JP2000120518A - Ignition device for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To simplify the constitution of a condenser discharge type ignition device having an excessive rotation prevention function. SOLUTION: The thyristor S1 for exciter short circuit for short circuiting the output voltage of the exciter coil 4 when the charge voltage of the condenser 2 for ignition exceeds a limit value is provided between the condenser 2 for ignition and exciter coil 4 provided on the primary side of an ignition coil 1. The rotation speed of an engine is detected and a rotation speed detection circuit 13 for the generation of a trigger signal Vt is provided when the detected rotation speed exceeds a set value and the trigger signal Vt generated by the rotation speed detection circuit 13 is given to the thyrister S1 for the exciter short circuit. By making the thyrister S1 conductive when the rotation speed of the engine exceeds the set value, the ignition is stopped and it is prevented that the rotation speed of the engine exceeds the set value.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ignition device for an internal combustion engine provided with a circuit for preventing an overspeed of the internal combustion engine.

[0002]

2. Description of the Related Art When the rotation speed of an internal combustion engine exceeds a predetermined safe speed, the engine may be damaged or the load may be damaged. Therefore, when there is such a possibility, the engine speed is prevented by detecting the rotation speed of the internal combustion engine and stopping the ignition of the internal combustion engine when the detected rotation speed exceeds the set value. An ignition device for an internal combustion engine is used.

FIG. 3 shows a circuit configuration of a conventional ignition device for an internal combustion engine of this type. In the figure, 10 '
Is charged to one polarity by the output voltage of one half cycle of the ignition coil 1 and one half cycle of the exciter coil 4 provided on the primary side of the ignition coil 1 and provided on the magnet generator attached to the internal combustion engine. An ignition capacitor 2; an ignition thyristor 3 which conducts when an ignition signal Vg is supplied to discharge the electric charge charged in the ignition capacitor 2 through the primary coil 1a of the ignition coil 1;
And an exciter short-circuit switch circuit 9 'having an exciter short-circuiting thyristor S1 for conducting when the voltage between both ends exceeds a set value and short-circuiting the output of one half cycle of the exciter coil 4.

[0004] Reference numeral 11 denotes a pulser coil provided in a signal generator attached to the internal combustion engine. This pulser coil generates a pulse signal Vs1 in the direction of the solid line shown in the figure at the ignition position of the internal combustion engine, and is delayed from the ignition position. A pulse signal Vs2 is generated at the position. The pulse signal Vs1 generated at the ignition position of the engine passes through an ignition signal supply circuit 12 'consisting of two diodes D7 and D8 connected in series with a bias circuit consisting of a parallel circuit of a capacitor C2 and a resistor R5. As an ignition signal Vg.

In the illustrated ignition circuit 10 ′, the ignition capacitor 2 is charged to the illustrated polarity by the output voltage of one half cycle of the exciter coil 4, and the thyristor 3 is charged.
When the ignition signal Vg is given to the thyristor 3, the thyristor 3 conducts and charges the capacitor 2 to the primary coil 1 of the ignition coil 1.
Discharge through a. As a result, a high voltage for ignition is induced in the secondary coil 1b of the ignition coil 1, so that a spark is generated in the ignition plug 8 and the engine is ignited.

When the rotation speed of the internal combustion engine exceeds a set value, the ignition operation of the ignition circuit 10 'is stopped to prevent the engine from over-rotating, so that the anode of the diode D7 of the ignition signal supply circuit 12' is connected to the ground. Is connected to a collector-emitter circuit of an NPN-type ignition signal bypass transistor 18,
When the engine speed exceeds the set value, the transistor 18
A rotation speed detection circuit 13 'is provided to apply a base current to the transistor to make the transistor conductive.

The illustrated rotational speed detecting circuit 13 'includes a speed detecting capacitor 15 which is charged through a resistor R7 by an output voltage of a power supply circuit 14 and generates a speed detecting voltage Vc1 at both ends, a transistor TR1, and resistors R8 to R10. The reset circuit 16 includes a capacitor C4 and a diode D5, and a base current supply circuit 17 'including comparators CM1 and CM2, resistors R11 to R15, and a capacitor C5. When the set value is exceeded, a base current is supplied to the ignition signal bypass transistor 18 to make the transistor 18 conductive.

