This paper presents an implementation of a bubble actuator array (BAA) based active robotic skin, a modular system, onto existing low cost robotic end-effectors or prosthetic hands for conformal grasping of objects. The active skin is comprised of pneumatically controlled polyurethane rubber bubbles with overlaid sensors for feedback control. Sensor feedback allows the BAA based robotic skin to conformally grasp an object with an explicit uniform force distribution. The bubble actuator array reported here is capable of applying up to 4N of force at each point of contact and tested for conformally grasping objects with a radius of curvature up to 57.15mm. Once integrated onto a two-finger gripper with one degree of freedom (DOF), the active skin was shown to reduce point of contact forces of up to 50% for grasped objects. © (2014) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE).

1. Conformal Grasping Using Feedback Controlled Bubble Actuator
Array
Wei Carrigan†, Richard Stein†, Manoj Mittal, Muthu B.J. Wijesundara*
University of Texas at Arlington Research Institute, 7300 Jack Newell Blvd. S, Fort Worth, TX,
USA 76118
ABSTRACT
This paper presents an implementation of a bubble actuator array (BAA) based active robotic skin, a modular system,
onto existing low cost robotic end-effectors or prosthetic hands for conformal grasping of objects. The active skin is
comprised of pneumatically controlled polyurethane rubber bubbles with overlaid sensors for feedback control. Sensor
feedback allows the BAA based robotic skin to conformally grasp an object with an explicit uniform force distribution.
The bubble actuator array reported here is capable of applying up to 4N of force at each point of contact and tested for
conformally grasping objects with a radius of curvature up to 57.15mm. Once integrated onto a two-finger gripper with
one degree of freedom (DOF), the active skin was shown to reduce point of contact forces of up to 50% for grasped
objects.
Keywords: bubble actuator, conformal grasping, sensorized, robotic gripper, end-effectors
1. INTRODUCTION
Conformal grasping is important for robotic manipulation and handling in dynamic environments and requires highly
dexterous and intelligent end-effectors. Conformal grasping, with maximized contact area, provides security and stability
for the grasped object1. Integration of sensor feedback to the conformal grasping operation is critical for handling objects
with different sizes, shapes, weights, and compliance. Several anthropomorphic active grippers capable of dexterous and
conformal grasping are commercially available, such as Cyber Hand2 and Smart Hand3, and are an active area of
research. These grippers rely on joints with high DOF and require high-dimensional control algorithms with feedback
controls for autonomous operations. Though attractive, the cost of these highly dexterous end-effectors is often between
15 and 30 times more expensive than low DOF and underactuated grippers; this impedes their adoption in large scale
commercial and domestic environments, where the additional functionality is unnecessary. The complex nature of these
end-effectors also serves as a formidable barrier to integrating them with prosthetic hands.
As a result, most current robotic devices and prosthetic hands use very simple end-effectors such as three finger and
parallel jaw grippers4 that are engineered to perform in a very limited set of operating scenarios. The lack of operational
flexibility requires frequent switching of end-effectors to interact with dynamic environments. It has been shown that
conformal grasping capabilities can be achieved in low DOF and underactuated systems using flexible polymers
deformed under pneumatic or hydraulic control5-9. For example, Martinez et al. demonstrated a pneumatically actuated
silicone rubber “tentacle” capable of conformally grasping a delicate object such as a flower5. Hydraulically actuated
flexible silicone rubber fingertips with controllable grasping force have also been demonstrated6,7. These grasping
mechanisms, however, are largely integrated into customized robotic platforms and cannot be directly translated to
existing systems.
Modifying existing end-effectors with soft pads or artificial skin is one approach for improving adaptability and safety10.
The compliant nature of the pads or artificial skin improves shape adaptation leading to an increase in contact area. This
increase helps to reduce point contact forces leading to safe handling of objects with different shapes. However, the
passive nature of these skin materials limits the level of control over the contact force distribution. An active skin, which
expands or contracts based on the size and the shape of the object, could significantly improve the versatility of current
end-effectors. An improvement of this nature would vastly change the current service robot landscape and open new
opportunities for existing low cost end-effectors.
