10 n March/Apr i l 2015 IEEE PotEntIals 0278-6648/15©2015IEEE

The early detection of wild-fires with monitoring sen-sors can improve the effec-tiveness of fire responders, decreasing the destruction of property and saving

lives. However, distributing these sensors across a wide area is an expensive and laborsome task. Fur-thermore, the cost of the sensors themselves may be high, limiting their practicality in instrumenting large wilderness areas. By integrat-ing aerodynamic structures into the sensor module printed circuit board, a reliable aerial deployment mecha-nism may be constructed.

These sensor-aircraft draw upon the maple seed samara for inspira-tion, using a monowing to autorotate to a soft landing. The same struc-tures found in the biological seed directly transfer to the design of the electroaeromechanical device. In practice, this structure is found to be very reliable and straightforward to produce.

the problemDistributed sensing of environmental conditions over a geographical area is a growing field of interest in land management, agriculture, and rural wildfire fighting. Popular national parks prone to wildfires, such as Yosemite, are of particular interest to fire managers and conservationists.

Prior to fire season, reliable mea-surements of growing conditions across a forested area enables accu-rate estimates of expected fuel loads over the season. This allows park managers to identify at-risk areas that may have heightened levels of dead wood, scrub, or grass, and effected fuel load reduction or clearing may be undertaken to reduce the potential fire hazard. At the onset of a fire, these sensors could detect elevated tempera-tures, alerting responders immedi-ately: a fire-alarm for the forest.

During the outbreak, emergency response—detection, monitoring and fighting—particularly relies upon up-to-date knowledge of environmental conditions. Rapidly shifting fire fronts pose a danger to ground crews who may suffer from degraded situational

awareness due to smoke and wind. Even after a fire has passed through, an area may not be safe for cleanup crews to enter due to residual ground heat, fumes, and smoldering embers. Aerial imagery taken from drones, he-licopters, and spotter planes is often obscured by smoke and can only sur-vey one area at a time.

Currently, obtaining ground-level environmental measurements over the expanse of a state park must be done manually, which is an expen-sive and labor-intensive task. Fur-thermore, state parks often consist of remote, rugged, and treacherous terrain, putting volunteers at risk. Conventional automatic monitoring stations are too expensive to get ubiq-uitous coverage or to put at risk in fire-prone areas.

Digital Object Identifier 10.1109/MPOT.2014.2359034 Date of publication: 5 March 2015

Image courtesy of WIkImedIa commons/

krzysztof zIarnak

Paul Pounds and surya singh

Samara: Biologically inspired self-deploying

sensor networks

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There is a need to automatically rapidly deploy sensors across a wide area, so that even the most difficult to reach places may be surveyed (Fig. 1). Our plan is to airdrop sen-sors from carrier aircraft so that only a limited number of complex vehicles are needed, and the sensors can be as simple as possible. The sensors must passively soft-land on their own so that instruments reach the ground intact. Existing soft-landing solutions such as complex parachutes or inflatable landing cushions are labor intensive and ex-pensive. However, each device must be sufficiently low cost so that per-vasive and redundant coverage of an area is economical, and the cost of units destroyed by fire is acceptable.

nature’s solutionA natural analog of this design prob-lem is the seed dispersal of maple trees. Maple trees thrive over a wide geographic area, and there is substan-tial survival benefit to the species in colonizing as many places as possible. However, there is a real biological cost paid by the organism to grow a disper-sal system where the the odds of a seed reaching fertile ground may be low. Consequently, maple trees have developed a low-cost solution to the problem that allows a multitude of units to be deployed over a wide area.

Each mature tree may release a multitude of seed pods or “samaras,” each equipped with a thin fibrous wing. The samaras autorotate as they fall from the tree, slowing their descent. If caught by the wind, a seed pod may fly for a considerable distance, greatly increasing the abil-ity of the maple to spread.

