Temperature Measuring Device for Recording Large Changes in Temperature

This invention relates to a temperature measuring device for recording large changes in temperature, especially temperatures in an internal combustion engine.

A known method for recording temperatures is the insertion of temperature dependent resistors such as NTC resistors into, or adjacent to, the item whose temperature is being monitored. A resistor of this type is connected to a constant source of voltage by way of a dropping resistor whereby the voltage drop at the temperature dependent resistor is a gauge for the recorded temperature or, as the case may be, change in temperature.

Temperature dependent resistors change their value in orders of magnitude to the power of ten. Thus, within a temperature range of -60.degree. to +200.degree. C., an NTC resistor changes its value from 160 KOhm to 20 Ohm (8000:1).

In order to achieve a sufficient degree of exactness in measuring at high temperatures, a low ohmic dropping resistor must be used. As can be gathered from curve 1 in FIG. 1, the change in voltage with a low ohmic dropping resistor in the range from -60.degree. C. to 0.degree. C. is almost nill, while in the case of high temperatures, there are sufficient changes in voltage which make a corresponding determination of temperature possible.

If a high ohmic dropping resistor is inserted to record lower temperatures, this results in curve 2, shown in FIG. 1, which is relatively flat in the higher temperature range so that an exact determination of temperatures in this range is not possible.

In practical use, further difficulties may be encountered including disorders such as breaks in conduction or short-circuiting. In the case of a low ohmic dropping resistor, an interruption in conduction to the NTC resistor in the low temperature range cannot be recognized, while in the case of a high ohmic resistor, a short-circuit of the NTC resistor in the high temperature ranges cannot be recognized.

It is a primary object of this invention to provide a temperature measuring device for measuring large changes in temperature which has a sufficient output signal to permit a positive determination of a break or a short-circuit in the electrical circuit at both low and high temperatures.

Switching from a high ohmic dropping resistor to a low ohmic dropping resistor, which is achieved by means responding to a strictly specified frequency, produces an output signal from the temperature measuring device which at a temperature point on the low ohmic curve (curve 1 in FIG. 1), corresponding to a predetermined frequency jumps to the high ohmic curve (curve 2 in FIG. 1). Accordingly, the output signal of the temperature measuring device is available at high as well as low temperatures and changes recognizably in its amplitude even with minor changes in temperature. The output signal reproduces both curves appropriately processed. Absence of the output signal, which normally changes dramatically in amplitude in a temperature measuring application, can indicate a break or short-circuit in the electrical circuit or a breakdown of the temperature dependent resistor.

By switching between a high ohmic and a low ohmic dropping resistor in accordance with this invention, the power which flows through the temperature dependent resistor is, in its average value, less than in the case of continuous operation with the low ohmic dropping resistor. The heat buildup of the temperature dependent resistor in the temperature measuring device of this invention is, therefore, lower. The temperature dependent resistor in a temperature measuring device of this invention is not heated to a greater extent, when producing high output signals, than when using the prior art device with a low ohmic dropping resistor.

The temperature measuring device of this invention makes advantageous use of a dropping resistor circuit which consists of parallel branches which include, respectively, a high ohmic resistor and a low ohmic resistor. One branch includes the low ohmic resistor and a switch which can be switched on and off by a specified frequency so that the dropping resistor circuit is switched at a set frequency between a high ohmic value, which is determined by the high ohmic resistor in the other parallel branch and a low ohmic value, which is determined primarily by the branch having the low ohmic resistor. An advantage lies in the fact that the switch is a transistor, which is operated as a switch by a frequency having a probe ratio of 1:1.

In one embodiment of the invention (FIG. 2), the dropping voltage at the temperature dependent resistor is plotted via a low pass filter so that at the outlet of the low pass filter a processed output signal is available which indicates a sufficiently great change in voltage over the entire temperature range.

