MILLIOHM IMPEDANCE MONITORING APPARATUS AND METHOD

MILLIOHM IMPEDANCE MONITORING APPARATUS AND METHOD

Technical Field

The present invention relates to electric circuitry used to monitor and evaluate the readiness of devices employing electrically operated ignition devices used in conjunction with sensors or actuators.

Prior Art

Electrically operated ignition devices (hereinafter also referred to as squibs) are used in a wide variety of applications. As employed in certain air bag safety systems for vehicles, for example, upon detection of an impact of a greater magnitude than a predetermined value, such as in a collision, they are exploded to trigger a high pressure gas source to inflate an air bag situated in the front part of the passenger compartment, thereby protecting the driver and/or passenger from impact.

Over the years a variety of sensor means and checking systems for these devices has been proposed, including that of Spies et al which uses a piezoelectric acceleration sensor, Oishi et al who advocate a transformer-coupled sensor, and that of Montaron, which employs accelerometers as the sensing means.

A common arrangement in other such apparatus is the use of a plurality of normally-open impact sensors series- connected to the squib and a power source or sources of sufficient energy to detonate the squib under the

SUBSTITUTE SHEET appropriate circumstances. Upon sensing such an occurrence, they are adapted to close for a predetermined period of time to allow a high current energy pulse through them to explode the squib, thereby creating the high pressure gas to inflate the air bag.

Because it is not possible to secure the safety of passengers in the event that a fault occurs in the air bag safety device, a supervisory or monitoring circuit is normally provided to check for shorts and opens in the squib, impact sensors and power source connections and provide malfunction information. Because checking the connection of the normally-open sensors is difficult without causing them to close, as in an impact, a resistor is often connected across them and a monitoring current is passed through them to check their connection as advocated by Yasui et al and as shown in prior art FIG. 1. In this figure, a power source, such as a battery, is applied through an ignition switch S_j„, through series connected sensors S-^ and S₂ and squib R_g. Monitoring resistors R^ and R₂ are paralleled across sensors S-_j_ and S₂ to provide monitoring current through the squib, sensors and interconnections. The current through the circuit must be small enough to not trigger the squib, yet large enough to develop enough voltage across the squib, connections, and wiring to allow them to be measured accurately. There are several drawbacks with this arrangement.

A principal problem is the small resistance presented by the squib (normally about 1 ohm) and the long, inductive winding which connects the driver's side squib, normally located in the vicinity of the steering wheel, to its power source and sensors. The effects of wiring resistance add to the series impedance which cuts down the current available to fire the squib. An increase in contact resistance of just a few ohms is enough to cause the squib to not fire, yet is very difficult to detect as a percentage variation of the sensor resistor values heretofore used. Another problem is that switch contact chatter occasionally causes the circuit to misfire if the

JUBSTITUTE SHEET ignition switch is rapidly cycled off and on again, which event may also occur on impact. Also, with the ignition switch off, as can occur with a vehicle just being parked, the protection circuit is not active, yet passengers and driver still in the vehicle may be subjected to an impact of sufficient force to require protection. If the circuit were kept on with the ignition switch on, battery drain would be excessive. In addition, as seen in prior art FIG. 1, with R_χ and R₂ connected as shown and power supplied, a constant DC current flows through the squib, causing gradual squib deterioration through aging, as long as power is supplied by the ignition switch being on. In an effort to help reduce the effects of this, the circuit of prior art FIG. 2, for example, has been employed. In this approach, the ignition switch S^_g, is used to apply battery power to the squib and sensors and a small metering switch, S_m, is used to deliver metering current through the circuit only when the ignition switch is on. Due to ignition switch chatter, however, false fault indications can be generated. The addition of the relay circuit adds an electromechanical part that can be expected to fail after a given number of mechanical cycles, can develop intermittent, resistive or welded contacts, is subject to shock and vibration, adds cost and requires care in its' driver circuitry to avoid problems with coil inductance.

As an attempt to alleviate ignition switch chatter and battery drain, the circuit of prior art FIG. 3 has been proposed by Kumasaka et al which provides for a delayed application of the DC monitoring current after ignition switch turn-on. It uses a relay, with the attendant problems noted above, to allow battery power across the sensors by opening the series connection of the paralleled resistors. It also checks for welded contacts during ignition switch turn-on which can cause squib deterioration.

