Abstract:
A movable code plate for an optical encoder is given a light path changing function by forming a series of indentations in one or more portions of one surface of the plate. Since these indentation are sufficiently shallow with respect to the thickness of the movable code plate, sufficient strength of the movable plate is maintained. These indentations can be formed on the surface of the movable code plate without a high accuracy. These indentations may be V-shaped grooves or matted surfaces.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical encoder having an improved structure for the optical detecting section thereof. 
     2. Description of the Related Art 
     Optical encoders include rotary type encoders which detect the position and speed of a rotational moving object, such as a motor, or the like, and linear type encoders which detect the position and speed of a linear moving object. Of these, one example of an encoder used widely as a rotary type optical encoder is now described with reference to FIG.  23 . 
     A movable code plate  100  is made from a glass plate onto which chrome has been vapour deposited, or a metal plate, such as stainless steel, nickel, copper, or the like, and light transmitting sections  101   a,    101   b  are formed by etching in portions of ring-shaped regions scanned by a light-emitting element (omitted from drawing) of a light source  1 . The areas outside the light transmitting sections  101   a ,  101   b  form light shielding sections. 
     A light source  1  and a light receiving section  2  are provided on either side of the movable code plate  100 . The light source  1  and light receiving section  2  are constituted respectively by a prescribed number of light-emitting elements (omitted from drawing) and photorecptor elements  21 ,  22 . When a light transmitting section  101   a ,  101   b  is positioned in front of the photoreceptor elements  21 ,  22 , light projected from the light source  1  to the movable code plate  100  is transmitted by the light transmitting section  101   a ,  101   b  and enters the corresponding photoreceptor element  21 ,  22 , and a signal representing the received light is output by the photoreceptor element  21 ,  22 . 
     The movable code plate  100  is installed on a rotating shaft  3  (the object of which the rotational position or speed is to be detected), and when it rotates about the axis of the rotating shaft  3 , a light reception signal is output periodically by the photoreceptor elements  21 ,  22 . By processing this light reception signal by means of a commonly known type of circuit, which is omitted from the drawing, it is possible to detect the rotational position, rotational speed, or the like, of the rotating shaft  3 . 
     When creating the conventional type of optical encoder described above, firstly, a light-shielding film made from chrome, or the like, is formed onto a movable code plate  100 , and the light-shielding film is then removed therefrom by etching in a prescribed code pattern. However, since etching uses chemical substances that require labour-consuming management and processing, a problem arises in that manufacturing costs are high and hence this method is not suitable for mass production. 
     Therefore, in order to resolve the aforementioned problem, it has been proposed that movable code plates can be manufactured by using an opaque resin material and forming transmitting holes in portions of the region scanned by light from a light source. By adopting movable code plates made from resin of this kind, advantages are obtained in that since no harmful chemical substances are used, manufacturing is simple and inexpensive, and good reproducibility is easy to achieve. However, problems arise in that it is difficult to form fine patterns, since adequate strength cannot be achieved, and therefore it is not possible to obtain the high resolution that can be achieved with code plates made from glass or metal. 
     Furthermore, as a further method of resolving the aforementioned problems associated with movable code plates made from resin, an optical encoder having a light path changing function has been proposed and a patent application has been made for same (Japanese Patent Application No. 11979/1998 issued as Japanese Patent No. 11-201229, U.S. patent application Ser. No. 09/225,516, European Patent Application No. 199990300085). As illustrated in FIG. 24, this optical encoder comprises a movable code plate  110  wherein the ring-shaped region scanned by a light-emitting element (omitted from drawing) of a light source is divided into a first type of region  110   a,    110   b  (regions indicated by hatching in FIG. 24) through which incident light from the light source  1  is transmitted after being bent at least once, and a second type of region through which incident light from the light source  1  is transmitted directly. 
     A light-receiving section  2  is positioned in such a manner that the state of light reception by photoreceptor elements  21 ,  22  in the light-receiving section  2  is different in cases where the light from the light-emitting element in the light source  1  is transmitted through the first type of region  110   a,    110   b  on the movable code plate  110 , and cases where the light is transmitted through the second type of region. 
