Practical Aerial Grasping of Unstructured Objects

Paul E. I. Pounds, Member, IEEE, Daniel R. Bersak and Aaron M. Dollar, Member, IEEE

Abstract— We aim to extend the functionality of UnmannedAerial Vehicles (UAVs) beyond passive observation to activeinteraction with objects. Of particular interest is graspingobjects with hovering robots. This task is difficult due to theunstable dynamics of flying vehicles and limited positionalaccuracy demonstrated by existing hovering vehicles. Con-ventional robot grippers require centimetre-level positioningaccuracy to successfully grasp objects. Our approach employspassive mechanical compliance and adaptive underactuation ina gripper to allow for large positional displacements between theaircraft and target object. In this paper, we present preliminaryanalysis and experiments for reliable grasping of unstructuredobjects with a robot helicopter. Key problems associated withthis task are discussed, including hover precision, flight stabilityin the presence of compliant object contact, and aerodynamicdisturbances. We evaluate performance of the initial proof-of-concept prototype and show that this approach to object captureand retrieval is viable.

I. INTRODUCTION

Unmanned Aerial Vehicles (UAVs) are rapidly evolving

into capable mobility platforms that manoeuver, navigate and

survey with much autonomy. However, UAVs ‘look, but don’t

touch’, avoiding contact with their surroundings. A natural

progression is to increase their scope beyond observation

to include interaction with objects. Rotorcraft, in particu-

lar, could hover over objects in order to manipulate them

(Fig. 1). Robust object retrieval and deposition capabilities

are compelling for aerial vehicles because of their unique

abilities — fast traversal of impassible terrain, movement

around 3D environments and unlimited vertical workspace.

Potential applications include object retrieval, intelligence

gathering, explosives disposal and courier services.

The idea of aerial payload retrieval is not new: early

aviators used long hooks to snare mail sacks on the ground as

the aircraft passed overhead [30], allowing towns lacking an

airstrip to send airmail. In the 1950s the Fulton skyhook was

developed by the CIA for retrieving packages and personnel

in the field [6]. The system consisted of a balloon tethered to

the payload. As the retrieving aircraft flew into the tether, a

special prong mounted to the nose snagged the line, drawing

the payload up into the fuselage slipstream for recovery.

Limited examples of autonomous aircraft interacting with

objects have been demonstrated, such as in-flight refueling

[27], [18] and the transport of slung loads, both individually

and cooperatively [3], [2], [20]. In these examples the

interacting object is either not contacted automatically (such

This work was supported in part by the Office of Naval Research grantN000141010737.

P. E. I. Pounds, D. R. Bersak and A. M. Dollar are with Dept. ofMechanical Engineering, Yale University, New Haven CT, 06511, USA.{firstname.lastname}@yale.edu

Fig. 1. Yale Aerial Manipulator Interacting with an Object.

as a load attached by a human operator on the ground) or

else highly structured (eg. refueling drogues with optical

markers). Efforts to develop autonomous helicopter UAV

payload acquisition have similarly relied on structuring of

the target object to simplify the task. One approach consists

of a hanging magnet at the end of a probe to collect ferrous

objects [1], [4]. Another method employs a hook at the end of

a probe to snag a hoop on an object [13]. The need for heavy

structuring of the object limits the potential utility of these

methods. We aim to demonstrate generalized object retrieval

and transport of unstructured objects from aerial platforms.

II. OBJECTIVES

We are developing the Yale Aerial Manipulator, a robot

helicopter integrated with a gripper, to hold and transport

objects (Fig. 1). Capturing an object with a helicopter UAV

poses challenges due to the precise hover positioning needed

to grasp a target with a conventional gripper, and the inherent

instability of helicopters.

Small UAVs are sensitive to gusts and disturbances during

hover which makes maintaining a single position difficult.