In another example of a relay opening the series connection of sensing resistors, the circuit of prior art FIG. 4 by Kamiji et al is presented. The major area of improvement is claimed to be in the trigger circuit, by a

SUBSTITUTE SHEET virtual direct connection of the power source to the sensors, irrespective of the ignition switch position. As will be shown in the present invention, a more optimal way of using AC pulsing rather than DC monitoring currents represents a simpler approach and appreciable improvement in impedance and contact monitoring over that of the prior art. This is also true with respect to more conventional means of impedance measurement, as shown in the prior art of Hall and Burch.

Disclosure of the Invention

It is, therefore, a primary objective of the present invention to provide a low-cost, accurate, highly reliable and temperature stable impedance and contact monitoring apparatus and method for use in evaluating and monitoring the readiness of devices using electrically operated ignition devices used in conjunction with sensors or actuators which may be enabled to initiate the ignition device. The major purpose of the invention is to check for shorts and opens in the interconnections of such devices and to measure the impedance of the electrically operated ignition devices used therein.

As contrasted with the prior art, an objective of the present invention is to allow a virtual direct connection of the electric power source to the sensors or actuator means and electrically operated ignition device while drawing virtually no current through the system when monitoring means is off by using a high reliability blocking capacitor. The purpose of this arrangement is to allow continuous operational readiness of the device without causing substantial power source drain and to eliminate switch chatter as a source of fault generation.

A further object of the invention is to increase the accuracy of testing the interconnection and firing device impedance by making the sensor's impedance appear high to a DC voltage and low to an induced pulse.

Another object of the invention is to use a single

SUBSTITUTE SHEET switch means to produce a single checking pulse which checks the impedance and connection of the triggering circuit. Another object of the invention is to disclose a simple low cost method of producing a transient DC voltage source to provide current for the aforementioned checking pulse.

According to the present invention, the impedance and contact monitoring circuit is provided with a DC power source and connection means from such source to an actuator or sensor connected in series with an electrically operated ignition device, a variable impedance monitoring means composed of a series connected resistor and capacitor connected across the sensor or actuator, an amplifier with feedback means and a biased input, and switch means used to change the impedance applied to the amplifier circuit. The method of producing the checking pulse used for the time base sampled measurement of the integrated and amplified RC waveform is achieved by utilizing the differential phase delay of the amplifier and switch means to create a current through the ignition device only when* instantaneously switching.

Inherent in the invention is the elimination of a DC monitoring current through the squib as shown in the prior art which reduces ignition device deterioration through aging and the provision for a delayed application of the monitoring pulse after ignition switch turn-on which eliminates switch chatter as a source of fault generation. The use of a high reliability DC blocking filter capacitor is employed instead of a relay to allow operational readiness with the monitoring circuit off, reducing circuit costs and increasing reliability. Provision is made for conversion means to control the checking pulse generation and switch impedance and time base sample the output waveform for measurement purposes. Provision for logical means such as a microcomputer or an application-specific IC (ASIC) , for example, is made to interpret the data, signal malfunctions or readiness, record data and perform other useful and desired functions.

SUBSTITUTE SHEET The above and further features, objects and advantages of the.present invention will more fully appear from the following detailed description of preferred embodiments of the invention when the same is read in conjunction with, and in reference to, the accompanying drawings.

Brief Description of the Drawings

FIGS. 1 and 2 are circuit diagrams of the conventional prior art contact monitoring arrangements for a vehicle safety device.

FIG. 3 shows a prior art circuit diagram for the same purpose which employs a delay means for contact supervision.

FIG. 4 shows a prior art circuit for the same purpose as FIG. 3 which allows a substantially direct connection of a power source to the sensors irrespective of ignition switch position.

FIG. 5 shows a preferred embodiment of the present invention configured for use in a vehicle safety device.

FIG. 6 shows an alternative arrangement of the circuit of FIG. 5 as adapted for a firing readiness monitoring circuit.

FIG. 7 is a diagram of operational waveforms of the circuit of the present invention depicting normal squib impedance, squib open and squib short indications.

FIG. 8 is a diagram of an impedance evaluating circuit.

Hode(s) for Carrying Out the invention

Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the embodiments, and more particularly FIGS. 5, 6 and 8, circuitry with dashed lines surrounding it is depicted as being part of an assembly or subassembly.

