Pressure and presence sensors in textile

Pressure and presence sensors in textile Eindhoven University of Technology Industrial Design Wearable Senses Admar Schoonen 2017-10-06 1

Contents Part I: Hard/soft connections Temporary connections Through hole components Single connections Multiple connections Part II: Textile sensor Theory of resistive pressure sensors Making a resistive pressure sensor Theory of capacitive touch sensors Making a capacitive touch sensor Making capacitive touch sensor sensing only one side Combining resistive and capacitive touch sensing 2

Part I: Temporary connections Alligator clips Great for quick tests When connecting conductive thread or thin copper wire: wind wire a few times around bottom part of the beak for more reliable connection (image: jensentools.com) 3

Part I: Temporary connections Kettelstiften (jewelry making materials): http://www.bykaro.nl/c-1277754/niet-en-kettelstiften/ Great for quick tests Fits in standard 0.1 header Available in bronze, silver, gold and rose Can be soldered onto NB: rose colored version seems to be a little thicker; could be too thick for some headers? (image: bykaro.nl/) 4

Part I: Temporary connections Simple connections can often be done very inconspicuous with existing haberdashery. Safety pins (image: globalsources.com) 5

Part I: Through hole components Components with long leads: bend leads outwards and into a curl, then hand-sew over them. (image: open softwear) 6

Part I: Through hole components Components with short leads: bend leads outwards then handsew over them. Test with a multimeter to make sure you have no short circuits! (image: open softwear) 7

Part I: Circuit boards Circuit boards (image: lilypadarduino.org) Circuit boards with standard 0.1 header holes require very fine needle 8

Part I: Haberdashery Simple connections can often be done very inconspicuous with existing haberdashery. Snap buttons (images: ayutrade.nl) (image: open softwear) 9

Part I: Haberdashery Simple connections can often be done very inconspicuous with existing haberdashery. Metal buttons (image: buttons4u.com) (image: open softwear) 10

Part I: Haberdashery Simple connections can often be done very inconspicuous with existing haberdashery. Zipper (image: ucanzippers.com) (image: open softwear) 11

Part I: Many connections When many connections are needed, a simple, efficient method is using a standard pin header Insulated conductive thread recommended Base fabric needs to be heat resistant (cotton: OK; nylon, polyester: not OK) Details at http://www.instructables.com/id/simple-e-textile-connector/ 12

Part I: Links http://www.kobakant.at/diy/ http://softwear.cc/book/files/open_softwear-be ta090712.pdf http://www.instructables.com/id/simple-e-textil e-connector/ 13

Part II: Pressure sensor circuit Use fixed resistor as reference R sense V =V cc R ref + R sense 5V + 5V + R ref =10 k Ω R ref =10 k Ω V =4 V R sense =40 k Ω V =3.75 V R sense =30 k Ω 14

Part II: Pressure sensor stackup Electrode 1 Resistive pressure sensitive material Electrode 2 15

Part II: Pressure sensitive material Unpressed Pressed 16

Part II: Actual structure Electrode 1 Resistive pressure sensitive material Electrode 2 17

Part II: Pressure sensitive material ESD foam image: conrad.com PU foam Impregnated with carbon / latex mixture Typically about 5 mm thick Very compressable Velostat / Linqstat image: aura-comms.com PE film Impregnated with carbon based material Typically about 0.2 mm thick Slightly compressible 18

Part II: Conductive textile Any conductive textile with good conductivity will work. Textiles with metal coating or metal woven into is fine Textiles with carbon coating or carbon woven into is not preferred (has higher resistivity) Textile should not have an insulating coating Examples: Sparkfun DEV-10056 (conductive fabric ripstop) Sparkfun DEV-10055 (conductive fabric MedTex180, strechable in 1 direction) Sparkfun DEV-10070 (conductive fabric MedTex130, strechable in 2 directions) 19

Part II: Sensor design Electrodes: Size is always a bit smaller than pressure sensitive material (prevents short circuit) Material: conductive fabric or uncoated conductive yarn / wire Conductive fabric based on nylon can easily fray watch out for short circuits Conductive fabric has lower resistance than yarn / wire Good sensor: > 40 kohm unpressed, < 4 kohm pressed Pressure sensitive material: resistance can be increased by: Stacking several layers of pressure sensitive material (increases the length of the resistor) Decreasing area Using material with lower conductivity Pressure sensitive materials usually have a hysteresis Material does not always immediately and completely recover sensor still sees some pressure after pressure is removed 20

