Construction of a Hyper Redundant Robotic Tentacle 

Manipulator

by Miles C. D. Pekala and Derek Scherer

ARL-TR-6019 June 2012

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ARL-TR-6019 June 2012

Construction of a Hyper Redundant Robotic Tentacle 

Manipulator

Miles C. D. Pekala and Derek Scherer

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Construction of a Hyper Redundant Robotic Tentacle Manipulator

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14. ABSTRACT

This report presents the design and development of a robotic tentacle manipulator research platform as a tool to explore new 

methods of enhancing the capabilities of remotely operated manipulators, autonomous manipulation, and autonomous search. 

We review the current state of dexterous manipulators and past milestones, choose manipulator morphology based on this 

review, and evaluate the morphology on the basis of design, control, and manufacturing constraints. A feasible architecture for 

a robotic tentacle manipulator research platform is conceived, designed, and built. Design and development of software, 

hardware, and mechanical components is presented. Applications of the robotic tentacle manipulator in autonomous robots are 

investigated. Potential effects of various design decisions are evaluated and strategies for platform improvements are 

suggested.

15. SUBJECT TERMS

robot, autonomy, manipulation, tentacle, dexterous manipulator, mechatronics

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Miles C. D. Pekala

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iii

Contents

List of Figures v

List of Tables vi

Acknowledgments vii

1. Introduction 1

2. Background 3

2.1 Manipulators and Artificial Hands..................................................................................3

2.2 Snakes, Tentacles, and Continuum Manipulators...........................................................4

3. Construction 5

3.1 Manipulator Morphology ................................................................................................5

4. Mechanics 6

4.1 Kinematic Chain Design .................................................................................................6

4.2 Servo Selection................................................................................................................7

4.3 Linkage Development .....................................................................................................8

5. Control System 10

5.1 System Configurations..................................................................................................11

5.2 User Interface Design....................................................................................................12

5.3 Current Controls............................................................................................................13

6. Electronics 14

6.1 Overall System Design..................................................................................................14

6.2 Joint Controllers............................................................................................................15

6.3 Communications Protocols............................................................................................17

6.4 Master Controller ..........................................................................................................18

6.5 Power System................................................................................................................19

7. Conclusions and Future Work 20
iv

8. References 21

Distribution List 25
v

List of Figures

Figure 1. Kinematic chain-free body diagram. ...............................................................................6

Figure 2. CAD drawings of linkages D, C, B, A. ...........................................................................8

Figure 3. Possible orientations of a kinematic chain. .....................................................................9

Figure 4. Different generations of rapidly prototyped linkages......................................................9

Figure 5. (a) The design of a three-fingered tentacle manipulator and (b) construction of a 

three-fingered manipulator.......................................................................................................10

Figure 6. Vicon-based system configuration. ...............................................................................11

Figure 7. Serial servo controller configuration. ............................................................................12

Figure 8. Typical operator interface in Labview. .........................................................................13

Figure 9. Simplified electronics system overview........................................................................14

Figure 10. Simplified serial controller. .........................................................................................16

Figure 11. Simplified parallel controller.......................................................................................16

Figure 12. Internals of the power supply unit. ..............................................................................20

Figure 13. Front of the power supply unit. ...................................................................................20
vi

List of Tables

Table 1. Data communications format..........................................................................................17

Table 2. Serial board commands...................................................................................................18

Table 3. Parallel board commands................................................................................................18
vii

Acknowledgments

This report presents the design and development of a robotic tentacle manipulator research 

platform as a tool to explore new methods of enhancing the capabilities of remotely operated 

manipulators, autonomous manipulation, and autonomous search. We review the current state of 

dexterous manipulators and past milestones, choose manipulator morphology based on this

review, and evaluate the morphology on the basis of design, control, and manufacturing

constraints. A feasible architecture for a robotic tentacle manipulator research platform is 

conceived, designed, and built. Design and development of software, hardware, and mechanical 

components is presented. Applications of the robotic tentacle manipulator in autonomous robots 

are investigated. Potential effects of various design decisions are evaluated, and strategies for 

platform improvements are suggested.
viii

INTENTIONALLY LEFT BLANK.
1

1. Introduction

Robust autonomous manipulation has long been sought for remotely operated vehicles operating 

in unstructured or unknown environments. Many remote manipulators are almost entirely teleoperated with only minor autonomous functions. Use of these remote manipulators requires a 

great deal of training to operate efficiently due to their reliance on the skill of a human operator. 

Since an operator is required to operate a robot, search teams may become less effective because

at least one human member has to focus on manipulation control. With the advent of 

autonomous manipulation and object classification, robots would gain the ability to clear paths, 

open doors, and search rubble piles with full autonomy. Tasks such as autonomous search

procedures would greatly improve, and the effectiveness of smaller teams would increase 

dramatically. Even partial autonomy of manipulators would provide benefits such as decreasing 

the skill and time required to perform complex remote manipulation tasks. Ultimately,

development of autonomous manipulation for these robots would allow smaller teams to have a 

greater impact. 

In the most general sense, a manipulator can be thought of as a part of a robot that interacts in 

some way with the environment in a controlled manner. Given this broad definition, many robot 

accessories can be thought of as manipulators. The definition can be refined somewhat to 

include only those parts of a robot that interact with the world and alter the environment. For 

example, wheels, though not commonly thought of as manipulators, interact with the ground in a 

specific controlled manner. Some might argue that wheels do not alter the environment, though 

researchers such Lynch and Mason (1) have taken advantage of how wheels interact with the 

environment to accomplish tasks such as manipulating sheets of paper. Other manipulation 

concepts include using multiple robots working in concert to achieve a task, such as moving an 

object or modifying their operational area. Each robot simply pushes or pulls using their chassis 

rather than an attached device. This type of distributed manipulation or cooperative 

manipulation accomplished between independent agents has been demonstrated in references 2

and 3.