Typical UAV station-keeping accuracy is tens of centime-

tres; the Rotomotion SR-100 UAV has a reported 200 mm

position-keeping accuracy [13]. Common positioning meth-

ods used by UAVs, such as GPS, are not yet accurate enough

to place the helicopter within the very small bounds required

for precise manipulation with rigid grippers. The amount of

positional error a typical end-effector may tolerate in order

to capture an object is significantly smaller than such an

aircraft can accommodate; misalignment of the gripper by

as little as 3 mm can lead to failed grasps and large contact

forces. As helicopters are inherently unstable, such forces

can potentially destabilize the aircraft, leading to a crash.

There are several obstacles to be overcome in developing

grasping functionality, specific to the aerial manipulation

task. A naive approach to aerial grasping would make use

of a conventional stiff robot gripper to take hold of objects.

978-1-61284-481-7/11/$26.00 ©2011 IEEE 99

However, this accentuates the factors that make the aerial

grasping task challenging: the need to precisely position the

aircraft over the target object for grasping in the presence

of aerodynamic disturbances, and the inherent instability of

helicopters.

Our approach is to instead use a compliant underactuated

manipulator based on the SDM gripper [8]. The character-

istics of the design — open-loop adaptive grasping, wide

finger span, insensitivity to positional error up to 15 cm —

closely match the challenges of the task. The compliance

in the grasping mechanism makes this problem tractable by

decoupling the motion of the aircraft from the object and

preventing transmitted forces from destabilizing the vehicle

[22]. Underactuation in the gripper allows for adaptivity in

finger joint configuration, with grasps conforming to the

surfaces of irregular objects, producing more robust grips.

Unlike rigid grippers, hooks, magnets or sticky pads, it

requires no preparation or structuring of the target object

(no catches, ferromagnetic material, flat surfaces) in order

to successfully capture payloads in the presence of large

positioning errors.

In this paper we identify key problems associated with

aerodynamics and flight control in aerial grasping. We de-

scribe how underactuated, compliant grippers can be used

to simplify the capture task. A proof-of-concept system for

use in grasping experiments, consisting of a gripper module

mated to a small-scale helicopter platform, is presented. The

system is used to grasp numerous objects and to capture op-

erational performance data. Results from these experiments

are given, along with an analysis of observed phenomena. A

brief conclusion summaries our findings.

III. AERIAL MANIPULATION CHALLENGES

There are several obstacles in developing grasping capabil-

ities hovering platforms. Limited hover positioning accuracy,

flight instability and shift in aircraft trim must all be over-

come to allow successful recovery of an object on the ground

by a helicopter.

A. Hover Station-Keeping Performance

The success of any grasp is highly dependent on placing

the gripper squarely over the target object. Small VTOL

UAVs are sensitive to gusts and disturbances during hover

which makes maintaining a single position difficult. Typical

station-keeping accuracy is tens of centimeters; the Rotomo-

tion SR-100 UAV has a reported 200 mm position-keeping

accuracy [13].

The amount of positional error a typical end-effector

may tolerate in order to capture an object is significantly

smaller than such an aircraft can accommodate. Existing

sensor modalities such as GPS, vision or inertial systems

are not accurate enough to localize the vehicle with sub-

centimeter precision. While it should be possible to im-

prove the position-keeping performance of VTOLs through

advancements in flight control, sensing and aircraft respon-

siveness, this requires substantial work outside the scope of

our current efforts.

Fig. 2. Helicopter Dynamic Model with Elastic Tether.

For an aerial grasping system to be practical, the gripper

must successfully grasp with offsets greater than the range

of landing position error. Compliance in the gripper reduces

sensitivity of the grasp to positional error. Previous test-

bench results of the SDM Hand demonstrated acceptable

object misalignment of up to 100 mm. Crucially, compliance

also reduces the forces transmitted to the airframe through

the gripper; there is little margin to compensate for distur-

bances introduced by contact with the object during landing.

B. Coupled Flight Stability

During contact with the environment, stability of the

aircraft in flight must be guaranteed [10]. When the aircraft

contacts an object, but has not yet lifted it, ground forces are

imparted to the airframe through the manipulator (Fig. 2).

These forces couple with the motion of the helicopter to

produce distinctly different dynamics from free flight. The

gripper can be treated as an elastic tether connecting the

helicopter to a fixed object.

