EP0279042B2 - Arrangement for protecting relay contacts - Google Patents

Description

Glow plug device for self-igniting internal combustion engines, in particular in motor vehicles

The invention relates to a preheating device for self-igniting internal combustion engines, in particular in motor vehicles, with a power source, with a glow plug, with a first switching device for connecting the glow plug to the power source via a resistor, with a second switching device, which is designed as a relay, for direct connection the glow plug with the current source and with a time switching device by means of which the first switching device can be switched on before the second switching device.

Such preheating devices are used for heating glow plugs via a heating current, in particular from the starter battery of motor vehicles. The glow plugs promote the ignition of the diesel fuel injected into the cold self-igniting internal combustion engine. The heating current can reach orders of magnitude up to 300 amperes and larger.

Such a preheating device is known from DE-OS 29 07 772, in which to protect relay contacts and glow plugs, the glow plugs as electrical consumers during a first heating period via a first switching device which is designed as a relay and connected to the power source via a series resistor and thereby be supplied with a correspondingly low heating current. Only after a predetermined period of time of a few seconds to a few minutes is the glow plug directly connected to the power source via a second switching device.

However, the known device has disadvantages. By supplying the glow plugs with a low heating current during a first heating period, the glow plugs heat up more slowly than with preheating devices which take the measures of not have known device. This means that the waiting time of the machine operator until the self-igniting internal combustion engine can be started increases noticeably compared to preheating devices that have no means to protect relay contacts. This leads to a loss of comfort when operating the self-igniting internal combustion engine.

Furthermore, the series resistor is additionally required in the known device. Such a series resistor with a correspondingly high load capacity is expensive to procure and takes up space, eg. B. in the engine compartment of motor vehicles. This means that the previously known device can only be produced in a complex and expensive manner.

From US-PS 35 58 910 a device for protecting relay contacts in AC switchgear is known in which the circuit is closed via a TRIAC before closing the relay contacts. However, the TRIAC with the circuitry described there can only be used in AC switchgear. In addition, this device, like the preheating device of DE-OS 29 07 772, has no measures at all to monitor the circuit for short circuits and line interruptions.

The invention has for its object to provide a preheating device which is simpler and less expensive to produce than the prior art, which, while protecting relay contacts, enables the electrical consumer to be supplied with the full supply voltage and which provides for monitoring the circuit for short circuit and power interruption.

This object is achieved in that the first switching device is a semiconductor component, that the resistance is the internal resistance of the switching path of the semiconductor component, that a monitoring device is provided which measures the voltage drop across the internal resistance of the switching path of the semiconductor component and with compares predetermined threshold values when the first switching device is switched on and the second switching device is switched off, that the monitoring device switches off the switching devices when the value falls below a first threshold value and / or delivers a first error signal for a first display device and / or that the monitoring device when monitoring a second threshold value switches off the switching devices and / or supplies a second spring signal to a second display device.

By designing the first switching device as a semiconductor component, unlike in the prior art, it is not possible for relay contacts of a first switching device to be overloaded because there are no relay contacts. The internal resistance of the switching path of the semiconductor component is small compared to the series resistance of the previously known device, so that the time until the glow plug heats up can be significantly reduced compared to the prior art. In contrast to the prior art, no additional expensive and space-consuming series resistor is required. When this measure is used, the protection of the relay contacts of the second switching device is nevertheless ensured, because no arc is formed when the relay contacts are closed.

The device according to the invention also has the advantage that the first period, during which the first switching device is switched on and the second switching device is still switched off, can be reduced to times of the order of 1 ms, inter alia, due to the shorter switching times of the semiconductor component. The result of this is that z. B. the time to complete heating of the glow plug can be shortened compared to the prior art and essentially corresponds to the time period with constant direct feeding of the glow plug from the power source.

