Sandin P.E. Robot mechanisms and mechanical devices illustrated

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154 Chapter 4 Wheeled Vehicle Suspensions and Drivetrains

can negotiate obstacles that are twice the wheel height. This figure shows

only the basic parts of the mobility layout. The part labeled “chassis” is

the backbone or main support piece for the main body, which is not

shown.

The very fact that each wheel is passively loaded by the rocker bogie

suspension reduces its negotiable chasm width. Lockable pivots on the

bogie can extend the negotiable chasm width by making the center

wheels able to support the weight of the entire vehicle. This adds yet

another actuator to this already complicated layout. This actuator can be

a simple band or disc brake.

The rocker bogie suspension can be skid steered, but the side forces

on the wheels produce moments in the rockers for which the rockers

must be designed. Since the wheels are at the end of arms that move rel-

ative to each other, the most common layout puts a motor in each wheel.

Steering is done by turning both the front and the rear wheels with their

own steering motors. This means that this layout uses 10 motors to

achieve its very high mobility. In this design, the large number of actua-

tors reduces the number of moving parts and over all complexity.

The steering geometry allows turning in place with no skidding at all.

This is the layout used on Sojourner, the robot that is now sitting on Mars

after completing an entirely successful exploration mission on the Red

Planet. Mobility experts claim this layout has the highest mobility possi-

Figure 4-26 Rocker bogie

Chapter 4 Wheeled Vehicle Suspensions and Drivetrains 155

ble in a wheeled vehicle, but this high mobility comes at the cost of those

ten actuators and all their associated control electronics and debug time.

There is a layout that is basically six-wheeled, but with an extra pair

of wheels mounted on flippers at the front. These wheels are powered

with the three on each side and the vehicle is skid steered, but the front

set of wheels are only placed on the ground for extra traction and stair

climbing. This layout is in the same category as several layouts of

tracked vehicles, as are several of the eight-wheeled layouts.

The next logical progression, already commercially available from

Remotec in a slight variation, is to put the four center wheels on the

ground, and put both end pairs on flippers. The center pair, instead of

wheels, could be tracks, as it is on Remotec’s Andros. The flippers

carry either wheels or short tracks. This vehicle is rather complicated,

but has great mobility since it can reconfigure itself into a long stair-

climbing or crevasse-crossing layout, or fold up into a short vehicle

about half as long.

EIGHT-WHEELED LAYOUTS

If six wheels are good then eight wheels are better, right? For a certain

set of requirements, eight wheels can be better than six. There is, theo-

retically, more surface area simply because there are more wheels, but

this is true mostly if there is a height limitation on the robot. If the robot

needs to be particularly low for its size, then eight wheels may be the

answer.

The most common layout for eight wheels, since inherently there are

more moving parts already, is to skid-steer with fixed wheels. Lowering

the center two pairs aids in skid steering just like on a six wheeled skid

steer, but the four wheels on the ground means there is less wobbling

when stopping and starting. Figure 4-27 shows this basic layout with the

center wheels lowered slightly.

With all the wheels fixed there are many times when several of the

wheels will be lifted off the ground, reducing traction greatly. A simple

step to reduce this problem is to put the wheels on rockers, in pairs on

each side. A set of wheels may still leave the ground in some terrains, but

the other six wheels should remain mostly in contact with the ground to

give some traction. Adding steering motors at the attachment point of

each rocker would produce four-corner steering with minimal skidding.

Since the bogie is a fairly simple arm connecting only a pair of wheels, a

single motor could potentially be mounted near the center of the bogie

and through a power transfer system, drive both wheels. This would

156 Chapter 4 Wheeled Vehicle Suspensions and Drivetrains

reduce the number of actuators, even with four corner steering, to eight.

No known instances of this layout, shown in Figure 4-28, have been built

for testing, though it seems like an effective layout.

With eight wheels, there is the possibility of dividing the vehicle into

two sections, each with four wheels. The two parts are then either con-

nected through a passive joint and individually skid steered, or the joint

is articulated and steering is done by bending the vehicle in the middle.

This is identical to the four tracked layouts discussed and shown in chap-

ter five. This can be a very effective layout for obstacle negotiation and

crevasse crossing, but cannot turn in place. Figure 4-29 shows an exam-

ple of a two-part passive joint eight-wheeled layout. Figure 4-30 adds a

roll joint to aid in keeping more wheels on the ground.

