Abstract:
In one embodiment, an apparatus includes a mount, a housing, and a sensor. The mount is adapted to be disposed at least partially underneath a shoelace of a shoe, and the housing is adapted to be releasably attached to the mount. There is a tongue on one of the mount and the housing and a groove on the other of the mount and the housing, wherein the tongue is adapted to releasably engage the groove when the housing is attached to the mount. The apparatus further comprises a sensor, that senses motion of the shoe, disposed within the housing such that the sensor remains disposed within the housing when the housing is separated from the mount. In another embodiment, a method involves providing a housing that houses a sensor that senses motion of a shoe, and attaching a mount to an instep portion of the shoe such that at least a portion of the mount is disposed underneath at least a portion of a shoelace of the shoe. The housing is attached to the mount by engaging a tongue on one of the mount and the housing with a groove on the other of the mount and the housing, and the housing is detached from the mount without separating the sensor from the housing.

Description:
This is a continuation of application Ser. No. 09/592,925, filed Jun. 13, 2000, and now U.S. Pat. No. 6,357,147, which is a continuation of application Ser. No. 09/164,654, filed Oct. 1, 1998, and now U.S. Pat. No. 6,122,340. 
    
    
     BACKGROUND OF THE INVENTION 
     Electronic devices for monitoring the locomotion (e.g., walking, jogging, or running) of people are known in the art. Most commonly, one or more impact-sensitive sensors, e.g., contact switches, piezoelectric sensors, etc., are placed in a sole of a shoe to sense when the shoe comes into contact with and/or leaves the ground. Typically, such sensors are embedded in the heel and/or toe portion(s) of the shoe&#39;s sole, and outputs thereof are fed to a processor or other electronic device. 
     In a device disclosed in U.S. Pat. No. 4,649,552 (“the &#39;552 device”), for example, a contact switch is fixed to the undersurface of a removable insole of a shoe, such that each footstep of the person wearing the shoe causes the contact switch to close. In the &#39;552 device, the two leads of the contact switch are fed (via a cable) to a mount base including a pair of electrical terminals, with a respective terminal being connected to each lead. In addition, an electronic counter is detachably mounted to the mount base such that, when mated together, a pair of terminals on the electronic counter electrically contact the pair of terminals on the mount base, so that the counter can count the number of footsteps the user takes by counting the number of times the contact switch closes. 
     It is also known to secure a sensor to the instep portion of a shoe to sense when the shoe flexes during footsteps of a person wearing the shoe. In a device disclosed in U.S. Pat. No. 4,651,446 (“the &#39;446 device”), for example, the two leads from an instep-mounted contact switch (which is closed when a person&#39;s shoe flexes during footsteps) are fed to a mount base including a pair of electrical terminals, with one lead being connected to each terminal. As with the &#39;552 device, the &#39;446 device includes an electronic counter which is detachably mounted to the mount base such that, when mated together, a pair of terminals on the electronic counter electrically contact the pair of terminals on the mount base, so that the electronic counter may count the number of times the instep-mounted contact switch closes when the user&#39;s foot flexes, thereby counting the number of footsteps taken by the user. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, an apparatus includes a mount, a housing, and a sensor. The mount is adapted to be disposed at least partially underneath a shoelace of a shoe, and the housing is adapted to be releasably attached to the mount. There is a tongue on one of the mount and the housing and a groove on the other of the mount and the housing, wherein the tongue is adapted to releasably engage the groove when the housing is attached to the mount. The apparatus further comprises a sensor, that senses motion of the shoe, disposed within the housing such that the sensor remains disposed within the housing when the housing is separated from the mount. 
     According to another aspect, a method involves providing a housing that houses a sensor that senses motion of a shoe, and attaching a mount to an instep portion of the shoe such that at least a portion of the mount is disposed underneath at least a portion of a shoelace of the shoe. The housing is attached to the mount by engaging a tongue on one of the mount and the housing with a groove on the other of the mount and the housing, and the housing is detached from the mount without separating the sensor from the housing. 
