AERODYNAMIC DAMPER FOR IMPACT PRINTING ACTUATOR

Abstract: Aerodynamic damper for impact printing actuator in which a movable armature (7) of a magnetic circuit has to resume, after actuation, with minimum rebounding and oscillations, a stable rest position defined by the contact of a flat surface (9) of the armature (7) with a flat surface (10) of a stop element (2), consisting in a recess (13) formed in at least one of the two flat surfaces and having peripheral side walls (14, 15) slanted as to the flat surface.

Description: The present invention relates to an aerodynamic damper for an impact printing actuator. The printing actuators used in high speed printers for data processing systems and particularly in dot matrix printers, consist of electromagnets where the actuation of a movable armature causes the printing operation. Two kinds of actuators and two operative modes can be distinguished, that is actuators provided with a simple electromagnet and actuators provided with a permanent magnet, as well as actuators operating by armature attraction and actuators operating by armature release. In actuators of the simple electromagnet type, operating in attraction mode, the movable armature, in rest position, is spaced apart from a magnetic pole and rests on a stop element. An air gap is present between the armature and the megnetic pole. The electromagnet energization causes the armature to be attracted against the magnetic pole and the deenergization causes the release and the return of the armature, imposed by resilient means, to the rest position. In actuators of the simple electromagnet type, operating in release mode, the movable armature is, in rest condition, attracted against a magnetic pole, owing to the electromagnet energization. The denergization of the electromagnet causes the release of the armature, owing to resilient means and the consequent print operation, whilst the energization causes the armature attraction in the rest position. In the permanent magnet type actuators, which generally operate in release mode only, the movable armature, when at rest, is attracted against a magnetic pole. The electromagnet energization causes the neutralization of the magnetic field produced by the permanent magnet and the consequent release of the armature. The deenergization of the electromagnet causes the armature to be pulled against the magnetic pole. In all mentioned cases it is required, to obtain high speed performances, that the armature returns to the rest position as fast as possible and without rebound. To this purpose, dampers of various type have been used, such as resilient, ballistic and pneumatic dampers. Among dampers which more properly relate to the present invention, the device disclosed in U.S. Pat. 4,202,638 is to be mentioned. In such patent, a stop element is provided which defines a flat surface against which a corresponding flat surface of the armature lays down. During the armature return phase at rest position, a compressed air cushion is formed between the two surfaces. This cushion performs a braking action on the moving armature, and damps its impact against the rest surface, reducing the rebound. For the efficacy of the device a relatively broad flat contacting surface is required, which is difficult to reconcile with the miniaturized structures of printing actuators currently used. This limitations is overcome by the aerodynamic damper of the present invention which achieves a much greater efficacy by exploiting the aerodynamical pressure developed by the air flow between surfaces slighty converging and consists in a recess provided in the armature or in the rest element (or both) and having slanted walls as to the contact plane. The features and the advantages of the invention will appear more clearly from the following description and the related drawings where: Figure 1 shows in section a preferred form of embodiment of printing actuator embedding the aerodynamic damper of the invention Figure 2 shows in section the shape of the aerodynamic damper of the invention Figure 3 shows in section an alternative shape of the aerodynamic damper of the invention Figure 4 shows in qualitative form the local pressure distribution caused by the aerodynamic damper of the invention compared with the one caused by a conventional damping device. Figure 1 shows in section a preferred form of embodiment of printing actuator embedding the aerodynamic damper of the invention. The actuator comprises a first magnetic joke 1, a cylindrical magnetic column 2, onto which a winding 3 is inserted, a magnetic column 4 consisting in a permanent magnet, a second magnetic joke 5, directing the magnetic flux to column 2, a magnetic spacer 6, and a resilient magnetic armature 7 cantilever mounted on spacer 6 and extending towards magnetic column 2. Armature 