     In the proposed optical encoder described above, the first type of region  110   a ,  110   b  is provided by forming a cut-out section  10  (see FIG. 25) in the movable code plate  110 . The cut-out section  10  must be formed to a significant depth in order that the light incident on the movable code plate  110  is bent through a prescribed angle. Therefore, when this type of cut-out section  10  is formed in the regions indicated by the hatching in FIG. 24, thus reducing the material thickness in those regions, the strength of the movable code plate  110  declines accordingly. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an optical encoder having a light path changing function, without impairing the strength of movable code plates. Additional objects and advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention. 
     The optical encoder according to the present invention comprises a movable code plate for converting input light to encoded output light, a light source provided with one or more light-emitting element for scanning one or more regions on the movable code plate, and one or more photoreceptor elements for receiving the output light encoded by the movable code plate. Moreover, in order to perform the encoding action, at least one of the regions scanned by the input light is divided into a first type of region and a second type of region generating a first type of output light and a second type of output light having mutually different light paths. At least one of either the first type of region and/or the second type of region comprises a light path changing function whereby the light path of the input light is bent at least once. Furthermore, the one or more photoreceptor elements are positioned in such a manner that a first light reception state is created in at least one of these photoreceptor elements when the input light is incident on the first type of region, and a second light reception state, which is different to the first light reception state, is created therein when the input light is incident on the second type of region. The light path changing function is created by forming a series of indentations which are sufficiently small with respect to the thickness of the movable code plate in one surface of the movable code plate, these indentations being formed in at least one of the first type and/or second type of region. 
     According to the present invention, adequate strength can be provided in a movable code plate for an optical encoder, and furthermore, it is possible to form movable code plates without requiring metal moulds having high registration accuracy, and it is also possible to manufacture movable code plates without depending on signal frequency. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects and features of the invention will become apparent from the following description of preferred embodiments of the invention with respect to the accompanying drawings, in which: 
     FIG. 1 is a general oblique view showing the principal composition of an optical detecting section of a rotary-type optical encoder relating to a first embodiment of the present invention; 
     FIG. 2 is a diagram showing an enlargement of a portion of cross section A—A in FIG. 1; 
     FIG. 3 is an enlarged sectional view of the movable code plate in FIG. 2; 
     FIG. 4 is an output signal diagram relating to the optical encoder in FIG. 1; 
     FIG. 5 is an approximate oblique diagram showing the principal composition of an optical detecting section of a rotary-type optical encoder relating to a second embodiment of the present invention; 
     FIG. 6 is a diagram for explaining the light path changing function of the movable code plate in the optical encoder in FIG. 5; 
     FIG. 7 is an output signal diagram of the optical encoder in FIG. 5; 
     FIG. 8 is a diagram for explaining the light path changing function of a movable plate in a rotary-type optical encoder relating to a third embodiment of the present invention; 
     FIG. 9 is an output signal diagram relating to the optical encoder in FIG. 8; 
     FIG. 10 is a diagram for explaining the light path changing function of a movable plate in a rotary-type optical encoder relating to a fourth embodiment of the present invention; 
     FIG. 11 is an enlarged view of the movable code plate in FIG. 10; 
     FIG. 12 is a diagram for explaining the light path changing function of a movable plate in a rotary-type optical encoder relating to a fifth embodiment of the present invention; 
     FIG. 13 is an enlarged view of the movable code plate in FIG. 12; 
     FIG. 14 is a diagram for explaining the light path changing function of a movable plate in a rotary-type optical encoder relating to a sixth embodiment of the present invention; 
     FIG. 15 is an enlarged view of the movable code plate in FIG. 14; 
     FIG. 16 is a diagram for explaining the light path changing function of a movable plate in a rotary-type optical encoder relating to a seventh embodiment of the present invention; 
     FIG. 17 is an output signal diagram relating to the optical encoder in FIG. 16; 
     FIG. 18 is a diagram for explaining the light path changing function of a movable plate in a rotary-type optical encoder relating to an eighth embodiment of the present invention; 
     FIG. 19 is an enlarged view of the movable code plate in FIG. 18; 
     FIG. 20 is an output signal diagram relating to the optical encoder in FIG. 18; 
     FIG. 21 is a diagram for explaining the light path changing function of a movable plate in a rotary-type optical encoder relating to a ninth embodiment of the present invention; 
     FIG. 22 is an output signal diagram relating to the optical encoder in FIG. 21; 
     FIG. 23 is an approximate oblique view showing the principal composition of an optical detecting section in a conventional rotary-type optical encoder; 
     FIG. 24 is an approximate oblique view showing the principal composition of an optical detecting section in a rotary-type optical encoder relating to a conventional technology; and 
     FIG. 25 is a diagram for explaining the light path changing function of the movable plate in the optical encoder in FIG.  24 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now made in detail to the present preferred embodiments of the present invention, examples of which are illustrated in the accompying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below in order to explain the present invention by referring to the figures. 