FIG. 5 illustrates an impedance and contact monitoring apparatus in accordance with the present invention as adapted for a vehicle passenger safety system. The apparatus includes an electrically operated ignition device 5, to be

SUBSTITUTESHEET fired to inflate an air bag and has a small internal resistance value (1 ohm, for example) .

The circuit comprises a pair of impact sensors 27 and 30 possibly located in each of the front corners of the vehicle in the head lamp area and an additional impact sensor 12, possibly located in the central front portion of the passenger compartment. Each of these sensors is in the form of a normally open switch which is adapted to close for a predetermined period of time (for example, 100 milliseconds) upon sensing a deceleration of a greater than predetermined magnitude. Impact sensor 12 is joined at one contact to filter resistor 13 and to the cathodes of blocking diodes 43 and 44. The other contact of impact sensor 12 is connected to filter and blocking capacitor 14 and through connection means 25 to electrically operated ignition device 5. It is also connected to overvoltage protection resistor 38, to gain setting amplifier feedback resistor 11 and to conversion means 6. Impact sensor 27 has one contact connected to filter capacitor 28 and through connection wiring 33 to the junction of bias level setting resistors 9 and 10, integrator low level setting resistor 8, overvoltage protection resistor 37, and to conversion means 6, and through connection means 26 to electrically operated ignition device 5. The other contact of impact sensor 27 is connected to filter resistor 29 and through connection wiring 34 to electric circuit supply return 24. Impact sensor 30 is essentially paralleled to impact sensor 27 with one of its contacts connected to filter capacitor 31 and through connection wiring 35 to the junction of connection wiring 33, connection means 26, the junction of bias level setting resistors 9 and 10, integrator low level setting resistor 8, overvoltage protection resistor 37 and conversion means 6. The other contact of impact sensor 30 is connected to filter resistor 32 and through connection wiring 36 to electric circuit supply return 24.

In FIG. 5 as shown, electric power source 1, a vehicle battery, is connected positive pole to connection means 2,

^•S .T3TUTS SHSST an ignition switch, and the anode of blocking diode 43. The negative pole of electric power source 1 is connected to electric power source return 23. The normally open connection of connection means 2 is connected to backup energy source 45, which may consist of the stored charge held on large aluminum electrolytic capacitors of large value fed by electric power source 1 or a stepped up switching regulated voltage (not shown) derived from electric power source 1, for example. The output of the backup electric power source 45 is applied to the anode of blocking diode 44, bias level setting resistor 9, the maximum power source potential 16 for amplifier 3, the cathodes of two terminal protection devices 39 and 41 and to conversion means 6. A purpose of this arrangement is to provide an alternative power source for the electrically operated ignition device 5 in the event that connections from electric power source 1 are broken, as could occur from a collision, for example. It is configured to allow power to be applied to the basic triggering circuit comprised of series-connected impact sensors 12, 27 and 30 and electrically operated ignition device 5. In the event that an impact of sufficient magnitude occurs to cause impact sensor 12 to close, and either impact sensor 27, or 30, or both to close, then either current from electric power source 1 will be applied through blocking diode 43 to activate electrically operated ignition device 5, and/or current from backup electric power source 45 will be applied through blocking diode 44 to cause electrically operated ignition device 5 to activate, causing the air bag to inflate.

The impedance and contact monitoring apparatus further comprises a switched impedance differential amplifier/buffer circuit including amplifier 3, whose output is connected to gain setting amplifier feedback resistor 11 and conversion means 6. The inverting input of amplifier 3 is connected to the junction of overvoltage protection resistor 38, the anode of two terminal protection device 39 and the cathode of two terminal protection device 40. The non inverting

