2018 IEEE International Conference on Soft Robotics (RoboSoft)
Livorno, Italy, April 24-28, 2018

An Addressable Pneumatic Regulator for Distributed Controlof Soft Robots
Joran W. Booth1,† , Jennifer C. Case1,2,† , Edward L. White2 , Dylan S. Shah1 , and Rebecca Kramer-Bottiglio1∗
Abstract— In this paper, we describe a digitally-controlled,miniature, multi-mode pressure regulator for integration directly into centimeter-scale soft robots. This regulator integrates
best-of-class commercially-available pneumatic valves and pressure sensors to achieve a small and lightweight servo-controlledpressure regulator. We demonstrate that the regulator is able to
track both step and ramp commands and quantify the error in
the resulting pressure. In order to facilitate integration of manysuch regulators into a single soft robot, we have implemented
an addressable digital communication system, based on theindustry-standard I2C bus. This allows us to connect up to
127 regulators on a single 4-line bus, significantly reducing themass and complexity of the required wiring.

I. INTRODUCTION
Soft robots use low-modulus and multi-functional materials to create highly deformable robotic systems. The
resulting systems possess many novel properties, such as
light weight, robust structures, and the ability to operate in
unstructured environments. The majority of research in thefield to-date has focused on these novel structures, while
comparatively little attention has been paid to the support
elements which enable these capabilities. In the case ofpneumatically-powered soft robots, pressure regulators are
required to control the inflation of the actuators. The majorityof current pneumatic robots use off-board pressure supplies
and regulators and require a tether between the robot and the
hardware. This strategy allows for the use of commerciallyavailable pressure regulators, but limits the range and utilityof the resulting robots.
An important goal for pneumatic soft robots is to havegeneralizable strategies for easily creating untethered robots.
In this paper, we present a miniature pneumatic regulator
capable of continuous pressure control, small enough to fit
on centimeter-scale soft robots (Fig. 1). The regulators canoperate in two modes: a servo-controlled mode for distributed
closed-loop operation and an manual mode to enable offboard control. In servo-controlled mode, the regulator can becommanded to achieve a specific pressure. In manual mode,
the regulator can be commanded to execute specific states,
including fill, release, and hold, allowing the valves within
the regulator to be activated from off-board. Each regulator iscommanded over an addressable digital bus, enabling groupsof regulators to achieve distributed control on a robotic body.

Fig. 1: The addressable pneumatic regulator. The regulator isable to imitate the function of a proportional valve, despite
its compact size. By using the I2C protocol, the regulator isable to achieve distributed actuation while staying in a size
small enough for centimeter-scale soft robots. The regulator
measures 22 x 50 x 20 mm. The scale bar represents 1 cm.

This work advances the state of the art in soft robotics
by simplifying valve control and distributed control in soft
robots.
II. BACKGROUND
Several groups have worked towards the goal of untethering pneumatically actuated soft robots. Tetherless robots
have been created using several strategies. One strategy is
to make the robot so large and correspondingly powerful,that it can lift its own power supply [1]. While this strategy
is successful for meter-sized robots, the size and weight
constraints of commercially available pneumatic components
make it infeasible for smaller scales. Another strategy has
the robot drag its power supply behind it on wheels [2],[3] or have an additional rigid robot carry the soft robot’s
power supply [4]. This strategy is good for testing robots,
but may limit their use for in-field applications, such as
search and rescue. An additional approach is to use chemicalpressure generation, often through the decomposition of amonopropellant such as hydrogen peroxide [5]–[9] rather
than to use an electrical air pump. This approach is easily
scaled to centimeter-sized robots or smaller and is moreweight efficient than a traditional pneumatic system. A final,

1 School of Engineering & Applied Science, Yale University, New Haven,
CT, USA2 School of Mechanical Engineering, Purdue University, West Lafayette,
IN, USA∗ To
whom correspondence should be addressed; E-mail:rebecca.kramer@yale.edu† J.W. Booth and J.C. Case are co-first authors.

