Integration Guide
Force Sensing Potentiometer
To be used in conjunction with current FSP series data-sheets available at www.ohmite.com
Ohmite FSP Series Integration Guide: Force Sensing Potentiometer
v1.0 Mar 2018
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Integration Guide
Force Sensing Potentiometer
CONTENTS
1 Force Sensing Potentiometer (FSP) Overview Page 3
2 FSP01CE/FSP02CE Introduction Page 4
3 FSP01CE/FSP02CE Construction Page 4
4 FSP01CE/FSP02CE Connection and Sampling Page 4
5 FSP01CE/FSP02CE Recommendations Page 6
6 FSP03CE Introduction Page 7
7 FSP03CE Construction Page 7
8 FSP03CE General Theory of Operation Page 8
9 Contact Page 9
Ohmite FSP Series Integration Guide: Force Sensing Potentiometer
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Integration Guide
Force Sensing Potentiometer
1 Force Sensing Potentiometer (FSP) Overview
This guide covers Ohmite’s standard Force Sensing Potentiometer offerings FSP01CE, FSP02CE and FSP03CE. These
sensors operate as both position and force sensors offering users the ability to control menu navigation, device
function, movement, audio control and many other HMI interaction in a more reliable and intuitive manner. Adding
additional opportunities for user interaction, haptic control lighting, and integration methods.
Interfacing to an FSP sensor is simple and can be achieved using a number of different methods either with a
dedicated microcontroller outputting serial data to a host controller, or directly linked to the host with a few simple
external passive components.
This guide provides all the necessary technical information for the successful integration of Ohmite’s force sensing
potentiometers into products such as:
• Media controllers
• Computer and peripherals
• E-readers
• Industrial, scientific or medical devices
• Home automation and lighting control
• Midi controllers
• White goods
• IOT devices
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Integration Guide
VREF_NEG
V1
Force Sensing Potentiometer
2 FSP01CE/FSP02CE Introduction
FSP01CE
Stacked view
A Top layer
B Spacer Adhesive
C Bottom layer
D Mount adhesive
A
B
C
D
Figure 1. Linear Sensor Structure
R
1
Figure 2. Resistance groups on an FSLP
R1
Ohmite's FSP01CE & FSP02CE Force Sensing Potentiometers (FSPs) are high-feature-set, cost-effective touch sensors
enabling intuitive control and navigation. FSPs are “single touch” devices that simultaneously report both touch
position and variable force. They are easy to integrate, high resolution, low-power, and ideal for a wide range of HMI/
MMI applications & markets. Interfacing is simple via a host processor without the need for a dedicated MCU. FSPs are
dynamically reconfi gurable in fi rmware enabling multiple functions from a single sensor.
3 FSP01CE/FSP02CE Construction
Force-Sensing Resistor (FSR) construction can generally be categorized into two types, Shunt Mode or Thru Mode*.
These alternate types exhibit different Force vs. Resistance characteristics. Ohmite's FSP01CE and FSP02CE are based
on Thru mode sensor construction which has solid top and bottom electrodes both over-printed with an FSR layer.
Current passes through the FSR ink from one layer to the other requiring electrical connections on both top and bottom
layers. (See Figure 1.)
4 FSP01CE/FSP02CE Connection and Sampling
Figure 2 shows the general resistance groups in a Force Sensing Potentiometer (FSP). R1 + R2 is the total resistance
of the resistive layer on the Sensor while RW is the Force resistance between the conductive and resistive layer when
force is applied on the Sensor. The actual values of R1 and R2 depend on the location along the length of the Sensor
where the force is applied.
R
W
R
2
V
1
R2
V2
Figure 3 shows the general schematic for how the FSP can be setup for measuring the force being applied to it.
V
WIPER
For best results, a microcontroller with an analog to digital converter (ADC) module should be used to measure the
position and relative force of touch along the length of the sensor.
V
2
The pins shown in Figure 3 need to be connected to the microcontroller as follows:
• V1 – Digital pin
• V2 – ADC pin
• V
– ADC pin
WIPER
• V
– Digital pin
REF_NEG
4.1 Position Measurement
RW
The position of the touch location can be measured similarly to measuring the position of a standard potentiometer.
