Sandin P.E. Robot mechanisms and mechanical devices illustrated

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144 Chapter 4 Wheeled Vehicle Suspensions and Drivetrains

ALL-TERRAIN VEHICLE WITH

SELF-RIGHTING AND POSE CONTROL

Wheels dr iven by gearmotors are mounted on pivoting struts.

NASA’s Jet Propulsion Laboratory, Pasadena, California

A small prototype robotic all-terrain vehicle features a unique drive and

suspension system that affords capabilities for self righting, pose control,

and enhanced maneuverability for passing over obstacles. The vehicle is

designed for exploration of planets and asteroids, and could just as well

be used on Earth to carry scientific instruments to remote, hostile, or oth-

erwise inaccessible locations on the ground. The drive and suspension

system enable the vehicle to perform such diverse maneuvers as flipping

itself over, traveling normal side up or upside down, orienting the main

vehicle body in a specified direction in all three dimensions, or setting

the main vehicle body down onto the ground, to name a few. Another

maneuver enables the vehicle to overcome a common weakness of tradi-

tional all-terrain vehicles—a limitation on traction and drive force that

makes it difficult or impossible to push wheels over some obstacles: This

vehicle can simply lift a wheel onto the top of an obstacle.

The basic mode of operation of the vehicle can be characterized as

four-wheel drive with skid steering. Each wheel is driven individually by

a dedicated gearmotor. Each wheel and its gearmotor are mounted at the

free end of a strut that pivots about a lateral axis through the center of

Figure 4-14 Each wheel Is driven by a dedicated gearmotor and is coupled to the idler

pulley. The pivot assembly imposes a constant frictional torque T, so that it is possible to

(a) turn both wheels in unison while both struts remain locked, (b) pivot one strut, or (c)

pivot both struts in opposite directions by energizing the gearmotors to apply various

combinations of torques T/2 or T.

Chapter 4 Wheeled Vehicle Suspensions and Drivetrains 145

gravity of the vehicle (see figure). Through pulleys or other mechanism

attached to their wheels, both gearmotors on each side of the vehicle

drive a single idler disk or pulley that turns about the pivot axis.

The design of the pivot assembly is crucial to the unique capabilities

of this system. The idler pulley and the pivot disks of the struts are made

of suitably chosen materials and spring-loaded together along the pivot

axis in such a way as to resist turning with a static frictional torque T; in

other words, it is necessary to apply a torque of T to rotate the idler pul-

ley or either strut with respect to each other or the vehicle body.

During ordinary backward or forward motion along the ground, both

wheels are turned in unison by their gearmotors, and the belt couplings

make the idler pulley turn along with the wheels. In this operational

mode, each gearmotor contributes a torque T/2 so that together, both gear-

motors provide torque T to overcome the locking friction on the idler pul-

ley. Each strut remains locked at its preset angle because the torque T/2

supplied by its motor is not sufficient to overcome its locking friction T.

If it is desired to change the angle between one strut and the main

vehicle body, then the gearmotor on that strut only is energized. In gen-

eral, a gearmotor acts as a brake when not energized. Since the gearmo-

tor on the other strut is not energized and since it is coupled to the idler

pulley, a torque greater than T would be needed to turn the idler pulley.

However, as soon as the gearmotor on the strut that one desires to turn is

energized, it develops enough torque (T) to begin pivoting the strut with

respect to the vehicle body.

It is also possible to pivot both struts simultaneously in opposite direc-

tions to change the angle between them. To accomplish this, one ener-

gizes the gearmotors to apply equal and opposite torques of magnitude

T: The net torque on the idler pulley balances out to zero, so that the idler

pulley and body remain locked, while the applied torques are just suffi-

cient to turn the struts against locking friction. If it is desired to pivot the

struts through unequal angles, then the gearmotor speeds are adjusted

accordingly.

The prototype vehicle has performed successfully in tests. Current

and future work is focused on designing a simple hub mechanism, which

is not sensitive to dust or other contamination, and on active control

techniques to allow autonomous planetary rovers to take advantage of

the flexibility of the mechanism.

This work was done by Brian H. Wilcox and Annette K. Nasif of

Caltech for NASA’s Jet Propulsion Laboratory.

