mobility performance evaluation of planetary rover with simi(2)

月球车珍贵资料——来自美国

typical systems to know what essential points are for improving mobility.

A. Conventional 4-wheel drive system

As you know well, a 4-wheel drive (4WD) shown in Fig.2 is the most popular system for an automobile to traverse rough terrain. We call this conventional 4WD here after. There are many rovers adopted this system for its locomotion, e.g., ISAS/Nissan rover [11-12], Nomad [13-14], AMSL Minirover [15], etc. Conventional 4WD has no mechanisms to improve its mobility, but has just one provides torque to each wheel distributively. It's simple and light in weight, however, it doesn't have high degree of mobility compared with following systems.

B. PEGASUS system

As above, the micro planetary rover is required to have both a simple and light weight mechanism like 4-wheel drive system and a high degree of mobility like rocker-bogie suspension system. In order to achieve these opposed requirements, we propose a new suspension system named “Pentad Grade Assist SUSpension (PEGASUS)”. A devel-oped rover prototype Micro5 equipped with PEGASUS system is shown in Fig.3 (a). PEGASUS consists of a conventional four-wheel drive system and fifth active wheel. As shown in Fig.3 (b), the fifth wheel is attached to the end of a link, and the other end of the link is attached to the body with a passive rotary joint. PEGASUS needs only one joint rather than the rocker-bogie that needs 4 joints. In general, joints are heavy parts and easily lead to trouble in space environment. So, the architecture called "Only-One-Joint" would be one of advantages.

(a) Frame model of Micro5 (b) Mechanism of PESASUS system

Fig.3 PEGASUS (PEntad Grade Assist SUSpention) system

The system is designed to distribute the load of weight equally to all five wheels when the rover climbs up on the step-alike terrain. It means that the fifth wheel supports the load taken to the front wheels when the front wheels climb up rocks, and it also supports that taken to the rear wheels when the rear wheels climb up the rocks. As shown in Fig.4, when the rear wheel climb a step, forward force generated by the traction of the fifth wheel (shown #1 in Fig.4 ) pushes the rear wheel backward as (#2). These forces produce nose-dive moment (#3), then the moment turns to a vertical force of the front wheel (#4) to support traction. This is the reason why PEGASUS has extremely high mobility. This system realized such high mobility in simple and light mechanism. [16]

In the following chapter, we discuss the performance evaluation compared with above system through the experi-

ment, which is carried out by some similarity models.

Fig.4 Kinematics of PEGASUS to climb a step

IV. Similarity Law

When it is difficult to experiment with the real model, the similarity model is often used. The similarity models must be physical similarity of its real phenomenon, e.g. Time, Power, Speed, Temperature, etc., as well as geometric similarity. By carrying out a model experiment, we'll infer the real phe-nomenon from the experimental result.

In the case of a lunar rover, we mate the relation of the physical parameters, which acts on the rover, between on the moon and on the earth. [17] When planetary rover moves on the moon, the physical powers taken into consideration are

shown bellow.

(1) Inertia force of rover Fir = Mrα = MrL/T2

(2) Inertia force of ground Fis = Msα =

ρL2V2 (3) Gravitational force rover Fgr = Mrg (4) Gravitational force ground Fgs = ρgL3 (5) Adhesive force of ground Fc = cL2 (6) External force F Where,

Mr : Mass of rover

[kg] Ms : Mass of ground [kg]

T : Time [s] L : Length [m] V : Velocity of rover [m/s] α : Acceleration of rover [m/s2] ρ : Density of ground [kg/m3] c : Adhesive stress of ground [N/m2] g : Gravitational acceleration [m/s2]

A sign with “ “ show a model. Because we use a soil similar to that on the moon, it's apparent that ρ=ρ′, c=c′.

Consequently, the similarity law can be defined as follows: