Signal conditioning for resistive strain gauge
sensors in mobile applications
Robert Wendlandt1, Christoph Warncke2, Jörg Müller2
1)
University Hospital Schleswig-Holstein
Biomechanics Laboratory, Ratzeburger Allee 160, D- 23538 Lübeck
2)
Hamburg University of Technology
Micro systems technology, Eissendorfer Strasse 42, D-21073 Hamburg
Robert.Wendlandt@uk-sh.de
Abstract—Resistive strain gauge sensors are used for
precise measurement of mechanical forces. Although the
sensor elements (strain gauge foils) are inexpensive, a
sensor requires costly manual trimming.
Signal conditioning and data acquisition ICs can render
manual trimming dispensable. Different systems based
upon a programmable system on chip and also dedicated
sensor signal conditioning ICs are analyzed for absolute
sensitivity and sampling rate.
The medical application of strain gauge sensors in
conjunction with a sensor signal conditioning IC in a
robotic external fixator is shown to deliver accurate force
measurements.
acquisition are compared with the single-chip
design using a reference force sensor.
The designs are discussed for the use in a robotic
hexapod external fixator [1] in terms of system
complexity, flexibility and sufficient data rate.
Index terms—strain gauge, sensor, mobile
I.
INTRODUCTION
Resistive strain gauge sensors are commonly
used for precise and reliable measurements of
mechanical forces in different areas of application
ranging from space and industrial applications to
your bathroom scale. The usually small change of
resistance of the strain gauge is related to the
mechanical strain of a supporting object.
Although the strain gauge foils are inexpensive to
manufacture, complete sensors are costly. Time
consuming manual processing steps, gluing the
sensors to the object and especially trimming a
Wheatstone-bridge with abrasive resistors for offset
and thermal shift, are setting the price rather than
material costs.
Offset compensation with operational amplifiers
or lock-in techniques and subsequent digital
processing can render manual bridge trimming
dispensable yet electronic complexity has to be
considered as well.
This paper describes the application of a singlechip signal conditioning and data acquisition
system that connects resistive strain gauge sensors
with a general purpose microcontroller for smart
sensors. Different system architectures for data
Fig. 1: Robotic hexapod external fixator
The sensor element of the robotic fixator (fig. 1)
is made from stainless steel tube with a diameter of
11mm. Two resistive strain gauge half-bridges
(Vishay Measurements Group GmbH) are glued to
the tube which is then integrated into the
mechanical structure of an external fixator system
for fracture treatment. The sensor shows a
sensitivity of 440µV/V at 1000N while the zeropoint offset is typically in the same order of
magnitude.
II. MATERIALS AND METHODS
Data acquisition systems (configuration A – E)
for untrimmed resistive strain gauge sensors for
mobile medical applications were designed using a
programmable
system-on-chip
(PSoC)
microcontroller
(CY8C29x66,
Cypress
Semiconductor) and two different external sensor
signal conditioning ICs with integrated ADC
(PGA309, Texas Instruments and SX8723, Semtech
Corporation). To compare the performance of the
different designs a commercially available force
sensor (8435, Burster Präzisionsmesstechnik GmbH
& Co KG) was connected to the systems. The
sensor has a sensitivity of 1 mV/V at the full load of
1 kN. It is assembled out of resistive strain gauge
foils in a full-bridge configuration. The bridge
resistance of the sensor is 350 Ω.
The overall system design is similar for all of the
five designs. The PSoC microcontroller is powered
by 3.3 V low dropout regulator also powering the
external sensor signal conditioning IC, if present, as
well as supplying bias to the sensor. The PSoC uses
its internal 12 MHz clock. A USB-to-serial
converter is used for data transmission between the
PSoC and a control PC.
Sections A – E describe the design differences of
the analyzed systems.
Measurements are performed at room
temperature. Weights from 0 kg up to 7 kg in
increments of 1 kg are attached to the sensor and
1000 samples of data are taken. The data samples
for the seven measurements are evaluated in Excel
(Microsoft Corporation). Mean value, standard
deviation and slope of the linear regression of the
mean values are determined. The absolute
sensitivity (slope of linear regression divided by
standard deviation) is calculated to compare the
different systems.
A. Internal amplifier and ADC
Only internal function blocks of the PSoC are
used to implement an amplification of the small
scale sensor signal. The instrumentation amplifier
block (INSAMP) has a maximum gain of 93. A
14 bit analog to digital converter (ADC) digitizes
the amplified signal. The data rate of the system is
40 Hz.
Fig. 2: Configuration A (internal amplifier and
ADC)
B. Two internal amplifiers and ADC
Configuration A and B are very similar. Yet the
amplified sensor signal is fed out and in of the
PSoC to a programmable gain amplifier (PGA)
block. Gain of the PGA is 4 to stay within the
system voltage supply resulting in a system gain of
372. A 14 bit analog to digital converter (ADC)
digitizes the amplified signal. The data rate of the
system is 40 Hz.
