Abstract:
A gas sensor comprises an atmosphere-side cover, ventilation filter, and filter cover as well as a sensing element and housing. The atmosphere-side cover has i) a first section of a first diameter located on a tip-end side in a axial direction of the housing and fixed on an axially base end of the housing, ii) a second section of a second diameter smaller than the first diameter, the second section being located on an axially base-end side, iii) a stepped section formed between the first and second sections, and iv) a rib protruding outward from the second section and being formed between the stepped section and the second section. The ventilation filter is mounted on the second section and has an axially tip-end section being in contact with an axially base-end section of the rib. The filter cover fixes the ventilation filter between the second section and the filter cover.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is based on and claims the benefit of priority from earlier Japanese Patent Application No. 2006-020354 filed on Jan. 30, 2006, the description of which is incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a gas sensor that can be used to control combustion in an internal combustion engine, such as an engine for a vehicle. 
   2. Description of the Related Art 
   Conventionally, there is a gas sensor that detects levels of various types of gas components within the exhaust gas. 
   One example of such gas sensors is shown in Japanese Patent Laid-open Publication No. 2002-82085. In this publication, as shown in  FIG. 12 , a gas sensor is installed in the exhaust system of an internal combustion engine, such as an engine for a vehicle. 
   Specifically, as shown in  FIG. 12 , the gas sensor  9  includes a sensing element  92 , a housing  93 , and an atmosphere-side cover  94 . The sensing element  92  detects a particular gas concentration within the gas to be measured. The sensing element  92  is inserted into the housing  93  and held therein. The atmosphere-side cover  94  is fixed onto the base-end side of the housing  93 . On the tip-end side of the housing  93 , a cover  97  serving as a gas-being-measured-side cover is secured to the housing  93 . This cover  97  is formed as a double-structure cover composed of an inner cover  971  and an outer cover  972 . Gas introducing openings  973  are formed through both inner and outer covers  971  and  972 . Hence, a gas  974  to be measured is introduced into the inside of the inner cover  71  through the openings  973 . 
   As shown in  FIG. 12 , the atmosphere-side cover  94  includes a large diameter section  941 , a small diameter section  942 , and a stepped section  943 . The large diameter section  941  is fixed onto the housing  93 . The small diameter section  942  is mounted on the base-end side of the atmosphere-side cover  94 . The stepped section  943  is formed to connect the large diameter section  941  and the small diameter section  942 . 
   The base-end side of the atmosphere-side cover  94  is covered by a filter cover  96 . In addition, as shown in the diagram, a waterproof ventilation filter  95  is sandwiched between the atmosphere-side cover  94  and the filter cover  96 . The ventilation filter  95  is fastened by fastening sections  960  in a caulking manner, provided in two locations in the axis direction. As shown in  FIG. 12 , the ventilation filter  95  is positioned so that the tip-end section of the ventilation filter  95  is in contact with the stepped section  943 . 
   Air-introducing openings  944  are formed through both covers  94  and  96 , so that atmosphere  945  is introduced into the inside of the atmosphere-side cover  94  through the openings  944 . The atmosphere  945  serves as a reference gas. 
   As is known, the sensing element  92  has the capability of sensing the concentration of a particular (i.e., designated) component of a gas to be measured. A sensed signal from the sensing element  92  is supplied to an external unit via lead portions  921 , connectors  922 , and lead wires  923 . 
   When the gas sensor  9  is in use, the temperature of the tip-end side of the gas sensor  9  becomes high. The heat is transmitted to the base-end side, and the temperature of the ventilation filter  85  also becomes high. Therefore, the ventilation filter  95  becomes deteriorated by the heat, through repeated use of the gas sensor  9  in the exhaust system of the internal combustion engine. There is risk of the deterioration of the waterproof properties of the ventilation filter  95 . 
   As a result, it is necessary for the ventilation filter  95  to be placed as close to the base end of the gas sensor  9  as possible. Further provision of a new stepped section on the base-end side of the stepped section  943  and disposition of the ventilation filter  95  near the base end of the gas sensor  9  can be considered. However, a narrow crevice is easily formed between the atmosphere-side cover  94  and the filter cover  96  when the stepped section is increased. When moisture or the like leaks into the crevice, it is difficult to drain the moisture or the like. Therefore, there is a risk of crevice corrosion caused by the moisture or the like. 