The fact that a monitoring device is provided which the voltage drop across the internal resistance of the switching path of the Semiconductor component measures and compares with predetermined threshold values when the first switching device is switched on and the second switching device is switched off, the monitoring device switching off the switching devices when the value falls below a first threshold value and / or supplying a first error signal to a first display device and / or the monitoring device when Exceeding a second threshold value, the switching device switches off and / or supplies a second error signal to a second display device, the heating circuit can be checked for short circuits and circuit interruptions. Due to the short switching times of the semiconductor component, a short first period of approximately 1 ms is also sufficient for this circuit monitoring.

Further advantageous refinements and developments of the subject matter of the invention emerge from the subclaims.

It is advantageous to use a field-effect transistor, in particular a MOS-FET, as the semiconductor component, because the internal resistance of the switching path is sufficiently small to withstand the heat loss which forms on the field-effect transistor without further aids, such as, for. B. heat sink to dissipate to the environment. A semiconductor device can use a bipolar transistor.

It can also be a MOS-FET with an integrated monitoring device, such as that used for Is currently freely available, so that the additional effort for the construction of the monitoring device is low.

In particular in order to reduce the circuit complexity, the time switch device and / or the monitoring device can be designed as part of a microcomputer.

In order to use the advantage of protecting the relay contacts of the second switching device at the end of the heating of the glow plugs, it is advantageous to design the time switching device in such a way that the second switching device can be switched off before the first switching device.

Finally, it is particularly advantageous to design the time switching device in such a way that it switches the switching devices on and off periodically, in particular at a constant frequency and depending on the supply voltage and the glow plug temperature of variable switch-on time, because this measure compensates for supply voltage fluctuations and controls the glow plug temperature Changing the switch-on times of the first and second switching device is possible. In particular when using this measure, the advantages of the preheating device according to the invention become particularly clear because the frequent switching on and off of the first and second switching devices with frequencies of z. B. 2 Hz, the load on the relay contacts is particularly high. Without the use of the preheating device according to the invention, the relay contacts can have burned down after only about 8 months of operation of a motor vehicle with a self-igniting internal combustion engine and over 2 million actuations of the relay contacts, so that attempts to start the internal combustion engine are no longer possible, as tests have shown.

An embodiment of the subject of the invention is shown in the drawings and is explained in more detail below.

• Figur 1 schematisch eine erfindungsgemäße Vorrichtung, die als Vorglüheinrichtung für selbstzündende Brennkraftmaschinen ausgebildet ist und
• Figur 2 ein Diagramm, in dem der Spannungsabfall am Innenwiderstand des Halbleiter-Bauelements abhängig von den Einschaltzeiten der ersten und zweiten Schalteinrichtung dargestellt ist.

Show it

• Figure 1 schematically shows a device according to the invention, which is designed as a preheating device for self-igniting internal combustion engines and
• Figure 2 is a diagram in which the voltage drop across the internal resistance of the semiconductor device depending on the Switch-on times of the first and second switching device is shown.

In Figure 1, the glow plug circuit is a current source (B), which can be designed as a motor vehicle battery, a glow plug (VS), which can be designed as the first catch of a glow plug of a motor vehicle, an electrical consumer (V), which is designed as a glow plug and a first switching device, which is designed as a semiconductor component (HL), in particular as a MOS-FET, and a second switching device, which is designed as a relay contact set (RK) of an electromagnetic relay. The semiconductor component (HL) and the relay contact set (RK) are connected in parallel in the glow plug circuit.

The semiconductor component (HL) and the electromagnetic relay (RK, RS) can be switched on and off by a time switch, which is designed as part of a microcomputer (MC). The microcomputer (MC) is connected in a known manner to the current source (B) for the power supply. To amplify the control current of the microcomputer (MC) and to switch the electromagnetic relay, an NPN driver transistor (TR1) is provided, the base of which can be controlled by the output signal of the microcomputer (MC) and whose emitter is connected to the negative pole of the current source (B) and its Collector is connected to the positive pole of the power source (B) via the electromagnetic relay coil (RS).

The semiconductor switch (HL) can also be controlled by the microcomputer (MC) via a voltage doubler circuit (SPV). The voltage doubler circuit (SPV) is required because an N-channel MOS-FET is used here as the semiconductor component and the potential at the gate input (G) must always be greater than the potential at the SOURCE- to control the MOS-FET. Port (S).

