Proposal and analysis of three closed double magnetic circuits to obtain a very long stroke for electrodynamic force generators

Electrodynamic force generators with a stroke of up to several hundred millimeters are in high demand in fields such as voice coil actuators and low frequency vibration metrology. The key is to extend the effective length of the long air gap (LAG) with required uniform magnetic flux density (MFD) and low magnetic flux leakage. This paper focuses on a proposal and comparative analysis of three closed double magnetic circuits (CDMCs) to obtain long/ultra-long strokes. Analyses are performed from the viewpoint of the magnetic line distribution, fringing effect, distribution of the MFD, and magnetic flux leakage in order to evaluate the stroke potential of three proposed CDMCs, while taking into account the vacuum annealing, material differences in permanent magnet pairs (PMs), and assembling errors in PM arrays. Analyses show that the CDMC with PMs assembled at the ends of the center yoke exhibits advantages over the other two schemes. An electrodynamic force generator with an air gap length of 200 mm is constructed with this structure. Experimental results reveal an MFD of up to 233.5 mT, MFD non-uniformity of 0.2% in the LAG, and magnetic flux leakage of 3 mT, which meet the strict requirements in low frequency vibration metrology.