Test and calibration of the resistive sensors FlexiForce®.
1. Task, employed sensors and tooling.
For measurement of the weight of the Barrel it is necessary the load sensors. It is need to
measure weight of the Barrel at 4 points; each point should be carry till 600kg with precision
20kg.
The resistive FlexiForce sensors (Tekscan Inc.) were offered, see fig. 1. On this figure
sensors HT201, A201 and sample custom #1207 are shown.
HT201
A201
#1207
fig. 1
FlexiForce sensors.
Technical parameters of sensors shown in the table
table 1
Thickness
Sensing area
A201 and HT201
#1207
Substrate
Standard force range
Linearity
Repeatability
Hysteresis
Drift
Response time
Non-destructive pressure
0.208mm
9.53 mm diameter
26 mm diameter
Polyester
0-100 lb
< ±5%
< ±2.5% of full scale
< 4.5% of full scale
< 3% of logarithmic time scale
< 5 µsec
10 000 PSI (lb per square inch)
Based on this value of non-destructive pressure we can calculate maximum power for sensor
F = 10000PSI * 0.078inch2 = 780lb ≈ 3500N,
where 0.078inch2 is square of the puck diameter 8mm. Puck is using for distribute of loading
to working area.
Recommend circuit for measurement of the FlexiForce sensors shown in fig. 2. Reference
resistance is Rref = 20kOm, reference voltage is Vref = 1Volt. Output voltage read by PC and
calculated the value 1/RFlexiForce which should have linear depentanizer of the loading.
Rref
Vref
RFlexiForce
Vout
+
fig. 2
Measurement circuit.
As the etalon pressure sensor the digital force gauge Mecmesin was used. This tooling was
calibrated at CERN metrology lab using UTS test system. Calibration plot is shown in fig. 3.
verification of mecmesin
12000
10000
mecmesin, N
8000
series 1
6000
series 2
series 3
4000
series 4
2000
0
0
2000
4000
6000
8000
10000
UTS, N
fig. 3
Verification of Mecmesin force gauge.
For calibrate FlexiForce sensors the tooling shown in fig. 4 was employed.
12000
fig. 4
The tooling for calibration of sensors.
The Mecmesin gauge and FlexiForce sensors clamped between bench-vice grips. The puck
glued on the FlexiForce sensor and clamped between two metal plates with plane surfaces.
2. The measurement results.
The purpose of the measurement was calibrating of FlexiForce sensors and understanding of
the “training” process. Important feature of the sensors is what it should be conditioned before
using. To condition a sensor it should be load for 110% of maximum measuring value 4-5 times.
Typical calibration curve acquired when measurement shown in fig. 5.
Sensor #1 FlexiForce
calibration curve
0.2
0.18
0.16
1/R, kOm-1
0.14
0.12
0.1
0.08
0.06
0.04
0.02
0
0
500
1000
1500
2000
Pressure, N
2500
fig. 5
The calibration curve for sensor #1.
It is seen that hysteresis amount to 10% and nonlinearity is present.
3000
3500
For understanding of conditioning process it was made set of measurements of the nonconditioned (new one) sensor. Sensor was loaded till ~3000N and then unloaded any times. In
each such cycle value 1/RFlexiForce was recorded at maximum load. The measurement results for
sensor #4 are shown in fig. 6. Yellow curve shows quotient of FlexiForce 1/R by loading force.
Factor 3000 is for bringing to scale only (right Y-axis).
Sensor #4, load-unload training
maximum load in each pass
3500
0.45
3000
0.4
0.35
2000
1500
0.3
FlexiForce, kOm-1
mecmesin, N
2500
1000
mecmesin
500
0.25
FlexiForce
FF/MM*3000
0
0.2
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41
measurements
fig. 6
Conditioning of the sensor #4.
It is seen that first 5 times it was big drift of 1/R value. Then drift becomes less to 1% per
measurement. In spite of big number of cycles there was not obtain horizontal line.
Points from #14 to #22 and from #28 till #40 is reading at lower pressure (2700N instead of
3000N) for examination of postulate that conditioning should be at 110% of maximum measured
loading. Evidently drift is present.
