Structure and Mobility--Michael Poteet

As of October 17th:           

           

The suspension and drive-train are crucial to the design of the rover.  Its main function will be to house all of the power and communication components of the vehicle, along with maintaining stability while the rover is in motion.  The primary goal for the chassis will be its ability to handle the rocky terrain of the Mars surface.  While this is a rigorous task in itself, the weight of the frame will need to be kept at a minimum for two reasons:  power and launch cost.  More power, and in turn even more weight in the form of larger power generation equipment, will be required to propel a heavier vehicle.  Also, the cost associated with putting the rover into space is tens of thousands of dollars per pound.  A heavier frame needing more power would cause the launch cost to skyrocket. 

Another design consideration involves the shock absorption of the suspension.  Due to the sensitive equipment aboard the rover, the forces due to movement will need to be kept to a minimum.  Next, the large amount of fine dust particles located in the Mars atmosphere will need to be kept out of the joints of the suspension.  Also, the lower temperature on Mars is much less than on Earth, so the materials used will need to be able to handle this harsh environment.  Finally, the rover will need to be relatively simple and easy to repair or replace any parts that might become damaged or fail during the mission.

The main focus of frame design will be on its weight and strength.  The materials considered for this so far are aluminum and steel.  Until recently, steel has been the most commonly used material in automotive design because of its extremely strong and durable characteristics.  However, aluminum has begun to make its way into the frames and bodies of cars, due to its light weight and strength.

            The suspension needs to incorporate some sort of spring-mass-damper configuration in order to absorb the impact of the rocky Mars surface.  A traditional shock and spring set-up would be light weight, durable, and easy to replace if it ever became damaged.  Multiple shock-spring modules may be needed to properly account for the vibrations made by the rover’s movement.  Another component in traditional vehicles used for absorbing road impacts is in the tires.  Considering the cold conditions on Mars, regular tires may become brittle and crack, which would be disastrous for the mission.  A way to solve this is to have the wheels be bigger in diameter and have a strip of tread attached to the rim.  This would minimize the use of rubber, while still getting the rover down the Martian road. 

Moving the rover on Mars can be dealt with in a couple of ways.  First, it could be driven by one larger electric motor which transfers power to a gear box, and then the power is delivered to either two or four wheels, depending on the needed power.  This configuration would allow for a simple steering configuration where the front or rear wheels would rotate similar to an automobile and allow the rover to turn.  Another option involves having two or four independent motors placed at the wheels of the vehicle and used to propel the rover.  This option would utilize differential steering, or skid steering, which turns the rover by giving more power to certain motors on the vehicle and having the other side “skid” to follow it. 

 

The chassis of the rover will be made of an Aluminum alloy.  With the low top speed of the rover (minimum of 10 km/h), the frame will be under much less stress due to lower impact forces acting upon it.  This reduction in materials will be crucial in minimizing the overall mass of the rover.

The frame will be modeled after that of a truck or sport utility vehicle, having two main members parallel to each other with at least three cross members.  More cross members may be added as needed for extra stability and weight distribution.  The main and cross members will be tubular in design, with some form of elliptical and/or rectangular cross section.

            The suspension will be a major focal point of the design, mainly due to the importance of the contents carried onboard the rover.  Many of the electrical components are extremely sensitive to vibration, so keeping them at a steady state will be of key importance in the design.  The suspension will be modeled after that of a traditional automobile as well.  It will consist of upper and lower control arms with appropriate bearings and bushings.  The shape of the arms will resemble that of an “A”, which is why they are also known as “A-arms.”  Only the lower control arm will be allowed to move, and its axis of rotation will be in line with the main frame members.  Each of the four wheels will have this set up, along with a spring/damper configuration on both sides of the arm.  The springs will be of a metallic material, while the damper will consist of a shock similar to ones used on cars and trucks.  Heaters may need to be placed around the shocks due to the colder conditions on Mars.  The wheels will also be similar to automobiles.  They will be approximately 24-30 inches in diameter, and will have a rubber strip around them, probably with an all-terrain type tread.  All of these components should help to minimize the vibration of the electrical instruments on the rover.

            The propulsion of the rover will be accomplished by electrically driven motors located at the individual wheels.  These will be mounted to the lower control arms, between the two shock/dampening apparatuses.  There will either be two motors, located at the rear, or four motors, one located at each wheel.  This decision will be made when a better estimate of required drive power has been determined.

