Abstract:
A sensor uses an accelerometer to measure acceleration values in two axes to detect if a tip-over angle of a system has exceeded a tip-over threshold angle α. Each acceleration value is respectively multiplied by a corresponding factor a, b. The two factors a, b are chosen as a function of the tip-over threshold angle α. Two values are calculated and each calculated value is compared to zero. Depending upon which values are greater than or less than zero determines whether the tip-over angle has been exceeded. The detector, upon sensing of a tipped-over condition, provides a signal indicative of that condition. The output signal can be employed to trigger an alarm or to shut down a device that has tipped over or to otherwise denote the tipped-over condition.

Description:
RELATED APPLICATION 
       [0001]    This application is a non-provisional application claiming priority under 35 U.S.C. §119(e) to U.S. provisional application Ser. No. 61/835,104, entitled “Tip-Over Sensor” filed on Jun. 14, 2013, the entire contents of which is incorporated herein by reference for all purposes. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    It is well-known that accelerometers can be used to measure an angle of inclination of an apparatus. A Digital Thermal Orientation Sensor (DTOS) device, available from MEMSIC, Inc., Andover, Mass. can be used to measure an inclination angle in order to determine if the apparatus is being operated within its parameters. An indication that the apparatus has tipped beyond some predetermined angle, i.e., it has “tipped over,” may require intervention in order to maintain safe operating conditions. 
         [0003]    An accelerometer used to determine an inclination angle is, oftentimes, placed in an environment that subjects it to noise, vibration and other adverse conditions. In addition, accelerometers are often used in handheld devices and, therefore, need to be as small as possible while meeting high levels of reliability. 
         [0004]    What is needed, therefore, is an accelerometer that is accurate and reliable under harsh conditions, easily configured and inexpensive. 
       BRIEF SUMMARY OF THE INVENTION 
       [0005]    A tip-over sensor or detector that, upon sensing of a tipping or tipped condition, provides a signal indicative of that condition. The output signal can be employed to trigger an alarm or to shut down a device, for example, but not limited to, a motorcycle, space heater, iron, etc., that has tipped over or to otherwise denote the tip-over condition. 
         [0006]    In one embodiment of the present invention, a method of detecting a tip-over condition includes setting a tip-over threshold angle value α and first and second multiplier values a, b as a function of the tip-over threshold angle value α. An acceleration value (Xm) along an X-axis and an acceleration value (Zm) along a Z-axis are measured. The method includes calculating a first summed value F 1 =(a*Zm−b*Xm) and a second summed value F 2 =(a*Zm+b*Xm) and then determining whether the tip-over condition has occurred as a function of the first and second summed values F 1  and F 2 . 
         [0007]    A tip-over sensor, according to another embodiment of the present invention, includes an accelerometer that measures and outputs an acceleration value Xm along an X-axis and an acceleration value Zm along a Z-axis. A multiplier multiplies the acceleration value Xm by a first multiplier value (b) and multiplies the acceleration value Zm by a second multiplier value (a) and outputs (b*Xm) and (a*Zm) where the first and second values (a) and (b) are set as a function of a tip-over threshold angle α. A summer outputs a first summed value F 1 =(a*Zm+b*Xm) and a second summed value F 2 =(a*Zm−b*Xm). An orientation detector determines if the tip-over threshold angle α has been reached as a function of the first and second summed values F 1 , F 2 . 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0008]    Various aspects of at least one embodiment of the present invention are discussed below with reference to the accompanying figures. It will be appreciated that for simplicity and clarity of illustration, elements shown in the drawings have not necessarily been drawn accurately or to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity or several physical components may be included in one functional block or element. Further, where considered appropriate, reference numerals may be repeated among the drawings to indicate corresponding or analogous elements. For purposes of clarity, not every component may be labeled in every drawing. The figures are provided for the purposes of illustration and explanation and are not intended as a definition of the limits of the invention. In the figures: 
