Design Challenge - June 2013

Brooks Stevens - Electro-rheological fluid

Brooks StevensThe BSI Magneto Forest Harvester is a unique and versatile machine that leverages new technologies to ensure smooth operation and a comfortable ride, allowing the operator to focus on the safe and efficient operation of the vehicle.  The suspension utilizes articulating arms and an electro-rheological fluid system to adapt to terrain and provide vibration damping.  The cab itself is magnetically levitated (on permanent magnets) and uses modulating electromagnets, driven by sensors in the cab, to isolate the operator from ancillary vibrations.  A high efficiency gas turbine generates power to drive the track motors, as well as the electromagnets, suspension arm servos, and other vehicle subsystems.

The main suspension utilizes a newly-developed substance known as electro-rheological (ER) fluid, whose viscosity can be modified by application of electric current.  The arms have two translucent cylindrical-section aluminum-oxynitride reservoirs filled with ER fluid at each joint.  The reservoirs utilize a fixed wall on one end and a sliding piston on the other, with a valve and flexible membrane in between.  When servo-motors on a pivot axis start moving, the valve opens and the pistons force fluid from one side to the other.  When the desired position is achieved the valve closes and the motors power down.  The flexible membrane allows limited articulation of the joints in the absence of applied power. The damping and spring effect of each joint can then be dynamically altered by modifying the viscosity of the ER fluid via the application of electrical current through the reservoirs.  As the vehicle moves over terrain, sensors on the tracks and throughout the arms feed data to a control system that varies the applied current to soften or harden the fluid as needed.  The mounting points on the chassis are able to rotate and pivot on two axes by hydraulic actuation, while the articulation of the arm itself provides rotation in the third axis, as well as a linear displacement.

The vehicle tracks can morph into multiple configurations, including circular wheels and elongated tracks.  The internal structure of the tracks is similar to that of the main suspension arms, with electric motors at each pivot joint that drive rollers to move the treads.  The tracks are connected to the arms by ball-joints that are locked by spring force.  They may be freed by a solenoid during main arm articulation, allowing re-orientation and then re-clamping prior to vehicle motion.  There are also pivoting blades, or paddles, embedded in each tread that can be rotated out, enabling the treads to dig into softer terrain or to assist whenever traction is otherwise insufficient.  The internal surfaces of the tracks are made of an abrasion-resistant ballistic-weave aramid fabric held taught by spring steel bands which maintain the desired shape across track configurations.

The cab is levitated and constrained by four sets of rare-earth permanent magnets located near the bottom corners of the cab, with four more sets on the top.  The sets of magnets are configured with their poles in an axially offset pattern to maintain levitation. The magnet located in each cab corner is repelled from the chassis by two magnets that are offset towards the outside edges, and spaced such that their fields create a net force towards the opposite corner (which is cancelled by the opposite corner’s mirrored set).  So, even without power the cab is levitated and in static equilibrium. The air gap is approximately 300 mm, requiring very large, powerful magnets that must be shielded with a thick layer of “mu-metal” nickel-iron alloy.  When the vehicle is running, power is wirelessly transferred to the cab via a pair of magnetically-resonant matched coils – one in the chassis and one in the base of the cab.  While in operation, the control computer reads from accelerometers in the cab and ultrasonic distance sensors.  Using this information, the computer then controls cab displacement and oscillations by modulating power to the auxiliary electromagnets.  In the event of a rollover or other situation under which the cab may overcome the force of the permanent magnets and move beyond allowable displacement limits, spring-loaded retention arms are released to contain the cab and prevent it from being ejected from the vehicle.

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Tou Yia Thao has been an industrial designer with Brooks Stevens in St. Paul, Minneapolis, for two years; mechanical engineer Joe Knutson has also worked there for two years





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