*muthuw@uta.edu; phone 1-817-272-5994; fax 817-272-5952 www.uta.edu/utari/research/biomedical-technologies
† authors contributed equally
Next-Generation Robots and Systems, edited by Dan O. Popa, Muthu B. J. Wijesundara, Proc. of SPIE
Vol. 9116, 911607 · © 2014 SPIE · CCC code: 0277-786X/14/$18 · doi: 10.1117/12.2058253
Proc. of SPIE Vol. 9116 911607-1
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2. This paper describes a novel concept for an active BAA based robotic skin that can be overlaid on existing low cost
robotic end-effectors or prosthetic hands. Details on the approach, design, fabrication, and initial experimental results on
a sensorized pneumatic BAA for conformal grasping are presented. Although integration of visual feedback or proximity
sensors is required for fully automated operation, this work only focuses on applicability of a pneumatically actuated
bubble array to improve the usefulness of widely used end-effectors. The results described here will highlight the ability
of the BAA based robotic skin for conformal grasping, force distribution as well as the capability of applying pre-determined
force to grasped objects. The paper will conclude with plans for future research and describe some near term
(a) (b)
Gripper
BAA
Grasped object
Sensor
applications.
2. SYSTEM DESIGN AND APPROACH
2.1 Approach
The purpose of BAA based robotic skin is to conformally grasp an object with known force through pneumatically
actuated bubbles. The bubbles are integrated with a sensor layer in order to provide closed loop control of the applied
force. BAA robotic skin is attached normal to the gripping surface of an end-effector or prosthetic hand. The sensors
measure the force applied on the object and the sensor feedback will ensure conformal contact is achieved. By presetting
the force value for a given sensorized region, a uniform pressure distribution can be applied to disparate objects. Figure 1
shows the two stage operation for conformal grasping with BAA. In the first stage shown in Figure 1(a), the existing
robotic system will use proximity, touch, and/or visual sensors to reach near the object and determine the optimal
configuration for grasping. BAA based robotic skin then takes over grasping from the existing system and inflates to
conformally grasp the object with a preset force value as shown in Figure 1(b).
Figure 1. Two stage grasping using BAA based robotic skin, (a) initial contact stage; (b) BAA assisted grasping
BAA based robotic skin is realized through a collection of polyurethane bubbles integrated with flexible force sensors.
The bubbles are controlled by either a computer or embedded controller which individually inflates each bubble in the
array through a small compressor and a manifold of micro solenoid valves as shown in Figure 2(a). The control system
monitors the force acting on each bubble and inflates or deflates the corresponding bubble until it matches a set point
value. This set point can be adjusted depending on the type and weight of the object that is to be grasped by the
gripper. Ideally, all the bubbles adjust their pressure until no gap between the bubble and object is obtained resulting in
perfect conformal contact with the object’s surface. This capability eliminates the need for complex mechanical grippers
with multiple joints and high DOF that requires complex control systems. Conformal contact also redistributes the force
around the object and reduces the contact force required at each point of contact. Increase in contact area reduces the
slippage in both translational and rotational directions.
For the system demonstrated in this paper, LabVIEW and a USB data acquisition module (LabJack) are used as a
supervisory controller for the BAA system. This setup reads the analog input signals from the force sensors and controls
the valves to pressurize the bubble array using a miniaturized DC compressor. The miniature 12V valves (Hargraves)
use 350 mW of power when actuated and all connect to a manifold. Force sensors (Tekscan) provide up to 110N of force
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3. measurement with less than a 3% linearity error. The control hardware components are low cost and suitable for rapid
and modular implementation. In future work, a microcontroller will replace the need for LabVIEW and a PC.
Microcontroller Compressor Zr
1 Relief Valve
16
-E /E]
10110 mil.
Pressure
Control
Pneumatic
Control
Plant
Force Sensor Bubble Actuator
Pneumatic Line
- - - - Electrical Line
(a) (b)
Force
Sensor
Array
To Bubble Array
(a) PUR Top mold
It 1 1 1 1 11
Bottom mold Sacrificial material
Ì 1 1 1 ,
(b)
611111111111d
Figure 2. (a) System block diagram (b) software control diagram
2.2 Fabrication Process
The bubble actuators are made using high strength flexible polyurethane rubber (PUR) (Reynolds Advanced Materials).
By adding a flexibilizer to the base and curing agent, patches with a Shore A hardness of 15 were fabricated. This
hardness was chosen because it allowed the bubbles to hold their shape when demolded but still provided a high degree
of compliance. Liquid compression molding and an overmolding process were used to fabricate the array of bubbles.
