Encoder for providing incremental and absolute position data

Details Encoder for providing incremental and absolute position data Encoder for providing incremental and absolute position data

1998-12-16

2001-01-16

Ham, Seungsook (Department: 2878)

Radiant energy

Photocells; circuits and apparatus

Optical or pre-photocell system

C250S231180

Reexamination Certificate

active

06175109

ABSTRACT:

This invention relates to position sensors or encoders which produce electrical signals indicative of the position of a moving object. In particular, the invention relates to an encoder that provides incremental position data as well as coarse absolute position data.
BACKGROUND OF THE INVENTION
From the prior art there are rotary encoders known which provide incremental output signals as well as coarse absolute output signals, e.g. commutation output signals. Commutation output signals are necessary in brushless motors where commutation is performed by electronically steering drive current in the motor to the appropriate winding. For this purpose the absolute rotor position has to be determined with a certain accuracy.
There are different ways known from the prior art how to generate the commutation output signals. In a typical method, Hall switches are used to determine the absolute rotor position. The Hall devices are arranged on a circuit board, which is aligned with a magnet on the rotor so that a defined relationship between the Hall outputs and the motor back EMF can be established. This solution has the disadvantage that additional room must be made for the commutation circuit board and for the encoder. In addition, Hall devices are sensitive to acoustic noise, current spikes, temperature and can be difficult to align.
Because of these disadvantages, encoders were developed which provide incremental output signals as well as commutation output signals from a single code wheel. Encoders of this type are sold, for example by the assignee of the present application. Pending patent application Ser. No. 08/980,047, U.S. Pat. No. 5,936,236 assigned to the present assignee describes such an encoder type.
The code wheel of these encoders usually has an incremental track, and at least three additional code tracks for generating the commutation output signals. This results in a minimum of four data tracks arranged concentrically on the code wheel. To provide the incremental output signals from the incremental track at least two photodetectors are necessary which are arranged in a spatial relationship so that two incremental signals are generated which have a phase shift of 90° with respect to each other. For the commutation output signals, each of the code tracks is scanned by a further photodetector which provides either a HIGH output or a LOW output. The combination of the three commutation output signals allows determination of the absolute rotor position within a certain range.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a very compact encoder which is able to generate incremental output signals as well as coarse absolute output signals.
It is another object of the invention to provide a very compact rotary encoder which generates incremental output signals as well as commutation output signals which can be used to commutate a brushless motor.
It is a further object of the present invention to provide an encoder which delivers incremental output signals as well as coarse absolute output signals and which allows very small optics to be incorporated.
An optical encoder according to the present invention is able to provide absolute and incremental positional information of an object which moves along a certain measuring direction. The encoder has a single data track positioned along the measuring direction, having a fine incremental track and a plurality of separate coarse code tracks overlying said incremental track. A light source is provided for emitting light in the direction of the data track. A first detecting unit is arranged with respect to the data track for receiving light modulated by said incremental graduation. The first detecting unit consists of an array of separate photodetectors, each of the photodetectors providing phase shifted incremental signals. A second detecting unit is arranged with respect to the data track for receiving light modulated by said at least three separate coarse code tracks. The second detecting unit has a number of separate photodetectors and the number of photodetectors is equal to the number of coarse code tracks.


REFERENCES:
patent: 4423958 (1984-01-01), Schmitt
patent: 4700062 (1987-10-01), Ernst
patent: 4712088 (1987-12-01), Ernst
patent: 5061073 (1991-10-01), Michel
patent: 5274229 (1993-12-01), Khan
patent: 5553390 (1996-09-01), Ernst et al.
patent: 5651187 (1997-07-01), Affa
patent: 5936236 (1999-08-01), Setbacken et al.
U.S. Patent Application Serial No. 08/782,050, filed Jan. 10, 1997, by Setbacken et al.
U.S. Patent Application Serial No. 08/929,539, filed Sep. 15, 1997 by Setbacken et al.
U.S. Patent Application Serial No. 08/980,047, filed Nov. 26, 1997, by Setbacken et al.

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