FLV Presentation--VIDEO 1

3-Component Piezoelectric Force Sensors and Analog / Digital signal conditioning to resolve Forces/Moment for FLV.

Bill Zwolinski Kistler Instrument Corp 4June 2015

Topics

n  Introduction

n  Piezoelectric Instrumentation and Force Sensor Installation

n  Measurement of Forces and Moments

n  Summary

FLV Presentation 2

Environmental Testing for Launch and Space Vehicles n  Space systems must endure a physically stressful journey from the launch pad

to their final destinations. Adequate testing can help ensure they survive the trip.

Ref: Environmental Testing for Launch and Space Vehicles, E. Perl, T.Do, A.Peterson, J.Welch FLV Presentation 3

Environmental Testing: FLV

n  Force Limiting makes vibration tests more realistic by

replicating the impedance characteristics of the flight mounting

interface by automatically notching or reducing the amplitude of

shaker input near the fixed base UUT resonances

n  3 component force sensors measure interface forces between

the UUT and shaker as inputs to shaker controller .

n  Usually the total in-axis force (ex. the sum of forces at all

mounting points in the shaker excitation axis) are measured

and limited and sometimes moments are used as well.

FLV Presentation 4 Ref: NASA-HDBK-7004C, Force Limited Vibration Testing

n  FLV may also involve measuring and limiting the acceleration and/or force responses at other critical positions (ex. resonant structures) on the UUT as well

n  FLV is best for highly resonant test items and more typically for UUT that are mounted on relatively lightweight flight structures, which have mechanical impedances comparable to that of the test item.

n  Use of a Shaker with Real time “Extremal Control” - controls based on the maximum of several inputs ( force or acceleration) with frequency

Quartz Piezoelectric Elements •  The quartz crystal acts as a very stiff, ideal spring

•  Very high rigidity (6…350 lbf/µin), high linearity and negligible hysteresis

•  High mechanical stiffness high natural frequency Wide freq. range

•  Ultra high insulation resistance (>1014 Ohms) = low freq. measurements (<1 Hz)

•  Very wide dynamic range

•  Temperature up to 400°C (752°F)

•  Not Pyroelectric

•  Long life with virtually no sensitivity shift over it's lifetime.

•  Prior to the cutting, a x-ray goniometer(X-ray crystallography) is used to determine the orientation of the major crystal axes to develop cuts

1 2

3 4

5

1- High Impedance Sensor 2- High Impedance Cable 3- Charge Amplifier ** 4- Low Impedance Cable 5 - Readout Equipment

0v

-10v

+10v

pC/mu 1 2

3 4

5

1- Low Impedance Sensor 2- Low Impedance Cable 3- Power Supply/Coupler 4- Low Impedance Cable 5 - Readout Equipment

~11v

0v

-5v

+5v

AC Coupled

mV/mu

Prior to AC Coupling

Typical Measuring Chains

No Internal Electronics

PE IEPE

Internal Electronics

•  Rangeable •  Dynamic and

Quasi-static •  Resettable

•  Fixed Measuring Range •  Fixed TC Dynamic •  Not resettable

** In-line Charge Converter to IEPE is sometimes used with a PE Sensor

Time Constant – Another name for High Pass Filtering

•  Single order High Pass Filter. Larger the time constant , , the lower the frequency (inversely related) •  f(-5%) = 0.5/ •  Long ( Quasi-Static), Medium, Short (Dynamic)

•  PE Measuring chains permit external adjustment of

TC ( Long, Med, Short) where the output is resettable •  IEPE Sensor measuring chains have a fixed low

frequency capability, dependent on the total system time constant •  total =a c /(a + c) •  (s) sensor •  (c) signal conditioner (ex. Coupler or DAQ)

•  High Pass affect •  Long TC optimizes low frequency response. Drift of

<6 lbs/hr ( ex. Sz=17.4pC/lb )

1 TC= 37% of peak

1 TC = 37% , 2TC = 13%. 3TC = 5%, 4TC=1.8%, 5TC = 0. 67%.....10TC = 0.005%

Example of a PE Systems’ Rangability Measuring 1 ton and 0.1 lb with one sensor

Load washer

Charge ampCharge

inputVoltageoutput

•  2000 lb force measurement

•  Hit “Reset” on Charge Amp

•  Set-up the charge amp for 19pC/lb and 400 lbs/volt.

•  Hit “Operate” on Charge Amp

•  FSO of 10 V = 4000 lbs.

•  Measurement Output ~ 5 volts or 2000lb.

•  BB Resolution = 4000*0.0003=1.2lbsrms.