If a differential is installed between the halves of a longitudinal rocker

layout, with the axles of the differential attached to each longitudinal

rocker, and interesting effect happens to the differential input gear as the

146 Chapter 4 Wheeled Vehicle Suspensions and Drivetrains

vehicle traverses bumpy terrain. If you attach the chassis to this gear, the

pitch angle of the chassis is half the pitch angle of either side rocker. This

pitch averaging effectively reduces the pitching motion of the chassis,

maintaining it at a more level pose as either side of the suspension sys-

tem travels over bumps. This can be advantageous in vehicles under

camera control, and even a fully autonomous sensor driven robot can

benefit from less rocking motion of the main chassis. This mechanism

also tends to distribute the weight more evenly on all four wheels,

increasing traction, and, therefore, mobility. Figure 4-15 shows the basic

mechanism and Figure 4-16 shows it installed in a vehicle.

Another mechanical linkage gives the same result as the differential-

based chassis pitch-averaging system. See figure 4-17. This design uses

a third rocker tied at each end to a point on the side rockers. The middle

of the third rocker is then tied to the middle of the rear (or front) of the

chassis, and, therefore, travels up and down only half the distance each

end of a given rocker travels. The third rocker design can be more volu-

metrically efficient and perhaps lighter than the differential layout.

Another layout that can commonly be found in large industrial vehi-

cles is one where the vehicle is divided into two sections, front and rear,

each with its own pair of wheels. The two sections are connected through

Figure 4-15 Pitch averaging

mechanism

Chapter 4 Wheeled Vehicle Suspensions and Drivetrains 147

an articulated (powered) vertical-axis joint. In the industrial truck ver-

sion, the front and rear sections’ wheels are driven through differentials,

but higher traction would be obtained if the differentials were limited

slip, or lockable. Even better would be to have each wheel driven with its

Figure 4-16 Chassis pitch aver-

aging mechanism using

differential

Figure 4-17 Chassis link-based

pitch averaging mechanism

148 Chapter 4 Wheeled Vehicle Suspensions and Drivetrains

own motor. This design cannot turn in place, but careful layout can pro-

duce a vehicle that can turn in little more than twice its width.

Greater mobility is achieved if the center joint also allows a rolling

motion between the two sections. This degree of freedom keeps all four

wheels on the ground while traversing uneven terrain or obstacles. It also

improves traction while turning on bumps. Highest mobility for this lay-

out would come from powering both the pivot and roll joints with their

own motors and each wheel individually powered for a total of six

motors. Alternatively, the wheels could be powered through limited slip

differentials and the roll axis left passive for less mobility, but only three

motors. Figures 4-18 and 4-19 show these two closely related layouts.

An unusual and unintuitive layout is the five-wheeled drivetrain. This

is basically the tricycle layout, but with an extra pair of wheels in the

back to increase traction and ground contact area. The front wheel is not

normally powered and is only for steering. Figure 4-20 shows this is a

fairly simple layout relative to its mobility, especially if the side wheel

pairs are driven together through a simple chain or belt drive. Although

the front wheels must be pushed over obstacles, there is ample traction

from all that rubber on the four rear wheels.

Figure 4-18 Two-sections con-

nected through vertical axis joint

Chapter 4 Wheeled Vehicle Suspensions and Drivetrains 149

Figure 4-19 Two sections con-

nected through both a vertical

axis and a longitudinal axis joint

Figure 4-20 Five wheels

150 Chapter 4 Wheeled Vehicle Suspensions and Drivetrains

SIX-WHEELED LAYOUTS

Beyond four- and five-wheeled vehicles is the large class of six-wheeled

layouts. There are many layouts, suspensions, and drivetrains based on

six wheels. Six wheels are generally the best compromise for high-

mobility wheeled vehicles. Six wheels put enough ground pressure, trac-

tion, steering mobility, and obstacle-negotiating ability on a vehicle

without, in most cases, very much complexity. Let’s take a look at the

more practical variations of six-wheeled layouts.

The most basic six-wheeled vehicle, shown in Figure 4-21, is the skid-

steered non-suspended design. This is very much like the four-wheeled

design with improved mobility simply because there is more traction and

less ground pressure because of the third wheel on each side. The wheels

can be driven with chains, belts, or bevel gearboxes in a simple way,

making for a robust system.