The bubbles were first molded as a single strip and cured using compression molding and are open at the base. A
thermally sensitive sacrificial material was as then poured into each bubble to hold the cavity while the base was
overmolded. Once the base has been overmolded, the sacrificial material was removed by submerging the bubbles in
boiling water. The number of bubbles in the array presented in this paper is six, but can be scaled up or down depending
on the application. The molds were printed using a sterolithography (SLA) 3D printer from 3D Systems Inc. All molds
were then coated with a thin layer of Parylene C which serves act as a release agent. The fabrication process flow is
shown in Figure 3(a) and fabricated actuator array is shown in Figure 3(b).
Figure 3. (a) BAA fabrication process (b) completed actuator array
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4. BAA consists of six individually controlled bubbles shown below in figure 3b. Bubbles are collinear with a gap of 1mm
between bubbles. The compact arrangement of the bubbles helps to maintain a nearly continuous surface coverage for
contoured shapes. The thickness of the walls of the bubble is between 0.7 and 1.2mm. The top and the base of the bubble
are circular and the inner diameter of the base is 16 mm. Each bubble is individually connected to the pneumatics
manifold using silicone tubing that allows fine control of the local interface pressure between the object and bubble
actuator array strip. Each bubble is then individually inflated up to 4.5 psi to ensure it is capable of holding pressure and
no internal leaks are present. The force sensors are then overlaid on each bubble and held in place using kapton tape.
3. CHARACTERIZATION AND DISCUSSIONS
3.1 Grasp Force Test of Bubble Actuator Array
The grip force BAA is capable of under different inflating pressures was tested using the setup shown below in Figure 4.
The bubble actuator patch was placed in between two parallel acrylic plates and held in place using bolts. Force sensors
were then placed in between the top of each bubble and the top acrylic plate. The bolts on the test setup were then
adjusted to ensure that the plate was making uniform contact with all of the sensors in this test. Pressure inside the
bubbles was then set between 0.0 and 4.5 psi in 0.5 increments. All sensors were pre-calibrated and the relationship
between force values (N) from sensor readouts (V) was obtained before performing this test.
Sensor
Bubble Actuator
Acrylic Plates
0.5
0.45
3- 0.4
.5 035
o 03
a) 0.25
ó 0.2
0.15
U)
Co 0.1
0.05
o
o 1 2 3 4
Inflation Pressure (psi)
4.675 mo
4.175 o
LL
3.675 p)
C
3.175 73
2.675 m
2.175 Z
1675 (j
1175 Ñ
0.675
o
0.175 p
5
Figure 4. Force characterization test setup
The generated force by each bubble was measured through the sensor at each given inflation pressure. As expected, the
applied force to the upper plate from each bubble increased as inflation pressure increased. A slight variation of the
generated force was observed from each bubble and that was attributed to the differences in wall the thickness of bubble
actuators. Figure 5 shows the representative data of sensor reading and generated force for corresponding inflated
pressures. By increasing the inflation pressure from 1.0 to 4.5 psi, we were able to generate forces ranging from 0.2 to
4N. The lateral expansion of the bubble limits further inflation of the actuators and consequently restricts it from
generating greater forces. By changing the bubble architecture for selective thickness expansion of the bubble or using
alternating the bubble fabrication materials such as heat sealable nylon, it is possible to generate higher forces. In order
to address the issue of increased force generation, bubbles fabricated from heat sealable nylon were created. These
bubbles were capable of holding pressures of up to 30 psi all while holding their original shape. Such bubbles will be
integrated in future development of the technology.
Figure 5. Grip Force vs. Inflation Pressure
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5. Based on previously published data using two-finger parallel-jaw grippers, the minimum contact force needed for
lifting a banana, egg and can of soda varies from 1 to 10N11. With the BAA based grasping system, the increased
number of points of contact between the gripper and the object means that the required force per contact is reduced.
More specifics in regards to grasping and contact force with regards to the BAA based grasping system can be
found in section 3.3.
3.2 Conformability testing
The next set of experiments run serve the purpose of demonstrating BAA based robotic skin’s ability to apply uniform
contact force to non-planar objects. The test setup shown in Figure 6 is used to demonstrate the aforementioned ability
using curved plates with different curvature. Figure 6(a) and 6(b) represent the before and after bubble inflation to apply
uniform pressure across a curved surface which have a radius of curvature of 378mm and 565mm, respectively. The
curvature of the plates was chosen such that one plate would result in a 2mm gap between the outermost bubbles and the
contact surface and the other results in a 3mm gap. Once the system is activated with a preset force (voltage) value, the
bubbles were inflated to make uniform conformal contact with the surface. Sensor readings for when both plates have
conformal contact with the actuator layer are shown in Figure 7. The figure shows the sensor readings across BAA based
robotic skin with a preset sensor value of 0.1 and 0.25V. The data show the preset sensor value is reached and conformal
contact to the surface has been made. The sensor to sensor variation is due to the built in error tolerance of +0.05V
relative to the set voltage on the LabVIEW control loop, which was implemented in order to reduce steady state
oscillations.