978-1-5386-4516-1/18/$31.00 ©2018 IEEE25

partial step, is to locate the control valves on the robot. Some
research groups have incorporated miniature valves into theirdesign [5], whereas others have created custom soft valves
that are built into the robot body [5], [6], [9]. All theseapproaches are still nascent and have to be custom-designed
for each new robot.
One problem with embedded valves, to date, is that they
are limited to on-off functionality only. In addition, theembedded soft valves are driven by pressure gradients andcurrent examples are passive and unable to be controlled bya central processor. While fluidic control has been demonstrated for a logic circuit [9] and as an addressable array ofvalves [10], each requires a highly customized system that is
not generalizable. Further, fluidic control generally requires
bulky and elaborate vascular systems for basic functions. It
is unlikely that fluidic control will be a significant strategy
for controlling pneumatic soft robots in the near-term. An
additional problem with the current state-of-the-art is thatlarge numbers of solenoid valves require complex wiring.
Finally, all the examples of embedded valves discussed aboveuse 2-way or 1-way valves. However, in many circumstances,
it is desirable to hold a specific pressure in the actuator,
which is why proportional valves are often used in off-board
setups. Our approach to address these shortcomings is aminiaturized board capable of distributed control for variable
pressure regulation.

(a)

(b)
Fig. 2: (a) Photo and diagram showing how airflow occurs
through the valves on the pressure regulator. The scale bar
represents 1 cm. Alternate airflow configurations are possible with minimal changes to the firmware. (b) Schematic
showing the functionality of the regulator.

III. M ATERIALSThe pneumatic regulator board is composed of a pressure
sensor (Honeywell, ASDXRRX030PDAA5), two miniature
2-way valves (Parker X-valve, 912-000001-003), and a custom circuit board (see github link [11])with an embedded microcontroller (Microchip,
PIC16F1825T-I/ST). Firmware for the PIC and for an
Arduino can also be found in [11]. The above pressure
sensor and 2-way valves are limited to 207 kPa (30 psi).
The resolution of the pressure sensor is 2% of thetotal range, which in the case shown in this paper is±8.3 kPa (1.2 psi). While the pressure sensors and 2-way
valves demonstrated in this paper are limited to 207 kPa,
we can achieve pressures up to 690 kPa (100 psi) by
swapping the pressure sensor and valves with 100 psi
versions (Honeywell ASDXRRX100PGAA5 and Parker
912-000001-021, respectively). The board uses I2C protocol
to communicate with an external microcontroller (e.g.,
Arduino, RaspberryPi). This architecture allows up to 127
regulators to connect to a single microprocessor. By using
I2C protocol, only four lines are needed (power, ground,clock, and data), resulting in a simplified wiring harness forlarge numbers of regulators. The valves and pressure sensor
are mounted on one side of the board, and the remaining
circuit is mounted on the other side. The complete regulator
assembly, including tubing, weighs 18 g. The regulator
assembly is 22.2 mm x 49.5 mm x 20.0 mm without tubing.
With tubing, it is about 22.2 mm x 56 mm x 30 mm, though
this could be improved for a specific application.

The regulator is capable of replicating the function of
a 3-way valve, which is not commercially available at thesize of our modules. It does so by opening the first valve,
which connects the supply in-line pressure to an actuator;
closing both valves to hold the air in the actuator; andopening the second valve to release the air from the actuatorto the atmosphere. A diagram of the port configuration isfound in Fig. 2. The regulator can operate in two modes:
manual and servo-controlled. In manual mode, the servo
receives state commands (release, hold, or inflate) over the
I2C communication lines and executes those states. Without
the hold state, the system would need to alternate between
fill and release to maintain a given pressure. The hold state
reduces chatter and energy consumption and, thus, increases
the lifetime of the regulator. Manual mode allows an external
controller to actuate regulators, and may be used for openloop control, or with an off-board logic controller that can benested inside a feedforward or sensor-based control loop. In
servo-controlled mode, an external microcontroller sends adesired pressure to the regulator over the I2C communication
line. The regulator then uses the on-board sensor and logic
controller to achieve the desired pressure. We implemented
a simple bang-bang controller with a minimum deadband of
1 bit, which corresponds to 2 kPa. The sample frequency isconstrained by the time it takes to operate each control loop,
26

