Building-plan MER

This building-plan gives a quick overview of the possible solutions to build the Mars Exploration

Rover (MER). The solution that seems most suited to complete the line-tracking and mars mission is made clear.

Possible issues for the MER are:

• Center of gravity for grip and stability: as low as possible

• Speed: small, light robot but it is easier to miss goals

• Wheels: big wheels will result in more speed, but less power

• Wheelbase: wide as possible to have stable robot and good straight line tracking/driving

• Turning-circle: small to easily turn on the limited space available

Chassis design

A number of chassis designs are possible, each with its advantages and disadvantages. A

couple of properties are given below:

• Three wheels

– two-wheeled robot with caster wheel

– each wheel driven by separate motor

– advantages: small turning-circle, turning easy to program, easy to build

– disadvantages: have to look at center of gravity with building, can be a bit unstable (especially at uneven surfaces)

• Four wheels

– four-wheeled robot, two wheels driven by separate motor, two wheels passive (can

also be four, with four motors)

– advantages: small turning-circle, turning easy to program, stable without problems

center of gravity

– disadvantages: skidding of passive wheels: lot of resistance. Possible solution: no tires on skidding wheels for less resistance

• Six wheels

– six-wheeled robot, two wheels driven by separate motor (center), two wheels steering,

two wheels passive (current design)

– advantages: stable, good for hill-climbing and rough surfaces

– disadvantages: large turning-circle, turning difficult to program, difficult to build, resistance with steering, straight line driving difficult

• Tracked robot

– rubber tracks, four (or six) wheels

– advantages: stable, low center of gravity, good on uneven surfaces and obstacles, can be capable of driving over a lake

– disadvantages: hard time traveling straight line, hop on smooth surface, rubber tension can be difficult (possible solution is to use a wheel in the middle for extra tension), a lot of friction

Steering

There are two possible ways of steering the MER

• Differential steering: compare with a wheelchair, so using two separate driven wheels

with different speeds/powers to steer; fast steering with small turning-circle

• Dual-wheel pivot steering drive system (current design): difficult steering mechanism,

need to calculate angles (with its deviations), furthermore you have to steer and steer

straight again; slow steering with large turning-circle.

Line tracking/Mars mission

For the line tracking we want a good straight-line tracking robot, with good steering properties

and reaction time. Furthermore we need to predict the length of the line as accurate as possible

(this means to zig-zag as less as possible). It is possible to use one light sensor (control on

the ’grey’ area between the line and environment colour), two light sensors (need to know the

width of the line) or three light sensors (same as two light sensors). I assume that the width

of the line is not known on beforehand, so we need to use one light sensor for the line tracking (and maybe one other to detect if the line starts/ends)

The best choice(s) for line-tracking in our opinion is (are):

• Best chassis: Robot with tree wheels (two wheels, separately driven, with a caster wheel).

A four wheel robot can also be a good choice but as the surface will be even a three-wheeled

robot will can be controlled most accurate

• Best steering: differential steering (wheelchair)

• Positioning light sensor: for following straight line, it is best to place it close to the pivot

point (less zig-zag behaviour). For following arcs it is best to place it far ahead of the

pivot point (current design).

• Check distance traveled: measure circumference of tire + rotation sensor(s)

For the mars mission the current design (six wheels) is not the optimal one. As the surface is

relatively even and pretty compact, a small turning-circle is wanted.

The three-wheeled robot can give stability issues, as the mars surface is not

entirely even. The best choice seems to be the four-wheeled robot. As the mars surface

is pretty rough, we need to check if the resistance with steering is not too big. Differential

steering seems to be the best choice in the small space that is available.

Design choice

As we don’t want to change the chassis for the line-tracking and mars mission, the best choice seems to be the four wheeled robot (two wheels driven by separate motor, two wheels passive). We are going to start building a light,stiff robot with a smart and low center of gravity, using smart and stiff suspensions with little friction and hysteresis to be able to drive straight lines when wanted. Design principle knowledge will be used here.

The light sensors can/will be positioned differently for the line-tracking competition and the mars mission, so they should be easy to (un)mount. We can change the transmission (gear ratio) when more speed (less accuracy!) is wanted.

If the four-wheeled design appears to have too many drawbacks/disadvantages in a later stadium, we can always change the design to a three-wheeled robot or even a tracked robot fairly easy.