Patent Publication Number: US-2007099735-A1

Title: Transmission system and method for measuring a drive force therein

Description:
The present invention relates in general to a transmission system of the span type. Such a system comprises two rotatable parts, which are jointly spanned by an endless transmission member closed in itself. This transmission member can e.g. be implemented as string, belt or chain, and the rotatable parts are accordingly implemented as discs, drums, pulleys, or chain wheels or the like. In the case of strings or belts, the transmission system exists as one piece; in the case of a chain, the transmission system exists as a system of links coupled to each other. Coupling between the transmission member and the rotatable parts can be based on friction, but also a form-coupling can be applied, wherein e.g. sprockets of a chain wheel engage in holes in the transmission member.  
      The present invention is particularly applicable to a chain transmission, wherein a chain couples two chain wheels. Therefore, simply the phrases “coupling chain”, or in short “chain”, and “chain wheel” will hereinafter be used. These phrases are however not used to limit the invention to this type, but are used here as phrases also including the embodiment as belt or string and corresponding pulleys or the like.  
      The present invention is particularly applicable to a chain transmission in a bicycle or another vehicle driven by human power, and to a chain transmission in a home trainer or the like. In such applications, but also in industrial force transmissions, there is a need for a measuring instrument for measuring the transmitted torque. A measure for that is the tension being present in the chain. Several measuring instruments based on measuring the force exerted on the driven wheel have already been described. Such measuring instruments presume however, that there is no bias tension in the chain, i.e. no force is exerted on the driven wheel if the driving wheel is not being driven itself. However, in situations where the chain in rest is also strongly tightened, in both chain halves the same tension is present, and the driven wheel experiences a force which equals the sum of those tensions, while in fact the drive torque equals zero.  
      Only when the driving wheel is driven, e.g. by the pedal force of a cyclist, the tension in one chain half becomes higher than the tension in the other chain half, whereby the driven wheel experiences a resulting drive torque which is substantially equal to the difference between the two tensions in the two chain halves, multiplied by the diameter of the driven wheel.  
      Furthermore, it has appeared in the case of bicycles that the chain wheels can have a certain eccentricity, whereby variations in the bias tension occur during use, which influence the measurement results of known measuring instruments. This occurs especially in applications where the chain is tightened strongly in order to absorb shocks by changes in the coupling direction, and in friction transmissions where a bias tension is necessary because of the required friction force.  
      The present invention therefore aims at providing a torque measuring instrument which is substantially insensitive to the magnitude of the bias tension in the chain.  
      More particularly, the present invention aims at providing a measuring instrument which is capable of measuring the tension difference. Such a measuring instrument will hereinafter be referred to as tension difference measuring device.  
      A tension difference measuring device has already been described in U.S. Pat. No. 4,909,086. The tension difference measuring device known from this publication comprises two freely rotating pulleys, arranged on the outer side of the chain and rotatably mounted on a common support. The mutual distance between those pulleys is smaller than the nominal distance between the chain halves, so that each chain half is forced to follow a part of the periphery of the corresponding pulley. As a result of the tension present in a chain half, this chain half exerts on the corresponding pulley an outwards directed force resultant; that force will hereinafter be referred to as transverse force. When the chain exerts a drive force and the tension increases in one chain half and decreases in the other chain half, the transverse force exerted on the corresponding pulley by the one chain half will increase and the transverse force exerted on the corresponding pulley by the other chain half will decrease, whereby the whole of the two pulleys and the common support is displaced in the direction of the transverse force exerted by the first chain half. The magnitude of the resulting displacement is a measure for the magnitude of the force exerted.  
      This known tension difference measuring device has some disadvantages. The number of components is fairly large, which makes the measuring device relatively expensive. Because the pulleys are placed at the outer side of the chain, their diameter must be fairly small, because otherwise the whole would take too much space; this drawback is particularly valid for a bicycle or a home trainer. Because of the small pulley diameter, the chain at the location of the pulleys is forced into a shape with a small curvature radius, which can lead to increased wear. Furthermore, the small pulley diameters imply that the pulleys rotate with a fairly high velocity, which is also a wear factor and is moreover accompanied with a fairly high sound production.  
      Moreover, each pulley is subjected to a fairly large force with respect to the support, namely the full transverse force, so that the pulley bearing of each pulley must be able to withstand this large transverse force, and is therefore relatively expensive. Moreover, the friction forces occurring are fairly large, whereby on one hand the performance of the transmission decreases, and whereby on the other hand a disturbing force is exerted on the sensor whereby the measurement accuracy decreases.  