An advantage of the third wheel in the skid-steer layout is that the

middle wheel on each side can be mounted slightly lower than the other

two, reducing the weight the front and rear wheel pairs carry. The lower

weight reduces the forces needed to skid them around when turning,

reducing turning power. The offset center axle can make the vehicle

wobble a bit. Careful planning of the location of the center of gravity is

required to minimize this problem. Figure 4-22 shows the basic concept.

Figure 4-21 Six wheels, all

fixed, skid steer

Chapter 4 Wheeled Vehicle Suspensions and Drivetrains 151

An even trickier layout adds two pairs of four-bar mechanisms sup-

porting the front and rear wheel pairs (Figure 4-23). These mechanisms

are moved by linear actuators, which raise and lower the wheels at each

corner independently. This semi-walking mechanism allows the vehicle

to negotiate obstacles that are taller than the wheels, and can aid in tra-

versing other difficult terrain by actively controlling the weight on each

wheel. This added mobility comes at the expense of many more moving

parts and four more actuators.

Skid steering can be improved by adding a steering mechanism to the

front pair of wheels, and grouping the rear pair more closely together.

Figure 4-22 Six wheels, all

fixed, skid steer, offset center axle

Figure 4-23 Six wheels, all cor-

ner wheels have adjustable

height, skid steer

152 Chapter 4 Wheeled Vehicle Suspensions and Drivetrains

This has better steering efficiency, but, surprisingly, not much better

mobility. Incorporating the Ackerman steering layout removes the ability

of the robot to turn in place. This can be a real handicap in tight places.

Figure 4-24 shows the basic layout. Remember that the relative sizes of

wheels and the spacing between them can be varied to produce different

mobility characteristics.

The epitome of complexity in a once commercially available six-

wheeled vehicle, not recommended to be copied for autonomous robot

use, is the Alvis Stalwart. This vehicle was designed with the goal of

going anywhere in any conditions. It was a six-wheeled (all independ-

ently suspended on parallel links with torsion arms) vehicle whose front

four wheels steered. Each bank of three wheels was driven together

through bevel gears off half-shafts. It had offset wheel hub reduction

gear boxes, a lockable central differential power transfer box with inte-

gral reversing gears, and twin water jet drives for amphibious propul-

sion. All six wheels could be locked together for ultimate straight-ahead

traction. No sketch is included for obvious reasons, but a website with

good information and pictures of this fantastically complicated machine

is www.4wdonline.com/Mil/alvis/stalwart.html.

The main problem with these simple layouts is that when one wheel is

up on a bump, the lack of suspension lifts the other wheels up, drastically

reducing traction and mobility. The ideal suspension would keep the load

Figure 4-24 Six wheels, front

pair steer

Chapter 4 Wheeled Vehicle Suspensions and Drivetrains 153

on each wheel the same no matter at what height any one wheel is. The

following suspension systems even out the load on each wheel—some

more than others.

An interesting layout that does a good job of maintaining an even load

distribution divides the robot into three sections connected by a single

degree of freedom joint between each section (Figure 4-25). The center

section has longitudinal joints on its front and back that attach to the

cross pieces of the front and rear sections. These joints allow each sec-

tion to roll independently. This movement keeps all six wheels on the

ground. The roll axes are passive, requiring no actuators, but the separa-

tion of the wheeled sections usually forces putting a motor at each wheel,

and the vehicle is skid steered. This layout has been experimented with

by researchers and has very high mobility. The only drawback is that the

roll joints must be sized to handle the large forces generated when skid

steering.

The rocker bogie suspension system shown in Figure 4-26 uses an

extension of the basic four-wheel rocker layout. By adding a bogie to

one end of the rocker arm, two wheels can be suspended from one end

and one from the other end. Although this layout looks like it would pro-

duce asymmetrical loads on the wheels, if the length of the bogie is half

that of the rocker, and the rocker is attached to the chassis one third of its

length from the bogie end, the load on each wheel is actually identical.

The proportions can be varied to produce uneven loads, which can

improve mobility incrementally for one travel direction, but the basic

layout has very good mobility. The rocker bogie’s big advantage is that it

Figure 4-25 Six wheels, three

sections, one DOF between each

section, skid steer