In another embodiment of the invention (FIG. 3), the voltage proportional to the voltage drop at the temperature dependent resistor and a control voltage of predetermined frequency and wave form are fed to a computer, whereby the output signal of the temperature measuring device and the frequency of the switch are synchronized. This increases the gradient of the output curve at its high and low temperature portions thereby increasing the accuracy of measuring and indicating temperature.

Additional features of the invention will be found in the following description and in the drawings, in which:

FIG. 1 shows a circuit for a temperature measuring device of the prior art;

FIG. 1a is a graph plotting the output signal versus the temperature using a prior art temperature measuring device with a high ohmic resistor and with a low ohmic resistor;

FIG. 2 shows a circuit of one embodiment of a temperature measuring device of this invention;

FIG. 2a is a graph showing the recorded output signal versus the temperature produced using the temperature measuring device of FIG. 2;

FIG. 3 shows a circuit of a second embodiment of a temperature measuring device of this invention; and

FIG. 3a is a graph showing the output signal versus temperature produced by using the circuit of FIG. 3.

In the case of the known temperature measuring device illustrated in FIG. 1, the temperature dependent resistor R.sub.T is subjected, via a dropping resistor R.sub.V, to a constant voltage source, of, for instance, 5 volts. The voltage drop U.sub.G at temperature dependent resistor R.sub.T is the output signal of the temperature measuring device. Using a low ohmic dropping resistor R.sub.V of, for instance, 120 Ohm, the output signal U.sub.G changes versus temperature as shown by curve 1. Using a high ohmic dropping resistor R.sub.V of, for instance, 4.7 KOhm, the output signal changes versus temperature as shown by curve 2. If a high ohmic dropping resistor is used, the change in the output signal in the area above 100.degree. C., plus or minus 20.degree. C., is relatively weak, so that an error-free, exact recording of temperature is not possible. In the case of a low ohmic dropping resistor, an exact determination of temperature in the range from -60.degree. C. to +20.degree. C. is not possible for the same reason.

The temperature measuring device of this invention illustrated in FIG. 2 consists of a temperature dependent resistor R.sub.T which is subjected to a constant voltage source of 5 volts through a dropping resistor circuit having a pair of parallel branches 11, 12 which include, respectively, one high ohmic dropping resistor R.sub.V2 and one low ohmic dropping resistor R.sub.V1. The high ohmic resistor has, for example, a value of 4.7 KOhm, and the low ohmic R.sub.V1 a value of 120 Ohm. The resistances of the resistors R.sub.V1 and R.sub.V2 have a ratio of at least 1 to 10. In circuit branch 12 of the low ohmic dropping resistor R.sub.V1, a transistor T is used as a switch or circuit breaker. The base of the control transistor T is loaded with a probe frequency (f) of, for instance, 50 Hertz. An advantageous frequency is one having a probe ratio of 1:1.

If the low ohmic branch is interrupted, the dropping resistor resistance of the dropping resistor circuit is determined by R.sub.V2 which is 4.7 KOhm.

In the case of a conducting low ohmic branch, the resistance value of the parallel dropping resistor circuit is determined by the equation: ##EQU1## and is calculated as about 117 Ohm. The resistance of the parallel dropping resistor circuit is primarily determined by the low ohmic dropping resistor R.sub.V1, which has a resistance value of 120 Ohm.

The dropping voltage at the temperature dependent resistor R.sub.T is plotted via a low pass filter provided by a resistor R.sub.C and a condenser C. The measuring circuit 13 connecting the resistor R.sub.C and the condenser C in series is connected at 14 with the dropping resistor parallel circuit branches 11, 12 and, thus, the circuit 13 is connected in parallel with the temperature dependent resistor R.sub.T. The voltage U.sub.G across the condenser C corresponds to the output signal of the temperature measuring device of this invention as is shown in FIG. 2a plotted versus the temperature. As can clearly be seen in FIG. 2a, the output signal changes markedly at low as well as at high temperatures so that the recorded temperatures can be determined with a high degree of precision from the changes in the output signal U.sub.G throughout a temperature range from -60.degree. C. to +200.degree. C. By using a dropping resistor R.sub.V1 which is disconnected by the switching transistor T at a set frequency, the heat buildup of the temperature dependent resistor is less than that of the prior art temperature measuring device of FIG. 1.

