Pergamon Mechafronics Vol. 7, No. 3, pp. 281-296, 1991

0 1997 Elsevier Science Ltd. All rights reserved. Printed in Great Britain.

09574158/97 $17.001-0.00

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A CONCEPTUAL WALKING VEHICLE FOR PLANETARY EXPLORATION

A. PREUMONT, P. ALEXANDRE, I. DOROFTEI and F. GOFFIN

Universitt Libre de Bruxelles, Department of Mechanical Engineering and Robotics, 50 Av. F. D. Roosevelt, B-1050, Brussels, Belgium

(Received 30 April 1966; accepted 23 October 1996)

Abstract-A lightweight, small size walking robot currently being developed at the Universite Libre de Bruxelles as a conceptual vehicle for planetary exploration is described. The robot consists of three articulated bodies connected by universal joints. Each body has two legs with two degrees of freedom each. Each universal joint is provided with two actuators and torque sensors; a compliant behaviour is achieved by force feedback. This particular device improves considerably the agility of the walking vehicle by allowing the central body to follow the ground profile. Further- more, the vehicle can walk on both sides and can recover from roll-over thanks to the actuated universal joints which allow an autonomous transfer from back to belly. 0 1997 Elsevier Science Ltd.

1. INTRODUCTION

Studies of future cometary and planetary exploration missions (ROSETTA and MARS) planned by the European Space Agency have identified the need for a small Mobile Instrument Deployment Device (MIDD) which is able to move some distance (say up to 20 m) from the lander and bring instrument sensors in contact with the soil [l ,2].

The basic requirements for the MIDD are a high packing density due to the limited space available for storage in the lander, a low mass (l-2 kg), the capability of climbing obstacles of 10 cm height and travelling at a speed of 5 m/h. The vehicle must also be capable of carrying a camera and a set of scientific instruments such as a spectrometer. Regarding the energy management, two possibilities are considered: in the first one, the MIDD will rely on the lander for power and high level control; in this case, the vehicle will include a tether mechanism. The second alternative is for it to be energetically autonomous and to rely on a radio link with the lander for high level control. The vehicle should be extremely stable and, if possible, have the capability to recover from a roll-over.

In addition to the above requirements, the design of the actual flight model will have to consider the reduced gravity on Mars (g = 3.74 m/s’) and especially on the comets (minigravity), the particularly harsh thermal environment and the exposure to dust.

Various concepts have been considered for the MIDD, including the MARSNET IDD

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[l], JPL’s ROCKY IV [3] and GO-FOR [4], or micro walking machines similar to MIT’s ATTILA II [5] (the last three have been funded by NASA). Their advantages and short- comings are reviewed in [6].

In this context and on the basis of its experience in walking machines [7], the Universite Libre de Bruxelles (ULB) has developed a new concept of lightweight six-legged walking vehicle called IOAN. The main innovative feature consists of three articulated bodies (each of them with two legs), connected by servo controlled universal joints also equipped with torque sensors; this configuration has two distinctive advantages, as follows.

??It is possible to program the joints to behave as active springs of arbitrary stiffness (including negative). This configuration improves considerably the agility of the vehicle by adapting the configuration of the main body to the ground profile.

??The vehicle can walk on both sides and transfer autonomously from back to belly and vice versa. This allows recovery from roll-over.

This paper is organized as follows: section 2 presents the architecture and the kinematics of the vehicle; the active compliance of the universal joints is discussed in section 3 and some control strategies are discussed in section 4.

2. OVERALL ARCHITECTURE, KINEMATICS

IOAN is a 1.2 kg, 40 cm long rectangular hexapod with three articulated bodies connected with servo controlled universal joints (Fig. 1). Each of the three bodies has two legs; each leg has two degrees of freedom organized as indicated in Fig. 2 and a contact switch at its tip. The leg kinematics is not new; it is identical to that used in a micro walking machine called GENGHIS, developed at MIT [8], and another previous prototype built at ULB [9]; it is in fact guided by the availability of low cost servo motors of rectangular shape (used in radio-commanded toys) which can be conveniently assembled to form the hip and the knee of the leg. Each degree of freedom is actuated by position controlled servo motors. A contact switch is mounted in the foot, for obstacle and pit detection. This very simple leg kinematics does of course make slippage unavoidable during walking, but this is no problem for such a lightweight vehicle; the simplicity of the device provides robustness and makes it extremely easy to control: the joint coordinate CI is related to the forwarddbackward motion of the leg that ensures propulsion when the leg is supporting, while /I operates the up-down motion of the leg that enables the lifting and placing of the leg. The up-down motion can be adapted to the terrain by the contact switch in the foot.

The actively articulated body is the innovative part of this concept. The objective is twofold: active suspension and recovery from roll-over.

