a new design for cassino hexapod robot

1 Copyright © 2010 by ASME

Proceedings of the 10 th Biennial ASME Conference on Engineering System Des ign and Analysis ESDA 2010

July 12-14, 2010, Istanbul, Turkey

ESDA2010-24020

A NEW DESIGN FOR CASSINO HEXAPOD ROBOT

Nestor Eduardo Nava Rodríguez , Luis Moreno Lorente

Giuseppe Carbone, Marco Ceccarelli

Robotics Lab

Carlos III University Calle Universidad 30,

28911 Leganes (Madrid), Spain email: [email protected],

[email protected].

LARM: Laboratory of Robotics and Mechatronics

DiMSAT – University of Cassino Via Di Biasio 43 - 03043 Cassino (Fr), Italy

email: [email protected]; [email protected]

ABSTRACT We present the design process of a new low-cost easy-

operation Cassino Hexapod robot. In a recent past, research

activities have been carried out for developing a six-legged

robot at LARM (LAboratory of Robotics and Mechatronics) of

Cassino University in Italy. The robot designs have been based

on suitable mechanism structures and architectures that can be

easy operated by a commercial on-off logic device. The first

prototype is composed of modular legs with three degrees of

freedom (DOF). A second prototype of a hexapod leg presents

two links and two DOFs. The new prototype Cassino Hexapod

robot has two DOFs, such as, one for pitch movement that

drives an articulated mechanism and other for yaw movement.

Additional peculiarity of the leg design is that the foot is

designed as a powered wheel with the possibility to regulate the

velocity and force at the contact during the support phases of

the leg. Simulation results of the proposed design operation of

the new hexapod robot are reported with the aim of

characterizing both operation performance and design features.

1. INTRODUCTION Walking machines have been attempted as technological

transportation machinery because the necessity to overpass the

limits of wheeled systems by looking at legged solutions in

nature. But only in the recent past efficient walking machines

have been conceived, designed, and built with performances

that are suitable for practical applications, [1]. Biped,

quadruped and hexapod as well as humanoid robots has been

developed in the last part of 20-th century around the world in

research centres and universities as machines that can help the

human being in dangerous or exhausting tasks, for example,

transportation of military staff, like Bigdog [2], mine detection

and grass cutting, like COMET II [1], or forest harvesting, like

the hexapod develop at Plustech Ltd [1]. Others examples of

existed hexapod robots in the world are RHEx robot [1] of

Ambulatory Robotic Lab. in Montreal, the Adaptive Suspension

Vehicle of Ohio State University [1], Boadicea [3] of AI Lab. in

MIT, SLAIR2 [4] of the Fraunhofer Institute for Factory

Operation Ottovon in Guericke University and the All–Terrain

Hex–Limbed Extra–Terrestrial Explorer (ATHLETE) [5] of

NASA.

Mobile robots can be structured of different types, first

ones are based on crawlers or wheels and second ones are

equipped with biologically inspired legs. This second type of

walking machines can be slow and more difficult to design and

operate with respect to the first ones. Nevertheless, legged

robots are more suitable for rough terrain, where obstacles of

any size can appear [6]. In fact, the use of wheels or crawlers

limits the size of the obstacle that can be climbed to half the

diameter of the wheels [7]. On the contrary, legged machines

can overcome obstacles that are comparable with the size of the

machine leg [8]. There is also a third type of waking machines

that is called hybrid robot since it has legs and wheels at the

same time. This type of walking machines may range from

wheeled devices to true walking machines with a set of wheels.

In one case, the suspensions are arms working like legs to

overcome particularly difficult obstacles, and in other case,

wheels are used to enhance the speed when moving on flat

terrain.

We report in this paper the design issue of a new six-legs

walking robot called Cassino Hexapod robot. The new design

has been conceived with commercial and no-complex feature

components that decrease cost and control complexity for the

robot system. An articulated mechanism has been considered

for the mechanical leg structure since it performs a suitable

pitch movement for a hexapod robot with only 1 DOF. A

mobile robot with wheels can displace fast and easily on a low

rough terrain. Thus, an actuated wheel has been proposed as the

new hexapod foot. The wheel also allows the possibility of

elaborating a strategy for the contact control between robot foot


and floor. The design has been validated by analysing the

results of kinematic simulations of basic robot operations.

