Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Patent
1983-02-15
1985-12-17
Nelms, David C.
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
250237R, 340347R, G01D 534
Patent
active
045594481
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a method and device, respectively, for measuring backlash arising in a drive mechanism comprising a screw and a nut device, the latter being connected to a unit which is movable in opposite directions along the screw. Preferably, the movable unit is a printing unit which is part of a printer or a typewriter. The drive mechanism comprises pulse generating means in which the number of emitted pulses for each displacement corresponds to the distance covered. A pulse counting means is provided for counting of pulses emitted by the pulse generating means.
BACKGROUND
In a drive mechanism of the above kind, an axial backlash must exist between the screw and the nut in order for the latter to be moved along the screw. In matrix printers, wherein the characters are formed by printing of dots and wherein the printing unit is moved in opposite directions along the screw, the backlash will cause the printing unit to take a first position when moving in one direction and to take a second position, different from the first position, when moving in the opposite direction.
In order to eliminate or reduce the influence of the inaccuracy connected with screw drive mechanisms, various nut means have been constructed which are spring-biassed so as always to bear on the same side of a groove joining two adjacent threads of the screw. However, it is not possible to completely eliminate the backlash because in such a case the nut would not be movable along the screw or would move at a speed which would be considerably reduced.
SUMMARY OF THE INVENTION
The invention represents a different way of thinking and its main object is to provide a measurement value, the magnitude of which represents the backlash, and to present this measurement value in a shape that can be easily treated bya backlash correcting equipment working digitally.
BRIEF DESCRIPTION OF THE FIGURES OF THE DRAWING
Other objects and advantages related to the invention will appear from the following description of an embodiment with reference to the accompanying drawings.
FIG. 1 is a schematic perspective view according to the invention of a drive mechanism including a screw and a nut as well as electric and electronic functional elements.
FIGS. 2a-2d, show four conditions which show the principle by which the backlash between the screw and nut influences the positioning of a movable unit.
FIGS. 3a-3e show conditions to illustrate the principle of measuring the backlash according to the invention.
FIG. 4 illustrates the hysteresis of a phototransistor used as a light detector.
FIG. 5 is a diagram illustrating the relation between light detected by the phototransistor of a photo-electric assembly as a function of the position of a screen member co-operating with the photo-electric assembly and supported by the movable unit.
FIG. 6 is a diagram related to the variation of light intensity, similar to FIG. 5, but related to the position of the movable unit and indicating the error in measurement due to the hysteresis of the phototransistor.
FIGS. 7 and 8 are diagrams similar to FIG. 6 and additionally show the technique of the invention in eliminating the error of measurement caused by the hysteresis of the phototransistor.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENT
With reference to FIG. 1 a drive mechanism 10 comprises an elongate screw 11 and a nut device, not shown, which co-operates with the screw and is non-rotatably connected to a movable unit 12, e.g. a printing unit which is movable in opposite directions along a platen, not shown. The movable unit 12 is guided by a rod 13 which runs parallel to the screw and is secured to end walls 14, 15 of a machine frame, not otherwise shown in detail. The screw 11 is mounted for rotation in bearings, not shown, in the end walls 14, 15 and is reversibly driven by a motor 16, the rotatable shaft 18 of which is connected via a cog belt 17 to a driving wheel 19 secured to the screw.
For determining the instant position of the movable unit 12 along the screw
REFERENCES:
patent: 3749925 (1973-07-01), Hertrich
patent: 3818224 (1974-06-01), Schmidt
patent: 4117320 (1978-09-01), Tomlinson et al.
patent: 4247214 (1981-01-01), Swan, Jr.
Facit Aktiebolag
Gatto J.
Nelms David C.
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