Sandin P.E. Robot mechanisms and mechanical devices illustrated

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184 Chapter 5 Tracked Vehicle Suspensions and Drivetrains

very capable layout results. The main chassis is geared to the shared axle

so it is always at the half-angle between the front and rear tracks, which

allows it to be raised up yet still be level when folding both tracks down.

This trick raises the entire chassis, but it also offloads the weight of the

robot from the track guide blades, increasing rolling efficiency when

high traction is not needed. This reconfigurable layout combines the high

mobility of tracks with good smooth-road rolling efficiency.

There are two basic layouts for four-tracked vehicles. They are both

train-like in that there are two two-tracked modules connected by some

sort of joint. The two modules must be able to move in several directions

relative to each other. They can pitch up and down, yaw left and right,

and, ideally, roll (twist).

The simplest connection that allows all three degrees of freedom is the

ball joint. If the joint is passive, steering is accomplished in the same

way as a two-tracked vehicle, except that now both modules must turn at

just the right time to keep skidding between the modules to a minimum.

This turns out to be tricky. The ball joint also limits the range of steering

angle simply because the socket must wrap around the ball enough to

adequately capture it. A universal joint has a greater range of motion, and

is easier to use if the joint is to be powered.

The articulated joint, an active universal joint, overcomes the steering

problem by allowing the tracks to rotate at whatever speed limits skid-

ding. This steering method makes this layout very agile. The Hagglund

Bv206, which uses this layout, is considered nearly unstoppable in

almost any terrain from soft snow to steep hills. It is even amphibious,

propelled through the water by the tracks. Because it cannot be skid

steered, it can’t turn in place. Nevertheless, it is a very capable layout.

Steering the Hagglund Bv206 is done with a standard steering wheel,

which turns the articulated joint and forces the two modules to bend. The

tracks are driven through limited slip differentials, allowing the inner and

outer tracks in each module to travel at different speeds just like in an

Ackerman steered wheeled vehicle.

Six-Tracked Drivetrains

There is at least one track layout (Figure 5-18) incorporated on an exist-

ing telerobotic vehicle that uses six tracks. It is an extension of the

Urbie design, but was actually invented before Urbie’s layout. The two-

tracked layout is augmented by flipper tracks on both the front and

back, independently tilted, but whose tracks are driven by the main

track motors. This layout allows the vehicle to stand up like the one

Chapter 5 Tracked Vehicle Suspensions and Drivetrains 185

shown in Figure 5-17. The double flippers extend the length of the two-

tracked base unit by almost a factor of two, facilitating crossing wide

crevasses and climbing stairs, yet still being able to turn in place in a

small aisle.

Figure 5-18 Six-tracked, double flippers

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Chapter 6 Steering History

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he Romans extensively used two wheeled carts, pulled by horses.

Pull on the right rein and the horse pulls the cart to the right, and vise

versa. The two wheels on the cart were mounted on the same axle, but

were attached in a way that each wheel could rotate at whatever speed

was needed depending on whether the cart was going straight or around

a corner. Carts got bigger and eventually had four wheels, two in front

and two in back. It became apparent (though it is unclear if it was the

Romans who figured this out) that this caused problems when trying to

turn. One or the other set of wheels would skid. The simplest method for

fixing this problem was to mount the front set of wheels on each end of

an axle that could swivel in the middle (Figure 6-1). A tongue was

attached to the axle and stuck out from the front of the vehicle, which in

turn was attached to a horse. Pulling on the tongue aligned the front

wheels with the turn. The back wheels followed. This method worked

well and, indeed, still does for four wheeled horse drawn buggies and

carriages.

189

Figure 6-1 Pivot mounted front

wheels

190 Chapter 6 Steering History

In the early 1800s, with the advent of steam engines (and, later, elec-

tric motors, gas engines, and diesel engines) this steering method began

to show its problems. Vehicles were hard to control at speeds much faster

than a few meters per second. The axle and tongue took up a lot of room

swinging back and forth under the front of the vehicle. An attempt

around this problem was to make the axle long enough so that the front

wheels didn’t hit the cart’s sides when turning, but it was not very con-

venient having the front wheels wider than the rest of the vehicle.

The first effective fix was to mount the two front wheels on a mecha-

nism that allowed each wheel to swivel closer to its own center. This

saved space and was easier to control and it appeared to work well. In

1816, George Lankensperger realized that when turning a corner with

the wheels mounted using that geometry the inside wheel swept a differ-

ent curve than the outside one, and that there needed to be some other

mechanical linkage that would allow this variation in alignment. He

teamed with Rudolph Ackerman, whose name is now synonymous with

this type of steering geometry. Although Ackerman steering is used on

almost every human controlled vehicle designed for use on roads, it is

actually not well suited for high mobility vehicles controlled by comput-

ers, but it feels right to a human and works very well at higher speeds. It

turns out there are many other methods for turning corners, some intu-

itive, some very complex and unintuitive.

