Case Study

First Principles Test Apparatus Case Study

First Principles Test Apparatus


  • Client was not confident in the robustness of the clinical supplies – specifically motor-gearbox and batteries
  • Because the design was locked, screening was the only viable way of ensuring capable clinical devices
  • Clinical supply team needed a simple, first-principles based, method of characterizing motor-gearbox

Our client was concerned about robustness of clinical supplies of a drug delivery device using a DC motor-gearbox.  We designed and built an inertial dynamometer for characterizing and screening the DC motor/gearboxes.  We considered other methods of measuring output torque and chose an inertial dynamometer due to its functional simplicity, relative ease of control, and robustness, and ease of data analysis.  We did basic modeling to identify the amount of inertia for the desired acceleration, and considered how to minimize the effect of potential loss terms – for instance the motor gearbox output gear was coupled to the inertial load using a zero loss (no relative motion), easy to engage, coupling.  We used an off the shelf, high resolution, encoder to measure acceleration and support the inertial load in order to minimize other losses.

Summary: We wanted to understand motor variability in the motor-gearbox intended for use in a clinical trial of a wearable pump

Test Apparatus


  • Preliminary modeling for basic understanding
  • Defined tester requirements
  • Developed simple inertial dynamometer
    • First principles T=J*α
    • Used OTS encoder to acquire θ vs t, and thereby ω and α, and support inertial load


  • Verified data analysis software and fixture performance
  • Qualified fixture
  • Developed understanding of key characteristics – e.g. gearbox wear-in
    • Arduino-based electronics were flexible and supported a companion battery tester
Apparatus Simulation

Key Takeaways

Simple first-principles based test apparatus provides quality data and enables fast screening without ongoing calibration challenges

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