Glow plug system, controlling device and method for controlling the power of a glow plug

The invention relates to a glow plug system for a Diesel engine of a motor vehicle, having the features presented including, a controlling device for controlling the power of an electric load, as well as a method for controlling the power of a glow plug.

In motor vehicles, the amplitude of the electrical supply voltage is, as a rule, subject to variations. For this reason, a measurement of the amplitude of the available supply voltage is required for precise control of the power of electric loads, such as glow plugs or heater elements. If, for example, the power is controlled by means of pulse width modulation, the pulse width, i.e. the duration of the time intervals in which the supply voltage is connected to the load, can be selected as a function of the value of the supply voltage to feed a desired power to the load.

Poor connection of a measuring or controlling device to a ground potential connected to the load may result in an incorrect value being determined in a measurement of the supply voltage because, in such a case, the supply voltage is measured in relation to a reference potential that deviates from the ground potential connected to the load. If an incorrect supply voltage value is used for power control, then the power provided is too high or too low which may result in damage. For example, glow plugs which overheat because the power is too high have a reduced service life. Glow plugs which fail to reach their specified end temperature result in a poorer ignition performance.

A poor connection of a measuring or controlling device to a ground potential is often caused by defective or aged pin and socket connections which may generate a considerable resistance.

An object of the invention is, therefore, to devise a way for improving the power control of glow plugs.

This object is met by a glow plug system having the features presented in as well as by a method for controlling the power of a glow plug, said method having the features specified.

The invention facilitates improved power control by measuring the supply voltage more precisely. It has been recognized within the scope of the invention that, while the supply voltage is being measured, this measurement is taken in relation to a reference potential, i.e. a value of the supply voltage is measured as the difference of the electrical potential of the positive terminal of a power source and the electrical reference potential. However, the reference potential might deviate from the ground potential connected to the load, for example due to poor contacts. As a consequence the actual value of the supply voltage connected to the load may deviate from a measured value according to the difference between the reference potential and the ground potential. The invention allows a possible deviation of the reference potential from the ground potential to be detected. The value of any deviation can then be used to correct a measured supply voltage value. In this manner, the value of the voltage connected to the load can be determined with a higher precision such that, if the power is controlled, for example, by means of pulse width modulation, the duration of voltage pulses which are applied to the load in order to supply a desired power can be calculated on a more reliable basis.

According to the invention, a first step involves measuring the supply voltage in relation to a reference potential. Since the value of said reference potential may deviate from the value of a ground potential connected to the load for various reasons, a further step involves checking whether the reference potential deviates from the ground potential and determining a value of a any deviation. Preferably, the value of a deviation of the reference potential from the ground potential is determined by means of a voltage divider circuit and a test voltage source. For example, the operating voltage required for electronic components can be used as test voltage. Suitable voltage sources are usually integrated in many electronic devices anyway so that, for example, a voltage of 5 volts can be easily and reliably provided largely free from voltage fluctuations.

In a glow plug system according to the invention, a value of the supply voltage that is connected to a measuring input of a measuring device is measured in relation to a reference potential present at a ground input of the measuring device that is integrated in the controlling device. In addition to the measuring input and in addition to the ground input, the controlling device has a test input which, during operation, is connected to a test voltage source via a first resistor. The test input is also connected to the positive terminal of the glow plug via a second resistor. Since the two resistors form a voltage divider circuit, the electrical potential difference which should be present between the electrical potential of the test input and the electrical potential of the ground input under ideal conditions, i.e. when the reference potential is identical to the ground potential, is known or can be easily determined. If the controlling device does not measure the expected reference value, a possible deviation is based on a difference between the reference potential and the ground potential. A deviation that might be determined can therefore be used to correct the measured supply voltage value.

An essential element of a glow plug system according to the invention is the controlling device described above, which can also easily be used to control the power of loads other than glow plugs. Therefore, the aforementioned object is also met according to the invention by a controlling device having the features specified.

Further details and advantages of the invention are illustrated in the following by means of an exemplary embodiment with reference being made to the enclosed drawing. The features described therein can be made the subject matter of claims either separately or in combination. In the figures:

FIG. 1 shows a circuit diagram of a glow plug system according to the invention.

The glow plug system shown in the diagram of FIG. 1 comprises a plurality of glow plugs which are shown as resistors RG 1 and RG 2 in FIG. 1 . Further glow plugs may be added to the system as required. Each of the glow plugs RG 1 , RG 2 has a positive terminal which can be connected to a supply voltage U 1 via a supply line G 1 or G 2 , respectively. A switch S 1 is arranged in each of the supply lines G 1 , G 2 . By opening and closing switch S 1 the glow plugs RG 1 , RG 2 , respectively, can be connected and disconnected to a car battery as required for power control by means of pulse width modulation. The switch S 1 , preferably a power transistor, is actuated by a control unit 1 for power control purposes.

The control unit 1 comprises a microprocessor with an analog-to-digital converter. The control unit 1 controls the power by means of a pulse width modulation procedure in which the switch S 1 is opened and closed for appropriate time intervals in order to generate the pulse widths in order to feed a desired electric power to the glow plug RG 1 . To feed the desired electric power, the control unit 1 controls the duration of the pulse widths as a function of the value of the supply voltage U 1 . The supply voltage U 1 of, for example, approximately 11 volts is provided by the electric system of the motor vehicle.