The configuration of each part of the ignition circuit 10 ', the power supply circuit 14, and the rotational speed detection circuit 13' is, except for a part, an ignition circuit 10 used in the ignition device of FIG. Since the configuration of each part of the power supply circuit 14 and the rotation speed detection circuit 13 is substantially the same, a detailed description of the configuration of each part is omitted here.

In the ignition device shown in FIG. 3, a speed detecting capacitor 15 is charged through a resistor R7 by a constant DC voltage output from a power supply circuit 14, and the transistor is turned on every time the pulser coil 11 generates a pulse signal Vs1. The charge of the capacitor 15 is discharged through TR1. The interval at which the pulse signal Vs1 is generated becomes shorter as the engine speed increases, and the interval at which the capacitor 15 is discharged becomes shorter as the engine speed increases. The detected speed detection voltage Vc1 decreases as the engine speed increases. If the rotation speed of the engine exceeds the set value, the speed detection voltage Vc1 cannot exceed the reference voltage Vr.
The output voltage of the comparator CM2 is at a high level, and a base current is supplied to the ignition signal bypass transistor 18. As a result, the application of the ignition signal Vg to the thyristor 3 is prevented, so that the conduction of the thyristor 3 is prevented, and the ignition operation is stopped. When the ignition stops,
The rotation speed of the engine decreases, and the speed detection voltage Vc1 across the capacitor 15 can exceed the reference voltage Vr. As a result, the output voltage of the comparator CM2 becomes low, and the supply of the base current to the ignition signal bypass transistor 18 is stopped. Thereby, transistor 1
8, and the ignition operation is restarted.

[0010]

In the above ignition device for an internal combustion engine, the ignition signal Vg supplied through the ignition signal supply circuit 12 'when the rotational speed exceeds the set value is ignited through the ignition signal bypass transistor 18. Thyristor 3
The thyristor 3 for ignition is prevented from conducting by bypassing the ignition thyristor 3 to stop the ignition operation.

Therefore, in addition to the rotation speed detecting circuit 13 ', not only is the ignition signal bypass transistor 18 necessary, but also when the ignition signal bypass transistor 18 becomes conductive, the collector-emitter of the transistor is turned off. In order to prevent the forward voltage drop between them from being applied to the gate of the thyristor 3 for ignition, it is necessary to provide two diodes D7 and D8 connected in series to the ignition signal supply circuit, which increases the number of parts. There was a problem.

An object of the present invention is to omit an ignition signal bypass transistor from a circuit for preventing an overspeed of an internal combustion engine and to connect a plurality of diodes connected in series to a circuit for supplying an ignition signal to an ignition thyristor. An object of the present invention is to provide a capacitor discharge type ignition device for an internal combustion engine, which eliminates the necessity of providing the ignition device and simplifies the circuit.

[0013]

SUMMARY OF THE INVENTION The present invention relates to an ignition coil and an output of one half cycle of an exciter coil provided on a primary side of the ignition coil and provided in a magnet generator mounted on an internal combustion engine. And an ignition thyristor provided so as to conduct when the ignition signal is given and to discharge the charge charged in the ignition capacitor through the primary coil of the ignition coil. An exciter short-circuit switch circuit including an exciter short-circuit thyristor that conducts when the voltage across the ignition capacitor exceeds a set value and short-circuits the output of one half cycle of the exciter coil; The output of the pulsar coil that generates a pulse signal at the rotation angle position is input, and the ignition signal is sent to the ignition thyristor at the ignition position of the internal combustion engine. Target ignition apparatus that includes a obtaining ignition signal supply circuit.

According to the present invention, there is provided a rotation speed detecting circuit for detecting a rotation speed of the internal combustion engine and supplying a trigger signal to the thyristor for short-circuiting the exciter when the detected rotation speed exceeds a set value.

The rotation speed detection circuit comprises a speed detection capacitor which is charged with a constant time constant by an output of a power supply circuit for generating a constant DC voltage with an output of the exciter coil as an input, and a pulser coil for generating a pulse signal. A reset circuit for discharging the speed detection capacitor every time, comparing the voltage across the speed detection capacitor with a reference voltage, and when the voltage across the speed detection capacitor does not exceed the reference voltage, A trigger signal generating circuit for generating a trigger signal for turning on the thyristor for exciter short circuit.

