Flexiforce Sensors Manual - Bio
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1. Tekscan FlexiForce Sensors User Manual 09 23 05 Flexiforce Sensor User Manual RevF Table Of Contents GETTING ASSISTANCE A a AE 3 SENSORS ia 4 APPLICATION a tt dt tt ridad 5 SENSOR LOADING CONSIDERATIONS 1senssnsnnzznssneensenzonsensensensonnenssensensensensensensensenneese SENSOR LOADING ab Saba bub a da Sa a A 6 SATURATIONI a ai G il e ie jena 6 CONDIMONING SENSOR Soi da es a a i ici 7 CALIBRATION GUIDELINES i ib ba couch bur ba a iia 8 SENSOR PERFORMANCE CHARACTERISTICS ccccssssssssssssssssssssssssssssssssssssssssseees Y REPEATABIIT Y a TEE O RNA IAN DI a E eo ELLE hat 9 EINEARI TY ora ie 9 TEMPERATURE SENSITIVITY ccsssssscscccccccccesssscscccccccceesssescccccssceeesssssesccssceeeesessescccseeesensnssosecs 9 SENSOR L1EE DURABILI 0 dr Es 10 SENSOR PROPERTIES iii ie line E ia 10 09 23 05 FlexiForce Sensor User Manual RevF 2 INTRODUCTION This manual describes how to use Tekscan s FlexiForce Sensors These sensors are ideal for designers researchers or anvone who needs to measure forces without disturbing the dvnamics of their tests The FlexiForce sensors can be used to measure both static and dynamic forces up to 1000 Ibf and are thin enough to enable non intrusive measurement 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 element2. S 6800 a 3 oo a 600 E 3 0 008 3 400 0 006 LU e 00 0 004 0 002 0 PRERESFERZZSZRS A La ra 0 20 40 60 80 100 120 140 Force Force CALIBRATION GUIDELINES The following guidelines should be considered when calibrating a sensor e Apply a calibration load that approximates the load to be applied during system use using dead weights or a testing device such as an MTS or Instron If you intend to use a puck during testing also use it when calibrating the sensor See Sensor Loading Considerations for more information on using a puck e Avoid loading the sensor to near saturation when calibrating If the sensor saturates at a lower load than desired adjust the Sensitivity e Distribute the applied load evenly across the sensing area to ensure accurate force readings Readings may vary slightly if the load distribution changes over the sensing area Note Read the Sensor Performance Characteristics section before performing a Calibration 09 23 05 FlexiForce Sensor User Manual RevF 8 SENSOR PERFORMANCE CHARACTERISTICS There are a number of characteristics of sensors which can affect your results This section contains a description of each of these conditions and recommendations on how to lessen their effects REPEATABILITY Repeatability is the ability of the sensor to respond in the same way to a repeatedly applied force As with most measurement devices it is customary to exercise or condition a s
3. do not become saturated during testing CONDITIONING SENSORS Exercising or Conditioning a sensor before calibration and testing is essential in achieving accurate results It helps to lessen the effects of drift and hysteresis Conditioning is required for new sensors and for sensors that have not been used for a length of time To condition a sensor place 110 of the test weight on the sensor allow the sensor to stabilize and then remove the weight Repeat this process four or five times The interface between the sensor and the test subject material should be the same during conditioning as during calibration and actual testing IMPORTANTI Sensors must be properly conditioned prior to calibration and use 09 23 05 FlexiForce Sensor User Manual RevF 7 CALIBRATION Calibration is the method bv which the sensor s electrical output is related to an actual engineering unit such as pounds or Newtons To calibrate applv a known force to the sensor and equate the sensor resistance output to this force Repeat this step with a number of known forces that approximate the load range to be used in testing Plot Force versus Conductance 1 R A linear interpolation can then be done between zero load and the known calibration loads to determine the actual force range that matches the sensor output range Resistance Curve Conductance Curve 100Ib Sensor B 0 02 0 018 1200 0 016 1000 0 014 tf E 8 0 012
4. 2 5 of full scale Repeatability conditioned sensor 80 force applied lt 4 5 of full scale Hysteresis conditioned sensor 80 force applied lt 3 per logarithmic time Drift scale constant load of 90 sensor rating Output variance up to 0 2 per degree F approximately 0 36 per degree C For loads gt 10 Ibs operating temperature can be increased to 165 F 74 C Temperature Sensitivity 09 23 05 FlexiForce Sensor User Manual RevF
5. SENSORS The FlexiForce sensor is an ultra thin and flexible printed circuit which can be easilv integrated into most applications With its paper thin construction flexibilitv and force measurement abilitv the FlexiForce force sensor can measure force between almost anv two surfaces and is durable enough to stand up to most environments FlexiForce has better force sensing properties linearity hysteresis drift and temperature sensitivity than any other thin film force sensors The active sensing area is a 0 375 diameter circle at the end of the sensor The A201 sensor is available in the following force ranges e Sensor A201 1 0 1 Ib force range e Sensor A201 25 0 25 Ib force range e Sensor A201 100 0 100 Ib force range In order to measure forces above 100 lbs up to 1000 lbs apply a lower drive voltage and reduce the resistance of the feedback resistor 1kQumin See the sample drive circuit below The sensors are constructed of two layers of substrate This substrate is composed of 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 sensor
6. 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 617 464 4500 Fax 617 464 4266 E mail flexiforce tekscan com Copyright O 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 09 23 05 FlexiForce Sensor User Manual RevF 3 OVERVIEW This section outlines Sensor Construction and Application
7. is driven by a 5 V DC excitation voltage This circuit uses an inverting operational amplifier arrangement to produce an analog output based on the sensor resistance and a fixed reference resistance Ry 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 Rp a lower reference resistance will make the sensor less sensitive and increase its active force range Va Vo R Rg where R R R out T R IKO R 20KQ R s1 SENSOR SENSOR 3 Pins on 3 Pin Sensor Connector V 5V lt U1 MC34071AP Plastic Dip Package U1 MC34071AD Gia Mount Package e Max recommended current 2 5 mA e No Load Resistance Approximately 20 MQ e Full Load Resistance gt 20 kO e Possible Overload Resistance gt 5 KQ The two supply voltages 9V and 9V and V 5V should remain constant In the circuit shown the dynamic force range of the sensor can be adjusted by changing the reference resistor Rr or by changing the Drive Voltage Vo Refer to the Saturation section for additional information 09 23 05 FlexiForce Sensor User Manual RevF SENSOR LOADING CONSIDERATIONS The following general sensor loading guidelines can be applied to most applications and will help you achieve the most accurate results from your tests It is important that you read the Sensor Performance Characteri