These results have illustrated feasible performances of the new

Cassino Hexapod in term of components interference, motor

synchronizations and versatility during legged and wheeled

operations.

2. THE ATTACHED PROBLEM Adequate movement of a hexapod robot is possible by

properly synchronising the motion of all six legs. Thus,

planning the movement of one leg is very important for the

successful performance of the robot. Each leg joint of the

Cassino Hexapod has been designed as actuated by a DC motor

which is activated by PLC, based on the digital logic. Operation

of a particular joint stops as soon as it reaches its extreme

position, which is sensed by the limit switches. Suitable

walking gait has been implemented for the locomotion of

Cassino Hexapod. For maintaining the stability of the robot, at

least three legs must be in contact with the ground

simultaneously [8-10]. Figure 1 shows a scheme with

movements of limbs in a six legs walking also with the footfall

formula representation. The limbs that are in contact with the

ground surface are shown as black circles in a table in which

the entries represent the possible foot contacts with the ground.

The arrow in Fig. 1 represents the moving forward direction of

the hexapod walking. In the tripod gait, the front and the rear

leg of one side and the middle leg of another side perform their

swing movements at the same time. Thus, the swings of right

and left tripods have to be synchronised by adding a software-

induced delay. This has to be done to implement a continuous

walk. Thus, the attached problem has considered how to

achieve a feasible movement for a hexapod gait through low-

cost structure composed of commercial components,

mechanical pieces with no complex features and suitable

mechanisms.

3. LARM DESIGN SOLUTIONS AND OPEN PROBLEMS

A single-link module can be defined as a link module

which contains in itself the needed actuators, transmissions and

sensors which can be connected together to form a mechanical

leg. Each module consists of a separate motor, hence increasing

the number of links which results in increasing the number of

robot DOF. Figure 2 shows the structure of the first hexapod

robot developed and built in Cassino [11]. For the current

design, a single leg is composed of three links (modules) and

four DOFs. The fourth DOF is arranged for the wheeled foot

enabling the smooth movement of the hexapod in a flat

environment. The first link motor can be modelled as the hip

joint, for turning the leg inside and out. The second link motor

is used to lift the leg up and down and finally the third link

motor is equivalent to the knee joint. Each motor is equipped

with a corresponding limit switch which signals to stop the

motion of the motor when joint limit is reached. The robotic leg

has been made up of commercially available components,

which have been assembled together to give the final form of

the leg, that is shown in Fig. 2, with a weight of 2.5 kg for each

leg.

Six legs have been assembled on a specifically designed

base frame for building the prototype of low-cost easy-

operation Cassino Hexapod of Fig. 2. The overall cost of this

built prototype has been less than 5000 Euros, including 18

commercial low-cost DC motors, 24 limit switches, 36 pulleys,

18 belts, about 150 screws of various sizes, PLC,

manufacturing of modules and other spare parts. Aluminium

alloy has been used as the material for constructing the hexapod

and hence leads to savings in weight and cost of the robot. The

weight of the robot without any payload or PLC installation is

17.7 kg. The weight of the hexapod with PLC and other circuits

is 21.6 kg. The operation of the hexapod has been achieved by

using PLC to control the different motors of the hexapod.

Siemens PLC Simatic S7–200, which is fixed onboard the

hexapod, is used for the operation of the robot. The program for

the hexapod operation has been written in a windows PC which

has STEP- 7/Micro WIN 32 installed into it. The program from

the PC can be downloaded onto the flash memory of the PLC

using a RS 232/PPI Cable.

Fig.1 Movements of limbs in a six legs walking with footfall formulas

representation (black circles stands for the limbs in contact with the ground

surface and arrow indicates moving forward direction).

Fig. 2. The first prototype of Cassino Hexapod robot.


