FSR® Integration Guide Interlink Electronics FSR® Force Sensing Resistors® FSR® Integration Guide Document part number 94-00004 Rev. C Interlink Electronics and the six dot logo are registered trademarks of Interlink Electronics, www.interlinkelectronics.
FSR® Integration Guide Table of Contents 1.0 Introduction ...............................................................................................1 2.0 Theory of Operation...................................................................................2 3.0 FSR® Force Sensing Resistor® Products..................................................5 4.0 Performance Specifications ....................................................................15 5.0 Environmental and Reliability Data......
FSR® Integration Guide 1.0 Introduction 1.1 Our Background Launched in 1985, Interlink Electronics is the world's leading innovator of cost effective polymeric force sensors. Our R&D team has developed a spectrum of technologies for “touch” and user interfaces solutions, and machine process controls.
FSR® Integration Guide 2.0 Theory of Operation The most basic FSR consists of two membranes separated by a thin air gap. The air gap is maintained by a spacer around the edges and by the rigidity of the two membranes. One of the membranes has two sets of interdigitated fingers that are electrically distinct, with each set connecting to one trace on a tail. The other membrane is coated with FSR ink. When pressed, the FSR ink shorts the two traces together with a resistance that depends on applied force.
FSR® Integration Guide Figure 2: FSR Ink Micrograph The conductive traces are typically screen printed from silver polymer thick film ink. However, these traces may also be formed out of gold plated copper as on flexible or standard circuit boards (FPC or PCB). Force may be applied to either substrate. One of the exterior surfaces typically includes a mounting adhesive layer to allow mounting to a clean, smooth, rigid surface. www.interlinkelectronics.
FSR® Integration Guide 2.2 Force Curve A typical resistance vs. force curve is shown in Figure 3. For interpretational convenience, the data is plotted on logarithmic scales. This particular force-resistance curve was measured from a model 402 sensor (12.7 mm diameter circular active area). A silicone rubber actuator with a 4 mm spherical radius tip and 60 Shore A durometer was used to press on the FSR).
FSR® Integration Guide 3.0 FSR® Force Sensing Resistor® Products Interlink designs and manufactures a broad array of sensor types. The basic FSR described in Figure 1 may be made in almost any shape or size and can even made to detect position in addition to force. All of these products may be combined into sensor arrays. Single Zone FSR 400 Series Single zone sensors can be made in a variety of shapes and sizes.
FSR® Integration Guide Custom Pressure Sensitive Snap Dome In applications requiring tactile feedback, such as buttons in consumer electronics, the usual method is to use a metallic snap dome. This basic switch function can be enhanced by adding force measurement with an FSR. The dome and FSR are built together into one sensor. Force can be measured both pre- and post-snap. This enables analog control functions such as zoom, scroll, volume, etc.
FSR® Integration Guide 3.1 Standard Standard FSRs deliver the most cost competitive solutions for a wide variety of applications. Cost savings are primarily achieved through reductions in tooling and engineering labor costs. The Interlink catalog of standard single zone FSRs is comprised of round, square, and strip sized sensors. PART TYPE DESCRIPTION Model 400 FSR, 0.2" [5.08mm] Circle Model 400 Short Tail FSR, 0.2" [5.08mm] Circle Model 402 FSR, 0.5" [12.
FSR® Integration Guide Standard round FSRs are offered in both Model 400 (Figures 5 & 6) and Model 402 (Figures 7 & 8) standard models. They are common and versatile products that can be incorporated into a variety of devices. Model 400 Round Exploded View Figure 5: Model 400 Round FSR www.interlinkelectronics.
FSR® Integration Guide Model 400 Round Short Tail Measurements: millimeters Exploded View Figure 6: Model 400 Short Tail Round FSR www.interlinkelectronics.
FSR® Integration Guide Model 402 Round Measurements: millimeters Exploded View Figure 7: Model 402 Short Tail Round FSR www.interlinkelectronics.
FSR® Integration Guide Model 402 Round Short Tail Measurements: millimeters Exploded View Figure 8: Model 402 Short Tail Round FSR www.interlinkelectronics.