8. E SENSITIVITY In general your results will vary if you combine high loads on the sensor with high temperatures To ensure accuracy calibrate the sensor at the temperature at which it will be used in the application If the sensor is being used at different temperatures perform a calibration at each of these temperatures save the calibration files then load the appropriate calibration file when using the sensor at that temperature 09 23 05 FlexiForce Sensor User Manual RevF 9 SENSOR LIFE DURABILITY Sensor life depends on the application in which it is used Sensors are reusable unless used in applications in which they are subjected to severe conditions such as against sharp edges or shear forces FlexiForce sensors have been successfully tested at over one million load cycles using a 50 lb force Rough handling of a sensor will also shorten its useful life For example a sensor that is repeatedly installed in a flanged joint will have a shorter life than a sensor installed in the same joint once and used to monitor loads over a prolonged period After each installation visually inspect your sensors for physical damage It is also important to keep the sensing area of the sensor clean Any deposits on this area will create uneven loading and will cause saturation to occur at lower applied forces Sensor Properties Model A201 Operating Range 15 F 9 C to 140 F 60 C Linearity Error lt 5 lt
9. ensor before calibrating it or using it for measurement This is done to reduce the amount of change in the sensor response due to repeated loading and unloading A sensor is conditioned by loading it to 110 of the test weight four or five times Follow the full procedure in the Conditioning Sensors section LINEARITY Linearity refers to the sensor s response digital output to the applied load over the range of the sensor This response should ideally be linear and any non linearity of the sensor is the amount that its output deviates from this line HYSTERESIS Hysteresis is the difference in the sensor output response during loading and unloading at the same force For static forces and applications in which force is only increased and not decreased the effects of hysteresis are minimal If an application includes load decreases as well as increases there may be error introduced by hysteresis that is not accounted for by calibration DRIFT Drift is the change in sensor output when a constant force is applied over a period of time If the sensor is kept under a constant load the resistance of the sensor will continually decrease and the output will gradually increase It is important to take drift into account when calibrating the sensor so that its effects can be minimized The simplest way to accomplish this is to perform the sensor calibration in a time frame similar to that which will be used in the application TEMPERATUR
10. s 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 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 a new connector must be attached This can be accomplished by purchasing staked pin connectors and a crimping tool A conductive epoxy can also be used to adhere small wires to each conductor 09 23 05 FlexiForce Sensor User Manual RevF 4 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 Sensors should be stored at temperatures in the range of 15 F 9 C to 165 F 74 C 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 below In this case it
11. stics section for further information on how to get the most accurate results from your sensor readings SENSOR LOADING The entire sensing area of the FlexiForce sensor is treated as a single contact point For this reason the applied load should be distributed evenly across the sensing area to ensure accurate and repeatable force readings Readings may vary slightly if the load distribution changes over the sensing area Note that the sensing area is the silver circle on the top of the sensor only It is also important that the sensor be loaded consistently or in the same way each time If the footprint of the applied load is smaller than the sensing area the load should not be placed near the edges of the sensing area to ensure an even load distribution It is also important to ensure that the sensing area is the entire load path and that the load is not supported by the area outside of the sensing area If the footprint of the applied load is larger than the sensing area it may be necessary to use a puck A puck is a piece of rigid material smaller than the sensing area that is placed on the sensing area to ensure that the entire load path goes through this area The puck must not touch any of the edges of the sensing area or these edges may support some of the load and give an erroneous reading The FlexiForce sensor reads forces that are perpendicular to the sensor plane Applications that impart shear forces could reduce
12. the life of the sensor If the application will place a shear force on the sensor 1t should be protected by covering it with a more resilient material If it is necessary to mount the sensor to a surface it is recommended that you use tape when possible Adhesives may also be used but make sure that the adhesive will not degrade the substrate polyester material of the sensor before using it in an application Adhesives should not be applied to the sensing area however if it is necessary ensure that the adhesive is spread evenly Otherwise any high spots may appear as load on the sensor SATURATION The Saturation force is the point at which the device output no longer varies with applied force The saturation force of each sensor is based on the maximum recommended force specified by Tekscan which is printed on the system packaging or the actual sensor along with the Sensitivity 09 23 05 FlexiForce Sensor User Manual RevF The saturation value is based on using the circuit and the values shown in the example circuit in the Application section In this example the saturation force maximum force of each sensor is related to the RF reference resistance and can be altered bv changing the sensitivitv The sensitivitv of the sensor would be adjusted bv changing the reference resistance RF a lower reference resistance will make the svstem less sensitive and increase its active force range It is essential that the sensor s
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