erik mueller michael white n.
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Erik Mueller Michael White. Microthruster Test Platform Design Presentation II. Introduction Problem Solution Load-cell Paddle-sensors Testing Q&A Sources. Contents. Background. Microthrusters : Produce <1N of thrust ( mN range)

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  • Problem
  • Solution
  • Load-cell
  • Paddle-sensors
  • Testing
  • Q&A
  • Sources
  • Microthrusters: Produce <1N of thrust (mN range)
  • Various propulsion systems (fuel/oxidant, monopropellant, ion drive)
the problem
The Problem
  • Thruster and forces involved are minute
  • Must be rugged and durable
  • Modular design preferable
our solution
Our Solution
  • Two methods of thrust measurement
  • Direct – load cell
  • Indirect – capacitive plate
  • Importance of two – better results, redundancy
system overview1
System Overview

Indirect Thrust Plume Pendulum Sensor

FM Modulator


Fire Control

Load Cell

Amplifier Circuit


Lab View


direct vs indirect
Direct vs. Indirect
  • Direct
    • Measurement of the net forces acting on thruster and any attached devices.
  • Indirect
    • Measures only reactive forces from the thrust plume.
  • Aluminum thruster bracket mounted directly to load cell
  • Mounting Chassis
    • Load cell will be mounted directly to an aluminum chassis to provide a stable platform free of mechanical vibration
    • Load cell mounted vertically to reduce the effect of gravity on the test.
direct force measurement
Direct Force Measurement
  • Load Cell
  • Strain Gauge
  • Signal Amplification
    • Gain = 1000
indirect thrust measurement
Indirect Thrust Measurement
  • Capacitive Plate System
  • Exhaust plume exerts force on a plate
  • Deflection corresponds to a change in capacitance

Induced Force


capacitive pendulum
Capacitive Pendulum
  • Uses electronic principle of capacitance
  • Two plates, moving and reference
  • Needle point fulcrums mechanically isolate
  • Given a distance of 2cm between plates
  • C = A ε/d = 4.43pF
  • A Frequency-Modulated system system is sensitive enough at this range
signal processing
Signal Processing
  • Using an FM generator, plates are a capacitor
  • Compare frequency shifts to determine deflection.
  • Advantage – more resistant to noise and distortion, very accurate
  • Disadvantage –more complex
circuit concept



Low-Pass Filter

Phase Detector


Circuit concept
  • Using a PLL to detect deviations in frequency, which are then read by a DAQ


Error signal

Error voltage




  • The NI USB-6008 DAQ will tie into a computer, along with the rocket ignition circuitry.
  • Both subsystems will be integrated into a single user-controlled program, using Labview
  • One rocket test has been performed to gauge ignition methods & rocket plume.
  • A calibration test was performed to verify strain subsystem linearity and determine transfer function.
  • A subsequent test was performed evaluate the strain subsystem.
  • Currently testing and revising software with the strain subsystem.
transfer function calculation
Transfer Function Calculation
  • y = 0.001x – 0.629 (x = Force in grams)
  • x = (y + 0.629/0.001) x 9.81 mN/g
  • x = y + 0.629 x 9810 (x = Force in newtons)
work breakdown structure
Work Breakdown Structure
  • Week 5-6 Subsystem and structure prototype
  • Week 6-7 Electrical circuit schematics
  • Week 5-9 Software composition and test
  • Week 7-10 Revision, second subsystem test
  • Week 9-12 Assembly, testing, revisions if needed
  • More details on website
works cited
Works Cited
  • Traceable calibration of the 3-axis thrust vector in the millinewton range, EB Hughes and S Oldfield, National Physical Laboratory
  • Direct Thrust Measurement of In-FEEP Clusters, IEPC-2005-235, K. Marhold and M. Tajmar, ARC Seibersdorf research GmbH
  • Rocket Thrust Measurement For an Estes B6-2 Model Rocket Engine, Peter Hyatt, Jeremy LeFevre, Russell Dibb, Bringham Young University
  • Thrust stand for ground tests of solid propellant microthrusters, S. Orieux and C. Rossi and D. Esteve, Review of Scientific Instruments, Volume73, Number 7, July 2002
  • A Ground Test Rocket Thrust Measurement System, Mary Fran Desrochers, Gary W. Olsen, M. K. Hudson, Department of Applied Science and the Graduate Institute of Technology, University of Arkansas
  • MilliNewton Thrust Stand Calibration Using Electrostatic Fins, Allen H. Yan, Bradley C. Appel, Jacob G. Gedrimas, Purdue University