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SIM MWG11 – Load Cells Tests by OIML R60
Buenos Aires, June 3010
Investigations of new silicon load cells
with thin-film strain gauges
Silicon load cells
Sascha Mäuselein, Oliver Mack
PTB
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
2/19
Silicon load cells
Sascha Mäuselein, Oliver Mack
PTB
Introduction
Dominant sensor technologies in weighing instruments:
Electromagnetic force
compensation load cells
•
Very high precision
•
Complex technology
•
Limited load range
3/19
Strain gauge load cells
•
Most common
•
Maximum number of
verification intervals: 6000
•
Limiting factors to step up
the precision:
 time depending effects
 hysteresis
Silicon load cells
Sascha Mäuselein, Oliver Mack
PTB
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
4/19
Sensor with
- High reproducibility
- Low time depending effects
Crystal growth procedure
- Good sensor properties
- High potential to improve
Sputtering
technique
the
properties
by digital
compensation
Silicon load cells
Sascha Mäuselein, Oliver Mack
PTB
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
5/19
Silicon load cells
Sascha Mäuselein, Oliver Mack
PTB
Investigations (I) – Experimental setup
Deformation measurements
Loading
Application of strain gauges in a later step
Fizeau Interferometer
• Dead loads
•Before:
3-D topology
data ofofthe
Investigation
thesurface
mechanical spring • Wire and pulley to switch
-> Tipping effects can be
the load force
-> Time dependent deformation after load change
calculated and corrected
Schematic arrangement of the experimental setup
6/19
Silicon load cells
Sascha Mäuselein, Oliver Mack
PTB
Investigations (I) – Experimental setup
Si spring
Pulley
Clamping
Wire
Masses
Interferometer
Picture of the experimental setup
7/19
Silicon load cells
Sascha Mäuselein, Oliver Mack
PTB
Investigations (I) – Results
Position of thin places
Deflection sensitivity
su = -65.2 nm/g
Surface topology as function of the
positions x and y for different load steps
8/19
Silicon load cells
Sascha Mäuselein, Oliver Mack
PTB
Investigations (I) – Results
Loading:
Influence of pulley
Low time depending effects of
silicon spring are verified
Unloading:
No detectable creep
behaviour
Mechanical after effect:
≤ 2·10-5
Normalised deflection uy,n as function of the time
for loading and unloading
9/19
Silicon load cells
Sascha Mäuselein, Oliver Mack
PTB
Application of thin film strain gauges
- Connection of four strain
gauges to a full bridge
- Analysis by precision
amplifier
Layer
composition
of the SGs
Si load cell
with thin film
strain gauges
10/19
Silicon load cells
Sascha Mäuselein, Oliver Mack
PTB
Investigations (II)
Load depending investigations
of the sensor signal
- Reproducibility
- Hysteresis
- Linearity
Time depending investigations
of the sensor signal
- Creep
11/19
Silicon load cells
Sascha Mäuselein, Oliver Mack
PTB
Investigations (II) – Experimental setup
Humidity
measurement
Chain masses
Si load cell
Temperature
measurement
Clamping
Piece of
hardwood
Connection of SGs
12/19
Picture of the experimental setup
Silicon load cells
Sascha Mäuselein, Oliver Mack
PTB
Investigations (II) – Reproducibility
Classes according to ISO 376:
By a factor of 10 better
than the requirements
for class 00
Relative repeatability error b as function of the load L
13/19
Silicon load cells
Sascha Mäuselein, Oliver Mack
PTB
Investigations (II) – Hysteresis
Classes according to ISO 376:
About a factor of 10
better than the
requirements for
class 00
Relative reversibility error u as function of the load L
14/19
Silicon load cells
Sascha Mäuselein, Oliver Mack
PTB
Investigations (II) – Linearity
Classes according to ISO 376:
Requirements for
class 1 are kept
Relative interpolation error I as function of the load L
15/19
Silicon load cells
Sascha Mäuselein, Oliver Mack
PTB
Investigations (II) – Creep
Relative creep C while loading
as function of the time t
• Relative creep < 2∙10-5
16/19
Relative creep C while unloading
as function of the time t
• After 7 minutes: No creep detectable
Silicon load cells
Sascha Mäuselein, Oliver Mack
PTB
Investigations (II) – Results
Reproducibility
++
2∙10-5
Hysteresis
Meaningful improvement
by digital compensation
is possible
+
8∙10-5
Linearity
o
9∙10-4
Creep
++
2∙10-5
17/19
Next step:
- Digital compensation of data
concerning linearity and
temperature
- Evaluation of data according to
OIML R60
Silicon load cells
Sascha Mäuselein, Oliver Mack
PTB
Evaluation – OIML R60
Precision
weighing instrument
Load cell error ELC as function of the load L
18/19
Silicon load cells
Sascha Mäuselein, Oliver Mack
PTB
Fields of application
• Load cells for precision measurements
• Transfer standard
19/19
Silicon load cells
Sascha Mäuselein, Oliver Mack
PTB