30056 FlexiForce Sensor Documentation v2.0

30056 FlexiForce Sensor Documentation v2.0
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FlexiForce Sensor Demo Kit (#30056)
The FlexiForce A201-25 Sensor from Tekscan is piezoresistive force sensor capable of measuring up to 25
lbs of force, perpendicular to its sensing plane. In an electrical circuit, this sensor will act as a variable
resistor, where the change in resistance is inversely proportional to the force applied. The change in
resistance can be measured by a simple RC decay circuit, readable by any Parallax microcontroller.
Application Ideas
Measure both static and dynamic
Simple RC circuit interface
Capable of measuring a total applied
force of 25 lbs
Requires at least 1 PSI on sensing
Versatile small size
Ultra-thin, flexible printed circuit
Compatible with all Parallax
Key Specifications
Touch pads
Alarm systems
Airbag force measurement
Compression moldings
Packing List
Verify that your FlexiForce Demo Kit has the
following components:
FlexiForce Sensor
220 Ω resistor
0.1 μF capacitor
0.01 μF capacitor
Communication: Variable resistance
inversely proportional to applied force
Operating temperature: 15 to 140 °F
(-9 to 60 °C)
Dimensions: 8.00 x 0.55 x 0.008 in
(203 x 9.53 x 0.208 mm)
Copyright © Parallax Inc.
Flexiforce Sensor Demo Kit (#30056)
v1.0 3/1/2010 Page 1 of 3
Device Information
Theory of Operation
The FlexiForce Sensor uses a resistive-based technology, and reads forces perpendicular to its sensing
plane. Its active sensing area is a 0.375” diameter circle located at the top of the sensor. When the
sensing area is unloaded, the resistance is very high (greater than 5 MΩ) and the resistance decreases to
approximately 20 kΩ as force is applied.
Connecting and Testing
The two outer pins of the connector are active, while the center pin is inactive. By connecting an
ohmmeter to the outer two pins, the output resistance of the sensor can be measured.
Before using the FlexiForce Sensor for the first time or after long periods of inactivity, it is highly
recommended that it be conditioned in order to stabilize the output resistance. The output resistance
can fluctuate over the first few tests, and conditioning the sensor ensures repeatable results.
To condition the FlexiForce sensor, place 110% of the maximum test weight on the active sensing area
and let it sit for a few minutes. Repeat this process another four to five times to guarantee accurate
Sensor Loading
The active sensing area of the FlexiForce Sensor should be treated as a single contact point, with the
load distributed evenly across the sensing area. For best results, the loaded area should cover 70-100%
of the sensing area and apply a minimum of 1 PSI.
If the load is smaller than the active sensing area, make sure the load is centered in the sensing area and
not near the edges.
If the load is larger than the active sensing area, you may need to make use of a “puck.” A “puck” is a
piece of rigid material smaller than the active sensing area that is placed between the object and the
sensing area. This will cause the load path go through the puck to increase the PSI, resulting in more
accurate measurements.
Connection Diagram
Copyright © Parallax Inc.
Flexiforce Sensor Demo Kit (#30056)
v1.0 3/1/2010 Page 2 of 3
BASIC Stamp® 2 Program
FlexiForce_Simple.bs2 uses the Debug Terminal to display the current decay time measurement of the
example circuit shown above. Larger decay time values indicate smaller forces, while smaller decay time
values indicate larger forces.
' Flexiforce_Simple.bs2
' Displays R/C Discharge Time in BASIC Stamp DEBUG Window
' {$STAMP BS2}
' {$PBASIC 2.5}
' -----[ Declarations ]---------------------------------------------------time
' Stores decay time output
' Flexiforce sensor circuit
' -----[ Main Routine ]---------------------------------------------------DO
HIGH sensorPin
RCTIME sensorPin, 1, time
DEBUG HOME, "time = ", DEC5 time, CR
' Discharge the capacitor
' Measure RC charge time
Resources and Downloads
You may download free documentation, example programs, and manufacturer datasheet from the
FlexiForce product page on www.parallax.com. Search “30056” from the home page.
See also http://www.tekscan.com/flexiforce.html for manufacturer information.
Copyright © Parallax Inc.
Flexiforce Sensor Demo Kit (#30056)
v1.0 3/1/2010 Page 3 of 3
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