Drift doesn’t depend on a measuring time, and it depends on amount of loading cycles. It
confirmed by the next fact. Time interval between points ##34 and 36 is more than 12 hours,
whereas delays between other points 1-2 minutes. Sensor remains under pressure ~2500N after
measuring at point #34 till point #36.
For confirmation of independence drift on time special measurement was made. Sensor A201
was loading with force 3000N and data were reading with 10sec time interval. Results shown at
plots (see fig. 7 and fig. 8). The yellow curve shows quotient 1/R value by applied force as in
previous plot.
Apparently transition processes is finishing at first seconds after loading.
Sensor response by time
0.3
3500
0.25
3000
2000
0.15
1500
0.1
FlexiForce
force, N
1/R, kOm-1
2500
0.2
1000
mecmesin, N
0.05
500
0
0
1000
2000
3000
4000
5000
6000
7000
0
8000
time, sec
fig. 7
0.3
4000
0.25
3500
0.2
3000
0.15
2500
FlexiForce
0.1
2000
FF/MM*3000
mecmesin, N
0.05
1500
0
1000
0
10
20
30
40
time, sec
fig. 8
50
60
70
80
mecmesin, N
1/Rff, kOm-1
Sensor response by time
With the purpose of decreasing unit pressure the sensor sample custom was used also. This
sensor #1207 has a sensing area 27mm diameter. Puck which used was 20mm diameter.
Maximum force in this case can calculated as
F = 10000PSI * 0.487inch2 = 4870lb ≈ 21700N,
where 0.487 inch2 is square of sensing area under puck.
This sensor was calibrated with pressure till ~2000N after conditioning 6 times with pressure
2540N. Results are shown in fig. 9.
FlexiForce #7
y = 0.0024x + 0.2228
2080
0.255
2070
0.25
2060
0.245
0.24
2040
0.235
2030
0.23
2020
0.225
2010
mecmesin, N
2000
FlexiForce, kOm-1
FF/MM*2000
mecmesin, N
2050
0.22
FlexiForce, kOm-1
0.215
FF/MM*2000
1990
Linear (FF/MM*2000)
0.21
1980
1970
0.205
1
2
3
4
5
6
7
8
9
10
11
measurements
fig. 9
Results of a test FlexiForce sensor sample custom #1207.
The drift is present and can be estimated as gradient of fit line as 2.4x10-3. It’s about 0.25%.
Also the sensor A201 tested with maximum load till 480N. Sensor was previously
conditioned 5 times with load 2560N. Sensor was measured 10 times with 2000N and 12 times
with 480N. Data are shown in fig. 10 and fig. 11.
2100
0.146
2080
0.144
2060
0.142
2040
0.14
2020
0.138
2000
0.136
1980
0.134
mecmesin
1960
0.132
FlexiForce #6
1940
FF/MM*3000
0.13
1920
Linear (FF/MM*3000)
0.128
1900
1/R FlexiForce, kOm-1
F mecmesin, N
loading of FlexiForce #6
after training at 2500N
max load 2000N
y = 0.0011x + 0.1332
0.126
1
2
3
4
5
6
7
measurements
8
9
10
fig. 10
y = 4E-05x + 0.0528
490
0.056
480
0.054
470
0.052
460
0.05
450
0.048
mecmesin, N
440
0.046
1/R FlexiForce, kOm-1
F mecmesin, N
loading of FlexiForce #6
max load 480N
FexiForce, kOm-1
FF/MM*500
430
0.044
Linear (FF/MM*500)
420
0.042
1
2
3
4
5
6
7
8
measurements
9
10
11
12
fig. 11
The drift with high loading (2000N) is 0.011 and with loading 480N is only 4x10-5. The
sensor behavior is good at pressure till 480N (100 lb for this sensing area) and sensor haven’t
good repeatability at higher loading.
3. Conclusion.
The FlexiForce resistive sensors have important properties:


…
The FlexiForce sensors have stable characteristics in range of loading 0-100 lb. In
the range 0-1000 lb sensors have a drift, i.e. changing of the response by each
loading.
Before the first using and when sensor didn’t use several weeks it needs to
recalibrate sensor.