            Due to the fine dust particles located on the Mars surface, extra measures will need to be taken to maintain a working vehicle.  The electrical motors will need to have a covering over them to prevent the dust from entering their cases.  Another concern involves the lower control arms.  The dust will hinder this motion if it gets into the bearings and bushings.  Rubber or an equivalent material will be used to create a “boot” over the joints to keep dust out.

As of November 14th:

Structure & Mobility

I. Design Requirements

The suspension and drive-train are crucial elements in the design of the rover.  This area’s main function will be to house all of the power and communication components of the vehicle, along with maintaining stability while the rover is in motion.  The chassis will need to be able to handle the rocky terrain of the Mars surface while minimizing its overall weight.   This strength to weight ratio is important not only to cut down on needed power for propulsion, but for keeping the launch cost to a minimum as well.  The expense associated with putting the rover into space is well over ten thousand dollars per pound.  Another chassis consideration with respect to the launcher is in the rover’s compactibility.  If the rover had the capability to “fold up” and have a lower volume when being transported to Mars, a smaller and cheaper launch vehicle could be utilized, effectively reducing the overall mission cost.

Another design consideration involves the shock absorption capabilities of the suspension.  Due to the sensitive equipment on board the rover, the forces due to impact will need to be kept to a minimum.  The suspension needs to incorporate some sort of spring-mass-damper configuration in order to tame the Martian terrain.  Another hazard of Mars is the large amount of fine dust particles located in the atmosphere.  Careful consideration will need to be taken when designing the joints of the suspension, as well as any other moving parts where the presence of dust could be a problem.  Also, the temperature on Mars is considerably less than it is on Earth, so the materials chosen will need to be able to handle the colder environment.  Finally, in addition to satisfying the design criteria set forth by TSGC, the rover will need to be relatively simple and easy to repair or replace any parts that may become damaged during the mission.

II. Final Design

The chassis of the rover will be made of an Aluminum alloy.  With the relatively low top speed of the rover (minimum of 10 km/h), the frame will be under much less stress due to lower impact forces acting upon it.  This reduction in materials will be essential in minimizing the overall mass of the rover.

The basic frame of the rover (shown below in Figure X) will be rectangular in shape and made up of three inch diameter hollow cylinders.  It will consist of two parallel ten foot pipes six feet apart from each other with three cross members, one placed in the center and the others located eighteen inches from

Figure X:  The Structural Frame of the Red Rover

the front and rear of the rover.  Four more members will be placed in the front half of the frame so that the seats have a stable base, as well as stabilizing beams to strengthen the rear suspension.  There will also be a roll cage made of the same cylindrical material in order to protect the power source, communication equipment, and passengers aboard the rover.  A half inch thick composite material will be placed directly on top of the frame and serve as a floor board for the vehicle. 

            As stated earlier, the rover will need to be as compact as possible so that it will not take up a lot of room on the launch vehicle.  Throughout the frame are hinges which allow the rover to fold up while being transported to Mars.  These pivot points are located in the middle of the ten foot main members and in various parts of the roll cage.  A picture of the frame in its “compact mode” is included below in Figure Y

Figure Y:  The Control or “A-Arm”

Sleeves will be placed at some of the junctures for added stability while the rover is in use. 

            The suspension will be a major focal point of the design, mainly due to the importance of the contents carried onboard the rover.  Many of the electrical components are extremely sensitive to vibration, so keeping them stable will be of key importance in the design.  The suspension will consist of four control arms similar to those used in automobiles.  The shape of the arms (shown in Figure Y) will resemble that of an “A”, which is why they are also known as “A-arms.”  These control arms will pivot about the middle of the frame.  Each of the four wheels will have this set up, along with a spring/damper configuration on both sides of the arm.  The springs will be made of a titanium alloy called Beta-C[1], which is ideal for these types of applications.  The dampening system to be used consists of a Magneto-Rheological[2] silicon fluid shock at each of the wheels.  This technology utilizes magnetizable particles in a fluid similar to that used in conventional shocks.  When a magnetic field is applied to the composition, the viscosity drastically changes depending on the magnitude of the field.  This would allow the ride of the rover to be as stiff or as gentle as is needed for an optimal ride. 

            One of the design complications due to the Martian environment is from the combination of cold temperatures and a dusty atmosphere on the pivoting surfaces.  The fine particle dust would get in between normal bearings or bushings and the cold weather would weaken any rubber used for general shock absorption or cushioning.  Therefore, Teflon bearings will be used at interfaces such as where the A-arms meet the chassis.  This material utilizes its naturally lubricated surface in combination with its durability[3] to produce favorable results, even in conditions as harsh as Mars.

