannon theorem.
  3. What’s the purpose of using a low-pass filter before digital sampling?
Go back Quiz