Thermal measuring and testing – Temperature measurement – By electrical or magnetic heat sensor
Patent
1997-06-26
2000-04-11
Gutierrez, Diego
Thermal measuring and testing
Temperature measurement
By electrical or magnetic heat sensor
327 78, 307116, 307117, 702 99, G01K 716, H03K 513, H01H 3500
Patent
active
060480947
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for measuring temperature using a negative temperature coefficient sensor. More particularly, a method of this type is intended to be employed in the field of automobile electronics.
2. Description of the Related Art
It is already known to use sensors whose resistance varies as a function of the temperature of the medium in which they are immersed. By measuring this resistance variation, and after processing the measured signal by a common signal processing means, a digital value N is determined which represents the temperature prevailing around the sensor.
Conventionally, as regards temperature sensors with negative temperature coefficient (also referred to as NTC sensors), a voltage source VA is applied to a sensor and a resistor which is connected in series (referred to as a "pull up" resistor) is connected to the positive pole of the voltage source. An analog measured signal, taken from a midpoint lying between the resistor and the sensor, is sent to an analog/digital converter. This converter also receives a reference voltage, so as to generate a digital value N representing the temperature measured by the sensor.
However, devices of this type have a major drawback, namely: it is difficult for them to measure with high precision a temperature which varies over an extended range.
In order to overcome this drawback, it is known to switch the reference voltage of the converter between a plurality of different reference voltages. One measured temperature range corresponds to each reference voltage. This provides a plurality of measurement scales for accurately monitoring a temperature which varies over an extended range. For example, in the automotive field, the temperature to be measured may vary from -40.degree. to +140.degree. C.
The drawback of devices of this type with multiple reference voltages is that the converter must be provided with a plurality of accurately calibrated voltage sources. Such a multiplicity of calibrated voltage sources is expensive.
It is therefore desired to create a method for measuring temperature which uses only a single voltage source, while ensuring that the measurement is highly accurate over the entire extent of the temperature range.
It is already known, to this end, to vary the resistance of the resistor connected in series with the sensor (by switching a transistor connected in parallel with this resistor), in order to improve temperature monitoring on different temperature scales. However, this technique has the drawback of changing the current flowing in the measurement branch and, in particular, it introduces errors due to the voltage drops and the leakage currents of the transistors.
SUMMARY OF THE INVENTION
The object of the present invention is therefore to use a single reference voltage, but to determine accurately the errors resulting from the presence of the transistors in the signal-processing stage, in order to take them into account and to eliminate errors associated with the presence of the transistors.
To this end, the present invention relates to a method for measuring temperature using a negative temperature coefficient sensor, said sensor having a variable internal resistance and being associated with a signal processing stage comprising at least one transistor connected in parallel with at least one so-called "pull up" resistor, a voltage through the signal processing means taken at a mid-point between the sensor and the "pull up" resistors being read on an analog input of an analog/digital converter, said method being one which consists in: transistor is alternately and regularly on, then off, when the transistor is on and when the transistor is off, two measurements by the signal processing means, at the analog input of the converter by the signal processing means and deducing from this corrected voltage the temperature prevailing around the sensor.
Thus, by measuring the voltage at the input of the converter when the transistor in parallel with the
REFERENCES:
patent: 3930256 (1975-12-01), Amemiya
patent: 5140302 (1992-08-01), Hara et al.
patent: 5313848 (1994-05-01), Santin et al.
Greenberg Laurence A.
Gutierrez Diego
Lerner Herbert L.
Siemens Automotive S.A.
Stemer Werner H.
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