Abstract:
A universal mount bicycle power meter module has a base member with first and second ends and a narrow central portion which is relatively compressible in response to applied forces. Firmly secured to the first end is an internally threaded mounting nut in registration with an aperture in the first end. The nut secures the first end of the base member to an externally threaded end portion of a bicycle axle. The second end of the base member is configured to be firmly secured to a rear frame portion of a bicycle using a mounting clamp. A strain gauge sensor assembly is mounted on the central portion of the base member to generate resistance values representative of the amount of compression in the central portion. The sensor assembly is coupled to a signal processing unit having circuitry for converting the resistance values and bicycle velocity signals from an associated bicycle speedometer to cyclist power signals.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to bicycle power meters used to indicate the amount of power expended by the bicyclist during cycling. More particularly, this invention relates to a universal mount bicycle power meter module capable of being fitted to any bicycle for enabling the generation of electrical signals from which power can be determined. 
     Bicycle power meters are being increasingly used by both professional and amateur cyclists as an aid in developmental training. Several different types of bicycle power meters are available, some of which use strain gauges to measure the force applied by the cyclist to the crankset, the bottom bracket or the rear wheel hub. While effective in providing electrical signals representative of applied force, known bicycle power meters using strain gauges are relatively expensive and somewhat difficult to install. 
     Commonly assigned U.S. Pat. No. 8,370,087 issued Feb. 5, 2013 for “Bicycle Power Meter With Frame Mounted Sensor” discloses a bicycle power meter which is relatively inexpensive and easy to install at the point of manufacture, at the retail level and by the end user. The bicycle power meter has a strain gauge sensor assembly mounted on a relatively compressible web portion of the end of the rear fork of the bicycle frame. The relatively compressible web portion is near the rear hub and subject to the forces exerted by the cyclist to the crankset, and transferred via the chain, and sprocket assembly to the hub. The sensor assembly has two ohmically interconnected stretch sensors each having a first layer bearing a variable resistance element, whose resistance changes with displacement of the compressible web portion, and a second layer for providing support for the first layer. The sensor assembly is connected in a bridge circuit to two other resistances to generate signals representative of cyclist applied force. These signals are processed along with velocity signals to generate power signals and the power signals are supplied to a display. While this bicycle power meter overcomes the disadvantages inherent in previously known bicycle power meters, the application is limited to those bicycles having the relatively compressible web portion of the bicycle frame. Not all bicycle frames have this construction. 
     SUMMARY OF THE INVENTION 
     The invention comprises a universal mount bicycle power meter module which can be mounted to any bicycle having a rear axle with an externally threaded end portion and a rear frame portion. The universal mount bicycle power meter is relatively inexpensive but effective in providing electrical signals representative of applied cyclist force, which signals can be combined with speed signals to generate real time power measurements. 
     In a first aspect the invention comprises a bicycle power meter module including an elongate base member having a first end with an aperture, a second end adapted to be secured to a bicycle frame portion adjacent a rear bicycle axle, and a compressible central portion; a strain gauge sensor assembly mounted on the compressible central portion, the strain gauge sensor assembly having first and second stretch sensors each including a first layer having a variable resistance element mounted thereon and a second layer for supporting the first layer, the variable resistance elements of the first and second stretch sensors being ohmically interconnected to present a total resistance value representative of cyclist force; a mounting nut firmly secured to the first end of the base member, the nut having a centrally located internally threaded through-hole in registration with the aperture in the first end of the base member so that the nut can be threaded onto an associated externally threaded bicycle axle end portion to secure the base member to the bicycle axle at the first end; and a signal processing unit mounted on the base member and electrically coupled to the strain gauge sensor assembly for converting the total resistance value to cyclist power signals. 
     The compressible central portion of the base member preferably has a width less that the width of the first and second ends of the base member to promote compression of the central portion under applied force. 
     The second end of the base member is preferably secured to the bicycle frame member using a mounting clamp secured to the second end of the base member, the mounting clamp having a mounting band adapted to firmly capture the bicycle frame portion. 
     The first and second stretch sensors may be arranged with each of the first layers in facing relation or with each of the second layers in facing relation. 
     The signal processing unit preferably includes a bridge circuit having the first and second stretch sensors connected in a first branch and a pair of fixed resistances connected in a second branch; an amplifier coupled to the bridge circuit for amplifying signals representative of the total resistance value; an analog-to-digital converter coupled to the amplifier for converting the signals output from the amplifier to digital signals; and a microcomputer coupled to the analog-to-digital converter for receiving the digital signals and bicycle velocity signals from an associated bicycle speedometer and converting the received signals to the power signals. In a preferred wireless embodiment, the signal processing unit further includes a transmitter coupled to the microcomputer for receiving the power signals and generating equivalent wireless signals; and a antenna coupled to the transmitter for broadcasting the equivalent wireless signals to an associated receiver. 
