Abstract:
An apparatus for detecting a continuous wave (CW) signal including a demodulator demodulating a received CW signal and providing a demodulated signal, an edge detector for detecting an edge of the demodulated signal and thereby, detecting the CW signal and a switchable short-circuit of the demodulator, edges being formed in the demodulated signal when the demodulator is temporarily short-circuited during receipt of the CW signal.

Description:
TECHNICAL FIELD 
       [0001]    The present invention is directed to a tire pressure monitoring system and is particularly directed to a method and an apparatus for detecting a continuous low frequency (“LF”) wave signal in a tire pressure monitoring system. 
       BACKGROUND 
       [0002]    A tire pressure monitoring system can be used to inform a vehicle driver of a tire condition problem such as improper tire pressure and/or temperature. Improper inflation pressure information is particularly useful to the driver of a vehicle having “run flat” tires. Before the use of run flat tires, in the event of sudden tire pressure lose (e.g. a tire is punctured) while driving, the driver could usually feel the condition due to a change in the handling characteristics of the vehicle. However, with the advent of the run flat tires, a driver might not detect sudden tire pressure lose. However, it is still important that the driver be informed, even with a run flat tire, when tire pressure and/or temperature values are not within predetermined limits. 
         [0003]    To implement a tire pressure monitoring (“TPM”) system, a vehicle based electronic control unit (“ECU”) can control a plurality of low frequency (“LF”) transmitters, each wheel well having an associated LF transmitter. The LF transmitters transmits a LF signal to an associated TPM sensor mounted within its associated tire assembly secured to the wheel, for example. When the LF signal is received by each TPM sensor, the TPM sensor will process the LF signal, sense the pressure of its tire, and transmits a radio frequency (“RF”) signal back to the vehicle based ECU. The ECU will process the RF signals from all the TPM&#39;s of the tires and provide an output signal to a display that is viewable by the vehicle&#39;s driver that will indicate an abnormal pressure status of any of the tires on the vehicle. 
         [0004]    In some instances, the LF signal provided to each TPM sensor is in the form of an amplitude shift-keying (“ASK”) modulated signal. The TPM sensor would include a demodulator that can demodulate the ASK signal such that the rising edges of the ASK signal can be detected. However, in other instances, the LF signal provided to each TPM sensor is an unmodulated continuous wave (“CW”) signal. Since the CW signal is unmodulated, a TPM sensor would not be able to detect a rising edge of the CW signal if the CW signal is present before the TPM sensor has already been activated, as a demodulated CW wave is a substantially constant waveform. If the CW signal is not present when the TPM sensor is initially activated, then the TPM sensor will detect exactly one rising edge, at the time when the TPM sensor first receives the CW signal. However, in some applications, the CW signal is present before the TPM sensor is activated, such that the TPM sensor will never detect the CW wave. In other applications, the TPM sensor may be activated before the CW signal is first received, but as stated above, the TPM sensor will detect only one rising edge, while many applications would require multiple rising edges to be detected. 
         [0005]    Previous attempts to design a TPM sensor that can detect an LF CW signal have proven to be expensive. Such previous attempts require a separate circuit specifically designed to detect the CW signal, and such circuits usually require a significant number of components and occupy valuable space on a printed circuit board (“PCB”). What is needed is a TPM sensor that can be programmed to detect both, amplitude modulated (“AM”) signals, and CW signals. 
       SUMMARY OF THE INVENTION 
       [0006]    An apparatus for detecting a continuous wave (CW) signal, the apparatus comprising a demodulator demodulating a received CW signal and providing a demodulated signal, an edge detector for detecting an edge of the demodulated signal and thereby, detecting the CW signal and a switchable short-circuit of the demodulator, edges being formed in the demodulated signal when the demodulator is temporarily short-circuited during receipt of the CW signal. 
         [0007]    The present invention can also be described as an apparatus for detecting a CW signal, the apparatus comprising receiving means for receiving the CW signal, demodulating means for determining an envelope of the received CW signal, edge detecting means for detecting an edge of the envelope and thereby detecting the CW wave signal and short-circuiting means for temporarily short-circuiting the demodulating means, edges being formed in the envelope when the demodulating means is temporarily short-circuited during the receipt of the CW signal. 
