FSR™ Integration Guide Interlink Electronics FSR™ Force Sensing Resistors™ FSR Integration Guide Document part number 94-00004 Rev B Interlink Electronics and the six dot logo are registered trademarks of Interlink Electronics, Inc.
FSR™ Integration Guide Table of Contents 1.0 Introduction ................................................................................................................... 1 1.1 Our Background................................................................................................................ 1 1.2 Intellectual Property and Other Legal Matters .................................................................. 1 2.0 Theory of Operation ..........................................................
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. 2.2 Force Curve A typical resistance vs. force curve is shown in Figure 3.
FSR™ Integration Guide Figure 3: Resistance vs. Force Immediately after turn-on, the resistance decreases very rapidly. At slightly higher and then intermediate forces, the resistance follows an inverse power law. At the high forces the response eventually saturates to a point where increases in force yield little or no decrease in resistance. Saturation can be pushed higher by spreading the applied force over a larger actuator.
FSR™ Integration Guide 3.0 FSR™ Force Sensing Resistor™ Products Interlink designs and manufactures a broad array of sensor types. The basic FSR described above 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 arrays: Single Zone Single zone sensors can be made in a variety of shapes and sizes. Interlink provides both custom sensors and a standard catalog of round, square, and strip shaped single zone parts.
FSR™ Integration Guide 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. In addition, these pressure sensitive domes can be put into arrays.
FSR™ Integration Guide Linear potentiometer strips can also be formed with arcs. The example below is fabricated on fpc with a board-to-board connector soldered on. Similar to straight linear pots, these can measure position along the arc and force. Full annular ring potentiometers are formed from overlapping sections of arc potentiometers to provide unbroken and smooth 360° detection. Simple measurement algorithms can be provided to interested customers.
FSR™ Integration Guide 3.1 Standard vs. Custom 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" [5mm] Circle Model 402 FSR, 0.5" [12.7mm] Circle Model 406 FSR,1.5" [38.1mm] Square Model 408 FSR, 24" [609.
FSR™ Integration Guide Round Standard round FSRs are offered in both Model 400 (Figure 5) and Model 402 (Figure 6) standard models. They are common and versatile products that can be incorporated into a variety of devices.
FSR™ Integration Guide Measurements: millimeters [inches] Figure 6: Model 402 Round FSR
FSR™ Integration Guide Square The standard Model 406 (Figure 7) square FSR, as compared to the round FSR, offers similar functionality within a larger electrically active area.
FSR™ Integration Guide Strip The standard Model 408 (Figure 8) strip FSR is useful for force detection in large devices.
FSR™ Integration Guide VersaPad The standard VersaPad sensor is useful for positional location and force detection.
FSR™ Integration Guide Ring Sensor The standard ring sensor is useful for radial position location and force detection.
FSR™ Integration Guide 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 your specific requirements. General PARAMETER VALUE NOTES Force Sensitivity Range 10 g to 1.0 kg (0.1 to 10N) Dependent on mechanics Break Force (Activation Force) 10 g (0.
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. 6.0 Measurement Techniques 6.
FSR™ Integration Guide FSR Voltage Divider For a simple force-to-voltage conversion, the FSR device is tied to a measuring resistor in a voltage divider configuration. The output is described by the 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. The measuring resistor, RM, is chosen to maximize the desired force sensitivity range and to limit current.
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 In this circuit, the FSR device is the input of a current-to-voltage converter. The output of this amplifier is described by the equation: With a positive reference voltage, the output of the op-amp must be able to swing below ground, from 0V to –VREF, therefore dual sided supplies are necessary. A negative reference voltage will yield a positive output swing, from 0V to +VREF.
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.
FSR™ Integration Guide Suggested op-amps are LM358 and LM324. Figure 17: Additional FSR Current-to-Voltage Converter FSR Schmitt Trigger Oscillator Figure 18: FSR Schmitt Trigger Oscillator In this circuit, an oscillator is made using the FSR device as the feedback element around a Schmitt Trigger. In this manner, a simple force-to-frequency converter is made. At zero force, the FSR is an open circuit. Depending on the last stage of the trigger, the output remains constant, either high or low.
FSR™ Integration Guide 7.0 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 Housed Female: A stitched on AMP connector with a receptacle (female) ending. A black plastic housing protects the contacts. Suitable for removable ribbon cable connector and header pin attachment. Hysteresis: In a dynamic measurement, the difference between instantaneous force measurements at a given force for an increasing load versus a decreasing load. Repeatability: The ability to repeat, within a tolerance, a previous response characteristic.
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 Can I fold the sensor? The sensor is designed to be flexible; however the sensing area should not be folded as this causes shearing. The traces should not be bent more than 90° as the silver conductive leads could break. Can I adhere the sensor to a surface? If you need to adhere the sensor to a surface, a thin, double-sided tape is recommended. Often the sensors are supplied with such a rear adhesive, covered with a removable liner.
FSR™ Integration Guide What are the minimum and maximum quantities you can do annually? Due to the cost involved, we typically do not design custom sensors for when expected volumes are low. The maximum quantities that can be produced depend on several factors. We have produced specific sensors in volumes as high as 20M pcs per year. What is the average cost of a custom design? Each request is different, depending on size, complexity of design, force ranges, quantities, etc.
FSR™ Integration Guide 9.0 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 10.0 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 11.0 Contact Interlink Electronics United States Corporate Offices 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: fsr@interlinkelectronics.com Japan Interlink Electronics Inc. Japan 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.
