A circuit for driving a signaling device

Abstract: A circuit for controlling the reserve lamp (LAMP) of a vehicle's fuel level indicator instrument (STR). The circuit uses a switch (SW1) controlled by an extremely asymmetric clock signal (CLK) periodically to switch, for a very short time, the signal (Rj) provided by the level sensor (SENS) towards a comparator (CMP) operable to compare this signal (Rj) with a threshold value (Rth) for the purpose of determining the state of the reserve lamp (LAMP).

Description: The present invention relates in general to circuits for controlling indicator devices, for example illuminable indicator devices, of the kind defined in the preamble of claim 1. More specifically the present invention concerns a circuit for controlling a lamp for indicating the achievement of a reserve level of fuel in a vehicle. The invention is applicable to cases in which the fuel level is displayed by means of a needle indicator instrument of crossed coil type, whilst the sensor for detecting the level in the tank is formed by a resistor the resistance of which varies as a function of the level itself. Hereinafter the term resistance will be used, for simplicity, to indicate both a resistor and its resistance. It is the value of this variable resistance which provides the information for controlling the lamp. In particular, this lamp must be lit when the variable resistance of the level sensor is above (or below) a predetermined threshold value. In known prior art systems there is typically provided an electronic interface between the indicator instrument and the level sensor for the purpose of damping variations caused by movements of the fuel during motion of the vehicle. In Figure 1 there is illustrated a system of the type described above. As can be noted this system essentially comprises three modules or units: a level sensor SENS, an electronic processing circuit EL and an indicator instrument STR. As previously stated the sensor SENS is of variable resistance type for which reason it is constituted by a resistance Rj which is variable as a function of the level of fuel in the tank. The indicator instrument STR, as expected, is of the crossed coil type, illustrated in the drawing as two resistances R1 and R2. These coils R1 and R2 are connected in series and supplied by the vehicle's supply voltage Val. The currents which flow in the coils R1 and R2 are indicated respectively I1 and I2. The information displayed by the indicator instrument STR, that is to say the position assumed by the needle of the instrument, depends on the ratio between the currents I1 and I2. This ratio depends on the current arriving at or leaving the common node between the coils R1 and R2. Originally the sensor SENS was connected directly to this common node so that the ratio between the currents I1 and I2 varied upon variation of the resistance Rj. Currently, the prior art systems use instead an electronic processor circuit EL having an input IN connected to the sensor SENS, and an output OUT connected to the common node of the instrument STR. The processor circuit EL typically comprises a current generator acting to generate a biasing current Ib which is injected into the resistance Rj of the sensor SENS supplied by the supply voltage Val. This biasing current Ib naturally induces a voltage across the resistance Rj of the sensor SENS. This voltage is sent to a low pass filter LPF the filtering effect of which is substantially to damp oscillations in the value of the resistance Rj due to oscillations of the fuel in the tank. The output of the filter LPF is then connected to an output circuit DR connected to the output OUT of the processor circuit EL, operable to control the indicator instrument STR. A circuit acting to control a reserve lamp, typically associated with the indicator instrument STR, typically also uses a signal generated by the sensor SENS, taken for example from within the processor circuit EL, to cause illumination of the reserve lamp when the fuel level in the tank falls below a predetermined threshold level. The characteristics which a reserve lamp control circuit must have are listed below. The precision of the control circuit must be independent of the values of the resistance of the coils R1 and R2 of the indicator instrument STR. Otherwise it would make calibration of the switching point of the lamp necessary in that the resistances R1 and R2 constitute a significant source of inaccuracy. The circuit must damp variations in the signal emitted by the sensor SENS due to movements of the fuel. This damping must be similar to that used to provide the level indication in such a way that the variations in the state of the reserve lamp are perfectly correlated with the movements of the needle of the indicator instrument STR. The circuit must allow for programmability of the switching threshold of the reserve lamp, for example by means of the selection of a value of a resistance. The circuit must have a programable hysteresis, for example by selection of the value of a resistance, to limit as much as possible the illuminations of the reserve lamp caused by successive switchings when the fuel level is close to the threshold level. Figure 2 illustrates an alternative prior art form of the system of Figure 1, starting from which the present invention has been developed. In Figure 2 and in the subsequent drawings, parts and elements already described with reference to the preceding figures have been assigned the same reference numerals and/or letters. In the circuit