SUBSTITUTESHEET input of amplifier 3 is connected to the junction of overvoltage protection resistor 37, the anode of two terminal protection device 41 and the cathode of two terminal protection device 42. The negative supply connection of amplifier 3 is connected to electric circuit supply return 24 as is also the anodes of two terminal protection devices 40 and 42. Conversion means 6 may be used, for example, to monitor the inputs and outputs of amplifier 3, backup electric power source 45 and malfunction indicators if desired (not shown) and also has an open circuit monitoring node 15 used for monitoring the voltage present at the junction of filter resistor 13 and filter and blocking capacitor 14. Conversion means 6 also generates a voltage applied as switch means control input voltage 20 to switch means 4. Switch means 4, in a preferred embodiment, is a small, low on-resistance n- channel MOSFET with its gate voltage controlled by conversion means 6, its source connected to electric circuit supply return 24 and bias level setting resistor 10, and its drain connected to integrator low level setting resistor 8. In alternative forms, switch means 4 may also be a bipolar transistor, a JFET or other switching device. Conversion means 6 has signal outputs 22 applied as inputs to logical means 7, while logical means 7 has signal outputs 21 applied as inputs to conversion means 6. Depending on the relative sophistication of the device in which this invention is used, logical means 7 may be made to control and read data from conversion means 6, to interpret and record data, to generate a time base for checking pulse generation and to signal and record malfunction information, for example.

Before a detailed description of the mode of operation is begun, it is relevant to note that in the preferred embodiment of FIG. 5, as illustrated, three sensors and one electrically operated ignition device are used. Numerous alternative arrangements of the basic circuit may be readily configured, such as using another electrically operated ignition device in parallel with electrically operated

SU! IST1T UTE SHEET ignition device 5 or in using greater or fewer sensors or actuators as shown in FIG. 6, or FIG. 8, or in using other combinations of connections as suggested by prior art FIG. 4, for example.

In FIG. 5 as illustrated, impact sensor 12 is included in a subassembly in a more benign environment than either impact sensor 27 or 30 and can therefore be reasonably expected to have fewer potential problems induced by cabling and interconnections. This assumption has therefore guided the description of relative component values which affect performance and accuracy of the measurements. The mode of operation of the embodiment will now be described in detail. Referring now to FIG. 5, with connection means 2 open as shown, the potential of electric power source 1 is applied through blocking diode 43 to one contact of impact sensor 12 and filler resistor 13 so long as the potential at the anode of blocking diode 43 is somewhat greater than the potential at the anode of blocking diode 44. With connection means 2 open, the output potential held by backup electric power source 45 will be allowed to discharge through the various circuit elements. Upon sensing the opening of connection means 2, or the dropoff in potential from backup electric source 45, conversion means 6 may be made to apply a switch means control input voltage 20 to switch means 4 consisting of a voltage gate-to-source of zero volts, for example, thereby holding off switching of the checking pulse signal, by not allowing current through integrator low level setting resistor 8. In a quiescent mode in an off-state, a potential of approximately one diode drop less than that of electric power source 1 will be applied to impact sensor 12 allowing the triggering circuit to remain active, without requiring a relay, as in the prior art. This is because if filter resistor 13 is held to a moderate value such as a few thousand ohms, and filter and blocking capacitor 14 is held to a low value such as a few hundred picofarads, for example, and conversion means 6 presents a high impedance to the voltage present at the node junction of filter resistor 13 and filter and blocking

_SUBSTITUTESHEET capacitor 14, then battery drain will be minimal since filter and blocking capacitor 14 and blocking diode 44 will block the DC voltage present from other paths of the circuit. As taught by Laplace transforms, the connections of filter resistor 13 and filter and blocking capacitor 14 may be reversed if the input impedance of conversion means 6 can not be made high enough to limit battery drain and/or prevent damage to conversion means 6 with connection means 2 open and electric power source 1 potential applied. In a preferred embodiment, filter and blocking capacitor 14 should be a non-polarized ceramic capacitor with a low temperature coefficient and of a value small in relation to that of filter capacitors 28 and 31, which in the preferred embodiment should also be ceramic, non-polarized low temperature coefficient capacitors. The values of filter and blocking capacitors 28 and 31 should be high in relation to the distributed capacitances of connection wiring 33, 34, 35 and 36.

Referring now to FIGS. 5 and 7, wherein FIG. 7 represents operational waveforms of the circuits of FIGS. 5, 6 and 8, the vertical axis of FIG. 7 is used to represent the operational voltage present at various circuit nodes, while the horizontal axis displays that voltage as a function of time, t, in microseconds. With the circuit powered up, FIG. 7a waveform corresponds to switch means control input voltage 20 applied to switch means 4. FIG. 7b waveform corresponds to the open circuit monitoring node 15 for impact sensor 12. FIG. 7c waveform depicts the voltage present at amplifier output 18 for normal circuit operation with FIG. 7d waveform the corresponding waveform for a shorted electrically operated ignition device 5. FIG. 7e waveform shows the voltage at amplifier output 18 for an electrically operated ignition device 5 that is open.