      In the case of bicycles and home trainers, the two chain wheels usually have mutually different diameters, so that the two chain halves are not mutually parallel. Because of this, the two transverse forces are not in line, with the result that the resultant of the two transverse forces exerts a net moment to the pulley support, which influences the measurement signal.  
      The present invention aims to provide a tension difference measuring device wherein the said disadvantages are absent or at least strongly reduced.  
      According to an important aspect of the present invention, a tension difference measuring device comprises a transverse force sensor located inside the chain, the transverse force sensor being at least partly spanned by both chain halves, and the tension difference measuring device is further provided with means for measuring the transverse force exerted on the sensor. In an important embodiment, this transverse force sensor is a wheel rotatably mounted with respect to a support, which rotates along with the moving chain.  
      In a first embodiment variation, the transverse force sensor is a separate measuring wheel, which is placed in the chain plane within the span of the chain. The diameter has been chosen to be so large that each chain half is forced to follow a part of the periphery of the measuring wheel. In this case, the curvature radius of the chain is relatively large. The measuring wheel rotates with a relatively low velocity. The measuring wheel is mounted on a supporting arm, which in turn is fixedly attached with respect to the frame in which the driving wheel and the driven wheel are mounted (the bicycle frame). Providing an electric measurement signal which is representative for the displacement of the support can take place by measuring the deformation of this supporting arm, e.g. by means of strain gauges or by measuring the displacement of this supporting arm, e.g. by means of a laser.  
      In a second embodiment variation, the transverse force sensor is the chain-driven wheel itself. The means for measuring the transverse force exerted on the driven wheel in this case comprise a sensor for measuring the force exerted in a direction perpendicular to the wheel axle. Such sensors are known per se. A certain type of such sensors is based on measuring the bending of the wheel axle, like for instance described in WO01/30643 and PCT/NL02/00867. In this publication, this force sensor is intended for measuring the chain force, and is therefore mounted in such a way that its sensitivity direction is substantially horizontal, namely substantially directed parallel to the chain. This same sensor can be used as sensor for application with the present invention if this is rotated over 90°, and is thus mounted in such a way that its sensitivity direction is substantially vertical, namely substantially directed perpendicular to the chain. 
    
    
      These and other aspects, features and advantages of the present invention will be further explained by the following description with reference to the drawings, in which same reference numbers indicate same or similar parts, and in which:  
       FIG. 1  is a side view schematically showing a transmission system, in a state of rest, provided with a transverse force sensor mounted on a supporting arm, wherein the supporting arm is directly attached to a frame;  
       FIG. 2  schematically shows the transmission system of  FIG. 1 , in an active state in which a drive force is exerted;  
       FIG. 3  schematically shows a variation of a transverse force sensor;  
       FIG. 4A  is a schematic side view of a part of the transmission system of  FIG. 1 , with a vertical supporting arm for the transverse force sensor;  
       FIG. 4B  is a schematic front view of a part of the transmission system of  FIG. 1 , with a horizontal supporting arm for the transverse force sensor, perpendicular to the plane of the chain;  
       FIG. 5  is a schematic side view of the transmission system comparable to  FIG. 1 , wherein as variation the supporting arm of the transverse force sensor is attached to the axle of the driven wheel;  
       FIG. 6  is a schematic side view of the transmission system comparable to  FIG. 1 , wherein as variation the axle of the driven wheel is used as transverse force sensor;  
      the  FIGS. 7A and 7B  show details of a supporting arm for the transverse force sensor. 
    
    
       FIG. 1  schematically shows a transmission system  1 , comprising a drive wheel  2  and a driven wheel  3 , coupled to a drive chain  4 . The transmission system  1  can be part of a bicycle, wherein the drive wheel  2  is driven by a user by means of pedals, but this is not shown in the figure for sake of simplicity. It is common practice that the drive wheel  2  then has a larger diameter than the driven wheel  3 .  
      The chain  4  successively comprises a first part  4 A extending along a part of the drive wheel  2 , a second part  4 B extending along a part of the driven wheel  3 , a third part  4 C extending between the wheels  2  and  3 , and a fourth part  4 D extending between the wheels  2  and  3 . In this example, it is assumed that the axles of the wheels  2  and  3  are situated next to each other in a horizontal plane, and that the third part  4 C is located above the fourth part  4 D. Hereinafter, the third and fourth chain parts  4 C and  4 D are also referred to as first and second chain half, respectively.  