Previous tethered helicopter stability research has consid-

ered the helicopter flying far from the tether point, where the

line tension and direction is approximately constant and the

tether cannot support moments [12], [26], [21]. In this case,

however, the elastic tether link has both prismatic and angular

reaction forces which change sign as the aircraft moves over

the equilibrium point. If not correctly compensated-for by the

flight controller, these forces will destabilize the helicopter,

leading to a crash. Rather than develop special controllers

solely to stabilize the aircraft during the aerial manipulation

task, it is desirable to show that a standard Proportional

Integral Derivative (PID) control architecture will remain

stable during hovering object capture for a given gripper and

helicopter configuration.

Passive ground effect and inflow damping produces in-

herently stable vertical dynamics in rotorcraft; added tether

elasticity does not destabilize vertical motion. However,

the combined dynamics of the object-helicopter system in

coupled horizontal and pitch motion can be destabilized by

certain tether stiffness configurations. It was shown in earlier

analysis that a hovering helicopter grasping an object fixed

to the ground with a compliant gripper under PID control

100

will be stable for if the ratio of lateral and angular tether

stiffnesses, kx and kθ are below some value:

kxk′θ

<mh

I(h+ q1dz)(1)

where m is the mass of the helicopter, h is the height of

the rotor above the helicopter center of gravity, I is the pitch

rotational inertia, q1 is rotor flapping angle linear velocity

coupling and dz is the tether length [22]. Intuitively, a

helicopter with zero lateral stiffness and free to slide whilst

held level by high angular stiffness will not be unstable,

whereas a helicopter held with a pin joint and free to rotate

will pivot into the ground.

C. Payload and Trim

Given a successful grasp, the helicopter must then trans-

port its payload. Helicopter rotors produce a limited amount

of thrust to lift the combined weight of the vehicle and cargo

plus an additional fraction to allow for vertical acceleration,

called the ‘thrust margin’. Typical thrust margin for vertical

take-off and landing UAVs in hover is 30 percent of the

vehicle weight [23], while specialized acrobatic model he-

licopters can have thrust margins in excess of 100 percent.

These aircraft can carry up to half of their unloaded weight,

and still maintain 30 percent thrust margin.

Conventional helicopter balancing places the payload’s

center of mass coaxial with the rotor shaft so that the thrust

force is aligned with the load. Due to the limited positioning

accuracy of current hovering platforms and the uncertainty

inherent with grasping a potentially unknown object, the

payload may be significantly off-center when grasped.

Small weight imbalances can be trimmed out by adjusting

the cyclic blade pitch inputs, so that the thrust vector is

tilted to pass through the true center of mass. One degree

of cyclic blade pitch results in one degree of thrust angle

deflection [15]. A typical small helicopter has a useful blade

pitch range of 20 degrees [24], which requires that the

center of mass of the helicopter/gripper/object system fall

within a cone of approximately 10 degrees from the rotor

axis (Fig. 3a). However, utilizing the full range of cyclic

control to actively trim out load offsets comes at the expense

of limited maneuvering control. An unbalanced helicopter

has less ‘maneuvering margin,’ the available cyclic range to

affect maneuvers [15]. If the combined center of mass is

sufficiently far from the rotor axis that it lies outside the

cyclic cone, the aircraft cannot counteract the offset and

regain trim (Fig. 3b).

IV. GRIPPER FOR AERIAL GRASPING

As described above, conventional robot graspers require

a level of precision incompatible with that of a typical

helicopter platform. The SDM Hand has two crucial fea-

tures that enable reliable grasping in the presence of large

positioning errors: passive mechanical compliance in the

joints, and an adaptive and underactuated transmission. These

have substantial advantages in facilitating the capture of

target objects from hovering platforms with poor positional

accuracy.

Fig. 3. Cyclic Control Limits of a Helicopter: Center of mass inside (a)and outside (b) cyclic range.