FSR® Integration Guide The standard Model 406 (Figure 9) square FSR, as compared to the round FSR, offers similar functionality within a larger electrically active area. Model 406 Square Exploded View Figure 9: Model 406 Square FSR www.interlinkelectronics.
FSR® Integration Guide The standard Model 408 (Figure 10) strip FSR is useful for force detection in large devices. Model 408 Strip Measurements: millimeters Exploded View Figure 10: Model 408 Strip FSR www.interlinkelectronics.
FSR® Integration Guide 3.2 Custom Sensors Custom sensors offer flexibility in meeting the needs of unique customer design requirements. All strip, ring, pad, pot, array, and 4 zone sensors are applicable. Below are some of the typical customization options available. Contact your Interlink representative for additional details, custom sensor examples, and to learn more about the Custom Design Process. Shapes and Sizes Interlink custom sensors come in a variety of shapes, sizes, and zone quantities.
FSR® Integration Guide 4.0 Performance Specifications Below are typical parameters. The FSR is a custom device and can be made for use outside these characteristics. Consult us for your specific requirements. General PARAMETER VALUE NOTES Force Sensitivity Range ~0.2 to 20N Dependent on mechanics Break Force (Activation Force) ~0.2N min Dependent on mechanics and FSR build Part-to-Part Force Repeatability ± 6% of established nominal With a repeatable actuation system, single lot.
FSR® Integration Guide Environmental Performance Specifications PARAMETER TYPICAL R CHANGE NOTES Hot Operation -15% 85°C after 1 hour Cold Operation -5% -40ºC after 1 hour soak Hot Humid Operation +10% +85°C, 95% RH, after 1hour Hot or Cold Storage -10% -25ºC to +85°C, 120hrs Hot Humid Storage Temperature + 30% of established nominal resistance +85°C, 95% RH, 240 hours Thermal Shock ± 2% typical -25ºC to +70°C, 10 Cycles, 15 minute dwell, 5 minute transitions Note: Specifications are d
FSR® Integration Guide Linear Pots PARAMETER VALUE NOTES Positional Resolution 0.075 to 0.5 mm (0.003” to 0.02”) Dependent on actuator size and electronics and exact design Positional Accuracy Better than ± 2% of full length 5.0 Environmental and Reliability Data Contact your Interlink Representative for full details. www.interlinkelectronics.
FSR® Integration Guide 6.0 6.1 Measurement Techniques Circuit Voltage Divider Figure 9: FSR Voltage Divider FSR Voltage Divider For a simple force-to-voltage conversion, the FSR device is tied to a measuring resistor in a voltage divider (see figure below) and the output is described by the following equation: VOUT R MV RM RFSR In the shown configuration, the output voltage increases with increasing force. If RFSR and RM are swapped, the output swing will decrease with increasing force.
FSR® Integration Guide Multi-Channel FSR-to-Digital Interface Figure 12: Multi-Channel FSR-to-Digital Interface Sampling Cycle (any FSR channel): The microcontroller switches to a specific FSR channel, toggling it high, while all other FSR channels are toggled low. The RESET channel is toggled high, a counter starts and the capacitor C1 charges, with its charging rate controlled by the resistance of the FSR (t ~ RC).
FSR® Integration Guide FSR Variable Force Threshold Switch Figure 13: FSR Variable Force Threshold Switch This simple circuit is ideal for applications that require on-off switching at a specified force, such as touch-sensitive membrane, cut-off, and limit switches. For a variation of this circuit that is designed to control relay switching, please see the next page. The FSR device is arranged in a voltage divider with RM. An op-amp, U1, is used as a comparator. The output of U1 is either high or low.
FSR® Integration Guide FSR Variable Force Threshold Relay Switch Figure 14: FSR Variable Force Threshold Relay Switch This circuit is a derivative of the simple FSR Variable Force Threshold Switch on the previous page. It has use where the element to be switched requires higher current, like automotive and industrial control relays. The FSR device is arranged in a voltage divider with RM. An op-amp, U1, is used as a comparator. The output of U1 is either high or low.