• Set all lines to 0 Volts to clear any existing charge from the sensor and reduce any noise on the readings
• Setup V1 as an output pin on the microcontroller and make it output a digital HIGH signal.
VWIPER
• Setup V2 as an output pin on the microcontroller and make it output a digital LOW signal.
• V
must be setup as an input pin on the microcontroller and set to LOW (this ensures that no current fl ows
REF_NEG
through R
• Setup V
an ADC measurement, A
) and drains any further charge due to setting the other pins
REF
as an input pin (which ensures that no current fl ows through RW) and wait a few microseconds then take
WIPER
, from the pin. A
POS
represents the voltage across R2 which will be directly proportional to the
POS
position of the touch.
Figure 3. Force measurement setup schematic
A second reading with V1 set to LOW and V2 set to HIGH can be taken to check the validity of the fi rst reading. The second
reading should be roughly equal to the bit count of the ADC - A
POS
For very light touches RW may have a high resistance of 500 Kohms or more therefore depending on the input resistance
of the ADC a high impedance buffer may improve positional measurement accuracy.
*Further details on FSR types can be found in Ohmite's FSR Integration Guide at www.ohmite.com
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Integration Guide
1
VOLTAGE (ADC) * C
1000
1
VOLTAGE (ADC) * C
1000
1
VOLTAGE (ADC) * C
1000
A
A
Force Sensing Potentiometer
4 FSP01CE/FSP02CE Connection and Sampling (continued)
Figure 4. Method 1 test results
Figure 5. Method 2 test results
Figure 6. Method 3 test results
900
800
700
600
500
400
300
200
100
0
0
POINT 1
FORCE (KG)
POINT 2 POINT 3
4.2 Force Measurement
The relative touch force will be proportional to RW. However it is not possible to measure RW independently of R1 and/or R2
and as R1 and R2 change depending on the location of touch the simplest approach of measuring V
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
0.90.80.70.60.4 0.50.30.20.1
• Setup V1 as an output pin on the microcontroller and make it output a digital HIGH signal.
• Setup V
• Setup V2 and V
• Take an ADC measurement, A+, from pin V
• Take an ADC measurement, A-, from pin V
as an output pin on the microcontroller and make it output a digital LOW signal.
REF_NEG
as an input pins
WIPER
2
WIPER
relative to V
WIPER
REF_NEG
• Calculate the relative force using the following formula
A
–
900
800
700
600
500
400
300
200
100
4.2.2 Method 2
0
0
POINT 1
FORCE (KG)
POINT 2 POINT 3
0.90.80.70.60.4 0.50.30.20.1
F =
–
A
(A
)
+
–
• Setup V1 as an output pin on the microcontroller and make it output a digital HIGH signal.
• Setup V
as an output pin on the microcontroller and make it output a digital LOW signal.
REF_NEG
• Setup V2 as an output pin on the microcontroller and make it output a digital HIGH signal.
• Setup V
• Take an ADC measurement, A
• Using the measured analog value of the position, A
as an input pin
WIPER
, from pin V
WIPER
WIPER
, the values for R1 and R2 can be approximated and the
POS
value of RW (the resistance which represents the inverse of the force) can be calculated
POS
p =
ADC
= (1 – p)(R1 + R2)
R
1
MAX
POS
=
1023
R2 = p(R1 + R2)
900
800
700
600
500
400
300
200
100
A
ADC
WIPER
MAX
RW = R
F
=
R1R
R1 + R
REF
1
R
W
R
REF
2
+ RW + R
2
ADC
MAX
V
WIPER
=
p(1 – p)(R1 + R2) + RW + R
REF
– p(1 – p)(R1 + R2) – R
4.2.3 Method 3
0
0
POINT 1
FORCE (KG)
POINT 2 POINT 3
0.90.80.70.60.4 0.50.30.20.1
This method first measures V
microcontroller switches V2 to a HIGH output voltage and V1 to a high impedance pin. V
again. The average of the two measurements will give an approximation for the force.
• Setup V
• Setup V
as an output pin on the microcontroller and make it output a digital LOW signal.
REF_NEG
as an input pin
WIPER
with V1 at a HIGH voltage and V2 as a high impedance pin. Then, the
WIPER
R
REF
REF
REF
will be measured
WIPER
• Setup V1 as an output pin on the microcontroller and make it output a digital HIGH signal.
