FSR Design Guide: Thin, Durable & Precise Sensors

1. The Ultimate Force Sensing Resistor Design Guide

2. Table of Contents Introduction Types of FSRs FSR Form Factors Design Limitations FSR Datasheet 3 3 3 4 4 6 8 9 Butler Technologies, Inc. What Is a Force Sensing Resistor (FSR)? ShuntMode vs. ThruMode High Resistance Ink vs Low Resistance Ink Choosing the Correct FSR 10 14 16 2

3. Introduction Butler Technologies, Inc. Prototyping and full-scale manufacturing Headquartered in Western Pennsylvania, Butler Technologies, Inc. (BTI) was founded in 1990 as a humble printing brokerage firm and has grown into an innovative printed electronics design firm and manufacturer. As a manufacturing facility, BTI plays a vital role as a developer, helping clients turn ideas and needs into functional product, with our proof- of-concept development, prototyping and R&D activities. Butler Technologies, Inc. is committed to delivering the highest-quality force sensing resistor (FSR) on the market. If your company requires a custom-designed FSR, whether as a standalone product or part of a larger concept, BTI has a solution for you.  If you would like to learn more about our collaborative design process, rigorous quality standards, and competitive pricing, give us a call today. We produce quality FSRs for a variety of industries including home healthcare, medical devices, fire & safety equipment, and performance athletics. What Is a Force Sensing Resistor (FSR)? A force sensing resistor (FSR) is a variable resistor, constructed of several thin flexible layers, that varies in resistance as pressure is applied and released. As pressure is applied, the resistance lowers and then returns to its original value as the pressure is removed. An FSR can take many shapes and sizes depending on the application’s demands. What Is Its Purpose? The FSR’s main purpose is to measure the force applied to a specific area and then relay that information via selected output electronics. Although force sensing is in the name, an FSR senses pressure (force per area, PSI) rather than force. This information is then relayed via selected output electronics. All FSRs use resistive carbon-based inks which, along with other design factors, can be re-formulated to alter the functionality. 3

4. Types of FSRs ShuntMode vs. ThruMode When deciding between the types of FSRs, there are two main types of construction that we can choose from: Shunt or Thru mode. The construction of each type of FSR is simple, but the technology behind each design will give us advantages for different applications. Single Zone ShuntMode FSR This type of FSR consists of printed silver interdigitated fingers that are covered by a single layer of printed carbon. This construction is then usually put on a flexible substrate for ease of production. As a force is applied to the surface, the silver fingers are pressed against the carbon layer to create a short. This resistance of the short is then interpreted as pressure on the FSR. ShuntMode FSR Cross Section 4

5. Types of FSRs Single Zone ThruMode FSR ThruMode construction is made by having a solid semi-conductive FSR element over a solid conductive area. They are then laid on top of each other facing one another. When a force is applied, the two conductive pads make contact and allow electricity to pass from one conductive pad to another. Lighter forces will produce a high resistive output while higher forces create lower resistive outputs. In general, the cost per unit is more than ShuntMode due to the increased ink that is needed to create the conductive pads. When a light force is applied to the carbon pads, the resistance output is relatively high. When a larger force is applied, the resistance output is lower. ThruMode FSR Cross Section 5

6. Types of FSRs High Resistance Ink vs Low Resistance Ink ShuntMode FSR • High resistance ink will give you a wider range of resistive outputs when a force is applied • Active sensing range: 2.5-85 psi (1” diameter FSR) • Low resistance ink will give you a smaller range of resistive outputs when a force is applied • Active sensing range: 2.5-20 psi (1” diameter FSR) 1” ShuntMode FSR Pressure (psi) vs Resistance output (ohms) for Different Resistant Inks 6

7. Types of FSRs ThruMode FSR • High resistance ink will give you a wider range of resistive outputs when a force is applied • Active sensing range: 3-40 psi (1” diameter FSR) • Low resistance ink will give you a smaller range of resistive outputs when a force is applied • Active sensing range: 2-18 psi (1” diameter FSR) 1” ThruMode FSR Pressure (psi) vs Resistance output (ohms) for Different Resistant Inks In comparing the different inks for ThruMode and ShuntMode, the relationship is consistent across both types of FSRs. High resistive ink will give you a wider range of resistive outputs while low resistive ink will give you a narrower range of resistive outputs. 7

