Abstract:
A bicycle power meter has a strain gauge sensor assembly mounted on a relatively compressible portion of the end of the rear fork of the bicycle frame. The relatively compressible 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 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.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    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 bicycle power meter using a frame mounted sensor for enabling the generation of electrical signals from which power can be determined. 
         [0002]    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. Due to these disadvantages, bicycle power meters have not found wide acceptance in the bicycling community. 
       SUMMARY OF THE INVENTION 
       [0003]    The invention comprises a bicycle power meter using one or more strain gauge sensor assemblies, which 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. 
         [0004]    In a broadest aspect, the invention comprises a bicycle power meter with a rear bicycle frame having a first fork with a relatively compressible rear portion adjacent a region to which a hub can be attached; and a strain gauge sensor assembly secured to the relatively compressible rear 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. 
         [0005]    The first and second stretch sensors are alternatively arranged with each first layer in facing relation, or with each second layer in facing relation. 
         [0006]    The bicycle power meter further 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; 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 power signals; and a display coupled to the microcomputer for displaying the power signals to a cyclist. 
         [0007]    The bicycle power meter can be configured as either a wired or a wireless system. In a wired system, the units are all ohmically interconnected. In a wireless system, a transmitter is coupled to the microcomputer for receiving the power signals and generating equivalent wireless signals; and a receiver coupled to the display receives the equivalent wireless signals and provides the equivalent wireless signals to the display. 
         [0008]    In an alternative embodiment, the rear bicycle frame has a second fork with a second relatively compressible rear portion adjacent a region to which a hub can be attached; and an additional strain gauge sensor assembly is secured to the second relatively compressible rear portion. The additional strain gauge sensor assembly has third and fourth 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 third and fourth stretch sensors being ohmically interconnected to present a total resistance value representative of cyclist force. 
         [0009]    The third and fourth stretch sensors are alternatively arranged with each first layer in facing relation, or with each second layer in facing relation. 
         [0010]    In this alternative embodiment, the third and fourth stretch sensors are connected in the second branch of the bridge circuit. 
         [0011]    The invention greatly facilitates the inclusion of a bicycle power meter with any bicycle having a relatively compressible structural portion at the end of the rear fork of the bicycle frame. The entire power meter system, or just the strain gauge sensor assemblies, can be easily secured to the bicycle components at the point of manufacture. Similarly, the entire system 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 system to a bicycle after purchase, at relatively low cost and effort. 
         [0012]    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 
         [0013]      FIG. 1  is a perspective partial view taken from above of a bicycle rear frame illustrating the fork ends, hub, chain and strain gauge sensor assembly of a first embodiment of the invention; 
           [0014]      FIG. 2  is an elevational view taken along lines A-A of  FIG. 1  illustrating the mounting position of the strain gauge sensor assembly on the inner surface of a compressible portion of the right rear fork; 
           [0015]      FIG. 3  is a block diagram of a bicycle power meter unit using a single strain gauge sensor assembly configured as a wired unit; 
           [0016]      FIG. 4  is a block diagram of a bicycle power meter unit using a single strain gauge sensor assembly configured as a wireless unit; 
           [0017]      FIG. 5  is a schematic view of a single strain gauge sensor illustrating the sensor in three different positions; 
           [0018]      FIG. 6  is a schematic view of a first embodiment of a dual element strain gauge sensor assembly; 
           [0019]      FIG. 7  is a schematic view of a second embodiment of a dual element strain gauge sensor assembly; 
           [0020]      FIG. 8  is a perspective partial view taken from above of a bicycle rear frame illustrating the fork ends, hub, chain and strain gauge sensor assembly of a second embodiment of the invention having two separate strain gauge sensor assemblies; 
           [0021]      FIG. 9  is an elevational view taken along lines B-B of  FIG. 8  illustrating the mounting position of the second strain gauge sensor assembly on the inner surface of a compressible portion of the left rear fork; 
           [0022]      FIG. 10  is a block diagram of a bicycle power meter unit using two strain gauge sensor assemblies configured as a wired unit; 
           [0023]      FIG. 11  is a block diagram of a bicycle power meter unit using two strain gauge sensor assemblies configured as a wireless unit; and 
           [0024]      FIG. 12  is a schematic diagram illustrating variation in cyclist power with crankset angular position. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0025]    Turning now to the drawings,  FIG. 1  is a perspective partial view taken from above of a bicycle rear frame illustrating the fork ends, hub, chain and strain gauge sensor assembly of a first embodiment of the invention. As seen in this Fig., a bicycle rear frame has a pair of terminating portions termed a right fork  12  and a left fork  14 . Secured between forks  12 ,  14  by means of an axle  16  and capture nuts  17 ,  18  are a rear hub  20  and a sprocket assembly  22 . A drive chain  24  passes around individual sprockets comprising sprocket assembly  22  in order to provide rotational movement of hub  20  is response to cycling effort by the cyclist. 