Other research has explored manipulation through the development of manipulation strategies 

and the design of manipulator mechanisms. Without a doubt, the most commonly derived 

manipulator is a robotic arm and an end effector. The pervasiveness of this is partly due to the 

human experience; we are innately familiar with our own anthropomorphic manipulators, our 

hands. However, mimicry of our own physiological machinery is not an expression of hubris but

far from it. Reasons for such ubiquity stem primarily from the utility of fingered hands from a 

functional standpoint. Anthropomorphic or multifingered hands are capable of complex 
2

dexterous manipulation of objects, using fingers cooperatively to grasp or otherwise manipulate 

objects. In-hand and fine-grain manipulation become possible with multifinger dexterous 

manipulators. 

A large percentage of manipulator research has focused on the development of manipulators for 

industrial purposes. Most industrial manipulators have a high degree of accuracy but little in the 

way of flexibility. This is due, in part, to specialized end effectors developed for accomplishing 

a specific task in a known manufacturing environment. Commonly, the end effectors in 

industrial robots are only capable so-called “fixturing,” whereby an object is constrained in an 

end effector and moved by a robotic arm. This differs from dexterous manipulation because

fixturing constrains an object in a specific position within a robot end effector while dexterous 

manipulation allows for in-hand manipulation of that object as defined in reference 4. Fixturingtype manipulation lends itself to the manufacture of products quite well but translates poorly to 

an unstructured environment such as a battlefield, disaster site, or even the everyday human 

world which we inhabit. 

Some currently fielded end effectors included are parallel-gripping pincers (5) and three-finger 

grippers (6), which manipulate objects with a closing-style grasp. The grasping force is 

orthogonal to the finger length, and gripping depends on the contact friction and normal force of 

the gasp to hold an object. Objects are gasped under the assumption that their center of mass 

exists somewhere between the fingers or that the torque generated by the object in the hand can 

be overcome by the actuators. Additionally, some end effectors possess a swiveling wrist 

(powered or not) to allow greater degrees of freedom and compliance. Unfortunately, these end 

effectors are limited due to their simplicity, only allowing for fixturing and not dexterous 

manipulation. Furthermore, only rudimentary control layers exist between the operator and the 

end effector, resulting in a system that requires significant training and practice by the operator 

to gain and maintain proficiency. 

Unfortunately, robotics platforms with rudimentary manipulation systems, i.e., those that have 

basic end effectors or manipulation control systems, are ultimately limited in their efficacy. 

Environments where fine dexterous manipulation is required for navigation or inspection are 

quite common. The most obvious example is a human living space where the manipulation of 

door handles, windows, light switches, cupboard latches, and many other simple machines is 

required for inspection and navigation. Other examples include unstructured environments such 

as rubble piles, forests, caves, and many others where fine manipulation may be required to clear 

a path or carry out a task where most fixturing graspers are unsuitable. For robust robotic teleoperation and autonomy, dexterous manipulation is required. 

Ultimately, our goal is to create a manipulation system that is fully autonomous. In an effort to 

reach this goal, we focus on several intermediary tasks, construction of a manipulation 

mechanism, development of a control architecture, research on low- and high-level control 

systems, development of an operator interface, and the challenges that come with these tasks.
3

This report describes our progress on the completion of the first and second tasks—development 

of a mechanism and development of a control architecture. 

With our goal being autonomous manipulation, a priority will be placed on mechanisms that 

assist autonomous manipulation tasks. The manipulation tasks that inform our development of 

the mechanism and control architecture are in-hand manipulation as well as autonomous 

manipulation of objects that are semi-fixed or constrained (such as a pipe under a rubble pile, a 

door, or other object). In order to explore the possibilities of dexterous manipulation 

mechanisms and strategies, particularly, autonomous strategies, we conduct a preliminary 

investigation into potentially useful dexterous manipulators to accomplish these tasks. With the 

purpose of increasing the dexterity of remotely controlled autonomous manipulation, our 

investigations are centered on manipulators that can be integrated into currently existing 

platforms.

We present a brief background of manipulators, snake-like robots, and graspers as an overview 

of current and past platforms for manipulation and as potential components. After some

analysis, we develop the Robotic Tentacle Manipulator concept and discuss the relevant 

mechanism design and construction. We also present the software design and development of a

control layer between the operator, human or autonomous agent, and the research platform. The 

electronic system implemented is documented, and all relevant protocols discussed. Finally, 

future and current work is discussed as well as possible improvements to the platform.

2. Background

2.1 Manipulators and Artificial Hands

Improvements in artificial limbs have been long sought after. Typically, they were looked upon 

as replacement prostheses for the unfortunate few who, by accident or birth, were denied use of 

their natural limbs. Unlike legs, which only need to support the weight of a user and be robust 

enough for use in daily life, artificial hands must also be capable of fine manipulation to provide 

utility to the user commensurate with their natural counterparts. The aspiration to replace 

passive prosthetic hands with controllable dexterous hands drove early development in dexterous 

manipulation and robotic manipulators. 

An early reference to an artificial “robotic” hand is the Iron Hand of Gottfried von Berlichingen,

who lost his hand to cannon fire in the early 1500s (7). The hand was a crude iron replacement 

but featured many interesting mechanical designs that were built upon later, such as poseable

fingers with locking joints for grasp generation. The Belgrade Hand (8), one of the first modern 

interpretations of an anthropomorphic robot hand, was a tendon-driven mechanism in the shape 

of a human hand. The hand was designed with external control and power signals, a significant 

departure from the shrug controls of previous mechanical prosthetic hands. It also had touch 
4

sensors made of sponges impregnated with conductive dust used in a resistive divider circuit that

allowed the hand to react to objects and grasp them without human command.

