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 by OIML R60 Buenos Aires, June 3010

Sascha Mäuselein, Oliver Mack P B T Silicon load cells 2/19 Table 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

Sascha Mäuselein, Oliver Mack P B T Silicon load cells 3/19 Introduction 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

Sascha Mäuselein, Oliver Mack P B T Silicon load cells 4/19 Introduction 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

Sascha Mäuselein, Oliver Mack P B T Silicon load cells 5/19 Spring made of single crystalline silicon Aspects 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 Si within the spring Mechanical spring made of silicon

Sascha Mäuselein, Oliver Mack P B T Silicon load cells 6/19 Investigations (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

Sascha Mäuselein, Oliver Mack P B T Silicon load cells 7/19 Investigations (I) – Experimental setup Picture of the experimental setup Pulley Interferometer Wire Si spring Masses Clamping

Sascha Mäuselein, Oliver Mack P B T Silicon load cells 8/19 Investigations (I) – Results Surface topology as function of the positions x and y for different load steps Deflection sensitivity s u = nm/g Position of thin places

Sascha Mäuselein, Oliver Mack P B T Silicon load cells 9/19 Investigations (I) – Results Normalised deflection u y,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

Sascha Mäuselein, Oliver Mack P B T Silicon load cells 10/19 Application of thin film strain gauges Si load cell with thin film strain gauges Layer composition of the SGs - Connection of four strain gauges to a full bridge - Analysis by precision amplifier

Sascha Mäuselein, Oliver Mack P B T Silicon load cells 11/19 Investigations (II) Load depending investigations of the sensor signal - Reproducibility - Hysteresis - Linearity Time depending investigations of the sensor signal - Creep

Sascha Mäuselein, Oliver Mack P B T Silicon load cells 12/19 Investigations (II) – Experimental setup Picture of the experimental setup Clamping Connection of SGs Si load cell Chain masses Temperature measurement Humidity measurement Piece of hardwood

Sascha Mäuselein, Oliver Mack P B T Silicon load cells 13/19 Investigations (II) – Reproducibility Relative repeatability error b as function of the load L By a factor of 10 better than the requirements for class 00 Classes according to ISO 376:

Sascha Mäuselein, Oliver Mack P B T Silicon load cells 14/19 Investigations (II) – Hysteresis Relative reversibility error u as function of the load L About a factor of 10 better than the requirements for class 00 Classes according to ISO 376:

Sascha Mäuselein, Oliver Mack P B T Silicon load cells 15/19 Investigations (II) – Linearity Relative interpolation error I as function of the load L Requirements for class 1 are kept Classes according to ISO 376:

Sascha Mäuselein, Oliver Mack P B T Silicon load cells 16/19 Investigations (II) – Creep Relative creep C while loading as function of the time t Relative creep C while unloading as function of the time t Relative creep < 2∙10 -5 After 7 minutes: No creep detectable Relative creep < 2∙10 -5

Sascha Mäuselein, Oliver Mack P B T Silicon load cells 17/19 Investigations (II) – Results Meaningful improvement by digital compensation is possible Reproducibility ++ + o 2∙10 -5 Hysteresis Linearity Creep 9∙ ∙ ∙10 -5 Next step: - Digital compensation of data concerning linearity and temperature - Evaluation of data according to OIML R60

Sascha Mäuselein, Oliver Mack P B T Silicon load cells 18/19 Evaluation – OIML R60 Load cell error E LC as function of the load L Precision weighing instrument

Sascha Mäuselein, Oliver Mack P B T Silicon load cells 19/19 Fields of application Load cells for precision measurements Transfer standard Thank you for your attention

Sascha Mäuselein, Oliver Mack P B T Silicon load cells 20/19 SiliziumAluminiumStahl (Ck67) Dichte in g/cm 3 2,332,707,85 Elastizitätsmodul in N/m 2 (13-18) · · ·10 7 Therm. Ausdehnungsk. in 1/K(2,3-2,6) ·10 -6 (23-25) · ·10 -6 Zugbruchfestigkeit in N/m 2 (5-20) ·10 7 (7-19) ·10 7 (53-160) ·10 7