     In a second aspect the invention comprises the combination of a bicycle having a frame with a rear portion and a rear axle with an externally threaded end portion adjacent said rear portion of said frame; and a bicycle power meter module comprising an elongate base member having a first end with an aperture, a second end secured to the rear portion of the frame, and a compressible central portion; a strain gauge sensor assembly mounted on the compressible central portion, the strain gauge sensor assembly having first and second stretch sensors each including a first layer having a variable resistance element mounted thereon and a second layer for supporting the first layer, the variable resistance elements of the first and second stretch sensors being ohmically interconnected to present a total resistance value representative of cyclist force; a mounting nut firmly secured to the first end of the base member, the nut having a centrally located internally threaded through-hole in registration with the aperture, the nut being threaded onto the externally threaded axle end portion so that the base member is secured to the axle end portion at the first end of the base member; and a signal processing unit mounted on the base member and electrically coupled to the strain gauge sensor assembly for converting the total resistance value to cyclist power signals. 
     The compressible central portion of the base member preferably has a width less that the width of the first and second ends of the base member to promote compression of the central portion under applied force. 
     The second end of the base member is preferably secured to the bicycle frame member using a mounting clamp secured to the second end of the base member, the mounting clamp having a mounting band adapted to firmly capture the bicycle frame portion. 
     The first and second stretch sensors may be arranged with each of the first layers in facing relation or with each of the second layers in facing relation. 
     The signal processing unit preferably includes a bridge circuit having the first and second stretch sensors connected in a first branch and a pair of fixed resistances connected in a second branch; an amplifier coupled to the bridge circuit for amplifying signals representative of the total resistance value; an analog-to-digital converter coupled to the amplifier for converting the signals output from the amplifier to digital signals; and a microcomputer coupled to the analog-to-digital converter for receiving the digital signals and bicycle velocity signals from an associated bicycle speedometer and converting the received signals to the power signals. In a preferred wireless embodiment, the signal processing unit further includes a transmitter coupled to the microcomputer for receiving the power signals and generating equivalent wireless signals; and an antenna coupled to the transmitter for broadcasting the equivalent wireless signals to an associated receiver. 
     The invention greatly facilitates the inclusion of a bicycle power meter with any bicycle having a rear axle with an externally threaded end portion and a rear frame portion. The bicycle power meter module can be easily secured to the bicycle components at the point of manufacture. Similarly, the bicycle power meter module can be readily secured to the bicycle at any point in the distribution chain, such as at the retailer as an add-on option. The bicyclist can also add the bicycle power meter module to a bicycle after purchase, at relatively low cost and effort. 
     For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view of a preferred embodiment of a bicycle power meter module incorporating the invention; 
         FIG. 2  is a partial plan view taken from the rear sprocket side of a bicycle illustrating the bicycle power meter module of  FIG. 1  mounted in place; 
         FIG. 3  is an enlarged sectional view taken along lines  3 - 3  of  FIG. 2  illustrating the mounting nut, frame and module base assembly; 
         FIG. 4  is an enlarged sectional view taken along lines  4 - 4  of  FIG. 2  illustrating the mounting clamp assembled to the bicycle frame; 
         FIG. 5  is a block diagram of a bicycle power meter module using a single strain gauge sensor assembly configured as a wired unit; 
         FIG. 6  is a block diagram of a bicycle power meter module using a single strain gauge sensor assembly configured as a wireless unit; 
         FIG. 7  is a schematic view of a single strain gauge sensor illustrating the sensor in three different positions; 
         FIG. 8  is a schematic view of a first embodiment of a dual element strain gauge sensor assembly; 
         FIG. 9  is a schematic view of a second embodiment of a dual element strain gauge sensor assembly; and 
         FIG. 10  is a schematic diagram illustrating variation in cyclist power with crankset angular position. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Turning now to the drawings,  FIG. 1  is plan view of a first embodiment of a bicycle power meter module incorporating the invention. As seen in this Figure, a bicycle power meter module assembly generally designated with reference numeral  10  includes an elongate base member  11  having a first end portion  12 , a second, opposite end portion  13 , and a central portion  14  preferably with a narrower width than end portions  12 ,  13 . Elongate base member  11  is fabricated from a relatively compressible, thin material, such as stainless steel having a thickness of 1.0 mm., in order to contract or expand in length in response to force applied to end  12 . 