• Setup V2 as an input pin
• Take an ADC measurement, A
• Setup V2 as an output pin on the microcontroller and make it output a digital HIGH signal.
WIPER_1
, from pin V
WIPER
• Setup V1 as an input pin
• Take an ADC measurement, A
• Take an average of A
WIPER_1
WIPER_2
and A
, from pin VW
to get an estimate for the force
WIPER_2
IPER
Ohmite FSP Series Integration Guide: Force Sensing Potentiometer
1
F
2
(A
WIPER_1
+
A
WIPER_2
)
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Force Sensing Potentiometer
4 FSP01CE/FSP02CE Connection and Sampling (continued)
109.00
3
2
1
10.00
8.00
46.50
44.50
10.00
4.2 Force Measurement (continued)
The test results from these three methods are shown in Figures 4, 5 and 6 and the test positions are shown in Figure
7. The advantages and disadvantages for the three methods are discussed in Table 1 along with the complexity of the
sampling firmware and hardware required. The best method for the project requirements should be chosen considering a
balance of required force accuracy, electronic complexity and cost.
Method Advantages Disadvantages Complexity
1
2
3
• Less dependent on the
position of the applied
force on the FSLP
than method 2 at low
forces
• Linear relationship
between applied force
and ADC output
• Linearity continues
beyond 1kg finger
force
• Only one analog
reading is needed,
which make the circuit
simpler and more
accurate
• More stable and
reliable data
• Produces more stable
and reliable data
• This method is least
dependent on the
position of applied
force
• Applying force near one end of
the pot where the voltage is high,
results in a different ADC output
comparing to other places on the
POT
• The measured data is noisier at
higher forces (can be resolved
by using an ADC with higher
resolution)
• The need to take two analog
readings can introduce
inaccuracies
• Force output is dependent on the
position of applied force on FSLP
• Force output has an exponential
characteristic and can saturate
beyond 1Kg finger force
• At higher forces greater than 500g
ADC output becomes dependent
on the position of applied force
on FSLP
• Data has an exponential
characteristic and starts to
saturate beyond 1kg finger force
2 ADC pins are required. The System
needs some form of averaging in order
to increase the resolution and read
more accurate data. Also, the nature
of the method demands working with
arithmetic and floating points. Hence,
a fast microcontroller (preferably more
than 8Mhz) and relatively complex
hardware is needed.
Only single ADC pin is needed which
makes both the firmware and the
hardware easy to implement.
Two GPIO pins and one ADC pin
are required. Furthermore, the
system needs to constantly toggle
the GPIO pins and use arithmetic
which demands fast microcontroller
(preferably 8Mhz and up) and relatively
complex hardware.
Table 1. Advantages and disadvantages of the three methods for measuring the force of an FSP
5 FSP01CE/FSP02CE Recommendations
65.00
10.00
1 2 3
10.00
Figure 7. Test results for the 3 methods with force measurements
taken at various locations along the FSP (units:mm)
Ohmite FSP Series Integration Guide: Force Sensing Potentiometer
For the majority of force sensing potentiometer implementations Method 1 is most likely the best compromise. It
is a simple approach electronically and outputs a very liner response to force. Even though it suffers from reduced
resolution at higher forces this is generally not a critical requirement for most applications. Furthermore it’s position
dependency is only relevant for finger forces greater than 200-300g which is sufficient for most applications.
Where increased resolution at higher forces is a requirement in the application Methods 2 or 3 can be employed, and if
high finger force consistency is relevant then Method 3 should be chosen.
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Integration Guide
Force Sensing Potentiometer
FSP03CE
Stacked view
A Top layer
B Spacer Adhesive
C Bottom layer
D Mount adhesive
A
B
C
D
Figure 8. Ring Sensor Structure
D3
R3 R1
D4 D2
R2
MCU
D2 D1D3 A0
D4
234 1
6 FSP03CE Introduction
Ohmite's FSP03CE Ring sensor is a force sensing potentiometer which allows highly accurate angular touch position
measurement as well as relative touch force detection. This is achieved with a continuous ring resistor with 3
electrodes placed at 120° around the circle. A wiper layer with FSR ink makes contact with this ring resistor at the
point of touch and the allows for various voltage measurements to be taken to determine the touch position and
relative force in a similar method as an FSP.
