v1.0 Mar 2018
5
Ohmite FSP Series Integration Guide: Force Sensing Potentiometer
Integration Guide
Force Sensing Potentiometer
4 FSP01CE/FSP02CE Connection and Sampling
(continued)
4.2 Force Measurement
The relative touch force will be proportional to R
W
. However it is not possible to measure R
W
independently of R
1
and/or R
2
and as R
1
and R
2
change depending on the location of touch the simplest approach of measuring V
WIPER
relative to V
REF_NEG
will yield a different result for the same relative force at different points along the sensor.
A number of different methods are explained below that can be used to measure the touch force, each of which has it’s
own advantages and disadvantages. These are further discussed in
Table 1
on the following page.
4.2.1
Method 1
• Setup V
1
as an output pin on the microcontroller and make it output a digital HIGH signal.
• Setup V
REF_NEG
as an output pin on the microcontroller and make it output a digital LOW signal.
• Setup V
2
and V
WIPER
as an input pins
• Take an ADC measurement, A
+
, from pin V
2
• Take an ADC measurement, A
-
, from pin V
WIPER
• Calculate the relative force using the following formula
4.2.2 Method 2
• Setup V
1
as an output pin on the microcontroller and make it output a digital HIGH signal.
• Setup V
REF_NEG
as an output pin on the microcontroller and make it output a digital LOW signal.
• Setup V
2
as an output pin on the microcontroller and make it output a digital HIGH signal.
• Setup V
WIPER
as an input pin
• Take an ADC measurement, A
WIPER
, from pin V
WIPER
• Using the measured analog value of the position, A
POS
, the values for R
1
and R
2
can be approximated and the
value of R
W
(the resistance which represents the inverse of the force) can be calculated
4.2.3
Method 3
This method first measures V
WIPER
with V
1
at a HIGH voltage and V
2
as a high impedance pin. Then, the
microcontroller switches V
2
to a HIGH output voltage and V
1
to a high impedance pin. V
WIPER
will be measured
again. The average of the two measurements will give an approximation for the force.
• Setup V
REF_NEG
as an output pin on the microcontroller and make it output a digital LOW signal.
• Setup V
WIPER
as an input pin
• Setup V
1
as an output pin on the microcontroller and make it output a digital HIGH signal.
• Setup V
2
as an input pin
• Take an ADC measurement, A
WIPER_1
, from pin V
WIPER
• Setup V
2
as an output pin on the microcontroller and make it output a digital HIGH signal.
• Setup V
1
as an input pin
• Take an ADC measurement, A
WIPER_2
, from pin VW
IPER
• Take an average of A
WIPER_1
and A
WIPER_2
to get an estimate for the force
Figure 4.
Method 1 test results
0
1
0.9
0.8
0.7
0.6
0.4 0.5
0.3
0.2
0.1
FORCE (KG)
POINT 1
POINT 2
POINT 3
VOLTAGE (ADC) * C
1000
0
100
200
300
400
500
600
700
800
900
Figure 5.
Method 2 test results
0
1
0.9
0.8
0.7
0.6
0.4 0.5
0.3
0.2
0.1
FORCE (KG)
POINT 1
POINT 2
POINT 3
VOLTAGE (ADC) * C
1000
0
100
200
300
400
500
600
700
800
900
Figure 6.
Method 3 test results
0
1
0.9
0.8
0.7
0.6
0.4 0.5
0.3
0.2
0.1
FORCE (KG)
POINT 1
POINT 2
POINT 3
VOLTAGE (ADC) * C
1000
0
100
200
300
400
500
600
700
800
900
F =
A
–
(A
+
– A
–
)
F
1
R
W
p =
A
POS
ADC
MAX
=
A
POS
1023
R
2
= p(R
1
+ R
2
)
R
1
= (1 – p)(R
1
+ R
2
)
=
R
REF
A
WIPER
ADC
MAX
=
R
REF
p(1 – p)(R
1
+ R
2
) + R
W
+ R
REF
+ R
W
+ R
REF
R
1
R
2
R
1
+ R
2
R
W
= R
REF
– p(1 – p)(R
1
+ R
2
) – R
REF
ADC
MAX
V
WIPER
1
2
(A
WIPER_1
+ A
WIPER_2
)
F