           [0009]      FIG. 1  is a conceptual representation of an embodiment of the present invention on a motorcycle; 
           [0010]      FIG. 2  is a representation of a coordinate system in accordance with an embodiment of the present invention; 
           [0011]      FIG. 3  is a functional block diagram of a tip sensor in accordance with an embodiment of the present invention; 
           [0012]      FIG. 4  is a schematic diagram of a tip sensor in accordance with an embodiment of the present invention corresponding to that shown in  FIG. 3 ; 
           [0013]      FIG. 5  is a block diagram of a programmable capacitor; 
           [0014]      FIGS. 6 and 7  are functional block diagrams of the tip sensor of  FIG. 4  at different stages of operation; 
           [0015]      FIG. 8  is a functional block diagram of a tip sensor in accordance with an embodiment of the present invention; 
           [0016]      FIGS. 9-11  are functional block diagrams of the tip sensor of  FIG. 8  at different stages of operation; 
           [0017]      FIG. 12  is a functional block diagram of a tip sensor in accordance with an embodiment of the present invention; and 
           [0018]      FIG. 13  is a flowchart of a method of operation of a tip sensor in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]    Provisional application Ser. No. 61/835,104, entitled “Tip-Over Sensor,” filed on Jun. 14, 2013, is incorporated herein by reference in its entirety for all purposes. 
         [0020]    In the following description, numerous specific details are set forth in order to provide a thorough understanding of the embodiments of the present invention. It will be understood by those of ordinary skill in the art that these embodiments of the present invention may be practiced without some of these specific details. In other instances, well-known methods, procedures, components and structures may not have been described in detail so as not to obscure the embodiments of the present invention. 
         [0021]    Prior to explaining at least one embodiment of the present invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments or of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein are for the purpose of description and should not be regarded as limiting. 
         [0022]    It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention, which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination. 
         [0023]    The present invention provides a tip-over sensor that utilizes, in one embodiment, a MEMS thermal accelerometer that is extremely robust, reliable and shock tolerant. The device is particularly well suited to harsh or high vibration environments such as for use in motorcycles and other vehicles. 
         [0024]    A tip-over sensor in accordance with the invention can be implemented, in one embodiment, to provide an interface for a user to program, i.e., to choose, a tip-over threshold angle from a plurality of thresholds. If the device orientation with respect to a reference exceeds the programmed tip-over threshold angle, a digital output changes state to provide an alert of a tip-over, or fall down, event. In another embodiment, the threshold angle can be preprogrammed, i.e., pre-selected or hard-wired, into the device. 
         [0025]    Referring now to  FIG. 1 , a two-axis accelerometer  105 , or sensor  105 , (not to scale), is mounted on, for example, a motorcycle  110 . It has two axes of sensitivity: X and Z. When the motorcycle  110  is in “normal” riding position, i.e., vertical, the sensor  105  senses 1 g of acceleration on the Z-axis and 0 g acceleration on the X-axis. If the motorcycle  110  leans to one side, the sensor  105  will register a smaller acceleration value on the Z-axis, and a nonzero acceleration value on the X-axis. The X-axis signal value may be positive or negative depending on whether the motorcycle  110  is leaning to the left or to the right, looking in the direction toward the front wheel, i.e., the normal orientation of an operator or rider. 
         [0026]    Now, consider, if a lean angle (LA) is equal to a predetermined tip-over threshold angle (α), the following relationship will hold if the motorcycle  110  is leaning to the right: 
         [0000]        Z= 1 g *cos(α)  Eq. (1)
 
         [0000]        X= 1 g *sin(α)  Eq. (2)
 
         [0027]    Equations 1 and 2 can be rewritten as: 
         [0000]        Z *sin(α)= X *cos(α)  Eq. (3)
 
         [0028]    For purposes of explaining an embodiment of the present invention, both sides of Eq. (3) are multiplied by a coefficient based on two scale factors, a and b. The values for a and b are chosen such that the following ratio is satisfied: 
         [0000]        a/b =tan(α)  Eq. (4)
 