•  0.1 lb force measurement

•  Hit “Reset” on Charge Amp

•  Set-up the charge amp for 19 pC/lb and 0.1 lbs/volt.

•  Hit “Operate” on Charge Amp

•  FSO of 10 V = 1 lb.

•  Measurement Output~ 1 volts or 0.1lb

•  BB Resolution = 1*0.0003=0.0003lbsrms.

FLV Presentation 8

AC coupling would further improve the resolution as 1/f noise has most power at low frequencies BB= Broadband

Preloading 3-Component Force Sensor n  Accuracy is exploited thru proper mechanical mounting. n  Standard Preload ~70% FS

�  Z-axis preload provides static frictional force to enable transmitting X and Y shear loads

�  Tensile and Compressive Measurement: Fx, Fy, Fz �  Optimize linearity with the closure of microgaps �  Utilizes much of the compressive measuring range

n  Fx, Fy Shear measuring range as a function of Fz Preload �  Higher internal sensor frictional coefficients provide

higher ratio of Fx, Fy Range to Fz Preload ( ex. 0.2) n  Measuring Range : Fz is typically 2x Fx, Fy n  Sensitivity X, Y is typically 2x Z (Shear vs. Compressive)

n  Preload bolt is structurally in parallel with the load washer and made of tool steel for low force shunt.(6% to 9%.)

n  Force Links are factory preloaded

FLV Presentation 9

Ring Nut

Preload

Bolt

Top Plate

Centering Sleeve

3- Component Force Sensor

Sliding Washer

Base Plate

Grease

Loctite® 221™

Load Bearing Surface Requirements n  Standard Preload bolts use ARMCO 17-4 PH (similar to

DIN 1.4542) high-strength steel, hardness HRC 46 …48.

n  As sensor is made from stainless steel, recommend a

similar material for the base plate and the cover plate. A

certain minimum tensile strength is necessary to be able to

withstand the large forces of the preloading bolt.

n  Rule of thumb~ top and bottom plate minimum thickness

1x…2x sensor height

FLV Presentation 10

n  Check Manufacturers data for requirements

Mounting Surfaces, Base and Cover Plate Bores

3-Component Force Sensors Alignment /Cross Talk, Height, Ground Isolation n  Sensor “flats” used to align the sensor, and hence the coordinate system.

n  An alignment error of α = 1 ° corresponds to an Fx ↔ Fy crosstalk of approximately 2 %.

n  Large sets of sensors can be ground to a common height to support FLV applications.

n  Ground Isolation – use one common grounding point to eliminate ground loops and related noise

FLV Presentation 11

Top and Bottom Integral Isolation Plates

FLV Presentation 12

Measurement of Force/Moments

8 Channel Charge Amplifier

•  A force Dynamometer is comprised an array of Force sensors between thick steel plates. •  Off-center forces applied to a single load washer cause moment loads that, at best,

degrade accuracy and, at worst, can break the sensor (cracked quartz). •  Dyno’s absorb moment loads by differential force reactions within the sensor array

•  A dynamometer allows consistent measurement of forces applied in varying locations.

6-Component Measurement Fx, FY, Fz, Mx, My, Mz

FLV Considerations n  Stiff vibration test fixture in the frequency range of interest to avoid fixture

resonance reactions.

n  High Preload and Load bearing surfaces per manufacturer

requirements prevent sensor damage + obtain accurate force reading

( ex. bending moments being falsely sensed as tension/compression.)

n  Preload considerations

•  Max. stress on the sensor does not exceed manufacturer limits

•  Preload is sufficient to carry the shear loads via friction, without slip

•  Preload is sufficient to prevent unloading related to the dynamic

forces and moments,

•  Force sensors used to measure the preload when torqueing bolts. .

FLV Presentation 13 Ref: NASA-HDBK-7004C, Force Limited Vibration Testing

Preloading force sensors in ring typically uses a star pattern

n  Preload bolt carries a portion of the dynamic load .

�  If standard preload bolt used – consult manufacturer Cal Cert force shunt sensitivity details .

�  If preload bolt is tailored to the UUT– In Situ Calibration to determine sensitivity correction

�  Dynamic: Low-level sine sweep or random to compare apparent mass with the known total

mass . Axial force more affected by shunt as preload bolts less stiff in shear

�  Static: Load sensors with weight, reset charge amp, remove weight to minimize any

transient overshoot associated with the load application.