A. Comparison
While many researchers do not report their pressure regulators, we compare our regulators to the fluidic control board
reported on the Soft Robotics Toolkit website and the few
reported hardware components we could find in the literature.
This comparison is shown in Table I. Our pressure regulator
is much smaller and lighter and performs the same numberof functions or more than other reported systems.
B. System Characterization
1) System Response: We characterized the system response of the pneumatic regulator servo by measuring settling time, percent overshoot, and steady state error of thesystem using a McKibben actuator. Ten trials were collected
at pressures ranging from 5 to 25 psi at 5 psi intervals (or 34,
69, 103, 138, 172 kPa). Each trial lasted 30 s. The settling
time was defined to be how long it takes the system to reachand stay within 5% of the set point. The percent overshootis found as the maximum value before the settling time.
The steady state error is the set point minus the averagepressure. This error was calculated using the average of thedata after the system is considered settled via the calculated
settling time. The results of the first second of these trials areshown in Fig. 5 and the settling times, percent overshoots,
and steady state errors are presented in Table II.
The settling times for the McKibben actuator are all under
0.3 s. The settling times for the low pressures (34 and 69 kPa)
were below 0.1 s. The settling time for the highest pressure(172 kPa) did not overshoot the desired pressure and, thus,
the settling time is based on how long it takes to inflate to
the desired pressure (0.17 s), which was very consistent, as
can be seen in Fig. 5. This may be due to the fact that thepressure is close to the in-line pressure, which was chosen
due to component limitations on the regulator. In contrast,
the settling times for the 103 and 138 kPa trials were longer
due to the system oscillating before coming to rest. We
suspect that while filling at these intermediate pressures, themomentum of the air entering the actuator creates a shockwave and resulting spike in pressure that is not sufficiently
damped by the elasticity of the air or the actuator, similar
to water hammer. An important consideration is that thesettling time for actuators is limited by the pressure, flowrate, and inflated volume of the actuator, which means thatlarger actuators will take a longer time to reach a desiredpressure.
Looking at percent overshoot, we see that the lower
pressures have larger overshoots than the higher pressures.
We suspect this is due to the larger difference in the desired
and in-line pressure for lower pressures. The steady state
error shows that for all the pressures that we tested, we geterrors within the range of the accuracy of the pressure sensor
itself (±8.3 kPa). Therefore, the limitation in our regulators
is due to the accuracy of the sensor rather than our controller.
2) Power Consumption: The power consumption of thepressure regulator depends on which state is active and iscomposed of two independent components: air and electrical
power. The air power consumed by a robot depends on theFig. 3: (a) The control loop used by the on-board microprocessor when in servo-controlled mode. (b) The state machineused for the logic controller. When filling or releasing, thesystem stops and holds the pressure when the input pressureis crossed. However, it resumes filling or releasing if thepressure reading falls outside the band around the input
pressure.

Fig. 4: Drawing of McKibben actuator with dimensions
labeled.

and is 17.8 kHz. Fig. 3 shows the logic controller and statemachine used by the on-board microprocessor. The following
sections highlight the performance of the regulator in servocontrolled mode, since this allows for distributed control ofmultiple actuators.
IV. R ESULTS & D ISCUSSION
In order to highlight the utility of our pressure regulators, we first compare various parameters of our system toother existing pneumatic systems that have been reported.
Additionally, we characterized the regulators looking at both
system response and power consumption. We also show thesystem performance in terms of step and ramp inputs aswell as response to disturbances. For our experiments, weused an in-line pressure of approximately 190 kPa and hada band of 2 bits on our servo-controlled pressure regulator,
which corresponds to 4 kPa. Our characterization and system
performance tests were done with a McKibben actuator thatwas 1 cm in diameter and 9 cm in active length when at rest,
as seen in Fig. 4. Finally, we demonstrate these regulators
on a five-actuator multi-gait soft robot [18].
27TABLE I: A comparison with other common pneumatic valve control systems used in soft robotics research. This comparison
is not comprehensive since few research groups publish details on their off-board hardware. The Parker X-Valve is the sameas on our regulator, but we include it in the comparison since it could be used independently. The Soft Robotics Toolkit(SRT) board includes more functions than our regulator provides, such as pressure generation, physical switches and an
Arduino. We omit these elements when calculating the volume for this board.
Valve and Controller
Faboratory Regulator
Soft Robotics Toolkit [12]COTS Festo CPV 14-VI 18210 [13]COTS SMC ITV1031-21F1N-Q [14], [15]Range0-207 kPa
0-700 kPa
0-1000 kPa
0-500 kPa

# Actuators
1
4
6
1

Package Volume
22 cc/actuator
770 cc/actuator
236 cc/actuator
244 cc/actuator

Flow Rate
6.4 Lpm
174 Lpm
800 Lpm
200 Lpm

Volume Specific
Flow Rate
290 s−1
226 s−1
3390 s−1
818 s−1

Weight
18g
–
1530g
350 g

Control
Logic
PWM
PLC
PLC

Valve Only
COTS Parker X-Valve (207 kPa) [16]COTS Parker Ten-X (207 kPa) [1], [17]0-207 kPa
0-207 kPa