With the above arrangement, when the rotation speed of the internal combustion engine exceeds a set value, the rotation speed detection circuit generates a trigger signal to turn on the exciter short-circuiting thyristor. Is short-circuited and the ignition capacitor is no longer charged. As a result, the ignition of the engine stops, and the rotation speed decreases.
When the rotation speed of the engine becomes lower than the set value, no trigger signal is generated, and the ignition of the engine is restarted.

As described above, in the present invention, the ignition is stopped by directly applying the trigger signal generated from the rotation speed detection circuit to the exciter short-circuit thyristor when the rotation speed of the internal combustion engine exceeds the set value. The ignition signal bypass transistor required for bypassing the ignition signal from the ignition thyristor can be omitted. Further, since the ignition signal bypass transistor can be omitted, a diode for preventing a voltage generated between the collector and the emitter when the transistor is turned on from being applied to the gate of the ignition thyristor can be omitted.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIGS.

In FIG. 1, reference numeral 1 denotes an ignition coil having a primary coil 1a and a secondary coil 1b, one end of which is grounded;
Is an ignition capacitor having one end connected to the non-ground side of the primary coil 1a of the ignition coil, and 3 is an ignition thyristor having a cathode grounded and an anode connected to the other end of the ignition capacitor 2. A capacitor C1 and a resistor R1 are connected in parallel between the gate and cathode. Reference numeral 4 denotes an exciter coil provided in a magnet generator attached to the internal combustion engine. One end of the exciter coil 4 is connected to the cathode of a diode 5 whose anode is grounded, and the other end is a diode whose anode faces the exciter coil. 6 is connected to a connection point between the ignition capacitor 2 and the anode of the ignition thyristor 3. A diode 7 whose cathode is directed to the ground side is connected in parallel to the primary coil 1a of the ignition coil, and a secondary coil 1b of the ignition coil is
A spark plug 8 attached to a cylinder of an engine (not shown) is connected.

9 is to set the charging voltage of the ignition capacitor 2 by short-circuiting the output of one half cycle of the exciter coil 4 in the direction of the solid line arrow when the voltage across the ignition capacitor 2 exceeds the set value. An exciter short-circuit switch circuit for limiting the value to a value, the exciter short-circuit switch circuit comprising an exciter short-circuit thyristor S1 having an anode connected between both ends of a series circuit of the exciter coil 4 and the diode 5 with the anode facing the other end of the exciter coil. A resistor R2 connected between the gate and the cathode of the thyristor, a voltage dividing circuit for dividing the voltage between the anode and the cathode of the thyristor S1 by the resistors R3 and R4, and a voltage dividing point of the voltage dividing circuit and the gate of the thyristor S1. Zener diode Z connected between
1 and a series circuit of a diode D1. When the charging voltage of the ignition capacitor exceeds a set value, the Zener diode Z1 conducts and a trigger signal is supplied to the ignition thyristor S1.

The diode D1 is provided to prevent a trigger signal Vt supplied from the rotation speed detecting circuit 13 described later from flowing to the Zener diode Z1.

In this example, the capacitor discharge type ignition circuit 10 is constituted by the above-mentioned components, and the diodes 6, 7 and 5 are driven by the voltage of one half cycle in the direction indicated by the solid arrow shown in FIG. The ignition capacitor 2 is charged to the polarity shown in FIG. When the ignition signal Vg is applied to the ignition thyristor 3 at the ignition position of the internal combustion engine, the thyristor 3 conducts and the ignition capacitor 2
Discharges through the primary coil 1a of the ignition coil,
At that time, sparks fly to the ignition plug 8 due to the high voltage for ignition induced in the secondary coil 1b of the ignition coil, and the engine is ignited.