     According to another aspect of the invention, an apparatus comprises a mount, a housing, and a sensor. The mount is adapted to be secured to a shoe. The housing is adapted to be releasably attached to the mount. The sensor is supported by the housing such that the sensor remains supported by the housing when the housing is separated from the mount. 
     According to another aspect of the invention, an apparatus comprises a mount and a motion-sensing device. The mount is adapted to be secured to a shoe, and the motion-sensing device is adapted to be releasably attached to the mount. 
     According to another aspect of the invention, an apparatus comprises a mount adapted to be secured to a shoe, and means for supporting a sensor, that senses motion of the shoe, with the mount. 
     According to another aspect of the invention, an apparatus comprises a mount, and a sensor supported by the mount. The mount is adapted to be secured to a shoe, and has an enclosed passageway through which a shoelace of the shoe can pass to secure the mount to the shoe when the shoelace is taut. The passageway is oriented such that, when the mount is secured to the shoe and the shoelace is taut, a line can pass through the passageway without intercepting the shoe. 
     According to another aspect of the invention, an apparatus comprises a mount, and a sensor supported by the mount. The mount has a passageway oriented such that, when the mount is secured to a shoe, a bottom surface of the mount contacts the shoe, and the passageway receives a shoelace of the shoe in a direction substantially parallel to the bottom surface of the mount. 
     According to another aspect of the invention, a method involves providing a housing that supports a sensor that senses motion of a shoe. According to the method, the housing is attached to a mount supported by the shoe, and is also detached from the mount without separating the sensor from the housing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an illustration showing an example of how an electronic device may be mounted to a shoe of the user according to one embodiment of the present invention; 
     FIGS. 2 and 3 show perspective views of separate units that may be used to support an electronic device with respect to a shoe according to an embodiment of the invention; 
     FIG. 4 shows an example of how the two separate units shown in FIGS. 2 and 3 may be releasably attached to one another according to one embodiment; 
     FIG. 5 shows an example of how the two separate units shown in FIGS. 2 and 3 may be detached from one another after having been attached together; 
     FIG. 6 shows a front view of the example embodiment of the invention shown in FIGS. 1-5; 
     FIG. 7 shows an example of how an accelerometer may be mounted at an acute angle with respect to a plane of a circuit board according to one embodiment of the invention; and 
     FIG. 8 shows an example of how an accelerometer having two perpendicular acceleration-sensing axes may be used to sense acceleration in a direction other than the directions in which the two acceleration-sensing axes are oriented. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In each of the &#39;552 and &#39;446 devices described above in which an electronic counter is detachably mounted to a mount base, it is necessary that the physical connections between the pair of terminals on the mount base and the pair of terminals on the electronic counter be complete and constant in order for the electronic counter to accurately count footsteps. Applicants have recognized that the extreme physical forces on a person&#39;s foot during locomotion can cause the integrity of the physical contact between the pairs of terminals to lessen such that the connections between the terminals can become loose, intermittent, or incomplete. If the connections between the terminals become loose, extra footsteps may be counted when the terminals bounce against one another during periods that the contact switch is closed. If the connections between the terminals become intermittent or incomplete, some or all of the footsteps of the user may not be counted by electronic counter. 
     Additionally, in each of the &#39;552 and 446 devices, the sensor is permanently connected to the mount base and remains secured to the shoe when the electronic counter is detached from the mount base. Therefore, a person may find it necessary to remove both the electronic counter and the mount base from his or her shoe before exercising in adverse weather conditions (e.g., rain or snow) or washing the shoe so as to prevent the sensor or associated circuitry from becoming damaged or rusting. 
     Further, the requirement that a sensor be connected to the mount base can increase the cost of manufacturing and retail price of the mount base, thereby prohibiting consumers from buying a separate mount base for each of several shoes. Therefore, to use the housing unit with different shoes, a person is required to transfer the mount base from one shoe to another, which can be inconvenient and time consuming. 