In the 1980s, several different manipulators were developed such as the Okada Hand (9), the 

Utah/MIT Hand (10), The Stanford/JPL (Salisbury) Hand (11), the NYU Planar Hand (12), and 

others. The hand Okada constructed possessed three fingers and a modular structure for each 

joint. Each joint axis could be reoriented with regard to previous orientations to produce various 

finger topologies. The MIT/Utah Hand, a semi-anthropomorphic tendon-driven manipulator 

with three fingers and a thumb, was developed primarily for research in manipulation. Both 

hands were controlled by large external actuation structures limiting similar designs from being 

integrated into small robot platforms. 

Though these and other hands provided an excellent platform for the development of 

manipulation, sensing, and grasping methods, there was the desire for simpler and more compact 

grippers for common tasks. These grippers could forgo the requirements of dexterous 

manipulation for speed and computational simplicity. An early example of this is the 

UPenn/Barrett Hand (6). This gripper was a “medium-complexity” end effector developed as a 

middle ground to high-complexity robotic hands and very low-complexity grippers. The hand 

featured an interesting mechanism for the repositioning of fingers around a single base and threefinger driven manipulators. Additional grippers include “Graspar” (13), the TIT Gripper (14), 

and others.

The development of low-control complexity manipulators has resulted in a variety of 

mechanically compliant or “mechanically intelligent” manipulators with peculiar clutch 

mechanisms and tendon routing. The majority of these mechanisms are specifically 

underactuated to take advantage of mechanical coupling tricks to generate their grasps or 

decrease the need for external computation to produce compliant grasps. For example, Jia et al. 

(15) designed a compact mechanism that automatically adapted to the shape of objects it 

grasped. It used a single actuator and a variety of clutches in its control architecture to drive the 

manipulator’s grasping mechanism (16). Other examples become even more conservative with 

control structures such as an anthropomorphic manipulator that uses one degree of freedom to a 

generate grasp as described in reference 17. With this grasper, only a single drawing force was 

required to generate conformable grasps. Several comprehensive reviews of conformal 

underactuated grippers and hands, in general, can be found in references 4, 18, and 19.

2.2 Snakes, Tentacles, and Continuum Manipulators

Snakes and snake-like robots have been of interest for the past 40 or so years. Though modern 

analysis of snake-like mechanisms was developed by Gray (20), what would most likely be 

considered the first modern snake-like robots began with the development of snake-inspired 

robots by Hirose in the mid 1970s (21). The original snake-like robots developed by Hirose 

were simple kinematic chains utilizing unactuated wheels and joint actuation to generate 

purposeful movement. By actuating the joints to change the pose of the robot, the snake
5

undulated from side to side and would generate snake-like motion. Additional snake robots 

using this principle of movement were developed by Chirikjian and Burdick (22), Miller (23),

and Crespi et al. (24). The idea was further extended to a three-dimensional snake that could 

twist and rotate by Mori and Hirose (25). Additional advancements to undriven snake robots 

added swimming methods such as those in references 26 and 27. 

Further snake-like robot development continued with the addition of driven links, that is actively 

driven joints as opposed to the passive wheels of the aforementioned snake robots. Examples of 

these are the GMD-SNAKE2 (27), the ACM-R4 (21), and the Koyru II (21). Snake robots using 

actively driven sections also included some that were developed with tank treads such as the 

Koyru I (21), JL-1 (28), MOIRA (29), and the OmniTread series (30, 31). An overview of these 

types of snake robots can be found by Hopkins et al. (32). 

However, while wheeled and treaded snake robots are an interesting platform, they do not really 

mimic the gait of a natural living snake. For snake-like robots that move like real snakes, they 

would need to move utilizing only skin friction and pose. Snakes developed by Dowling (33), 

Wolf et al. (34), Brown et al. (35), and Worst and Linnemann (36) possessed these traits, relying 

only on pose and body contact to generate purposeful locomotion. With only undulation or 

expansion methods to generate motion, these snake robots possessed a more “authentic” snakelike motion than the previous generations. Thorough reviews of these snakes and others can be 

found in references 32 and 37. 

Snake-like robots developed for locomotion represent only some possibilities of hyperredundant 

tentacles. Another interesting application of snake robots and tentacles, in general, is 

manipulation. Typically, most tentacle manipulators have been continuum devices utilizing the 

entire trunk of the tentacle to grasp and manipulate objects. Continuum mechanisms can be 

differentiated from hyperredundant mechanisms by virtue of their degrees of freedom and lack of 

an atomic unit. Primitive continuum mechanisms can be thought of as cord-driven fingers, as 

previously mentioned. Cord- and tendon-driving methods present the most ubiquitous method 

for driving continuum robots, outpacing pneumatic and electric designs. Examples of continuum 

robots include the KSI manipulator (38), the OctArm (39), and the TAKO grasper (40). 

3. Construction

3.1 Manipulator Morphology 

Based upon the previous review of snake robots and robotic manipulators and graspers, a servoactuated tentacle hand concept was selected. This was not the only design under consideration,

and various manipulator concepts may be pursued in future work. For in-hand dexterous 

manipulation of objects, a multifingered tentacle manipulator provides a robust platform to 

develop control methods. The tentacles are oriented on a palm for hand-like interactions as well
6

as potential tentacle-like interactions with objects. The palm and fingers can be reconfigured at 

will if certain finger interactions are necessary for any particular control method. An additional

benefit of a hand-like tentacle manipulator was the morphologies we considered: a tentacle 

manipulator was the most straightforward one to develop, and most parts could be bought off the 

shelf or easily manufactured in-house. 

