Tekscan FlexiForce User manual

FlexiForce

Sensors
User Manual
02/26/04 Flexiforce Sensor User Manual (RevC)

Table Of Contents
INTRODUCTION......................................................................................................................... 3
GETTING ASSISTANCE.................................................................................................................. 3
OVERVIEW.................................................................................................................................. 4
SENSORS ...................................................................................................................................... 4
APPLICATION ............................................................................................................................... 4
SENSOR LOADING CONSIDERATIONS............................................................................... 6
SENSOR LOADING ........................................................................................................................ 6
SATURATION................................................................................................................................6
CONDITIONING SENSORS.............................................................................................................. 7
CALIBRATION............................................................................................................................ 8
CALIBRATION GUIDELINES .......................................................................................................... 8
SENSOR PERFORMANCE CHARACTERISTICS................................................................ 9
REPEATABILITY ........................................................................................................................... 9
LINEARITY ................................................................................................................................... 9
HYSTERESIS ................................................................................................................................. 9
DRIFT........................................................................................................................................... 9
TEMPERATURE SENSITIVITY ........................................................................................................ 9
SENSOR LIFE / DURABILITY ....................................................................................................... 10
02/26/04 FlexiForce Sensor User Manual (RevC) 2

INTRODUCTION
This manual describes how to use Tekscan's FlexiForce Sensors. These systems are ideal for
designers, researchers, or anyone who needs to measure forces without disturbing the dynamics
of their tests. The FlexiForce sensors can be used to measure both static and dynamic forces (up
to 1000 lbf.), and are thin enough to enable non-intrusive measurement. System hardware
includes the sensor handle and sensor. The electronics has a 9-pin R3232 (serial) connector. One
of the major advantages of this system is that it is simple to install, and requires very little
hardware.
The FlexiForce sensors use a resistive-based technology. The application of a force to the active
sensing area of the sensor results in a change in the resistance of the sensing element in inverse
proportion to the force applied.
GETTING ASSISTANCE
Tekscan, Inc. will provide technical assistance for any difficulties you may experience using
your FlexiForce system.
Write, call or fax us with any concerns or questions. Our knowledgeable support staff will be
happy to help you. Comments and suggestions are always welcome.
FlexiForce
a division of Tekscan, Inc.
307 West First Street
South Boston, MA 02127-1309
Phone: [phone]
Fax: [phone]
E-mail: [email protected]
Copyright © 2004 by Tekscan, Incorporated. All rights reserved. No part of this publication
may be reproduced, transmitted, transcribed, stored in a retrieval system, or translated into any
language or computer language, in any form or by any means without the prior written
permission of Tekscan, Inc., 307 West First Street, South Boston, MA 02127-1309.
Tekscan, Inc. makes no representation or warranties with respect to this manual. Further,
Tekscan, Inc. reserves the right to make changes in the specifications of the product described
within this manual at any time without notice and without obligation to notify any person of such
revision or changes.
FlexiForce is a registered trademarks of Tekscan, Inc.
Windows 95/98/ME/2000/XP, MS-DOS, Word, Notepad, and Excel are registered trademarks of
Microsoft Corporation.
02/26/04 FlexiForce Sensor User Manual (RevC) 3

OVERVIEW
This section outlines Sensor Construction and Application.
SENSORS
The FlexiForce sensor is an ultra-thin and flexible printed circuit. Sensors are available in three
full-scale force ranges: Low (25 lbf ), Medium (150 lbf ), and High (1000 lbf ). The length of the
sensors can be trimmed by Tekscan to predefined lengths of 2”, 4” and 6” or can be trimmed by
the customer. If the customer trims the sensor they will have to attach a connector to the sensor.
(This can be accomplished by purchasing staked pin connectors and a crimping tool. They can
also use a conductive epoxy to adhere small wires to each conductor). The "active sensing area"
is a 0.375” diameter circle at the end of the sensor. The sensors are constructed of two layers of
substrate, such as a polyester film. On each layer, a conductive material (silver) is applied,
followed by a layer of pressure-sensitive ink. Adhesive is then used to laminate the two layers of
substrate together to form the sensor. The silver circle on top of the pressure-sensitive ink defines
the “active sensing area”. Silver extends from the sensing area to the connectors at the other end
of the sensor, forming the conductive leads. FlexiForce sensors are terminated with a solderable
male square pin connector, which allows them to be incorporated into a circuit. The two outer
pins of the connector are active and the center pin is inactive.
The sensor acts as a variable resistor in an electrical circuit. When the sensor is unloaded, its
resistance is very high (greater than 5 Meg-ohm); when a force is applied to the sensor, the
resistance decreases. Connecting an ohmmeter to the outer two pins of the sensor connector and
applying a force to the sensing area can read the change in resistance.
APPLICATION
There are many ways to integrate the FlexiForce
sensor into an application. One way is to
incorporate it into a force-to-voltage circuit. A
means of calibration must then be established to
convert the output into the appropriate
engineering units. Depending on the setup, an
adjustment could then be done to increase or
decrease the sensitivity of the sensor.
An example circuit is shown above. In this case, a
5 V DC excitation voltage drives it. This circuit uses an inverting operational amplifier
arrangement to produce an analog output based on the sensor resistance and a fixed reference
resistance. An analog-to-digital converter can be used to change this voltage to a digital output.
In this circuit, the sensitivity of the sensor could be adjusted by changing the reference resistance
(RF); a lower reference resistance will make the sensor less sensitive, and increase its active
force range.
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* When used in the circuit shown. However, the dynamic force range of the sensor can
be adjusted by changing the reference resistor (RF) or by changing the Drive Voltage
(VO). (refer to the Saturation section)
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