Samara: Low-cost Deployment for Environmental Sensing Using PassiveAutorotation

Samuel Valdes, Ignacio Urza, Paul E. I. Pounds∗ and Surya Singh

Abstract— Distributed sensing offers a variety of benefitsfor environmental monitoring applications such as bushfirefighting. The cost and delay in distributing sensors over a largegeographic area makes ad hoc aerial deployment via air-drop acompelling technology. However, the need to ruggardise sensorsto survive free-fall impact, or the cost of including aerodynamicarrest devices such as parachutes for soft-landing, makes aerialdeployment less viable. We present a method for constructinglow-cost light-weight sensor modules that incorporate a mono-wing into the structure of the sensor PCB. This allows thesensors to autorotate to a soft-landing after release from theair, without additional processing or fabrication steps.

I. INTRODUCTION

Distributed sensing of remote areas is being increasinglyrecognised as a vital capability for agriculture, environmentalmonitoring, and bushfire fighting [1]. In regions such asAustralia and California, where wildfires pose a significanthazard to life and property, the ability to determine the localmoisture content and temperature of an area is potentiallyvaluable. Accurate geographic data of this kind allows valu-able fire-fighting and clearing units to be deployed ahead ofa fire front to create firebreaks through particularly dry andwarm areas considered at-risk, rather than committing themto regions where fire is less likely to take hold [2]. Thisis complementary to, but distinct from, the use of UAVsfor remote sensing (e.g., using hyperspectral sensors) in thatthe deployed sensors are able to monitor conditions overextended periods [3].

One of the large costs associated with this capability isthe labour and time involved in deploying ad hoc sensornetworks throughout a large geographic region. During timesof fire crisis, manpower is at an all-time premium andvolunteers cannot be spared to trek though countryside takingmeasurements, especially when there is an ever-present riskof being overrun by the fire front.

Aerial deployment of monitoring networks eliminates theneed to expose humans near a wildfire, and reduces the time-lag in placing sensors over a wide area. However, sensorsdropped from an aircraft (manned or unmanned) must be

This work was supported by University of Queensland CONICYT Re-search Skills Training Program.

Samuel Valdes and Ignacio Urza are research exchange students at Pontif-ica Universidad Catlica De Chile, Avenida Libertador Bernardo O’Higgins340, Santiago.sovaldes@uc.cl, ilurzua@uc.clPaul Pounds and Surya Singh are lecturers at the University of Queens-

land, St. Lucia, QLD, Australia 4072.paul.pounds@uq.edu.au, spns@uq.edu.au*Corresponding author.

Fig. 1. Samara sensor PCB monowing and stand-alone sensor PCB.

robust to a fall from great height; a simple sensor falling ona rock can have impact accelerations over 100 g. Thus, air-dropped sensor modules must either be very rugged or elseemploy an arrest system such as a parachute, streamer oraerodynamic propulsion. As a consequence, this makes air-dropped sensors heavier, bulkier, and more expensive thanequivalent units deployed from the ground. In particular, theexpense of a sensor module that is likely to be consumedby fire and destroyed is a serious drawback for regional fireservices that may not have the budget to discard valuableequipment.

A natural example of aerial distribution of pods under costconstraints is in the form of maple seed, or ‘samara’ seedpods. These well-known seeds consist of a fibrous fruit that iselongated into a wing shape [4]. When the seed falls from thetree, the wing autorotates under passive aerodynamic forcesthat slows the its descent. This increases the seeds chancesof being caught by a gust and distributed far from the parenttree and confers a competitive advantage. The relative growth‘cost’ of the wing is low, due to its integration in the existingstructures of the seed.

The concept of using a rotating monowing as a roboticsplatform is not new. Extensive work was undertaken atUniversity at Maryland by Ulrich et al, who developed aseries of increasingly small UAVs using rotary flight [5], [6].This work included analysis of the autorotation mode of thesevehicles [7], [8]. Similar work on actively propelled UAVflight has also been carried out by Lockheed (‘Samarai’) [9]and VeraTech (‘Phantom’) [10]. In all of these cases, thevehicle is quite large and relatively complex; Ulrich et alhave the smallest demonstrator, at 270 mm wingspan. Thesevehicles would not be classed as expendable.

In contrast, our approach is to use the same rationale as thetree to reduce the cost of an aerial deployment mechanism.We demonstrate that the PCB substrate that forms the sensorcan be repurposed to act as a wing (see Fig. 1), reducingthe complexity and fabrication cost of the sensor system.Potentially, the cost of the system could be decreased to thepoint where the entire sensor-vehicle could be reasonablydiscarded after deployment.