Our design concept can be broadly stated as a multifingered tentacle manipulator with tentacles 

of indeterminate lengths and potentially heterogeneous linkages. Mechanically, each tentacle is 

a number of servos with specially designed linkages connected in a serial chain. Since the servos 

and their linkages can be serially mounted, tentacles of a variety of lengths are possible. Both 

parallel and serial control methods were implemented to allow direct servo control. Tentacles 

were mounted to a large base plate attached to a custom test stand that allowed the manipulator 

to be tested independently of an arm or robot. Custom adapters were required to connect the 

manipulator to a robotic arm or other platform. The entire manipulator was controlled serially 

through an interface to a personal computer (PC). Custom-designed circuit boards were 

embedded in the manipulator to handle the majority of control processing. A Labview 

environment provided a simple interface to develop grasp poses and control systems using the 

hand.

4. Mechanics

4.1 Kinematic Chain Design

The first step of the tentacle design, i.e., the design of each finger, was determining the required 

torques for each finger to accomplish a dexterous manipulation task. The objects to be 

manipulated, cardboard models, weighed ~30–100 g, giving a small range of torques in which 

the tentacles needed to function. For any manipulation task, a number of motions are needed that 

require torque at a specific joint. In the worst case, illustrated in figure 1, the manipulated object

would be as far from the base servo as possible on a finger tip and would need a torque from the 

base servo. Using this basis, we can choose our servo based upon the particulars of the 

kinematic chain.

Figure 1. Kinematic chain-free body diagram.

 
7

In our design, each five-link tentacle is designed to be able to lift 30 g when fully extended and 

against gravity. To do this, the base actuator must be able to torque the entire finger under the 

weight without the assistance of other actuators. From the proximal to the distal linkage, each 

actuator must torque a smaller linkage/actuator assembly to achieve this goal. 

Torque requirements on each actuator decrease from the leading actuators to smaller actuators 

still fulfilling this requirement. For the tentacle, we have a torque generated at the base,

expressed as

 base= object + tentacle= dnFobj + diFli

i=n

i=0

,

(1)

where d is the distance between linkages, n is the number of linkages, F is the force on the 

object by gravity, and F is the force on the ith linkage. For a variety of manipulations, a servo 

needs to be able to counter this force. For each link that is not the base servo, we have the 

following torque : 

 a= dn − da

 Fobj + di − da Fli

i=n

i=a

.

(2)

4.2 Servo Selection

In addition to the required torques, various other issues influenced the design and the selection of 

servos for the kinematic chain. Since the tentacles are essentially fingers on the end of a 

manipulator palm, one goal was to make each tentacle as small as possible. This was driven 

mostly by a desire for the manipulator to be capable of interacting with common household 

objects that required anthropomorphic manipulators like human hands. As a consequence of this 

goal, efforts were focused on selecting servos which were small as possible while remaining 

strong enough to do any desired manipulations. Other considerations included the option to 

modify a servo with custom electronics to suit the needs of the manipulator (should that 

requirement arise), the availability of the servos, and the cost. Since the restrictions on servo 

strength decrease and size restrictions increase as one travels away from the base, the possibility 

of utilizing multiple types of servos in one kinematic chain was also considered. The kinematic 

chain has no requirement to be homogenous, and a variety of servos could be used along its 

length if it proves advantageous. 

A variety of small and large servos was investigated—all commercially available hobby servos. 

Due to the previously mentioned motivations regarding size, standard-sized servos like the Hitec 

HS-311, Futaba S3151, or the Dynamixel AX-12+ were considered for integration into the 

tentacle at the base, while smaller servos were considered for use in the links. (Analysis of the 

commercially available servos integrated into particular kinematics chains can be found in 

appendix A.) In the end, the selection dwindled down to the Hitec HS-65 and HS-85 because of
8

cost and availability restraints. Kinematic chains consisting of a greater variation of servos or 

different servos were left to potential future work. 

4.3 Linkage Development

Utilizing these servos as a basis, a kinematic chain of linked actuators was developed. Each 

linkage connected to the previous servo by the servo horn and an axial nub on the previous 

linkages. Additionally, each linkage also had a mount point for a servo control board and sensor 

circuitry. Cabling for the mounted servo actuator was routed around each linkage to prevent 

restriction. The linkages, designed using the Solidworks computer-aided design (CAD) suite,

were from access to a variety of rapid prototyping machines manufactured using an fused 

deposition modeling (FDM) process. Due, in part, to access to prototyping equipment, the time 

between iterations of the kinematic chain was on the order of days. Minor design changes from

alterations in control electronics or servo selection could be implemented very rapidly using this 

method. This allowed us to generate a functional kinematic chain relatively quickly. 

Our design contains seven types of linkages between the servos: A, B, C, D, E, F, and fingertips

shown in figure 2. With the exception of the fingertips, all the linkages connect two servos 

together to form a single revolute joint between them. This is accomplished by mounting a servo 

in the linkage and then attaching the linkage to the servo horn of a driving servo. Linkages A, B, 

C, and D are all mounted by this method using micro fasteners. These linkages are designed to 

be mounted to Hitec HS-65 servos and hold Hitec HS-65 servos, while linkages E and F are 

designed to be mounted to Hitec HS-85 servos and hold Hitec HS-65 servos. Linkages A and B 

orient the servo actuator in a 90° fashion to make the next connected link rotation axis 

orthogonal to the previous rotation axis. Linkages C and D both orient the servo actuator to 

make the actuation axis of the connected link parallel to the previous rotation axis. Due to this 

configuration, A and C have fasteners on the right side, while B and D have them on the left. 

