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Put the brakes on pneumatics
Magneto-rheological, fluid-filled brakes controlsair-cylinder position, velocity, and acceleration
By John Lewis, Northeast Technical Editor -- Design News, July 5, 1999
Cary, NC--Pneumatics are a relatively simple and economical way to transfer something from one point to another. But when you need to stop the load somewhere between the end stops, step motors or servo systems are often the way to go.
Such electromechanical systems provide control of position and speed, something two-position pneumatic cylinders just can't do. Lord Corp. changed all that when it unveiled two working pneumatic motion-control devices at the National Design Engineering Show in Chicago in March.
By combining patented RheoneticTM technology in parallel with pneumatic actuators, Lord's new designs offer multiple-position control, velocity, and acceleration control that far surpass that of conventional pneumatic devices.
Magneto-rheological (MR) fluids, developed half a century ago, are essentially suspensions of micron-sized, magnetizable particles in oil. Under normal conditions, MR fluid is a free-flowing liquid with a consistency similar to that of motor oil. Exposure to a magnetic field, however, transforms the fluid into a near-solid in milliseconds.
Just as quickly, the fluid can be returned to its liquid state with the removal of the field. The degree of change in an MR fluid is proportional to the magnitude of the applied magnetic field. When subjected to the field, Rheonetic fluids actually develop a yield strength and behave as a Bingham solid or ideal plastic. The change appears as a very large change in effective viscosity.
Lord overcame the fluid's abrasive quality, and the tendency for magnetizable particles to settle out of solution, and introduced the first commercially viable MR fluid in 1994. Recent applications include Motion MasterTM, a new type of seat suspension system that combines MR fluid with electronic controls for improved ride comfort, and in Carrera Shocks' (Atlanta, GA) MagnashockTM. By turning a dial inside the cockpit during a race, the driver changes the magnetic field within the Magneshock to adapt the suspension to changing track conditions.
Lord's new products for pneumatic motion-control applications, provide more flexible yet simple design solutions, says Product Manager Eddy Houchin. A linear/rotary actuator, used for higher force applications, achieves a maximum thrust of 85 lb, while the light-duty linear actuator puts out 16 to 32 lb of thrust.
Lord's linear/rotary actuator uses a MR-fluid filled, rotary-brake device to apply drag force to a pneumatic cylinder. Between the cylinder's two piston heads, a rack-and-pinion mechanism translates the linear motion to the brakes' rotor. The brake housing acts as a stator, and MR fluid in the gap between shears to create drag force.
When the brakes' coil is unenergized, drag force is near zero. As current increases to the coil, the magnetic field across the fluid-filled gap increases, as does fluid's viscosity. This effectively increases shear force and the brakes' applied torque. Both the high-force and low-force devices operate with industrial-type controllers that modulate current to the MR-fluid brake's coil, and controls the pneumatic solenoid valves.
Light duty actuator. The light-duty actuator consists of a standard bearing block with attached tool plate, an air cylinder with integral sensor, and two low-force MR dampers mounted on either side of the air cylinder. In the annular space between the MR damper's piston head and cylinder, an MR-fluid soaked foam material shears as the piston moves. An electromagnet inside the piston head applies a magnetic field across the MR fluid/foam combination, creating variable drag force.
"The 'sponge-like' foam material keeps the MR fluid where it's needed," says Lead Engineer Mark Jolly, "but most of the stress is carried in the fluid. Although the foam wears with use, it expands as it wears to fill the gap." The rugged, modular, and low-cost design operates at pressures between 20 to 40 psi. Operating temperature is from 0 to 160F, and electrical power is 2A (72W) maximum.
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| When unenergized, the brakes’ drag force is near zero. As current increases to the coil, the magnetic field across the fluid-filled gap increases, as does fluid’s viscosity and drag force. |
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Additional details…Contact Eddy Houchin, Lord Corp., Box 8012, Cary, NC 27512; Tel: (919) 469-2500 ext. 2118; FAX: (919) 469-5991.
Other Applications
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Tension control
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Vibration control
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Coil winding
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