Abstract:
Multiple function stable circuitry measures both pressure and temperature for example. It includes both a pressure sensitive capacitor, and fixed reference capacitor, and also includes both a constant current source and a temperature variable current source. The complete cycle includes at least two phases, with one phase of the cycle utilizing one reference capacitor and one pressure variable capacitor; and at least one other phase including the reference capacitor and at least one temperature variable charging source. Other multiple slope multiple functions may also be implemented.

Description:
FIELD  
       [0001]     This invention relates to sensing systems, such as pressure and/or temperature sensing systems.  
       BACKGROUND  
       [0002]     In the field of pressure sensors, it is known to provide a diaphragm type variable capacitor, in which the capacitance varies with applied pressure. A fixed reference capacitor is also provided, and pressure is determined by circuitry which compares the capacitance of the variable capacitor and the reference capacitor. Two representative patents disclosing this type of system are U.S. Pat. Nos. 4,398,426 and 6,199,575.  
         [0003]     For applications such as automobile or truck tire pressure sensors, it would be useful to also measure the temperature. However, it is relatively costly to provide both a pressure sensor and a temperature sensor.  
       SUMMARY  
       [0004]     In accordance with the present invention both pressure and temperature may be measured using a single circuit which is significantly less expensive than the cost of separate pressure and temperature sensing systems.  
         [0005]     In accordance with one sensing system illustrating the principles of the invention, a reference capacitor and a pressure variable capacitor are provided; and both a constant reference charging current source and a temperature varying charging current source are also provided. Initially the reference capacitor is charged to a predetermined reference voltage level from the constant current source, and then the system is switched so that the pressure variable capacitor is charged by the same constant reference current until a reference voltage is reached. The same sequence is then followed using the temperature variable current source. Comparator circuits are provided for indicating when the capacitors are charged to the reference levels.  
         [0006]     The time for each of these charging intervals are indicative of both the pressure and the temperature. The output may be in the form of pulse width modulated signals, or digital signals, or may initially be in one form and converted to the other. Digital control and counter circuits, including a source of clock pulse signals may be employed to count the time periods for each interval included in the sequences set forth above. The counting circuitry can include well known circuitry which counts the number of clock pulses which occur between specified events and hence measures the time interval between those events.  
         [0007]     The pressure is determined by the ratio of the time for charging the pressure variable capacitor to the time required for charging the reference capacitor. This output may be provided either digitally, or as a pulse width modulated signal, or both.  
         [0008]     Further, the temperature is determined by comparing the time for the cycle using the temperature varying charging current source, with the time for the cycle using the fixed current charging source.  
         [0009]     In the implementation of the foregoing, a single reference capacitor, a single pressure variable capacitor, a single integrator, and a single set of comparator circuits are used for both pressure and temperature calculations, thereby providing both pressure and temperature output signals which is significantly less expensive than separate circuits for determining pressure and temperature, separately.  
         [0010]     In accordance with another feature of the illustrative system, the charging interval may involve charging (and discharging) from an initial starting voltage point to a different reference level and then back to the starting point. In the present specification and claims the phrase “charging or supplying current until a predetermined voltage level is reached”, encompasses the “down-up” or “up-down” charging as well as charging in one polarity only, either up or down.  
         [0011]     One advantage of the system for measuring both pressure and temperature is the low cost and relative simplicity of the system as compared with providing two separate circuits for measuring temperature and pressure. Thus, as noted above, many of the circuit elements, such as the integrator, the comparators, the microprocessor and other circuit components may be employed for both the pressure and the temperature sensing.  
         [0012]     Other objects, features and advantages of the invention will become apparent from a consideration of the following detailed description, and from the associated drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]     The invention may be more readily understood by referring to the accompanying drawings in which:  
         [0014]      FIG. 1  is a schematic showing of a system illustrating an application of the invention;  
         [0015]      FIG. 2  shows a semiconductor chip which may be employed in the implementation of the invention.  