      The wheels  2  and  3  are rotatably mounted to a frame  5 , in such a way that in rest there is a bias tension in the chain  4 . The tension in the first chain half  4 C is referred to as F C , and the tension in the second chain half  4 D is referred to as F D .  
      The transmission system  1  is provided with a measuring system  6 , adapted for measuring the forces F c  en F D  in the chain  4 , which are a measure for the torque transmitted by the chain  4 . This measuring system  6  comprises a transverse force measuring wheel  10 , arranged within the span of the chain  4 , substantially in the same plane as the wheels  2  and  3 . The diameter thereof is so large that both the first chain half  4 C and the second chain half  4 D follow a curved trajectory between the wheels  2  and  3 , and extend for a part along the periphery of the measuring wheel  10 . In a possible embodiment, the measuring wheel  10  has the same diameter as the largest of the wheels  2  and  3 , and, in case of a measuring wheel provided with sprockets, the measuring wheel can be equal to the largest of the wheels  2  and  3 . A first transverse force F DC  is exerted to the measuring wheel  10  by the tension F C  in the first chain half  4 C, and a second transverse force F DD  is exerted to the measuring wheel  10  by the tension F D  in the second chain half  4 D. These two forces go through the center of the measuring wheel  10 .  
      The measuring wheel  10  is rotatably mounted to a supporting arm  20 , which in turn is fixed with respect to the frame. Preferably, the center point of the measuring wheel  10  in rest is located on a line L connecting the rotation center points of the wheels  2  and  3 . Fixation of that supporting arm  20  takes place when the system is in rest, i.e. when no drive force is exerted. Then the tensions F C  en F D  in the two chain halves  4 C and  4 D are equal to each other, and the resultant F DR  of the two transverse forces F DC  and F DD  lies in the horizontal plane, represented in the figure by said line L. The center of the measuring wheel  10  is then situated, as said, on said line L. In this situation the supporting arm  20  is fixed to the frame.  
      When a drive force is exerted to the drive wheel  2 , which is transmitted by the chain  4 , the tension in one chain half becomes higher than the tension in the other chain half. Assume that the drive wheel  2  is driven counter clockwise, as indicated in  FIG. 2 . The tension F C  in the first chain half  4 C then becomes higher than the tension F D  in the second chain half  4 D. Consequently, also the corresponding transverse force F DC  becomes larger than F DD , so that the resultant F DR  of these two transverse forces gets a component F V  directed perpendicular to the horizontal line L, in this case directed downward. Hereby, a bending of the supporting arm  20  occurs. This bending is measurable by a deformation sensor  30  mounted on the supporting arm  20 , which can be implemented as a strain gauge or a system of strain gauges, as known per se.  
      The electric measurement signal S M  given by the measuring sensor  30  is a measure for the force exerted, and is supplied to a processor  40  for further processing. In case of a home trainer, this processor  40  can for instance be adapted for calculating the amount of calories consumed.  
      The rotatable measuring wheel  10  is mounted on the supporting arm  20  by means of a bearing which is not shown for the sake of simplicity. This can be a relatively simple bearing, since the measuring wheel  10  does not experience a large force with respect to the supporting arm  20 : this bearing is only loaded by the resultant F DR .  
      Although it is preferred that the measuring wheel  10  is rotatable, this is not necessary for the operation of the measuring wheel  10  within the scope of the present invention. If the measuring wheel  10  is fixed, and the chain  4  slides over the measuring wheel  10 , a force resultant F DR  as described above emerges as well.  
      In case of a non-rotatable force sensor  10 , it does not need to have a circular outline. The force sensor  10  can then for instance have a four-sided outline, of which two contact faces  11  and  12  situated opposite each other can have a convex shape, for instance the shape of an arc, and of which the other sides  13  and  14  can have an arbitrary shape, for instance a straight shape, as illustrated in  FIG. 3 . Instead of a convex shape with constant curvature radius, said contact faces  11  and  12  situated opposite each other may also have a curvature radius varying as function of the location, and they can for instance have the shape of a sinusoid or hyperboloid.  
      Preferably, the force sensor  10  is made of a material which counteracts sound production, at least that part of the force sensor which comes into contact with the chain is provided with such a layer. An example of a suitable material is synthetic material.  
