THE USE OF FLEXIFORCE SENSORS FOR THE SCT END-CAPS
Stephen Haywood – 10 March 2006
1. INTRODUCTION
Encouraged by Marco Olcese, Patrick Werneke and Andrea Catannacio, the SCT End-cap will use FlexiForce
A201 (0-100lbs) sensors to determine the load on the four “Mechanisms” which support the End-cap on the TRT
Rails.
It is expected that the weights will be around 30 kg on each of the Front Mechansisms and around 70 kg for each
of the Rear Mechanisms1. While these loads are outside the nominal range of the FlexiForce sensors, it is
apparent from the Tekscan literature2 that the sensors should cope fairly well.
The FlexiForce sensors are resistive devices. The sensitive part of the device is a disc approximately 1 cm in
diameter. As this is compressed, the resistance falls. The resistance is approximately inversely proportional to
the applied force. The sensitive area is surrounded by a few millimetres of clear plastic, which continues to one
side with conductive tracks which are read out at the far end through metallic pins – see Fig 0.
The primary role in the case of the SCT is to ensure that the loads on the Mechanisms are well balanced, namely
that the End-cap is supported on 4 points rather than 3. The sensors will be sandwiched between the flat shims
which are used to adjust the height of the Mechanisms. The TRT Community would like to go further and use
their capacitive sensors to determine the supported mass of the detector, including the SCT when inserted.
These sensors were tested by the TRT Community3 and found to not be very satisfactory due to their instability
with time, and the need to recalibrate. Subsequently, the TRT Community has moved to using capacitive sensors
and has developed a good understanding of these, although it has taken many days of work4. Eric Perrin has also
tested the sensors for use with the Barrel. Eric reports similar problems to those discussed in this report.
Fig 0: FlexiForce Sensor
A B
2. MAKING MEASUREMENTS
Measurements were made using a Load Cell at RAL (Testometric AX500), provided by the Advanced Materials
Group. The Load Cell was accurate to 1%.
The FlexiForce sensors were inserted between the flat faces of two metal blocks, between the pistons of the Load
Cell. The sensors were inserted such that the blocks covered up to the edge of the first little squares printed on
the sensor (line A in Fig 0). In practice, the sensors will probably not be inserted so far into the Mechanisms.
To make a measurement, the Load Cell was changed to the required load (this was difficult to control accurately,
because of the large rate of change of force used to move from one measurement to the next). The FlexiForce
sensors exhibit significant drift as a function of time, as the plastic relaxes and compresses, and consequently the
resistance falls. It was found that the fall was most significant in the first 20-30 seconds (see Fig 1) – consistent
http://hepwww.rl.ac.uk/atlas-sct/engineering/material_budget/crude_Mass/EC_crude_Mass.xls
http://www.tekscan.com/flexiforce.html
3 http://hepwww.rl.ac.uk/atlas-sct/engineering/ec/endcap/Integration/Russian_report.doc
4 Contact: Neil Dixon at CERN.
1
2
with the measurements made by the TRT Community. After this, the resistance stabilised somewhat and
readings were typically taken 30-60 seconds after the load was first appplied.
Time Variation
54.5
54
Resistance (kOhm)
53.5
53
52.5
F=257 N
F=904 N /scaled
52
51.5
51
50.5
50
0
20
40
60
80
Time (s)
Fig 1: Time dependence of resistance for two different forces (second curve has been scaled). Note the
significantly offset vertical scale.
2.1 Effect of Sensitive Area
The sensors have a sensitive disc through which all the force should be transmitted. To test this, a sensor was
tested in the standard position (described earlier) and then with some foil discs (each 110 m thick) the same
size as the sensitive disc placed over this area. It was found that the discs made little difference – if anything, one
would expect that more of the force would be transmitted to the sensitive region, lowering the resistance – this
was not observed:
Conditions
Resistance (kOhm) for 1480 N
No discs
11.2
1 disc
12.8
5 disc
12.4
The reason that the discs have little effects is probably because that the sensitive discs are actually a bit thicker
(220 m) than the surrounding inert plastic (200 m).
Because we don’t want to introduce any mechanically instability into the support, we will not use any discs over
the sensitive area.
A test was done, placing the sensor in a different location between the metal blocks used for the measurements.
The sensor was pulled out so that the blocks crossed the sensor at the shoulders on the FlexiForce (line B in
Fig 0). The measurements (Fig 2) gave results which differed by about 10% from the standard measurements,
although rather than increasing the apparent force giving a reduced resistance and increased conductance, the
reverse was seen. To check this wasn’t a repeatability problem, another set of measurements was taken in the
standard position and these agreed with the original measurements.
This is not understood. It is clear that if possible, the sensors should be used as they have been used for these
calibration measurements; otherwise they should be used in a consistent way between various measurements.
FlexiForce Sensors
0.0900
Conductance (mmho)
0.0800
0.0700
0.0600
#7
0.0500
#7, but cover to shoulder
near disc
0.0400
#7, standard position
0.0300
0.0200
0.0100
0.0000
0
500
1000
1500
Force (N)
Fig 2: The effect of positioning the sensors in different locations (see text). The third measurement is a repeat of
the first, and shows consistency.
2.2 Effect of Conditioning
It is advised to condition the sensors 4-5 times to 110% of their max operating load. I found that this didn’t have
a huge effect on the performance, although the TRT Community showed that the performance continues to vary
even after 40 conditioning loads.
Fig 3 shows the set of measurements to calibrate a typical sensor. A set of 4 conditioning loads were applied,
going from 0 N to around 1200 N each time. Then the calibration run was started from 0 N going to ~1200 N in
steps of approximately 100 N.