         [0016]      FIG. 3  is a circuit diagram illustrating the principles of the invention;  
         [0017]      FIG. 4  shows waveforms illustrating the mode of operation of  FIG. 3 ;  
         [0018]      FIG. 5  is a program flow diagram indicating the program steps employed in analyzing the output signals involving the circuitry of  FIGS. 1, 3  and  4 ; and  
         [0019]      FIG. 6  indicates one possible way of utilizing pulse width modulation signals, or converting them to another format. 
     
    
       [0020]     Like numerals refer to like parts throughout the several views of the drawings.  
       DETAILED DESCRIPTION  
       [0021]     While the specification describes particular embodiments of the present invention, those of ordinary skill can devise variations of the present invention without departing from the inventive concepts.  
         [0022]     Referring now to  FIG. 1  of the drawings, an automobile or a truck tire  12  is provided with a sensor chip  14  which is exposed to the air contained within the tire  12 . The sensor chip  14  is coupled to the microprocessor  16  mounted in the vehicle by radio frequency or other known arrangements. The microprocessor  16  includes a data processing and control section  18  including counters, a Read Only Memory or ROM  20  and a Random Access Memory or RAM  22 . A display and alarm circuit  24  provides a visual output displaying pressure and temperature along with an alarm signal  26  to indicate pressure or temperature levels exceeding predetermined limits.  
         [0023]     The ROM  20  contains a program for calculating the pressure and temperature from the signals provided from the sensor chip  14 , as developed in detail in connection with  FIGS. 1-4  of the drawings.  
         [0024]      FIG. 2  of the drawings is a semiconductor chip  32  included in the sensor  14  of  FIG. 1 .  
         [0025]     The semiconductor chip includes a variable capacitance diaphragm  34 , which deflects with applied pressure, changing the spacing between electrodes to vary the capacitance. The symbol C p  indicating capacitance varying with pressure will be employed in parts of the following specification. Also visible in  FIG. 2  are the fixed reference capacitor  36  and output coupling pads  38 .  
         [0026]     The chip  32  and its associated variable and reference capacitors  34  and  36  are described in greater detail in U.S. patent application Ser. No. 10/872,055, filed Jun. 18, 2004, and assigned to the assignee of this invention and application. The foregoing patent application is hereby incorporated by reference into this specification.  
         [0027]     Consideration will now be given to the circuit of  FIG. 3  and the companion plots of  FIG. 4  which show the various electrical wave forms present in the circuit of  FIG. 3 .  
         [0028]     Initially, it may be noted that capacitors C p  and C R  are shown somewhat to the left of center in  FIG. 3 . These two capacitors are initially charged to a predetermined reference voltage level as indicated at point  40  in  FIG. 4 . The biasing, or charging/discharging circuit  42  includes source  44  of reference current I REF  for charging the two capacitors C R  and C p ;  
         [0029]     and also includes a companion source  46  of current I REF  for discharging the two capacitors.  
         [0030]     The first step in the cycle is to linearly discharge the reference capacitor, as indicated at reference numeral  48  in  FIG. 4  of the drawings with the discharging bias current source  46  being coupled to C R . Parenthetically, the variable capacitor C p  is not being actively charged or discharged at this time.  
         [0031]     The integrator  49  senses the I REF  discharge current, and provides an output equal to the voltage level on reference capacitor C R . The comparator circuit  50  includes comparator  52  which has two inputs, one being from integrator  49  and the other being a high reference input voltage V REFH . A second comparator  54  has as one input the output from integrator  49 , and has a low reference voltage V REFL  applied to its other input. The high and the low reference voltage levels correspond to the voltage levels  56  and  58  as shown in the plots of  FIG. 4 .  
         [0032]     When the reference capacitor C R  is discharged to the lower reference level  58 , as detected by comparator  54 , it provides an output switching signal on lead  60 . This switching signal is connected to the bias or charging/discharging circuit  42  (see reference numeral  60 ′) and switches the reference current from discharge source  46  to the charge reference current source  44  by the actuation of switching circuitry  62 . The reference capacitor is then linearly charged back up to the high reference level  56  as indicated at reference numeral  64  in  FIG. 4 .  