The design of samaras is a high-ly integrated structure. Each seed is actually an entire fruit, in which the embryonic seed (the payload) is contained by the exocarp, the tough outer casing that protects it. The exo-carp forms the aerodynamic wing and has a thickened leading edge and vasculature extending across the sur-face of the wing, which helps stiffen the structure in flight [Fig. 2(a)]. By reusing existing parts of the fruit to perform multiple functions, the maple tree multiplies its competitive repro-ductive advantage without increasing generative effort.

Economics of mass- produced sensor modulesFor humans, the economics of building low-cost sensors are like-wise nontrivial: tens of thousands of devices will be required for a complete air-deployable network,

and destruction rates of up to 10% may be expected annually. The per-unit cost must be minimized. Meth-ods for doing this are to integrate as many functions into as few com-ponents as possible and to use mature mass-production methods that are amenable to automated assembly processes. This provides numerous benefits such as reduc-ing the amount of material used and the total labor required for assembly (Fig. 3).

There are several cost drivers for technology products: design, parts, processing, and labor. Design is a one-off fixed cost, amortized over the number of units made; for very large fabrication runs, the incremen-tal cost goes to zero. Prefabricated parts—electrical components, bat-teries, sensors, and connectors—scale nonlinearly with the number of units ordered; as order numbers

fig1 the aerial deployment of bushfire detection sensor modules.

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fig2 the (a) samara fruit of Acer nigrum michx. f. by Hurst and (b) the electroaerome-chanical sensor aircraft.

Wing Root

Sensors

Seed Pod

Vasculature 1 cm

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(b)

Leading Edge Weight/Stiffener

Leading Edge Weight/Stiffener

Stringer Vasculature

Strain ReliefCopper Levee

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increase, price breaks reduce per-unit cost. The processing and la-bor involved in assembling custom aerodynamic, mechanical, and elec-tronic fabrications scale linearly with production volume and for large quantities may constitute the bulk

of expense in building a technology product. Thus, an optimal approach is to reduce the number of parts, minimize the number and variety of fabrication processes, and eliminate labor where possible.

However, as the amount of inte-gration in a component increases, the challenges and effort of develop-ing the system may increase sub-stantially, complicating the task of the designer.

an electroaeromechanical aircraftWe took inspiration from the samara fruit to reuse the sensor module cir-cuit board as a wing, in the same way that the samara uses its exo-carp [Fig. 2(b)]. The sensor circuit board is a required part of the device, which typically performs only one job: carrying current between electronic devices. By using a flex-

rigid structure, the board can be made to perform a variety of tasks simultaneously; the entire aircraft consists of a single component.

The flexible substrate of the board can be used to form the aero-dynamic lifting surface of the wing.

The thin-section and light weight of the polyimide membrane is an ef-fective airfoil at the small-scale low Reynolds number flows encountered by an autorotating monowing. As part of its fabrication, the polyimide substrate is clad with copper traces that are typically etched away. In the case of the sensor module, we selectively retain strips of copper running transverse to the wing to act as “stringers” that help maintain the shape of the wing. These are ho-mologous to, and perform the same function as, the vasculature of the samara wing.

The rigid section of the board is extended around the leading edge of the wing—this section itself has multiple roles. First, it acts as a stiffener to make the wing rigid and transfer the load of the aerody-namic surface to the sensor module “seed.” It also acts as a mechanical

counterweight to move the center of mass of the wing ahead of its aero-dynamic center of pressure, allow-ing it to be passively stable in pitch, much like a glider. A long straight conductive trace along the top of the stiffener forms a radiating el-ement for the sensor’s telemetry transmitting system.

The main rigid section of the board supports the electronics that form the payload of the device. Each device consists of sensors, a micro-controller, battery, and radio mod-ule. The weight of the battery forms a counter balance for the wing, sized so that the barycenter of rotation is located at the wing root. The current prototype employs tropospheric sen-sors for its environmental monitor-ing task—temperature, humidity, and pressure—but the design is eas-ily adaptable to use any low-weight surface mount sensor. An optional GPS daughter board measures the release and landed positions of the device, allowing local wind velocity to be estimated (at the expense of shortened battery life).