     In one embodiment of the present invention, a mounting unit (which may be relatively inexpensive to manufacture) is adapted to be secured to a shoe (e.g., on the shoelace or elsewhere on the shoe), and a housing unit is releasably attached to the mounting unit. A motion-sensing device, which is used to monitor a parameter associated with the locomotion of a person, may be attached to or housed by the housing unit such that the motion-sensing device is separated from the mounting unit and the shoe when the housing unit is separated from the mounting unit. By associating the motion-sensing device with the housing unit in this manner, no physical connections are required to be made between terminals on the mounting unit and terminals on the housing unit each time the housing unit is releasably attached to the mounting unit, thereby eliminating the risk that the physical forces on the shoe will cause the physical connections between such terminals to become loose, intermittent, or incomplete. Additionally, in such an embodiment, the risk that the motion-sensing device will be damaged due to washing, adverse weather conditions, etc., is eliminated simply by detaching the housing unit from the mounting unit so that the motion-sensing device is separated from the shoe. Further, because the mounting unit is not required to have a motion-sensing device attached to it, the mounting unit can be manufactured relatively inexpensively and sold at a reasonable cost. Therefore, a consumer may purchase several mounting units and secure a respective mounting unit to each of several shoes, so that a single housing unit may be readily transferred to any one of the respective mounting units. 
     In one embodiment, the housing unit may be entirely self-contained, with all of the electrical components that operate in connection with the motion-sensing device to monitor a parameter associated with the locomotion of the person, except being housed by the housing unit for components such as a wrist-mounted display, etc., to which information may be transmitted via a wireless communication link. 
     In each of the &#39;552 and &#39;446 devices described above in which an electronic counter is detachably mounted to a mount base, the sensor requires some sort of external physical force (e.g., a compression or bending force) to be exerted on it in order to sense motion, and can become worn or damaged over time as a result of this physical force. U.S. Pat. No. 5,485,402 discloses the use of a motion-sensing device that does not require external physical forces thereon to sense motion (i.e., an electronic accelerometer) to monitor foot strikes of a person. This accelerometer is mounted in a housing which is strapped to the person&#39;s ankle so as to generate a signal indicative of the acceleration of the ankle. Because it is not subjected to external physical forces, this accelerometer is less likely to become damaged than are pressure-sensitive switches, etc. The ankle-mounted nature of the housing, however, can be uncomfortable for wearers who exercise vigorously, e.g., joggers and runners. For example, when an ankle-mounted device is tightened sufficiently to remain in place during jogging or running, blood flow to the wearer&#39;s foot can become restricted. Also, when worn during strenuous exercise, the straps used to secure such a device to a person&#39;s ankle tend to accumulate perspiration, and therefore can develop an unpleasant odor over time. 
     In one embodiment of the invention, an electronic device including a motion-sensing device that does not require external physical forces thereon to sense motion (e.g., a solid-state accelerometer) is secured to a person&#39;s shoe, rather than the person&#39;s ankle, so as to overcome the disadvantages of the ankle-mounted device described above. In one embodiment, the motion-sensing device is included in a housing unit which is secured to the instep portion of the person&#39;s shoe. The housing unit may be releasably attached to a mounting unit, and the mounting unit may be secured to the shoe, e.g., using a shoelace of the shoe, so that the housing unit may be readily separated from the mounting unit and the shoe. When an accelerometer is used as the motion-sensing device, the accelerometer may be oriented within the housing unit such that an acceleration-sensing axis of the accelerometer is oriented perpendicular to or parallel with the ground, even though the housing unit is secured to a portion of the shoe that is not oriented perpendicular to or parallel with the ground, e.g., on the instep of the shoe. In one embodiment, this orientation of the acceleration-sensing axis of the accelerometer is accomplished by mounting the accelerometer on a circuit board such that an acceleration-sensing axis of the accelerometer forms an acute angle with a plane of the circuit board. 