1
1

2.2cc/actuator
9.6cc/actuator

6 Lpm
12 Lpm2730s−1
1250s−1

4.5g
12g

N/A
N/A

TABLE III: Power characteristics for the pressure regulator,
by state. Power expenditure due to air is not shown since it
is dependent on the application and can be estimated withthe flow rate.
State
Hold
Fill
ReleaseTABLE II: Characterization for 9 cm McKibben actuator atgiven desired pressures. Ten trials are used to calculate means
and standard deviations.
Settling Time(s)
0.08 ± 0.04
0.09 ± 0.01
0.27 ± 0.12
0.21 ± 0.14
0.17 ± 0.00

% Overshoot(%)
10 ± 612 ± 4
6±4
3±2
0

Amps
0.022A
0.105A
0.105A

Power
0.264W
1.26W
1.26W

application, but can be easily estimated if the flow rate is
known. As a result, we do not report power consumption
due to air, but as an aid to future researchers we report the
maximum flow rate that our regulator can achieve. In our
experiments with the complete regulator assembly at room
temperature, we measured a flow rate of 6.4 Lpm for 138 kPaover 10 trials using a 30 cm length of 3.2 mm (1/8”) innerdiameter (ID) tubing, but measured a flow rate of 4.7 Lpm
when we used 90 cm length of 1.6 mm (1/16”) ID tubing.
The electrical power consumed by the module is dependent on which state it is in. The regulator has threestates: fill, hold, and release the air. The hold state consumes
less power than the fill and release states. The fill andrelease states actuate a solenoid valve (X-Valve 912-000001021, by Parker) which consumes 1 W when activated. The
control board consumes 0.022 A, resulting in a power drawof 0.264 W. When in the hold state, the board consumes
0.264 W. When in the fill or release states, the regulator
consumes 1.26 W. These results are summarized in Table III.

Fig. 5: System response for various pressures. The desiredpressure is shown as black dashed lines with a 5% bandshown in a gray cloud around the dashed line. The averaged
response for 34 kPa is shown as blue; 69 kPa is shown as
red; 103 kPa is shown as green; 138 kPa is shown as cyan;
and 172 kPa is shown as magenta. The colored clouds aroundthe averages represent 95% confidence intervals for 10 trials.

Pressure
(kPa)
34
69
103
138
172

Volts
12V
12V
12V

C. System Performance
To demonstrate the capabilities of the regulator to follow
commanded pressures, we tested the regulator using bothstep and ramp inputs. First, the regulator was commanded
to hold each pressure (0, 34, 69, 103, 138, and 172 kPa)for 5 s stepping both up and down. Then the regulator wascommanded to ramp up from 0 kPa to 172 kPa and backdown over the course of 45 s. This procedure was done for10 trials using our McKibben actuator. The results are shownin Fig. 6. This shows the capabilities of the regulator to not

Steady State
Error (kPa)−0.0 ± 0.4−1.0 ± 0.2−1.2 ± 0.1−1.5 ± 0.1−1.8 ± 0.1

28

Fig. 6: Step and ramp responses of pressure regulator. The
black line shows the desired pressure while the blue line
shows the achieved pressure. The colored cloud around theaverage represent 95% confidence intervals for ten trials.

Fig. 8: Images of a soft locomotion robot with five regulators
on board. Note that the tether consists of only one supply
line and four wires. The scale bars represent 5 cm.

these disturbances for 10 trials. When the pressure increase
was imposed on the actuator, the servo-controlled regulator
was not able to fully compensate and we observed a pressure
increase on the order of 10 kPa. This was because the airflow
into the actuator from the manual regulator was higher
than the airflow out from the servo-controlled regulator. The
inflow rate and outflow rate are different in all pneumatic
valves, so this behavior is expected. Further, the pressure
plateaus, suggesting that the increased pressure and therefore increased inflow rate comes into equilibrium with theoutflow rate. When we used the manual regulator to vent theactuator, we observed an immediate disturbance that quickly
settled back to 103 kPa. The servo-controlled regulator fullycompensated for the leaking. The results of this experimentdemonstrate that the regulators are able to compensate forminor leaks in actuators and other disturbances.