In order to provide an ignition signal Vg to the ignition thyristor 3, a pulser coil 11 for generating a pulse signal at a predetermined rotational angle position of the engine and a positive pulse signal Vs1 output from the pulser coil in the direction of a solid arrow shown in FIG. There is provided an ignition signal supply circuit 12 which supplies the ignition signal Vg to the gate of the ignition thyristor 3. The illustrated ignition signal supply circuit 12 comprises a bias circuit comprising a parallel circuit of a capacitor C2 and a resistor R5.
The pulse signal of positive polarity generated by 1 is given to the gate of the ignition thyristor 3 through the bias circuit. A diode D2 whose anode is directed to the ground side is connected to both ends of the pulsar coil 11, so that a negative pulse signal Vs2 generated by the pulsar coil 11 in the direction of the dashed arrow shown in the figure is short-circuited through the diode D2. .

The ignition signal supply circuit is based on the rotation angle information and the rotation speed information of the engine obtained from the pulse signals Vs1 and Vs2 output from the pulser coil 11.
It is also possible to use a circuit which calculates the ignition position at each rotational speed of the internal combustion engine by a calculation means such as a microcomputer and outputs an ignition signal to the ignition thyristor 3 at the calculated ignition position. When such an ignition signal supply circuit is used, the positive and negative pulse signals Vs1 and Vs1 generated by the pulser coil 11 are connected without connecting the diode D2.
s2 is input to the ignition signal supply circuit.

Reference numeral 14 denotes a power supply circuit. The power supply circuit includes a power supply capacitor C3 which is charged to the illustrated polarity through diodes D3 and D4 at an output voltage of the exciter coil 4 in a half cycle in the direction of the dashed arrow shown in FIG. The power supply thyristor S2 is turned on when the voltage at both ends exceeds the Zener voltage of the Zener diode Z2 and limits the voltage at both ends of the capacitor C3 to a constant value. A DC voltage B having a constant polarity is output.

Reference numeral 13 denotes a rotation speed detection circuit which detects a rotation speed of the internal combustion engine and provides a trigger signal to the thyristor S1 for short-circuiting the exciter when the detected rotation speed exceeds a set value. A speed detecting capacitor 15 charged through the resistor R7 by the output voltage of the circuit 14, a reset circuit 16 for instantaneously discharging the capacitor 15 each time the pulser coil 11 generates the positive pulse signal Vs1, The detected speed detection voltage Vc1 is compared with the reference voltage Vr, and when the rotation speed of the engine exceeds the set value and the speed detection voltage cannot exceed the reference voltage, the thyristor S1 for short-circuiting the exciter.
And a trigger signal generating circuit 17 for applying a trigger signal to the trigger signal.

More specifically, the reset circuit 16 includes a transistor TR1 having a collector-emitter circuit connected in parallel between both ends of a speed detecting capacitor 15, a bias circuit including a capacitor C4 and a resistor R8, a diode D5 and a resistor R9. And a base current supply circuit comprising a pulse signal Vs1 and a pulse signal Vs1.
Is generated, the transistor TR1 is turned on to discharge the speed detecting capacitor 15 instantaneously.

The trigger signal generation circuit 17 includes a power supply circuit 14
A voltage dividing circuit that divides the output voltage by the resistors R11 and R12 to generate the reference voltage Vr, the speed detection voltage Vc1 is input to the inverting input terminal, and the reference voltage Vr is input to the non-inverting input terminal. No. 1 comparator CM1, an integrating capacitor C5 connected between the output terminal of the first comparator and ground and charged with a constant time constant through the charging resistor R13 by the output voltage of the power supply circuit 14, and the integrating capacitor C5. A second comparator CM2 in which a voltage Vc2 across C5 is input to a non-inverting input terminal and a reference voltage Vr is input to an inverting input terminal;
It comprises a resistor R14 connected between the output terminal of M2 and the output terminal of the power supply circuit 14, and a diode D6 having an anode connected to the output terminal of the comparator CM2. The cathode of the diode D6 is the thyristor S1 for short-circuiting the exciter.
And the exciter short-circuit thyristor S1 from the power supply circuit 14 through the resistor R14 and the diode D6 when the potential of the output terminal of the comparator CM2 becomes high.
Is supplied with a trigger signal.

FIG. 2 shows the voltage waveform of each part of the rotation speed detecting circuit 13 with respect to time t. The left half of FIG. 2 shows a state where the rotation speed of the internal combustion engine is equal to or less than a set value, and the right half. The section indicates a state where the rotation speed of the engine has exceeded the set value.

