Abstract:
A tire pressure measurement system (TPMS) includes a capacitor and an integrated circuit configured to receive a supply voltage. The integrated circuit includes a voltage regulator and a measurement unit. The voltage regulator is configured to be turned on and off for predetermined periods of time such that the capacitor is charged and discharged, respectively. The voltage regulator and the capacitor are connected to the measurement unit in order to selectively provide electric charge at a voltage between predetermined upper and lower limits.

Description:
RELATED APPLICATIONS 
       [0001]    This application is a Continuation of application Ser. No. 12/105,853 filed Apr. 18, 2008, entitled “TIRE PRESSURE MEASUREMENT SYSTEM WITH REDUCED CURRENT CONSUMPTION,” which claims priority to Application No. EP 07107174.0 filed Apr. 27, 2007, entitled “TIRE PRESSURE MEASUREMENT SYSTEM WITH REDUCED CURRENT CONSUMPTION,” which is incorporated herein by reference in its entirety. 
     
    
     FIELD OF INVENTION 
       [0002]    The present invention relates to a tire pressure measurement system (TPMS) where a wheel module, including a measurement unit, is located inside the tire, and is supplied from a power source of limited capacity. Examples of such power sources are electrochemical batteries, piezoelectric power generators or inductive power generators. 
       BACKGROUND 
       [0003]    Wheel modules include one or more integrated circuits to perform measurement and signal processing tasks required by the system. The integrated circuit is programmed to wake up for less than a second at intervals of typically 10-30 seconds, to perform measurements and transmit data. The rest of the time is spent in the sleep (powerdown) mode, where only a timer circuit is active. Since more than 99% of the time may be spent in sleep mode, the power consumption of the device in this mode is obviously of importance. 
         [0004]    When produced in the currently used technologies, such integrated circuits have a maximum limit for the internal supply voltage of the digital core circuits which is less than the voltages provided by the energy sources mentioned above. A voltage regulator or converter is therefore required to adapt the supply voltage to a level that is compatible with the digital core of the integrated circuits. 
         [0005]    It is often the case that the integrated circuit includes special higher voltage elements, so that the voltage regulator can be provided on the same chip as the low voltage digital core. 
         [0006]    Due to the limited capacity of the power sources, it is important that the power consumption of the wheel module is minimized. Typically, voltage regulators are used for voltage conversion. The voltage regulator requires a certain bias current to operate, and this current constitutes a significant part of the sleep mode power consumption. 
       SUMMARY OF THE INVENTION 
       [0007]    A tire pressure measurement system (TPMS) is described herein. The system comprises: a capacitor; and an integrated circuit configured to receive a supply voltage, the integrated circuit comprising: a voltage regulator; and a measurement unit; wherein the voltage regulator is configured to be turned on and off for predetermined periods of time and arranged such that the capacitor is charged and discharged, respectively. The voltage regulator and the capacitor are connected to the measurement unit in order to selectively provide the measurement unit electric charge at a voltage between predetermined upper and lower limits. The average bias current of the voltage regulator is reduced by turning it on and off at fixed intervals. While the regulator is turned off, current is supplied from the capacitor. 
         [0008]    The above and still further features and advantages of the present invention will become apparent upon consideration of the following definitions, descriptions and descriptive figures of specific embodiments thereof, wherein like reference numerals in the various figures are utilized to designate like components. While these descriptions go into specific details of the invention, it should be understood that variations may and do exist and would be apparent to those skilled in the art based on the descriptions herein. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    Examples of the present invention will now be described with reference to the accompanying drawings, in which: 
           [0010]      FIG. 1   a  shows an integrated circuit of a tire pressure measurement system (TPMS); 
           [0011]      FIG. 1   b  shows an integrated circuit of a TPMS according to an embodiment; 
           [0012]      FIG. 2  shows a comparison of the various voltages of the circuit according to an embodiment; 
           [0013]      FIG. 3  shows an example of a voltage regulator of the integrated circuit of  FIG. 1   b ; and 
           [0014]      FIG. 4  shows an example of an arrangement of a TMPS according to an embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]    With reference to  FIG. 1   a , a TPMS wheel module comprises an integrated circuit  1  that is supplied with an external supply voltage V in  which varies between the voltages V in     —     max  and V in     —     min . The integrated circuit  1  comprises a voltage regulator  2  configured to receive the external supply voltage, in the range between V in     —     max  to V in     —     min , inclusive, and produces a regulated output voltage V reg  within the limits V reg     —     max  and V reg     —     min . The integrated circuit  1  further comprises a digital core  3  requiring a supply voltage between the levels V core     —     max  and V core     —     min , inclusive. 
         [0016]    When V in     —     max &gt;V core     —     max , the digital core  3  cannot be connected directly to the external supply voltage, and a voltage regulator  2  must be inserted between the external supply voltage and the digital core  3 . The voltage regulator  2  creates a voltage drop between the external supply voltage and the digital core  3 , ensuring that V core     —     max &gt;V reg     —     max . For proper operation of the device, it is also necessary to have V in     —     min &gt;V core     —     min , and V reg     —     min &gt;V core     —     min . 
