Abstract:
A circuit for evaluation of a measurement signal of a thermocouple (S 1 ), especially a thermocouple which is used as a flame detector (F) of a motor vehicle heater, with an amplifier (U 1 ) for amplifying the measurement signal and with a microcontroller (μC) which is supplied with the amplified measurement signal as the input signal. A switch (T 2 ) is provided for short circuiting the input ( 15 ) of the amplifier (U 1 ) during a pause in the measurement cycle, and the microcontroller (μC) has a memory in which the input signal is deposited during the measurement cycle pause, and the microcontroller (μC) has a correction algorithm to correct the measurement signal using the signal deposited in the memory before evaluation. This measure enables an economical operational amplifier to be used as the amplifier (U 1 ) while still attaining reliable information about the temperature prevailing at the measurement site.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a circuit for evaluation of a measurement signal of a thermocouple, especially a thermocouple which is used as a flame detector of a motor vehicle heater, with an amplifier for amplifying the measurement signal and with a microcontroller which is supplied with the amplified measurement signal as the input signal. 
     2. Description of Related Art 
     In the evaluation of thermocouple measurement signals, typically a series of problems rise. To make it possible to evaluate these measurement signals with a voltage in the millivolt range by a microcontroller, they must be amplified into the volt range. Based on the required evaluation accuracy, expensive amplifiers, specifically measuring amplifiers, especially in the form of integrated components, are ordinarily used for this purposes. 
     The problem in the evaluation of thermocouple measurement signals is, furthermore, that a defect of the thermocouple, for example, its interruption, cannot be detected without higher circuit cost. Another problem is the fact that the thermocouple measurement signal, depending on the temperature difference between the thermocouple and the control device or amplifier, has a positive or negative sign, while the microcontroller can typically process only positive voltages. Finally, on principle, only temperature differences between two points can be measured with a thermocouple. To determine the absolute temperature by means of a thermocouple, therefore additional, conventionally complex measures must be taken. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, a primary object of this invention is to devise a circuit for evaluating a measurement signal of a thermocouple of the initially mentioned type which ensures the required accuracy using economical components. 
     This object is achieved in a circuit for evaluation of a measurement signal of a thermocouple, especially of a thermocouple which is used as a flame detector of a motor vehicle heater, with an amplifier for amplifying the measurement signal and with a microcontroller which is supplied with the amplified measurement signal as the input signal, by there being a switch for short circuiting the input of the amplifier during a pause in the measurement cycle, the microcontroller having a memory in which the input signal is deposited during the measurement cycle pause, and the microcontroller having a correction algorithm to correct the measurement signal using the signal deposited in the memory before evaluation. 
     In other words, the invention calls for acquisition of construction-induced errors of the amplifier, which acquired errors are thereupon taken into account in the microcontroller in the evaluation of the thermocouple measurement signal, so that a measurement signal which has been corrected with respect to amplifier errors is evaluated. The measures in accordance with the invention thus allow use of an economical, faulty amplifier for amplification of the thermocouple measurement signal instead of the expensive measuring amplifiers which have been used in the past. Typically, an economical, less precise standard operational amplifier costs much less than, for example, roughly only one tenth as much, a high precision measuring amplifier. The measures in accordance with the invention, for error correction, i.e., the components and programming measures used for this in the microcontroller increase the costs only slightly, so that the circuit according to the invention is, in any case, much more economical than those existing heretofore. Furthermore, it has been ascertained that the accuracy which can be achieved by the error correction of the invention, when using an economical amplifier, ensures a reliable conclusion about the temperature prevailing at the measurement site without recourse to the accuracy made available by a measuring amplifier. 
     The amplifier of the inventive evaluation circuit is advantageously connected as a difference amplifier with a noninverting input and an inverting input which are connected to two terminals of the thermocouple and are bridged by a switch, by the closing of which pauses the measurement cycle, the voltage inherent in the absence of an amplifier being relayed to the microcontroller for subsequent error correction of the measurement signal. 
     The switch is preferably a semiconductor switch, advantageously in the form of a field effect transistor (FET). In the case of connecting the amplifier as a difference amplifier, this switch is connected between its noninverting input and its inverting input. 
     The switch is activated advantageously via the control output of the microcontroller which carries the measurement cycle. 