            The rover will also make use of rack and pinion steering in the front end of the vehicle, which is illustrated below in Figure Z.  When the pinion gear is turned, it forces the rack to move either to the left

Figure Z:  Illustration of Rack and Pinion Steering[4]

or to the right.  This action rotates the rover’s wheel via the steering arm and tie rod configuration.  The steering assembly will be covered by a composite material and Teflon bearings/seals will be placed on the outputs of the rack.

While the front wheels will be used to direct the rover, the rear wheels will be doing the moving.  Both the right rear and left rear A-arm will have a ¾ hp DC motor bolted onto it that will effectively propel the vehicle.  Assuming a motor efficiency of 80%, the two motors will consume a maximum of about 1400 W.  The choice to have the rover be rear wheel drive was made due to the towing operations that were mentioned in the design specifications.  Regenerative breaking, which is the act of placing a reverse load on the motors to slow them down, will be used to stop the rover.  In this process the motor actually becomes a generator and recharges the power source.  The output shaft of the motor will have a gear on it, which will drive a planetary gear box with a ratio of 36:1.  An illustration on what a planetary gear box looks like is included below as Figure M.  This ratio will give the wheels enough torque to climb a 35º

Figure M:  Example of Planetary Gear System[5]

incline while fully loaded (estimated at 1000 kg) while still allowing for a top speed of over the aforementioned requirement of 10 km\h.  The motor itself will have a thin covering to keep the dust out but still allow heat to escape.  The gear box will be sealed with a composite casing accompanied by Teflon seals for the motor shaft.  Cryogenic temperature rated grease will be used to lubricate the moving parts.  The front wheels will also run on Teflon bearings.

The wheels themselves will be 28” in diameter, 9” wide and made of the same aluminum alloy as the chassis.  The wheel has a five spoke configuration with a light weight composite shield on the outside to protect the moving parts from any kind of debris.  Around the rim there will be a two inch thick solid rubber compound with titanium chevrons to aid in traction[6].  Also, to keep any projectiles off of the astronauts, fenders will be covering each of the four wheels.  To protect the power source located in the front compartment of the rover, a hood with left and right front fender made of a composite material will be added.  The hood and both fenders will pivot about the front of the vehicle, allowing access to the power source. 

The “cab” of the rover will have two seats and a control panel that displays the speed of the vehicle and in which direction it is traveling in relation to Martian north.  The seats will have an aluminum frame with nylon coverings[7] and will accommodate an astronaut’s suit and the life support system carried on their back.  The seats will also be able to recline and will have the ability to fold flat for when the rover is being transported to Mars.  A simple Velcro lap belt will be used in keeping the astronauts from floating away.

The movement of the rover will be controlled by a motion controller similar in aesthetics to a manual transmission shifter.  This will be located between the two seats.  Pushing the lever forward will direct more power to the motors and cause the rover to accelerate, while pulling back will reverse the polarity of the motors, causing the rover to go into reverse.  When the joystick is moved to the side, it actuates a servo motor that acts as the pinion gear on the steering system illustrated in Figure Z.  Another lever located near the motion controller will, when pulled back, actuate a pin in the planetary gear to hold the vehicle in park. 

The electrical components will be located underneath and behind the seats.  The entire rear half of the rover will be used for any samples that may be collected or for any tools that will be needed on Mars.  Also, a trailer hitch is located at the rear of the frame and different types of attachment media can be used.


 

[1] Broge, Jean L.  TIMET’s Titanium Alloy for Springs..  Society of Automotive Engineers.  March 2001.              <http://www.sae.org/automag/material/03-2001/>

[2] Magneto-Rheological (MR) Fluids.  Lord Corporation.  ©2003.      <http://www.mrfluid.com/fluid_begin.htm>

[3] BOStonE F-1 Glass Filled Teflon Bearings.  Boston Gear.  ©2003             <http://bostongear.com/pdf/bearing_catalog_sections/bearings_28-46.pdf>

[4] Nice, Karim.  How Car Steering Works.  How Stuff Works. ©2001            <http://auto.howstuffworks.com/steering2.htm>

[5] Nice, Karim.  How Hybrid Cars Work.  How Stuff Works.  ©2000.            <http://auto.howstuffworks.com/hybrid-car9.htm>

[6] Williams, Dr. David R.  The Apollo Lunar Rover Vehicle.  6 Aug. 2001.              <http://nssdc.gsfc.nasa.gov/planetary/lunar/apollo_lrv.html>

[7] Williams, Dr. David R.  The Apollo Lunar Rover Vehicle.  6 Aug. 2001.              <http://nssdc.gsfc.nasa.gov/planetary/lunar/apollo_lrv.html>