In FIG. 5, as connection means 2 is closed, the potential of electric power source 1 is applied to backup electric power source 45, causing the maximum power source potential 16 for amplifier 3 to rise to its predetermined value. This causes the potential applied to

SUBSTITUTESHEET the protected non inverting input 19 for amplifier 3 to charge to a value determined by the voltage divider ratio of bias level setting resistors 9 and 10 and the potential of backup electric power source 45. With each resistor having a value of 5000 ohms, for example, the voltage at protected non inverting input 19 will be approximately one- half of the differential in potential between maximum power source potential 16 and electric circuit supply return 24.

Because amplifier 3 draws virtually no input current except small error currents such as input bias current and input offset current, the voltage present at protected non inverting input 19, in a quiescent state, will be buffered by amplifier 3 to develop virtually the same voltage at protected inverting input 17, thus, with amplifier 3 having no input offset voltage or induced input offset voltage developed by input offset currents through overvoltage protection resistors 37 and 38, the voltage at amplifier output 18 will mirror that present at the junction of bias level setting resistors 9 and 10, and there will be no current through electrically operated ignition device 5. This is because filter capacitors 28 and 31 isolate and block the DC voltage from filter resistors 29 and 32, and DC blocking and filter capacitor 14 blocks the voltage developed at its junction with filter resistor 13. This arrangement allows the sensors to appear as a high impedance to a DC voltage and a low impedance to a pulsed input. Also, with switch means 4 off, then integrator low level setting resistor 8 will have no current through it. Therefore, with no checking pulse applied, and no offset voltage between the amplifier inputs, there will be no monitoring current through electrically operated ignition device 5.

In selecting appropriate values for protection resistors 37 and 38, they should be of an equal value and be as small as practical to protect the amplifier and add a series impedance to protect against possible fault mode activation of electrically operated ignition device 5. Also, a set of back-to-back diodes, (not shown) may be added across the inverting and non inverting inputs of amplifier 3, close to the device, to limit the differential input voltage applied during possible fault modes. These diodes and two terminal protection devices 39, 40, 41 and 42, in a preferred embodiment, should be small Schottky barrier diodes with a low forward voltage drop and small capacitance across them. It should be obvious to one skilled in the art that the protection schemes employed herein are intended to limit the effects of overvoltage and are of a general nature. Because of this, the various protection devices may not all be required for all applications in which the invention may be used. It should also be obvious to one skilled in the art that filter resistors used herein are a preferred embodiment of the impedance forming network for the sensors, and that other current limiting devices may be adapted to perform a similar function. The major considerations involving such adaptations should involve factors concerning the reliability, temperature stability, and cost of the devices employed. In the preferred embodiment, the values of filter resistors 29 and 32 should be held to a value greater than the lowest value practical to provide AC current limiting corresponding directly to the lowest level DC current required to activate the electrically operated ignition device 5. This allows possible contact resistances of a few ohms to represent a greater percentage variation from the fixed value across the sensor compared to the prior art, since the AC impedance of the series capacitor can be a fraction of 1 ohm, for example. This is allowable since the electrically operated ignition devices generally include a small RF choke in series with their resistance and require application of a minimum DC current for a minimum period of time to cause them to heat and ignite. Short, infrequently pulsed AC currents cause virtually no heating and reduce aging of electrically operated ignition device 5 as compared to a DC current of the same value.

Referring now to FIGS. 5 and 7, the method by which the checking pulse is generated will become apparent and is

SUBSTITUTESHEET hereby described. As seen earlier in a quiescent state, no current will flow through integrator low level setting resistor 8 with switch means 4 held off with a low voltage, approximately zero volts, applied to its control input. With switch means 4 on, however, the series impedance of it and that of integrator low level setting resistor 8 are placed in parallel with the resistance of bias level resistor 10. Therefore, if the value of integrator low level setting resistor 8 is made low with respect to that of bias level setting resistor 10, (for example, 15 ohms) and if the on resistance of switch means 4 is made low also (such as a few ohms) , then the bias level applied to the protected non inverting input 19 of amplifier 3 can be caused to switch from a high voltage level to a much lower voltage level. This lower voltage level will be primarily defined by the voltage divider determined by the value of bias level setting resistor 9 and the parallel impedance of bias level setting resistor 10 and the series connection of integrator low level setting resistor 8 and the on resistance of switch means 4. In the preferred embodiment, the switch means control input voltage 20 will be sufficiently high to cause conduction of switch means 4 for low currents, but be of a sufficiently low value to limit conduction so that the highest current possible through it is held to a value lower than that required to activate electrically operated ignition device 5.

