Research On A Millinewton Thrust Test Device

In the deep space exploration mission,microsatellites have received extensive attention because of their outstanding advantages such as small size,light weight,short development cycle and low cost.In the tasks of orbit maintenance,position-keeping,constellation phase control,attitude maneuvering and stability,and resistance compensation,the thrust of microsatellite is in milli-newton or even micro-newton.Compared with the traditional chemical thruster,the electric thruster can produce a smaller thrust,and has the advantages of higher specific impulse,smaller volume,lighter weight and less consumption of propellant.The experimentally evaluation of milli-newton or micro-newton electric thruster output characteristics is the key to practical application of thruster.,so it is of great practical significance to carry out the research on the micro thrust measurement.In order to accurately measure the thrust output of milli-newton electric thruster,a milli-newton thrust measuring device based on the micro-thrust measurement method and lever principle was designed.The milli-newton thrust measurement device can be calibrated by a device generating milli-newton electromagnetic force,which featured at simple structure,controlled force value,and wide calibration range.The milli-newton thrust measuring device is a two-point supporting torsion pendulum structure,so the thrust direction from the gravity direction of the thruster can be easily split.Initially,according to the estimated value of the thrust output of the tested electric thruster,a sensitive element with large axial stiffness(supports the mass of the thruster,counterweight,etc.)and small torsion stiffness(high sensitivity in the torsional direction)should be selected in comprehensive consideration of the measurement range,resolution of the small measured force,the quality of the thruster,and so on.Based on the lever principle,the entire measuring device can be balanced by adjusting the counterweights,thrusters and other related devices.Additionally,based on the current magnetic effect and the mutual transformation between magnetic field energy and electromagnetic force,this paper has established two mathematical models.The first one is a model of the electromagnetic force and current flowing in the electromagnetic coil,and then,the other one illustrate the relationship in electromagnetic force and the air gap between the electromagnetic coil and the magnetic block.Furthermore,a tiny device generating electromagnetic force was designed.This device mainly uses electromagnetic coil to attract magnetic block,and eventually generates micro forces.After this force value was calibrated by a high-precision electronic balance,two experimental curve was depicted finally,which include the relation of electromagnetic force and current,meanwhile,the curve of electromagnetic force and the air gap between the electromagnetic coil and the magnetic block was also obtained.At last,the measured tiny thrust force value was amplified and measured by the principle of optical lever.Due to the measured force value is relatively small,the measuring condition should be altered in experimentation.Firstly,the tiny thrust generated by the electric propeller is converted into the deflection of the beam,and then,the function relationship between the number of the external force and the displacement of the spot on the PSD can also be obtained.As the experimental results shows,the miscellaneous factors of measured small power is listed as follows: range is 0-11.18 mN,the resolution is 0.065 mN,the sensitivity is 0.67mm/mN,the nonlinearity is 2.92%,and the repeatability error is 1.45%,the stability is 0.01mm/h.