A. Mechanical Compliance

One approach to dealing with positional uncertainty is

through compliance in the grasping mechanism, so that offset

errors do not result in large forces and the mechanism

conforms to the object.

Robot compliance has often been considered in the context

of active control, where sensors and actuators are used to

achieve a desired force-deflection relationship, with many

studies devoted to impedance analysis and control (e.g.

[25], [19], [7]). However, this requires the active use of

position/velocity and force/torque sensor signals in the robot

joints or end-effector. This approach fails in impacts, where

the speed of the force transients is too fast to use sensing

and control to avoid damage, as well as when contact occurs

at locations on the mechanism that are not sensorized.

In contrast, passive mechanical compliance, implemented

through springs in robot joints, allows for large joint deflec-

tions and can ensure low contact forces. This can minimize

disturbance to objects during the first phases of acquisition

and in impacts, where active stiffness control often fails. The

elimination of the sensing required for active compliance can

also lower weight and increase durability.

B. Adaptive Underactuation

It is desirable for a gripper to have many links and

joints to allow for object conformation and form closure.

However, due to limitations in space, manipulator payload

and available control channels, there is a need to simplify

the number of actuators and, hence, controlled degrees of

freedom of robot hands.

Mechanical coupling allows many joints with few actua-

tors and can take the form of both fixed-motion joint coupling

and underactuation. A ‘fixed-motion coupled’ hand has more

joints than degrees-of-freedom, with each degree-of-freedom

controlled by a dedicated actuator, affording the mechanism

no adaptability (e.g. [5], [17]). In these hands, motion of

one joint always results in a fixed proportional motion of

the joint(s) coupled to it. In the same way, if contact occurs

on one joint thus fixing its position, all coupled joints are

thereby fixed. An underactuated hand has fewer actuators

than degrees-of-freedom, and therefore demonstrates adap-

tive behavior. In these hands, motion of the distal links can

continue after contact on the coupled proximal links occurs,

allowing the finger to passively adapt to the object shape

101

Fig. 4. Yale Aerial Manipulator Systems.

(e.g. [9], [28], [29], [11], [14], [16]). By carefully selecting

joint coupling schemes, much of the functionality of a hand

can be retained while reducing the number of actuators and

the overall complexity of the grasping mechanism.

A key aspect of the hand’s tendon transmission design

is that it allows the hand to be very compliant during

approach and initial contact, and stiffer during grasp due

to the compliance in the fingers being placed in parallel

with the actuator. Before the hand is actuated, the tendon

cables remain slack and the hand is in its most compliant

state, keeping contact forces low and maximizing passive

adaptability. After actuation, the tendon is tensioned, taking

much of the compliance out of the fingers, resulting in a

stiffer grasp with greater stability. This method permits use

of actuators that are not backdrivable and prevents inertial

load of the actuator from increasing the passive stiffness.

V. HARDWARE PLATFORM

A proof-of-concept prototype was constructed to test the

feasibility of aerial grasping with a gripper. The demonstrator

consists of a compliant underactuated gripper fitted to a

commercial radio-controlled helicopter.

The helicopter platform is a T-Rex 600 ESP (Align RC,

Taiwan), a large high-end fully-electric aircraft with a 1.35 m

diameter rotor driven by a 1.6 kW brushless motor, boasting

a maximum carry weight of 1.8 kg (Fig. 4). The gripper

is mounted ventrally between the skids. The skids retract,

allowing the gripper unobstructed access to objects on the

ground. The aircraft is stabilised by Helicommand RIGID

commercial off-the-shelf control system (Captron, Germany),

and directed by a human pilot. The Helicommand systems

regulate aircraft flight attitude with inertial sensors and

use a downward-pointing optical system to compensate for

drift. The system was later upgraded to a Helicommand

Profi, which adds additional optics and a pressure sensor

to allow hover position-keeping and height stabilisation. The

T-Rex has a cyclic control range of 20◦; when fitted with

the gripper, this gives the platform a maximum allowable

payload offset of 53 mm for a 1 kg load.

