Abstract:
An encoder unit is disposed facing a scale. The encoder unit is constructed such that a processing circuit and a read head are integrally formed on one and the same semiconductor substrate. This construction results in size reduction and integral formation of the encoder unit.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a relative-displacement detecting unit and a relative-displacement detecting device; and more particularly to a technique to achieve the size reduction and high accuracy detection of a unit for outputting its displacement relative to a scale in the form of an electrical signal. 
     2. Description of the Related Art 
     In related art, a transducer or an encoder for detecting relative displacement is known. In a capacitance-type encoder, a transmission electrode and a reception electrode are provided on a grid (unit), and a signal electrode is provided on a scale opposing this unit. The transmission electrode and the reception electrode on the unit are capacity-coupled with the signal electrode on the scale. A drive signal is supplied to the transmission electrode, and a detected signal occurring in the reception electrode in correspondence with the relative position of the unit and the scale is processed by a processing circuit. Thus, it is possible to detect the movement or the position of the unit with respect to the scale. In an induction-type encoder, the relative position is detected on the basis of the electromagnetic interaction (electromagnetic induction) between the unit and the scale. Namely, a transmission coil (excitation coil) and a detection coil are disposed on the unit, and a scale coil is formed on the scale. As current is fed to the excitation coil on the unit, a magnetic flux occurs, and an induced current is generated in the scale coil on the scale by electromagnetic induction. A magnetic flux is generated by the induced current generated in the scale coil, and an induced current (induced voltage) is generated in the detection coil on the unit by the magnetic flux. Since the induced voltage varies in correspondence with the relative position of the excitation coil and the scale coil, the relative position of the unit and the scale can be detected by detecting the induced voltage generated in the detection coil. In the encoder as mentioned above, much effort has been made to reduce the size of both the unit and scale, with an intention of increasing a detection accuracy and reducing the size of the unit and scale. 
     Even if the unit size is reduced, stray inductance and capacitance (stray LC) are present among the wires connecting the unit to its peripheral electric circuits and will degrade the encoder performance unless the peripheral electric circuits are integrated together with the unit. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to reduce the stray LC by integrating the encoder unit and its peripheral electric circuits on a substrate. 
     The above-mentioned object can be achieved by a relative-displacement detecting unit, according to the present invention, disposed facing a scale, for detecting its displacement relative to the scale and outputting a detected relative-displacement in the form of an electrical signal. The relative-displacement detecting unit includes a read head and a processing circuit. The read head detects a displacement of the relative-displacement detecting unit relative to the scale. The processing circuit drives the read head, processes signal output from the read head, and outputs the processed signal to exterior. The read head and the processing circuit are integrally formed on a semiconductor substrate. 
     Since the read head and the processing circuit are both integrally formed on a semiconductor substrate, the stray LC caused by wiring is suppressed and hence size reduction and high accuracy detection are realized. Here, “to integrally form” means not only to form those circuits on one and the same surface of the substrate, but also to form those circuits on different layers of the substrate. 
     The relative-displacement detecting unit may further comprise a magnetic shielding layer provided between the read head and the processing circuit. In the invention, the read head and the processing circuit are formed close to each other. A magnetic field developed from the read head directly affects the processing circuit (This phenomenon is called cross talk.). Provision of the magnetic shielding layer between the read head and the processing circuit prevents the cross talk. This results in increase of detection sensitivity. The magnetic shielding layer is made of high magnetic permeability material, e.g., ferrite, or may be a metal layer. 
     In the above-mentioned relative-displacement detecting unit, the processing circuit is preferably formed by a patterning process, and the read head is preferably formed by a resin buildup process. The relative-displacement detecting unit of the invention is integrally formed on a semiconductor substrate. In this case, the same forming process is not always used for forming the processing circuit and the read head. Rather, an active element portion (processing circuit portion) and a passive element portion (read head portion) may be formed by different forming processes. 