     In order to economically and simply overcome the initially addressed problem of detection of a thermocouple defect, according to one advantageous development of the invention, there is additionally a circuit for supplying a test current to the thermocouple during a pause in the measurement cycle, and the microcontroller comprises a means for evaluating its input signal which is present in the pause of the measurement cycle. Preferably, this circuit is coupled on the input side to the amplifier such that no noticeable voltage is produced at the amplifier output by the test current when the thermocouple is working properly, while when the thermocouple is defective, an indicative voltage which differs from the previous one occurs at the amplifier output. 
     The test current for detecting the thermocouple state is preferably sent to the thermocouple by means of a switch which, in turn, is designed advantageously as a semiconductor switch, for example, as a FET, and is triggered via the control output of the microcontroller, i.e., the output which carries the measurement cycle. 
     According to an advantageous development of the invention, the initially mentioned problem, according to which a microcontroller can process typically only positive voltages, is surmounted, in accordance with the invention, by a positive DC voltage being added to the amplified measurement signal, the DC voltage being higher than the amount of the negative measurement signal voltage at the addition point. This ensures that the voltage at the microcontroller input is always in the positive range. 
     Finally, a measure is in accordance with the invention to solve the initially mentioned problem of acquiring the absolute temperature by means of the thermocouple. Accordingly, there is a means for measuring the absolute temperature of the board which bears the amplifier and the temperature difference between the measurement point of the thermocouple and the board, from which the microcontroller computes the absolute temperature of the measurement point. 
     In the following, a preferred embodiment of the invention is explained using the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 schematically shows one embodiment of the circuit in accordance with the invention; 
     FIG. 2 is an partial overhead view of a heater for which the circuit of FIG. 1 is suited; and 
     FIG. 3 shows a partial section of the heater. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 schematically shows, to the left of a broken line  10 , a thermocouple S 1  and to the right of the line  10 , a board  9   a  with an evaluation circuit for the thermocouple S 1  which forms, for example, part of a control device  9 , especially for a motor vehicle heater  1  as shown in FIGS. 2 &amp; 3. The actual evaluation of the thermocouple measurement signal takes place in a measurement action by a microcontroller which is labeled μC in FIG.  1 . Upstream of the microcontroller μC, an operational amplifier U 1  is connected to amplify the measurement signals, which are in the millivolt range, to a level which can be processed in a D/A converter (not shown) and which follows the microcontroller input. 
     In FIGS. 2 &amp; 3 the heater is labeled  1 . As an example, an auxiliary vehicle heater is shown which works with a liquid heat transfer medium, for example, water. Such heater is incorporated, for example, into the coolant loop of a motor vehicle. The liquid heat transfer medium flows through an intermediate space  2  which is formed between a heat exchanger  3  and a jacket-shaped external housing  4 . Furthermore, the heater  1  has a schematically shown burner  5  which projects into a combustion chamber  7  which is formed by a combustion pipe  6 . The inlets and outlets which form connections in the external housing  4  for the liquid heat transfer medium for the heater  1  are not shown in the drawings. As follows especially from FIG. 2, a receiving space  8  which is advantageously cast on the exterior of a jacket-shaped external housing  4  as part thereof. The receiving space  8  houses a control device  9  which comprises one or more boards, in this example, a board  9   a,  which can be designed as a printed circuit board and/or can be equipped with the corresponding electrical or electronic components. 
     In particular, the thermocouple S 1  is connected via two supply leads  11 ,  12  to a measurement point which comprises two terminals  13 ,  14  on the board  9   a  on which the electronics of the control device  9  is implemented. The measurement point terminal  13  is connected via a resistor R 4  to the noninverting input (+) of the operational amplifier U 1 . The measurement point terminal  14  is connected via a transistor T 3  and a resistor R 5  to the inverting input (−) of the operational amplifier U 1 . To adjust the gain of the operational amplifier U 1 , its inverting input is connected to its output via a resistor R 6 . The output of the operational amplifier U 1  is, moreover, connected via a resistor R 7  to an input  15  of the microcontroller μC. 
     The capacitors C 1 , C 2 , and C 3  are designed as filter elements or for EMC suppression. Accordingly, the capacitors C 1  and C 2  connected to ground are connected to the measurement point terminals  14  and  13 , respectively, and the capacitor C 3  which is connected to ground is connected to the input  15  of the microcontroller. 
     A DC voltage at a level of +5V is applied via a voltage divider, comprised of resistors R 1  and R 2 , via a connecting line connected between the measurement point terminal  14  and the transistor T 3 . 