of Figure 2, the decision to change the state of the reserve lamp, indicated LAMP, is taken by a comparator CMP on the basis of the measurement of the output resistance, also indicated Rj, with which the output circuit DR controls the indicator instrument STR. In the drawing, for completeness, there have also been shown an output current IL and an output voltage VL of the processor circuit EL. In fact, a voltage divider Rp1, Rp2 on the one hand, and a resistor Rs having output resistance Rj on the other, form a Wheatstone bridge which, in dependence on the direction in which it is unbalanced, sends to the comparator CMP a differential input voltage which can be positive or negative and as a consequence of which the reserve lamp LAMP is extinguished or illuminated. Since the output resistance of the processor circuit EL is equal to the resistance Rj of the sensor SENS averaged in time by the low pass filter LPF, the change in state of the lamp LAMP is effected on the basis of an average resistance Rj, as well as by the constant resistances Rs, Rp1 and Rp2, according to the following equation: Rj = Rs·Rp2/Rp1 In this way all the characteristics for controlling the reserve lamp LAMP listed above are satisfied. The control circuit for the reserve lamp LAMP has a precision which is independent of the indicator instrument STR in that only the processor circuit EL plays a part in generating the output resistance Rj of the output circuit DR, which normally would have to correspond to the average value of the resistance Rj of the sensor SENS from which the switching threshold depends. As far as the correlation is concerned it is seen that there is a single filter LPF common both to the circuit for controlling the lamp LAMP and the processor circuit EL of the indicator instrument STR, and therefore their dynamic behaviour is rigorously correlated. As far as the switching threshold is concerned, this can be easily programmed by choosing the values of the resistances Rs, Rp1 and Rp2. As far as the hysteresis is concerned, however, the circuit illustrated in Figure 2 does not provide for it. This can easily be achieved, however, for example by modifying the value of the resistor Rp1 or Rp2 or both of these, on the basis of the state of the lamp LAMP, utilising controlled switches which add or subtract resistances in series or in parallel according to techniques of well known design. However the prior art arrangement, illustrated in Figure 2, has some disadvantages. These disadvantages consist principally in the lack of precision and in the errors introduced by the circuit for controlling the reserve lamp LAMP and will be further discussed hereinafter. The object of the present invention is that of providing a control circuit for an indicator device which makes it possible to resolve all the above indicated problems satisfactorily. According to the present invention this object is achieved by a control circuit,for an indicator device having the features defined in claim 1. Further advantages and characteristics of the present invention will become evident from the following detailed description, given with the aid of the attached drawings, provided purely by way of non-limitative example, in which: Figure 1 is a schematic circuit representation of a prior art detector circuit capable of using a control circuit of the type in question, and has already been described; Figure 2 is a schematic circuit representation of a prior art control circuit and has already been described; Figure 3 is a schematic circuit representation of one embodiment of the control circuit according to the present invention; Figure 4 is a schematic circuit representation of an alternative embodiment of the control circuit according to the invention; Figure 5 is a schematic circuit representation of a portion of the detector system of Figure 1, usable in association with the control circuit according to the present invention; and Figures 6 to 8 are schematic circuit representations of alternative embodiments of the control circuit according to the invention. Figure 3 represents the basic embodiment of the present invention. As can be seen, with respect to the system illustrated in Figure 2, the following additional components are present: a reference resistance Rth which, on the basis of a comparison with the variable resistance Rj makes it possible to establish the state of the lamp LAMP, a diverter switch SW1 which can divert the biasing current Ib from the variable resistance Rj to the reference resistance Rth and vice versa, a flip-flop FF which has the purpose of memorising a logic signal output from the comparator CMP, a synchronisation signal generator circuit (not illustrated) for generating clock signals CLK with a very asymmetric duty cycle, for example 100µs/500ms, for controlling the diverter switch SW1 and the flip-flop FF. Moreover, as can be seen, the comparator CMP in this case compares the output voltage of the low pass filter LPF with that of the reference resistor Rth. The operation of the lamp control circuit for controlling the lamp LAMP illustrated in Figure 3 will now be described. The clock signal CLK controls the diverter switch SW1 in such a way as to direct the biasing current Ib for the majority of the time, given the extremely asymmetric duty cycle, towards the variable resistance Rj, allowing the operation of the indicator instrument STR without any alteration with respect to the prior art system illustrated in Figure 1. At relatively long time intervals, for example 500ms, the clock signal