Optimizing 3-Component Force Sensor Installation for Satellite Force Limited Vibration Testing Bob Metz PCB Piezotronics, Inc. Depew, NY USA

Overview Introduction Preloading & Mounting of Piezoelectric Force Sensors Preload Examples Signal Conditioning Requirements Summary

Introduction Force Limited Vibration Testing (FLVT) helps limit reaction force between shaker & UUT 3-Component force sensors provide convenient measurement of input force Payloads fitted with piezoelectric force sensors using flight hardware or adapter rings Discussion: –Preload selection criteria –Effects on gage sensitivity –Case studies

Preloading & Mounting of Piezoelectric Force Sensors Preload Requirements Piezoelectric sensors have exceptional linearity Selection of sensor depends upon size and mechanical constraints of UUT Two types of piezoelectric sensors: –Ring Style - externally preloaded, preload in situ –Force Links - internally preloaded, preload at factory Ring style requires preloading of 20% or more of measuring range simply for optimal linearity

Preload Requirements Region of linear output Additional preload for –Limits slippage –Tension measurements –Required for Shear force measurements Resulting friction allows for force transmission FSO 20% of FSO Linear Portion of Output Applied Load Sensor Output Equation 1: F preload = F shear   where  is the coefficient of friction Figure 1 Output vs. Preload Force

Preload Hardware Force rings calibrated & ship with mounting stud, designed to stretch –Standard stud material: BeCu –Steel bolts shunt ~20-50% of force Instrumented Horseshoe –Preloaded with steel bolts –Individual sensor sensitivity correction between 4% and 13% less than factory calibration for the z-axis –x & y axes have negligible change Not stretching in the x-y direction

Preload Hardware Method in which threads couple to unit under test is important –Stud/bolt not directly connected to top plate by threads –Does not shunt any force in compression Figure 4 Preload Stud Thread Engagement Examples

Preload Calibration Effects Calibration study using 3-component force ring ICP ® PCB Model 260A01 Determine sensitivity correlation between shunting and non-shunting preload methods Figure 5 James Webb Space Telescope NIRSpec Bracket Assembly

Preload Calibration Effects Mounting arrangement for shunt calibration Load applied in vertical direction via precision strain gage reference cell Z-Direction, force enters through ball bearing to account for misalignment Figure 6 Shunt Method Calibration Z-axis Force Application through Threads (Left), X-Y Axis across Bolt (Right)

Preload Calibration Effects Force path in z-axis is shunted ~13% through threads before reaching sensor Friction from preload –Keeps sensor plates from sliding & making contact with bolt –X-Y axes have little shunt –Calculate bolt ratio for in situ shaker calibration

Preload Examples Adaptor Rings CubeSat (NPSCul) Launcher –Prepared for Naval Post Graduate School, Monterey, CA NPSCul – 5 sided structure carries 24 CubeSats into orbit Adapter ring consists of –(4) Model 260A03 ICP ® force rings –Preload was 35,000 lbs. –Large wrench required for smooth control of torque to load Figure 7 Typical Section View of Instrumented Adaptor Ring Courtesy of Skybox Imaging, Inc.

Adapter Rings Preload procedure for values between 30k-35k lbs. −Force ring connected to DC coupled signal analyzer −Voltage converted to a force using factory calibration sensitivity −Torqueing one sensor in small increments, had little effect on preload of other three sensors Entire mass of NPSCuL considered to be moving as rigid body −Transfer function analysis finds mass of all hardware above force sensors −Result within 5% of expected mass Figure 8 Naval Postgraduate School FLVT Preloaded Adaptor Ring (Left) and OUTSat Acceptance Test (Right)

Force Links Eliminates preload requirement of force ring Construction –3-component force ring sensor –BeCu stud holds assembly together –Preload between two mounting plates Links are factory preloaded, used for: −Compression & tension in z-axis −Positive/negative shear forces in x & y-axes Figure 10 Section View of Model 261A03 ICP  3-Component Force Link

Force Links Force link assembled & preloaded like adaptor ring –Preload stud screwed into base –Pilot bushing installed over center of stud –Top plate placed –Thread locking epoxy applied Figure 11 Preload Stud/Pilot Bushing Installation (Left), Thread Locker (Right)

SIGNAL CONDITIONING REQUIREMENTS Low Frequency Characteristic of First-Order, High-Pass Filter Preloading has time limit set by system Discharge Time Constant (DTC) –Low frequency response for quartz piezoelectric force sensors resemble a high-pass filter –Quasi-static measurements can be made w/sufficiently long DTC –Signal from lower DTC value dissipates faster than signal with higher DTC Discharge Time Constants

Signal Conditioning Requirements When preloading ICP  output sensors for static calibration –Use DC coupled signal conditioners(PCB  Model 484B) & readout –DC bias voltage to be zeroed –Do not apply any additional high-pass filter effects to system Output signal loss & time elapsed over first 10% of a DTC, have linear, one-to-one relationship

Summary Proper test planning for 3-component force sensor use is vital to a successful FLVT Preload is required for –Linear operation of gage –Tension measurement –Friction for shear forces Calibrate with the bolt Large adapter rings with multiple sensors –Preload in small increments DC coupling critical for ‘steady state’ signals