The PhytoSense façade system is an interactive green facade that uses swept frequency capacitive sensing to play with light depending on touch.  An array of plants selected by their texture lets the user interact and be in “touch” with them by fading light to the plants being touched. But how does it work? How can the plant “feel” when it is being touched? As always, our friend Arduino gives us a hand.

PhytoSense – Vimeo

Swept Frequency Capacitive Touch Sensor

The circuit senses changes in voltage and capacitance from current being run through a swept frequency capacitive sensor (SFCS) attached to a conductor (copper plate) buried in a dielectric (planter and plant). With the circuit on this state, current runs freely through it until it finds ground and nothing is sensed. Yet, when a second conductor (human hand) approaches or touches the plant, a capacitor is made between the copper plate and the hand, with the plant acting as its dielectric inert material. Introducing the capacitor to the AC circuit changes the frequency of the voltage and current after it goes through the capacitor, a change in frequency that is measured by the SFCS.

Touch Dependent Capacitance Variation

The SFCS can also measure the change in capacitance of the system. This change is created by the way the plant is touched. The main variables that change it are the distance from the hand to the copper plate and the amount of area touching the plant (second conductive area). A decrease in the distance of the conductors and an increase in the area change the potential difference between the conductors. This increases the electrical field in between them, distorting the frequency further. The capacitor is also affected by the “non-conductive” capacity of the dielectric, this makes some plants perform better or worse depending on factors such as the moisture content of the soil the amount of tissue on the plant membrane, and the conductivity of the user.

Both the change in frequency and the increase of capacitance sensed by the SFCS allows for the plant to “feel” how it is being touched and react in a certain way depending on it. This value (in arbitrary units) is the one which controls how and where the light is being directed.

Experimentation and Calibration

The system is delicate in terms of the sensed value changing due to several factors as frequency noise, voltage applied, and resistance among others. Several tests using different sensor configurations where conducted before getting a stable reading. The first tests conducted used a simple system with a 100 KOhm resistor and various fruits. The fruitDuino experimentation was done on plants but yielded very different values from species to species. Further testing and switching to a new SFCS circuit gave more stable and reliable results, allowing us to implement the technology into a facade system. The SFCS was connected to a single plant in one experiment as well several plants together until the final version was created. The SFCS sensor works by measuring the change in value in capacitance. Electrical flow runs through the SFCS sensor to the plant measuring the change in capacitance. The circuit is then plugged to digital and analog pins on the Arduino which then controls the addressable LEDs on the facade.  The pallet is divided into four quadrants because the Arduino Mega supports four-16 bit timers, ideal for controlling the addressable LEDs.

Building and Addressing

Built on a reused pallet, PhytoSense is comprised of 12 planters fastened to the back of the pallet, allowing plants to grow through the exposed gaps. A copper plate lines the inside of each planter and is attached to an SFCS. Four SFCS sensors control the four quarters of the pallet independently. Depending on which quadrant is touched, addressable LEDs gradually fade from green to red. The absence of touch returns the LEDs to their previous color. The whole prototype is controlled by one 16-bit Arduino Mega, which reads the sensor values in real time. PhytoSense uses a variety of local plants and their ability to measure capacitance differs depending on various factors, such as leaf length, water retention, and soil moisture. The many and different values led us to simplify the interaction.

Applications

This facade system has great potential when taken to a larger scale. The use of addressable LED strips makes it a potential screen facade. This would allow the display of any image with a pixel based format throughout the facade. As for the plants, several information about the species or their watering needs could be identified by touching the desired plant. Furthermore, a world map could point out the touched plant’s origin and growing locations.

References

– King Walrus, SweepingCapSense – GithubInc. ; 

https://github.com/KingWalrus/SweepingCapSense/blob/master/SweepingCap.h

– “Arduino do the Touché Dance”, DLZ Evil Genius Liar Blog; 

http://dzlsevilgeniuslair.blogspot.it/2012/05/arduino-do-touche-dance.html

– “Touche for Arduino; Advanced Touch Sensing”, Madshobye, Instructables; 

http://www.instructables.com/id/Touche-for-Arduino-Advanced-touch-sensing/

– ” Touché: Enhancing Touch Interaction on Humans, Screens, Liquids, and Everyday Objects”, Munehiko Sato, Ivan Poupyrev, Chris Harrison, Disney Research; 

– “Cultivating Frequencies 1.0”, Colin Honigman;