         [0008]    The present invention can also be described as a method for detecting and processing a CW signal, the method comprising receiving the CW signal at an antenna, providing the CW wave to an input of a demodulator, providing an output of the demodulator to an edge detector, activating a short-circuiting switch to short-circuit the demodulator at predetermined periodical intervals, deactivating the short-circuiting switch to create a rising edge in the output of the demodulator and detecting the rising edge at the edge detector. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The foregoing and other features and advantages of the invention will become apparent to one skilled in the art upon consideration of the following description of the invention and the accompanying drawings in which: 
           [0010]      FIG. 1  illustrates a system that implements an embodiment of the present invention. 
           [0011]      FIG. 2  illustrates an embodiment of the present invention. 
           [0012]      FIG. 3  illustrates another embodiment of the present invention. 
           [0013]      FIG. 4  illustrates an example of a waveform that could be used in an embodiment of the present invention. 
           [0014]      FIG. 5  illustrates a flow chart of a process used in the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]      FIG. 1  illustrates an example embodiment of the present invention. A vehicle  100  includes four tires wells  102 ,  104 ,  106 ,  108 . It is to be understood that the vehicle could include more-or less tire wells. The vehicle also includes at least one electronic control unit (ECU)  110 . Each wheel well  102 ,  104 ,  106 ,  108  includes at least one low frequency (LF) transmitter  112 . Each wheel well further includes at least one tire pressure monitoring (TPM) sensor  114 . The ECU  110  includes at least one antenna  116  for receiving radio frequency (RF) signals. The ECU  110  is connected to each of the LF transmitters  112 . The ECU  110  is also connected to a display  118  that is viewable by a vehicle occupant. 
         [0016]    The ECU  110  provides each LF transmitter  112  with an LF signal. The LF transmitters  112  transmit LF signals to the TPM sensors  114 . The TPM sensors  114  receive the LF signal, process the signal in the processor, sense the current tire pressure of a tire in the corresponding tire well, and generate an RF signal based on that sensed pressure. The ECU  110  receives the RF signals at the antenna  116 , and processes the RF signals. The ECU  110  will then provide a signal to the display  118  to indicate the tire pressure detected by the TPM sensors  114 . 
         [0017]    By way of example, the ECU  110  can provide the display  118  a signal to indicate a tire pressure of “LOW” when the tire pressure detected by the TPM sensors falls below a predetermined value. Additionally or alternatively, the ECU  110  can provide the display  118  a signal to indicate a tire pressure of “HIGH” when the tire pressure detected by the TPM sensors rises above a predetermined value. The display  118  can be, for example, an liquid crystal display (LCD) screen, a constellation of light emitting diodes (LEDs), or a single indicator. Obviously, increasing the complexity of the display system increases the information ascertainable by the vehicle occupant concerning the pressure of the vehicle tires. 
         [0018]    In the present example, the LF signal transmitted to the TPM sensors  114  can be an amplitude modulated (AM) signal or an unmodulated continuous wave (CW) signal. Typically, the AM signal is in the form of an Amplitude. Shift-Keying (ASK) signal. For clarity and convenience, henceforth, the LF signal will be in the form of an unmodulated CW signal, unless otherwise specified. Accordingly, it will be assumed that the CW signals have a substantially constant peak voltage and a substantially constant frequency. However, it is to be understood that the present invention could be designed to detect CW waves at any frequency, and LF signals are shown simply as one method of implementation. Additionally, it is to be assumed that in the present examples, the CW signal is present before the TPM sensor is activated. 