         [0026]    The end portion  26 ,  28  of each of forks  12 ,  14  has a thinner lateral thickness dimension than the remaining major portion of each fork  12 ,  14 . Secured to the inner surface of end portion  26  of right fork  12  is a strain gauge sensor assembly  30  described more fully below. As best shown in  FIG. 2 , strain gauge sensor assembly  30  is attached to a thin web portion  32  extending along end portion  26  of right fork  12 . Web portion  32  has the mechanical property of being relatively compressible when compared to the more robust structure of right rear fork  12 , so that the physical dimensions of strain gauge sensor assembly  30  can change with different force magnitudes applied to web portion  32  via chain  24 , sprocket assembly  22 , and axle  16 . 
         [0027]      FIG. 5  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. 5 , 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). 
         [0028]      FIG. 6  illustrates a strain gauge sensor assembly  30  of the type incorporated into the power meter configuration shown in  FIGS. 1 and 2 . As seen in this Fig., sensor assembly  30  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  30  is displaced by forces applied to web portion  32  of right rear fork  12 , the total resistance of each stretch sensor will vary in equal and opposite directions. 
         [0029]      FIG. 7  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 Fig., 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 web portion  32  of right rear fork  12 , the total resistance of each stretch sensor will vary in equal and opposite directions. 
         [0030]      FIG. 3  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 Fig., the stretch sensors  40   a,    40   b  comprising strain gauge sensor assembly  30  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 a battery. 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. 
         [0031]    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 16F73 unit available from Microchip Corporation. Velocity signals from a bicycle speedmeter (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. 3  embodiment the units are coupled together by ohmic wire connections. 
         [0032]      FIG. 4  is a block diagram of a bicycle power meter unit using a single strain gauge sensor assembly configured as a wireless unit. In this Fig., elements corresponding to the same elements in the system of  FIG. 3  are designated with the same reference numerals. In the  FIG. 4  system, the processed power signals are coupled to the input of an r.f transmitter  62  located near the sensor assembly  30  ( 50 ). Transmitter  62  transmits the power signals to a receiver  64  located near the multifunction display  60 , which supplies these signals to the multifunction display  60 . 
         [0033]      FIG. 8  is a perspective partial view taken from above of a bicycle rear frame illustrating the fork ends, hub, chain and strain gauge sensor assembly of a second embodiment of the invention having two separate strain gauge sensor assemblies. In this Fig., elements corresponding to the same elements shown in  FIG. 1  are designated with the same reference numerals, with the exception of sensor assembly  30  which is designated with reference  30   a.  Secured to the inner surface of end portion  28  of left fork  14  is a second strain gauge sensor assembly  30   b.  Sensor assembly  30   b  has the same structure and function as sensor assembly  30  described above. As best shown in  FIG. 9 , strain gauge sensor assembly  30   b  is attached to a thin web portion  33  extending along end portion  28  of left fork  14 . Web portion  33  has the mechanical property of being relatively compressible when compared to the more robust structure of left rear fork  14 , so that the physical dimensions of strain gauge sensor assembly  30   b  can change with different force magnitudes applied to web portion  33  via chain  24 , sprocket assembly  22 , and axle  16 . 
         [0034]      FIG. 10  is a block diagram of a bicycle power meter unit using two strain gauge sensor assemblies  30   a,    30   b  configured as a wired unit.  FIG. 11  is a block diagram of a bicycle power meter unit using two strain gauge sensor assemblies  30   a,    30   b  configured as a wireless unit. The principal elements shown in each Fig. are essentially the same as those shown in  FIGS. 3 and 4 , with the exception of the configuration of the bridge circuit. In both  FIGS. 10 and 11 , the fixed resistances  52 ,  54  are replaced by the individual stretch sensors  40   a,    40   b  comprising the second sensor assembly  30   b,  with electrical connections as shown. Thus, the bridge comprises four individual variable resistance stretch sensors  40 . 
         [0035]    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. 12  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. 
         [0036]    As will now be apparent, bicycle power meters fabricated according to the teachings of the invention offer cost and ease of installation advantages over known bicycle power meters using strain gauges. Firstly, the strain gauges are relatively simple to install on any bicycle frame having the relatively compressible thin web portion adjacent the rear hub. 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 meters 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. Further, by employing the two layer dual strain gauge assemblies, greater sensitivity is achieved over single strain gauge designs. Lastly, by employing the four strain gauge configuration shown in  FIGS. 8-11 , a high level of insensitivity to temperature variations encountered during cycling can be achieved. 
         [0037]    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.