      The supporting arm  20  can in principle have an arbitrary direction. In a first embodiment variation, schematically illustrated in  FIG. 4A , the supporting arm  20  is situated in the plane of the chain wheels  2  and  3  and the chain  4 , substantially directed horizontally, i.e. perpendicular to the wheel axle connection line L. In that case, the deformation occurring in the supporting arm  20  as a result of the force component F V  to be measured will mainly be a change of length, and the deformation sensor  30  needs to be adapted for measuring change of length, as will be clear to a person skilled in the art.  
      In a second embodiment variation, schematically illustrated in  FIG. 4B , the supporting arm  20  is situated perpendicular to the plane of the chain wheels  2  and  3  and the chain  4 . In that case, the deformation occurring in the supporting arm  20  as a result of the force component F V  to be measured will mainly be a bending, and the deformation sensor  30  needs to be adapted for measuring bending, as will be clear to a person skilled in the art.  
      In a third embodiment variation, schematically illustrated in the  FIGS. 1 and 2 , the supporting arm  20  is situated in the plane of the chain wheels  2  and  3  and the chain  4 , substantially directed horizontally, i.e. directed according to the wheel axle connection line L. In that case, the deformation occurring in the supporting arm  20  as a result of the force component F V  to be measured will mainly be a bending, and the deformation sensor  30  needs to be adapted for measuring bending, as will be clear to a person skilled in the art.  
      This third embodiment variation is preferred. In this case, it is preferred that the supporting arm  20  is not directly attached to the frame, but to the axle of the driving wheel  2  or the driven wheel  3  fixed with respect to the frame, as schematically illustrated in  FIG. 5 . Here, the advantage is achieved that, if the wheel is adjusted with respect to the frame, for instance to tighten the chain, it is not necessary to adjust the mounting of the force sensor  10  as well. Moreover, in this case no extra mounting point is necessary, and the measuring arrangement, when a connection bolt of the wheel is loosened, automatically seeks the position where the bending beam  20  is unloaded.  
      In another embodiment variation of the present invention, schematically illustrated in  FIG. 6 , no separate measuring wheel  10  is necessary for measuring the vertical force component F V  if the diameters of the driving wheel  2  and the driven wheel  3  are mutually different. The explanation given in the preceding with respect to the occurrence of a force resultant F V  is also applicable to the driving wheel  2  and the driven wheel  3 : also these wheels experience a vertical force component which is a measure for the tension difference. In this case, however, this vertical force component occurs as resultant of the normal forces being exerted by the first chain part  4 A and the second chain part  4 B, respectively. This force component can be measured with a force sensor  130  associated with the wheel  2  and  3  concerned, which is schematically shown in  FIG. 6  for the driven wheel  3 .  
      For measuring the forces exerted to a wheel, several force sensors have been developed. An example of such a force sensor is described in WO01/30643 and PCT/NL02/00867. Since these known force sensors can all in principle be used for application as force sensor  130 , it is not necessary here to give an extensive description of their design and operation.  
      The known force sensors are intended for measuring the chain force itself, i.e. the tension in the first chain half  4 C, based on the thought that there is no tension in the second chain half. Therefore, the known force sensors are mounted in such a way that their sensitivity direction is directed substantially horizontally, at least substantially corresponds with the direction of the first chain half  4 D. For application as force sensor  130  within the scope of the present invention the mounting needs to be such, that their sensitivity direction is directed substantially vertically. With respect to the known mounting, this only means a rotation over approximately 90°.  
      It will be clear to a person skilled in the art that the invention is not limited to the above discussed exemplary embodiments, but that several variations and modifications are possible within the scope of protection of the invention as defined in the attached claims.  
      For instance, the force sensor  10  can e.g. be provided with a groove in its surface over which the chain runs.  
      Furthermore, it is possible that the drive wheel  2  and/or the driven wheel  3  are attached to the frame  5  through a supporting arm, in a comparable way as the supporting arm  20  for the measuring wheel  10 .  
      Furthermore, it is possible that a force sensor for measuring the force exerted to the measuring wheel  10  is mounted on the axle of the measuring wheel or in the bearing of the measuring wheel; for such a force sensor, a force sensor as described in WO01/30643 and/or PCT/NL02/00867 can advantageously be applied. In case of mounting on the axle of the measuring wheel, the force sensor can be adapted for measuring the bending of this axle. In case of mounting on the bearing of the measuring wheel, the force sensor can be adapted for measuring the resulting force exerted to the measuring wheel.  