Figure 2. CAD drawings of linkages D, C, B, A.

The A, B, C, and D linkage configurations allow the development of specific kinematic 

orientations of the kinematic chain. Previously, Okada (9) demonstrated a large number of 

possible orientations of kinematic chains consisting of only revolute joints, i.e., the rotation axis 

of each joint with respect to the previous joint and the base reference frame. The linkage design 

replicates the ability to generate alternate orientations of the revolute joints in the kinematic 

chain. Linkages E and F joints are similar to A and B in orientation but differ in that they are
9

adapters from HS-85 servos to HS-65 servos. While the orientation of each kinematic chain can be 

changed relatively easily, the orientation most often used was of every other servo being rotated by 

90° relative to the base frame if fully extended. This orientation is expressed in figure 3 as 

orientation 6. Also note that with five linkages, there are 16 possible orientations and 256 possible 

configurations using the linkages since ientations 

 

and on i ations 

 

.

Figure 3. Possible orientations of a kinematic chain.

Machines used for manufacture of the linkages are the Objet Eden (41), Stratasys Fortus (42),

and Stratasys Dimension (43) three-dimensional (3-D) printing machines. The Fortus and 

Dimension use an FDM process and acrylonitrile butadiene styrene (ABS) plastic as the 

prototyping medium and the Objet uses a proprietary photopolymer and stereo lithography

(SLA) process. While the Objet Printer has greater accuracy, parts from it suffer from plasticity 

due to undercuring and thermal-related deformation issues. Parts manufactured with FDM ABS 

suffer from middling resolution and strength issues from the FDM process. Both types of 

manufactured parts age poorly; photopolymer parts become brittle from overcuring and ABS due 

to apparent brittleness associated with bead surface interfaces. While these parts would not be a 

substitute for traditional machined parts due to the low stresses and experimental nature of the 

linkages in each serial chain, they proved sufficient. Figure 4 shows several different 

generations of rapidly prototyped linkages in ABS and photopolymer materials.

Figure 4. Different generations of rapidly prototyped linkages.

The fingertip linkage lacks any mounting point for a servo actuator and only mounts to the 

previous linkage by the previous linkage’s servo horn. However, the fingertip linkages can be 

equipped with a variety of sensors and rubberized fingertips to aid in the manipulation of objects. 

A simple pressure sensor was developed using a pressure sensitive resistor and a soft capacitive 

sheet. 
10

The final kinematic chain solution allowed minimal modification of the servo joint, rapid 

replacement of servos should they become broken, and speedy development of kinematic chains 

of various sizes. Additionally, kinematic chains of various orientations and joint servos can be 

rapidly developed, regardless of their case geometry or internal electronics.

Once built, the kinematic chains are mounted to a common platform known as the palm. This 

palm provides the end effector reference frame as well as a physical connection to the robot arm. 

The palm consists of a mounting layer, a support layer, and an interface layer. The mounting 

layer consists of a round platform, with a number of mount points for each tentacle. Each mount 

point connects with a kinematic chain via a servo actuator mounted to the mounting layer. 

Servos mount to the mounting layer in a manner similar to the link-to-link connections, i.e.,

using micro fasteners to servo horn and an axial nub. The support layer is structural support for 

the mounting layer. Since the mounting layer is typically constructed via FDM or SLA, it 

generally requires support to prevent warping. The interface layer allows the hand to be attached 

to any test fixture or robot hand; it is designed to be quickly changed. The interface layer also 

contains mounting points for control electronics and interface electronics. Possibilities for palm 

actuation, specifically, folding and possibly, a soft palm, may be covered in potential future 

work.

To facilitate testing, we built a test stand consisting of an electronics layer, a stand, and a base. 

The electronics layer is simply a box for attaching various electronics boards while the stand and 

base are aluminum structures to support the hand during testing; they mimic a robot arm. The 

design and implemented setup for manipulation testing is shown in figure 5.

Note: the construction is only five joints per finger, while the design is seven.

Figure 5. (a) The design of a three-fingered tentacle manipulator and (b) 

construction of a three-fingered manipulator. 

5. Control System

Development of the control architecture for the manipulator was driven mostly by ease of use, 

ease of modification, and adaptability. Motivating this decision was the desire to alter 

components on the fly without significant hardware modifications to the architecture aside from 
11

reorganization of linkages, sensors, and arm adapters. A flexible foundation allows numerous 

control methods to be examined and shortens the development time for a large variety of 

mechanisms. For this reason, exact control methods applied to the manipulator will only be 

discussed in passing and not in depth; they are part of ongoing research, and no single one 

represents anything but a subset of work done. Rather, the focus shall be on the design and 

implementation of the generalized control architecture for the development of manipulation 

strategies and manipulation mechanics. This will include preliminary simulations, development 

of an operator interface for the manipulator, and development of lower level control systems in 

the manipulator.

Overall, the system design is very simple. A PC connects to a master control board (MCB) 

which connects to either a set of parallel servo controllers or several chains of serial servo 

controllers. These servo controllers connect directly to hobby servos or, possibly, a motor driver 

circuit. Since each serial servo controller has sensor feedback ports, servos can be customized or 

constructed entirely from scratch, utilizing a simple proportional-integrative-derivative 

controller. Additionally, the sensor ports on each serial servo controller allow a variety of 

temperature, pressure, or other tactile sensors to be implemented on each linkage. The MCB also 

has sensor inputs, allowing tactile, force, pressure, and orientation sensors, but sensors read by 

the master controller are relegated to the palm and interface layers due to routing considerations. 