A measurement input ADC 1 of an analog-to-digital converter integrated in the controlling device 1 is provided for measuring the supply voltage U 1 . According to FIG. 1 , the supply voltage U 1 is fed both to the measurement input ADC 1 and the positive terminals of the glow plugs RG 1 , RG 2 . The controlling device 1 measures the supply voltage U 1 in relation to a reference potential GND′ which is applied to a ground input ADC 2 of the controlling device 1 , i.e. the supply voltage is measured as the difference of the electrical potentials present at the measurement input ADC 1 and the ground input ADC 2 , respectively. The reference potential GND′ can deviate from the ground potential GND which is present at the ground terminal of the glow plugs RG 1 , RG 2 , for example because of poor contacts which may be caused by defective pin and socket connectors. Pin and socket connectors are customarily used for connecting electronic components in automotive engineering. For that reason, the resistance R 3 that is caused by a poor connection of the ground input ADC 2 of the controlling device 1 to the ground potential GND is indicated between the reference potential GND′ and the ground potential GND in FIG. 1 .

The residual resistance R 3 causes the reference potential GND′ to deviate from the ground potential GND and, as a consequence, the value of the supply voltage U 1 determined by the controlling device 1 to deviate from the value dropping out from the glow plug RG 1 , RG 2 .

In order to be able to measure a possible deviation of the reference potential GND′ from the ground potential GND, the controlling device 1 has a test input ADC 3 which is connected via a first fixed resistor R 1 to a test voltage source supplying the constant test voltage U 2 . Furthermore, the test input ADC 3 is connected to the positive terminal of the glow plug RG 1 , RG 2 via a second fixed resistor R 2 . In the exemplary embodiment shown, the fixed resistor R 2 is connected in parallel to one glow plug RG 1 , RG 2 each.

In order to check whether the reference potential GND′ present at the ground input ADC 2 deviates from the ground potential GND present at the ground terminal or ground connection of the glow plug RG 1 , RG 2 , the supply voltage U 1 is deconnected from the positive terminal of the glow plug RG 1 , RG 2 . For this purpose, the switch S 1 is opened by the controlling device 1 .

The resistors R 1 and R 2 form a voltage divider circuit such that a voltage U 3 is applied to the test input ADC 3 . The value of the voltage U 3 in relation to the ground potential GND is known very accurately because, in a first approximation, it is only dependent on the value of the test voltage U 2 and the fixed resistors R 1 and R 2 . As a matter of principle, the exact value of the voltage U 3 is, of course, also influenced by further factors, for example by the electric resistance of the glow plugs RG 1 , RG 2 . Since the electric resistance of a glow plug including supply lines at room temperature is typically only approximately 0.5 ohms, these influences can usually be neglected and, if necessary, be taken into account by means of a more accurate calculation. Typically, the fixed resistors R 1 and R 2 have a value of several hundreds of ohms, while the value of the test voltage U 2 is preferred to be 5 volts. Preferably, the test voltage source also supplies the operating voltage required by the controlling device. Therefore, the voltage supply required for the microprocessor of the controlling device 1 anyway can be used as test voltage source.

Preferably, the values of the first resistor R 1 and the second resistor R 2 are each at least 50-fold, preferably at least 200-fold, in particular at least 1000-fold, larger than the resistance of the glow plug RG 1 , RG 2 . This is advantageous in that, in good approximation, the electric resistance of the glow plug RG 1 , RG 2 then does not have any influence on the potential U 3 applied to the test input ADC 3 .

If the controlling device 1 detects that, the value of the potential U 3 present at the test input ADC 3 in relation to the reference potential GND′ deviates from a reference value which, in a first approximation, is only dependent on the fixed resistors R 1 , R 2 and the value of the test voltage U 2 , as illustrated above, then the reference potential GND′ also deviates from the ground potential GND. The controlling device 1 uses the value of any detected deviation to correct the measured value of the supply voltage U 1 that was measured in relation to the reference potential GND′ applied to the ground input. It is, thus, possible to supply the desired electric power to the glow plug RG 1 , RG 2 for power control purposes with higher precision.

Preferably, the test voltage U 2 is lower than the supply voltage U 1 which is, for example, 11 volts. For this reason, a diode D 1 or D 2 is arranged between the second fixed resistor R 2 and the positive terminal of the glow plug RG 1 , RG 2 , respectively, in the exemplary embodiment shown. The diode D 1 , D 2 prevents the supply voltage U 1 from having an influence on the value of the potential U 3 applied to the test input ADC 3 of the controlling device 1 .

RG 1 Glow plug

RG 2 Glow plug

G 1 Supply line

G 2 Supply line

U 1 Supply voltage

S 1 Switch

1 Control unit

ADC 1 Measuring input

GND′ Reference potential

ADC 2 Ground input

GND Ground potential

R 3 Disturbing resistance

R 1 Fixed resistor

U 2 Test voltage

ADC 3 Test input

R 2 Fixed resistor

U 3 Voltage

D 1 Diode

D 2 Diode