Part II: Sensor design: conductive fabric electrodes Front electrode Back electrode Prepare electrodes: Cut out 2 pieces of conductive fabric, about 7 x 7 cm Tip: ripstop fabric easily frays, which can cause short circuits. If you use a sewing machine, this will be taken care of in step 3. If sewing by hand: Laser cutter (melts the material no fraying) Sew a hem Tape edges (when you're lazy) 21

Part II: Sensor design: conductive fabric electrodes Front electrode Back electrode Prepare electrodes: Place uncoated conductive thread / wire on electrode and fixate with stitches Alternatively: sew thread directly through electrode 22

Part II: Sensor design: conductive fabric electrodes Electrode Cotton patch Cut a patch of nonconductive fabric (such as cotton); must be larger than the electrode Optionally: trace outlines of conductive fabric on cotton patch, such that it is visible on both sides Trace of conductive fabric 23

Part II: Sensor design: conductive fabric electrodes Front Back Sew each electrode to its own cotton patch When using sewing machine: use zigzag to keep in any frays 24

Part II: Sensor design: conductive fabric electrodes Front Pressure sensitive material Cut out pressure sensitive material Must be slightly larger than electrode but smaller than cotton patch 25

Part II: Sensor design: conductive fabric electrodes Front Side Place pressure sensitive material in between electrodes Electrodes must face inwards 26

Part II: Sensor design: conductive fabric electrodes Sew sensor together Good Bad Be careful to not stitch inside the outline, to prevent stitching through conductive fabric and causing short circuit Do not pull the stitches too tight to prevent too must base pressure 27

Part II: Wiring it all up void setup() { Serial.begin(115200); } void loop() { int x; } x = analogread(0); Serial.println(x); 28

Part II: Simplifying things Arduino has internal pull-up resistors on analog input (20 50 kohm) void setup() { Serial.begin(115200); pinmode(a0, INPUT); digitalwrite(a0, HIGH); } void loop() { int x; } x = analogread(0); Serial.println(x); 29

Part II: Schematic vs reality 5V + 5V + R ref R ref Switch to enable pull-up Switch to enable pull-up To ADC R sense To ADC R sense 30

Part II: Resistor or capacitor? Pressure sensitive material has too low resistance to work as proper insulator. Instead, both sides of conductive fabric + pressure sensitive material can act as one large (and thick) plate of a capacitor. Human hand acts as 2nd plate of capacitor. Capacitance increases if hand is brought closer to sensor. Details on how this type of capacitive sensing works are out of scope for workshop. 31

Part II: Combining capacitive and resistive sensing 5V + 5V + R ref Closed switch to enable pullup R sense Closed switch to connect ground R ref To ADC Open switch to disable pull-up To ADC C sense Open switch to disconnect ground 32

Part II: Wiring it all up (without guard) Software: rapidly switch between capacitive sensing (for distance information) and resistive sensing (for pressure information) Download: https://github.com/admarschoonen/touchlib 33

Part II: Summary Resistive sensor: reduce resistance by compressing medium resistive material, reducing path length for electrons ESD foam and ESD foil are very suitable Influence sensitivity with number of layers and area Capacitive sensor: increase capacitance to ground by bringing hand / finger close to sensor, increasing field lines Resistive sensor with floating pins is very suitable as capacitive sensor Influence sensitivity with area Capacitive and resistive sensor can be combined in one to measure both distance and pressure 34

Part II: Hands-on Sew sensor (slides 21-27) Download Arduino code (slide 33) Install as library in Arduino Open Example00SemiAutoTuning Set Arduino serial monitor speed to 115200 baud Answer questions Copy + paste generated code in new Arduino sketch Upload sketch and test sensor More details: http://etextile-summercamp.org/2017/summercamp/presence-and-pressure-sensor/ http://etextile-summercamp.org/2017/summercamp/hardsoft-connections/ 35