Referring now to FIG. 7a, as such a voltage is applied to switch means 4 control input 20, a series of actions occur to cause the checking pulse to develop a waveform representing the condition of the triggering circuit. As the voltage of FIG. 7a rises to a value to cause switch means 4 to conduct, the stored charge held across filter capacitors 29 and 32 is discharged rapidly through the switched impedance comprised of bias level setting resistor 10, integrator low level setting resistor 8 and the on resistance of switch means 4. Since the voltage differential across electrically operated ignition device 5 is no longer zero volts, a current is induced through it. This is due to

_SUBSTITUTESHEET the fact that the voltage held at the protected inverting input 17 of amplifier 3 represents the quiescent value of the voltage applied to the protected non inverting input 19 of amplifier 3 before being switched, but not instantaneously after. As depicted in FIG. 7c, the amplifier output 18 begins slewing to reflect the change. Also, as illustrated in FIG. 7b, the voltage present at the junction of filter resistor 13 and filter and blocking capacitor 14 is also brought low as the charge across filter and blocking capacitor 14 is discharged through electrically operated ignition device 5. Thus, the connection of sensor 12 can be verified by monitoring the voltage present at open circuit monitoring node 15.

In a manner to be described, amplifier 3 serves as a transient voltage source to supply the bulk of the switched current drawn through electrically operated ignition device 5. This is due to the inherent differential phase delay between the switch means 4 MOSFET, which can switch in a few nanoseconds, and that of the amplifier, which has several stages, typically, through which the signal must propagate, and which is biased for linear, not switched mode operation. Therefore, on an instantaneous basis, amplifier output 18 voltage will not reflect the voltage impressed at its inputs, but will continue to hold its previous value until its' internal nodes have charged and stabilized.

This action is reflected in the difference of the waveforms of FIGS. 7c, 7d, and 7e. With the waveform of FIG. 7c representing a normal (1 ohm, for example) impedance for electrically operated ignition device 5, FIG. 7d shows the waveform for a shorted device and FIG. 7e for an open device, with connection means 25 and 26 having, in this illustration, a 1.2 ohm resistance. Thus a slightly larger resistance value for the electrically operated ignition device 5 provides a much greater reduction of the feedback signal from amplifier 3, and is reflective of the difference in the apparent RC charge/discharge times illustrated in the waveforms of FIGS. 7c, 7d, and 7e.

SUBSTITUTESHEET If, for example the pulse width of the signal of FIG. 7a waveform is 100 microseconds, then, in normal operation pulsed current will flow through the electrically controlled ignition device during the transition time of about 10 microseconds, as shown in FIG. 7c. When the amplifier reaches the integrator low level setting threshold, current flow through electrically operated ignition device 5 will effectively cease.

Measurement of the various impedances and connections in the triggering circuit is performed by sampling the voltage at amplifier output 18 after opening switch means 4 by apply ^a signal of zero volts to switch means control input 20. This causes charging current through bias level setting resistor 9 to be applied to filter capacitors 28 and 31, allowing amplifier output 18 to rise from the lower voltage level established by integrator low level setting resistor 8 and the on resistance of switch means 4, to the value established by the ratio of bias level setting resistors 9 and 10.

As shown under the horizontal axis of FIG. 7, after turning off switch means 4, the time, t-_|_, is set for approximately 1 time constant of the RC charge time to set the time delay at which amplifier output 18 is sampled. This allows factors such as amplifier slew rate and sample-hold acquisition error considerations to conform to Nyquist sampling criteria for an accurate measurement. The sampled voltage may be digitized for analysis and recording for example, or used as an input to strobed window comparators to indicate fault conditions. How the sampled voltage value reflects the triggering circuit condition is next described.

