Geometrical instruments – Miscellaneous – Light direction
Patent
1989-09-29
1991-06-25
Will, Thomas B.
Geometrical instruments
Miscellaneous
Light direction
33 1PT, 901 25, G01B 730
Patent
active
050255630
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to angle transducers of the multiturn type, where the absolute angular position of a shaft which can rotate is measured, i.e. the total number of revolutions plus an angle less than 360.degree. is measured from a fixed initial, reference position of the shaft.
For reasons of safety, angle transducers of the kind mentioned above are coming into greater and greater use with industrial robots. The reasons for this is that should a current failure occur it is essential to keep track of the absolute angular positions of the respective rotating shafts in the robot, because when power returns the robot must continue its motion from where it left off when current failed. In other words, a current failure must not mean that the respective angle transducers record zero values, since this would mean that when current returns the respective shafts in it would start from their respective zero angular positions, which could give the robot a completely incorrect motion and could mean danger to life for persons in the vicinity of the robot. Such undesired zero setting of angular position occurs if the respective angle transducer does not measure the shaft angle absolutely but only relative to the angle at which the shaft happens to stop.
Angle transducers of the kind mentioned are known, and usually comprise two transducers, e.g. resolvers or electro-optical transducers arranged side by side, and mutually connected via a spur gear. One of the transducers is connected to a rotating shaft of the robot. The gear comprises two gear wheels, one of which has 64 teeth, for example, and the other 65 teeth. The differential rotation of the resolvers enables absolute measurement of the total angle passed through by the rotating shaft. Any angular position of the two resolvers corresponds to an unambiguously determined total turning angle of the rotating shaft. The differential turning angle is determined by conventional electrical circuits, which may be of the phase-sensitive type or the amplitude sensitive type.
The angle transducer unit described in the last paragraph has plurality of disadvantages. The transducers in it are situated side by side and are mutually connected to the gear, resulting in that the cylindrical housing into which the whole transducer unit is usually built has a large diameter. In certain applications, the space taken up by the transducer housing on the machine detail where the rotating shaft is situated is disadvantageously large. The gear, which solely comprises two externally toothed spur gears in mesh, is not free from backlash. To prevent this backlash, one of the gear wheels is provided with a springbiased gear wheel which urges the gear wheels in the gear into mesh with each other during all rotational phases of the shaft. The teeth of the gear are subjected to large loads due to the large moment of inertia of the driven gear wheel and the high momentary accelerations occuring for rapid positional changes, or when the rotating shaft changes its direction of rotation. Such large accelerations cause wear on the teeth after a while. Only one tooth on one gear wheel is simultaneously in engagement with a tooth on the other gear wheel of the gear, resulting in that only these two teeth take up the acceleration stresses. This causes wear on the teeth of the gear. The meshing factor of a conventional spur gear is normally about 1.6-2. Both gear wheels in a conventional spur gear rotate at high absolute speed and the mutual, relative tooth speed is very high. The high relative tooth speed causes wear on the teeth. All this reduces the life of the known angle transducer. Due to the high tooth speeds and thereby the high centrifugal force, the gear lubricant will be urged away from the teeth. To obtain good, durable lubrication it is necessary for the gear wheels to be disposed in an oil bath.
The present invention has the object of providing an angle transducer of the kind mentioned in the introduction, which, while avoiding the above mentioned disadvantages has long life, and f
REFERENCES:
patent: 3634946 (1972-01-01), Star
patent: 3793731 (1974-02-01), Hyyppa et al.
patent: 4123653 (1978-10-01), Bovie
patent: 4660288 (1987-04-01), Dangschat
patent: 4771545 (1988-09-01), Hisayasu et al.
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