      Preferably, the horizontal position of the measuring wheel  10  is adjustable along the said line L. This can for instance offer advantages in case of the measuring wheel  10  being a sprocket wheel, the sprockets of which engaging in the links of the coupling chain  4 . Such an adjustability can be attained in a relatively easy way by providing the supporting arm  20  with a somewhat elongated hole (slotted hole), in which an attachment member for the measuring wheel  10  is fastened.  
       FIG. 7A  shows a schematic front view of a preferred embodiment of the supporting arm  20 , which in general has the shape of a beam with a rectangular cross section, wherein a first mounting hole  202  is arranged at a first end  201  for mounting the supporting arm  20  to a frame or to a bicycle axle or the like, and wherein an elongated mounting hole  204  for the measuring wheel  10  is arranged at a second end  203 , the longitudinal direction of said elongated mounting hole  204  substantially overlapping with a center line of the supporting arm  20 . In this preferred embodiment, the supporting arm  20  comprises a cut-away  209  extending over almost the entire width (i.e.: vertical direction) of the arm  20 , in this case a cut-away with a substantially U-shaped outline, which divides the arm  20  into a primary arm part  210  which contains the first mounting hole  202  and a secondary arm part  220  which contains the said elongated mounting hole for the measuring wheel  10 . The cut-away  209  leaves free two bridge parts  230 ,  240 , which connect the secondary arm part  220  with the primary arm part  210 . Each bridge part  230 ,  240  has, viewed in the longitudinal direction, a first bridge end part  231 ,  241 , a middle bridge part  232 ,  242 , and a second bridge end part  233 ,  243 , wherein the middle bridge part  232 ,  242  in the illustrated preferred embodiment is a little thicker than the adjoining first and second bridge end parts. On a side face  234  of one of the bridge parts, in this case bridge part  230 , a deformation sensor  250  comprising two strain gauges  251 ,  252  is mounted, wherein the two strain gauges  251 ,  252  are substantially aligned with said bridge end parts  231  and  233 .  FIG. 7B  schematically shows a perspective view hereof. The side face  234  is a face of which the normal direction is substantially directed vertically, i.e. parallel to the direction of the force component F V  to be measured.  
      This design offers the following advantages.  
      When a transverse force is exerted on the measuring wheel (not shown in the  FIGS. 7A and 7B ) as discussed above, the secondary arm part  220  will be displaced substantially along a straight line in a direction perpendicular to the longitudinal direction of the supporting arm  20 , while the two bridge parts  230 ,  240  are deformed to an S-shaped outline. The deformation mainly occurs in the thinnest parts of the two bridge parts  230 ,  240 , i.e. the first and second bridge end parts. As a result, one of the strain gauges  251 ,  252  experiences a lengthening while the other one experiences a shortening, and the signals generated thereby can be processed so that they enhance each other. When the supporting arm is subjected to another deformation, like for instance a lengthening as a result of external load or as a result of temperature variations, the variations in the output signals of the measuring strips cancel each other out.  
      If a solid supporting arm would be used, which is as a whole loaded for bending, then two measuring strips compensating one another must be attached on surfaces situated opposite each other (top surface and bottom surface) of the supporting arm. This means that operations like surface treatments and attaching strain gauges would then have to be executed twice, against only once in the case of the design proposed by the present invention.  
      A further advantage is attained in manufacturing the supporting arm itself. In order to be able to be used as measuring arm, the bending-sensitive part of the measuring arm must be manufactured precisely. In the case of a solid supporting arm, bending takes place over a relatively long length, so that the precise manufacturing must be applied to a large length part, which is cumbersome and makes the supporting arm relatively expensive. In the case of the supporting arm proposed by the present invention, only a single bridge part  230  needs to be manufactured precisely, which is simpler. In a possible embodiment, the elongated hole  204  and the cut-away  209  are manufactured by a single punch treatment.  
      A further advantage of the supporting arm proposed by the present invention is attained when using it. Because the secondary arm part  220  is substantially displaced along a line in a direction perpendicular to the longitudinal direction of the supporting arm  20 , the deformation of the bridge parts, and thereby the sensor signal generated, is substantially independent from the precise location of the axle of the measuring wheel in the elongated hole  204 .  
      It is noted that the number of bridge parts between the primary and secondary arm parts may also be higher than two.