8. Types of FSRs Choosing the Correct FSR It is imperative that right FSR is chosen for the application. If not, you may get a consistent resistance output rather than a variable resistance output that will tell us the correlating pressure. If you look below, you can see that the ThruMode FSR saturates at a lower force than the ShuntMode FSR. If you were looking to measure between 3,000 grams and 3,500 grams with the ThruMode FSR, you would get a consistent output that would be of no use. Therefore, the correct FSR needs to be chosen for the application. 8

9. FSR Form Factors Styles of FSRs Single Zone Matrix Array Single point of measurement and output. In a matrix array, a large quantity of sensing elements is arranged in a grid, with each sensor element located at the intersection of a row and column. Rows and columns are pinned out, rather than individual sensors (as in a discrete array). Matrix arrays require multiplexed scanning electronics, but allow for very high sensor counts (often 10K+ sensor cells) using limited I/O pins. Discrete Array Force Sensing Linear Potentiometer FSR sensing an unidirectional force. A discrete array is simply a collection of any number of single-zone elements, printed together on a single substrate. The two terminals of each sensor element may be pinned out individually or connected to a common trace at one end to reduce connector contacts. 9

10. Design Design Considerations Spacer Height (Thickness) & Inside Diameter (ID) The upper circuit and lower circuit of an FSR are separated by the spacer adhesive (usually 0.002’ to 0.005’ thick). The spacer’s thickness and inside diameter (ID), which is the open area of the spacer, as well as the upper circuit’s film thickness determine the amount of force required to activate the FSR. Sometimes a very thin spacer is required to create a “pre-loaded” FSR. A pre-loaded FSR is in a very high resistive state and requires only a small force to activate. With a pre-loaded FSR, a threshold circuit is used to set the limit at which the device is considered in “contact” (activated). Dielectric Dots Other names for dielectric dots include spacer or insulative dots. They can also be used for spacing apart the upper and lower circuits. This is useful with very large sensor areas or when the sensor may be flexed or bent. The dielectric dots can also be added to modify the actuation force of the FSR. The frequency or spacing of the dots determines the amount of force needed for activation: the closer the dots are to each other, the more force required to activate the sensor. Mounting The FSR device works best when mounted to a rigid or semi-rigid surface so that when a force is applied, there is a surface to push against. FSR devices can be mounted to surfaces with pressure sensitive adhesive such as 0.002” thick 3M 467MP. 10

11. Design Actuator The actuator is the device that touches the surface and applies force to the FSR device. The actuator system is critical for improving the part-to-part reproductivity of the FSR device. As the actuator applies force to the FSR device, the upper circuit deflects under the load. The actuator could be something uniform like in the image to the right. Or something more variable like a human finger or foot. Initially there is a small amount of contact between the FSR elements of the circuit. As the force is increased, the area of contact also increases, and the output becomes more conductive (less resistance). If the force is applied consistently, cycle-to-cycle repeatability is maintained. A thin elastomer, such as silicone rubber, placed between the actuator and the sensor can be used to absorb some error from inconsistent force distribution. Designing the actuator to ensure proper loading of the sensor is critical to a consistent FSR device. The actuator material is chosen specifically for the application. The actuator should be 20% smaller than the ID of the spacer opening. For thicker spacers, the actuator should be even smaller. If the actuator is too close to the spacer wall, it can block the upper circuit from deflecting properly. Inks Inks affect the output of the FSR device. For example, the wider the printed silver traces and spaces, the more resistive the output. More resistive FSR inks tend to be more linear and will tolerate higher force whereas more conductive FSR inks can work better for some finger activation devices. 11

12. Design Thermal Drift Like any resistive sensor, FSRs are affected to a certain extent by the ambient temperature. In general, FSRs become increasingly resistive as the ambient temperature increases. The exact relationship of resistance vs. temperature depends on ink composition and surface area of the specific FSR and must be characterized/compensated for in low drift applications. Below is a chart to help demonstrate examples of thermal drift on an FSR device. The chart shows the thermal drift of an FSR device (under constant load) as the ambient temperature varies over a four-hour period. In the case of this chart, the output being plotted is the voltage. As ambient temperature is ramped up, the voltage peaks at about 5V and then drifts to a reading of about 4V as the temperature stabilizes over the hour or so. The chart demonstrates that the output of the FSR device changes with fluctuations in ambient temperature. 12