     A temperature measurement means  17  is connected to the temperature acquisition input  18  of the microcontroller μC. The temperature measurement means  17  is made, for example, in the form of a commercial, economical IC which, if necessary, inputs the measured temperature already in digital form into the microcontroller μC or in analog form into an integrated A/D converter. The temperature measurement means  17  measures the absolute temperature on the board of the control device, and thus, also on the measurement point terminals  13 ,  14  on the board. In the microcontroller μC, the absolute temperature at the measurement points is.computed from the temperature difference between the temperature at the measurement point and the absolute temperature on the board of the control device. For the case in which the thermocouple is used as a flame detector F in a motor vehicle heater, information about the absolute temperature is useful in the evaluation of the states flame ON and flame OFF. 
     To be able to use an economical, but moderately accurate standard operational amplifier as the operational amplifier U 1 , which standard operational amplifier is, in principle, subject to errors, there is a semiconductor switch in the form of a transistor, preferably a FET, specifically the transistor T 2 , which is connected via the resistors R 4  and R 5  between the noninverting and inverting input of the operational amplifier U 1  and is controlled by a first control output  19  of the microcontroller μC. The transistor T 2  is triggered such that the operational amplifier U 1  is short circuited on the input side during a pause of the measurement cycle. The microcontroller μC comprises a memory  22  in which the output signal of the operational amplifier U 1 , i.e., the signal which is at its input  15 , is deposited during the measurement cycle pause. In another memory  23  of the microcontroller μC, a correction algorithm is stored in order to correct the measurement signal supplied in amplified form to the input  15  during a measurement cycle based on the deposited signal, before actual evaluation. This results in that the error of inaccuracy of the operational amplifier U 1  is taken into account in the evaluation so that it is unnecessary to use an expensive measuring amplifier. 
     Since the error inherent in the operational amplifier U 1  can change over the period of operation, especially in conjunction with a heat-generating heater and the control device located in its vicinity, the above explained error measurement is taken preferably cyclically. 
     To detect the serviceability of the thermocouple S 1  or a defect thereof, moreover, there is a circuit for supplying a test current to the thermocouple S 1 . This circuit comprises a transistor T 1  which is triggered via a second control output  20  of the microcontroller μC, and which, on the other hand, is supplied via a resistor R 3  with positive voltage, specifically with a voltage of +5V. Moreover, the transistor T 1  is connected using the connecting line between the measurement point terminal  13  and the resistor R 4  to the noninverting input of the microcontroller μC. 
     When the transistor T 1  is forcibly tripped in a measurement cycle pause which has been communicated via the second control output  20 , the DC voltage of +5V is applied via the transistor T 1  to the measurement point terminal  13 , and thus, to the thermocouple S 1 , via which the test current flows to the other measurement point terminal  14  when the thermocouple S 1  is intact, from which the test current drains to ground via the resistor R 2 . In this case, i.e., with the thermocouple S 1  intact, at the output of the operational amplifier U 1 , there is no signal or a very small signal. 
     Conversely, if the thermocouple S 1  is defective, and thus the test current is not relayed, essentially the +5V DC voltage is on the noninverting input of the operational amplifier U 1 , so that there is a relatively high voltage at the output of the operational amplifier U 1 , which is detected in the microcontroller μC and which is treated as an indication that a defect exists in the sensor S 1 . 
     The other transistor T 3 , which is in the connecting line between the measurement point  14  and the resistor R 5  and the inverting input of the operational amplifier U 1 , is triggered via a third control input  21  of the microcontroller μC. Conventionally, the transistor T 3  is forcibly tripped to relay the signal from the measurement point  14  to the operational amplifier U 1 . However, depending on the practical use of the thermocouple S 1 , it can be desirable, at a certain point in the measurement cycle, to electrically separate the thermocouple S 1  from the downstream circuit, i.e., to not relay a measurement signal to the operational amplifier U 1 . 
     The voltage divider formed of R 1 , R 2  adds an exactly known positive voltage to the measurement signal delivered by the thermocouple S 1  in order to ensure that there is always a positive voltage at the input  15  of the microcontroller μC. This is important since microcontrollers conventionally process only positive voltages, while the thermocouple S 1  delivers positive and negative voltages. Therefore, the added positive voltage is chosen to be quantitatively higher than the maximum amount of the measurement voltage deliverable by the thermocouple S 1 .