WO1997000432A1

WO1997000432A1 - Temperature measuring assembly

Info

Publication number: WO1997000432A1
Application number: PCT/GB1996/001077
Authority: WO
Inventors: Debra Ann Bezzina; Marsha Wenbel; Eric Blaine Ferch; Roberto Anthony Rossi; Robert Joseph Bolduc
Original assignee: Ford Motor Co Ltd; Ford Motor Co
Current assignee: Ford Motor Co Ltd; Ford Motor Co
Priority date: 1995-05-05
Filing date: 1996-05-07
Publication date: 1997-01-03
Anticipated expiration: 1997-11-05

Abstract

A temperature measuring assembly for accurately measuring temperature using a voltage source supplying a constant voltage signal includes a thermistor (12); an input port (14) for receiving an input operatively connected to the thermistor (12); an input conditioning circuit (16) electrically connected to the voltage source, the input conditioning circuit (16) and the thermistor (12) defining a voltage divider to variably divide the constant voltage signal to create a variable voltage signal, an analog-to-digital converter (18) electrically connected to the input conditioning circuit (16) wherein the A/D converter (18) converts the variable voltage signal into a digital signal; a control unit (20) electrically connected to the analog-to-digital converter (18) including first and second transforms to transform the digital signal into a temperature signal; and a resistive circuit (22) operatively connected between the control unit (20) and the input conditioning circuit (16) switchable between open and close conditions such that the resistive circuit (22) changes the variable voltage signal received by the A/D converter (18).

Description

TEMPERATURE MEASURING ASSEMBLY

The present invention relates generally to electronic controls for internal combustion engines and, more particularly, to temperature measuring systems for components of the internal combustion engine.

It is known to measure the temperature of cylinder heads over a limited range with acceptable accuracy. It would, however, be desirable to measure temperatures over a much larger range with improved accuracy. Such improvements have been unobtainable without providing temperature sensors with multiple sensing units over varying ranges of temperatures. Costs associated with inventory, production and maintenance of these assemblies prevent their use.

Accordingly, the present invention is a temperature measuring assembly for measuring temperature. The temperature measuring assembly uses a voltage source supplying a constant voltage signal. The temperature measuring assembly includes a thermistor and an input port operatively connected to the thermistor. An input conditioning circuit is electrically connected to the voltage source and the input port . The input conditioning circuit has a predetermined impedance. The input conditioning circuit and said thermistor sensor define a voltage divider to variably divide the constant voltage signal to create a variable voltage signal. An analog to digital converter is electrically connected to the input conditioning circuit and converts the variable voltage signal to a digital signal . A control unit is electrically connected to the analog to digital converter and includes first and second transforms to transform the digital signal into a temperature signal. A resistive circuit is operatively connected between said control unit and said input conditioning circuit and is switchable between open and close conditions such that the resistive circuit changes the variable voltage signal received by the analog to digital converter.

One advantage associated with the present invention includes providing a temperature measuring assembly to measure a temperature with improved accuracy over an extended range of temperatures without the addition of extra sensors and, thus, without the addition of additional costs. The present invention has the ability to apply the temperature sensing assembly to any thermistor measurement system, e.g., transmission fluid temperature, intake air temperature, and the like.

The invention will now be described further, by way of example, with reference to the accompanying drawings, in which : -

FIG. 1 is a schematic of a temperature measuring assembly according to the present invention,

FIG. 2 is a flowchart of a method of temperature measuring assembly according to the present invention and

FIG. 3 is a graphic representation of sensor voltage as a function of temperature.

Referring to FIG. 1, one embodiment of a temperature measuring assembly, according to the present invention, is generally indicated at 10. The temperature measuring assembly 10 is used to measure the temperature of a cylinder head of a internal combustion engine (neither shown) using a cylinder head temperature (CHT) sensor 12. In one embodiment, the CHT sensor 12 is a thermistor, it may be appreciated that the temperature measuring assembly 10 may be used to measure the temperature of any solid, fluid, or gas in or out of the motor vehicle environment.

An input port 14 operatively connects the CHT sensor 12 to an input conditioning circuit 16 which is, in turn, connected to a voltage source, five volts in one embodiment as shown in FIG. 1. It may be appreciated that the voltage source may vary its output and may even be a low frequency source. The input conditioning circuit 16 and the CHT sensor 12 create a voltage divider for the voltage source which is variably dependent on the resistive value of the CHT sensor 12. The variability of the voltage divider is not dependent upon the input conditioning circuit 16 because it defines a predetermined constant impedance. The input conditioning circuit 16 and the CHT sensor 12 create a variable voltage signal from the voltage signal . The variable voltage signal is suitable for reception by an analog to digital (A/D) converter 18, discussed subsequently. The predetermined constant impedance is suitably matched with the A/D converter 18.