Compact routing of the adaptive gripper transmission is

required to reduce weight and size for mounting in the

available space under the helicopter. The prototype gripper

is actuated by a pair of HSR-5990TG high-performance

hobby servos (Hitec, South Korea) operated in parallel and

controlled by an onboard microprocessor. A pair of sense

Fig. 5. Flexible Compliant Gripper Module.

resistors measures current to each servo and the micropro-

cessor modulates the servo position commands to regulate

constant effort applied by the gripper. The servo controller

also limits the stall current of the servos, preventing damage

from overheating. The complete gripper assembly, including

servos, batteries and transmission, weighs less than 750 g.

As the helicopter can only approach objects from above,

all grasps are finger-tip or power grasps with opposing

finger forces. Buckling modes in finger-tip grasps can lead

to azimuthal instability, due to the limited torsion stiffness

of the proximal joints. Early experiments showed that this

twisting would regularly eject objects from a fingertip grasp

as, unlike in a power-grasp, the flats of the finger pads had

insufficient contact to stabilize the twist motion. To improve

grasp stability, ‘syndactylic bars’ joining the proximal links

of adjacent digits together were added, constraining them

to move as a pair while still allowing the distal links to

tension independently. This adaptation prevents the proximal

joints from twisting due to a force couple induced by

the intermeshing finger configuration. The addition of the

syndactylic bars has largely eliminated this effect, but at the

expense of lower overall adaptability of the gripper.

To acquire an object, the helicopter approaches the target,

descends vertically to land over the target and then closes its

gripper. Once a solid grasp is achieved, the aircraft ascends

with the object. The UAV manipulation system has thus far

demonstrated grasp and transportation of objects up to 1.5 kg.

In bench-tests, the gripper was able to stably grasp a range

of sizes and shapes of objects, with sufficient force to lift

masses well above the helicopter’ lifting capacity (>2 kg).

VI. EXPERIMENTAL PERFORMANCE EVALUATION

The ability of the system to grasp a variety of objects

(Fig. 6), and the reliability of the system grasping a single

object repeatedly, were tested experimentally. For these tests,

an expert pilot directed the helicopter with a radio handset,

taking care to avoid ground vortex phenomena. While this

introduces non-repeatable human influences, the expert pilot

is considered to be a near optimal controller with perfor-

mance representational of that of an equivalent autonomous

system.

102

Fig. 6. Target Objects. Fig. 7. Flight Trajectory for Capture. Fig. 8. Onboard Camera View.

For each grasping attempt, the pilot flew the helicopter

from a starting position greater than 1 m away from the

target, attempted to come to a hovering stand-still above the

target, and descend vertically to the ground (viz. Fig. 7). Less

than 30 seconds were spent on each attempt. The pilot stood

4.25 m up-range from the target, behind the helicopter.

The system was used to attempt to grip a series of

objects of different sizes, shapes and masses (Fig. 9b): a

softball (160 g, 89 mm), a PVC tube (390 g, 280 mm),

a water bottle (600 g, 220 mm), a wood block (700 g,

265 mm) and a cylinder (900 g, 390 mm). Each object was

attempted several times, recorded by an onboard camera and

an offboard camera. In each trial, the lateral positioning error

was extracted from the onboard camera, and the longitudinal

error from the off-board camera, utilizing known reference

points on the helicopter and the object edges.

The grasps were completed over two sets of trials. A

grasp was considered ‘successful’ if the gripper held the

object and the helicopter could lift, transport and return the

object without dropping it. Two objects, the block and hollow

tube, had no failed grasps. The water bottle, cylinder, and

softball had 67 per cent, 70 per cent and 80 per cent success

rates respectively. The gripper demonstrated object capture

even with significant positional error, successfully grasping

objects with as much as 103.5 mm longitudinal offset and

46.7 mm lateral offset.

The reliability of the gripper was tested by landing on an

object known to demonstrate good grasping performance, the

700 g wood block (Fig. 9c). The block center of mass was

determined using the onboard camera.