In this paper we consider a passive deployment techniqueusing an autorotating monowing constructed out of circuitboard material. The circuit board includes the sensor andelectronic systems for performing an environmental monitor-ing task. Section II describes the concept in more detail, andsection III discusses the key challenges associated with em-ploying circuit board material as a wing structure. Section IVreports early results demonstrating the functionality of themonowing as a descent arrest mechanism, and section Vdescribes future work.

II. DESIGN CONCEPT

The key design goal of the samara sensor is low costfabrication and aerial deployment. One of the largest ex-penses in producing an aerial system is in the manufacturingcomponents and processing. To reduce cost, the number ofparts and fabrication steps should be minimised. One way tothis is to synthesize as many functions into each component,thereby reducing the overall number of systems (and theassociated processes involved in making them).

The approach taken here is to combine the two mostcomplex components—the printed circuit board and theaerodynamic structure—into a single dual-purpose part. Theprinted circuit board is a light-weight polyamide substratecommonly used in flexible electronics applications, intothe shape of a seed wing. The wing section performs theaerodynamic braking needed to arrest the fall of the sensorwhen deployed from the air; this eliminates the need foradditional aerodynamic devices such as parachutes or rotors,and any additional actuators.

Printed circuit board manufacture is a mature mass-production technique. The unit fabrication cost of a hybridPCB-wing structure is extremely economical, especially inthe quantities of thousands required for widespread area sens-ing. As the aerodynamic structure is a single piece integratedwith the electronics, the only assembly step is automaticloading and soldering of the electronic components—again, amature mass-production process. Thus, the major cost driverof the sensor pod becomes the unit cost of the onboardembedded sensors themselves, comprising an estimated 66%of the overall cost of the aircraft in units of 1000.

A. Electronics

The proof of concept samara sensor consists of a stand-alone sensor module adapted to include a wing built into aflexible PCB. The sensor module is equipped with a SensironSHT-25 relative humidity and temperature sensor, controlled

by a Microchip 16F690 microprocessor. Sensor measure-ments are stored in the microprocessor’s flash memory untilread out over a three-wire serial port.

The standard module is powered by a 250 mAh lithiumpolymer battery that typically provides nearly a week moni-toring time. The need to reduce weight for airborne deploy-ment limits the size of the battery to a sub-1 g 20 mAh cellwith a light-weight magnetic Bahoma-style connector. Thisgives the module up to 12 hours of passive sensing time.

The samara sensor pod also includes an ST L3G4200three-axis mems gyroscope for detecting descent spiral mo-tion and subsequent landing. This allows the sensor to onlybegin taking measurements and transmitting data once de-ployment is complete. Sensor feedback on the demonstrationunit is currently limited to an RGB LED that provides acolour indication of local environmental conditions. In thefuture, this will be replaces by a 2.4 GHz wireless modulethat will periodically communicate with the deploying air-craft.

The entire sensor, microprocessor, battery and wing as-sembly weighs 15 grams. A mid-size fixed wing UAVequipped with a release mechanism could easily carry anddeploy up to 100 samara sensors in a single flight.

III. AEROMECHANICAL DESIGN

A whirling seed pod operates in a fashion akin to aconventional helicopter rotor in auto-rotation, where the rotoris a monowing spinning about a barycenter very close to therelatively massive seed. The principles of a falling mapleseed are well understood [4], [8], but the fabrication of thewing structure from flat flexible circuit board material createsunique challenges for the sensor pod.

A. Stability

A conventional helicopter rotor is fixed to a rotor shaftwith a swashplate that enforces the orientation of the bladesrelative to the fuselage. Careful adjustment of the rotorblades’ cyclic pitch controls maintains the flight attitude ofthe helicopter [11]. In a free-floating autorotating monowing,however, there is no control input or reference, and thestability of blade pitch must be maintained through passiveaerodynamic means.

This can be thought of as analogous to a gliding flying-wing descending in circles. As with a passively-stable flyingwing, the aerodynamic and mass centres must be configuredsuch that the pitching moment of the airfoil is balancedby the force couple induced by the horizontal separationbetween the centre of lift and the centre of mass. Flyingwings often use special ‘reflex’ airfoil shapes that movecentre of lift of the wing aft, to reduce the pitching moment[12].

The samara sensor is constructed from a single piece offlat material of uniform density, the wing is limited to a flat-plate airfoil and it is not possible to employ reflex. The liftgenerated by a radial element of the wing is proportionalto the chord, as is the mass; for any given uniform chordelement, the local centre of mass will be at the half-chord,

while the centre of lift will always be ahead of the quarter-chord. Consequently, there is no chordal or radial planformthat will stabilize a wing of uniform density.