It is within the scope of this invention to provide a switch in the temperature measuring device whereby, in response to a control frequency, the temperature dependent resistor R.sub.T is subjected to the constant voltage source alternatively through the high ohmic resistor R.sub.V2 or through the low ohmic resistor R.sub.V1 .

The temperature measuring device of FIG. 2 operates with a probe ratio of 1. Therefore, the high ohmic dropping resistor R.sub.V2 is switched on 50% of the time, and the low ohmic dropping resistor R.sub.V1 is switched on 50% of the time resulting in curves 1 and 2 in FIG. 1. At temperatures lower than 140.degree. C., the voltage fluctuation of curve 2 is approximately 0; however, since the output indicated in curve 1 is switched on only 50% of the time, its voltage, and consequently any fluctuation thereof, is cut in half. The same disadvantage occurs below 0.degree. C. for the voltage fluctuation of curve 2. By using a temperature measuring device of the embodiment shown in FIG. 3, the voltage shortage can be substantially eliminated by regulating the probe ratio, with which the two dropping resistors are switched, via the output voltage in such a way that with low output voltage, curve 1 is switched on for a long time and curve 2 for a short time and with high output voltage, curve 2 is switched on for a long time and curve 1 for a short time. Thus, as shown in FIG. 3a, the average value curve 4 at high temperatures passes above the curve 3, shown in broken lines in FIG. 3a and in a solid line in FIG. 2a, and at low temperatures the curve 4 passes below the curve 3. Thus, almost twice the gradient is attained at both ends of the measuring range of the average value curve 4 as exists in the curve 3. The increased gradient greatly increases the accuracy of measurement and recording of temperature changes at high and low temperatures.

Referring to FIG. 3, the resistors R.sub.1 and R.sub.2 are the dropping resistors used to produce curves 1 and 2 and the switching transistor T is the same as the one used in FIG. 2. The series connected resistor R and condenser C constitute a low pass filter by which the average value of both curves 1 and 2 is obtained. A signal from the temperature measuring circuit 13 is fed to a first input 16 of the comparer computer K via lead 17, which is connected to the measuring circuit 13 intermediate the resistor R.sub.C and the condenser C. A delta voltage wave form 18 with values fluctuating between O and U.sub.B is fed to the input 19 of the comparer computer K through a lead 21.

The comparer computer K averages the voltage drop U.sub.G received at its input 16 via lead 17, compares the average value of U.sub.G with the delta wave voltage received via input 19 and delivers at its output 22 a square wave voltage U.sub.GK of the form 23 which is supplied to the base of the switching transistor T via lead 24 in which a resistor R.sub.V3 is interposed. The comparer computer K will cause the probe ratio of its output (square wave voltage U.sub.GK) to change in response to changes in the average value of the voltage U.sub.G fed to its input 16. Thus, when measuring high temperatures, the probe ratio will be greater than 1 and the switching transistor T will be switched "on" for a longer time interval than it is "off". When measuring low temperatures, the probe ratio is less than 1 and the switching transistor is switched "on" for a shorter time than it is switched "off".

The computer can also be used to identify a break in the electrical conduction or a breakdown of temperature dependent resistor R.sub.T by analyzing the output signal, which will change erratically compared to the output signal when the components and circuits are not defective.

It may be advantageous to replace the constant source of voltage and the dropping resistors R.sub.V1 and R.sub.V2 which are located between NTC resistor R.sub.T and the voltage source, with two different constant voltage sources of appropriate values. The two constant sources of voltage would be alternatively connected with the NTC resistor R.sub.T in response to a control frequency.