Configurations and Key Issues of Low Thrust Measurement for Microspacecrafts

Microthrusters on microspacecrafts are of interest and its output thrust must be characterized for a particular applicaton. The technology of micro thrust measurement can evaluate the performance of microthrusters. The measurement principles of steady-state thrust, pulse impulse, average thrust and instantaneous thrust are introduced. The classical thrust stands at home and broad are summarized and they are classified into torsional arm type, balance type, hanging pendulum type and flexure vibration type according to their dynamics configurations. The characters, purpose, advantages and disadvantages are analyzed. The key problems, such as error influence factors and its countermeasure, calibration, and measurement automatization, are pointed out.


Introduction
Microspacecrafts can potentially reduce launch cost and can complete a variety of new space-based missions, so it has attracted much attention in recent years.And then, miniaturization of every component on the microspacecraft is required.Among all the components, microthruster, which can produce very small (on the order of μN or nN) and highly accurate thrust for stabilization, pointing and position keeping of microspacecraft is of current interest.Solid Propellant Microthruster (μSPT), Pulsed Plasma Microthruster (μPPT), Field Emission Electric Propulsion (FEEP), Colloid Thruster, Laser Ablation Microthruster (μLAT) and so on is generally recognized as potential microthrusters.But, the accurate measurement of their performance is the basic problem to be solved at first.And the most critical performance measurement is that of thrust.For the purpose of thrust measurement, many thrust measurement stands already exist.In this paper, the principles of thrust measurement are concluded.Several typical configurations of microthrust measurement system are summarized.And the key issues of microthrust measurement are discussed.The goal of this paper is to help users choose best configuration for a particular microthruster, and provide issues which are must pay attention to for particular micro thrust measurements.

Principles of Microthrust Measurement
Microthrusters can be classified into continuous thruster and pulse thruster according to the thrust output mode.For continuous thrust, the steady-state thrust is generally measured.For pulse thruster, a Corresponding author: changhao5976911@163.com pulse impulse, average thrust and instantaneous thrust is generally measured.The measurement principles are summarized in Table 1.Thrust is linear with the angular acceleration of the executive unit.deflection angle The precision is restricted by the sampling rate, the numerical differentiation algorithm, etc. Thrust is compensated with feedback force in time.
deflection angle The responsive bandwidth is low because of the inertia.

Conclusion and Analysis of Classical Microthrust Measuring System
For the purpose of thrust measurement, many thrust measurement stands already exist.And they can be classified into torsional arm type, balance type, hanging pendulum type and flexure vibration type according to their dynamics configurations.Typical applications in various configurations are shown in Table 2 to Table 5.And test characteristics, advantages and disadvantages are analyzed in Table 6.1999 [3] C. R.  2005 [9] University of Colorado Pulsed Plasma Thruster LVDT Impact hammer -2011 [10] University of Southern California Micro Cathode Arc Thruster LVDT -Impulse bit: 0.72±0.02Ns 2012 [11] Beijing The thrust is not separated from the gravity.

Hanging pendulum type
The thruster is mounted on the pendulum.The pendulum response in vertical plane by rotating about its axis.The restoring force is provided by gravity.

steady-state thrust pulse impulse average thrust
The restoring force is not influenced by the temperature.
The restoring force is provided by the gravity so that the nonlinear effect exists.And the variation of the thruster can influence the restoring force.

Flexure vibration type
The thruster is mounted on the elastic element.And the elastic element vibrates when the thruster fires.

pulse impulse instantaneous thrust
The configuration is simple.
The noise interference is serious so that it is difficult to filter.(a) Error Analysis.Environmental factors that may have an impact on measurement are: (i) Vibration.Footsteps, passing vehicles, air flow and so on can cause measurement signal noise.So, firstly, the good testing environment, such as at night, must be taken into account.And then corresponding vibration reduction measures, such as vibration isolation foundations and vibration isolation platform, can be considered.At last, data processing model for noise reduction, such as the use of Fourier transform spectral analysis, adaptive filtering and so on, can be researched.(ii) Temperature.Temperature changing can cause changes in the spring coefficient of the elastic element.And this can affect measurement accuracy.So the characteristics of the impact must be studied.And the higher heat capacity and thermal conductivity material can be taken into account.Also, the spring coefficient can be calibrated before each experiment.(iii) Electromagnetic environment.Electromagnetic environment has a certain effect on some component (such as the electromagnetic dampers, coil permanent magnet) or thrusters (Pulsed Plasma Microthruster).Electromagnetic intensity can be monitored real-time so that the interference source can be checked out and removed.A certain amount of shielding measures can be carried out.Because of different configurations of thrust stand, the environmental interference factors must be analyzed specifically and then corresponding denoising measures is to be carried out.(b) Automation Control.Thrust measuring system involves level control, zero control, calibration and so on.Because of particular measurement requirements in vacuum and the flexibility, the above operations had better automate as much as possible.(c) Calibration.Calibration is the key issue in the validation of the measurement system.For thrust measurement, calibration commonly uses weights, electrostatic force and electromagnetic force.The friction caused by pulley can arouse error when the weight method is carried out.Electrostatic force and electromagnetic force has no mechanical contact, and can be controlled easily by program.But they are sensitive to electromagnetic interference.For pulse impulse measurement, calibration commonly uses ball falling method, the impact pendulum method and the force hammer method.

Summary
With the development of the microspacecraft, micro propulsion becomes a research field of current interest.The micro thrust measurement is very important for the performance validation of the microthrusters.But it is a complex and challenging technology.The working mode and the configuration of all kinds of microthrusters are different from each other so that the dynamical configuration of the measuring system must be chosen carefully according to the specific case.Interference is easy to be introduced into the measuring data.So denoising is the key technology to be solved all the time.And lastly, the implementation of the micro thrust measuring system is a complex and repeating work which needs patience and confidence.

Table 1 .
Measurement principles of micro thrust

Table 2 .
Thrust stands based on torsional arm

Table 3 .
Thrust stands based on balance

Table 4 .
Thrust stands based on hanging pendulum

Table 5 .
Thrust stands based on flexure vibration

Table 6 .
Characters of all types of thrust stands