Patent GB-1504105-A (GB)
Title: Solenoid assembly incorporating non-linear spring for use in matrix printing
IPC: H01F7/13
Filed: 1976-06-07
Abstract: 1504105 Electromagnets CENTRONICS DATA COMPUTER CORP 7 June 1976 [18 June 1975] 23426/76 Heading H1P [Also in Division F2] In an electromagnetic actuator for a dot matrix printer (Fig. 1) a solenoid assembly is mounted within a hollow cylindrical ferromagnetic case 11 having a recessed inwardly extending shoulder 11a spaced from the lower and, while the upper end is internally tapped at 11b to engage a threaded endcap 12; the connecting leads of solenoid 14 extending through aperture 11c of the case which is filled with epoxy resin. Internal shoulder 11d supports flux ring 16, and core stem 17 has an elongated threaded portion 17a engaged by 18 and also threaded into the rear wall of a print head housing (not shown). Annular flange 17b of the stem 17 abuts flange 19a of solenoid bobbin 19 and is spot welded or adhesively secured by epoxy resin to case 11. Nose portion 17c of stem 17 extends into the hollow core of bobbin 19 and has therein an elongate bore 17d with a portion of reduced diameter 17e. Within the bore is fitted a tubular elongate non-magnetic wire guide resting on tapered shoulder 17g and secured in position by, e.g., epoxy resin or welding 20a. A print wire 21 is slidable in the guide and extends through portion 17e to be fixed, e.g. by solder, to opening 22a of armature 22, whose end is formed with a radial head 22b having a flat underside 22c (Fig. 1a) abutting the portion of spring member 26 surrounding opening 26a accommodating the armature. The flat portion is surrounded by a curved portion extending outwardly therefrom with progressive separation from the spring face. The spring (Fig. 2a) has radial spokes 26c tapered towards the free ends which carry spaced sectors 26d resting on the flux ring 16 carried by shoulder 11d. The ring, the spring and the armature are ferromagnetic. End cap 12 has an adjusting screw-driver slot for spring preloading by axial movement of armature 22, and after adjustment is secured by epoxy resin or a silicone or rubber adhesive. On de-energization the armature is held at rest abutting the end cap and the spring 26 is slightly flexed. On energization the armature is moved axially against spring 26, the operative lengths of whose spokes initially remains constant and supported by flat portion 22c of head 22b, until with movement the points of support move outwardly over the curved portion as the spring flexes so as to confer greater stiffness and bias against movement of the armature. The combined effect is to allow the armature first to acquire a substantial velocity against a linear spring force so as to impact the print wire on the recording platen and then to move against a non-linear increasing spring force storing energy for its rapid return on do-energization. The pull-in force curve of the solenoid combined with the nonlinear spring force curve is substantially linear between force and displacement (Figs. 3a, 3b, not shown). The solenoid is energized by square wave pulses, and opening 22f of the face of the armature head provides with end cap 12c a dashpot effect limiting armature bounce. In a modification (Figs. 4a 4b not shown) the armature has a cylindrical projection extending through the spring and clamped by a surrounding ring shaped retainer which is swaged over against a bevelled surface. The surface of the flux ring is formed as a flat portion within its circumference, and a curved portion extending therefrom to the central aperture against which the spring can bear on progressive deflection. The operation is similar.