CLK forces the diverter switch SW1 to divert the biasing current Ib from the variable resistance Rj to the reference resistance Rth. The dwell time of the biasing current Ib on the reference resistance Rth, with consequent absence of biasing of the variable resistance Rj is, conversely, very short, for example 100µs. In this way the control of the lamp LAMP interferes in an extremely limited manner, that is to say for 100µs every 500ms, equal to 0.02% = 200ppm, on that of the indicator instrument STR. The voltage on the sensor SENS has a value given by the product Ib Rj, where the biasing current Ib is to be considered as constant whilst the resistance Rj is variable in time with an oscillatory variation consequent on the movements of the level of fuel. At the output of the filter LPF there is thus the average value of this voltage Ib Rj, which is therefore equivalent to the product of the biasing current Ib by the average value of the variable resistance Rj. The output voltage from the filter LPF maintains the said value even during the phase in which the reference resistance Rth is biased and the input of the filter LPF assumes a zero voltage. This utilises the property that the filter LPF memorises and maintains its output voltage to allow comparison with the reference resistance Rth. In fact the comparator CMP, receiving at one input the voltage Ib Rth and at the other input the average voltage Ib Rj, provides at its output a logic signal which indicates if the average variable resistance Rj is greater than or less than the reference resistance Rth. An instant before the end of the phase in which the reference resistance Rth is biased the flip-flop FF, controlled by the clock signal CLCK stores the logic output signal from the comparator CMP, since at this moment this logic signal is valid for controlling the lamp LAMP. The state of this is thus updated and then maintained by the flip-flop FF until the end of the subsequent biasing cycle of the reference resistance Rth. Naturally, for a greater security against erroneous indications, it is always possible to interpose between the output of the flip-flop FF and the lamp LAMP a circuit, which can be made using known techniques, which checks that the results of a certain number of successive measurement cycles are consistent before enabling any change of state of the lamp LAMP. As can be noted, the circuit according to the invention satisfies the requirements of controlling the lamp LAMP described initially. In fact, the switching threshold or change of state of the lamp LAMP depends, as is seen, on a comparison between the reference resistance Rth and the variable resistance Rj which is not connected in any way with the indicator instrument STR and its resistance values R1, R2. Damping of the signal generated by the sensor SENS is effected with the same filter LPF as is used for the indicator instrument STR, and therefore the correlation of the two dynamic behaviours is guaranteed. The programmability of the switching threshold is obviously achieved with the choice of the value of the reference resistance Rth. Hysteresis is not implemented in the circuit of Figure 3, but again it is simple to implement using known techniques by modifying the value of the reference resistance Rth as a function of the state of the lamp LAMP. Figure 4 shows an exemplary embodiment of a circuit with hysteresis achieved by the insertion of a hysteresis resistance Rhys in series with the reference resistance Rth. This insertion is conditioned by the state of the lamp LAMP and is effected by means of a diverter switch SWH controlled by the output of the flip-flop FF, which itself determines the state of the lamp LAMP. The control circuit according to the present invention has the following important advantages. First of all, there is an extremely limited reduction in performance of the processor circuit EL which controls the indicator instrument STR. This reduction is quantifiable as a loss of precision of 200ppm in the example considered, but can be reduced at will by increasing the asymmetry between the biasing times of the sensor SENS and the reference resistance Rth, that is to say the duty cycle of the clock signal CLK. On the other hand the flow of output current is not in any way reduced or altered by additional series resistances. In the prior art systems, moreover, all the elements of the processor circuit EL contribute to the generation of the average variable resistance Rj which controls the indicator instrument STR so that all these elements contribute to the generation of an error in the switching threshold of the lamp LAMP. In a circuit according to the invention on the other hand only one of these elements, that is to say the filter LPF, makes a contribution to the error. In fact, the biasing current Ib is common both to the reference resistance Rth and to the variable resistance Rj such that its absolute value does not influence the measurement. The output circuit DR moreover is not in any way involved in controlling the lamp LAMP and therefore this, too, has no influence. In the second prior art circuit of Figure 2 the comparator CMP is called upon to compare voltages of very low value in that the voltage drop across the resistance Rs is, typically, as low as 100mV at the switching threshold. This occurs because the resistance Rs typically 10 Ohm at most, must not significantly disturb the output resistance Rj which controls the indicator instrument STR when the current in this is at a maximum, for example 100mA, whilst the