   SUMMARY OF THE INVENTION 
   The present invention has been achieved in light of the foregoing issues. An object of the present invention is to provide a gas sensor that secures ventilation between an atmosphere-side cover and a filter cover and has improved heat-resistance. 
   The present invention is a gas sensor including a sensing element, a housing, an atmosphere-side cover, a ventilation filter, and a filter cover. The sensing element detects a particular gas concentration within the gas to be measured. The sensing element is inserted into the housing and held therein. The atmosphere-side cover is fixed onto the base-end side of the housing. The ventilation filter is mounted on the outer periphery of the base-end section of the atmosphere-side cover. The filter cover fixes the ventilation filter between the atmosphere-side cover and the filter cover. 
   In the present invention, the side to be installed within the exhaust pipe of the internal combustion engine of an automobile or the like is described as the tip-end side. The side opposite of the tip-end side is described as the base-end side. 
   The atmosphere-side cover includes a large-diameter section (i.e., a first section according to the present invention) on the tip-end side and a small-diameter section (i.e., a second section according to the present invention) on the base-end side. The large-diameter section is fixed onto the housing. The filter cover is disposed on the outer periphery of the small-diameter section. A stepped section (i.e., a step-wise section) is formed between the large-diameter section and the small-diameter section. 
   A rib that protrudes outward from the small-diameter section is formed between the stepped section and the small-diameter section. The tip-end section of the ventilation filter is in contact with the base-end section of the rib. 
   In this way, the rib is formed between the stepped section and the small-diameter section, and the tip-end section of the ventilation filter is in contact with the base-end section of the rib. Therefore, the ventilation filter can be disposed closer to the base end of the gas sensor by the length of the rib in the axis direction. The ventilation filter can be separated from the tip-end section of the gas sensor that has been heated to a high temperature. As a result, transmission of heat from the tip-end section of the gas sensor to the ventilation filter and heat deterioration of the ventilation filter can be suppressed. A gas sensor having superior heat-resistance can be obtained. 
   In addition, by the implementation of the above-described configuration, a space that connects to the outside can be formed between the atmosphere-side cover and the tip-end section of the filter cover. Therefore, ventilation between the atmosphere-side cover and the tip-end section of the filter rover can be secured. Thus, corrosion degradation of the ventilation filter, the atmosphere-side cover, and the like can be prevented. 
   By the tip-end section of the ventilation filter being in contact with the base-end section of the rib, the ventilation filter can be easily positioned and, in addition, disposed at a predetermined position in a stable state. 
   In this way, according to the present invention, a gas sensor that secures ventilation between an atmosphere-side cover and a filter cover and has superior heat-resistance can be provided. 
   The length of the rib in the axis direction is preferably 3 to 10 mm. In this case, a gas sensor that prevents the decline in rigidity and strength on the base-end side while having a sufficiently superior heat-resistance can be acquired. 
   At the same time, when the length in the axis direction is less than 3 mm, it becomes difficult to dispose the ventilation filter so that tip-end section of the gas sensor, which has a high temperature, and the ventilation filter are sufficiently separated. There is also risk of it becoming difficult to prevent the heat-deterioration of the ventilation filter. 
   In addition, when the length in the axis direction exceeds 10 mm, the measurement of the base-end side of the gas sensor becomes long. There is risk of decline in the rigidity and the strength of the gas sensor on the base-end side. 
   In addition, three to eight ribs are preferably formed. In this case, the ventilation filter can be disposed with stability, and the ribs can be easily formed. 
   At the same time, when the number of formed ribs is less than three, there is risk of it becoming difficult for the ventilation filter to be in contact with the base-end section of the rib in a stable state. 
   In addition, when the number of formed ribs exceeds eight, there is risk of it becoming difficult to form the ribs. 
   Length A in the circumferential direction of the formation area of the rib and length B in the circumferential direction of the non-formation area between ribs of the atmosphere-side cover preferably have a relationship of 0.15≦A/B≦1. 
   In this case, the ventilation between the atmosphere-side cover and the tip-end section of the filter cover can be sufficiently secured, while positioning the ventilation filter easily and in a stable state. 
   At the same time, when A/B is less than 0.15, there is a risk that it becomes difficult to position the ventilation filter in a stable state. 