     A first embodiment of the present invention is now described with reference to FIG. 1 to FIG.  4 . 
     The movable code plate  4  used in the rotary-type optical encoder is made from transparent optical glass or plastic material. On this movable code plate  4 , a ring-shaped region containing a plurality of regions  41   a,    41   b,    42   a,    42   b  indicated by hatching in FIG. 1 is scanned by light output from a light source  1 . 
     A light source  1  and a light-receiving section  2  are provided on either side of the movable code plate  4 . The light-receiving section  2  comprises two photoreceptor elements  21 ,  22 . The photoreceptor element  21  is positioned such that it corresponds to regions  41   a,    41   b,  and the photoreceptor element  22  is positioned such that it corresponds to regions  42   a,    42   b.    
     The regions  41   a,    41   b,    42   a,    42   b  in the ring-shaped scanning region of the movable code plate  4  are constituted by forming a continuous plurality of V-shaped grooves which are sufficiently shallow with respect to the thickness of the movable code plate  4  (in other words, of a depth which does not impair the strength of the movable code plate  4 ), on one side of the movable code plate  4 . 
     FIG. 2 shows a cross section of region  42   a,  of the aforementioned regions  41   a,    41   b,    42   a,    42   b  (hereinafter, referred to as first-type regions). The other regions  41   a,    41   b  and  42   b  have the same structure as  42   a.    
     Of the ring-shaped scanning region of the movable code plate  4 , the portion where V-shaped grooves are not formed, that is, the portion other than the aforementioned first-type region  41   a,    41   b,    42   a,    42   b,  constitutes a second-type region. 
     As shown in FIG. 2, light from the light source  1  that is incident on the first-type region  42   a  (the same applies to regions  41   a,    41   b,    42   b ) is fully reflected by the surfaces of the V-shaped groove and returns in the direction of the light source  1 , as illustrated in FIG.  3 . In other words, the light does not proceed towards the light-receiving section  2 . On the other hand, light from the light source  1  that is incident on region sp, which is of a second type (see FIG.  2 ), is transmitted without alteration and proceeds directly towards the light-receiving section  2 . 
     Since the first-type regions are obtained by forming a plurality of small V-shaped grooves in a continuous fashion on one side of the movable code plate  4 , there is only a slight reduction in the strength of the movable code plate  4  due to forming of the first-type regions, when compared to a case where cut-outs  10  (see FIG. 25) are formed in order to obtain first-type regions as in the prior art. 
     As described above, light incident on the movable code plate  4  from the light source  1  is shielded from passing to the light-receiving section  2  in the first-type regions  41   a,    41   b,    42   a,    42   b,  whereas it is allowed to proceed to the light-receiving section  2  in the second-type regions, and therefore the outputs of the photoreceptor elements  21 ,  22  in the light-receiving section  2  are as shown respectively in FIGS.  4 ( a ) and ( b ). FIG. 4 shows rectangular waveforms which were transformed from triangular waveforms of the photoreceptor elements  21 ,  22  by a well-known signal processing circuit (not shown). Photoreceptor element  21  and photoreceptor element  22  are positioned such that they correspond respectively to the inner side and the outer side of the region scanned by the output light from the light source  1 . 
     A second embodiment of the present invention is now described with reference to FIG. 5 to FIG.  7 . 
     The region scanned by the light output from the light source  1  covers two ring-shaped regions  51 ,  52  indicated by hatching in FIG.  5 . 
