Apparatus for centering an actuator coil

Abstract

An automatic method for optimizing coil/gap centering in the manufacture of an electro-magnetic actuator or transducer such as a loudspeaker or microphone eliminates the use of shims. Capacitance measurements between the coil and adjacent magnet poles are utilized to evaluate and optimize concentricity of the coil and its bobbin in the magnetic gap prior to finally affixing the flexible suspension member(s) such as the spider and/or surround to the actuator mainframe, typically by adhesive bonding around the outer edge of the suspension member(s). Optimal concentricity corresponds with the minimum capacitance reading found from a series of capacitance measurements at different locations of the coil assembly within the constraints of interference between the coil with its bobbin and the pole pieces. This principle may be utilized in a work station that constrains the mainframe portion while automatically moving the coil assembly laterally in a search pattern to acquire the concentric location corresponding to minimum capacitance so that the suspension can be affixed to a portion of the mainframe, typically by adhesive bonding, with the coil concentricity thus optimized. Capacitance measurement can further serve as a valuable quality control test: requiring the minimized capacitance to fall below a statistically based limit provides assurance that the coil is correctly shaped and oriented and is optimally centered.