Twenty attempts were made of which all grasps succeeded,

even with position errors in excess of 100 mm longitudinally

and 40 mm laterally. Although the increasing experience of

the pilot in performing the grasping task may play a role in

the perfect result of this experiment, all grasp attempts of

the block in the previous trials were also successful.

As the gripper is constrained to always grasp objects

from above, all grasp attempts are functionally similar.

Consequently, with one exception, the failure mode observed

during gripping consisted of the object being grasped and

lifted, but slipping out of one pair of fingers, and rotating

downwards before sliding out of the grasp entirely. In the

exception, the softball had an displacement error 90 percent

of its diameter, which instead caused the gripper to eject it

during the grasp attempt.

Fig. 9. Assorted Object Grasp Scatter Plot (a), 700 g Block Grasp ScatterPlot (b).

In no trials did the gripper entirely fail to grip and lift an

object which was within its grasp, even if it did not retain

it. Only one object within 40 mm of center failed grasping.

Objects were generally more sensitive to lateral error than

to longitudinal error, primarily due to the aspect ratio of the

tested objects, with most successful attempts being within

0–40 mm of the aircraft centerline.

The geometries that gripped best were rough textured

objects with flat sides that allowed the gripper to resist slip

and rotation. The block performed especially well, as the top

edges of the block contact the finger pads of the proximal

links as the block rotated downwards, producing a stable grip

akin to a power grasp. The water bottle performed poorly

because its smooth sides and uneven contours promoted

slipping, and as the bottle rotated the grasping pair of fingers

would squeeze the object out of the grip. The heavy cylinder

slipped easily out of grasp on one end, but was too big to be

ejected by the grip; instead, its great weight pulled it free of

the two gripping fingers due to insufficient friction to keep

it in place.

Across the objects attempted, the helicopter grasping sys-

tem demonstrated 85 per cent success out of more than

50 grasp attempts, rising to 100 per cent success for the

best performing objects. However, the objects chosen did

not span the entire range of potential target objects. The

current gripper design was not optimized for the described

application, and significant future work will be devoted to

enable grasping a wider range of object size and shape,

particularly for small objects.

103

A. Observed Phenomena

During the experiments the helicopter flight controller

maintained control of the vehicle at all stages. Although the

grasp offset expected from the target landing trials would

suggest that there is a high likelihood of grasping a heavy

object outside the range where the cyclic control margin

would saturate, this did not occur during the tests. Once

grasped, the weight of off-center objects caused the gripper

to droop, bringing the mass of the payload closer to the rotor

axis, and inside the limits of the control margin.

The greatest applied torque during the test was from the

900 g cylinder with an offset of 91 mm (applying a 0.8 Nm

bias load) which was carried without ill effect. This same

object, the largest object, could be released by the operator

mid-flight without producing large pitch or roll excursions.

VII. CONCLUSION

We have demonstrated a helicopter UAV-gripper system

capable of reliably grasping and retrieving objects under

human control. This system exploits the unique performance

capabilities of an underactuated, compliant gripper to directly

address the particular challenges of helicopter imprecision.

The system was able to grasp a variety of objects including

blocks, balls, bottles and cylinders, ranging from 160 g to

900 g. To our knowledge, this is the first time a helicopter

UAV has directly captured and retrieved objects with a

gripper. As preliminary approach to the problem, the gripper

design is not fully optimized for this task.

The most difficult object could be grasped 67 percent of

the time, and the easiest object 100 percent of the time. Load

bias disturbances of 0.8 Nm applied to the airframe by the

payload were rejected by the flight controller around hover.

Future work will consider tuning the gripper for improved

grasping and considering control aspects of grasping during

flight and from unstable platforms.

VIII. ACKNOWLEDGEMENT

The authors would like to thank Joe Acosta of Build

Right Fly Right Hobbies, Wallingford CT, USA, and Gre-

gory Brown for their support of this project. This project

was funded in part by US Office of Naval Research grant

#N000141010737.

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[30] U.S. Centennial of Flight Commission, “Air-mail and the Growth of the Airlines,”http://www.centennialofflight.gov/essay/Government Role/1930-airmail/POL6.htm (Sept. 2010).

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