The Cassino Hexapod design of Fig. 2 has presented some

problem during the operation from the point of view of high

weight and power consumption. The modular configuration

gives high number of actuators that increase versatility but as

well as motor power requirements. Therefore, a further leg

design for the hexapod has been studied obtaining the second

prototype of Fig. 3. The proposed leg consists of only two

modules, with a cross-section of 35.5 x 35.5 mm for upper-

module and 29.5 mm x 29.5 mm for lower-module, and one

wheel. The total leg height is 540 mm with a weight of 850

grams and the maximum step size of 200 mm. The leg contains

one motor, a screw-nut transmission, limit switcher, support for

the motor, proximal module, distal module and ball bearings for

reduce the friction in the joints. Some problems of the hexapod

leg design have been resolved for the second prototype of Fig.

3. Reducing the module quantity form 3 to 2 has decreased the

structure weight and including screw-nut transmission allows

installing the leg actuation on the hip sub-system. Moreover,

the screw-nut transmission operates as brake system when the

robot is static reducing power consumption. Nevertheless, even

if the second leg prototype presents a more zoomorphic design,

the screw-nut transmission is not a suitable solution for legged

mobile robots since it provides low velocities for small screw

paces and low precision for high screw paces. Thus, a new leg

design for the Cassino Hexapod Robot has been developed

considering problems and solution of the previous prototypes,

as following reported.

4. A NEW DESIGN SOLUTION The new solution for a Cassino Hexapod Robot is based

on millipede anatomy and walking. When millipede walking,

each leg takes a step that consists of two stages [12]. The first is

when the claw is in contact with the ground and is moving

backward and the animal body moves forward. The second is

the recovery stage when the leg moves forward in the air. In

millipede, the propulsive stage is longer than the recovery stage

since the greater is the thrust for pushing. Based on the

millipede anatomy, the new hexapod leg must perform mainly

pitch and yaw movements for walking. For pitch movement, an

articulated mechanism has been used as transmission solution,

as well as the leg structure, such the cross-four-bar linkage

mechanism of Fig. 4.

Fig. 3. A operating sequence of the second leg robot design.

Fig. 4. Kinematic scheme of a new leg for the Cassino Hexapod Robot.

Table 1. Kinematic parameters of the mechanism architecture of Fig. 3.

Links (mm) Angles (deg)

l11 58.3 θ1 Input

l51 375.0 θ0 59.1

l21 460.0 γ1 5.9

l12 50.0 λ1 50.2

l31 405.0 θ21 Variable


l32 240.0 γ2 130.0

l22 260.0 θ2 Output


l42 55.0 θ3 Output

l53 198.4 γ3 127.2

Similar solution has also been used in robot hand

applications [13] since the mechanism characteristic of

performing a three-DOFs movement with only one actuator.

Therefore, downward and upward movements for the walking

stages (and more operations, as reported in the simulation part)

can be performed by only one motor reducing the robot power

requirements. The mechanism of Fig. 4 has been properly

modified in order to obtain a feasible leg configuration. Table 1

list the values of the leg mechanism parameters. Note that θij (i

= 1, 2, 3 and j = 1, 2) values of Table 1 are variable since

represents input, outputs and moving component angles of Fig.

4 architecture. The other fixed values represent the angles of

component features of the internal body of this mechanism. The

yaw movement is performing by actuating directly the hip joint

with a commercial DC motor in a robust assembly, as shown in

Fig. 5(a). Finally, a wheel has been installed in the robot foot in

order to obtain a higher versatility with wheeled displacements

as well as improve force and velocity control at the contact

point with the ground.

Figure 5 shows a 3D-CAD model of the new design of the

Cassino hexapod robot in Pro-Engineer environment. Figure

5(a) shows a detail of a leg structure, in which the mechanism


of Fig. 4 can be recognized. Note in the leg base that a yaw

joint is included with a DC motor in vertical position and ball

bearings, in order to reduce friction, in a compact and suitable

assembly. The wheeled foot of the proposed leg structure can

be also recognized in Fig. 5. A similar DC motor assembly to

hip joint has been used for the foot joint, in which the actuator

is directly installed actuating the wheel. Finally, Fig. 5(b)

shows an overview of the whole hexapod model. The trunk

sub-system of the new design is similar to the first Cassino

Hexapod prototype of Fig. 2 since this shape provides a legs

configuration that keeps the operating leg closer to the robot

support area, formed by three legs in contact to the floor, than a

rectangular trunk shape.