Fig. 3: Configuration B (two internal amplifiers and
ADC)
C. Carrier frequency excitation
Configuration C also uses internal blocks of the
PSoC only yet carrier frequency excitation is used
in this design. The sensor is supplied with a
sinusoidal signal generated by a pulse width
modulator (PWM) block generating a rectangular
signal of 5 kHz which is then filtered by a band
pass filter (BPF) block with a center frequency of
5 kHz an a bandwidth of 300 Hz. The sinusoidal
output voltage of the sensor is amplified by an
INSAMP block with a gain of 93 and also filtered
by a BPF. A mixer is used for phase-sensitive
rectification. The rectified signal is sampled by the
integration ADC over 125 full periods effecting a
low pass filtering of the digitized signal. The data
rate of the system is 40 Hz.
the ADC is set to 15 Hz for a filter notch close to
50 Hz.
Fig. 6: Configuration E (SX8723 external amplifier
and ADC)
Fig. 4: Configuration C (carrier frequency
excitation)
D. PGA309 external amplifier
A PGA309 [2] sensor signal conditioning IC is
used in addition to the PSoC in this configuration.
The sensor’s output is connected to the PGA309
and a common power supply is used for the sensor,
the PGA309 and the PSoC. Gain of the PGA309 is
set to the maximum of 1152 and offset registers are
programmed for 0 V output at no sensor load. The
sensor output is digitized by an internal 15 Bit ADC
of the PGA309, which is read out via an OneWire
Bus. Sampling time of the ADC is 100 ms. The low
data rate on the OneWire Bus limits the effective
data rate to approximately 8 Hz.
III. RESULTS
For each of the configurations A – E seven
measurements with different weights are made and
1000 samples of data are taken for each
measurement. Absolute sensitivity is calculated
from the slope of the linear regression divided by
the standard deviation. The results are given in
table 1.
Tab. 1: Measurement result of configurations A – E
Configuration
A
B
C
D
E
Slope
[N-1]
1.77
6.63
1.68
42.79
66.53
Standard
deviation
2.26
8.92
1.46
2.99
5.88
Absolute
sensitivity
[N-1]
0.78
0.74
1.15
14.31
11.31
IV. DISCUSSION
Fig. 5: Configuration D (PGA309 external amplifier
and ADC)
E. SX8723 external amplifier
The SX8723 [3] data acquisition IC is used in
addition to the PSoC in this configuration. Gain of
the SX8723 is set to the maximum of 1000 and
offset registers are programmed for a near 0 output
at no sensor load. The sensor’s output is digitized
by the internal 16 Bit ADC of the SX8723, which is
read out via I²C-Bus at 400 kHz. Sampling rate of
The different system designs differ largely in
their absolute sensitivity. The designs with an
external sensor signal conditioning ICs show an
absolute sensitivity 10 times better than the designs
realized with the internal PSoC blocks.
Zero point offset of the sensor is a large concern
for the configurations A – C. Although a
commercially available sensor with a very low zero
point offset was used, it was still the limiting factor
for the gain thus limiting the absolute sensitivity to
the given values.
The PGA309 sensor signal conditioning IC has
the highest system gain of 1152 although only a
15 Bit ADC is used. The slowest data rate of the
compared systems also gives the highest absolute
sensitivity. System complexity and board space
required is highest in this design as many passive
components are used and a leaded package is
available only. The internal ADC is intended for
calibration purposes only therefore the data rate on
the OneWire Bus is low as a polling method has to
be used to monitor an end of conversion flag.
The absolute sensitivity of the SX8723 is slightly
worse than of the PGA309. But system complexity
and extra board space is minimal in this design.
Only one capacitor and two external resistors are
used for the I²C Bus. Data rate was chosen with
15 Hz in this analysis but can be configured with
same standard deviation and accuracy to 60 Hz.
With digital filters the standard deviation can be
reduced while the data rate is still sufficient for the
intended application. Offset compensation range is
highest in this design with up to 15 times the full
scale input range of the sensor.
V.
CONCLUSION
A data acquisition system for untrimmed resistive
strain gauge sensors for mobile medical
applications was designed using a microcontroller
and the SX8723 (Semtech Corporation, CA) signal
conditioning system. The SX8723 provides a
complete low power acquisition path and uses least
extra board space.
The sensor element of the robotic external fixator
is made from stainless steel tube with a diameter of
11mm. Two resistive strain gauge half-bridges
(Vishay Measurements Group GmbH) are glued to
the tube which is then integrated into the
mechanical structure of an external fixator system
for fracture treatment.
The complete sensor system has a board space of
10 mm * 20 mm on a one side, two layer PCB. The
complete
system
is
housed
in
polyurethane/composite enclosure made with a
rapid prototyping system (ZPrinter 450 Z
Corporation).
The sensor shows a sensitivity of 440µV/V at
1000 N while the zero-point offset is typically in
the same order of magnitude. With an ADC data
rate of 60 Hz and digital filtering the absolute
sensitivity of the system is 16.26 N-1 allowing a
resolution better than 0.1 N with a data rate of
7.5 Hz.
VI. REFERENCES
[1]
K. Seide, U.-J. Gerlach, R. Wendlandt, N. Weinrich, J. Müller, C.
Jürgens: “Intelligent external fixator for fracture treatment and
deformity correction”
in Trauma und Berufskrankheit,
Volume 9, Number 2, 2007, pp. 109–116
[2]
[3]
Texas Instruments: “PGA309 Datasheet (SBOS292B)”, 2005,
http://www.ti.com/lit/gpn/pga309
Semtech Corporation: “SX8723 Datasheet (V1.8)”, 2009,
http://www.semtech.com/pc/downloadDocument.do?navId=H0,C
1,C193,C195,C199,P3361&id=2652