LaunchPoint TechBlog

Magnetic Spring Technology Enables High-Speed Rotary and Linear Actuators

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LaunchPoint Technologies has invented a magnetic spring technology (U.S. Patent 7,265,470) that enables high-speed motion of mechanical devices with minimum energy consumption. LaunchPoint has been funded to apply this technology to a filter switching mechanism, to electronic engine valves, and foresees applications in a wide range of industrial valves and other mechanisms.

Prior to this invention, high speed point-to-point motion was only accomplished with relatively high power motors. The motor converts electrical energy to kinetic energy in a moving mass and then extracts that kinetic energy to bring the mass to a stop. Such an approach requires a large motor that adds inertia to the system and consumes ~25% of the total energy in motor heating during the acceleration and deceleration. Thus, the large motors increase both cost and energy consumption.

The idea behind the magnetic spring technology is to use the energy stored in a magnetic field to move an object back and forth rather than using a motor. While conventional mechanical springs can accomplish this same task, the maximum force occurs at the end-points, introducing jerky motions that cause noise, wear, and unwanted vibrations.  LaunchPoint’s magnetic spring technology, however, allows for “soft landings” at each end-point because the restoring force drops precisely at the end-points.

The spring can be fabricated in either a rotary or linear form. Figure 1 depicts a rotary version of the magnetic spring (left panel) together with a finite-element model of the magnetic fields (right panel). The outer pink magnet array is fixed and the inner array rotates. Looking closely at the magnetization of the magnet segments, one sees that there is an alternating pole pattern on both rings with 3 pole pairs per revolution. In addition to inward and outward oriented magnets, there are additional magnet orientations chosen to maximize the stiffness of the spring.

Figure 1. (left panel) Solid model of magnetic spring having 3 pole pairs on the inner and outer magnet arrays. (right panel) Finite-element model of the associated magnetic fields.

Unlike a conventional torsional spring having a linear torque versus angle characteristic, the magnetic spring has a periodic torque versus angle characteristic as show in Figure 2. When the north poles on the inner array align with the south poles of the outer array and visa versa, the magnetic spring applies zero torque and lies in a stable equilibrium. When the north poles align (and the south poles align too), the torque is zero and spring is an unstable equilibrium. This torque profile is very similar to a pendulum which has one stable and one unstable equilibrium point. Such a characteristic can be used to affect rapid motion with very little external force. Think of the pendulum in the inverted, unstable, configuration. With a very small forces provided by, say, a motor, the pendulum can be maintained in the inverted position. Likewise, with a small “nudge” the pendulum will swing to the right and then back up on the left, returning to the vertical position. In this process, potential energy is converted to kinetic energy and back again. In the magnetic spring the potential energy is created with the magnetic fields rather than the gravitational field. Further, in the magnetic spring we can effectively change the length of the pendulum and the force of gravity to achieve very high-speed motions.

Figure 2. Torque versus angle for the rotary magnetic spring of Figure 1 and a corresponding pendulum analogy for switching between unstable equilibria.

LaunchPoint received funding from Raytheon Company for a rotary version of the device to use in a fast filter-switch mechanism.  Figure 3 depicts two rotating filter wheels developed in collaboration with Raytheon Company that rapidly change orientation using LaunchPoint’s magnetic spring technology. The wheels counter-rotate to cancel torque disturbances to the system supports.

Figure 3. Implementation of two magnetic springs in a filter switch mechanism designed in collaboration with Raytheon Company.

Additional funding was awarded by the National Science Foundation (Award No. IIP-0945595) to evaluate a linear version of the magnetic spring as an electronic engine valve actuator. The electronic engine valve is the ‘Holy Grail’ of engine technology.  At each engine speed and operating condition, the valve timing can be optimized, enabling a 20% improvement in fuel mileage.

The first bench-top prototype of the valve mechanism is shown in Figure 4.  This prototype has a stroke of 8mm and switches position in ~3 milliseconds.  A follow-up SBIR Phase II grant (Award No. IIP-1058556) from the National Science Foundation will fund a speed-up to 2 milliseconds and a demonstration in a single-cylinder test engine.

Figure 4. Prototype electronic valve actuator developed with National Science Foundation funding. This linear magnetic spring has unstable equilibria spaced by 8mm and switches in ~3 milliseconds.

The implementations we have introduced above are just two of the many potential applications requiring rapid point-to-point motion. The reader is encouraged to contact us about integrating the magnetic spring technology into their own application.