A solution to this problem is to shift the centre of massof the wing forward by adding material to the front of thewing—in this case, a ridge of solder is added to the top andbottom sides of the leading edge. The rounded surface ofthe solder also acts to alleviate the sharpness of the leadingedge, and so reduce the tendency for airflow over the wingto separate and induce stall. Likewise, by lengthening thechord at the outboard third of the wing, the overall centre oflift can be moved aft, maximising the restorative stability ofthe configuration.

B. Disc-Loading

A descending seed pod autorotates in what is calledthe ‘windmill brake state’—where the rotation of the rotorremoves energy from the on-coming vertical wind and dis-sipates it, slowing the descent of the aircraft. The rate ofdescent in this mode can be shown to be twice the inducedvelocity of the airflow in hover [13, p109]. This indicatesthat the disc-loading of the rotor (the mass of the aircraft,divided by circular planform swept of the rotor) should bekept as low as possible.

As the structure of the sensor pod comprises the rotoritself, increasing the diameter (and thus area) of the rotornecessarily increases the mass of the aircraft. Naively, in-creasing the size of the wing without limit should decreasethe relative contribution of the deadweight of the sensor andpower source. However, as the dimensions of a thin wingincreases, its ability mechanically to support itself underaerodynamic loads degrades and the aerodynamic shape losesintegrity [14]. Consequently, the wing must be sized largeenough such that the disc-loading is sufficient to provide aslow enough descent, but no larger. This is in contrast to thecounter-intuitive case of helicopters, where a heavier aircraftwith a given disc-loading will descend slower than a lighteraircraft of equal disc-loading [13, p139].

As helicopter gross mass decreases, the disc-loading mustalso decrease. The disc-loading of a conventional helicopteris typically greater than 12 kgm−2. An average quadrotorhas a disc-loading of 8kgm−2. In contrast, the 99 mm longsamara sensor wing has a disc-loading of 0.49 kgm−2.

C. Aeroelasticity

The mechanical strength of the wing sets a practical lowerbound on disc-loading; increasing the stiffness of the wingthus increases the overall achievable performance of thedesign. The solder counterweight added to the leading edgeperforms a secondary function of increasing the bendingstiffness of the wing and reducing torsional twisting. Tofurther improve the stiffness of the design without contribut-ing significantly to the mass of the structure, parts of theconducting copper layer have been left unetched over thesurface of the wing to form transverse stringers. These mimicthe vasculature seen in natural seed pods, and the authors be-lieve they serve the same purpose—rigidising the lightweight

Fig. 2. Bare PCB layout and features.

flexible aerodynamic structure. The stringers converges at theroot of the wing section to provide maximum stiffness at thepart of the structure under the most load.

Additional solder structure is added at the wing root tofurther stiffen the wing interface with the sensor circuitry.The aerodynamic loads of the wing can cause small defor-mations leading to solder cracking and failure of the circuitcomponent connections. This leads to premature failure ofthe device, even under the relatively small cyclic forces ofautorotation. By placing a solder levee around the functionalcircuit traces as strain relief the integrity of the electronicsis protected.

IV. PRELIMINARY RESULTS

There are two stable descent modes: spiral descent andplunge. Natural seeds use small ripples in the trailing edgeof the wing to create a aerodynamic imbalance that leadsrotation. As the prototype samara sensors currently lack thisfeature, the device will plunge if dropped seed-end down.To avoid this, the sensor is always dropped ‘seed-end up’, sothat the device will undergo at least one rotation to turn end-over-end; this rotation is often enough to initiate the spiralmode.

In plunge, the aerodynamic centre of the seed is alignedvertically above the centre of mass (‘seed-end down’), whichdoes not generate a rotational moment. This offers a muchsmaller cross-section to the vertical on-coming wind, and sodoes little to arrest the descent of the seed (falling 2.5 m inless than 0.75 s).

In the spiral mode, the wing autorotates as expected. Droptests using a strobing LED at 15 Hz and a long-exposurephotograph show the seed falling 2.5 m (see Fig. 3). In spiralmode the seedpod falls at approximately 0.65 ms−1, with arotational velocity of 110 RPM. This compares favorablywith the high velocity of the seed falling the same distancein freefall (approximately 7 ms−1).

V. FUTURE WORK

Future work on the system will increase spiral reliability,and add functionality to prepare the units for a trial de-ployment. Foremost will be the exploration of trailing edgeripples to transition the seed out of plunge mode and into

Fig. 3. Stroboscopic Descent Spirals with 15 Hz LED flash.

spiral descent. Additional systems will include a light-weightlow-speed wireless communication system, and miniatureGPS sensor on a daughter board that solders to the side ofthe PCB opposite the battery. An array of these sensors willbe air-dropped by a helicopter UAV flying a fixed waypointtrajectory; seed pods will report their survival, location andenvironment conditions over the monitoring period.

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