Each time a pulse signal Vs1 (see FIG. 2A) is generated in the pulsar coil 11 in the direction of the solid line shown in FIG. 2, the reset circuit 16 discharges the speed detecting capacitor 15 and the speed detecting voltage Vc1 (FIG. 2B) at both ends of the capacitor 15. ) To zero and the next pulse signal Vs1 after the pulse signal Vs1 disappears.
The capacitor 15 is charged during the period until the occurrence of, and the speed detection voltage Vc1 rises. Therefore, the speed detection voltage Vc1
Is a sawtooth waveform as shown in FIG.
As the rotational speed of the internal combustion engine increases, the condenser 1
Since the time interval at which the discharge of No. 5 is performed becomes shorter, the peak value of the speed detection voltage Vc1 at both ends of the capacitor (the value immediately before being reset) becomes lower as the rotation speed of the engine increases.

The speed detection voltage Vc1 obtained at both ends of the speed detection capacitor 15 is compared with the reference voltage Vr (see FIG. 2B) by the first comparator CM1, and when the speed detection voltage Vc1 exceeds the reference voltage Vr. The potential of the output terminal of the first comparator CM1 becomes low (ground level), and the integrating capacitor C5 is discharged instantaneously. When the speed detecting capacitor 15 is discharged and the potential of the output terminal of the first comparator CM1 becomes high level, charging of the integrating capacitor C5 is resumed, and the capacitor C5 is charged with a constant time constant (FIG. 2C). reference).

The voltage Vc2 across the integrating capacitor C5 and the reference voltage Vr are compared by a second comparator CM2 (FIG. 2C).
When the voltage Vc2 exceeds the reference voltage Vr, the potential of the output terminal of the second comparator CM2 becomes high level, and the exciter short-circuits from the output terminal of the rotation speed detecting circuit 13 (the cathode terminal of the diode D6). The trigger signal Vt (FIG. 2D) is given to the thyristor S1.

In the illustrated example, the reference voltage input to the first comparator CM1 and the reference voltage input to the second comparator CM2 are set to the same value, and the charging time constant of the integration capacitor C5 is set to the speed. The value is selected to be larger than the charging time constant of the detection capacitor 15. Therefore, the speed detection voltage V
When c1 exceeds the reference voltage Vr, the voltage Vc2 across the integrating capacitor C5 is lower than the reference voltage Vr. Further, when the rotation speed of the internal combustion engine is equal to the set value, the charging time constant of the capacitor 15 and the reference voltage Vr are set so that the speed detection voltage Vc1 immediately before the reset of the speed detection capacitor 15 is performed becomes equal to the reference voltage Vr. Is selected. Therefore, when the rotation speed of the internal combustion engine exceeds the set value, the speed detection voltage Vc1 obtained at both ends of the speed detection capacitor 15 cannot exceed the reference voltage Vr (the potential of the output terminal of the comparator CM1 is low). ), And the charging of the integrating capacitor C5 is continued. The voltage Vc2 across the integration capacitor C5
Rises above the reference voltage Vr, the potential of the output terminal of the comparator CM2 becomes high level, so that the trigger signal Vt is given to the thyristor S1 for short-circuiting the exciter.

When the trigger signal Vt is applied to the thyristor S1 for short-circuiting the exciter and the thyristor S1 becomes conductive,
The output voltage of the exciter coil 4 in the direction of the solid arrow shown in the drawing is short-circuited, and charging of the ignition capacitor 2 is prevented, so that the ignition operation is stopped. When the rotation speed of the internal combustion engine falls below the set value, the speed detection voltage Vc1 again exceeds the reference voltage Vr, so that the trigger signal Vt is no longer generated and the ignition operation of the engine is restarted. By repeating the above operation, the rotation speed of the internal combustion engine is limited to substantially the set value or less.

