Force Sensing Resistor (FSR) Design Guide: Thin, Durable & Precise Sensors

Explore our Force Sensing Resistor (FSR) Design Guide to learn about construction, ink types, and design considerations for precise pressure sensing. To know the basics of FSRs, visit: https://butlertechnologies.com/force-sensing-resistors

1. The Ultimate
Force Sensing Resistor
Design Guide

2. Table of Contents
2
Introduction 3

Butler Technologies, Inc. 3

What Is a Force Sensing Resistor (FSR)? 3
Types of FSRs 4

ShuntMode vs. ThruMode 4

High Resistance Ink vs Low Resistance Ink 6

Choosing the Correct FSR 8
FSR Form Factors 9
Design 10
Limitations 14
FSR Datasheet 16

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.
4
ShuntMode FSR Cross Section

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.
5
ThruMode FSR Cross Section

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)
6
1” ShuntMode FSR Pressure (psi) vs Resistance output (ohms) for Different Resistant Inks

7. Types of FSRs
7
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)
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.
1” ThruMode FSR Pressure (psi) vs Resistance output (ohms) for Different Resistant Inks

8. 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
Types of FSRs

9. Styles of FSRs
9
Single Zone
Single point of measurement and output.
Matrix Array
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
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.
Force Sensing Linear Potentiometer
FSR sensing an unidirectional force.
FSR Form Factors

10. 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).
10
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.
Design

11. 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.
11
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.
Design

12. 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
Design

13. 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
Design

14. 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
Limitations

15. 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
Limitations

16. 16
Typical Force Sensing Resistor Characteristics
Property Value Notes
Size
limited only by fabrication
equipment and raw material sizes
BTI can produce FSR sizes of 31” x
47” approx. (including tail length)
Force Sensitivity
Range
1 oz to 20 lbs Mechanical interface dependent
Pressure
Sensitivity Range
1 psi to 125 psi Mechanical interface dependent
Part-to-Part
Repeatability
Approx. ± 15% of average resistanceWith consistent actuation
Maximum
Current
0.5mA
Single Part Force
Repeatability
± 5% of established nominal
resistance
With consistent actuation
Force Resolution1% 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 Time1 msec
Lifecycle 1,000,000+ actuations
Operational
Temp. Range
-15ºF to +200ºF Substrate specification limitations
Device ThicknessConstruction specific
Typically 0.012” to 0.021” (0.017”
most common)
General guideline sheet for a Force Sensing Resistor (FSR)
FSR Datasheet