FlexiForce Sensors
0.0800
0.0700
Conductance (mmho)
0.0600
0.0500
0.0400
#4
0.0300
0.0200
0.0100
0.0000
-500
0
500
1000
1500
-0.0100
Force (N)
Fig 3: Calibration measurements, preceded by conditioning loads.
2.3 Drift Effects
The drift is reported to be 3% in logarithmic time (base 10). This means:
Rmeasured = R0 (1 – (1+log10(t/))
Where a = 0.03, and  is a time-constant. Time t is measured from the time when the force is first applied. It can
be seen that this formula is not straightforward – at t=0, the resistance is infinite. Also, it is not clear what the
value of the time constant is.
If calibration measurements are made after a time tc, then fractional change of resistance having waited for a
time t is
Rmeasured/R  – log10(t/tc)
Leaving the sensor under a load of 1200 N for a period of 18 hours, the fall in R was about 13%, to be compared
with falls of 10% and 9% found with calibration times of 30s and 60s respectively. Taking tc = 30s, to reproduce
this observation, we scale  to 0.04.
3. CALIBRATIONS
We possess 16 FlexiForce sensors, all of which have been calibrated. Fig 4 shows the fits to the calibration data
for one of the sensors.
FlexiForce Sensors
0.0800
0.0700
0.0700
0.0600
0.0600
0.0500
#4
0.0400
Linear
0.0300
0.0200
Conductance (mmho)
Conductance (mmho)
FlexiForce Sensors
0.0500
0.0400
#4
Parabolic
0.0300
0.0200
0.0100
0.0100
0.0000
0.0000
0
500
1000
Force (N)
1500
0
500
1000
1500
Force (N)
Fig 4: Fits to calibration data: linear fit (left), quadratic fit (right).
The relationship between the measured conductance (reciprocal of resistance) and the applied force is
approximately linear. It has been parametrised as:
1/R = F + F2
This can be inverted to give the force as a function of the resistance.
Using this formula for R = 25 k, the corresponding forces have been evaluated for each sensor to allow
comparison. The values for each of the 16 sensors is shown in Fig 5.
Fit at 25 kOhm
800
700
Force (N)
600
500
400
Fit at 25 kOhm
300
200
100
0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Sensor
Fig 5:
4. USING THE SENSORS
Using the FlexiForce sensors is not trivial:
 There are reproducibility problems – I have seen effects around 10%.
 The effect of positioning the sensors in between two surfaces is unclear and indicates effects O(10)%.
 There are drift effects, which affect the calibration and the actual measurements.
 There are hysteresis effects which appear to be O(10)%, depending on the exact definitions and use.
In principle, the sensors should be re-trained and re-calibrated just before use – this is not practical.
I propose to use the sensors as they are, bearing in mind there will be effects of the order of 10-20%, although it
is to be hoped these may be common for various measurements and hence be less important when comparing
measurements. However, this does make it difficult to use the sensors for an absolute measurement.
Using the calibration constants derived from the calibration curves, I have created an inverse function
(“CALCULATOR”) within the calibration spreadsheets5 to calculate for each sensor, the force corresponding to a
given measurement of resistance.
To correct for the drift effects, the resistance should be increased by:
Rcorrection/R   log10(t/30s)
This can be incorporated by supplying the number of hours since the sensor was inserted under the load.
With more time, it would be possible to obtain a better understanding of these sensors, but I do not have this
time. Nevertheless, I think they should be satisfactory for the job in hand, namely ensuring that the End-caps are
well supported by the Mechanisms on the TRT Rails.
5. ACKNOWLEDGEMENTS
Thanks to
 Martin Gibson & Richard Holt for helping me with the DVM and connecting to the sensors.
 Steve Robertson & Simon Canfer for helping me to use the Load Cell at RAL.
 Neil Dixon for explaining about the TRT work.
5
http://hepwww.rl.ac.uk/atlas-sct/engineering/ec/endcap/Integration/Flexiforce_calib.xls
6. MEASUREMENTS WHILE TESTING THE SUPPORTS & MECHANISMS
Brain Smith and Chris Nelson tested the strength of the Front & Rear Supports, as well as the Mechanisms6. At
the same time, they used the FleixForce sensors to compare with the known weights.
4 sensors (#1-4) were used under the Mechanisms at the four corners. The loading was such that each sensor
should have seen much the same weight; although there was no absolute guarantee of this, since it depends on
the precise geometry of the set-up. As it was, a significant weight was measured by each sensor, although these
were far from equal. The fractional weights recorded on all sensors were: 18%, 23%, 28% and 30%.
The total weights recorded are shown in the following table.
Setting up
Without Blankets
With Blankets
Blankets removed
One hour later
Actual mass (kg)
132.9
132.9
332.9
132.9
132.9
FlexiForce value (kg)
42.3
48.0
99.5
36.7
32.8
Ratio
0.68
0.64
0.70
0.72
0.75
It is unclear why the weights recorded by the FlexiForce sensors are so much less than the actual weights – I
would have guessed the determinations would be good to at least 10%.
The most likely explanation is that this arises due to the way in which the Mechanisms were transmitting the
force to the FlexiForce sensors.
Unfortunately immediately after the measurements, the Front & Rear Supports had to be sent to CERN, so it is
impossible to understand this better (combined with the fact that I do not have the time to do so).
My conclusions are:
 The sensors are capable of giving weight measurements with an accuracy of ~10%, with all things
considered.
 For some unknown reason, they do not give a very reliable measurement when used in the Mechanisms.
 At the very least, they can indicate that the weigh is shared between the Mechanisms and the End-cap is
not supported just on 3 points.
6
http://hepwww.rl.ac.uk/atlas-sct/engineering/ec/endcap/Integration/014-TDR061.doc (not yet available)