5. SIMULATION MODELS AND RESULTS Kinematic simulations of the new hexapod operations

have been carried out by using the 3D-CAD model of Fig. 4.

The simulations have consisted in developing basic robot

performances in a virtual environment in order to check the

design feasibility before prototyping. Mechanism Toolbox of

Pro/Engineer has been used for simulations due to the

convenient features for simulating the operation of multi-body

systems that have been modelled in this environment.

(a)

(b)

Fig. 5. A 3D-CAD model of the New Cassino Hexapod Robot: (a) leg design;

(b) whole robot assembly.

The Mechanism Toolbox of Pro/Engineer used computes

backward differentially the algebraic equations of the dynamic

model [14]. The Cassino hexapod model in Pro-Engineer takes

into account several aspects such as, external forces, gravity,

contact constraints, friction and inertia properties. The model

has been elaborated by introducing each component with its

specific characteristics in terms of material, mass, density,

shape and mechanical design. Almost all hexapod components

have been modelled as made of aluminium alloy. Components,

such as screws and nuts, have been considered as made of steel

alloy. Ball bearings and motors have been modelled introducing

the mechanical properties reported in their datasheets

respectively [15-16]. Finally, wheels have been modelled as

made of commercial rubber. Friction effects are assumed

negligible for joints in the simulation since low-friction

bearings [15] have been used in the robot design. The kinematic

simulations involve the new Cassino hexapod in three

operations such as, walking on a flat terrain, moving along

vertical axis and wheeled operations. The simulation results

have been used in order to check the feasibility of the new

design assembly, avoid interferences among robot components

during operations and validate the performance of the above-

mentioned three functional modes, as following illustrated.

5.1. Walking on a flat terrain Figure 6 shows a sequence of the walking simulation

on a flat terrain by the new Cassino Hexapod. In this

simulation, the robot performs a linear displacement by using

its legs during 12 sec of simulation time making two steps with

each leg. The time interval of 0.01 sec (100 intervals in 1 sec)

has been selected for the simulations in order to obtain suitable

results within reasonable computational periods. The wheeled

feet are not actuated in this early simulation, but walking

accuracy of the robot can be improved by controlling the feet

motors in terms of contact force and velocity, as will be

elaborated in futures works on the Cassino Hexapod

development. Figure 7 shows the time evolutions of positions

inputted for the motors of hexapod legs in the walking

simulation of Fig. 6. In particular, Fig. 7(a) shows the position

plot of input for leg pitch actuator and Fig. 7(b) shows the input

for yaw actuator. Note that inputs of the all six robot legs have

the same evolution for both pitch and yaw actuators with a 1

sec offset that provides the correct walking movement. Figure 8

shows the configuration of robot legs, in which each leg is

numbered from 1 to 6 in order of suitable actuation sequence, it

means, number 1 is the first leg moving forward, number 2 is

the second leg moving forward and so on.

The robot step can be recognized in Fig. 7 by

analyzing the shape of pitch and yaw input curves. The peak

from 0 deg to - 15 deg of Fig. 7(a) represents the pitch

movement in which the robot leg goes through the air from the

fully-back to fully-front position in 1 sec. From 1 sec to 7 sec

the contact between foot and floor is constant, representing by 0

deg value of pitch movement, in which leg is going backward

performing the whole robot forward displacement.


(a)

(b)

(c)

(d)

Fig. 6. Result sequence for the walking on a flat terrain simulation: (a) initial position; (b) after 4 sec; (c) after 8 sec; (d) final position.

From 7 sec to simulation end the position sequence is

repeated, so that the hexapod step time is 8 sec. From Fig. 7(b)

the ramp with positive slope form -15 deg to 15 deg represents

the forward leg movement through the air and the ramp with

negative slope from 1 sec to7 sec represents the backward leg

movement in contact to the terrain.