     The light-receiving section  2  comprises two photoreceptors elements  23 A,  23 B facing the inner portion of the ring-shaped region  51 , and two photoreceptor elements  24 A,  24 B facing the outer portion of the ring-shaped region  52 . The photoreceptor elements  23 A,  23 B and the photoreceptor elements  24 A,  24 B are positioned at a prescribed interval from each other in the circumferential direction of the ring-shaped regions  51 ,  52 . 
     On the other hand, in the ring-shaped region  51  of the movable code plate  5 , as illustrated in FIG. 6, a first-type region  51   a  and a second-type region  51   b  respectively having the same width as the photoreceptor elements  23 A,  23 B of the light-receiving section  2  are provided alternately at a prescribed pitch in the scanning direction of the movable code plate  5 . 
     Furthermore, although not shown in the drawings, in the ring-shaped region  52  of the movable code plate  5 , a first-type region  52   a  and a second-type region  52   b  respectively having the same width as the photoreceptor elements  24 A,  24 B of the light-receiving section  2  are provided alternately at a prescribed pitch in the scanning direction of the movable code plate  5 . 
     Similarly to the first-type region  42   a  shown in FIG. 2, a series of V-shaped grooves are formed in the first-type regions  51   a,    52   a.  Similarly to the second-type region  41  shown in FIG. 2, the second-type regions  51   b,    52   b  are areas where no V-shaped grooves are formed. (Incidentally, the regions  52   a,    52   b  are not shown in these drawings.) Consequently, the first-type regions  51   a,    52   a  and the second-type regions  51   b,    52   b  of the movable code plate  5  have a similar function to that of the first-type region  42   a  and the second-type region  42   b  of the movable code plate  4  illustrated in FIG.  2 . 
     The output of the photoreceptor elements  23 A,  23 B in the light-receiving section  2  are as shown respectively in FIGS.  7 ( a ) and ( b ), (similarly to the output of the photoreceptor elements  24 A,  24 B). FIG. 7 shows rectangular waveforms which were transformed from triangular waveforms of the photoreceptor elements  23 A and  23 B by a well-known signal processing circuit (not shown). 
     A third embodiment of the present invention is now described with reference to FIG.  8  and FIG.  9 . 
     In this embodiment, the composition is the same as the second embodiment (FIG.  6 ), with the exception that the intervals between the photoreceptor elements  23 A and  23 B (and the photoreceptor elements  24  and  24 B) are different. Specifically, whereas in FIG. 6 the interval between the photoreceptor elements  23 A and  23 B of the light-receiving section  2  is taken as the width of the first-type region  51   a  plus the width of the second-type region  52   b  (= 1  pitch), in the present embodiment, as shown in FIG. 8, this interval is taken as 1 pitch and a half, meaning that it is half a pitch larger than in the second embodiment. 
     Consequently, in this embodiment, as illustrated in FIG. 8, when one of the photoreceptor elements  23 A faces the second region  61   b  and receives light, the other photoreceptor element  23 B faces the first region  61   a  and does not receive light. 
     Therefore, the outputs of the photoreceptor elements  23 A,  23 B of the light-receiving section  2  are as shown in FIG.  9 ( a ) and ( b ), respectively. In other words, when one of the photoreceptor elements  23 A and  23 B outputs a signal, the other photoreceptor element produces no output. FIG. 9 shows rectangular waveforms which were transformed from triangular waveforms of the photoreceptor elements  23 A and  23 B by a well-known signal processing circuit (not shown). 
     A fourth embodiment of the present invention is now described with reference to FIG.  10  and FIG.  11 . 
     In this embodiment, instead of obtaining the first-type regions on the movable code plate by forming a series of V-shaped grooves, these regions are formed by matte finish. 
     As shown in FIG. 10, when light from a light source  1  is incident on the movable code plate  7 , this light is scattered at the first-type region  70   a  processed to have a matte finish, so that it does not reach the photoreceptor elements  23 A,  23 B of the light-receiving section  2 . On the other hand, light that is incident on the second-type region  70   b,  which does not have a matte finish, travels on to the photoreceptor element  23 B (or photoreceptor element  23 A) of the light-receiving section  2 . 
     By forming the first-type regions with a matte finish, it is possible further to reduce the depth of grooves formed in the movable code plate, compared to cases where the first-type regions are created by forming V-shaped grooves, and hence any decline in the strength of the movable code plate due to forming of the first-type regions can be further suppressed. 