(b)
EJE
Figure 6. Schematic demonstration and experimental testing of conformability of BAA based robotic skin; (a) initial contact before
actuation; (b) after actuation
0.4
0.35
0.3
ó
0.25
to 0.1
0.05
Sensor Readouts from Each Bubble Actuator
#1 #2 #3 #4 #5 #6
x
X X
X
x x X
0
0 2 3 4 5
Distributed Sensors
6
A Radius of Curvature: 0.565 m
x Radius of Curvature: 0.378 m
-error range for 0.1 V
-- -error range for 0.25 V
7
Figure 7. Sensor readouts from each bubble actuator exposed to different curvatures
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6. Currently the system cannot detect if the grasped surface is beyond the range of travel of the actuator. Future designs
seek to address this problem by incorporating MEMS pressure sensors that can be used to couple internal pressure to the
applied force so that it can detect that contact cannot be made. Another option is to use proximity sensors on the gripper
to evaluate the distance between bubble and contact surface, then selectively inflating the bubbles that are capable of
reaching the contact surface.
3.3 Testing of BAA Based Robotic Skin on a Gripper
In order to assess and compare the bubble actuator arrays for grasping objects, two types of experiments were performed
using the two-fingered jaw gripper shown in the Figure 8(a). The gripper is manufactured in-house using an SLA 3D
printer. The first test was performed without the addition of the BAA to collect sensor reading at the interface between
the gripper and the cylindrical object. When grasping the cylindrical container pictured, only four contact points were
made, two on each side, which is expected due to the shape of the gripper. Force sensors were directly mounted at each
contact point on both sides of the gripper. Sensing data at each contact point were collected when it became stable.
.
(a) (b)
Sensor
Gripper
BAA
Grasped object
Figure 8. Test setup of the BAA based robotic skin on a 1 DOF gripper, (a) griping test without BAA robotic skin;
(b) gripping test with BAA robotic skin
The second experiment was carried out with two BAA robotic skin patches attached to the gripping faces of the finger.
Force sensors were attached on top of the actuators and were normal to the contact area. Gripping of the cylindrical
container with actuator arrays at both sides of the jaw gripper is shown in shown in Figure 8(b). After initial contact was
made the BAAs were inflated and eight actuators made contact with the object. The sensor data were collected once the
actuators reached a stable state. In both cases the gripper was able to hold the object without slipping.
An additional test was conducted to investigate the grasping capability of the BAA on the 1 DOF gripper. A weight of
280g was added to the cylindrical container and the minimum force required to hold the object from slipping was
obtained through the sensor data. Using the BAA resulted in a 50% reduction in the average contact force. Increase of
the number of contact points significantly reduces the contact force per contact point. In these experiments, we were able
to show the viability of integrating bubble actuators to existing low cost grippers to achieve conformal grasping. We
were able show the significant contact force reduction however a high density sensor array is required to obtain the
quantitative measures of contact force and its distribution.
4. CONCLUSIONS AND FUTURE WORK
Based on the preliminary experimental results shown in this work, it was concluded that integration of BAA based
robotic skin to existing low cost and end effectors and grippers can potentially improve the conformal grasping ability.
We were able to show significant contact force reduction due to increase in contact surface area and that would be highly
beneficial when handling fragile objects. The system can be underacted to allow for the use of simple non high-order
controllers and complex kinematics are not required to grasp an object. However, further development and experiments
are needed in order to fully understand the potential of the sensorized BAA. Our future investigation will also include
exploring new materials and/or novel bubble architectures for applications with wide force range. Different dimensions
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7. and distributions of the bubble actuator will be designed and tested to obtain an optimal design of BAA for a wide range
of existing end-effectors and prosthetic hand. Numerical analysis will be performed to determine sensor granularity
based on bubble architecture to fully characterize the force distribution as well as to develop mathematical model for use
in future control schemes. Future efforts will also focus on the reduction of control system size and weight. Finally, we
plan to implement this system to current robotic service platforms such as the PR2 (Willow Garage) and Kuka youBot
(KUKA Robotics Corp.) and compare the grasping capabilities.
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