     Secured to the upper surface of central portion  14  is a strain gauge assembly  15  described more fully below with reference to  FIGS. 7-9 . Strain gauge assembly  15  has a pair of ohmic conductors which are electrically connected to the input terminals of a signal processing unit described more fully below with reference to  FIG. 5  (wired version) and  FIG. 6  (wireless version) mounted on a circuit board  17 , which in turn is bonded to the upper surface of end portion  13  of base member  11 . The signal processing unit is powered by a small battery  18  removably secured to circuit board  17  (as illustrated) or directly to the upper surface of end portion  13  of base member  11 . Power from battery  18  to the signal processing unit is controlled by a manually operable on/off switch  19 . 
     Secured to the upper surface of first end portion  12  of base member  11  is an internally threaded nut  20 , which is used to secure bicycle power meter module  10  to one end of the rear axle of a bicycle. Nut  20  can be secured using a variety of techniques, such as welding, brazing, or gluing with a strong adhesive. What is important to the process of securing is nut  20  must be firmly secured to base member  11  so that relative motion between nut  20  and base member  11  is prevented. Stated differently, when the end of the bicycle axle to which nut  20  is threadably attached is deflected due to force applied by the bicycle chain, this deflection must be transferred to base member  11  via nut  20  so that strain gauge assembly  15  senses the amount of deflection. 
     The second, opposite end  13  of base member  11  is provided with a pair of mounting apertures  22 ,  23 . A mounting clamp generally designated with reference numeral  25 , preferably a conventional automotive hose clamp, is provided with a mating pair of mounting apertures  26 ,  27  so that end portion  13  can be secured to mounting clamp  25  in the manner depicted in  FIGS. 2 and 4 . 
       FIG. 2  is a partial plan view taken from the rear sprocket side of a bicycle illustrating the bicycle power meter module  10  of  FIG. 1  mounted in place. With reference to this Figure, and  FIGS. 3 and 4 , mounting clamp  25  is first secured to opposite end  13  of the base member  11  using threaded fasteners  28  passing through apertures  22 ,  23 ,  26 ,  27 . Next, module  10  is attached to the sprocket end of a threaded axle  28  by threading nut  20  onto the end of axle  28  until the base member  11  is firmly fixed in place with opposite end portion  13  aligned with portion  29  of the bicycle frame in the manner shown in  FIG. 2 . Next, opposite end  13  of base member is secured to portion  29  of the bicycle frame by passing the mounting band  31  of mounting clamp  25  around frame portion  29  and tightening the band  31  against the periphery of frame portion  29  until opposite end portion  13  is firmly secured to frame portion  29 . This completes the mechanical mounting of bicycle power meter module  10 . 
     In use, as a bicyclist applies force to the pedals attached to the crankset, drive chain  35  ( FIG. 2 ) experiences a force which is transferred via sprocket  36  to axle  28 , causing axle  28  to deflect in the horizontal direction. This deflection of axle  28  is transferred via base member  11  to strain gauge assembly  15 , causing a change in the signal output from strain gauge assembly  15 . The output signal is processed in the manner described below to generate power magnitude signals which can be displayed to the bicyclist. 
       FIG. 7  illustrates a simple stretch sensor  40  having the property of an ohmic resistance which varies in a predictable amount with linear longitudinal displacement of the sensor body. Stretch sensor  40  has a first layer  42  on which a thin variable resistance element  43  is mounted, and a second base layer  44  which carries the first layer and provides additional mechanical strength for sensor  40 . The resistance value of sensor  40  depends upon the longitudinal displacement of the sensor body. As shown in  FIG. 7 , when sensor  40  is displaced in one direction (illustrated as flexing) in a first direction, the value of the resistance increases (R+r), where R is the at rest resistance value of sensor  40  and r is the additional resistance value due to the displacement in the first direction. Similarly, when sensor  40  is displaced in the opposite direction, the value of the resistance decreases (R−r). 
       FIG. 8  illustrates a strain gauge sensor assembly  15  of the type incorporated into the power meter configuration shown in  FIGS. 1 and 2 . As seen in this Figure, sensor assembly  15  comprises two two layer stretch sensors  40   a ,  40   b  having first layers  42   a ,  42   b , and second layers  44   a ,  44   b . Stretch sensors  40   a ,  40   b  are arranged with the first layers  42   a ,  42   b  in facing relation in an (R+r), (R−r) relation. As sensor assembly  15  is displaced by forces applied to axle  28  of right rear fork  12 , the total resistance of each stretch sensor will vary in equal and opposite directions. 
       FIG. 9  illustrates an alternate strain gauge sensor assembly  50  of the type incorporated into the power meter configuration shown in  FIGS. 1 and 2 . As seen in this Figure, sensor assembly  50  comprises two two layer stretch sensors  40   a ,  40   b  having first layers  42   a ,  42   b , and second layers  44   a ,  44   b . Stretch sensors  40   a ,  40   b  are arranged with the second layers  44   a ,  44   b  in facing relation in an (R−r), (R+r) relation. As sensor assembly  50  is displaced by forces applied to axle  28  of right rear fork  12 , the total resistance of each stretch sensor will vary in equal and opposite directions. 