same resistance Rs must perform its switching function on the lamp LAMP when the current in the indicator instrument STR is low, for example 10mA corresponding to a low fuel level. It is clear therefore that an input offset of the comparator CMP, typically 1mV, gives a consistent contribution to the imprecision, typically 1% of that of the control circuit. In the circuit according to the invention illustrated in Figure 3, on the other hand, the input voltages at the comparator CMP are decidedly higher, for example 1V in that the variable resistance Rj is in the vicinity of, for example, 200 Ohm at its highest when the fuel level is close to the minimum and therefore to the switching threshold of the lamp LAMP. The result of this therefore is that the same comparator CMP utilised in the circuit according to the invention, gives a significantly reduced contribution to the error with respect to the prior art, for example 0.1%. In a further embodiment, illustrated in Figure 5, a low pass filter LPF of a type known in the art is used in the control circuit according to the invention. As can be seen the filter of Figure 5 is a feedback filter in which a part of the feedback loop is digital. The filter comprises, at its input, a so-called anti-aliasing filter AAF. This anti-aliasing filter AAF can be made, for example, by means of a low pass filter constituted by a resistance and a capacitance. The output of the anti-aliasing filter AAF is connected to the input of a comparator CMP. The other input of the comparator CMP is connected to the output of the filter LPF itself. The output of the comparator CMP controls a circuit CTRL acting to generate digital signals. The digital output of the circuit CTRL is connected to a digital/analogue convertor D/A. The output of the digital/analogue convertor D/A also constitutes the output of the filter LPF. In this case a further improvement in the performance of the circuit is possible in consideration of the fact that the comparator CMP employed in the filter LPF (Figure 5) can also fulfil the function of the comparator CMP utilised by the control circuit for the lamp LAMP. This is possible due to the fact that the two comparators, if they were separate, would no longer be utilised contemporaneously. In fact, when the reference resistance Rth is biased it is necessary only that the comparator CMP compares the output of the filter LPF with the voltage present on the reference resistance Rth, whilst when the variable resistance Rj is biased only the comparator CMP of the filter LPF, illustrated in Figure 5, is utilised, which compares input and output of the filter LPF. In Figure 6 is illustrated a control circuit according to the invention equipped with the filter LPF of Figure 5 in which the comparator CMP is utilised, in time division, to perform the function of controlling the lamp LAMP. The inverting input of the comparator CMP is maintained permanently connected to the output of the D/A convertor in that this configuration is common to its two functions. On the other hand the non-inverting input is switched between the reference resistance Rth and the output of the anti-aliasing filter AAF of the filter LPF in synchronism with the clock signal CLK by means of a diverter switch SW2. This switch SW2 has a low cost in that it acts on a signal of low current and voltage. It is observed that with this system the offset of the comparator CMP is cancelled in comparison with the controlling of the lamp LAMP. In fact, the output voltage of the digital/analogue convertor D/A is displaced with respect to its theoretical value by a quantity equal to the offset of the comparator CMP because when this is itself used to evaluate the voltage of the reference resistance Rth there is a comparison performed with an identical error, in value and sign, to that contained in the voltage, namely the output of the D/A convertor considered as reference for the comparison itself. The circuit illustrated in Figure 6 has a further additional characteristic with respect to that of Figure 3 given by the use of a switch SW3. The purpose of this is to disconnect the input of the anti-aliasing filter AAF from the sensor SENS at the moment in which this loses the biasing current Ib in favour of the reference resistance Rth. Consequently, in this time interval the incorrect voltage of the sensor SENS does not influence the anti-aliasing filter AAF which maintains unaltered its state, for example by means of its capacitance. Therefore the loss of precision caused by the duty cycle of the clock signal CLK, quantifiable as 200ppm in the preceding example, thus converts into a single loss of information that the sensor SENS would provide in the time interval in which it is not biased. In other words if, during this time, the variable resistance Rj of the sensor SENS were to remain substantially constant there would be no loss of precision from this cause. The resolution of the D/A convertor is the single source of imprecision for control of the lamp LAMP in the invention described up to this point apart from the previously-mentioned contribution of the duty cycle which is quantifiable at 200ppm and which, however, can be statistically nullified. In fact, this D/A convertor, by memorising the voltage on the sensor SENS to be compared with that on the reference resistance Rth, involves an error which can be up to half of its least significant bit. If, for example, there are ten bits the maximum error becomes: 0.5/1024 = 0.05% = 500 ppm. It can therefore be concluded that the precision