     FIG. 1 shows a shoe-mount device (attached to a shoe) according to one embodiment of the present invention. As shown, the shoe-mount device may include two separable portions: a housing unit  100 , and a mounting unit  102 . In the FIG. 1 embodiment, the mounting unit  102  is secured to an instep portion of an athletic shoe  104  using exposed portions  110   a-e  of a shoe lace  110 . It should be appreciated, however, that the shoe with which the present invention is employed may be any of numerous types of shoes, and the invention is not limited to a device adapted for use with any particular type of shoe. For example, the mounting unit  102  may alternatively be adapted for use with a boot, a sandal, a sneaker, a dress shoe, etc. With shoes that employ one or more straps which are fastened, for example, using buckles or hook and loop fasteners in lieu of or in addition to a shoelace, the mounting unit  102  may be adapted to be secured to the shoe&#39;s strap(s) instead of the shoelace  110 . 
     It should also be appreciated that the mounting unit  102  may be secured to the shoe  104  at any of numerous locations, and may take on any of numerous forms so as to permit it to be secured properly to the shoe. The invention is not limited to a mounting device  102  that is adapted to be secured to any particular location on the shoe. The mounting device  102  may, for example, be adapted to be secured to a side or back portion of the shoe  104 , rather than to the shoe&#39;s instep. 
     In the illustrative embodiment shown in FIG. 1 (in which the mounting unit  102  is secured to an instep portion of the shoe  104 ), the mounting unit  102  includes an opening  112  through which the portions  110   d  and  110   e  of the shoelace  110  may pass, and includes an attachment member  114  that may be inserted underneath the shoelace portions  110   a-c . The tension of the shoelace portions  110   a-c  across a top surface of the attachment member  114  serves to secure a bottom surface of the attachment member  114  to the instep portion of the shoe. In embodiments in which a shoelace or strap is used to secure the mounting unit  102  to the shoe  104 , any number of openings and/or attachment members may be provided in or on the mounting unit  102  to permit shoelace or strap portions to secure it to the shoe, and the invention is not limited to the use of any particular types or number of openings and/or attachment members. In one alternative embodiment, for example, a mounting unit  102  is employed that includes a pair of attachment members that can be moved (e.g., folded or slid) with respect to each other such that both members can be slid underneath one or more exposed portions of the shoelace  110  to secure the mounting unit  102  to the instep of the shoe  104 . In some embodiments, the mounting unit  102  may also include gripping members that affirmatively clamp the mounting unit  102  onto portions of the shoelace  110 . 
     In alternative embodiments of the invention, the mounting unit  102  may be secured to the shoe  104  without the use of the shoelace  110  or one or more straps. Such embodiments may be used, for example, with shoes that are merely slipped onto the foot and do not employ shoelaces or straps. In one embodiment, for example, an adhesive substance may be used to secure the mounting unit  102  to a side or back portion of the shoe  104 , or the mounting unit  102  may be sewn to or embedded in a portion of the shoe to secure it thereto. In another embodiment, the mounting unit  102  may be secured to the shoe  104  using one or more laces or straps (other than the shoelace  110 ) that may be attached in some fashion to the shoe  104 . For example, a lace or strap secured to the mounting device  102  may pass through one or more grommets embedded in a portion of the shoe to secure the mounting unit  102  thereto. 
     The mounting unit  102  may be made of any of numerous types of materials, and the invention is not limited to any particular type of material. In the embodiment shown in FIG. 1, for example, the mounting unit  102  may be made of a semi-rigid polymer-based or metal material. In alternative embodiments, the mounting unit  102  may be made of a cloth material (e.g., a cloth pouch) that is sewn to or otherwise attached to the shoe  104 . In one embodiment, the mounting unit may include the loop portion of a hook and loop fastener, with the housing unit including the hook portion, or vice-versa. 
     In the embodiment of the invention shown in FIG. 1, the housing unit  100  is releasably attached to the mounting unit  102 . The releasable interface between the housing unit  100  and the mounting unit  102  may be accomplished in any of numerous ways, and the invention is not limited to any particular attachment technique. It is important only that the housing unit  100  be attachable to the mounting unit  102  such that, when attached together, the user can manipulate some sort of quick-release mechanism located on the housing unit  100  and/or the mounting unit  102  that permits the housing unit  100  to be separated from the mounting unit  102 . In alternative embodiments, for example, one or more clips, straps, strings, pins, levers, hook and loop fasteners, etc., may be used to secure the housing unit  100  to the mounting unit  102  such that the housing unit  100  may be readily disengaged from the mounting unit  102 . 