   In addition, when A/B exceeds 1, the space formed between the atmosphere-side cover and the tip-end section of the filter cover becomes small. There is risk of it becoming difficult to sufficiently secure ventilation between the atmosphere-side cover and the filter cover. 
   In addition, the tip-end section of the filter cover can be in contact with the base-end section of the rib. In this case, a space than can ventilate to the outside can be formed between the atmosphere-side cover and the tip-end section of the filter cover. Therefore, ventilation between the atmosphere-side cover and the tip-end section of the filter cover can be sufficiently secured. 
   In addition, the filter cover can be configured to be disposed so that the tip-end section of the filter cover is in contact the outer surface of the rib. In this case, the ventilation filter can be easily positioned in the base-end section of the rib when assembling the gas sensor. 
   In addition, the filter cover can be disposed so that the tip-end section of the filter cover does not contact the atmosphere-side cover. In this case, a space than can ventilate to the outside can be formed between the atmosphere-side cover and the tip-end section of the filter cover. Therefore, ventilation between the atmosphere-side cover and the tip-end section of the filter cover can be sufficiently secured. 
   In the present invention, for example, an O 2  sensor, an air/fuel (A/F) sensor, an NOx (nitrogen oxides), or a CO (carbon monoxide) sensor is used as the above-described gas sensor. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings: 
       FIG. 1  is a cross-sectional explanatory view of a gas sensor according to a first embodiment of the present invention; 
       FIG. 2  is a cross-sectional explanatory view taken along line X-X in  FIG. 1 ; 
       FIG. 3  is a cross-sectional explanatory view taken along line Y-Y in  FIG. 2 ; 
       FIG. 4  is a cross-sectional explanatory view of the base-end section of a gas sensor according to a second embodiment of the present invention; 
       FIG. 5  is a cross-sectional explanatory view taken along line Z-Z in  FIG. 4 ; 
       FIG. 6  is a cross-sectional explanatory view of the base-end section of a gas sensor according to a third embodiment of the present invention; 
       FIG. 7  is a cross-sectional explanatory view of the atmosphere-side cover in a direction perpendicular to the axis direction when the cross-sectional shape of the rib is changed in a fourth embodiment of the present invention; 
       FIG. 8  is a cross-sectional explanatory view of the atmosphere-side cover in the direction perpendicular to the axis direction when the cross-sectional shape of the rib is changed in the fourth embodiment; 
       FIG. 9  is a cross-sectional explanatory view of the atmosphere-side cover in the direction perpendicular to the axis direction when the cross-sectional shape of the rib is changed in the fourth embodiment; 
       FIG. 10  is a cross-sectional explanatory diagram of the base-end section of a gas sensor according to a fifth embodiment of the present invention; 
       FIG. 11  is a line graph showing the measurement results according to the fifth embodiment; and 
       FIG. 12  is a cross-sectional explanatory diagram of a gas sensor according to a conventional example. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to the drawings, various embodiments of the gas sensor according to the present invention will now be described. 
   First Embodiment 
   The gas sensor according to a first embodiment of the present invention will be described with reference to  FIG. 1  to  FIG. 3 . 
   As shown in  FIG. 1 , a gas sensor  1  according to the present embodiment includes a sensing element  2 , a housing  3 , an atmosphere-side cover  4 , a ventilation filter  5 , and a filter cover  6 . 
   In the present embodiment, the sensing element  2  is formed as a tubular member having a length providing an axial direction of the gas sensor  1 . In  FIG. 1 , the lower side is described as a tip-end side to be installed within the exhaust pipe of the internal combustion engine of an automobile or the like, while the upper side is described as a base-end side. 
   The sensing element  2  detects the concentration of a particular gas within a gas to be measured. The sensing element  2  is inserted inside the housing  3  and held therein. The atmosphere-side cover  4  is fixed onto the base-end side of the housing  3 . The ventilation filter  5  is mounted on the outer periphery of the base-end section of the atmosphere-side cover  4 . The filter cover  6  fixes the ventilation filter  5  between the atmosphere-side cover  4  and the filter cover  6 . 
   On the tip-end side of the housing  3 , a cover  7  serving as a gas-being-measured-side cover is secured to the housing  3 . This cover  7  is formed as a double-structure cover composed of an inner cover  71  and an outer cover  72 . Gas introducing openings  73  are formed through both inner and outer covers  71  and  72 . Hence, a gas  74  to be measured is introduced into the inside of the inner cover  71  through the openings  73 . 