         [0029]    Eq. (3) can be re-written as: 
         [0000]        a*Z=b*X   Eq. (5)
 
         [0030]    Or, as: 
         [0000]        a*Z−b*X= 0  Eq. (6)
 
         [0031]    If the motorcycle  110  is leaning to the left, and the tip-over threshold angle is still α (or − 60  to be mathematically correct), Eqs. 5 and 6 become: 
         [0000]        a*Z=−b*X   Eq. (7)
 
         [0000]      and: 
         [0000]        a*Z+b*X= 0  Eq. (8)
 
         [0032]    For discussion purposes, an orientation vector of the motorcycle  110  is defined as going through the spine of the rider, starting at her seat and exiting at her head, i.e., the Z-axis. 
         [0033]    Referring now to  FIG. 2 , the possible orientation of the motorcycle  110  is considered to be in one of four quadrants: TOP, RIGHT, BOTTOM and LEFT. These quadrants are limited by the boundaries defined by the tip-over threshold angle α. 
         [0034]    If the orientation vector of the motorcycle  110 , i.e., the lean angle (LA) is in the TOP quadrant, the following is true: 
         [0000]        a*Z−b*X&gt; 0  Eq. (9)
 
         [0000]        a*Z+b*X&gt; 0  Eq. (10)
 
         [0035]    If the orientation vector of the motorcycle  110  is in the RIGHT quadrant, the following is true: 
         [0000]        a*Z−b*X&lt; 0  Eq. (11)
 
         [0000]        a*Z+b*X&gt; 0  Eq. (10)
 
         [0036]    If the orientation vector of the motorcycle  110  is in the BOTTOM quadrant, the following is true: 
         [0000]        a*Z−b*X&lt; 0  Eq. (11)
 
         [0000]        a*Z+b*X&lt; 0  Eq. (12)
 
         [0037]    The BOTTOM orientation is unlikely to occur with a motorcycle, but is easily achievable with a watercraft, such as a jet ski or similar vehicle, where this orientation most likely indicates the watercraft is upside-down in the water and the operator, hopefully, unhurt and nearby treading water. 
         [0038]    If the orientation vector of the motorcycle  110  is in the LEFT quadrant, the following is true: 
         [0000]        a*Z−b*X&gt; 0  Eq. (9)
 
         [0000]        a*Z+b*X&lt; 0  Eq. (12)
 