7 is provided with a cylindrical post 8 for reclosure of the magnetic circuit, the post facing the pole end of magnetic column 2 and being coaxial thereto. Post 8 inserts in an opening of joke 5 and faces with a flat surfaces 9 the upper face 10 of pole 2. When the magnetic field of permanent magnet 4 is neutralized by a current flowing in winding 3, the resilient armature is unbent and in a released position and flat surface 9 of post 8 is spaced apart from upper face 10 of column 2 by an air gap having a whidth in the order of 0,5 mm. Missing the demagnetizing current, the armature 7 is elastically bent and attracted towards column 2 and surface 9 is in contact with the upper face 10 of column 2. Column 2 and post 8 have a diameter in the order of 4:5 mm. Armature 7 supports, at its free end, an actuation arm 11 to which a printing needle 12 is fixed. The above structure is entirely similar to the one shown and described by way of example in european patent application published with N 228589 but differs from it for the reason that the top of column 2 has a recess 13 as better shown in Figure 2. Such recess has for instance the shape of a spherical segment having a depth (height) in the order of 30-60 micron and diameter in the order of 3-4 mm. In other words the flat surface stopping the armature in rest position, and formed by the upper surface 10 of pole 2 has a recess 13, drawn in the pole and having peripheral walls 14,15 slanted relative to the flat surface 10 Alternatively the recess 13, as shown in Figure 3 may have frusto conical shape having a depth (height) in the order of 30-60 microns and maximum diameter in the order of 3-4 mm. This very simple expedient produces a damping effect, somehow unexpected, much greater than the one provided by the compressed air cushion which is formed between two flat surfaces of equal size when they approach each other. Without entering into a complex analytical description of the phoenomena, it may be observed, with reference to Fig. 4, that in the case of two circular flat surfaces approaching each other a pressure distribution results which is shown in qualitative form by diagram A. The pressure at the border is obviously equal to the environment pressure and has a peak at the surface center. In case one of the two surfaces or both have a slight recess as the described one, the pressure distribution is subjected to a radical change and takes the form qualitatively shown in diagram B. In other words, the compressed air flow which from the center of the recess tends to flow to the outside, exerts a dynamical push on the side walls, which is converted in a pressure increase. Not only the pressure peak at the center is increased, but also and more important, it is broadened in width, giving rise to a much greater damping action. It is clear that the preceding description is related to a preferred form of embodiment of the invention, but the invention may be used in printing actuator of different kind, for instance actuator of the simple electromagnet type operating both in release as well as in attraction mode. Even the positioning of the recess on the upper surface of a magnetic column is preferred, for manufacturing reasons, but from a functional stand point the recess may be located indifferently in the movable armature, as well as in the stop element or both. In addition the suggested sizing is largely susceptible of change depending on the size of the armature and stop surface, being clear that in case the recess affects a section of the magnetic circuit it is advisable that the depth of the recess be limited in the order of few hundredth of millimeter so as not to cause substantive non uniformity in the distribution of the magnetic flux.

Claims: Aerodynamic damper for impact printing actuator in which a movable armature (7,8) of a magnetic circuit (1,2,4,5,6) has to resume, after actuation, a stable rest position defined by the contact of a flat surface (9) of said armature (8) with a flat surface (10) of a pole element (2) of said magnetic circuit (1,2,4,5,6) characterized in that at least one between said armature (8) and said pole element (2) is provided, on said flat surface (9,10), with a recess (13) formed in said flat surface (9,10) and having surrounding side walls (14,15) slanted as to said flat surface (9,10). Aerodynamic damper, as in claim 1, wherein said recess (13) has the form of a spherical segment with a ratio between diameter and height comprised within 20 and 100. Aerodynamic damper, as in claim 1, wherein said recess (13) has the form of a frustum of cone with a ratio between maximum diameter and height comprised within 20 and 100.

Inventors: FARE, CARLO, Farè, Carlo
Applicants: BULL HN INFORMATION SYSTEMS ITALIA S.P.A.
Application year: 1988
Main classification: B41J9
IPC codes: B41J2, B41J9