     As will be discussed in more detail below, in the FIG. 1 embodiment, the user may detach the housing unit  100  from the mounting unit  102  by pressing a cantilever  116  of the mounting unit  102  towards the attachment member  114  of the mounting unit  102  to permit a back end  128  of the housing unit  100  to be slid towards an ankle of the user and away from the mounting unit  102 . 
     Any of numerous electronic devices that monitor the locomotion of a person may be included in the housing unit  100 , and the invention is not limited to any particular type of device. In one embodiment, for example, an accelerometer-based device that senses acceleration of the shoe  104  to analyze locomotion of the user is included in the housing unit  100 . An example of such an accelerometer-based device is described in co-pending patent application Ser. No. 08/942,802, which is assigned to the assignee of the present invention, and which is incorporated herein by reference in its entirety. 
     In some embodiments, the housing unit  100  may include only a motion-sensing device, a transmitter, and a power source (if necessary), and may transmit a signal (e.g., via a radio-frequency (RF) or other wireless communications link) from the motion-sensing device in the housing unit  100  to a receiving unit remote from the housing unit  100 . In other embodiments, the housing unit  100  may include additional circuitry, such as an amplifier, filter, and processor, which may process an output signal from a motion-sensing device, perform calculations based on this processed output signal, and transmit a signal to a receiving unit (e.g., using a wireless communications link) indicating results of such calculations. A receiving unit used in connection with such embodiments may, for example, process and analyze the signal received from the circuitry in the housing unit  100  to extract information therefrom and/or may perform calculations based thereon. Such a receiving unit may be located, for example, in a wrist-mounted housing, and may include a processor to process and analyze the signal received from the circuitry in the housing unit  100 . 
     In one embodiment, the circuitry in the housing unit  100  includes a solid state accelerometer that is used to measure moments that a foot of a person leaves and comes into contact with the ground, and includes a processor that receives an output signal from the accelerometer, determines an average foot contact time of the person wearing the device based on the output signal, and calculates a pace of the person wearing the device based on the determined foot contact time. In one embodiment, a signal, indicative of the calculated pace of the person, is transmitted by circuitry in the housing unit  100  to a receiver located in a wrist-mounted device positioned on the wrist of the person wearing the foot-mounted device. Circuitry in the wrist-mounted device may receive this signal from the circuitry in the housing unit  100 , and display the calculated pace of the person on a display located on the wrist-mounted device (e.g., a light crystal display (LCD)). 
     As shown in FIG. 1, in one embodiment of the invention, an accelerometer is mounted within the housing unit  100  such that an acceleration-sensing axis  118  of the accelerometer is substantially parallel to the ground when a bottom surface of the shoe  104  is flat on the ground. This orientation of the acceleration-sensing axis  118  with respect to the ground has been found to be particularly advantageous in determining both the moment that the foot leaves the ground and the moment that the foot comes in contact with the ground. To orient the acceleration-sensing axis  118  as shown in FIG. 1, an accelerometer may be mounted within the housing unit  100  such that the acceleration-sensing axis  118  is at an angle θ a  with respect to a plane of the housing unit  100  (which plane is indicated by the dashed line  130 ). Examples of how an accelerometer may be mounted within the housing unit  100  to orient the acceleration axis  118  as shown in FIG. 1 are described in detail below in connection with FIG.  7 . 
     In the FIG. 1 embodiment, the foot-mount device comprising the housing unit  100  and the mounting unit  102  is located above a bottom surface  108  of the user&#39;s foot. Also, in the embodiment shown, the housing unit  104  is self-contained such that, in order to monitor the locomotion of the user, circuitry in the housing unit  100  is not required to interface with any circuitry located within the mounting unit  102 , the sole  120 , or elsewhere in or on the shoe  104 . It should be appreciated, however, that the housing unit  100 , while being self-contained, may interface with other components, e.g., a wrist-mounted display, via a wireless communication link. 
