This Small Business Innovation Research (SBIR) Phase I project will develop a linear, interleaved magnetic motor (LIMM). Magnetic motors are transducers, which convert electrical power into mechanical power according to the Lorentzian force function,F=B×li, where F is Force, B is magnetic flux, l is the length of wire in the gap, and i is the current. Magnetic motors are used in devices such as electric motors in hybrid vehicles, disk drives, linear actuators, and loudspeakers. The LIMM is a unique topological arrangement of the magnetic gap, which creates a long, narrow, serpentine gap. The LIMM's topology allows for the design of transducers, which increase the force, F, faster than the system adds mass, without affecting the width of the air gap. This allows for compact, high power transducers, with almost any quality factor, Q, needed for the system. In order to realize the potential of the LIMM, this Phase I project will focus on selecting an appropriate flex circuit drive coil for use in the serpentine gap. A cascaded design of experiments will be used to identify the correct flex circuit substrate and adhesive, conductor, and forming method, amongst other attributes, to create a repeatable, high-quality transducer.
The broader impact/commercial potential of this project is the ability to create compact, high power, low Q transducers using ferrite, rather than neodymium, magnets. China controls 97% of the world supply for neodymium and, recently, has artificially limited supplies. China's cutback in supplies during 2010 sent market prices soaring-neodymium jumped from $41 per kilogram in April to $92 per kilogram in October. The United States has stable access to ferrite magnets. By creating a transducer that exceeds current performance demands while using ferrite, instead of neodymium, magnets, the LIMM will reduce our dependence on China as a source of raw material. Additionally, the efficiency gains inherent in the LIMM will also lower the power consumption of the transducer in high current applications. For example, a loudspeaker built from a LIMM can easily attain a sensitivity of 96 db at 1 W and 1 m. This is a 6 db increase (or 4x the acoustic power) in output power from the current state of the art loudspeaker. Due to the ease of implementation, speed to market, and performance improvement over current technologies, the first implementation of the LIMM will be in a loudspeaker.
In order for electrical energy to be useful to the end-user, there must be a means for converting electrical energy into mechanical energy, and vice versa. Transducers are the class of devices which convert electrical energy to mechanical energy. At the core of a transducer is a magnetic motor, which is comprised of a permanent magnet, metal components to complete a magnetic circuit, and a coil of wire. The magnet and metal components are arranged in such a way that they create a narrow air-gap. In practice, the air gap, in current technology, is almost always a cylinder. Traditional magnetic motors are highly inefficient, because, inter alia, they operate at high temperatures, resulting in significantly increased impedance in their wire coil. A Linear Interleaved Magnetic Motor ("LIMM") is a new type of magnetic motor, which possesses a greatly elongated, serpentine, linear gap. The long gap is made possible by ganging two magnetic circuits together, so that they create a unitary, efficient magnetic circuit. Instead of using a traditional coil of wire as a drive coil, the LIMM uses a flexible printed circuit ("FPC"). A FPC is comprised of a substrate, such as polyimide, PET, or PEN, and a conductor. Typically, copper is used as the conductor. In order to be useful to a LIMM, the FPC must be precision formed to fit in the elongated air gap (See Figure 1 and Figure 2). The goal of this Phase I Project was to identify a FPC that would make a commercially viable loudspeaker voice coil for a LIMM. The ideal FPC would be easily formable, dimensionally stable, robust, repeatable, and inexpensive. The project substantially succeeded. The preferred prescription for a FPC for a LIMM was defined. The preferred substrate is PET, laminated without adhesive. The preferred conductor cross-section is ½ oz/ft2, although 1 oz/ft2 copper doesn’t adversely impact the performance. The preferred bending geometry is a 3 mm radius with a 2" leg. The preferred forming temperature is 85°C. The preferred dwell time during forming is 60 seconds, although longer dwell times do not negatively affect the performance. The FPC needs to be forced-air-cooled, while still in the fixture, in order to properly form. No grounding methodology is required to meet the design targets. The only Technical Objective which was not achieved was an operating rise at 20W of less than 25°C. The typical thermal rise at 20W is slightly over 30°C. This is still well below the thermal rise of competitive technology, which has a thermal rise of 100°C to 130°C at 20W. This Technical Objective might be achievable with small adjustments in trace thickness.