         [0039]    Thus, by evaluating the signs of each of the two quantities (a*Z−b*X) and (a*Z+b*X), it can be determined whether or not the lean angle LA exceeds the tip-over threshold angle α. 
         [0040]    In an actual implementation, e.g., on the motorcycle referenced above, because earth&#39;s gravity vector g points “down,” a sensor in accordance with the embodiments described herein would be mounted with its positive Z-axis orientation also directed “downward.” One of ordinary skill in the art would understand, therefore, that the Z-axis would be pointed downward in  FIG. 2  and that the formulae for the TOP and BOTTOM quadrants would be swapped. 
         [0041]    Referring now to  FIG. 3 , in one embodiment of the present invention a tip-over sensor system  300  includes a two-axis accelerometer  302  that generates two voltages proportional to the X- and Z-axis accelerations, respectively. Two multipliers  304 - 1  and  304 - 2  are provided along with two programmable coefficient generators  306 - 1  and  306 - 2 . The first programmable coefficient generator  306 - 1  is set to the “b” value while the second programmable coefficient generator  306 - 2  is set to the “a” coefficient value. The first multiplier provides the value b*X and the second multiplier  304 - 2  provides the value a*Z. 
         [0042]    The outputs of the two-axis accelerometer  302  may be presented to signal conditioning circuits, e.g., pre-amplifiers, CN converters, filters, offset adjustment circuits, etc., prior to being sent to the multipliers. Such signal conditioning circuits may either be externally provided with respect to the accelerometer or internally integrated. 
         [0043]    In addition, the two multipliers  304 - 1  and  304 - 2  may be implemented using amplifiers with different gain values as is understood by one of ordinary skill in the art. 
         [0044]    First and second summers  308 - 1 ,  308 - 2  are provided where the first summer  308 - 1  provides an output of (a*Z+b*X) and the second summer  308 - 2  provides an output equal to (a*Z−b*X) as the second summer  308 - 2  has an inverting input at which it receives the value b*X. A first sign detector  310 - 1  determines a sign of the output from the first summer  308 - 1  while a second sign detector  310 - 2  determines a sign of the output from the second summer  308 - 2 . The respective outputs from the first and second sign detectors  310 - 1 ,  310 - 2  are received at an orientation detector  312  that provides a binary signal indicating the leaning condition of the sensor  302  with respect to the coefficient values a, b. 
         [0045]    In addition, first and second amplifiers  314 - 1  and  314 - 2  are used to provide Z and X acceleration values directly. 
         [0046]    Alternatively, the orientation detector  312  may include a logic function to determine which quadrant the device is in, e.g., generate a two-bit output based on the outputs of the first and second sign detectors  310 - 1 ,  310 - 2  and then a functional logic block to determine if the quadrant is allowed or not. 
         [0047]    In an alternate embodiment of the present invention, referring now to  FIG. 4 , another tip-over sensor system  400  is provided that operates in similar fashion as the tip-over sensor system  300  shown in  FIG. 3 , however, the multipliers  304  and programmable coefficient generators  306  are replaced with adjustable capacitors and switches as will be described in more detail below. 
         [0048]    The first multiplier  304 - 1  and corresponding programmable coefficient generator  306 - 1  are replaced by an adjustable capacitor  402 - 1  having a value of C b  and a number of switches  404 ,  406 ,  408  and  410 . Similarly, the second multiplier  304 - 2  and programmable coefficient generator  306 - 2  are replaced by a second adjustable capacitor  402 - 2  having a value of C a  and switches  412 ,  414 ,  416  and  418 . With respect to switches  406 ,  410 ,  414  and  418 , one end is coupled to a node of the respective capacitor while the other end is coupled to an analog ground  420 . Analog ground  420 , however, is not necessarily a zero volt reference. 
         [0049]    The adjustable capacitors  402 - 1 ,  402 - 2 , and their corresponding switches, operate as a sampling capacitor to sample both positive and negative X and Z output voltages from the sensor  302 . With this circuit of switched capacitors, the variable of interest is the charge built up on each of the capacitors. In order to establish the a/b ratio, the first capacitor  402 - 1  is set to a value of b picoFarads and the second capacitor  402 - 2  is set to a value of a picoFarads. 