         [0019]      FIG. 2  illustrates an example of an embodiment of a TPM sensor  200  used in  FIG. 1 . The TPM sensor  200  includes a first antenna  202  and a second antenna  204 . The TPM sensor  200  also includes an optional low pass filter  206  connected between the first antenna  202  and an application specific integrated circuit (ASIC)  208  at first and second contact points  214 ,  216 , wherein the first and second contact points  214 ,  216  are both connected to the same node  226 . It is to be understood that the ASIC  208  could alternatively be implemented as other devices including, but not limited to, a microcontroller, or a series of interconnected circuit components. In the present example, the ASIC  208  is software programmable. The ASIC  208  is connected to the second antenna  204 . The ASIC  208  is also connected a pressure sensor  210 . 
         [0020]    In the present example, the ASIC  208  includes a carrier detector (CD) that is shown to be implemented as a demodulator  220  coupled with a processor  222  that can act as an edge detector. Accordingly, the CD could detect a carrier signal by demodulating the carrier signal, and then detecting a rising edge of the demodulated signal. Additionally, in the present example, the ASIC  208  is capable of changing the impedance of at least one of the contact points  214 ,  216  from a high impedance to a low impedance, and then back to a high impedance. 
         [0021]    The first antenna. 202  receives the LF signal  212  and provides the LF signal  212  to the filter  206 . The filter  206  provides a filtered LF signal to the ASIC  208  at the first contact point  214  that has a high impedance. The filter  206  also provides the filtered LF signal to the ASIC  208  at the second contact point  216  that initially has a high impedance. 
         [0022]    The ASIC  208  processes the filtered LF signal by demodulating the filtered LF signal and detecting a rising edge of the demodulated LF waveform. A rising edge on the filtered LF waveform signals the ASIC  208  to initiate a tire pressure measuring sequence wherein the ASIC  208  will signal the pressure sensor  210  to measure the pressure in its corresponding tire (not shown), and then the ASIC  208  will provide the tire pressure information via an RF wave  218  through the second antenna  204  to the ECU  110  shown in  FIG. 1 . However, as stated above, in the present example, the LF signal  212  received by the TPM sensor  200  is unmodulated. Accordingly, the demodulated signal received by the processor  222  has no rising edge. The demodulated signal will be in the form of a substantially constant direct current (DC) signal. Thus, the demodulated signal will not cause the ASIC  208  to initiate the tire pressure measuring sequence. 
         [0023]    As stated above, the ASIC  208  has first and second contact points  214 ,  216  that receive the filtered LF signal. The first contact point  214  has a high impedance. Periodically, the ASIC  208  will change the impedance of the second contact point  216  from a high impedance to a low impedance, such that substantially all of the current from the filtered LF signal will flow into the second contact point  216 , thereby short-circuiting the first contact point  214  and the demodulator  220 . The short-circuiting of the first contact point  214  will reduce the voltage at the first contact point  214  to a level that is approximately zero (electrical neutral). Then, after waiting a predetermined amount of time (e.g. 5 milliseconds), the ASIC  208  will change the impedance of the second contact point  216  back to a high impedance from a low impedance. Then the filtered LF signal will be demodulated by the demodulator  220  in the ASIC  208  such that the demodulated signal will appear to the processor of the ASIC  208  as having a rising edge about the time that the ASIC  208  switches the second contact point  216  from a low impedance to a high impedance. Accordingly, the filtered LF signal received at the first contact point  214  will have a rising edge, detectable by the processor  222  in the ASIC  208 . As stated above, the rising edge will cause the ASIC  208  to initiate the tire pressure measuring sequence, and as such, the ASIC  208  will then signal the pressure sensor  210  to measure the pressure of the corresponding tire (not shown) and provide the RF signal  218  to the ECU  110  of  FIG. 1  indicative of the measured pressure. The frequency of this switching of the second contact point  216  from a high impedance to a low impedance and then back to a high impedance can be programmed to meet the needs of specific applications, and can typically range between several minutes and several hours. 
         [0024]      FIG. 3  illustrates another embodiment of a TPM sensor  300  that could be implemented in the system illustrated in  FIG. 1 . The TPM sensor  300  includes a first antenna  302 , and an optional low pass filter  304  connected to an ASIC  306  at a first, second, third and fourth contact point  308 ,  310 ,  312 ,  314 . Initially, all four of the ASIC contact points  308 ,  310 ,  312 ,  314  are input ports with a high impedance. The first and third contact points  308 ,  312  are both connected to a first node  330 , while the second and fourth contact points are both connected to a second node  332 . The ASIC  306  is also connected to a pressure sensor  316  and a second antenna  318 . 