The first Zener diode (Z1) and the second Zener diode (Z2) also serve to protect the MOS-FET (HL) and the NPN transistor (TR1).

The first Zener diode (Z1) protects the NPN transistor (TR1) against overvoltage and is reverse polarity protection. The second Zener diode (Z2) serves to limit the voltage at the GATE input compared to the voltage at the SOURCE connection of the MOS-FET (HL). This voltage difference must not exceed a value specified by the design of the MOS-FET.

The SOURCE connection of the MOS-FET (HL) is thus via the glow plug (V) with the negative coil of the current source (B) and via the DRAIN connection (D) and the preheat switch (VS) with the positive pole of the current source (B) conductively connected. The potential or the voltage drop (UDS) on the switching path of the MOS-FET (HL) is tapped between the SOURCE connection (S) and the DRAIN connection (D) and fed to an analog-to-digital converter (ADC), which, however, is also used as Comparator can be formed. The output signal of the analog-to-digital converter (ADC) is fed to the microcomputer (MC) to determine contact actuation, line interruptions and short circuits. The analog-to-digital converter (ADC) is part of a monitoring device, the other part of which is formed in the microcomputer (MC).

The monitoring device compares the voltage drop (UDS) across the internal resistance of the switching path of the semiconductor component (HL) with predetermined threshold values when the first switching device, i.e. the MOS-FET (HL), is switched on and the second switching device, i.e. the electromagnetic relay, is switched off . If the voltage drop (UDS) falls below a first threshold value, the monitoring device switches off the first switching device (HL) and prevents the second switching device (RK) from being switched on and outputs a first error signal to a first display device (FL1), which is shown in FIG simple indicator lamp is shown. If the voltage drop (UDS) exceeds a second threshold value, the monitoring device switches the first switching device (HL) off and prevents the second switching device (RK) from being switched on, and a second error signal is delivered to a second display device (FL2), which is also in 1 as a simple indicator lamp is shown. However, the error signals can also be used to control other devices on the self-igniting internal combustion engine or the motor vehicle.

The function of the preheating device according to the invention according to FIG. 1 will now be explained in more detail with reference to FIG. 2:

At the beginning, the preheat switch (VS) is in the open position shown in FIG. 1. Then the entire preheating device according to the invention is de-energized and no potential differences can be measured. If the preheating switch (VS) is now closed, the first switching device (HL) and the second switching device (RK) remain open from this point in time, which is marked with the time zero in FIG. 2, until the point in time (T1) Position so that no current can flow through the glow plug (V). A potential is then measured at the switching path of the semiconductor switch as a voltage drop (UDS) that corresponds to the battery voltage (UB) of the current source (B).

If the semiconductor switch (HL) is now switched on at time (T1), the current flow from the positive pole of the battery via the glow plug (VS), the switching path of the semiconductor switch (HL) and the glow plug (V) to the negative pole of the current source (B) is made possible . If the glow plug (V) is in an electrically perfect condition, a small amount (U1) of the total voltage applied drops across the switching path of the semiconductor switch (HL). This voltage drop (U1) can be measured over the entire period (T), which extends from T1 to T2 and in which the semiconductor switch (HL) is switched on and the second switching device (RK) is switched off. This voltage drop (U1) is measured by the analog-to-digital converter (ADC) and passed on in a converted form to the microcomputer (MC).

If the second switching device (RK) is additionally switched on at the time (T2) after the time period (T) has elapsed, then because of the low contact resistance between the Relay contacts (RK) only a negligible voltage drop (UDS) on the switching path of the semiconductor switch (HL) can be measured.

At time (T3), the second switching device (RK) is opened so that the current flows again over the switching path of the semiconductor component (HL). Correspondingly, a voltage amount (U1) drops across the switching path of the semiconductor component (HL). After the time period (T) has elapsed, that is to say at time T4 in FIG. 2, the semiconductor component (HL) is also switched off. The current flow is interrupted. The entire battery voltage (UB) is now again applied to the switching path of the semiconductor switch (HL).