         [0050]    Each of the first and second capacitors  402 - 1 ,  402 - 2  is itself an adjustable capacitor module  500  as shown in  FIG. 5 . Here, in one non-limiting example, seven capacitors C 1 -C 7  are arranged amongst six pairs of switches SW 1 A, SW 1 B; SW 2 A, SW 2 B; SW 3 A, SW 3 B; SW 4 A, SW 4 B; SW 5 A, SW 5 B; and SW 6 A, SW 6 B. As is understood by one of ordinary skill in the art, by opening and closing specific switches, in order to place capacitors in parallel and/or in series with one another, or in equivalent t-network or π-network configurations, a number of different capacitive values can then be chosen, i.e., the a and b values described above, and presented across the output terminals C IN  and C OUT . 
         [0051]    Control signals are sent to the switches to open and close the respective switches to obtain the desired capacitance value. In this embodiment, the six pairs of switches SW 1 A, SW 1 B, SW 2 A, SW 2 B, SW 3 A, SW 3 B, SW 4 A, SW 4 B, SW 5 A, SW 5 B, SW 6 A, SW 6 B are operated such that, in each pair, one is opened when the other is closed. Thus, for example, switches SW 1 A, SW 1 B are operated such that a first control signal is sent to switch SW 1 A and an inverted version of the first control signal is sent to SW 1 B. 
         [0052]    In one embodiment, the capacitors C 1 -C 7  are implemented as CMOS parallel plate capacitors and the switches are implemented with, for example, CMOS FET devices or NMOS transistors as is well understood. Of course, any number of capacitors and switches, and associated technologies, can be used. Further, the control signals (not shown) could be provided to the module  500  to open and close the switches, via any one of a number of known signaling schemes including, but not limited to, I 2 C or parallel logic pins or inputs as needed and as is well understood by one of ordinary skill in the art. 
         [0053]    Advantageously, if capacitors C 3 , C 6  and C 7  are each 50 fF and capacitors C 1  and C 4  are each 200 fF and capacitors C 2  and C 5  are each 100 fF, then based on the operation of the switches, the output capacitance can be adjusted in a range from 0 to 393.75 fF in steps of 6.25 fF. Of course, this is just an example and not intended to be limiting. 
         [0054]    Referring now to  FIG. 6 , the switches  404 - 410  and  412 - 418  are alternately opened and closed in order to build up charge on the sampling capacitors  402 - 1 ,  402 - 2 . As shown in  FIG. 6 , when acquiring charge on the sampling capacitors, switches  404  and  410  are closed while switches  406  and  408  are open. Similarly, switches  412 ,  418  are closed and switches  414 ,  416  are open in order to build up charge on the second sampling capacitor  402 - 2 . The timing of the opening and closing of the switches is under the control of a device, not shown here, but the operation of which is easily understood by one of ordinary skill in the art. 
         [0055]    The charge built up on the sampling capacitors is then transferred to the summers  308 - 1 ,  308 - 2  by opening the switches that are closed and then closing the switches that are open. It should be noted that the switches operate, generally, in a break-before-make mode of operation, as is understood by one of ordinary skill in the art. The control signals of the switches can be provided by a clock generator providing appropriate non-overlapping signals as understood by one of ordinary skill in the art. 
         [0056]    Referring now to  FIG. 7 , switches  404 ,  410  are opened and switches  406 ,  408  are closed while switches  412 ,  418  are opened and switches  414  and  416  are closed. As a result, the charge values on the sampling capacitors are presented to the summers for the tip-over determination as has been described above. 
         [0057]    In another embodiment of the present invention, a tip-over sensor apparatus  800 , as shown in  FIG. 8 , includes an analog-to-digital converter (ADC)  802 , a positive summing junction (SJP)  804  and a negative summing junction (SJN)  806 . The same programmable capacitors, as described above in  FIG. 7 , and their corresponding switches, are used and function as has already been described. It should be noted that the embodiment shown in  FIG. 8  depicts the sensor  302  as having differential X and Z outputs. Such differential outputs are well known to those of ordinary skill in the art and the operations of the previously described embodiments, although described as single-ended signals for convenience, could be easily modified for differential values as is well understood. 
         [0058]    In one embodiment of the present invention, the positive and negative summing junctions SJP, SJN and the first and second summers  308 - 1 ,  308 - 2  may be implemented, using analog techniques, with the summing junctions of opamps, as known to one of ordinary skill in the art. 
         [0059]    Returning to the tip-over sensor system  800 , additional switches  810 - 824  are provided to couple the charges on the sampling capacitors to either of the positive or negative summing junctions  804 ,  806  as will be described below. 