Though the master controller currently only possesses the previously mentioned sensor 

interfaces, development of video and scanning interfaces is under way and may be implemented 

in future manipulators.

5.1 System Configurations

Our first configuration, shown in figure 6, is an open loop, with the master control being used as 

a USB pass through. Each servo chain is controlled by a parallel servo controller. In this 

configuration, the servos are controlled directly by the PC via Labview and do not have direct 

feedback through onboard sensing. A Vicon Motion System provides external feedback on the 

location of each linkage of the system. While this system provided a sufficient test bed for 

mechanics, it proved to be difficult to implement various control methods. This limitation was 

primarily due to the bulky and messy nature of wiring harnesses, line-of-sight issues with the 

Vicon Motion System, and communications processing issues. Implementation of this initial 

manipulator on a test robot proved impracticable, but the system was sufficient for test bench

work.

Figure 6. Vicon-based system configuration.
12

The second configuration, shown in figure 7, utilizes the serial servo controllers, with the master 

controller being used as a USB pass through. Each servo chain is connected to the master 

controller through an RS-485 bus. The servos are controlled directly by the PC via Labview 

similar to the previous configuration; however, actuation and sensor feedback is provided from a 

servo controller mounted to each linkage. A Vicon Motion system is no longer strictly necessary 

to provide feedback but can be implemented if convenient. This system is currently being 

implemented in the manipulator. 

Figure 7. Serial servo controller configuration.

5.2 User Interface Design

A generalized user interface was designed during development of the manipulator to facilitate

human-controlled testing procedures. The interface is designed in Labview and features full 

control of the manipulator through low- or high-level commands. A customizable control loop 

running on the MCB communicates to the servo controllers via USB, allowing direct control of 

each servo linkage. Additionally, generalized commands can be issued to the MCB and allow 

manipulation procedures to be processed entirely onboard without intervention of the operator or 

direct control kinematic through a PC.

The user interface also features a 3-D model of the current manipulator. Since the mechanics 

and construction of the manipulator may change, the configuration of the 3-D model is 

designated by the user during setup of the control system. This allows manipulators of differing 

morphologies to be correctly represented on the PC without needing to generate new models for 

each configuration. The 3-D model reflects either the desired or actual pose of the manipulator,

depending on sensor conditions and user settings. If there is no actuator position feedback, the 

desired pose is displayed. With actuator position feedback and the appropriate user-controlled 

settings, the actual pose is presented. Optional displays for sensor readings and hand 

characteristics can be displayed or hidden, depending on user preference. A typical operator 

interface setup can be seen in figure 8.
13

Figure 8. Typical operator interface in Labview.

The interface allows quick alterations in manipulator mechanics, system layout, and controls. 

For hobby servo-based mechanisms, servo trims can be manually set for each servo through 

direct commands or through the parallel or serial servo controller for a trim specification. Direct 

user control over the manipulator is accomplished through a 3Dconnexion SpaceNavigator 

mouse integrated into the user interface, allowing manipulation of each individual finger, 

tentacle, group of fingers, grasped object, or the manipulator as a whole through the control loop. 

Segments can be added or subtracted from each finger on the fly or during setup and calibration. 

The interface is designed to allow quick implementation of manipulation strategies without 

developing software drivers for specific actuators or sensors. Working in concert with the 

interface electronics, all actuators and sensors are abstracted to allow rapid development of 

control strategies. Interfaces for peripherals and sensor circuits are integrated into the control 

loop instead of requiring a separate interface for each one. All sensor information is fed through 

a bus into the loop, and actuation information is output from the control loop through a separate 

bus. This system allows the preservation of state data and provides user input, sensor input, and 

an output method to fully integrate the manipulator controls into the overall architecture. 

5.3 Current Controls

A variety of test commands and controls was implemented for mechanism analysis as well as 

demonstrations of the current user interface and control system. These include motive macros 

for actuation and mechanism analysis, control loops for servo and actuator response testing, and 

interactive controllers for use with a human operator. 
14

The macros come in two varieties—motive and sensory. The motive macros are sets of 

manipulator poses expressed through actuator positioning values. These allow the control 

system to generate poses to test mechanisms without the need for a specific control system for 

calibration and testing of each linkage actuator. The sensory macros are recordings of user input 

or sensor data that are fed into the control loop, allowing for control system development 

independent of the manipulator itself. 

Both macros can directly act upon the actuators or go through a servo intermediary. The 

intermediary interface loops are designed to trim out actuators, prevent actuator commands from 

moving actuators beyond their built-in limitations, bring actuators up to speed without exceeding 

acceleration limitations, and ensure accuracy. For hobby servo actuators and manipulation 

configurations that utilize the master and serial controllers, these functions are performed at the 

actuator level and not specifically required. 

6. Electronics

6.1 Overall System Design

During the design phase of manipulator mechanics, it was necessary to develop accompanying 

interface and drive electronics for actuation control and sensor data. All communications to and 

from the electronics were required to be quickly and easily interfaced with a PC to use 

previously developed control systems. Intersystem communication was held to this standard as it 

would simplify testing procedures while providing a flexible basis for developing electronic 

systems for any possible manipulator configuration. Each board was either connected directly to 

a power source or drew power from a serialized communications and power bus. The resulting 

boards were integrated into the physical structure of each linkage and the palm of the 

manipulator. In figure 9, we see a manipulator system implementation using these electronics, 

specifically the system shown previously in figure 7.

Figure 9. Simplified electronics system overview.
15

The electronics system consists of several types of control boards. Among these are a serial 

servo controller, parallel servo controller, MCB, several sensor interface boards, PC interface 

boards, and some power converters. All boards use the same communications protocol, which is 

run over a RS-232, RS-485, or RS-422 serial network.