Example Global Precipitation Measurement (GPM) Satellite n  GPM Monitor rain and snow activity worldwide

n  Payload mass approx. 7000 lbs. ( 3,175kg)

n  The force ring approx. 350 lbs.(158.8kg)

n  Frequency range of interest 5Hz to 70Hz

n  3 inch thick top and bottom plate

n  Ring Diameter ~102 inch

n  Qty 12, Type 9077C load Cells

n  Preload to 60,000lb ( 267kN) , star pattern, 4 step preload ( 25%, 50%, 75% and 100%)

n  Custom 1” diameter bolts with Delran spacers to center Force sensor FLV Presentation 14

12 Sensor FLV Equation Implementation

FLV Presentation 15

Stanford Research Systems (SRS) Configurable –Commercial Solution

SIM900 Mainframe Sum-Diff Amp

SIM980 SIM983

Scaling Amp

FLV Presentation 16

n  Charge Amp Outputs for PE Force

n  Coupler Outputs for IEPE Force

Candidate Measuring Chains to Resolve Forces and Moments

n  Common request is to utilize existing legacy equipment.

n  Various implementations possible

FLV Presentation 17

PE 3-Comp Force

PE 3-Comp with in-line Charge Converter to IEPE or IEPE 3-Comp Force

Optional Charge Summing

Multi-Channel Charge Amp

Multi-Channel IEPE Coupler Voltage Sum-Difference

Amplifiers and Scaling

Voltage Sum-Difference Amplifiers and Scaling

Analog Forces and Moments

Analog Forces and Moments

Analog Forces

PE Force Sensors: Charge Summing

n  For PE Force Sum’s Directly – no need for electronics to perform charge summing � Reduce number of Charge Amplifiers for Fx, Fy and Fz to 3 Channels � Use Average Sensitivity Fx, Fy, Fz

FLV Presentation 18

Fx, Fy, Fz Resolution n  Generate Analog signals Fx, Fy, Fz, Mx, My

FLV Presentation 19

12:1 Charge Sum

12:1 Charge Sum

12:1 Charge Sum

Xi

Yi

Zi

Fx

Fy

Fz

1Ch Charge-Amp

1Ch Charge-Amp

1Ch Charge-Amp

Mx, Mz Resolution n  Generate Analog signals Fx, Fy, Fz, Mx, My

�  Mx = d [(Z4+Z9)-(Z3+Z10)] + e [(Z5+Z8)-(Z2+Z11)] + f [(Z6+Z7)-(Z1+Z12)]

�  My = a [(Z7+Z12)- (Z1+Z6)] + b [(Z8+Z11)- (Z2+Z5)] + c [(Z9+Z10)- (Z3+Z4)]

FLV Presentation 20

Mx

My

Zi

12Ch Charge-Amp

NI CompactRIO to Resolve Analog Forces and Moments

n  NI CompactRIO controllers provide stand-alone/networked execution of LabVIEW Real-Time applications

n  Each CompactRIO system contains a reconfigurable FPGA for custom timing, triggering, and processing

n  Implement Force and Moment Equations in Real time

n  PE and IEPE Modules available with integrated onboard DAQ

n  Expandable Chassis to support a scalable solution for FLV

n  Analog Output to provide inputs to User Vibration Controller.

FLV Presentation 21

NI 9234 ±5 V, IEPE AC/DC Analog Input 51.2 kS/s/ch, 4 Ch Module

Kistler 5171A4, Charge ranges upto ± 1,000,000 pC, 50.8 kS/s/ch, 24 bit, 4 Ch Charge Amplifier Module

Expandable Chassis options with Controller

NI 9263, 4 chn, 100 kS/s/ch ±10 V output , NI 9264, 16 chn, 25 kS/s/ch ,±10 V output

Force + Moment Video

FLV Presentation 22

Summary n  PE Sensors can be ranged externally with scalable resolution. TC can be changed from Long for Set-up/

Checkout to Medium or Short for the dynamic Testing.

n  IEPE Sensors have Fixed Range and Low Frequency

n  Quasi-static capability during set-up and check out is convenient.

n  PE Sensor with Inline Charge Converter to IEPE. Option to use a Charge Amp to PE sensor for

preloading

n  Analog Force and Moment measurement as inputs to Vibration Controller.

�  PE: �  Charge Summing and Charge Amplifiers to resolve analog Forces only �  Forces and Moments: Charge Summing and Charge Amplifiers with Voltage amplifier/Scaling

amplifiers such as Stanford Research

�  IEPE �  Forces and Moments: Couplers with voltage amplifiers/scaling amplifiers such Stanford

Research

n  Real time calculation of Moments and Forces with Analog Output to Controller , for example with the NI

cRio platform.

FLV Presentation 23