(a)

(b)

Fig. 7. Inputs for the robot actuation during the simulation of Fig. 5: (a) pitch of leg, (b) yaw of leg.

Fig. 8. Leg sequence for a correct walking displacement in a flat terrain.

The effectiveness of the leg mechanisms has been validated

through kinematic results of the walking simulations. Figure

9(a) shows the time evolution of angular position, velocity and

acceleration of pitch movement of leg end-effector and Fig.

9(b) shows the angular position, velocity and acceleration of

yaw movement of leg end-effector. In both plots, position is

represented in solid line, velocity in dotted line and acceleration

in dashed line.


(a)

(b)

Fig. 9. Numerical results of the kinematic simulation of walking operation in Fig. 6: (a) position, velocity and acceleration of leg pitch movement; (b)

position, velocity and acceleration of leg yaw movement. (Position is in solid

line, velocity is in dotted line and acceleration is in dashed line)

In the plots of Fig. 9(a), the end-effector position goes up

to 8 deg even if the pitch input has been up to 15 deg. It means

the leg mechanism drives 8 deg of pitch displacement for end-

effector with 15 deg of input movement. Maximum velocity

and acceleration have been computed as about 20 deg/sec and

about 100 deg/sec^2, respectively, when maximum

displacement is reached. In the plots of Fig. 9(b), the computed

position of end-effector has shown the same time evolution of

input curve of Fig. 7(b) since the yaw motor actuates directly

the whole leg structure. Similar to the pitch case, maximum

yaw velocity and acceleration have been computed at

maximum displacement position with magnitudes of about 40

deg/sec and about 80 deg/sec^2, respectively.

5.2. Moving along a vertical axis Figure 10 shows a photo sequence of the virtual

hexapod simulating a vertical movement under kinematic

conditions. Vertical movement is an important functionality for

a mobile robot since it provides the possibility of reaching

caves, holes and difficult accessible places for multiple tasks

like exploration, inspection and recovering objects or even

injured people. For instants, an application of mobile legged

robot is archaeology and basic features for robotic systems in

this field can be summarized as follows: the robot should be

able to move inside the archaeological site carrying objects and

it can also use laser systems and thermographic sensors to

obtain more precise information about the pavement [17]. The

vertical movement characteristic allows the new Cassino

hexapod robot the capability of working in application like the

above-illustrated.

The vertical movement of the new hexapod consists in a

synchronized operation between legs pitch and feet wheels.

Figure 11 shows the input plots of leg pitch motors and wheel

motors for performing a correct upward and downward

movement. In particular, Fig. 11(a) shows the time evolution of

pitch input of leg and Fig. 11(b) shows the input plot of the

wheel motor for 10 sec of simulation time. Inputs for all six-

legs of the robot have been the same with the curve evolution

of Fig. 11. The pitch plot presents a curve with sinusoidal

evolution that represents the downward movement (from 0 sec

to 5 sec) and upward movement (from 5 sec to 10 sec). Note an

input value of pitch position near to zero at about 5 sec that

represent the transition between downward and upward

movements. The fully upper position has occurred at about 2.5

sec and the fully lower position has occurred at about 7.5 sec.

(a)

(b)

(c)

Fig. 10. Sequence of simulation results of moving along vertical axis: (a) initial

position; (b) minimum lower position; (c) maximum upper position.


(a)

(b)

Fig. 11. Robot inputs for the simulation of Fig. 7: (a) pitch of leg; (b) wheel

actuator.

The plot of wheel actuator represents the linear

displacement of wheel during vertical operation of the new

hexapod. It presents also sinusoidal evolution, as input plot of

leg pitch, but the curve is not symmetric respect to the time

axis. To obtain the same magnitude for upward and downward

displacements, the wheel must move more distance for upward

than for downward movement. Indeed, the maximum wheel

position for downward position has been about 150 mm, while

the maximum position for upward movement has been about

225 mm.

The vertical movement capacity of the new hexapod can be

used preventing a compliance system for robot legs during

walking and wheeled operations on rough surfaces. Suitable

algorithm can be implemented for the pitch motors controlling

input positions by feeding back trunk rotations about Y and X

axes. The aim of this control architecture will be to keep robot

trunk parallel to the terrain that is an important feature for robot

stabilization.