     Because, in the FIG. 1 embodiment, a solid-state accelerometer may be employed (in the housing unit  100 ) to sense motion of the shoe  104 , no devices that require external physical forces on them to sense motion (e.g., pressure-sensitive switches or piezoelectric sensors) need to be employed in connection with the FIG. 1 embodiment. Therefore, no components need to be employed that may be damaged due to the forceful impacts of foot strikes. 
     FIGS. 2 and 3 show top and bottom perspective views, respectively, of one embodiment of a shoe-mount device (such as that shown in FIG. 1) when the housing unit  100  and the mounting unit  102  are separate from one another. As shown in FIGS. 2 and 3, the mounting unit  102  may include a pedestal  102   a  and a camber (i.e., a slightly curved surface)  102   b.    
     The pedestal  102   a  may be made of a material of sufficient flexibility such that the cantilever  116  is permitted to bend slightly when the housing unit  100  is attached to the mounting unit, as described below. The camber  102   b  may be curved so as to conform to a shape of the instep of the shoe  104  on which it is to be mounted. As shown in FIGS. 1-3, the positioning of the camber  102   b  about the rounded instep of the shoe  104  and the tension of the shoelace portions  110   a-e  (when laced tightly) serve to prevent the mounting unit  102  from moving in all six degrees of freedom (i.e., three degrees of translation and three degrees of rotation) with respect to the shoe  104 . It should understood, however, that alternative embodiments of the invention may employ mounting units that, when mounted to the shoe  104 , have fewer that all six degrees of freedom restricted, and that the invention is not limited to embodiments in which all six degrees of freedom of the mounting device  102  with respect to the shoe  104  are restricted. 
     As shown in FIGS. 2 and 3, the housing unit  100  may include a tapered groove  302  and the mounting unit  202  may include a tapered tongue  202 , which tongue  202  and groove  302  are shaped so as mate with one another when the two units are attached together. An example of a suitable taper in the tongue  202  and groove  302  is illustrated, for example, by the angle θ b  (see FIG.  6 ). As shown, the tongue  202  and groove  302  may be tapered a sufficient amount that the housing unit  100  is prevented from being separated vertically from the mounting unit  102  when the two units are attached together. 
     Also show in FIGS. 2 and 3 are: (1) a ramp  206  on an upper surface  204  of the pedestal  102   a , (2) a notch  306  on a lower surface  306  of the housing unit  100 , and (3) the curved back end  128  of the housing unit  100 . When the housing unit  100  is attached to the mounting unit  102 , as described below, the tongue  202  is inserted fully into the groove  302  and a shoulder  208  of the ramp  206  engages an inner wall  308  of the groove  306  so as to prevent the tongue  202  from being removed from the groove  302 . The back end  128  is curved inwardly in the embodiment shown so as to permit the back end  128  of the housing unit  100  to wrap slightly around the instep of the shoe  104 . 
     FIG. 4 shows an example of how the housing unit  100  and the mounting unit  102  of the example embodiment of FIGS. 1-3 may be releasably attached to one another. As shown, the housing unit  100  and the mounting unit  102  may be oriented such that the bottom surface  304  of the housing unit  100  is in approximately the same plane as the top surface  204  of the mounting unit  102 , and the housing unit  100  may be slid toward the mounting unit  102  (in a direction indicated by the arrow  402 ) when the devices are so oriented. The tongue  202  may be slid into the groove  302  until a top section  210  of the ramp  206  contacts a front portion  310  of the bottom surface  304 , thereby causing the cantilever  116  to bend slightly toward the attachment member  114 . When the cantilever  116  bends downward, the top section  210  of the ramp  206  may be caused to slide across the front portion  310  of the bottom surface  304  until the shoulder  208  of the ramp  206  engages the inner wall  308  of the notch  306 . 
     When the housing unit  100  and the mounting unit  102  are attached together as shown in FIGS. 5 and 6, the engagement of the shoulder  208  with the inner wall  308 , in addition to the engagement of the tapered tongue  202  within the tapered groove  302 , prevents the housing unit  100  from moving with respect to the mounting unit  102  in all six degrees of freedom. It should appreciated, however, that alternative embodiments of the invention may employ housing units and mounting units that, when attached together, restrict fewer that all six degrees of freedom of the housing units with respect to the mounting units, and that the invention is not limited to embodiments in which all six degrees of freedom are restricted. 