         [0060]    Referring to  FIG. 9 , switches  404 ,  404 - 1 ,  410 ,  410 - 1 ,  412 ,  412 - 1 ,  418  and  418 - 1  are closed in order to acquire charge on the sampling capacitors. Subsequently, those switches are opened and switches  406 ,  406 - 1 ,  412 ,  412 - 1 ,  408 ,  408 - 1 ,  416  and  416 - 1  are closed as shown in  FIG. 10 . Additionally, as shown in  FIG. 9 , switches  812  and  824  are closed in order to place the X+ and Z− signals on the negative input of the ADC  802  via the SJN  806 . Further, switches  814  and  818  are closed in order to provide the X− and Z+ signal on the positive input of the ADC  802 , as shown in  FIG. 9 . As a result, the calculation of (a*Z−b*X) is determined by the ADC  802  via the SJP  804 . One of ordinary skill in the art will understand that one or more switches shown herein may not be necessary and could be removed. The representations herein are exemplary in order to aid in the understanding of the operation of the various embodiments of the present invention and, therefore, the circuits shown are not intended to be limiting. 
         [0061]    In one embodiment of the present invention, the operation of the device transitions between the states shown in  FIGS. 9 and 10  multiple times in order to provide a delta-sigma, or oversampling, operation with the ADC. 
         [0062]    Subsequently, referring to  FIG. 11 , switches  812  and  814  are opened and switches  810  and  816  are closed. As a result, the X+ signal and the Z+ signal are provided to the positive summing junction  804  and to the plus input of the ADC  802  and the X− and Z− charges are provided onto the negative summing junction  806  and to the negative input of the ADC  802  which then determines the equation a*Z+b*X in order to then determine whether the tip-over threshold angle α has been exceeded. 
         [0063]    In operation, the device&#39;s states transition from that shown in  FIG. 10  to the state shown in  FIG. 9  and then to the state shown in  FIG. 11 . There is a return to the  FIG. 9  state in order to recharge the capacitors, i.e., to take another sample of acceleration values, because presenting a capacitor to a summing junction transfers the charge from the capacitor to the junction and, at the end of this process, the capacitor is discharged and has lost its information. 
         [0064]    As set forth above, the JSP  804  and JSM  806  are provided as inputs to the ADC  802 . The ADC  802  is being used to determine whether the charge values (a*Z−b*X) or (a*Z+b*X) are positive or negative. In operation, the determination may be made by observing the most significant bit (MSB) of the output of the ADC  802 . Alternatively, a comparator could be used in place of the ADC. 
         [0065]    A matrix/ADC module  830  can be defined as including the ADC  802  and the sampling capacitors  402  and the corresponding switches as outlined within the dotted line shown in  FIG. 8 . As a result, a further embodiment as shown in  FIG. 12  includes the differential acceleration sensor  302  feeding the differential X, Z signals to a first matrix/ADC module  830 - 1  that receives a/b control signals to set the variables a, b in order to determine the tip-over threshold angle, as has been described above, as well as to a second matrix/ADC module  830 - 2  that receives a separate set of a, b control signals to output the X, Z signals. Here, for the second matrix/ADC module  830 - 2 , the value (a) would be set equal to the value (b) in order to provide the measures of the magnitudes of accelerations in the X and Z-axes. 
         [0066]    The output of the first matrix/ADC  830 - 1  is sent to a digital low-pass filter  902  in order to remove the effects of vibration and other extraneous conditions on the measured signals. Subsequently, the output of the low-pass filter  902  is sent to the sign detector  310  and to the orientation detector  312  to operate as has already been described above. 
         [0067]    In an alternate embodiment of the system shown in  FIG. 12 , the second matrix/ADC module  830 - 2  is not included. In that case, the tip-over system operates in four phases. During phases one and two, the first matrix/ADC module  830 - 1  is operated with the values a, b set in the ratio a/b, as described, to calculate the quantities (a*Z−b*X) and (a*Z+b*X) which are then evaluated and the orientation is determined. The first matrix/ADC module  830 - 1  is then used to measure the X raw acceleration value during phase three when the values a, b are set to 0, 1, respectively, and during phase four the values a, b are set to 1, 0, respectively, to measure the Z-axis acceleration. 
         [0068]    Further, instead of two ADCs, a single ADC may be implemented to sequentially measure X, Z, (a*Z−b*X) and (a*Z+b*X). Further still, four ADCs could be used, one for each of the variables or measurements. One of ordinary skill in the art will understand how this would be implemented. 