Together, the f lex-rigid circuit boards carry out aerodynamic, structural, and electronic functions concurrently; it is a single multi-physics construction. We call such highly integrated aircraft aeroelec-tromechanical systems—the inter-section between mechatronic and aeromechanical structures (Fig. 4). This approach has been applied to a variety of small-scale aerial robotics problems, such as gliders and pow-ered flight, as well as different ma-terials including paper or aluminum foil composites. More sophisticated processes include forged bends, cor-rugations and rigidized hinges for stronger, precision three-dimensional aerodynamic shapes.

Most importantly, it is manufac-tured using existing circuit board fabrication processes and suitable for automatic pick-and-place loading of electronic parts. Once a manufac-turing line is established, no human labor is required beyond testing and packing. This makes each device substantially cheaper than compa-rable miniature aircraft.

fig3 the outcomes of sensor deployment in unmanned aerial vehicle functional integration.

Multifunction Structure

Fewer Discrete Parts

Reduced Material Usage

Reduced Aircraft Usage

Improved Performance

Simpler Fabrication

Less Labor

Lower Cost

Highly Integrated Parts

Increased Complexity

More Design Effort

Distributed sensing of environmental conditions over a geographical area is a growing field of interest

in land management, agriculture, and rural wildfire fighting.

IEEE PotEntIals March/Apr i l 2015 n 13

In quantities of 10,000, the per-unit cost for each de-vice may fall as low as US$15; as many as 50 devices to be purchased (each covering up to one hectare) for the same cost of one hour’s flight time in an observation helicopter. For scale, Yosemite National Park could be instrumented for less than US$5 million, while Yosemite Valley itself could cost less than US$50,000 for deployment. Wide area instal-lations can be carried out in stages, allowing the efficacy of the network to be tested prior to full-scale deployment.

Proof of concept trialsA basic deployment method for pro-totype wings consists of a container attached to the underside of a com-mercial off-the-shelf quadrotor plat-form capable of navigating GPS way-points (Fig. 5). The container incor-porates a motor that drives an auger coil, in which the sensor modules

are loaded. When the quadrotor reaches its next waypoint, the motor is activated to release a single wing. A photointerrupter senses when a wing has deployed, and the auger direction is reversed to reduce the likelihood of the next device in the queue shaking out.

Early testing in which the sen-sor modules were dropped from a height of 20 m found that the spi-

raling flight was very reliable. On deployment, the sensors initially free-fall and tumble. The sensors enter autorotation automatically: unsteady aero-dynamic forces quickly induce rotation due to the asymmetry of the wing planform. Once spinning, the autorotative de-scent is passively stable and the sensors slow themselves through windmill braking. The center of mass is set low so that the modules preferentially land with the sensors and antennae facing skyward. In static drop tests, a terminal rotational rate

of 1.8 Hz and terminal velocity of 0.65 m/s were observed.

From 20 m altitude, the sen-sors can be carried by the wind for some distance—by as much as 45 m in moderate wind. This places a natural constraint on landing pre-cision for completely passive sensor aircraft. However, with each deploy-ment’s added GPS landing data, a prevailing wind velocity estimate

fig5 a sensor deploying from a carrier quadrotor with a mechanism schematic (inset).

Battery

Motor

Controller

Photointerrupter

fig4 an integrated mechanical-aerodynamic-electronic device Venn diagram.

Electro-Aeromechanical Systems

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can be updated, and each subse-quent deployment coordinate ad-justed to decrease the error in land-ing coordinate.

Challenges aheadWhile the integrated aircraft design, transport system, and release mech-anism have shown strong reliability, other parts of the system require further development. There are a broad range of minor developments needed to bring the system into commercial readiness—environmen-tal sealing to keep moisture out,

insulation against cold where neces-sary, and fine-tuning of the autoro-tation system so that the devices always land with antennas pointing skyward—but these are problems that are resolvable with conscien-tious engineering. More challenging-ly, the seedpod sensor modules, like many remote sensing applications, suffer from two fundamental limita-tions: power and communications.