         [0017]    For earlier semiconductor technologies using batteries as a power source, the voltage regulator was in many cases not needed since V core     —     max &gt;V in     —     max  (i.e., the supply voltage range included the battery voltage). For designs using current semiconductor technologies, or using other types of power generators, this is in general not the case, and a voltage regulator is required. This problem also arises in the field of energy harvesting devices, as these frequently generate relatively high voltages. In order to operate correctly, the voltage regulator draws a certain amount of current in its control circuits. This current, called the regulator bias current, is drawn from the battery in addition to the current required by the digital core. Several design techniques are available to obtain sufficient circuit performance with very low current consumption. Nevertheless, the voltage regulator bias current is of the same order of magnitude as the supply current to the digital core when in sleep mode. 
         [0018]    Thus, the regulator increases the load on the power source, which in turn means that the power source must be increased, leading to added size, weight, and cost. Since TPMS systems must minimize all these parameters in order to be efficient, a way to minimize or eliminate the voltage regulator bias current is desirable. 
         [0019]      FIG. 1   b  shows an example of the present invention in which a voltage regulator  2  is turned on for brief periods to charge a capacitor  4 . When the voltage regulator  2  is in an off state, a digital core  3  (e.g., measurement unit) is supplied by the charge stored on the capacitor  4 . By correctly choosing the on and off times of the voltage regulator  2 , the V core  voltage can be maintained between the limits V core     —     max  and V core     —     min , while at the same time reducing the average bias current of the voltage regulator  2 . Additionally, a switch  5  is closed and opened under control of the digital core. Typically, the present invention reduces the average bias current by 75% or more, resulting in a significant battery saving over the lifetime of the device. 
         [0020]      FIG. 2  shows how the V core  voltage will vary between V core     —     max  and V core     —     min  as the voltage regulator is turned on and off (i.e., duty cycled) and the switch  5  is closed and opened, respectively. The switch  5  on (closed) time must be sufficient to charge the capacitor to the V reg  voltage while the voltage regulator  2  is in the on state, and the switch  5  off (open) time must be short enough to ensure that the V core  voltage will never fall below V core     —     min  while the voltage regulator  2  is in the off state and the capacitor  4  is discharging. 
         [0021]    The purpose of the switch is to avoid leakage of charge from the capacitor back into the voltage regulator when the latter is turned off. Therefore, the switch can be dispensed with if the voltage regulator presents a high impedance in the off state. However, if the switch is needed, then the switch must be closed (conducting) when the regulator is on, and open (isolating) when the regulator is off. The control signal(s) from the digital core must change the state of the switch and the voltage regulator in synchronism. 
         [0022]    The voltage regulator can be realized in a number of ways, for example, by a conventional linear regulator with a fixed output voltage, or a combination of a comparator and a switch. 
         [0023]    An example of a voltage regulator  2  is shown in  FIG. 3 . The two exemplary types of regulator mentioned above share the majority of the circuit elements, such as a voltage reference V ref , a comparator/error amplifier  6 , and a switch/pass transistor  7 . The main difference is in the type of regulation. The linear regulator must be designed to be stable, while the switching circuit is by design unstable. 
         [0024]    In the case of a comparator, when turned on, the switch passes current to the capacitor, until the comparator decides that the capacitor voltage has reached its upper limit and turns off the switch. While the comparator is used to turn off the charging current to the capacitor, the turn-on time has to be determined by the control logic, as previously described. 
         [0025]    There are several possible solutions to control the on and off timing to ensure a V reg  between V core     —     max  and V core     —     min . 
         [0026]    The on and off times can be calculated based on simulations or measurement data, and fixed in the digital core  3 . By using fixed timing, the need for a circuit to detect if V core  is close to V core     —     min  is avoided, thus avoiding another potential current consumer. 
         [0027]    To optimize for lowest possible charge consumption, several different timing schedules can be selected by the digital core  3  before it enters the sleep mode based on, for example, the expected current load in sleep mode. The timing schedule can be influenced by parameters (e.g., calculated current consumption based on activated modules during sleep mode, measured temperature, and measured battery voltage). 
         [0028]    Those skilled in the art will take proper design practice into account, such as allowing for temperature variation of current, considering the effect of the capacitor on the stability of the voltage regulator  2 , and considering the effect of timing difference between the turn-on of the voltage regulator  2  and the closing of the switch  5 . It should also be appreciated that other types of voltage regulators may be employed to implement the invention. 
         [0029]      FIG. 4  shows an implementation of a TPMS according to an embodiment of the present invention in a vehicle wheel. The system comprises a wheel module  8  that houses the integrated circuit  1  that performs measurements for determining the tire pressure and transmits data by RF electromagnetic waves  9  to an external receiver  10 . 
         [0030]    While the invention has been described in detail with reference to specific embodiments thereof, it will be apparent to one of ordinary skill in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof. Accordingly, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.