Abstract:
A method wherein (a) a voltage to be measured (U e ) is amplified with a gain proportional to the ratio between the reference voltage (V ref ) of an analogue-to-digital converter (3) and the extent of the voltage range; (b) the reference voltage (V ref ) is amplified with a gain proportional to the ratio between the lower terminal (U 1 ) of the voltage range and the extent of the voltage range; and (c) the analogue input of the converter (3) is supplied with the resulting amplified voltage difference. The method is useful for measuring a temperature by means of a thermocouple and measuring the temperature of an oxygen sensor in a motor vehicle exhaust line.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a method and a device for digitally measuring an analog voltage that varies over a predetermined voltage range, and more particularly to the use of such a device for measuring a temperature. 
     In automotive electronics, it is often necessary to measure the temperature of certain fluids, oil or coolant, or devices such as a catalytic converter for oxide reduction of the exhaust gases from a motor vehicle, or an oxygen sensor used in a device for regulating the air and fuel mixture supplied to the engine of such a vehicle. Currently, a suitable temperature pickup furnishes an analog electrical signal to an analog/digital converter, which furnishes a digital expression of the temperature measurement that can be used by a computer that forms part of an open- or closed-loop control unit provided in the vehicle. 
     The signal output by the pickup is normally an electrical voltage. When the temperature monitored becomes the subject of closed-loop control, this control keeps the signal within a predetermined voltage range. Thus oxygen sensors are known that are associated with a heating resistor actuated in such a way as to maintain the sensor temperature within a very narrow temperature range, for example 650° to 750° C., in which range the signal output by the sensor can be used. If the sensor temperature is evaluated on the basis of the temperature of the heating resistor, whose resistance is a function of that temperature, then the voltage picked up at its terminals to do so also varies within a range whose values are fixed by the limits of the aforementioned temperature range. 
     It has also been desirable for the temperature of the catalytic converter, around an optimal operating temperature of 850° C., for example, to be known by a computer with a fixed precision of several percent. If a thermocouple is used to do so, then its output voltage varies in a corresponding manner within a predetermined voltage range. 
     The problem then arises of the precision of the digital measurement of the image voltage of the temperature observed. This precision is a function of the reference voltage V ref  of the converter and of the number of bits n of the measurement N that it furnishes. In the aforementioned applications, measuring the voltage is of interest only within a predetermined voltage range, which a priori does not correspond to the dynamics of the digital measurement chain used, which depends on parameters (V ref , n) that are independent of this range. 
     SUMMARY OF THE INVENTION 
     Accordingly, the object of the present invention is to furnish a method of digital measurement of a voltage that varies within a predetermined range, with the aid of a measurement chain including an analog/digital converter, with which method a maximum measurement precision within this range can be attained. 
     Another object of the present invention is to furnish such a method that is suitable for measuring an analog voltage affected by the voltage furnished by an electrical power source, which may possibly fluctuate, as is the case of the voltage furnished by the battery of a motor vehicle, by which method a digital measurement freed of any influence of the fluctuations of the voltage can be obtained. 
     Furthermore, an object of the invention is to use a device for implementing this method that can be used particularly in a temperature measurement chain. 
     These objects of the invention as well as others that will become apparent in the ensuing description, are attained with a method by which 
     a) the voltage to be measured is amplified with a gain proportional to the ratio between the reference voltage of the analog/digital converter used and the length of said voltage range; 
     b) the reference voltage is amplified with a gain proportional to the ratio between the lower value of the voltage range to the breadth of this voltage range; and 
     c) the analog input of the converter is supplied with the difference between the thus-amplified voltages. 
     As will be noted in further detail hereinafter, this method makes it possible to adapt the converter dynamics to the length of the voltage range, so as to advantageously maximize the precision of the measurement furnished by the converter. 
     In a variant of the method according to the invention, adapted for measuring an analog voltage affected by the voltage furnished by a possibly fluctuating electrical supply source, the converter is supplied with a reference voltage proportional to the fluctuating electrical voltage, in such a way as to make the digital measurement furnished by the converter independent of the fluctuations in said voltage. 
     This variant is especially useful in motor vehicle electronics, because as is well known, the source of electrical voltage normally used there is the vehicle battery, whose output voltage currently exhibits very major fluctuations, for example from 4 to 15 V for a nominal voltage of 12 V. 
     For implementing this method, the invention furnishes a device, including an analog/digital converter and a source of reference voltage (V ref ) for supply to this converter; this device is notable in that it includes amplification means supplied with the analog voltage (U e ) to be measured and with the reference voltage (V ref ) in order to furnish to the analog input of the converter an output voltage (U s ) such that: ##EQU1## where U 1  and U 2 , respectively, are the lower and upper values of the predetermined voltage range. 
     Further characteristics and advantages of the present invention will become more apparent from the ensuing description and from a study of the accompanying drawing, in which: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a graph useful for illustrating the method of the present invention; 
     FIG. 2 is a functional diagram of a device for implementing the method of the present invention; 
     FIG. 3 illustrated one embodiment of the device schematically shown in FIG. 2; 
     FIG. 4 is a diagram of a device for measuring a temperature with the aid of a thermocouple, implementing the method of the invention; and 
     FIG. 5 is a diagram of a device for measuring the temperature of an oxygen sensor, corresponding to another use of the method of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     On the U e  axis of FIG. 1, a predetermined voltage range (U 1 , U 2 ), within which a voltage U e  to be measured varies, is plotted. Also shown in FIG. 1, on the other axis, is the graph of the output N of an analog/digital converter supplied with a reference voltage V ref . 
     If U 1  &lt;U 2  &lt;V ref , then one can see that the range of variation dN of the output of the analog/digital converter corresponding to the range (U 1 , U 2 ) is much narrower than the dynamics ΔN of the converter, at the expense of the precision of the digital measurement of the voltage to be measured, whose resolution is proportional to the reference voltage V ref  and inversely proportional to 2 n , where n is the number of bits of the output of the converter. 
     In the present invention, the precision of the voltage measurement to be made is maximized by adapting the voltage range [0, V ref  ] of the converter to the voltage range [U 1 , U 2  ]. To do so, the voltage U 1  is made to correspond to 0 in the range to be measured of the converter, and the voltage U 2  is made to correspond to the voltage V ref . This measurement range is then graduated over the totality of the 2 n  levels of the output N of the converter, not over a fraction of these 2 n  levels, which thus improves the precision of measurement of a voltage U e  that is within this measurement range. A computer, supplied with the output of the converter and loaded with the voltage value U 1  corresponding to the 0 of the converter output, thus establishes the voltage U e  with the greatest possible precision, taking into account the characteristics of the analog/digital converter used. 
     Turning to FIG. 2 of the accompanying drawing, a device will be described that is designed for the implementation of the present invention. A pickup 1 furnishing a voltage signal U e  to be measured is connected to amplification means 2, whose output U s  supplies an analog/digital converter 3, which in turn is supplied with a reference voltage V ref . 
     According to the present invention, the amplification means 2 are further supplied by the reference voltage V ref  and furnish an output voltage U s  such that: 
     