         [0069]    In accordance with a method of operation as shown in  FIG. 13 , a tip-over threshold angle α is set in step  1302  and as a result the values a, b are set in step  1304 . At step  1306 , the X- and Z-axis accelerations are measured and the two calculations (a*Z−b*X) and (a*Z+b*X) are calculated in step  1308 . Although shown as separate steps  1306 ,  1308 , these occur simultaneously and are only shown separated for explanation purposes. The results of those calculations are each compared to a threshold value, i.e., zero, as described above, in step  1314 , where the orientation is identified, i.e., the quadrant is identified by the comparisons of the two equations with zero. Subsequently, in step  1316 , if the identified quadrant is determined as being an allowed quadrant, then control passes back to step  1306  for continued measure of the lean angle. If, however, at step  1316  it is determined that the device is now oriented in a “not allowed” quadrant then control passes to step  1318  where a signal indicating such condition may be asserted. 
         [0070]    In the application to a motorcycle, one would have a single “allowed” quadrant and consider the remaining three as “prohibited.” In the jet ski application, one may also want to have a single “upside down” quadrant as “prohibited,” where the three other remaining quadrants are “allowed.” In that case, if the sensor is mounted “upside down” and the output polarity is inverted, such a condition can be detected. By identifying the quadrant and then determining whether the identified quadrant is allowed or not, both cases can be implemented. 
         [0071]    Returning now to step  1308 , in an alternate embodiment, additional steps  1310  and  1312  are inserted between steps  1308  and  1314 . In step  1310  the values of the X and Z accelerations are each compared to a threshold level. And if, at step  1312  these values are within the appropriate threshold, then control passes back to step  1314  to compare the tip-over angle calculations. If, however, at step  1312  it is determined that X and Z are not within the appropriate thresholds, then control passes back to step  1306  without a tip-over determination because the X, Z signals are not sufficient. 
         [0072]    The determinations in steps  1310 ,  1312  are provided to make sure that the acceleration signals are valid for determining the tip-over angle. In one embodiment of the present invention, if the larger of the X and Z acceleration value is less than ⅜ g, then it is determined that the signals being measured by the sensor are not strong enough to make a valid determination as to the lean angle of the apparatus to which the tip-over angle sensor is connected. 
         [0073]    In the foregoing description, one embodiment of the present invention included two variable capacitors that could be programmed with different capacitances. In an alternate embodiment, one of the capacitors may be of a fixed value and the other variable. This will provide a simpler device but may be limited as to the number of different tip-over threshold angles that can be selected. 
         [0074]    The embodiments of the present invention may be implemented in a single device, for example, an eight-pin device in an LCC package with inputs and outputs running under, e.g., the I 2 C protocol. Of course, the necessary I/O components, clock, power and bias generators, signal conditioning, etc., although not described herein, nor needed to understand the present invention, would be included. Advantageously, the values a, b and, therefore, the tip-over threshold angle α, could be set via input pins not requiring any operating protocol, and the associated circuitry, thus simplifying the interface. Of course, the values a, b could be pre-set at the factory, and the input pins disabled, in order to provide a device with an already-set tip-over threshold angle α. Further, an ASIC may be provided in the device to operate the timing signals for the opening and closing of the switches described above, in addition to the other functions also described above, as well as any I/O operations that might be needed, as is understood by one of ordinary skill in the art. 
         [0075]    Further, while the two-axis acceleration sensor has been described, in one embodiment, as being a thermal accelerometer, it is envisioned that other types of acceleration sensors could be used. Still further, two single-axis acceleration sensors may be used. One of ordinary skill in the art would understand how this would be implemented. 
         [0076]    Having thus particularly shown and described several features of at least one embodiment of the present invention, it is to be appreciated that various alterations, modifications and improvements will readily occur to those skilled in the art. Such alterations, modifications and improvements are intended to be part of this disclosure and are intended to be within the scope of the invention. Accordingly, the foregoing description and drawings are by way of example only and the scope of the invention should be determined from proper construction of the appended claims, and their equivalents.