     A fifth embodiment of the present invention is now described with reference to FIG.  12  and FIG.  13 . 
     In the ring-shaped area of a movable code plate  8  scanned by a light-emitting element (omitted from drawing) of a light source  1 , on the opposite side of the plate to the light source  1 , portions where a series of V-shaped grooves are formed constitute a first-type region  80   a,  while portions where no grooves are formed constitute a second type region  80   b.    
     Light from the light source  1  is incident obliquely on the movable code plate  8 , as illustrated in FIG.  12 . In the first-type region  80   a,  this incident light is travelling in a perpendicular direction to the oblique faces of the V-shaped grooves, as illustrated in FIG. 13, and therefore the incident light is transmitted. On the other hand, in the second-type region  80   b,  the light is reflected at the face of the movable code plate  8  on the opposite side to the light source  1 , as illustrated in FIG. 12, and hence the light enters the photoreceptor element  23 B (or photoreceptor element  23 A) of the light-receiving section  2 . 
     The position of the light-receiving section  2  may be changed in such a manner that it receives light transmitted by the first-type region  80   a.    
     A sixth embodiment of the present invention is now described with reference to FIG.  14  and FIG.  15 . 
     In the ring-shaped area of a movable code plate  8  scanned by a light-emitting element (omitted from drawing) of a light source  1 , on the opposite side of the plate to the light source  1 , portions where a series of V-shaped grooves are formed constitute a first-type region  81   a,  while portions where no grooves are formed constitute a second type region  81   b.    
     As shown in FIG. 15, the V-shaped grooves which constitute the first-type region  81   a  each comprise a single oblique face for reflecting incident light from the light source  1 . The light reflected at the oblique faces of these V-shaped grooves enters the photoreceptor element  23 B (or photoreceptor element  23 A) of the light-receiving section  2 . 
     On the other hand, in the second-type region  81   b  where none of the aforementioned V-shaped grooves are formed, the incident light from the light source  1  is transmitted by the movable code plate  8 , and therefore it does not reach the photoreceptor elements  23 A,  23 B of the light-receiving section  2 . 
     The position of the light-receiving section  2  may also be changed in such a manner that it receives light transmitted by the second-type region  81   b.    
     A seventh embodiment of the present invention is now described with reference to FIG.  16  and FIG.  17 . 
     In this embodiment, light transmitted by the first-type region  80  in the fifth embodiment (FIG. 12) is also received by a further light-receiving section  2 ′ which is separate from the light-receiving section  2 . In other words, incident light from the light source  1  that is reflected by the second-type region  80   b  is received by the photoreceptor element  23 B (or photoreceptor element  23 A) of the light-receiving section  2 , and furthermore, light that is transmitted by the first-type region  80   a  is received by a photoreceptor element  23 D (or photoreceptor element  23 C) of the light-receiving section  2 ′. 
     By using these two light-receiving sections  2 ,  2 ′, it is possible to obtain complementary signals. Namely, since photoreceptor element  28 A and photoreceptor element  23 C, and photoreceptor element  23 B and photoreceptor element  23 D, output complementary signals, respectively, it is possible to raise detection accuracy. 
     The outputs from photoreceptor elements  23 A,  23 B,  23 C,  23 D are as shown respectively in FIGS.  17 ( a ), ( b ), ( c ), ( d ). FIG. 17 shows rectangular waveforms which were transformed from triangular waveforms of the photoreceptor elements  23 A,  23 B,  23 C,  23 D by a well-known signal processing circuit (not shown). 
     An eighth embodiment of the present invention is now described with reference to FIG. 18 to FIG.  20 . 
     In the ring-shaped area of a movable code plate  9  scanned by a light-emitting element (omitted from drawing) of a light source  1 , on the side of the, plate facing the light source  1 , portions where a series of V-shaped grooves are formed constitute a first-type region  90   a,  while portions where no grooves are formed constitute a second type region  90   b.    
     In the first-type region  90   a  where V-shaped grooves are formed, as illustrated in FIG. 19, incident light from the light source  1  is refracted in different directions by the left and right-hand oblique faces of the V-shaped grooves, and the refracted light is transmitted by the movable code plate  9 . 