         [0025]    In the present example, the ASIC  308  includes a CD that can be in the form of an envelope detector  326  coupled with a processor  328  that can act as an edge detector. Accordingly, the ASIC  306  can detect a carrier signal by demodulating a signal using the envelope detector  326  providing the demodulated signal (or envelope of the filtered CW) signal to the processor  328  and then detecting a rising edge of the demodulated signal using the processor  328 . Additionally, in the present example, the ASIC  306  is software programmable, and is capable of switching at least two of the contact points from input ports to output ports. 
         [0026]    In the present embodiment, the first antenna  302  is an inductor  320 . Connected in parallel with the inductor  320  is the optional low pass filter  304 . In the present embodiment, the low pass filter  304  is shown as a resistor  322  and a capacitor  324  connected in parallel. It is to be understood that other components could comprise the low pass filter  304 , and  FIG. 3  illustrates only a resistor  322  and a capacitor  324  for purposes of simplicity. 
         [0027]    The first antenna  302  receives an LF signal. The low pass filter  304  is connected in parallel with the antenna  302  such that the low pass filter  304  filters the LF signal and provides a filtered LF signal to the ASIC  306 . The ASIC  306  receives the filtered LF signal at the first, second, third and fourth contact points  308 ,  310 ,  312 ,  314 . The ASIC  306  will demodulate the filtered LF signal using the ASIC&#39;s  306  envelope detector  326 . The ASIC  306  will also detect a rising edge of the demodulated signal using the ASIC&#39;s processor  328 . 
         [0028]    When the ASIC  306  detects a rising edge of the demodulated signal, the ASIC  306  initiates a tire measure sequence. In this tire measure sequence, the ASIC  306  signals the pressure sensor  316  to measure the tire pressure in a corresponding tire (not show). The ASIC  306  then processes the information received from the pressure sensor  316 , and provides an RF signal indicative of the tire pressure to the second antenna  318  for transmission to the ECU  110  shown in  FIG. 1 . 
         [0029]    As stated above, the LF signal is an unmodulated CW signal. Accordingly, the demodulated signal in the present example does not have a rising edge that is detectable by the ASIC  306 . In the present embodiment, the ASIC  306  will periodically change the third and fourth contact points  312 ,  314  from input ports with a high impedance to output ports driving a “LOW” signal, at a low impedance, while maintaining the first and second contact points  308 ,  310  as input ports with a high impedance. Accordingly, substantially all of the current flowing into the ASIC  306  will flow into the third and fourth contact points  312 ,  314 , and the voltage at the first and second contact points  308 ,  310  will reduce to a value of about zero (electrical neutral), effectively short-circuiting the first and second contact points  308 ,  310  as well as the demodulator  326 . Then, after predetermined amount of time, (e.g. about 1-4 milliseconds) wherein the predetermined amount of time varies based on the values of the various circuit components used as well as the frequency of the LF signal, the ASIC  306  will switch the third and fourth contact points  312 ,  314  back from output ports with a low impedance to input ports with a high impedance. This second switching of the third and fourth contact points  312 ,  314  will cause the first and second contact points  308 ,  310  to receive a waveform that when demodulated by the ASIC&#39;s  306  demodulator  326 , appears to have a rising edge. 
         [0030]    When the ASIC  306  detects the rising edge, the ASIC  306  will initiate the tire pressure measuring sequence and will signal the pressure sensor  316  to detect the pressure in the corresponding tire (not shown), and then the ASIC  306  will process the information received from the pressure sensor  316 . The ASIC  306  will then send a signal indicative of the tire pressure via an RF signal through the second antenna  318  to the ECU  110  shown in  FIG. 1 . After sending the RF signal, the ASIC  306  will wait a predetermined amount of time that can be, for example, programmed into the ASIC  306 , before repeating its switching of the third and fourth contact points  312 ,  314  from high impedance input ports to low impedance output ports. This predetermined period of time could range, for example, between several minutes and a several hours. 