       FIG. 5  is a block diagram of a bicycle power meter unit using a single strain gauge sensor assembly configured as a wired unit. As seen in this Figure, the stretch sensors  40   a ,  40   b  comprising strain gauge sensor assembly  15  or  50  are connected to a pair of fixed resistances  52 ,  54  in a well-known Wheatstone bridge circuit configuration. The top node of the bridge is connected to a source of electrical potential Vc supplied by battery  18 . The bottom node of the bridge is connected to circuit ground. The right node is connected to one end of the fixed resistances  52 ,  54  and serves as one output terminal of the bridge circuit. The second end of fixed resistance  52  is connected to one end of stretch sensor  40   a  and to supply voltage Vc. The second end of fixed resistance  54  is connected to one end of stretch sensor  40   b  and to circuit ground. The other ends of stretch sensors  40   a ,  40   b  are connected together and serve as the other output terminal of the bridge circuit. 
     The bridge circuit output terminals are coupled to the input terminals of an amplifier  55 , where the bridge signals are amplified. Amplifier  55  is preferably a type MAX4197 unit available from MAXIM Corporation. The amplified signals output from amplifier  55  are coupled to the input of an analog-to-digital converter  56  which converts the amplified analog signals to digital equivalent signals. The digital signals output from analog-to-digital converter  56  are coupled to an input port of a microcomputer  58 . Analog-to-digital converter  56  and microcomputer  58  are preferably combined in a type PIC 10F202 unit available from Microchip Corporation. Velocity signals from a bicycle speedometer (not shown) are also coupled to microcomputer  58 . Microcomputer  58  processes the force signals and the velocity signals using a known algorithm to provide power magnitude signals. The power magnitude signals are coupled to a multifunction display  60 , which displays the current power value in readable form by the bicyclist. In the  FIG. 5  embodiment the units are coupled together by ohmic wire connections. 
       FIG. 6  is a block diagram of a bicycle power meter unit using a single strain gauge sensor assembly configured as a wireless unit. In this Figure, elements corresponding to the same elements in the system of  FIG. 5  are designated with the same reference numerals. In the  FIG. 6  system, the processed power signals are coupled to the input of an r.f transmitter  62  located on circuit board  17 . Transmitter  62  is preferably a type nRF24AP2 ANT+ module available from Nordic Semiconductor Co. of Norway. Transmitter  62  transmits the power signals via an antenna  63  ( FIG. 1 ) to a receiver  64  located near the multifunction display  60 , which supplies these signals to the multifunction display  60 . 
     In both the wired and wireless versions of the bicycle power meter unit, the strain gauge assembly  15 ,  50 , amplifier  55 , A/D converter  56 , microcomputer  58 , and transmitter  62  are all mounted on circuit board  17 ; while display  60  and receiver  64  are mounted in a convenient location for the cyclist to view the display, typically somewhere on the handle bars of the bicycle. 
     In use, as the cyclist applies force to the bicycle pedals, the magnitude of the force is monitored by the bridge circuit and converted to visible power display signals for the bicyclist to observe.  FIG. 10  is a schematic diagram illustrating variation in cyclist power with crankset angular position. In position (a) the pedals are essentially horizontal and the cyclist is applying maximum force with the forward pedal. In position (b) the pedals are essentially vertical and the cyclist is applying minimum force. In position (c) the pedals are again essentially horizontal and the cyclist is applying maximum force with the forward pedal; while in position (d) the pedals are again essentially vertical and the cyclist is applying minimum force. 
     As will now be apparent, bicycle power meter modules fabricated according to the teachings of the invention offer cost and ease of installation advantages over known bicycle power meters using strain gauges. More particularly, the bicycle power meter module  10  is relatively simple to install on any bicycle having a threaded rear axle end portion onto which the nut  20  can be threaded and a rear frame portion around which the mounting band  31  of mounting clamp  25  can be secured. This installation can be done at the bicycle factory or elsewhere in the chain of commerce (e.g., by the retailer or the user-bicyclist). In addition, bicycle power meter modules fabricated according to the teachings of the invention can be configured in either a wired or a wireless mode, which affords great flexibility in the installation process. 
     While the invention has been described with reference to particular embodiments, various modifications, alternate constructions and equivalents may be employed without departing from the spirit of the invention. For example, while certain circuit components have been disclosed, other equivalent units may be employed, as desired. Therefore, the above should not be construed as limiting the invention, which is defined by the appended claims.