   As shown in  FIG. 1  and  FIG. 3 , the atmosphere-side cover  4  includes a large-diameter section  41  on the tip-end side and a small-diameter section  42  on the base-end side. The large-diameter section  41  and small-diameter section  42  correspond to a first section and a second section according to the present invention, respectively. The large-diameter section  41  is fixed onto the housing  3 . The filter cover  6  is disposed on the outer periphery of the small-diameter section  42 . A stepped section  43  is formed between the large-diameter section  41  and the small-diameter section  42 . 
   As shown in  FIG. 1  and  FIG. 3 , a rib  44  that protrudes outward from the small-diameter section  42  is formed between the stepped section  43  and the small-diameter section  42 . 
   The tip-end section of the ventilation filter  5  is in contact with the base-end section of the rib  44 . 
   In addition, the filter cover  6  is disposed so that the tip-end section of the filter cover  6  is in contact with the outer surface of the rib  44 . 
   As shown in  FIG. 1  and  FIG. 2 , the above-described rib  44  is formed so as to protrude outward from the small-diameter section  42 , between the stepped section  43  and the small-diameter section  42 . The length h of the rib  44  in the axis direction is 3 to 10 mm. In the present embodiment, as shown in  FIG. 2 , four equally-sized, arc-shape ribs  44  are formed equal distances apart, on the small-diameter section  43 . 
   A (=4° a.) is the total of the lengths in the circumferential direction of the four formation areas, when the length in the circumferential direction of the formation area of one rib  44  is “a.” B (=4b) is the total of the lengths in the circumferential direction of the four non-formation areas, when the length in the circumferential direction of one non-formation area between the ribs  44  is “b.” The dimensions A and B have a relationship of 0.15≦A/B≦1. In other words, in the present embodiment, the ribs  44  are formed to be equal in size and equal distances apart. Therefore, the length a in the circumferential direction of the formation area of the rib and the length b in the circumferential direction of the non-formation area between the ribs have a relationship of 0.15≦a/b≦1. 
   After the atmosphere-side cover  4  is formed, the rib  44  can be formed, for example, by the atmosphere-side cover  4  being cast into a mold, and pressed and spread. 
   As shown in  FIG. 1  and  FIG. 2 , the ventilation filter  5  is sandwiched between the atmosphere-side cover  4  and the filter cover  6 . The ventilation filter  5  is fastened and fixed by fastening sections  60  provided in two locations in the axis direction. The ventilation section that is waterproof and introduces air (atmosphere)  45  is formed by an air introduction opening  600 , the ventilation filter  5 , and an atmosphere communication opening  400 . The atmosphere  45  serves as a reference gas. The air introduction opening  600  is provided on the filter cover  6 . The atmosphere communication opening  400  is provided on the atmosphere-side filter  4 . As the ventilation filter  5 , a filter formed from, for example, polytetrafluoroethylene (PTFE) can be used. 
   The sensing element  2  is known as shown in U.S. Pat. No. 5,573,650 and has the capability of sensing the concentration of a particular (i.e., designated) component of a gas to be measured. For example, the sensing element  2  is a laminated type of heater-incorporated tubular member with two electrodes respectively exposed in the gas  74  to be measured and the atmosphere  45 . Ionic current and a potential difference caused between both electrodes provide a sensing signal for measurement of the concentration of a particular gas component of the gas to be measured. The sensing signal from the sensing element  2  is supplied to, for example, an electronic control unit mounted in an automobile, via lead portions  21 , connectors  22 , and lead wires  23 . 
   In addition, as shown in  FIG. 1 , the gas sensor  1  includes, within, an element-side electrical porcelain  12 , an atmosphere-side electrical porcelain  13 , and a Belleville spring  14 . The sensing element  2  is inserted into the element-side electrical porcelain  12  and held therein. The atmosphere-side electrical porcelain  13  is disposed on the base-end side of the element-side electrical porcelain  12 . The Belleville spring  14  is disposed on the base-end surface of the atmosphere-side electrical porcelain  13 . The Belleville spring  14  is disposed between the atmosphere-side electrical porcelain  13  and the stepped section  43  in a state in which the atmosphere-side electrical porcelain  13  is biased to be pressed in the direction of the element-side electrical porcelain  12 . The heat of the tip-end section of the gas sensor  1  that has been heated by a high-temperature gas to be measured is transmitted to the Belleville spring  14 , via the element-side electrical porcelain  12  and the atmosphere-side electrical porcelain  13 . Subsequently, the heat is transmitted from the Belleville spring  14  to the atmosphere-side cover  4  that sandwiches the ventilation filter  5 , via the stepped section  43 . 
