Electricity: motive power systems – Positional servo systems – Antibacklash systems
Patent
1983-05-11
1985-03-26
Dobeck, B.
Electricity: motive power systems
Positional servo systems
Antibacklash systems
318632, 318663, G05B 1101
Patent
active
045075956
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to a device for correcting of backlash, which arises on a change in the direction of movement in a drive mechanism and more particularly a power driven screw and a nut means co-operating with the screw, the nut means being connected to a unit which is movable back and forth along the screw. The device includes preferably a printing unit of a printer or a typewriter pulse generating means of the incremental type which in synchronism with the movement of the screw is arranged to generate two pulse trains having the same quadrangular shape but being 90.degree. out of phase. The pulse trains have a period corresponding to the displacement of the movable unit a predetermined distance along the screw and are arranged to be applied to a means which utilizes the pulse trains determine the position and the direction of the movable unit.
It is known in printers in connection with screw drive mechanisms of the kind referred to above to provide an incremental code disc that rotates with the screw and co-operates with a photoelectric device, in the usual way comprising light-emitting diodes and phototransistors, to generate two pulse trains being 90.degree. out of phase. From the two pulse trains information is derived about the position and the direction of movement of a printing unit movable back and forth along the screw.
In order for the movable unit to be easily moved along the screw a certain play or backlash must be present between the screw and a nut means co-operating with the screw. Usually, the backlash is compensated in a mechanical way, for example the nut means may comprise two nuts which are spring biassed away from each other and towards the flanks of the helical groove of the screw.
SUMMARY OF THE INVENTION
An object of the invention is without using mechanical arrangements, to correct for the play or backlash that necessarily must be present between the nut means and the screw. In accordance with the principle of the invention a measured or estimated value of the existing backlash is stored and used for the correction of control signals which are applied to an electric motor provided for driving of the screw. By this kind of correction the drive arrangement can be mechanically constructed such that the nut means will move along the screw with a minimum of friction at the same time as the backlash can be compensated at a higher degree than would be possible with mechanical arrangements. Correction must take place every time the movable unit changes its direction of movement. If in that connection the backlash is regularly measured the wear, appearing in the screw drive mechanism as time goes on and involving a gradual increase of the backlash, will be taken into consideration. Variations in magnitude of the backlash caused by wear cannot be equally corrected by mechanical correcting arrangements.
A further object of the invention is to achieve correction of the existing backlash in steps corresponding to a quarter of the period of the pulse trains utilized for determining of position and direction of movement.
The objects indicated will be achieved in a device having the characterizing features stated in the appending claims.
BRIEF DESCRIPTION OF THE DRAWING
An embodiment of the invention will now be described with reference to the accompanying drawings.
FIG. 1 is a block diagram of a correcting device according to the invention as applied to a printer, shown schematically.
FIG. 2 is a detail view showing the block 23 of FIG. 1.
FIG. 2a shows the time diagrams relating to the signal level at two inputs X, Y of the block 23.
FIG. 3 shows how to combine two pulse trains in order to move a predetermined flank of one of the pulse trains in a selected direction.
FIG. 4 is a block diagram showing a device for combining the pulse trains according to FIG. 3.
FIGS. 5 and 5a show schematically the order in correcting for backlash in both directions of movement of the movable unit shown in FIG. 1.
FIG. 6, finally, is a circuit diagram of a useful mo
REFERENCES:
patent: 3705400 (1972-12-01), Cordes, Jr.
patent: 3708736 (1973-01-01), Hoshina et al.
patent: 3794902 (1974-02-01), Nishimura et al.
patent: 3886424 (1975-05-01), Hoshina et al.
patent: 4296365 (1981-10-01), Yoshikawa
Dobeck B.
Facit Aktiebolag
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