v1.0 Mar 2018
8
Ohmite FSP Series Integration Guide: Force Sensing Potentiometer
Integration Guide
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:
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
maxAngle = 120 and minAngle = 0.
• minADC and maxADC are the minimum and maximum achievable ADC values. For example for an 8-bit controller
these can be assumed to 0 and 256, but for a more accurate touch position these should be measured for a particular
electronic configuration.
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.
angle =
(rawAngle – minADC) x (maxAngle – minAngle)
(maxADC – minADC)
+ minAngle1