     The gradient of the V-shaped grooves formed in the first-type region  90   a  and the positioning of the photoreceptor elements  23 A and  23 B with respect to the movable code plate  9  are devised in such a manner that, when either of photoreceptor elements  28 A or  23 B comes directly under the first-type region  90   a,  the light refracted by the first-type region  90   a  and transmitted by the movable code plate  9  does not reach either of the photoreceptor elements  23 A or  23 B. 
     FIG.  20 ( a ) shows the signal output by the photoreceptor element  23 A when that photoreceptor element  23 A comes directly under the second-type region  90   b.  FIG.  20 ( b ) shows the signal output by photoreceptor element  23 B when it comes directly under the second-type region  90   b  (at this time, the photoreceptor element  23 A comes directly under the first-type region  90   a  and produces no signal). FIG. 20 shows rectangular waveforms which were transformed from triangular waveforms of the photoreceptor elements  23 A and  23 B by a well-known signal processing circuit (not shown). 
     A ninth embodiment of the present invention is now described with reference to FIG.  21  and FIG.  22 . 
     In the eighth embodiment described above, light refracted by the first-type region  90   a  where V-shaped grooves are formed is not received by either of the photoreceptor elements  23 A,  23 B of the light-receiving section  2 , but this embodiment is devised such that this light is received by one or other of the photoreceptor elements. 
     Consequently, the intensity of the light received by the light-receiving section comprises a plurality of steps, depending on the positioning of the region where the V-shaped grooves are formed, and hence it is possible to obtain a signal corresponding to a position, by detecting the intensity of the received light. The photoreceptor elements  23 A′,  23 B′ of the light-receiving section  2  are positioned in such a manner that they can receive both a portion of the light refracted by the first-type region  90   a,  and the light travelling straight onwards through the second-type region  90   b.    
     Supposing a case where the positioning of the photoreceptor elements  23 A′,  23 B′ of the light-receiving section  2  with respect to the movable code plate  9  is as shown in FIG.  22 ( a ), (the movable code plate  9  having the same structure as the movable code plate  9  shown in FIG.  18 ,) then the intensity of light received by the photoreceptor elements  23 A′,  23 B′ will be as shown in FIGS.  22 ( c ) and ( e ), respectively. 
     For example, let it be supposed that the first-type region  90   a  shown by diagonal hatching and the second-type region  90   b  shown as blank are positioned at a prescribed pitch over two columns on the movable code plate  9 . 
     When light from the light-receiving section  1  is incident on the position on the movable code plate  9  indicated by “a”, only light travelling directly through the second region  90   b  is incident at the photoreceptor elements  23 A′,  23 B′. 
     When light from the light-receiving section  1  is incident on the position on the movable code plate  9  indicated by “b”, no light is incident at the photoreceptor element  23 A′. However, the light travelling directly through the second region  90   b,  and (a portion of) the light refracted by the first region  90   a,  is incident at the photoreceptor element  23 B′. The intensity of this refracted light is weaker than the intensity of the direct light. 
     When light from the light-receiving section  1  is incident on the position on the movable code plate  9  indicated by “c”, light refracted by the first-type region  90   a  directly above the photoreceptor element  23 B′ is incident at the photoreceptor element  23 A′, and light refracted by the first-type region  90   a  directly above the photoreceptor element  23 A′ is incident at the photoreceptor element  23 B′. 
     When light from the light-receiving section  1  is incident on the position on the movable code plate  9  indicated by “d”, then conversely to the case where it arrives at position “b”, the photoreceptor element  23 A′ receives the light travelling directly through the second region  90   b  and also (a portion of) the light refracted by the first region  90   a.  The photoreceptor element  23 B′ receives no light. 
     Therefore, in this ninth embodiment, the intensity of the received light detected by the light-receiving section can be classified into four steps corresponding to position, thereby making it possible to determine a position by detecting the intensity of received light. FIGS.  22 ( c ) and ( e ) show examples where the received light intensities in FIGS.  22 ( b ) and ( d ) have been assigned numerical values, thereby making it possible to determine coded positions. Although a few preferred embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes may be made in this embodiment without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.