         [0033]     When the reference capacitor C R  is charged back up to the high reference level indicated at  56  in  FIG. 4 , the comparator  52  provides an output signal on lead  66 .  
         [0034]     The signal on lead  66  is applied to the control circuit  74 , and output signals are applied on circuits  76  and  78  to operate switches  80  and  82 , to disconnect the reference capacitor C R  from the circuit, and to switch in the pressure variable capacitor C p . With capacitor C p  in the circuit, and starting charged from the high reference voltage level  56  (see  FIG. 4 ), the same sequence of discharging C p  and then applying current to charge it up to the high voltage level (see level  56  in  FIG. 4 ) takes place. This is indicated by the V-shaped characteristic  84  as shown in  FIG. 4 .  
         [0035]     Upon completion of this second sequence, a second “up” signal is provided on lead  66 . This is connected to the bias or charge/discharge circuit  42  at lead  66 ′; with the result of switching to the temperature varying charge and discharge current sources  68  and  70  (with circuits  44  and  46  being temporarily inactive).  
         [0036]     The cycle of first discharging and then charging the reference capacitor CR, and then charging and discharging the variable capacitor C p  is then accomplished, as indicated by the V-shaped plots at reference numerals  92  and  94  in  FIG. 4 .  
         [0037]     In the previous section of the specification, the detailed mode of operation of the circuit of  FIG. 3  has been set forth. We will now consider the surprising results which have been achieved. Specifically, as will be detailed below, both temperature and pressure information is available from the operation of the circuit, while using much of the circuit of  FIG. 3  for obtaining both pressure and temperature information. This is of course much simpler and less expensive than having two complete circuits, one for measuring pressure and the other for measuring temperature.  
         [0038]     First, considering pressure information, the control circuit  74  includes at least one bistable circuit connected to output lead  78 . This bistable circuit is responsive to “up” signals applied to control circuit  74 , to change state as indicted by the pulse width modulated plot shown at reference numeral  102  in  FIG. 4 . The bistable circuit is set to its low output state whenever the reference capacitor C R  is being discharged and charged; and  
         [0039]     is set to its high output state when the variable capacitor C p  is being charged or discharged.  
         [0040]     It is particularly to be noted that the mode of operation set forth in the preceding paragraph occurs both when the basic reference current sources  44  and  46  are active, and also when the temperature varying current sources  68  and  70  are being employed. In each case, the ratio of the time for charging (and discharging) the variable capacitor C p  to the time for charging the reference capacitor C R , provides the pressure information. In this regard, it may be noted that this ratio will be the same whether the reference current sources  44 ,  46 , or the temperature sensitive current sources  68 ,  70  are used. Of course, to determine the actual pressure from the pulse width modulated signals, offset and slope factors must be employed.  
         [0041]     Concerning the temperature determination, the ratio of the complete cycle using the temperature sensitive current sources  68 ,  70  to the complete cycle using the reference current sources  44 ,  46  is indicative of the temperature. An additional bistable circuit included in control circuit  74  provides a second pulse width modulated signal shown at  104  in  FIG. 4  on output lead  106  from control circuit  74 .  
         [0042]     Consideration will now be given to the program flow diagram of  FIG. 5  as associated with the electrical wave forms of  FIG. 4 . Initially, the start of the program is indicated at reference numeral  202  and the “Power-On” block  204  starts the initialization interval  206  (see  FIG. 4 ). The cycles described hereinabove are then enabled by the “chip select” or “sensor select” signal  208 , see wave form  208 ′ in  FIG. 4 . Following initialization, a control voltage shifts from a positive voltage level to a low or ground voltage level  58  as indicated at reference numeral  210  in  FIG. 4 . During the initialization interval the two capacitors C R  and C p  are set to the desired (high) reference voltage level, and the other circuits are to their initial states. The charge mode select block  212  (see  FIG. 3 ) is set to use the current source  44 ; and the triangular wave form of  FIG. 4  starting at point  40  begins.  