     Since the relative-displacement detecting unit is integrally formed on the semiconductor substrate, various mounting methods may selectively be used when the unit is mounted on a board. The relative-displacement detecting unit is mounted on a board by wire bonding, thereby forming a relative-displacement detecting device. The relative-displacement detecting unit may be mounted on a board by use of flip chips. Further, the relative-displacement detecting unit may be incorporated into a package mounted on a board. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and further features of the invention will be apparent with reference to the following description and drawings, wherein: 
     FIG. 1 is a diagram showing a scheme of an embodiment of the present invention; 
     FIG. 2 is a diagram schematically showing a read head used in the arrangement of FIG. 1; 
     FIG. 3 is a circuit diagram showing an electrical circuit arrangement of the embodiment of FIG. 1; 
     FIG. 4 is a diagram for explaining a high magnetic permeability film used in the embodiments of the invention; 
     FIG. 5 is a diagram for explaining a metal film used in the embodiments of the invention; 
     FIG. 6 is a diagram showing a mounting structure in the embodiments; 
     FIG. 7 is a diagram showing another mounting structure in the embodiments; 
     FIG. 8 is a diagram showing still another mounting structure in the embodiments; 
     FIG. 9 is a diagram showing yet another mounting structure in the embodiments; 
     FIG. 10 is a diagram showing a further mounting structure in the embodiments; and 
     FIG. 11 is a schematic illustration of another encoder unit in the embodiments. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The preferred embodiments of the present invention will be described with reference to the accompanying drawings. In the description, the invention is implemented into an induction-type encoder (magnetic-type encoder), by way of example. 
     FIG. 1 shows an arrangement of an induction-type encoder that is an embodiment of the invention. The induction-type encoder includes an encoder unit (relative-displacement detecting unit)  10  and a scale  12  located facing the encoder unit  10 . The encoder unit  10  includes a processing circuit  14 , a metal film  18 , a high magnetic permeability film  20 , and a read head  16 , which are integrally formed on a silicon substrate. The processing circuit  14  includes circuits for feeding a drive current to an excitation coil (transmission coil) of the read head  16  and circuits for processing a detected signal derived from a detection coil of the read head  16  and outputting the processed signal to exterior. Specifically, the processing circuit  14  includes a multiplexer  14   a,  an amplifier  14   b,  a relative-displacement detecting circuit  14   c,  an A/D converter  14   d,  an output circuit  14   e,  an oscillator circuit  14   f,  and a drive circuit  14   g.  The multiplexer  14   a  selectively outputs detected signals of different phases output from the detection coil. The amplifier  14   b  amplifies the detected signal from the multiplexer  14   a . The relative-displacement detecting circuit  14   c  detects a displacement of the encoder unit relative to the scale  12  by using the amplified detected signal. The A/D converter  14   d  converts detected analog signal into digital data. The output circuit  14   e  outputs the digital data to exterior. The oscillator circuit  14   f  and the drive circuit  14   g  feed the drive current to the transmission coil. The processing circuit  14  is formed on a silicon substrate by a known IC forming process. The metal film  18  and the high magnetic permeability film  20  are layered on the processing circuit. These films have a magnetic shielding function for shielding the processing circuit from a magnetic flux developed from the transmission coil in the read head  16 . The metal film  18  may be made of aluminum, copper, or gold, and the high magnetic permeability film  20  may be made of ferrite or Permalloy. 
     As shown in FIG. 2, the read head  16  includes a transmission coil  16   e , and reception coil groups  16   x  and  16   y,  which are differentially related. The reception coil group  16   x  consists of a plurality of reception coils  16   f  to  16   i.  The reception coils  16   f  to  16   i  are arranged at an interval of (¼)λ(λ=wave length of a scale coil) in a length measuring direction indicated by an arrow in FIG.  1 . Accordingly, the reception coils produce signals whose phases are 0°, 90°, 180° and 270°. The reception coil  16   f  and  16   h  are connected to each other, and the reception coils  16   g  and  16   i  are also connected to each other. Those interconnected reception coils produce detected signals of different phases (0° and 90°). Also in the reception coil group  16   y,  the reception coils  16   j  to  16   n  are connected as in the reception coil group  16   x.  The reception coil group  16   y  produces signals that are shifted by 180° from those by the reception coil group  16   x . Those signals function as differential signals. 
     FIG. 3 shows a circuit arrangement including the processing circuit  14  and the read head  16  of the induction-type encoder shown in FIGS. 1 and 2. As already stated, the reception coils  16   f  and  16   h  of the reception coil group  16   x  are interconnected to output a detected signal of 0° in phase, and the reception coils  16   g  and  16   i  are interconnected to output a detected signal of 90° in phase. The reception coils  16   f  and  16   h  of the reception coil group  16   x  are respectively connected to the reception coils  16   j  and  16   m  of the reception coil group  16   y.  The reception coils  16   g  and  16   i  of the reception coil group  16   x  are respectively connected to the reception coils  16   k  and  16   n  of the reception coil group  16   y.    