   Next, the effect of the present embodiment will be described. 
   As shown in  FIG. 1  and  FIG. 2 , the rib  44  is formed between the stepped section  43  and the small-diameter section  42 . The tip-end section of the ventilation filter  5  is in contact with the base-end section of the rib  44 . Therefore, the ventilation filter  5  can be disposed closer to the base-end of the gas sensor  1  by the length h of the rib  44  in the axis direction. The ventilation filter  5  can be separated from the tip-end section of the gas sensor  1  that has been heated to a high temperature. As a result, the transmission of the heat from the tip-end section of the gas sensor  1  to the ventilation filter  5  and the heat deterioration of the ventilation filter  5  can be suppressed. A gas sensor  1  having superior heat-resistance can be acquired. 
   In addition, after the heat of the tip-end section of the gas sensor  1  is transmitted to the Belleville spring  14 , as described above, the heat is further transmitted to the atmosphere-side cover  4 , via the stepped section  43 . Here, through the implementation of the above-described configuration, the area in the stepped section  43  in which the rib  44  is formed is not in contact with the Belleville spring  14 . Therefore, the area of the entire stepped section  43  that is in contact with the Belleville spring  14  can be reduced. As a result, the transmission of heat from the tip-end of the gas sensor  1  to the ventilation filter  5  can be suppressed and the heat deterioration of the ventilation filter  5  can be further suppressed. 
   In addition, through the implementation of the above-described configuration, as shown in  FIG. 2  and  FIG. 3 , a space  7  that connects to the outside can be formed between the atmosphere-side cover  4  and the tip-end section of the filter cover  6 . Therefore, ventilation between the atmosphere-side cover  4  and the tip-end section of the filter cover  6  can be secured. Thus, corrosion degradation of the ventilation filter  5 , the atmosphere-side cover  4 , and the like can be prevented. 
   By the tip-end section of the ventilation filter  5  being in contact with the base-end section of the rib  44 , the ventilation filter  5  can be easily positioned and, in addition, disposed at a predetermined position in a stable state. 
   As shown in  FIG. 1 , the length h of the rib  44  in the axis direction is 3 to 10 mm. Therefore, a gas sensor  1  that prevents the decline in rigidity and strength on the base-end side while having a sufficiently superior heat-resistance can be acquired. 
   In addition, as shown in  FIG. 2 , four ribs  44  are formed. Therefore, the ventilation filter  5  can be disposed with stability, and the ribs  44  can be easily formed. 
   In addition, in the atmosphere-side cover  4  according to the present embodiment, the length A in the circumferential direction of the formation area of the rib  44  and the length B in the circumferential direction of the non-formation area between the ribs  44  have a relationship of 0.15≦A/B≦1. Therefore, ventilation between the atmosphere-side cover  4  and the tip-end section of the filter cover  6  can be sufficiently secured while positioning the ventilation filter  5  easily and in a stable state. 
   In addition, as shown in  FIG. 1  to  FIG. 3 , the filter cover  6  is disposed so that the tip-end section of the filter cover  6  is in contact with the outer surface of the rib  44 . As a result, the ventilation filter  5  can be easily positioned in the base-end section of the rib  44  when assembling the gas sensor  1 . 
   As described above, according to the present embodiment, a gas sensor that can secure ventilation between the atmosphere-side cover and the filter cover and has superior heat-resistance can be provided. 
   The configuration of the present invention can be applied not only to the gas sensor  1  having the stacked-type sensing element  2 , as in the first embodiment, but also a gas sensor having a cup-shaped sensing element, which is formed into a cylinder with a bottom. 
   In addition, the ribs  44  are not limited to four ribs as in the first embodiment. The effects of the present invention can be achieved if one or more ribs are formed. 