         U.sub.s =K·U.sub.e -K&#39;·V.sub.ref         (1) 
    
     where K and K&#39; are constants established as follows. 
     According to the invention, the voltage U 1  and U 2  correspond respectively to the voltages 0 and V ref  at the output of the amplification means 2, and hence by employing the equation (1) above: 
     
         0=K·U.sub.1 -K&#39;·V.sub.ref 
    
     
         V.sub.ref=K·U.sub.2 -K&#39;·V.sub.ref 
    
     The following relationships can be found: ##EQU2## 
     The relationship (1) is then expressed as follows: ##EQU3## 
     Thus by amplifying the voltages U e  and V ref  in the means 2 with the amplification gains K and K&#39;, which are calculated as indicated above, and by supplying the analog input of the converter 3 with the difference U s  between these amplified voltages, a measurement of the voltage U e  whose precision is as high as possible can be taken from the output N of the converter and from a recorded digital measurement of the voltage U 1  ; the pitch of this precise measurement of the voltage U e  is equal to V ref  /2 n , where n is the number of bits of the output N of the converter 3. Thanks to the amplification means 2, an output N=0 of the converter 3 corresponds to an input voltage U e  =U 1 , and N=2 n  corresponds to a voltage U e  =U 2 . 
     One embodiment of the device schematically shown in FIG. 2 has been shown in FIG. 3 of the accompanying drawing. The amplification means comprise a differential amplifier 2&#39;, supplied on the one hand with the voltage U e  to be measured, via charge resistors R 1  and R&#39; 1 , between its inverting and noninverting inputs, respectively, and on the other is supplied with an offset voltage as a function of the reference voltage V ref  of the converter. A reaction resistor R 2  is mounted between the output of the amplifier and the inverting input, in order in combination with the resistor R 1  to establish the gain of the amplifier relative to the input voltage U e , while the reference voltage V ref  of the converter is connected to one terminal of a resistor R 3 , whose other terminal, which is in common with the resistor R 2 , is connected to the inverting input of the amplifier. The noninverting input is connected to ground through a resistor R&#39; 2 . Under these conditions, it can be demonstrated that the relationship (2) above becomes as follows: ##EQU4## 
     A first use of the invention, for measuring a voltage that varies within a predetermined range furnished by a thermocouple, is illustrated in FIG. 4. The measurement chain represented includes a thermocouple whose &#34;hot&#34; junction 4 is at a temperature Θ 1  to be measured, while its reference junction 5 is at a temperature Θ 2 . It is known that such a thermocouple furnishes a voltage as follows: 
     