The active compliance of the chassis increases the vehicle’s capability to get over obstacles by adapting its shape to the terrain profile. The idea was first tested on an earlier prototype where the three bodies were articulated by 1 d.o.f. rotational joints (pitch); this design proved useful for obstacles like steps or a corrugated type of ground. This motivated the extension to the current design, which is equipped with universal joints (pitch and roll; see detail in Fig. 1). Once again, the mechanical design of this prototype is governed by the use of the same servo motors as those used for the legs; each degree of freedom operates independently, the transmission uses a four bar mechanism including a flexible element equipped with strain gauges, for force measurement purposes. The kinematic analysis of

A conceptual walking vehicle for planetary exploration 289

Detail of the active

Fig. 1. General view of the robot.

Chassis =._ I

Fig. 2. Kinematics of a leg.

290 A. PREUMONT el al.

Y (degree) 240 ,

I 210 1.

150 +

120 _i____ _____I___ _--_+~ ~~~~ i __-_t_ _----i--___- _

0 30 60 90 120 150 180

6 (degree)

Fig. 3. Kinematics of the pitch joint.

the transmission mechanism (Fig. 3) allows the strain measurements to be related to the torques about the two axes of the universal joints.

The storage configuration of the vehicle is also very important for a planetary exploration vehicle, because of the limited space available on the lander. Figure 4 shows a possible storage configuration; the walking vehicle stands up by actuating the /I joints of the legs.

The available stroke of the universal joints gives this vehicle a tremendous mobility; in particular, it is possible to transfer from one side to the other by repeating a sequence of steps which have been carried out by the operator and recorded on the on-board computer (Fig. 5).

Although the robot architecture looks complicated, there are only two critical com- ponents which will require careful design for a future flight model:

??the leg, including the actuators and sensors of the two degrees of freedom, and the ground detector at the tip;

??the active universal joint, including the actuators, and the torque sensors.

3. ACTIVE COMPLIANCE

The principle of the active compliance consists of measuring the error of the position and the force distribution with respect to steady state reference values and enforcing the following equilibrium condition:

.Y, -s +.q(.f; -.f’, = 0. (1)

A conceptual walking vehicle for planetary exploration 291

Fig. 4. Storage configuration.

wherefand x are the current values of the force and the position,f, and x, are the reference values, and g is the compliance of the system. This requires the capability to measure the force f and to compute the reference value fr. Active leg compliance to accommodate the differential sinkage between the various legs when moving on soft soil has already been used successfully [ 10, 111. In this study, the active compliance has not been implemented in the leg, for lack of a suitable sensor in the foot, but in the universal joints connecting the articulated bodies. For each degree of freedom, we have

e,-e-g(T,-T) = 0, (2)

where 0 and Prefer to the angular position and torque, respectively. We shall now examine how the reference torque T, can be computed.

As a first step, the force distribution for a given gait state can be calculated by expressing the static equilibrium equations of the whole vehicle; if we assume that the ground support forces are vertical (Fig. 6), we obtain

cFzixi = 0 id

where P is the weight of the vehicle; I is the set of supporting legs; and xi, yi are the

292 A. PREUMONT PI ul.

Fig. 5. Sequence of transfer from belly to back

coordinates of the feet with respect to the center of mass. Whenever there are more than three legs on the ground, this system of equations (AF, = B) is under-determinated (there are more unknowns than equations); the minimum norm solution is given by

F. = ‘4 r B. (4)

where A + = AT(AA7) ’ is the pseudo-inverse of matrix A. This solution minimizes J = Z Fz, while satisfying the equality constraints (3) of the static equilibrium.

Once the vertical force distribution has been obtained, the reference torques in the two degrees of freedom of the universal joints can be obtained by considering the rotation equilibrium about the corresponding axis. A precise knowledge of the reference torque is not necessary and the active compliance (2) can just as well be implemented with T, = 0.

A conceptual walking vehicle for planetary exploration 293

Fig. 6. Force distribution for a given gait state.

4. CONTROL

4.1. Control architecture

The control of the walking vehicle is based on a microcontroller board (Intel 87C196KC) located on the central part of the body. This board generates the PWM signals for the 16 servo motors of the robot (12 for the legs and four to drive the universal joints), performs the acquisition of the four force signals and reads the state of the contact switches. Some electronics and wiring are included in the robot frame and legs which are made out of printed circuit boards.

In its present state, the control board is linked to a PC (supposed to be located in the lander) through a serial line which is included in the tether, along with the power line. The PC acts as Man-Machine Interface, gait generator, and manages the active compliance of the joints. The last two functions could easily be implemented at the lower level (mic- rocontroller) in a future version of the control software.

4.2. Gait control, steering and turning

A wide variety of regular gaits have been implemented and tested; theforward wave gait has been selected because it maximises the front stability margin of the vehicle; the duty factor is adjusted to fit the forward velocity selected by the operator. Fine steering to keep the trajectory straight or for wide curves is achieved by adjusting the size of the steps on each side of the vehicle while faster changes of direction are obtained by stopping the vehicle and using a special tripod gait where the left and right legs move in opposite directions.