5.3. Wheeled operation mode The wheeled foot of the new design gives the characteristic

of moving as a vehicle to the hexapod robot. Two significant

operations have been simulated by using the CAD model of the

new Cassino hexapod, such as forward and lateral linear

movements. Figure 12 shows pictures of a wheeled movement

simulation of the hexapod straight forward. In particular, Fig.

12(a) shows a picture in which the robot is positioning its legs

in a configuration that allows the straight forward displacement.

This configuration is four legs parallel to the sagittal plane and

the others two in opposite position, as shown in Fig. 12(b).

Trunk shape permits to obtain this proper legs configuration

without interferences among components that represents an

extra novelty for the sub-system design. Figure 12(b) shows a

robot picture in a middle position during the straight forward

movement by using wheels.

Wheels and leg yaws have been the actuated DOFs for

the simulation of straight forward movement. Figure 12 shows

the input plots for these DOFs, in which Fig. 13(a) is the plot of

leg yaws and Fig. 13(b) is the plot of wheel linear displacement

for 10 sec of simulation time. Similar to the previous simulated

modes, the input plots of Fig. 13 are the same for all six robot

legs. Nevertheless, the ramp of Fig. 13(a) from 0 sec to about 3

sec, which represent the positioning movement of leg for a

proper configuration, has positive slope for four legs and

negative slope for the others two. At about 3 sec, all legs reach

the correct position and remain stable at 90 deg to the operation

end. Diagonal displacement can be performed by robot if the

leg yaws reach a different input angel θ from 1 deg to 89 deg.

Figure 13(b) shows, from 0 sec to 4 sec, values of wheel

displacements near to zero that represent the phase in which

legs are positioning in proper configuration and robot remains

at the beginning location. The growing up ramp, from 4 sec to

10 sec, represents the linear straight forward movement to a

final position that is located at about 1500 mm from the

beginning.

(a)

(b)

Fig. 12. Result sequence for a wheeled operating simulation: (a) positioning legs for wheeled translation; (b) forward movement.


(a)

(b)

Fig. 13. Actuation Inputs for the operating simulation of Fig. 9: (a) yaw of leg;

(b) wheel motor.

Figure 14 shows the hexapod simulation performing a

lateral movement by using its wheels. This robot capacity can

be useful for obstacle avoidance during walking or wheeled

displacement since the robot can move rapidly and easily to

another position, as shown in the simulation results. A big

sphere has been modelled as fixed obstacle that interferes in the

robot trajectory, as shown in Fig. 14(a). Figure 14(b) shows

how the hexapod avoids successfully the obstacle by using only

wheels; no more DOFs are required to be actuated. Simulation

time has been 11 sec for the operating simulation of lateral

wheeled movement. Figure 14(c) shows the input for wheel

motors that has been the same for all robot feet. In this

simulation mode, pitch and yaw of legs are not actuated and

their motors have been modelled as fixed in the initial position.

The growing up ramp of Fig. 14(c) represents the robot lateral

translation of 2000 mm avoiding the sphere obstacle.

6. CONCLUSIONS This paper presents results of analysis and design activity

for improving Cassino Hexapod Robot. In particular, the

current leg design and its operation have been considered, in

previous works, both experimentally and numerically through

simulations and tests identifying problems and possibilities of

enhancing the Cassino leg design. A novel design solution has

been conceived specifically for the Cassino Hexapod Robot in

order to achieve better performance of the operation modes

than the original structure. The feasibility of the proposed new

design and easy operation modes have been tested by means of

numerical simulations with suitable detailed model of a

mechanical design. Results have shown efficiency in the system

operation, limited power consumption, robust and compact

design as well as suitable gait planning.

Dynamic simulations of robot operation and control

architecture design will be future works as part of this research

activity on developing a successful hexapod robot.

(a)

(b)

(c)

Fig. 14. Simulation results of a wheeled lateral translation: (a) initial position;

(b) avoid a spherical obstacle; (c) input for the wheel actuator.


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a new design for cassino hexapod robot

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