         U.sub.e=f (Θ.sub.1 -Θ.sub.2) 
    
     The voltage U e  supplies a device that is identical to that of FIG. 3. The same reference numeral is used for devices or elements that are identical or similar in FIGS. 3-5. It is understood that a computer can determine the voltage of the temperature Θ 1  from the function f and from a measurement of the voltage U e , with the temperature of Θ 2  being assumed to be fixed and known. 
     When the temperature Θ 2  the reference junction is likely to vary (as is the case for example when it is at ambient temperature), then according to the invention means are provided that are sensitive to the temperature of this junction and are capable of furnishing a correction signal, enabling the computer to suitably correct the calculation of the temperature Θ 1 . 
     To do so, a temperature pickup, such as a thermistor 6, is disposed in proximity with the reference junction 5, and the variations in the resistance of the thermistor 6 cause the voltage at the middle point of a resistor bridge (R 4 ,6), which is supplied with the voltage V ref  for example, to vary. The computer periodically orders a switching means 7 to connect the middle point of the bridge to the input of the converter 3, which then furnishes the computer with a digital measurement of the temperature Θ 2 . Such a device is useful especially when a temperature sensor must be located at a distance far from the computer, as is the case for instance when the temperature of a catalytic converter for oxide reduction of the exhaust gases from an internal combustion motor driving a motor vehicle is measured. 
     Another &#34;motor vehicle&#34; use of the present invention is shown in FIG. 5. This is the measurement of the temperature of an oxygen sensor 8 heated by a heating resistor R ch . As has been seen above, the temperature of such a sensor must be kept within a relatively narrow range. To that end, a device for closed-loop control of this temperature controls the supply to the heating resistor. This resistor may be considered in thermal equilibrium with the sensor 8 that it heats. The temperature of the sensor can be known from that of the heating resistor assumed to be in thermal equilibrium with the sensor, which resistor varies with the temperature, as is well known. This measurement is made by picking up the voltage U e  from the middle point of a bridge comprising the resistor R ch  and a fixed resistor R 5 , the resistors being connected in series between the positive pole of the vehicle battery and ground. This voltage varies inversely with the variation in the resistance of R ch , when this resistor has a positive temperature coefficient, as is conventionally the case. The measurement of this voltage by a computer supplied with the output of the converter 3 of a device that is also in accordance with those shown in FIGS. 3 and 4 accordingly makes it possible to arrive at the temperature of the sensor 8. 
     In a motor vehicle, the voltage +V bat  furnished by the battery is capable of very major fluctuation, as has been seen above. Such fluctuations accordingly adulterate the measurement made, since they affect the supply to the heating resistor. 
     This disadvantage is overcome, according to the present invention, by taking the reference voltage V ref  of the converter from the voltage +V bat  furnished by the battery, in such a way that these voltages vary in the same direction. Automatic compensation for the effects of fluctuation in the battery voltage is thus obtained. Since the battery voltage is generally higher than the maximum voltage that the converter can withstand at its reference voltage terminal, it is only a fraction αV bat  (α&lt;1) of the output voltage of the battery that supplies the device according to the invention. It is understood that the present invention is not limited to the embodiments described and shown, which are given solely by way of example. The invention is accordingly advantageously applicable as well to the digital measurement of any variable that is accessible by means of a voltage varying within a predetermined range, and not only to measuring a temperature.