13. Design Loading Hysteresis Loading hysteresis describes the effect previously applied forces have on the current FSR resistive output. While it may be possible to characterize and compensate for loading hysteresis in very complex software algorithms, it is often sufficient to simply limit load magnitude and duration to values which will not impose excessive hysteresis. The chart above shows the hysteresis of an FSR device before and after a life cycle test under a 30psi load. The blue response curve shows the original output of the FSR device (prior to life cycle test). The orange response curve shows the new (altered) response curve of the FSR device (after the life cycle test). In summary, the loading and unloading of an FSR device over time has a permanent effect on its performance. Other Design Considerations • An FSR is not a strain gauge or a load cell. It will consistently deliver a characteristic curve and can achieve 2% accuracy with a well-designed actuator system. • A calibration system is suggested for applications where higher accuracy is required. • Most applications will achieve between 5%-15% accuracy depending on the actuation system. • Keep the current low, below 0.5mA. Overheating the sensor will destroy the device. 13

14. Limitations Limitations to Consider Throughout analyzing all the types of FSRs and how they function, we must also understand what limitations we have in design. If we are not careful in considering these limitations, we can jeopardize the functionality of the entire FSR. The largest limitation is the physical size of the FSR. An FSR has two conductive pads that need to have the ability to touch and rebound back to their stationary position in order to function properly. If we increase the size too dramatically, we could have pads in contact in the middle of the FSR due to the lack of static support for the middle of the substrate. This would cause a consistent resistance reading that will not be accurate to the amount of pressure being applied. If we are designing a larger FSR, we need to think about how we could play spacers throughout the parts of the FSR that are susceptible to sinking. In some cases, the functionality of the FSR will be a trial-and-error sequence that must be proved to confirm the reliability of the design. Another limitation is the pressure range that the FSR will be sensing. When we do the calculation for pressure that an FSR can sustain, our basic equation is shown below. If we are looking to measure higher forces, we would likely go to a larger size FSR to account for the force increase. In a smaller FSR, we are limited to lower pressure ranges due to the mechanical limitations of the design. In most cases, we are limited to 125 psi for a standard design FSR. 14

15. Limitations Limitations Continued The accuracy of the FSR is another limitation. Force vs. resistance accuracy varies by sensor model but is generally limited to around +/- 10% even in a well-designed mechanical system with consistent actuation. FSRs are not intended to replace strain gauges or load cells in designs where high absolute accuracy is required. FSRs do, however, provide significant advantages over overload cells in terms of low physical profile and cost-effectiveness (no need for bridge circuits or instrumentation amplifiers), in applications where relative or course absolute force measurements are acceptable. excel, for instance, in a wide variety of human touch applications, where 10% variance in absolute force is virtually imperceptible. The relative accuracy of FSRs is quite good, so they’re also ideal in force mapping applications, in which the distribution of force is of interest, but absolute force/weight is not particularly relevant. 15

16. FSR Datasheet Typical Force Sensing Resistor Characteristics Property Value Notes limited only by fabrication equipment and raw material sizes BTI can produce FSR sizes of 31” x 47” approx. (including tail length) Size Force Sensitivity Range 1 oz to 20 lbs Mechanical interface dependent Pressure Sensitivity Range1 psi to 125 psi Mechanical interface dependent Part-to-Part Repeatability Approx. ± 15% of average resistance With consistent actuation Maximum Current 0.5mA Single Part Force Repeatability ± 5% of established nominal resistance With consistent actuation Force Resolution 1% full scale Stand-off Resistance 100K Ohms to 1M Ohms No load, FSR ink formula dependent Switch Travel Zero to thickness of spacer Typically 0.002” to 0.006” Devise Rise Time 1 msec Lifecycle 1,000,000+ actuations Operational Temp. Range -15ºF to +200ºF Substrate specification limitations Typically 0.012” to 0.021” (0.017” most common) Device Thickness Construction specific General guideline sheet for a Force Sensing Resistor (FSR) 16