         [0031]      FIG. 4  shows waveforms  400 ,  402  in accordance with an example embodiment of the present invention. The first waveform  400  is a filtered LF signal that illustrates a signal received by an ASIC during a short circuiting of the a first contact point on the ASIC, as described above. The first waveform  400  is shown in the form of a sine wave that will have characteristics dependent upon the original LF signal received by an antenna, as well the characteristics of any intervening filter. Initially, the waveform alternates in a sinusoidal manner between Vmin and Vmax. After a predetermined amount of time, t 0 , the ASIC switches the impedance of at least one contact point that is coupled to the first contact point from a low impedance to a high impedance, as described above. Thus, the voltage received by the ASIC at the first contact point with a high impedance will reduce to a value of approximately zero (electrical neutral). Then, after a second predetermined amount of time, t 1 , the ASIC will switch the impedance of the at least one contact point back to its original state. Thus, the waveform  400  that the ASIC will receive at the first contact point will have a “gap” during the short circuiting-period. After the short-circuiting period, the waveform  400  resumes its oscillation between Vmin and Vmax. 
         [0032]    The second waveform  402  illustrates the first waveform  400  after the first waveform  400  has been demodulated by the ASIC. The second waveform  402  is generally a square wave that provides a signal of Vmax (or “HIGH”) until t 0 . At t 0 , the second waveform  402  falls to zero (electrical neutral) until t 1 , at which time, the second waveform  402  rises back to Vmax. Thus, an edge detector in the ASIC will detect a rising edge  404  about the time of t 1 . After the short circuiting period, the waveform  402  returns to Vmax (or “HIGH”). It is too be understood that the waveforms illustrated are not drawn with any particular application in mind, and should be used exclusively as an example for understanding the present invention. 
         [0033]      FIG. 5  is a flow chart illustrating an process for a TPM sensor as described in  FIG. 1 , in accordance with an example embodiment of the present invention. The process  500  is a method for detecting a CW signal. The process begins at step  502  and immediately moves to step  504 . At step  504 , an LF signal is received. The process then moves to optional step  506  where the LF signal is filtered. At step  508 , an attempt is made to detect a rising edge of the filtered signal at an ASIC with a demodulator and an edge detector. At step  510 , a determination is made on whether a rising edge has been detected, if one has been detected, the process moves to step  520  (discussed below). If a rising edge has not been detected, the process moves to step  512 . At step  512  another determination is made. If sufficient time has elapsed, then the process moves to step  514 , otherwise, step  512  is repeated until sufficient time has elapsed. 
         [0034]    At step  514 , a short-circuit is activated to short-circuit the filtered LF signal. The process then moves to step  516  where a determination is made. If sufficient tinge has elapsed since the activation of the short-circuit, the process moves to step  518 . If sufficient time has not elapsed, the process repeats step  516  until sufficient time has elapsed. At step  518 , the short-circuit is deactivated and the process moves to step back to step  508 . It should be noted that the activation and deactivation of the short-circuit will cause a rising edge to form in the demodulated signal within the ASIC. 
         [0035]    As stated above, if a rising edge is detected the process moves to step  520 . At step  520 , a pressure sensor is signaled to sense pressure in a tire. The process then moves to step  522  wherein a signal is provided that is indicative of the sensed pressure. The process then returns to the first step,  502  and repeats. 
         [0036]    The various embodiments and methods described above provide a number of ways to carry out the invention. Of course, it is to be understood that not necessarily all objectives or advantages described may be achieved in accordance with any particular embodiment described herein. Also, although the invention has been disclosed in the context of certain embodiments and examples, it will be understood by those skilled in the art that the invention extends beyond the specifically disclosed embodiments to other alternative embodiments and/or uses and obvious modifications and equivalents thereof. Accordingly, the invention is not intended to be limited by the specific disclosures of embodiments herein.