STEERING BASICS

When a vehicle is going straight the wheels or tracks all point in the

same direction and rotate at the same speed, but only if they are all the

same diameter. Turning requires some change in this system. A two-

wheeled bicycle (Figure 6-2) shows the most intuitive mechanism for

performing this change. Turn the front wheel to a new heading and it

rolls in that direction. The back wheel simply follows. Straighten out the

front wheel, and the bicycle goes straight again.

Close observation of a tricycle’s two rear wheels demonstrates

another important fact when turning a corner: the wheel on the inside of

the corner rotates slower than the outside wheel, since the inside wheel is

going around a smaller circle in the same amount of time. This important

detail, shown in Figure 6-3, occurs on all wheeled and tracked vehicles.

If the vehicle’s wheels are inline, there must be some way to allow the

wheels to point in different directions. If there are wheels on either side,

they must be able to rotate at different speeds. Any deviation from this

Chapter 6 Steering History 191

Figure 6-2 Bicycle steering

Figure 6-3 Tricycle steering

192 Chapter 6 Steering History

and some part of the drive train in contact with the ground will have to

slide or skid.

Driving straight in one direction requires at least one single direction

actuator. A wind-up toy is a good demonstration of this ultra-simple

drive system. Driving straight in both directions requires at least one bi-

directional actuator or two single-direction actuators. One of those single

direction actuators can power either a steering mechanism or a second

drive motor. Add one more simple single-direction motor to the wind-up

toy, and it can turn to go in any new direction. This shows that the least

number of actuators required to travel in any direction is two, and both

can be single-direction motors.

In practice, this turns out to be quite limiting, at least partly because it

is tricky to turn in place with only two single direction actuators, but

mostly because there aren’t enough drive and steer options to pick from

to get out of a tight spot. Let’s investigate the many varieties of steering

commonly used in wheeled and tracked robots.

The simplest statically stable vehicle has either three wheels or two

tracks, and the simplest power system to drive and steer uses only two

single-direction motors. It turns out that there are only two ways to steer

these very simple vehicles:

1. Two single-direction motors powering a combined drive/steer wheel

or combined drive/steer track with some other passive wheels or

tracks

2. Two single-direction motors, each driving a track or wheel (the third

wheel on the wheeled layout is a passive swivel caster)

The simplest version of the first steering geometry is a single-wheel

drive/steer module mounted on a robot with two fixed wheels. The com-

mon tricycle uses this exact layout, but so do some automatic guided

vehicles (AGVs) used in automated warehouses. Mobility is limited

because there is only one wheel providing the motive force, while drag-

ging two passive wheels. This layout works well for the AGV application

because the warehouse’s floor is flat and clean and the aisles are

designed for this type of vehicle. In an AGV, the drive/steer module usu-

ally has a bi-directional steering motor to remove the need to turn the

drive wheel past 180° but single direction steer motors are possible.

There are many versions of AGVs—the most complicated types have

four drive/steer modules. These vehicles can steer with, what effectively

amounts to, any common steering geometry; translate in any direction

without rotating (commonly called “crabbing”), pseudo-Ackerman steer,

turn about any point, or rotate in place with no skidding. Wheel modules

Chapter 6 Steering History 193

for AGVs are available independently, and come in several sizes ranging

from about 30 cm tall to nearly a meter tall.

The second two-single-direction motor steering layout has been suc-

cessfully tried in research robots and toys, but it doesn’t provide enough

options for a vehicle moving around in anything but benign environ-

ments. It can be used on tracked vehicles, but without being able to drive

the tracks backwards, the robot can not turn in place and must turn about

one track. Figure 6-4 shows this limitation in turning. This may be

acceptable for some applications, and the simplicity of single direction

electronic motor-driver may make up for the loss of mobility. The

biggest advantage of both of these drive/steering systems is extreme sim-

plicity, something not to be taken lightly.

The Next Step Up

The next most effective steering method is to have one of the actuators

bi-directional, and, better than that, to have both bi-directional. The Rug

Warrior educational robot uses two bi-directional motors—one at each

wheel. This steering geometry (Figure 6-5a, 6-5b) is called differential

steering. Varying the relative speed, between the two wheels turns the

robot. On some ultra-simple robots, like the Rug Warrior, the third

wheel does not even swivel, it simply rolls passively on a fixed axle and

skids when the robot makes a turn. Virtually all modern two-tracked

Figure 6-4 Turning about one

track