Boards designed for direct actuation control are the serial servo controller (SSC) and the parallel 

servo controller (PSC). The SSC attaches to a single hobby style servo and has been designed to 

be serially chained to other SSCs. This is in contrast to the PSC, which connects to each servo 

individually. Both controllers use a standard serial communication protocol to communicate 

through a USB to serial converter or to an additional control board. 

MCBs were designed for overseeing multiple serial chains, providing an interface between serial 

chains and a PC or other robot control system, and potentially offloading a portion of sensor 

processing or system processing from the linkage actuator control boards. Sensor circuits may 

be integrated into the tip of each finger and may process pressure, temperature, or distance. The 

choice of servos or transmission protocol in the kinematic chain could require signal repeaters; 

however, for the chain we selected, they were not necessary. Additional boards being developed 

or not yet integrated into the manipulator include a field programmable gate array sensor 

processor board, a lightweight servo control board, a servo fail-safe/switch, and a motor driver 

board. 

Overall, the electronics system was designed to be reconfigurable to allow different tentacle 

manipulators to be implemented. Furthermore, electronics were designed to interface quickly 

and smoothly with electronics developed in-house for other projects. For example, the 

lightweight servo control board was designed to be capable of interfacing with a micro autopilot 

as well as a master controller. Most components of the system can be removed or added for 

different system configurations. Using the PC as an interface, a variety of common sensors and 

operator interfaces can be integrated into the control system with minimum effort. Through use 

of the communications protocol, each serial chain theoretically hosts up to 127 servos, though, in 

reality, the maximum is far less due to interference and mechanical feasibility problems. An 

additional constraint on our design was our power distribution system, which was only capable 

of hosting up to five servo chains with approximately eight servos per chain. It should be noted 

that this limitation was based on the power draw and the power supply, whose design was driven 

by the strength of the servos we chose. For simplicity sake, only three- and five-finger 

manipulators were implemented, with five joints and seven joints for each finger, respectively.

6.2 Joint Controllers

In the serial configuration, each servo actuator is connected to a joint controller board to allow 

the actuator to communicate via serial commands and send position and sensor data. The 

controller board is mounted on each servo linkage alongside the servo actuator. The servo itself 

connects to the joint control board via the standard hobby servo interface, a three–wire,
16

PWM-driven actuation signal. Additionally, a small hole is drilled in the case, and a wire from 

the center tap of the servo pot connects to a sensor input on each board to allow position 

feedback. Utilizing an external control board rather than replacing the servo electronics 

decreases the amount of modification that each servo must undergo. Minimum servo 

modification allows speedy replacement of broken servos. With further modifications to the 

servo, internal servo electronics can be completely replaced with a motor driving board and the 

joint controller. However, using our current configuration, complete replacement of servo 

electronics is not required as position can be controlled and sensed using the joint controller. 

This version of the serial controller can be seen in figure 10. In the final version of the SSC

circuit, a motor control chip is integrated into the board, allowing the circuit to drive motors 

directly. 

Figure 10. Simplified serial 

controller.

If each servo is not individually controlled by an onboard controller, an offboard parallel servo 

controller is used. The parallel servo controller we used in this setup is an eight-servo controller 

that connects to and commands eight individual servos. This control board connects directly to 

USB or RS232 and uses a communications protocol similar to the serial controller. The block 

diagram for this controller is shown in figure 11. However, unlike the serial controller, which 

was designed as a stand-alone motor controller/command interpreter, the parallel control board 

was designed solely for interfacing with hobby servos via pulse width modulation (PWM) 

signals. This means that the parallel controller acts simply as a pass-through for PC actuation 

commands and no processing is done on the parallel servo controller. Another significant 

difference is that the parallel servo controller lacks sensor input from both potentiometers on 

each servo and tactile sensors on each linkage. Processing is done by a PC in the control loop, 

with sensory feedback coming from a Vicon motion system through a Labview interface. Due to 

issues with this setup, parallel configuration was used mostly for mechanical testing purposes.

Figure 11. Simplified parallel controller.
17

6.3 Communications Protocols

The communications interface operates at 9600, 19200, 38400, or 57600 Bd and is provided 

through simple serial commands. Each command header is designed to fit into two bytes, with 

additional bytes for specific commands. Since the protocol is built on top of RS232, there are no 

ACKs or NACKs. The basic format for a command is shown in table 1. 

Table 1. Data communications format.

The C bit is set when identifying a command byte. Address and command bytes have the C bit 

set, while data bytes do not. In the command byte, the upper (U) bits are the upper address bits 

of the controller while the lower (L) bits are the lower address bits. In the parallel controller, the 

U bits control which parallel controller you are addressing and the L bits specify which servo 

you are addressing. If the K bit is set, the control board enters alternative command mode;

otherwise, it is in standard command mode. In standard command mode, the controller can be 

addressed the full command byte. If in alternative command mode, the controller’s lower 

address (the L bits) specifies the command. This configuration is only valid on the parallel 

controller and is done for timing reasons. In the serial controller, the all-address bits allow you 

to address any particular serial controller while the second byte specifies the command. It should 

be noted that the parallel controllers can command up to eight servos a piece, with eight

controllers on a single bus, while the serial controllers can control up to 128 servos or other 

circuits on a single bus. The commands listed in table 2 are the commands for the serial control 

board. The commands in table 3 are those for the parallel control board.
18

Table 2. Serial board commands.