As in the above example, the pulse signal Vs1 generated by the pulsar coil 11 is used as the ignition signal Vg and the thyristor 3
When the configuration is such that the ignition signal Vg is bypassed from the thyristor 3 by the ignition signal bypass transistor 18 as in the conventional example shown in FIG. When the collector current starts flowing through the ignition signal bypass transistor 18 beyond the threshold value, or when the collector current of the ignition signal bypass transistor 18 decreases due to a decrease in rotation speed, the collector current flows through the pulsar coil 11. As a result, the rising of the waveform of the pulse signal Vs1 becomes slow, and thus there is a problem that the ignition position is delayed when the ignition operation is stopped or when the ignition operation is restarted. According to the result of actual measurement, the delay of the ignition position may reach as much as 10 ° in the rotation angle of the crankshaft. With the configuration shown in FIG. 1, a large current does not flow through the pulser coil when the ignition operation is stopped or when the ignition operation is restarted, so that the ignition position can be prevented from being delayed as described above.

[0036]

As described above, in the ignition device for an internal combustion engine according to the present invention, the engine speed is detected and the engine is misfired when the detected rotation speed exceeds a set value.
It is possible to prevent the engine and the load driven by the engine from being damaged due to the engine over-rotating.

According to the present invention, when the rotation speed of the internal combustion engine exceeds a set value, a trigger signal is supplied from the rotation speed detection circuit to the exciter short-circuit thyristor to make the thyristor conductive, thereby charging the ignition capacitor. Since the engine is stopped, it is possible to prevent overspeed of the engine without separately providing a bypass switch such as an ignition signal bypass transistor required in the conventional ignition device.
Therefore, it is possible to provide the capacitor discharge type ignition device with an overspeed prevention function without increasing the number of parts and without causing a phenomenon that the ignition position is delayed when the ignition is stopped and restarted. There is.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a configuration example of an ignition device for an internal combustion engine according to the present invention.

FIG. 2 is a waveform diagram showing voltage waveforms at various parts of the ignition device of FIG.

FIG. 3 is a circuit diagram showing a conventional example.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Ignition coil, 1a ... Primary coil, 1b ... Secondary coil, 2 ... Ignition capacitor, 3 ... Ignition thyristor, 4
... Exciter coil, 5-7 ... Diode, 8 ... Spark plug, 9,9 '... Exciter short circuit switch circuit, 10,
10 ': ignition circuit, 11: pulser coil, 12, 12'
... Ignition signal supply circuit, 13, 13 '... Rotation speed detection circuit, 14 ... Power supply circuit, 15 ... Speed detection capacitor, 1
6 ... reset circuit, 17 ... trigger signal generation circuit, 18 ...
Ignition signal bypass transistor, S1 ... Exciter short circuit thyristor, S2 ... Power supply thyristor, CM1 ... First comparator, CM2 ... Second comparator, TR1 ... Transistor, C1-C4 ... Capacitor, C5 ... Integrating capacitor ,
Z1, Z2: Zener diode, D1 to D8: Diode, R1 to R15: Resistance.

Claims (2)

[Claims] 1. An ignition coil, which is provided on a primary side of the ignition coil and is charged to one polarity by an output of one half cycle of an exciter coil provided in a magnet generator attached to an internal combustion engine. An ignition capacitor, an ignition thyristor provided so as to conduct when an ignition signal is given and discharging the electric charge charged in the ignition capacitor through a primary coil of the ignition coil, and both ends of the ignition capacitor An exciter short-circuit switch circuit including an exciter short-circuit thyristor that conducts when the voltage exceeds a set value and short-circuits the output of one half cycle of the exciter coil, and a pulse signal at a predetermined rotational angle position of the internal combustion engine Inputting an output of a pulsar coil for generating an ignition signal to the ignition thyristor at an ignition position of the internal combustion engine. An ignition device for an internal combustion engine, comprising: a signal supply circuit; and a rotation speed detection circuit that detects a rotation speed of the internal combustion engine and provides a trigger signal to the exciter short-circuit thyristor when the detected rotation speed exceeds a set value. An ignition device for an internal combustion engine, comprising: 2. The method according to claim 1, wherein the rotation speed detection circuit includes a speed detection capacitor that is charged with a constant time constant by an output of a power supply circuit that generates a constant DC voltage with an output of the exciter coil as an input, A reset circuit for discharging the speed detection capacitor every time a signal is generated, and comparing the voltage across the speed detection capacitor with a reference voltage, the voltage across the speed detection capacitor may exceed the reference voltage. 2. The ignition device for an internal combustion engine according to claim 1, further comprising: a trigger signal generation circuit that generates the trigger signal when the state becomes impossible.