         [0043]     The signal  66  to the control circuit  74  is read periodically, as indicated by block  216  in  FIG. 5 . During the initialization interval, the output voltage should be high and block  218  indicates an inquiry as to the state of the control input to control circuit  74 , during initialization. If the output is not high (NO), sensor blocks  220  and  222  indicate a malfunction and the program is aborted. If the output is HIGH indicated by “YES” at the output of block  218 , the program proceeds to block  224 . If the output remains high, indicated by a “NO” answer to the block  226  inquiry, the program recycles through timing diamond  228  to block  224 . However, if the control signal remains high beyond an established time period, a sensor failure is indicated and the program aborts, as indicated at blocks  230  and  232 . However, a “YES” indication indicates normal initiation of the cycle, and the program continues to program step  234 , corresponding to the cycle initiation point  40  in  FIG. 4 .  
         [0044]     The next few blocks of the program follow the saw tooth wave form  48 ,  64 ,  84 , etc., shown in  FIG. 4 . Specifically, block  236  indicates reading the output from comparator  52  on lead  66  to the control circuit  74 . Program step  238  inquires “Output goes high?” to see if the charging cycle has increased the voltage from one of the capacitors C R  or C p  to the reference level V REFH  at the input to comparator  52  (level  56  on  FIG. 4 ), causing an output on lead  66 . The program recirculates as indicated by line  240  until the output on lead  66  of  FIG. 5  goes high, and then proceeds to program step  342 . This completes the initial timing cycle using C R  and switches pressure variable capacitor C p  into the charging and discharging cycle.  
         [0045]     Program steps  344 ,  346 ,  348  and  350  complete the saw tooth wave charging (and discharging) cycle using capacitor C p  and the reference current. During the interval from block  234  through  342 , the pulse width modulated output remains low but during the second cycle, using capacitor C p  the pulse width modulated pressure signal on plot  102  ( FIG. 4 ) remains high, as indicated by the legend in block  350 .  
         [0046]     Following program step  350 , the circuit of  FIG. 3  (1) switches over to a temperature variable charging current and (2) switches C R  back into the circuit, and during this portion of the cycle the PWM signal is low. The program steps  352 ,  354 ,  356  and  358  implement this cycle. Finally, still using the temperature varying charging current, the program steps  360 ,  362 ,  364  and  366  complete one cycle of pressure and temperature signal measurement. The PWM signal continues to be low when the reference capacitor C R  is employed but high when the capacitor C p  is used.  
         [0047]     The ratio of the high square wave pulses to the low intervals between pulses is indicative of the pressure.  
         [0048]     Further, the ratio of (1) the longer time intervals during which the temperature variable charging current is employed to (2) the total time period of the cycle using the reference charging circuit, is indicative of the temperature.  
         [0049]     In each case the offset and slope of the function permits ready calculation of the pressure and the temperature from these timed intervals, as indicated by the program steps.  
         [0050]     These last program steps are indicated by program steps  368  and  370 . Of course, in the usual case the program steps  226 - 370  will be repeated continuously prior to the final program step  372 .  
         [0051]     Referring now to  FIG. 6  of the drawings, pulse width modulated signals  402  are supplied from circuitry  404  to the low pass filters  406 . The low pass filter circuit  406  has changed the pulse width modulated signals from low pass filter  406  into a slowly varying D.C. signal. This maybe accomplished by selecting the filter components, such as the capacitance and resistance of an R-C filter circuit so that the time constant of the filter is very low, thus eliminating the pulse configuration. Analog display and Alarm circuitry  408  is then coupled to the low pass filter.  
         [0052]     As will be appreciated, various modifications can be made without departing from the scope or spirit of the invention. For example, the order in which the temperature and pressure parameters is measured is not critical, and the methodology of the present invention could be applied to sensing other parameters.