The present invention relates to an encoder device for generating pulses necessary for controlling the rotational speed and position of a motor. 
A miniature motor used, for example, for driving an optical disc such as a CD or DVD is required to incorporate therein a device for detecting the rotational speed and position thereof.  Known examples of such a detecting device include a magnetic device which performs magnetic detection by use of a magnet and a Hall device, an optical device which detects reflected light by use of a photodiode and a phototransistor, and a mechanical device which mechanically turns on and off electrical conduction between a pair of brushes.  The present invention relates to this mechanical encoder device. 
FIG.  7 is a pair of views for explaining a mechanical motor-rotation detection device based on the prior art (refer to Japanese Utility Model Application Laid-Open (kokai) No.  S57-15681).  As shown in FIG.  7, a rotation-detecting rotary contact portion is provided on a rotor shaft, in sliding contact with a pair of brushes.  The rotation-detecting rotary contact portion includes a large-angle conductive portion and a small-angle conductive portion, which are formed of an electrically conductive material such as copper and are provided along the outer circumference thereof.  Slits are formed in the boundaries between the large-angle conductive portion and the small-angle conductive portion so as to electrically insulate them from each other. 
Thus, when, as shown in FIG.  7(A), one brush is on the small-angle conductive portion, and the other brush is on the large-angle conductive portion, the circuit extending from one brush to the other brush is shut OFF.  When, as a result of progress in motor rotation, the state of FIG.  7(B) is reached, the two brushes are on the large-angle conductive portion; thus, the circuit extending from one brush to the other brush is turned ON.  In this manner, while the motor rotates one revolution, two ON pulses are generated. 
Hence, pulse signals are generated by alternation between an electrically conductive state in which the two brushes are in sliding contact with the large-diameter conductive portion, and an electrically nonconductive state in which one of the brushes is in sliding contact with the small-diameter conductive portion.  Rotation can be detected from the thus-generated pulse signals, and the number of revolutions can be detected by counting the pulse signals. 
Alternatively, the number of revolutions can be detected according to a similar principle through employment of a rotation-detecting rotary contact portion which, as shown in FIG.  8, includes, in place of the small-angle conductive portion, a nonconductive portion made of an electrically insulative resin material or the like (refer to Japanese Utility Model Publication (kokoku) No.  H06-49090 or Japanese Patent Application Laid-Open (kokai) No.  H07-284258). 
However, in either case, the prior art employs a carbon brush as a rotation-detecting brush.  With the progress of wear or the like, a portion of the carbon brush in sliding contact with a conductive portion increases in sliding area.  FIG.  9(A) shows an initial state of the carbon brush, in which the carbon brush is in point contact with the conductive portion, since the carbon brush is not worn.  By contrast, with increasing time of use of the carbon brush, as shown in FIG.  9(B), the brush undergoes wearing and comes into surface contact with the conductive portion.  In this state, the brush cannot be brought in point contact with the conductive portion, and the angle between the two brushes cannot be reduced.  Thus, even when, in order to obtain a number of pulses, the number of conductive portions is increased, and the angle between the brushes is reduced, wear on the carbon brushes causes loss of the angle between the brushes, resulting in a failure to function as an encoder device.  Since wear on the carbon brush causes an increase in the area of a sliding portion of the brush, the duty ratio of pulses changes. 
As described above, since the prior art employs carbon brushes as rotation-detecting brushes, it is impossible to obtain a number of pulses whose duty ratio does not change.  A failure to obtain a number of pulses, and involvement of change of duty ratio lead to a failure in applications requiring precision control.  The prior art fails to satisfy the need for not only controlling the rotational speed of motor but also accurately controlling the rotational, angular position of motor, on the basis of a detected signal. 
The prior art employs a mold structure achieved by use of insert molding in which resin is poured into a mold while a conductive portion or the like is set within the mold.  The prior art, employing a mold structure, fails to facilitate a process for manufacturing a motor rotor. 
An object of the present invention is to solve the above-mentioned problems, and to provide a mechanical encoder device for use in a miniature motor, which encoder can provide, for each revolution of the motor, a plurality of pulses whose duty ratio does not change with time, and which facilitates generation of a number of pulses, such as six pulses per revolution. 
Another object of the present invention is to provide an encoder device allowing easy incorporation into a miniature motor. 
An encoder device incorporated in a miniature motor of the present invention is incorporated in a miniature motor having an end cap fitted to a metallic case in such a manner as to close an opening of the metallic case, a rotor configured such that a rotor magnetic-pole and a motor commutator are integrally mounted on a rotatably supported shaft, and two motor brushes abutting the commutator.  The encoder device comprises a rotary contact portion provided on the rotor shaft, and a pair of rotation-detecting brushes in sliding contact with the rotary contact portion.  A plurality of rotary contact pieces are arranged on an outer circumferential surface of the rotary contact portion such that slits are formed therebetween.  The paired rotation-detecting brushes are each formed of a resilient metal and each assume the shape of a cantilever plate or wire spring whose side surface portion is subjected to sliding contact.  An inter-brush angle corresponding to an arcuate angle between contact points at which the paired rotation-detecting brushes abut the rotary contact portion is less than an arcuate angle corresponding to a single rotary contact piece, whereby a plurality of pulses are generated while the rotor shaft rotates one revolution. 
Linear recesses can be formed by means of grooving or corrugating, or linear through-holes can be formed by means of slitting, along or obliquely with respect to the axial direction of the shaft of the motor on a surface portion of each of the rotary contact pieces which comes in sliding contact with the rotation-detecting brushes. 
Furthermore, the rotary contact portion can be mounted on the shaft at an arbitrary position, such as at the same side as the motor commutator or at the side opposite the motor commutator with respect to the rotor magnetic-pole, or at the exterior of the end cap.