The A/D converter 18 is a ratiometric converter and is electrically connected to the input conditioning circuit 16. The A/D converter 18 uses the voltage source as a reference and converts the variable voltage signal into a digital signal. More specifically, the A/D converter 18 converts the analog signal received from the voltage divider created by the input conditioning circuit 16 and the CHT sensor 12, having units of volts, into a digital signal.

The digital signal is received by a control unit 20 which is electrically connected to the A/D converter 18. The control unit 20 includes first and second transforms to transform the digital signal into a temperature signal . Referring to Table 1 and Table 2, the control unit 20 converts the number of pulses or counts received from the A/D converter 18 into a corresponding temperature reading based on the experimental temperature values. The determination as to which transform is to be used will be discussed subsequently. In one embodiment, the control unit 20 is a microprocessor. Cold End Transfer Function

R = 20 kΩ

Counts Temp (°F)

1023 -40

1002 -40

995 -31

985 -22

956 -4

911 14

847 32

666 68

360 122

244 149

162 176

124 194

108 203

0 212

Table 1

Hot End Transfer Function

R = 20kΩ in parallel with IkΩ

Counts Temp (°F)

1023 168

817 176

789 185

729 203

664 221

527 257

430 284

318 320

294 329

272 338

232 356

182 383

155 401

0 500

Table 2

10 A resistive circuit 22 is. operatively connected between the control unit 20 and the input conditioning circuit 16. The resistive circuit 22 is switchable between open and close conditions such that the resistive circuit 22 changes the effective impedance of the CHT sensor 12, the input conditioning circuit 16 and the resistive circuit 22 which results in a change of the variable voltage signal received by the A/D converter 18. More specifically, the resistive circuit 22 is switchable between open and close conditions to change the voltage division created by the voltage divider, i.e., the CHT sensor 12, the input conditioning circuit 16, and the resistive circuit 22 when in the close condition, which, in turn, changes the effective impedance at the A/D converter 18. The effective impedance change at the A/D converter 18 results in the change of the variable voltage signal received thereby. The resistive circuit 22 includes a MOSFET transistor 24 which is connected to a first resistor 26.

A switching circuit 28 is electrically connected between the control unit 20 and the resistive circuit 22. The switching circuit 28 is controlled by the control unit 20 and switches the MOSFET transistor 24 when the temperature sensed by the CHT sensor 12 has increased or decreased beyond predetermined limits. The predetermined limits are set by the control algorithm, discussed subsequently. A graphic representation of these transforms is shown in FIG. 3. More specifically, when the CHT sensor 12 senses a temperature at approximately 200°F, the control unit 20 switches the MOSFET transistor 24 to close the resistive circuit 22 and, at the same time, operates using the second transform. Therefore, when using the second transform, a reading of approximately 4.200 V corresponds to a temperature reading of 200°F.

Likewise, when the temperature of the cylinder head is decreasing from the 400°F range, the second transform is used until approximately 160°F at which time the control unit 20 opens the MOSFET transistor 24 and returns to using the first transform. Changing both the impedance at the A/D converter 18 and the transform used by the control unit 20 effectively doubles the range of sensing capabilities while still using one CHT sensor 12. The hysteresis loop created by the two limits (200°F and 160°F) reduces excessive switching between the two impedance levels and the two transforms by requiring the temperature to change over 40°F before a switch occurs.

As an example of the operation of the present invention, the CHT sensor 12 senses the temperature of a cylinder head to be 50°F at startup. The sensor emits an input voltage signal of approximately 3.250V. In FIG. 3, this point is labelled at 30 along line 32. As the temperature of the cylinder head increases, the CHT sensor 12 produces an input voltage signal of decreasing voltage. At 200°F, the second transform and resistive circuit 22 are switched into operation and the 4.300V signal, represented at 34 represents 200°F as opposed to 40°F at 36, which would be the indication had the control unit 20 been using the first transform (Table 1) and the resistive circuit 22 had been open. The CHT sensor 12 continues along the path of line 38 and does not continue along line 40, as it would had the resistive circuit 22 not be closed. Upon cooling of the cylinder head, the CHT sensor 12 reads signals along lines 38 and 42 until approximately 160°F where the resistive circuit 22 is open and the first transform is used to determine the temperature of the cylinder head. The control unit 20 determines the temperature of the cylinder head to be the temperature corresponding to the input voltage signal received by the CHT sensor 12 according to line 32.