     The detected signal (whose phase is 0°) of the reception coils  16   f  and  16   h  and the detected signal (whose phase is 90°) of the reception coils  16   g  and  16   i  are both input to the multiplexer  14   a  of the processing circuit  14 . The multiplexer  14   a  alternately selects one of those detected signals, and outputs the selected one to the amplifier  14   b.  The amplifier  14   b  amplifies the detected signal, and outputs it to the relative-displacement detecting circuit  14   c.  The detected relative-displacement data is supplied, through the A/D converter  14   d,  to the output circuit  14   e,  which, in turn, supplies the received data to exterior. 
     Thus, in the embodiment, the processing circuit  14  and the read head  16  are integrally formed on one and the same silicon substrate. Accordingly, the encoder unit  10  may be reduced in size. Further, the feature of the reduced distance between the processing circuit  14  and the read head  16  accrues to reduction of the stray LC among the wires between the processing circuit  14  and the read head  16  and, hence, to no generation of noise and cross talk and securing a high level accuracy. 
     As already stated, in the present embodiment, the high magnetic permeability film  20 , which is typically made of ferrite, and the metal film  18 , which has low electric resistance, are provided between the read head  16  and the processing circuit  14 . Provision of these films contributes to increase the detection sensitivity. If required, the high magnetic permeability film  20  or the metal film  18  may be used instead. 
     FIG. 4 depicts a magnetic field in a structural arrangement of a case where the high magnetic permeability film  20  of a ferrite film, for example, is provided between the read head  16 , which faces the scale  12 , and the processing circuit  14 . If the high magnetic permeability film  20  is not present in the structure, a magnetic field developed from the transmission coil in the read head  16  directly reaches the processing circuit  14 . As a result, a so-called cross talk occurs to possibly generate noise. Use of the high magnetic permeability film  20 , however, reduces an intensity of the magnetic field reaching the processing circuit  14 , thereby suppressing the generation of the cross talk. 
     FIG. 5 illustrates a magnetic field distributed in a structural arrangement in which the metal film  18  made of copper, for example, is provided between the read head  16  and the processing circuit  14 . As seen, a magnetic field developed from the transmission coil in the read head  16  reaches the metal film  18 , so that an eddy current is induced in the metal film  18  by the magnetic field. The eddy current generated has such a direction as to suppress the magnetic field. Hence, this leads to the suppressing of the magnetic field directly reaching the processing circuit  14 . 
     While the embodiment that is believed to be preferred has been described, it should be understood that the invention is not limited to the above-mentioned one, but may variously be modified, altered and changed within the true spirit and scope of the invention. It is noted that in the embodiment, the encoder unit  10  is integrally formed on the silicon substrate. This feature creates the following advantage: it is easily mounted on another printed circuit board, a ceramic board, a glass board or the like by wire bonding or another suitable technique. Accordingly, it is easy to apply the invention to other relative-displacement detecting devices, in addition to the linear encoder. 
     FIG. 6 illustrates a case where an encoder unit  10  constructed according to the invention is mounted on a board  30  by wire bonding technique. The board  30  may be any of the printed circuit board, the. glass board, the ceramic board, and the like. 
     FIG. 7 illustrates a case where an encoder unit  10  of the invention is mounted on a board  30  by use of flip chips. Terminals are gathered on one side of the encoder unit  10 , and connected to the board. Combination of the encoder unit  10  and the flip chips will facilitate a further size reduction of the device. 
     FIG. 8 illustrates another case where the encoder unit  10  is mounted on a board by use of flip chips. As shown, a glass board  32  is provided facing a scale  12 . An encoder unit  10  is mounted on the reverse side (opposite to the side of the glass board facing the scale  12 ) of the glass board by use of flip chips. The glass board  32  is connected to an external processor:device by use of a FPC (flexible print circuit)  34 . 
     FIG. 9 illustrates a case where the encoder unit  10  of the embodiment is connected to a tape-like FPC  34  by TAB (tape automated bonding) process. 
     Further, the encoder unit  10  of the embodiment, as shown in FIG. 10, may be incorporated into a package (e.g., QPF package)  36  mounted on a board  30 . 
     In each embodiment, the encoder unit  10  is integrally formed onto the silicon substrate. In this case, it is not essential to form the encoder unit by one forming process. The encoder unit may also be formed in the following manner. As shown in FIG. 11, a portion of the processing circuit  14  is formed on a substrate by a known IC forming process while another portion including the read head  16 , which includes the transmission and reception coils, the metal film  18  and others is formed by layering resin (as a build-up board). 
     As seen from the foregoing description, the present invention succeeds in reducing the size of the encoder unit and integrally forming the same, and hence in suppressing a stray LC appearing among the wires and realizing high accuracy detection.