4.3. Pit detection mode

The contact switches on the front legs can be used for pit detection: if the switch fails to indicate the contact when the joint angle /I reaches its nominal value, a new reference value, /I + A/I, is generated and the operation is repeated. If the contact is established, the walking

294 A. PREUMONT et al.

proceeds normally; if not, a pit has been detected and the forward motion is stopped. Autonomous exploration can be easily programmed with the following set of commands:

do

Step_Forward( ); if (Pit Detected)

I

Step_Back( ); Turn_90( );

while (Exploration In-Progress);

With this simple strategy, the machine can be left on a table; it will explore it without falling off, with a seemingly intelligent behaviour. Such a behaviour would be much more difficult to achieve with a wheeled or tracked vehicle.

4.4. Actice compliance

By using a constant gain y, the active compliance can be made to behave like an ordinary spring of programmable stiffness. Whether the reference torque is specified accurately or not is of relatively minor importance, because errors on T simply produce small changes in the relative orientation of the three articulated bodies.

When moving on an irregular surface, several strategies are possible to combine the information from the contact switches with the active compliance: the simplest one consists of letting the active compliance alone take care of the adaptation to the terrain; the stroke pr of the up and down motion of the leg is kept constant. Another approach consists of sharing the ground irregularity between the legs and the active compliance; this can be achieved by updating the reference value /Ir of the up and down motion of the legs when the contact is established with the ground according to

/)‘T= /I*+# -fl*,. (5)

where plis the new reference value. B* is the joint angle when the leg touches the ground and 0 < E < 1 is an adjustable parameter. These strategies are under evaluation.

The following difficulty has been observed with the active compliance when the vehicle moves down a slope (Fig. 7a): the horizontal component of the contact force produces a torque which tends to decrease that of the vertical component; if the slope is steep, the net torque may even change its sign, producing a deformation of the body which is in fact opposite to the desired one. This situation can be detected by checking the sign of the torque, and corrected by changing the sign of the active compliance to achieve the desired behaviour (Fig. 7b). This procedure can easily be integrated in a rule-based logic.

4.5. Trans&r,jLom back to bell)

As already mentioned, the vehicle is capable of walking on both sides. In principle, this capability should eliminate the need for transferring from one side to the other. This is not

A conceptual walking vehicle for planetary exploration 295

(a)

(b)

Fig. 7. Moving down a step: (a) with positive compliance; (b) with negative compliance.

true, because the scientific instruments will not be located symmetrically on the body of the vehicle.

The transfer from back to belly and the other way round proceeds according to reach-in: extensive experiments have been performed with the vehicle and the sequence of operations has been recorded in the on-board computer. The macro instruction Transfer_From_Back_To_Belly( )

gansfer_From_Belly_To_Back() simply reads the corresponding recorded sequences.

5. CONCLUSION

A conceptual walking vehicle for planetary exploration has been built. It consists of three articulated bodies connected with two active universal joints. A compliant behaviour is

296 A. PREUMONT PI (I/.

achieved by force feedback. This improves considerably its agility by allowing the vehicle to adapt its configuration to the ground profile and to recover from roll-over. The forward motion of the vehicle follows a forward wave gait which maximizes the front stability margin; the velocity is adjusted by selecting the duty factor. Fine steering is achieved by adjusting the size of the steps on each side of the vehicle, while sudden changes of direction are obtained with a special gait. A pit detection mode using the contact switches on the front legs has been tested successfully. Various rule-based control logics of the active compliance are under evaluation. to maximize the agility of the vehicle.

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September. 12-15. 4. Willcox, B. H., Matthies, L., Gennery, D.. Cooper. B., Nguyen. T.. Litwin, T., Mishkin,

A. and Stone. H., Robotic vehicles for planetary exploration. Proceedings of’the 1992 IEEE Internationul Cogf&rence on Robotirs and Automation, Nice, France, May 1992.

5. Gomi. T.. Subsumption robots and the application of intelligent robots to the service industry. Applied AI Systems. Inc.. Ontario, Canada, July 1993.

6. Schilling, K. and Jungius. C , Mobile robots for planetary exploration. 2nd IFAC Workshop on Intelligent Autonomous Vehicles, Espoo, Finland, 1995.

7. Preumont, A. and Alexandre, P., Some trends in walking robots in Europe. 2nrl Internationul Cocferencc on Motion and Vibration Control MOVIC, Keynote lecture, Yokohama, Japan, 1994.

8. Brooks, R. A., A robot that walks: emergent behaviours from a carefully evolved network. IEEE International C’or$&wce on Robotics and Automution. Scottsdale, AZ, May 1989.

9. Preumont, A., Ghuys. D. and Malekian, C., On the static stability of hexapods. 3rd International Workshop on Adumccs in Robot Kinematics, Ferrara, Italy, September 1992. pp. 126-133.

10. Gorinevsky, D. M. and Schneider. A. Yu.. Force control in locomotion of legged vehicles over rigid and soft surfaces. The Intrrnationul Journal qf Robotics Research. 1990, 9(2), 423.

I I. Klein, C. A. and Briggs, R. L., Use of active compliance in the control of legged vehicles. IEEE Transactions on Systems, Mun, and Cybernetics, 1980, 10(7), 393400.