Sascha Mäuselein, Oliver Mack P B T Silicon load cells Investigations of new silicon load cells with thin-film strain gauges SIM MWG11 – Load Cells Tests

Silicon load cellsSascha Mäuselein, Oliver Mack P B T

Investigations of new silicon load cells

with thin-film strain gauges

SIM MWG11 – Load Cells Tests by OIML R60SIM MWG11 – Load Cells Tests by OIML R60Buenos Aires, June 3010Buenos Aires, June 3010

Silicon load cells2/19Sascha Mäuselein, Oliver Mack P B T

Table of contentsTable of contents

• Introduction

• Mechanical spring made of silicon

• Investigations (I)

• Application of strain gauges

• Investigations (II)

-> characteristic line

-> time depending effects

• Evaluation according to OIML R60

• Applications


IntroductionIntroduction

Dominant sensor technologies in weighing instruments:

Electromagnetic force

compensation load cells

• Very high precision

• Complex technology

• Limited load range

Strain gauge load cells

• Most common

• Maximum number of verification intervals: 6000

• Limiting factors to step up the precision: time depending effects hysteresis


IntroductionIntroduction

Single crystalline material (silicon) for the mechanical spring

- High purity

- Ideal elastic properties

- Less mechanical after effects

Thin film strain gauges

- Direct connection

- Less creep effects

- High reproducibility

Sensor with

- High reproducibility

- Low time depending effects

- Good sensor properties

- High potential to improve

the properties by digital

compensation

Sputtering technique

Crystal growth procedure


Spring made of single crystalline siliconSpring made of single crystalline siliconAspects of design:

Nominal load Thin film application Material properties of Si

double bending beam geometry

Numerical simulations to optimise

• the geometry parameters

• the orientation of Siwithin the spring

Mechanical spring made of silicon


Investigations (I) – Experimental setupInvestigations (I) – Experimental setup

Schematic arrangement of the experimental setup

Deformation measurements

Fizeau Interferometer

• 3-D topology data of the surface

-> Tipping effects can be

calculated and corrected

Loading

• Dead loads

• Wire and pulley to switch

the load force

Application of strain gauges in a later step

Before: Investigation of the mechanical spring

-> Time dependent deformation after load change


Investigations (I) – Experimental setupInvestigations (I) – Experimental setup

Picture of the experimental setup

Pulley

Interferometer

Wire

Si spring

Masses

Clamping


Investigations (I) – ResultsInvestigations (I) – Results

Surface topology as function of thepositions x and y for different load steps

Deflection sensitivity

su = -65.2 nm/g

Position of thin places


Investigations (I) – ResultsInvestigations (I) – Results

Normalised deflection uy,n as function of the time

for loading and unloading

Loading:

Influence of pulley

Unloading:

No detectable creep

behaviour

Not suitable

Mechanical after effect:

≤ 2·10-5

Low time depending effects of

silicon spring are verified


Application of thin film strain gaugesApplication of thin film strain gauges

Si load cellwith thin filmstrain gauges

Layercomposition

of the SGs

- Connection of four strain gauges to a full bridge

- Analysis by precision amplifier


Investigations (II)Investigations (II)

Load depending investigations

of the sensor signal

- Reproducibility

- Hysteresis

- Linearity

Time depending investigations

of the sensor signal

- Creep


Investigations (II) – Experimental setupInvestigations (II) – Experimental setup

Picture of the experimental setup

Clamping

Connection of SGs

Si load cell

Chain masses

Temperaturemeasurement

Humiditymeasurement

Piece ofhardwood


Investigations (II) – ReproducibilityInvestigations (II) – Reproducibility

Relative repeatability error b as function of the load L

By a factor of 10 betterthan the requirements

for class 00

Classes according to ISO 376:


Investigations (II) – HysteresisInvestigations (II) – Hysteresis

Relative reversibility error u as function of the load L

About a factor of 10better than the

requirements forclass 00



Investigations (II) – LinearityInvestigations (II) – Linearity

Relative interpolation error I as function of the load L

Requirements forclass 1 are kept



Investigations (II) – CreepInvestigations (II) – Creep

Relative creep C while loadingas function of the time t

Relative creep C while unloadingas function of the time t

• Relative creep < 2∙10-5 • After 7 minutes: No creep detectable• Relative creep < 2∙10-5


Investigations (II) – ResultsInvestigations (II) – Results

Meaningful improvement

by digital compensation

is possible

Reproducibility ++

+

o

++2∙10-5

Hysteresis

Linearity

Creep

9∙10-4

8∙10-5

2∙10-5

Next step:

- Digital compensation of data concerning linearity and temperature

- Evaluation of data according to OIML R60


Evaluation – OIML R60Evaluation – OIML R60

Load cell error ELC as function of the load L

Precisionweighing instrument


Fields of applicationFields of application

• Load cells for precision measurements

• Transfer standard

Thank you for your attention




Documents

Sascha Mäuselein, Oliver Mack P B T Silicon load cells Investigations of new silicon load cells with thin-film strain gauges SIM MWG11 – Load Cells Tests