In one embodiment shown in FIG. 1, the input conditioning circuit 16 includes a second resistor 44 which is connected between the constant voltage source, a 5V power supply, and the first resistor 26. The two resistors 26, 44 are also connected to a third resistor 46 and a capacitor 48. The three resistors 26, 44, 46, and the capacitor 48 are all connected to the input port 14. The capacitor 48 and a second capacitor 50 are connected to ground. The second capacitor 50 and the third resistor 46 are connected to the input of the A/D converter 18. The A/D converter 18 is connected to the control unit 20 which is connected to the switching circuit 28. More particularly, the control unit 20 is connected to a fourth resistor 52. The fourth resistor 52 is also connected a fifth resistor 54 and the base of a transistor 56. The emitter of the transistor 56 and the fifth resistor 54 are connected to ground. A sixth resistor 58 and a seventh resistor 60 are connected to each other and the collector of the transistor 56. The resistor 58 is also connected to a power supply Vp. The seventh resistor 60 is connected to the gate of the MOSFET transistor 24. The drain of the MOSFET transistor 24 is connected to the constant voltage source. The source of the MOSFET transistor 24 is connected to the first resistor 26.

Referring to FIG. 2, a flowchart of the method used by the control unit 20 is shown. A determination 27 is made as to whether the two resistors 26,44 in parallel are being used. if both resistors 26,44 are being used, i.e., if the MOSFET transistor 24 is closed, the second transfer function is used 29. If, however, the MOSFET transistor 24 is open, the first transfer function is used 31. If the measured temperature is greater than a predetermined value 33, 200°F in one embodiment, the second transfer function is used 35 as well as the use of the two resistors 26,44 in parallel given the MOSFET transistor 24 provides minimal resistance. The resistor 26 is used when the MOSFET transistor 24 is closed. If the measured temperature is not greater than the predetermined value, it is determined whether the measured temperature is less than the return value 37. If the measured temperature is below the return value, the MOSFET transistor 24 is opened whereafter the second resistor 44 is used 39. If the measured temperature has not cooled to a temperature below the return value, the MOSFET transistor 24 remains closed and the impedance at the A/D converter 18 is maintained at a level using the two resistors 26,44 in parallel.

In one embodiment, the accuracy 41 of the system, which includes the temperature measuring assembly 10 and the CHT sensor 12, is approximately ± 10°F between -40°F and 335°F and approximately 15°F between 335°F and 450°F. The accuracy of a temperature measuring assembly 10 is far superior to that found conventionally.

Claims

1. A temperature measuring assembly for measuring temperature using a voltage source supplying a constant voltage signal, said temperature measuring assembly comprising: a thermistor sensor (12) ; an input port (14) operatively connected to said thermistor sensor (12) ; an input conditioning circuit (16) electrically connected to the voltage source and said input port (14) , said input conditioning circuit (16) and said thermistor sensor (12) defining a voltage divider to variably divide the constant voltage signal to create a variable voltage signal; an analog to digital converter (18) electrically connected to said input conditioning circuit (16) to receive the variable voltage signal, said analog to digital converter (18) converting the variable voltage signal to a digital signal; a control unit (20) electrically connected to said analog to digital converter (18) , said control unit (20) including first and second transforms to transform the digital signal into a temperature signal; and a resistive circuit (22) operatively connected between said control unit (20) and said input conditioning circuit (16_ switchable between open and close conditions such that said resistive circuit (22) changes the variable voltage signal received by said analog to digital converter.

2. A temperature measuring assembly as claimed in claim 1, including a switching circuit electrically connected between said control unit and said resistive circuit .

3. A temperature measuring assembly as claimed in claim 2, wherein said control unit includes a microprocessor.

4. A temperature measuring assembly as claimed in any one of claims 1 to 3, wherein said first and second transforms create hysteresis loop.

5. A temperature measuring assembly as claimed in any preceding claim, wherein said resistive circuit includes a switch to switch said resistive circuit between said open and close conditions.

6. A temperature measuring assembly for measuring temperature of a cylinder head of an internal combustion engine using a cylinder head temperature sensor and a voltage source supplying a constant voltage signal, said temperature measuring assembly comprising: an input port electrically connected to the cylinder head temperature sensor; an input conditioning circuit electrically connected to said input port, said input conditioning circuit and the cylinder head temperature sensor defining a voltage divider to variably divide the constant voltage signal to create a variable voltage signal; an analog to digital converter electrically connected to said input conditioning circuit to receive the variable voltage signal, said analog to digital converter converting the variable voltage signal to a digital signal; a control unit electrically connected to said analog to digital converter, said control unit including first and second transforms to transform the digital signal into a temperature signal; and a resistive circuit operatively connected between said control unit and said input conditioning circuit switchable between open and close conditions such that said resistive circuit changes the variable voltage signal received by said analog to digital converter.

7. A method for measuring temperature of a cylinder head of an internal combustion engine using a cylinder head temperature sensor, an input conditioning circuit, and a resistive circuit switchable between open and close condition, the method comprising the steps of: measuring the temperature of the cylinder heads using the cylinder head temperature sensor; comparing the temperature to a predetermined value; and switching the resistive circuit when the temperature exceeds the predetermined value.

8. A method as claimed in claim 7, including the step of defining a return value differing from the predetermined value.

9. A method as claimed in claim 7 or 8, including the step of switching the resistive circuit when the temperature falls below the return value.