     FIG. 5 shows an example of how the housing unit  100  and the mounting unit  102  of the example embodiment shown in FIGS. 1-3 may be disengaged from one another. As shown, the user may push the cantilever  116  toward the attachment member  114  of the mounting unit  102  (as indicated by the arrow  502 ) such that the shoulder  208  of the ramp  206  is disengaged from the inner wall  308  of the notch  306 , and the housing unit  100  may simultaneously be slid away from the mounting unit  102  (in a direction indicated by the arrow  504 ) so that the upper surface  210  of the ramp  206  slides a slight distance across the front portion  310  of the bottom surface  304  of the housing unit  100 . In this manner, the tongue  202  of the mounting unit  102  may be completely removed from the groove  302  of the housing unit  100 , and the two devices may be separated from one another. 
     FIG. 6 shows a front view of the example shoe-mount device shown in FIGS. 1-5 when the housing unit  100  is attached to the mounting unit  102 . FIG. 6 illustrates, among other things, the curved surface of the camber  102   b  that may mate with the instep of the shoe  104 , and the tapering of the tongue  202  and the groove  302  at the angle θ b , which prevents the housing unit  100  from being separated vertically from the mounting unit  102  when these two units are attached together. 
     FIG. 7 shows an example of how an accelerometer  704  may be mounted with respect to the housing unit  100  such that the acceleration-sensing axis  118  of the accelerometer  704  forms an acute angle θ a  with respect to a plane of the housing unit. In the FIG. 7 example, the accelerometer  704  is mounted on an mounting device  702 , which is secured to a circuit board  700  such that an acute angle θ a  is formed between a plane of the mounting device  702  and a plane of the circuit board  700 . Alternatively, the accelerometer  704  may be mounted flush with the circuit board  700 , and the circuit board  700  may itself be oriented at an acute angle θ a  with respect to the plane of the housing unit  100 . As mentioned above, this orientation of the acceleration-sensing axis  118  at an acute angle with respect to the plane of the housing unit  100  permits the housing unit  100  to be secured to the instep of the shoe  104 , with the acceleration-sensing axis  118  being oriented substantially parallel with the ground when the shoe is flush on the ground. 
     It should be appreciated that, in some embodiments wherein an accelerometer is used to measure the impact of the shoe  104  with the ground, the accelerometer  704  may be oriented within the mounting unit  100  such that the acceleration-sensing axis  118  forms an acute angle with respect to the plane of the mounting unit and/or the plane of the circuit board  700 , and is oriented perpendicular to the ground when the shoe  104  is flat on the ground. 
     FIG. 8 shows an example of how an accelerometer having at least two orthogonal acceleration-sensing axes may be used to measure the acceleration in a direction other than the directions in which the acceleration-sensing axes are oriented. As shown, the accelerometer  704  may be mounted flush on the circuit board  700 , which circuit board  700  is oriented parallel to a plane of the housing unit  100 . In the embodiment shown, components of both an “x” acceleration-sensing axis  802  and a “z” acceleration-sensing axis  804  are combined using a trigonometric function (z*sin (θ a )+x*cos (θ a )) in order to sense the acceleration in the direction indicated by the arrow  806 , which extends at an acute angle θ a  with respect to the “x” acceleration-sensing axis  802 . It should be appreciated that the trigonometric function used in the FIG. 8 embodiment is only one example of an equation that may be used to combine the components of two acceleration-sensing axes according to this aspect of the invention, and that the invention is not limited to the use of the particular equation shown. In one embodiment, for example, the components of the “x” and “z” acceleration-sensing axes  802  and  804  are merely added together without using a trigonometric function to weigh the contribution of each. 
     Having described at least one embodiment in detail, various modifications and improvements will readily occur to those skilled in the art. Such modifications and improvements are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description is by way of example only, and is not intended to be limiting.