FSR® Integration Guide FSR Current-to-Voltage Converter Figure 15: FSR Current-to-Voltage 5 VOUT (V) 4 3 2 7.5 k 4.7 k 2.5 k 1 1.5 k 0 0 200 400 600 800 1000 FORCE (g) In this circuit, the FSR device is the input of a current-to-voltage converter.
FSR® Integration Guide The current through the FSR device should be limited to less than 1 mA/square cm of applied force. As with the voltage divider circuit, adding a resistor in parallel with RFSR will give a definite rest voltage, which is essentially a zero-force intercept value. This can be useful when resolution at low forces is desired. Additional FSR Current-to-Voltage Converters These circuits are a slightly modified version of the current-to-voltage converter detailed on the previous page.
FSR® Integration Guide The output swing of this circuit is from (VREF/2) to VREF. In the case where RG is greater than RFSR, the output will go into positive saturation. For either of these configurations, a zener diode placed in parallel with RG will limit the voltage built up across RG. These designs yield one-half the output swing of the previous circuit, but only require single sided supplies and positive reference voltages.
FSR® Integration Guide 7.0 Performance Optimization For best results, follow these seven steps when beginning any new product design, proof-of-concept, technology evaluation, or first prototype implementation: 1. Start with Reasonable Expectations (Know Your Sensor) The FSR sensor is not a strain gauge, load cell, or pressure transducer. While it can be used for dynamic measurement, only qualitative results are generally obtainable.
FSR® Integration Guide Keep actuation cycle time consistent. Because of the time dependence of the FSR resistance to an applied force (drift), it is important when characterizing the sensor system to assure that increasing loads (e.g. force ramps) are applied at consistent rates (cycle-to-cycle). Likewise, static force measurements must take into account FSR mechanical setting time.
FSR® Integration Guide 8.0 FAQ Below are answers to our most frequently asked questions: What are some applications in which the Interlink sensors have been used? Interlink sensors provide economical solutions and OEM tools to a variety of force measurement applications. Our sensors have been integrated into drug delivery devices, QA/QC equipment, industrial controls, sports and recreational gear, and more.
FSR® Integration Guide What drive voltages can I apply to the sensor? Electrically the sensors look like passive resistors. Any voltage that suits your circuit is fine. From 0.1V (as long as signal-to-noise (S/N) ratio remains acceptable) to 5V is the typical range. What is the resistance range of the sensor? The resistance range of the sensor is typically from >1MΩ at no load to approximately 1kΩ at full load. This can vary depending on the details of the sensor and actuating mechanics.
FSR® Integration Guide 9.0 FSR Usage: The Do’s and Don’ts Do follow the seven steps of the FSR Integration Guide. Do, if possible, use a firm, flat and smooth mounting surface. Do be careful if applying FSR devices to curved surfaces. Pre-loading of the device can occur as the two opposed layers are forced into contact by the bending tension. The device will still function, but the dynamic range may be reduced and resistance drift could occur.
FSR® Integration Guide 10.0 Glossary Terminology Active Area: The area of an FSR device that responds to normal force with a decrease in resistance. This is typically the central area of the sensor more than 0.5mm from the inside edge of the spacer. Actuator: An object that contacts the sensor surface and applies force to FSRs. Applied Force: The force applied by the actuator on the sensor active area.
FSR® Integration Guide Response Characteristic: The relationship of force or pressure vs. resistance. Saturation Pressure: The pressure level beyond with the FSR response characteristic deviates from its inverse power law characteristic. Past the saturation pressure, increases in force yield little or no decrease in resistance. Spacer Adhesive: The adhesive used to laminate FSR devices tighter. Dictates standoff.
FSR® Integration Guide 12.0 Contact Interlink Electronics United States Corporate Office Interlink Electronics, Inc. 546 Flynn Road Camarillo, CA 93012, USA Phone: +1-805-484-8855 Fax: +1-805-484-9457 Web: www.interlinkelectronics.com Sales and support: sales@interlinkelectronics.com Japan Japan Sales Office Kannai-Keihin Bldg. 10F/1004 2-4-2 Ougi-cyo, Naka-ku Yokohama-shi, Kanagawa-ken 231-0027 Japan Phone: +81-45-263-6500 Fax: +81-45-263-6501 Web: www.interlinkelec.co.jp www.interlinkelectronics.