   Second Embodiment 
   Referring to  FIGS. 4 and 5 , a second embodiment of the present invention will now be described. In the second and subsequent embodiments, the similar or identical components to those in the first embodiment are given the same reference numerals for the sake of a simplified explanation. 
   As shown in  FIG. 4  and  FIG. 5 , there is provided a gas sensor  1  according to a second embodiment, in which the tip-end section of the filter cover  6  is in contact with the base-end section of the rib  44 . 
   The rib  44  is formed to protrude farther than the ribs in the first embodiment (refer to reference number  44  in  FIG. 1  and  FIG. 2 ), in the base-end side direction of the axis direction and the outside-direction of the circumferential direction. In the present embodiment, the outer surface of the ribs  44  in the circumferential direction and the outer periphery surface of the large-diameter section  41  have the same amount of protrusion in the circumferential direction. 
   This embodiment is otherwise the same as the first embodiment. 
   In the present embodiment, the space  7  that can allow ventilation with the outside can be formed between the atmosphere-side cover  4  and the tip-end section of the filter cover  6 . Therefore, ventilation between the atmosphere-side cover  4  and the tip-end section of the filter cover  6  can be sufficiently secured. 
   The effects of this embodiment are otherwise the same as those of the first embodiment. 
   Third Embodiment 
   Referring to  FIG. 6 , a third embodiment of the present invention will now be described. 
   As shown in  FIG. 6 , there is provided a gas sensor  1  according to a third embodiment, in which the filter cover  6  is disposed so that the tip-end section of the filter cover  6  does not contact the atmosphere-side cover  4   
   This embodiment is otherwise the same as the first embodiment. 
   In the present embodiment, the space  7  that can allow ventilation with the outside can be formed between the atmosphere-side cover  4  and the tip-end section of the filter cover  6 . Therefore, ventilation between the atmosphere-side cover  4  and the filter cover  6  can be sufficiently secured. 
   The effects of this embodiment are otherwise the same as those of the first embodiment. 
   Fourth Embodiment 
   Referring to  FIGS. 7 to 9 , a fourth embodiment of the present invention will now be described. 
   As shown in  FIG. 7  to  FIG. 9 , a gas sensor  1  according to a fourth embodiment relates to the atmosphere-side cover  4  including the rib  44  of which the cross-section in the direction perpendicular in the axis direction is variably changed. In other words, the cross-section of the rib  44  can be elliptical as shown in  FIG. 7 , rectangular as shown in  FIG. 8 , or triangular as shown in  FIG. 9 . 
   The configuration and the effects of this embodiment are otherwise the same as those of the first embodiment. 
   Fifth Embodiment 
   Referring to  FIGS. 10 and 11 , a fifth embodiment of the present invention will now be described. 
   As shown in  FIG. 10  and  FIG. 11 , in a fifth embodiment, the length h of the rib  44  in the axis direction is variably changed and the amount of decrease in the temperature of the ventilation filter  5  is studied. 
   The amount of decrease in the temperature is the difference between the temperature of the ventilation filter in the conventional gas sensor (refer to the reference number  9  in  FIG. 12 ) in which the length h in the axis direction is 0 and the temperature of the ventilation filter  5  in the gas sensor  1  when the length h in the axis direction is variably changed, as shown in  FIG. 10 . The amount of decrease in the temperature is measured under conditions for actual use of the gas sensor. 
   The measurement results of the decrease amount in the temperature are shown in  FIG. 11 . As can be seen from the diagram, it is clear that the amount of decrease in the temperature of the ventilation filter  5  becomes larger by the length of the rib  44  in the axis direction  44  being increased. In other words, as the position of the tip-end section of the ventilation filter  5  is moved to the base-end side, it is clear that the transmission of heat from the tip-end section of the gas sensor  1  to the ventilation filter  5  can be suppressed. Furthermore, when the length h in the axis direction is 3 mm or more, the amount of decrease in temperature is 28° C. or more. The effects of the present invention can be sufficiently achieved. 
   The present invention may be embodied in several other forms without departing from the spirit thereof. The embodiments and modifications described so far are therefore intended to be only illustrative and not restrictive, since the scope of the invention is defined by the appended claims rather than by the description preceding them. All changes that fall within the metes and bounds of the claims, or equivalents of such metes and bounds, are therefore intended to be embraced by the claims.