Fig. 7: Disturbance response of actuator (shown as the blueline) when exposed to approximately 190 kPa pressure for
1 s (green) and atmospheric pressure for 1 s (red). The blue
clouds represent the 95% confidence intervals for 10 trials.

D. Robot Application
In order to demonstrate the regulators on a soft robot, wehave utilized the multi-gait soft robot design from Shepherd
et al. [18]. We chose this robot because it has multiple actuators and is well-known. Previously, this robot (approximately
14 cm in length) required five pneumatic tethers going to
off-board control of its four legs and central body. A larger
scale version of this robot (approximately 65 cm in length)
has been previously made untethered by having the robot
itself carry its power supply [1]. In contrast, the robot we
employ here is approximately 20 cm in length and requires
only a single pneumatic tether and a set of four wires forcommunication with the regulators. To position our pressure
regulators on-board the quadruped soft robot, we strapped
the regulators onto its back, as seen in Fig. 8. While thisregulator placement is non-ideal since it limits the inflation of
the central actuator, we are still able to show that regulators
control the inflation of the limbs (Fig. 8). Future robots that

only hold a specific pressure, but to track desired pressure
over time.
Because the logic controller is on-board, we are able totake advantage of the distributed control. The regulators
are able to locally respond to disturbances, such as thosecaused by external conditions. To demonstrate this feature,
we took our McKibben actuator and briefly exposed it to both
190 kPa and atmospheric pressure. In order to do this, weused two regulators that connected to the same actuator. One
regulator was run in servo-controlled mode to maintain an
actuator pressure of 103 kPa while the other regulator was
run in manual mode to introduce the changes in pressurefor 1 s intervals. Fig. 7 shows the regulator’s response to29

use these regulators should consider the placement of thecomponents and accommodate for them during the designof the robot.
One important case where our regulator design may beespecially useful is in robot implementations where there
is significant variability between actuators. For example, inthe case of the multi-gait soft robot, each leg actuates toa slightly different position in response to the same input
pressure. This problem is due to inherent imperfections in
the manufacturing or material defects and is common insoft actuators. Using our regulators and a calibration, thevariations between actuators could be corrected using onlya single pressure supply to the robot. Furthermore, onboard pressure regulators would allow actuators with vastlydifferent pressure requirements to use the same pressure
supply.

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V. C ONCLUSION
In this paper, we have demonstrated the functionalityof pressure regulators that can be used for soft robots.
These regulators are designed to be compact enough to be
put on-board centimeter-scale soft robots while also giving
full control of pressure and allowing for addressability of
individual actuators. We revealed basic characteristics of thepressure regulator and demonstrated its ability to follow adesired pressure over time, as well as its ability to respondto disturbances in pressure. Finally, we showed briefly how
these regulators can be placed on a soft robot to reduce
the number of pneumatic tethers in the system. One impactof this work is that the regulators can be used to accountfor the variations that typically arise in the manufacture ofsoft actuators. Additionally, because these regulators can becontrolled remotely using I2C communication, they can bescaled to large arrays while maintaining a single pneumatic
tether and a single bus of four wires coming from the robot.
While not fully untethering a robot, this regulator brings us
one step closer to untethered centimeter-scale soft robots bygeneralizing pressure control in a way that can be easily
adapted to on-board pressure generation methods.
ACKNOWLEDGEMENTS
This work was supported by the National Aeronautics
and Space Administration under the Early Career Faculty
program (80NSSC17K0553), and by the Office of Naval
Research under the Young Investigator Program (N0001417-1-2604). JCC and DSS are supported by NASA’s SpaceTechnology Research Fellowships (Grants NNX15AQ75H
and 80NSSC17K0164, respectively). ELW is supported by a
National Science Foundation Graduate Research Fellowship(Grant DGE-1333468). We would also like to thank R. Adam
Bilodeau for assistance with photos, Robert Shepherd forproviding the CAD files for the multi-gait soft robot, andRobert Baines for assistance with manufacturing the robot.
R EFERENCES[1] M. T. Tolley, R. F. Shepherd, B. Mosadegh, K. C. Galloway,
M. Wehner, M. Karpelson, R. J. Wood, and G. M. Whitesides, “A
Resilient, Untethered Soft Robot,” Soft Robotics, vol. 1, pp. 213–223,
Aug. 2014.

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