The FSP03CE ring sensor can be used for advanced HMI and MMI applications where circular motion and gestures are
required to be used, for example menu navigation, rotation control, or radial position detection.
7 FSP03CE Construction
The FSP03CE Ring Sensor is similar in construction to the FSP01CE and FSP02CE Sensors. It is constructed of 4
primary layers (see Figure 8):
• A top PET layer with graphic, conductive, dielectric, and an FSR ink print,
• A spacer adhesive layer,
• A bottom PET layer also with conductive, dielectric and an FSR ink print
• A mounting adhesive on the rear.
VWIPER
RW
2K
The main active area of the Ring Sensor is a ring of printed carbon ink divided in three arcs by three electrodes placed
on the ring 120° from each other.
Pin name Pin number Description
1 2 3 4
Drive 1 3 First drive electrode on the ring
Drive 1 2 Second drive electrode on the ring
Drive 3 4 Third drive electrode on the ring
Wiper 1 Wiper PIN
Table 2. Pin Out
Figure 10. Ring Sensor Pin Numbers
Any microcontroller which provides the required GPIO pins and an ADC can be used to interface to the sensor.
Figure 9 shows the circuit diagram the ring sensor and connection to the MCU. In this diagram, digital pins 2, 3 and
4 of the MCU are connected to pins 2, 3 and 4 of the sensor respectively. Pin 1, is connected to an analog pin of the
MCU and is also connected to digital pin 1 via a 2KΩ resistor. This pin acts as a virtual ground for measuring the force,
and will be fl oating when calculating the position. Please note that pin 3 of the sensor, is in fact the fi rst electrode (0°
reference).
Figure 9. Circuit Diagram for the Ring Sensor and MCU Connection
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Force Sensing Potentiometer
8 FSP03CE General Theory of Operation
There are 3 basic stages to the scanning procedure for the Ohmite Ring Sensor:
• Stage 1: Detect which two pins are closest to the touch position
• Stage 2: Use these 2 pins to measure the relative position of touch between the pins
• Stage 3: Measure the relative force of the touch by using similar techniques as described above for the FSP
force measurements
8.1 Identifying the pins closest to the touch position
• Drive pin 3 to low voltage, while pins 2 and 4 are high.
• Measure the ADC value of the wiper and save it as a variable, V1 for example.
• Repeat the same process for pins 3 and 4 as well.
• Once you have all the 3 ADC values, comparing them can detect the closest 2 pins. The lowest value would be for
the closest and the second lowest value would be related to the second closest pin. The highest value indicates the
furthest pin from the point of touch.
8.2 Position Measurement
To calculate the angle, the pin furthest from the point of the touch which was determined in the previous section, will be
left floating. Thus, if the furthest pin is pin 4 (as shown in Figure 9 on the previous page), then:
• Configure pin4 as an input pin, so that it floats.
• Drive pin 2 to high voltage and pin 3 to low voltage. This way, the potential is increasing clockwise in the 120°
interval, where the touch is happening.
• Save ADC value of wiper pin as rawAngle.
• Map the rawAngle to angle using this equation:
angle =
Where:
• maxAngle and minAngle are the angles of the 2 closest pins i.e. in Figure 9 D3 is at 0° and D2 is at 120° therefore
• minADC and maxADC are the minimum and maximum achievable ADC values. For example for an 8-bit controller
8.3 Force Measurement
To measure the force all the bottom pins D2, D3 and D4 should be driven high and D1 should be driven low. This creates
a voltage divider circuit with the 2K reference resistor and the ADC value measured will be relative to the force applied to
the sensor. The different force sensing methods described above for use with an FSP sensor can then be used depending
on the required accuracy and constraints of the electronics as discussed.
(rawAngle – minADC) x (maxAngle – minAngle)
(maxADC – minADC)
maxAngle = 120 and minAngle = 0.
these can be assumed to 0 and 256, but for a more accurate touch position these should be measured for a particular
electronic configuration.
+ minAngle1
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Integration Guide
Force Sensing Potentiometer
9 Contact
Headquarters 27501 Bella Vista Parkway, Warrenville IL, 60555 - USA
Toll-Free 1-866-9-OHMITE
International 1-847-258-0300
Fax 1-847-574-7522
Web www.ohmite.com
Email info@ohmite.com
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