of the commutation threshold of the lamp LAMP, achieved by means of the control circuit according to the invention, is equal to that of the reference resistance Rth in that all the other sources of error contribute in an entirely negligible manner considering the precision, typically 1%, of the best resistors which are normally used in the manufacture of electronic circuits of limited cost. Figure 7 shows a possible application of the present invention to the processor circuit EL which is the subject of an earlier invention. The switch SW1 for the biasing current Ib is provided with a pair of transistors M1a and M1b. The signal switch SW1 then provides for the transmission of the output of an operational amplifier OP1 to the transistor M1a or M1b in dependence on the direction in which the biasing current Ib must flow. Note that all the components Ra1, Ra2, Rab2, OP1 which determine the precision of the biasing current Ib do not change their function whatever the state of the switch SW1, thereby guaranteeing that in both cases the biasing current Ib is exactly the same. Figure 8 on the other hand shows how the present invention can be applied to a processor circuit EL which also effects control of ratiometric type. This circuit EL subdivides the output current range into several sub-intervals each of which is managed by a pair of power transistors as well as by a pair of current measuring resistors. Note that the control of the lamp LAMP does not require duplication of all the transistors which deliver the biasing current Ib to the sensor SENS in that if the biasing current Ib is high then the fuel level indicated by the instrument STR is certainly high and therefore the lamp LAMP must remain extinguished, without the necessity of performing the reference resistance sampling procedure on the resistance Rth. As seen by Figure 8 the control circuit in this embodiment requires only the duplication of power transistors for controlling small currents Ib with evident economies in materials.

Claims: A control circuit for an indicator device (LAMP), comprising a sensor resistor (Rj) adapted to be coupled to a constant current source (Ib) for providing an information signal (Ib * Rj) indicative of a detected physical parameter, said information signal (Ib * Rj) being sent to an indicator instrument (STR) operable to display information indicative of the said physical parameter; characterised by a reference resistor (Rth); a switch (SW1) operable to periodically couple the current source (Ib) to the sensor resistor (Rj) for a first time interval so as to allow the indicator instrument (STR) to display the said information, and to the reference resistor (Rth) for a second period of time for providing a reference signal (Ib * Rth); memory means (LPF; AAF) adapted to memorise a value of said information signal (Ib * Rj) during said second period of time; a comparator (CMP) adapted to compare in said second period of time the reference signal (Ib * Rth) with the memorised value of said information signal (Ib * Rj) to provide an output signal for controlling the state of the indicator device (LAMP); and a memory device (FF) interposed between the comparator (CMP) and the indicator device (LAMP) and capable of storing the output signal from the comparator (CMP) at the end of said second period of time. A circuit according to Claim 1, characterised in that the said first time interval is relatively long with respect to the said second time interval. A circuit according to Claim 2, characterised in that the said second time interval is less than or equal to a thousandth of the said first time interval. A circuit according to any of Claims 1 to 3, characterised in that the said reference resistor (Rth) has a programmable resistance. A circuit according to any of Claims 1 to 4, characterised in that it is configured in such a way as to compare the said information signal (Ib*Rj) with a first threshold value to determine activation of the said indicator device (LAMP), and with a second threshold value to determine disactivation of the said indicator device (LAMP) for the purpose of activating commutation of the said indicator device (LAMP) with hysteresis. A circuit according to Claim 5, characterised in that the said first and second threshold value are programmable for programming the said hysteresis. A circuit according to any of Claims 1 to 6, characterised in that it comprises a low pass filter (LPF) having digital feedback for control of the aid indicator instrument (STR) to compare the said information signal (Ib*Rj) with the said reference signal (Ib*Rth). A circuit according to any of Claims 1 to 7, characterised in that it is configured to determine commutation of the state of the said indicator device (LAMP) only after a predetermined number of comparisons performed in a corresponding number of time periods composed of the said first and second time intervals, having corresponding results. A circuit according to any of Claims 1 to 8, characterised in that the said information signal (Ib*Rj) is indicative of a level of a liquid in a tank and the said indicator device is an illuminable indicator (LAMP) operable to indicate the achievement of a predetermined level of the said liquid in the said tank.

Inventors: MORELLI MARCO, POLETTO VANNI, POMA ALBERTO, MORELLI, MARCO, POLETTO, VANNI, POMA, ALBERTO
Applicants: MAGNETI MARELLI SPA, ST MICROELECTRONICS SRL, MAGNETI MARELLI S.p.A., STMICROELECTRONICS S.R.L.
Application year: 1996
Main classification: H03K17
IPC codes: H03K17, G01F23