If the glow plug (V) is not in perfect condition, this can be detected by the monitoring device during the period (T). If the glow plug (V) had a short circuit, the entire voltage (UB) at the switching path of the semiconductor component (HL) would drop during the period (T). This means that the measured voltage drop (UDS) would be above the first threshold value (US1) shown in FIG. 2. This increased voltage drop would be recognized by the monitoring device and a first error signal delivered to the first display device (FL1). At the same time, the semiconductor switch (HL) is then switched off in order to prevent the semiconductor components (HL) from being overloaded.

If there is a line interruption in the glow plug circuit or on the glow plug (V), only a negligibly small amount of voltage would drop across the switching path of the semiconductor component (HL) in the time period (T). This means that the measured voltage drop is then below the second voltage threshold (US2) shown in FIG. 2. This fact is also determined by the monitoring device and leads to the monitoring device supplying a second error signal to the second display device (FL2).

It turns out that the preheating device according to the invention not only protects the relay contacts (RK) easily and inexpensively, but also enables the glow plug circuit to be monitored for short circuits and line interruptions in a simple manner.

It is also possible, in particular due to the short switching time of the semiconductor component (HL), to shorten the time period (T) to a value of approximately 1 ms compared to the prior art. This ensures that the time it takes for the glow plugs (V) to heat up completely to be shortened compared to the prior art. This means a great gain in comfort for the operation of the self-igniting internal combustion engine.

Finally, the glow plug (V) can advantageously be switched on and off periodically by the preheating device according to the invention. This has the advantage that
Supply voltage fluctuations and the glow plug temperature can be changed by changing the switch-on times. It is particularly advantageous to carry out this periodic switch-on and switch-off with a constant switch-on time and a switch-off time that is variable depending on the supply voltage and the glow plug temperature, because the clock signal required for this can be provided in a simple manner by using the microcomputer (MC). This means that the switch-on and switch-off process shown in FIG. 2 would be repeated periodically.

Because of the large number of switching operations that are required for periodic heating of the glow plug (V), the advantages of the preheating device according to the invention become particularly noticeable because the preheating device according to the invention means that the relay contacts (RK) are hardly noticeable.

Claims (7)

1. Pre-heating device for compression-ignition combustion engimes, particularly in motor vehicles, having a power source, a glow plug, a first switching device for connecting the glow plug to the power source via a resistor, a second switching device, which is configured as a relay, for the direct connection of the glow plug to the power source and having a timing device, which allows the first switching device to be switched on before the second switching device, characterised in that the first switching device is a semiconductor module (HL), in that the resistance is the internal resistance through the switching element of the semiconductor module (HL) in that a monitoring device is provided, which measures the voltage drop (UDS) across the internal resistance of the switching element of the semiconductor module (HL) and compares this with specified thresholds, when the first switching device is switched on and the second switching device is switched off, in that the monitoring device switches off the switching devices (HL, RK) if a first threshold is undershot, and/or supplies a first fault signal to a first indicator device (FL1) and/or in that the monitoring device switches off the switching devices (HL, RK) if a second threshold is exceeded and/or supplies a second fault signal to a second indicator device (FL2).
2. Pre-heating device according to Claim 1, characterised in that the semiconductor module is a field-effect transistor, particularly a MOSFET.
3. Pre-heating device according to Claim 1, characterised in that the semiconductor module is a bipolar transistor.
4. Pre-heating device according to Claim 1, characterised in that a MOSFET with integrated monitoring device is used.
5. Pre-heating device according to Claim 1, characterised in that the timing device and/or the monitoring device are configured as part of a- microprocessor (MC).
6. Pre-heating device according to Claim 1, characterised in that the second switching device (RK) can be switched off by the timing device before the first switching device (HL).
7. Pre-heating device according to Claim 1, characterised in that the timing device switches the switching devices (HL, RK) on and off periodically, particularly with a constant switch-on time and a variable switch-off time depending on the supply voltage (UB) and the glow plug temperature.

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