As the air-dropped sensor mod-ules’ weight must be kept to a mini-mum for soft-landing during autoro-tation, the available battery power will be strictly limited. While the sen-sors used are low power, the long op-erating period required for seasonal monitoring demands an energy sup-ply that cannot be provided by the best batteries currently available. A natural solution to this problem will be to repurpose the wing surface area to be a photovoltaic collector, to recharge batteries throughout the day provided it lands in a sufficiently insolated spot.

The challenge of transmitting data from the sensors back to the user is coupled with that of power supply. As less power is available, range and reliability decrease. This problem is an active area of

research in wireless sensor net-works. In the case of environmen-tal monitoring and bushfire detec-tion, it may be possible that each device returns only a single mea-surement per day over the course of the season. However, if a fire is detected by a node, that device may immediately switch to transmit-ting a warning signal continuously at maximum power in the expecta-tion that it will soon be destroyed. As fire spreads, more nodes will sound alerts, allowing the fire to be tracked in real time.

Beyond basic functionality, we also foresee the need to further develop the underlying materials technology of the aerial systems. In particular, the environmental impact of polyimide circuit boards and the chemicals contained with-in the circuit devices themselves is recognized as a key consideration in the deployment of the technol-ogy in ecologically sensitive areas. To this end, we are also exploring paper circuits that biodegrade after their mission is complete and also sealed ceramic modules that con-tain the more hazardous elements of the circuit (such as the battery) and slow the leaching of chemicals into the ecosystem.

ConclusionWhile there is substantial work still to be done in making sensors practi-cal for fire detection and monitoring, electroaeromechanical aircraft con-stitute a promising and cost-effec-tive deployment mechanism for such systems. In particular, their use of mature low-cost, mass-man-ufacturing processes make them eminently suitable for devices that are economically sensitive. Early experiments have shown the inte-

grated monowing system to be a very reliable descent arrest system, with no failed landings observed during any flight testing. Similarly, the deployment automation ele-ments of the technology have proven straightforward. It is hoped that this technology will rapidly evolve, lead-ing to effective large-scale field deployments.

Read more about it • C. Gamage, K. Bicakci, B. Crispo,

and A. Tanenbaum, “Security for the mythical air-dropped sensor network,” in Proc. 11th IEEE Symp. Computers Communications, 2006, pp. 41–47.

• R. Lorenz, Spinning Flight: Dynamics of Frisbees, Boomerangs, Samaras and Skipping Stones. New York: Copernicus, 2006.

• S. Hurst, “Acer nigrum Michx. f.,” USDA-NRCS PLANTS Database, 2006.

• P. Pounds and S. Singh, “Integrat-ed electro-aeromechanical structures for low-cost, self-deploying environment sensors and disposable UAVs,” in Proc. IEEE/RAS Int. Conf. Robotics Automa-tion, 2013, pp. 4459– 4466.

• P. Pounds, T. Potie, F. Kendoul, S. Singh, R. Jurdak, and J. Roberts, “Automatic distribution of disposable self-deploying sensor modules,” in Proc. Int. Symp. Experimental Robotics, 2014.

about the authorsPaul Pounds (paul.pounds@uq.edu.au) earned the B.E. degree in systems engineering in 2002 and the Ph.D. degree in robotics in 2008 from the Australian National University. He is currently a lecturer (assistant professor) at the Univer-sity of Queensland, Australia. He is a member of the IEEE Robotics and Automation Society.

Surya Singh (spns@uq.edu.au) earned the B.Sc. degree in mechani-cal engineering at the University of Tennessee, the M.Sc. degree at Carnegie Mellon University, and the Ph.D. degree in robotics from Stan-ford University. He is currently a se-nior lecturer (assistant professor) at the University of Queensland, Aus-tralia. He is a member of the IEEE Robotics and Automation Society.

We took inspiration from the samara fruit to reuse the sensor module circuit board as a wing, in the same way that the samara uses its exocarp.