Another eight-wheeled layout, also applicable to a four-tracked vehi-

cle, uses a transverse pivot, which allows the two halves to pitch up and

down. It is skid steered, and is suited for bumpy terrain, but which has

few obstacles it must go around. Imagine the vehicle in Figure 4-30, but

with the pivot axis on its side. This layout is similar to the double rocker

layout, with similar mobility and fewer moving parts.

The two halves of an eight-wheeled layout can also be coupled

together with a ball joint. The ball joint allows pitch, roll, and yaw

between the two parts which facilitates keeping all eight wheels on the

ground most of the time. The ball joint is a simple joint and can be made

robust. It has a limited range of motion around two of the axis, but the

third axis can rotate three hundred sixty degrees. Aligning this axis verti-

cally aligns it in the steering axis. This allows the vehicle to have a

tighter steering radius, but it cannot turn in place. Figure 4-31 shows the

four-wheeled sections connected through a vertical axis ball joint. The

ball joint is difficult to use with a four-wheeled vehicle because the

Figure 4-27 Eight wheels, all fixed, center axles offset

Chapter 4 Wheeled Vehicle Suspensions and Drivetrains 157

wheel torque would try to spin the section around the wheels. This prob-

lem can be reduced if the wheels are coupled together so their torques

are always nearly indentical.

Figure 4-28 Eight wheels, dou-

ble bogie

Figure 4-29 Two part, eight

wheeled, vertical center pivot

158 Chapter 4 Wheeled Vehicle Suspensions and Drivetrains

For a truly complicated wheeled drive mechanism, the Tri-star Land-

Master from the movie Damnation Alley is probably the most impres-

sive. This vehicle, of which only one was built, is a two-section, center

pivot steered layout with a Tri-star wheel at each corner. The Tri-star

wheels consist of three wheels, all driven together, arranged in a three-

Figure 4-30 Two part, eight

wheeled, vertical and roll joints

Figure 4-31 Two part, eight

wheeled, vertical ball joint

Chapter 4 Wheeled Vehicle Suspensions and Drivetrains 159

pointed star on a shared hub that is also driven by the same shaft that

drives the wheels. When a bump or ditch is encountered that the wheels

alone cannot traverse, the whole three-wheeled system rotates around the

center hub and the wheels essentially become very large cleats. The Tri-

star wheels are driven through differentials on the Land-Master, but pow-

ering each with its own motor would increase mobility even further.

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Chapter 5 Tracked Vehicle

Suspensions

and Drivetrains

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here has long been a belief that tracks have inherently better mobil-

ity than wheels and anyone intending to design a high mobility vehi-

cle should use tracks. While tracks can breeze through situations where

wheels would struggle, there are only a few obstacles and terrains which

would stop a six wheeled rocker bogie vehicle, but not stop a similar

sized tracked vehicle. They are

• very soft terrain: loose sand, deep mud, and soft powder snow

• obstacles of a size that can get jammed between wheels

• crevasses

They get this higher mobility at a cost of greater complexity and lower

drive efficiency, so tracks are better for these situations, but not inher-

ently better overall.

Tracked vehicles first started to appear in the early 1900s and were

used extensively in WWI. The basic layout used then is still in use today

on heavy construction equipment; a drive sprocket at one end, an idler

wheel at the other that usually serves as a tensioner, and something in

between to support the tracks on the ground. This basic, simple layout is

robust and easy to control. Even in its most simple form, this track layout

has all of the improvements over wheels previously listed.

The continuous surface in contact with the ground is what produces

the benefits of tracks. The long surface combined with widths similar to

wheels puts a large surface on the ground. This lowers ground pressure,

allowing traveling on softer surfaces. It also provides more area for

treads, increasing the number of teeth on the ground.

The continuous surface eliminates a wheeled vehicle’s problem of

becoming high centered between the wheels on one side. A correctly

sized obstacle can get caught between the wheels on one side, but the

track stays on top. The wheeled vehicle can get stuck in these situations,

where the track would simply roll over the obstacle.

Perhaps the most important capability the continuous surface facili-

tates that a wheeled suspension cannot match (without undue complex-

ity) is the ability to cross crevasses. Clever suspension components can

be added to a six-wheeled or eight-wheeled vehicle to increase its nego-

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