Command Byte Code

Get S1 Reading CMD: CUUU LLLL, B1: X000 0001; Response: XRRR RRRR, X--- -RRR

Get S2 Reading CMD: CUUU LLLL, B1: X000 0011; Response: XRRR RRRR, X--- -RRR

Get S2 Reading CMD: CUUU LLLL, B1: X000 0101; Response: XRRR RRRR, X--- -RRR

Set Trim CMD: CUUU LLLL, B1: X000 0111; Trim: XDDD DDDD, X--- ---D

Set Transient CMD: CUUU LLLL, B1: X000 1001; Transient: XDDD DDDD, X--- ---D

Set Initial Position CMD: CUUU LLLL, B1: X000 1011; Position: XDDD DDDD, X--- -DDD

Set Address CMD: CUUU LLLL, B1: X000 1101; Address: XUUU LLLL

Get Trim CMD: CUUU LLLL, B1: X000 1111; Response: XRRR RRRR, X--- ---R

Get Transient CMD: CUUU LLLL, B1: X001 0001; Response: XRRR RRRR, X--- ---R

Get initial position CMD: CUUU LLLL, B1: X001 0011; Response: XRRR RRRR, X--- -RRR

Table 3. Parallel board commands.

Command Byte Code

Set Trims CMD: CUUU 0001, Servo1: XDDD DDDD, Servo 2: XDDD DDDD, ...

Set Transient CMD: CUUU 0011, Servo1: XDDD DDDD, Servo 2: XDDD DDDD, ...

Set Initial Positions CMD: CUUU 0101, Servo1: XDDD DDDD, X--- -DDD, Servo 2: XDDD DDDD, X--- -

DDD,...

Set Address CMD: CUUU 0111, Address: X--- -UUU

Get Trims CMD: CUUU 1001; Response: XDDD DDDD, Response 2: XDDD DDDD, ...

Get Transient CMD: CUUU 1011, Response: XDDD DDDD, Response 2: XDDD DDDD, ...

Get Initial Positions CMD: CUUU 1101, Response: XDDD DDDD, X--- -DDD, Response 2: XDDD DDDD, X-

-- -DDD,...

Set Position CMD: CUUU LLL0, Data Byte1: XDDD DDDD, Data Byte2: X--- -DDD

6.4 Master Controller

The MCB is an embedded replacement for a full PC. This will allow an operator to install the 

tentacle manipulator on a variety of robot platforms without the need for a large, cumbersome 

PC connection. 

The MCB functions at midlevel and provides an intermediary from a full featured PC or robot 

controller to the lower level linkage servo controllers. This is evident from the system setup 

illustrated in figure 9. Each serial chain is allotted a dedicated port on the microcontroller, which 

keeps the master controller updated with all necessary information from each servo control
19

board. Using these ports, the master controller can quickly transmit different messages to each 

bus, allowing more pertinent communication. When required, the master controller can take 

high-level commands such as end effector locations, translate these commands into actuator 

control signals, and transmit the required control signals to each chain. Using this method, the 

MCB acts as a subprocessor for kinematic commands. Additionally, the master controller is able 

to keep tabs on each servo controller. This way, the master controller ensures that the kinematic 

chain functions as desired and, if not, can take steps to “remove” individual servos from the 

chain and adapt to broken linkages. 

To supplement or substitute the sensor circuitry near each linkage controller, the master 

controller is outfitted with 16 channels of analog-to-digital conversion as well as a large parallel 

input port. While centralized sensor processing is more cumbersome than a modular 

arrangement of sensors at each joint, it does off-load the size and complexity of the joints. An 

important implication of centralized sensor processing is that the linkage controllers are not 

bogged down with large amounts of sensor data and can therefore provide timelier actuation and 

more reliable communication among themselves and with the master controller. High bandwidth 

sensors, such as vision and object scanning, can be implemented through the master controller in 

this fashion. Additionally, these sensor channels may be used for data that is gathered from a 

broader system view, such as ambient temperature, platform orientation, etc.

If the manipulator is integrated into a host platform, the master controller board acts as the 

interface. This setup allows the manipulator to remain platform agnostic and retain its 

specialized control. The master controller can interpret host platform-level messages in the 

appropriate context and then communicate with the linkage controllers as usual. Likewise, any 

data requested by the host platform is issued by the MCB.

6.5 Power System

The power system consists of a Meanwell LPS-100-5 5V 30A power supply, a distribution 

board, an IEC C14 male socket, and a 25A switch. Since the servos and electronics of the 

robotic tentacle manipulator are driven off the same voltage, they are fed by the same line and 

separated only by a diode isolator. Additionally, the case for the power supply is designed to 

house both the master controller and the power supply. If the robotic tentacle manipulator is 

being driven solely through a computer, this can possibly be replaced by a USB to RS485 

converter or a USB to transistor level logic converter (if using parallel servo drivers).

The power is routed to the master controller and to the five 0.1-in pitch polarized and shrouded 

connectors on the front panel. Additionally, a USB port is on the front panel for connecting to 

the internal master controller or serial converter. A 60-mm case fan is also integrated into the 

power supply to ensure the assembly does not overheat. The assembly can be seen in figures 12 

and 13.
20

Figure 12. Internals of the power supply unit.

Figure 13. Front of the power supply unit.

7. Conclusions and Future Work

The robot tentacle manipulator is a framework for future research into new tentacle-based 

manipulators and tentacle manipulation strategies. The system comprises a highly modular 

tentacle manipulator with a reasonably flexible control system. In the short term, further 

development of the platform will include integration of new boards, such as an upgrade of the 

master controller, an upgrade of the serial servo boards, and the integration of simple tactile 

sensors. In the long term, goals, such as the development of mechanisms and methods for 

autonomous manipulation and continued development of intuitive user interfaces, are envisioned. 
21

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