Referring now to FIGS. 5 and 7, it has been shown that in a quiescent condition, the amplifier output 18 voltage mirrors that voltage present at its protected non inverting input 19. In this quiescent state, impact sensors 12, 27, and 30 all appear as open circuits due to their filter capacitors blocking the DC voltage across them. It can be readily seen that opening, for example, connection wiring

SUBSTITUTESHEET 33, 34, 35, or 36 will cause the voltage sampled at time, t_lf to change due to the decreased capacitive load, but that a later time, t₂, the amplifier output 18 will settle to the value established by the voltage divider of bias level setting resistors 9 and 10. This is true for open sensors, but not true for shorted sensors. A short or partial short across impact sensors 27 or 30, for example, will appear as a parallel impedance across bias level setting resistor 10, thereby causing the sampled voltage at time t₂ to be less then established by the value set by bias level setting resistors 9 and 10 alone. In a similar manner, a short across impact sensor 12 causes a parallel impedance to be placed across bias level setting resistor 9, thereby causing the amplifier output 18 to be higher than expected.

A consideration in the rate at which the triggering circuit is monitored is related to the device in which the invention is used. In an automobile, for example, where the device may be subjected to RF power -line noise and where the malfunction indicator is a light bulb, it may not be desirable to sample the triggering circuit more than once a second, for example, to allow for driver response considerations. In other uses, a more frequent monitoring rate may be desirable. In any event, attention should be paid to insure that switch means 4 is not activated for a long time if impact sensor 12 is shorted, which could cause a higher than desirable current through the electrically operated ignition device 5. The potential for this occurrence is minimized by a variety of means embodied in the invention to preclude this event. The likelihood of this occurring is greatly diminished by sampling at t₂ time before generating the checking pulse and inhibiting the checking pulse generation if the measured value is much higher than that expected. Another precautionary measure is to keep the checking pulse duration very short compared to what is needed to activate the electrically operated ignition device. Thus, the 100 microsecond pulse is very short compared to the 20 milliseconds often required to heat

SUB^STITUTESHEET the electrically operated ignition device sufficiently to cause it to explode. Another preventative measure is the use of a MOSFET switch for switch means 4 which increases on resistance with increased currents, thereby providing current limiting. Also, by holding the voltage gate-to- source of the MOSFET to an appropriately low level, the device can be made to automatically current limit to a value insufficient to activate the ignition device. Yet another approach may be to lock out the switch means control input 20 signal, by sensing the voltage at protected inverting input 17, comparing it to what is normally set by the voltage divider of bias level setting resistors 9 and 10 and, if it is much too high, then inhibit the pulse. Also, it should be obvious that the likelihood of an occurrence is directly related to the monitoring rate of the circuit, so reducing the monitoring rate directly reduces the potential for the occurrence.

Industrial Applicability

Referring now to FIG. 6, there is shown an alternative embodiment of the present invention as adapted for a firing readiness monitoring circuit. Like reference numerals to those of FIG. 5 refer to identical or corresponding parts in FIG. 6. In this case, the major difference is that a backup electric power source is not used, so blocking diodes 43 and 44 are not required. This causes the voltage at the open circuit monitoring node 15 of FIG. 6 to be less than that at the corresponding point in FIG. 5 with identical power sources, since the addition of the blocking diodes of FIG. 5 causes the circuit to act as a charge pump with inductive connection means 25 and 26. Also to be noted is the impact sensors have been replaced by another type of normally open switch, such as a microswitch actuator. Both switches are located in close proximity to each other with the monitoring circuit some distance away.

This arrangement allows different scaling of the values of filter resistor 13 and filter and blocking capacitor 14

SUBSTITUTESHEET due to this remote location and the elimination of the parallel connection across microswitch actuator 27. FIG. 8 shows an alternative embodiment with fewer actuators for use as an impedance evaluating circuit.

Obviously, numerous modifications and variations of the present invention are possible within light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein, without departing from the spirit of the invention.

_SUBSTI_TUTESHEET FIG. 5 Reference Number List

Electric power source

Connection means - ignition switch

Amplifier

Switch means

Electrically operated ignition device

Conversion means

Logical means

Integrator low level setting resistor

Bias level setting resistor

Bias level setting resistor

Gain setting amplifier feedback resistor

Impact sensor - safing

Filter resistor

Filter and blocking capacitor

Open circuit monitoring node for 12

Maximum power source potential for 3

Protected inverting input of 3

Output of 3

Protected non inverting input of 3

Switch means control input

Signal outputs from 7 applied as inputs to 6

Signal outputs from 6 applied as inputs to 7

Electric power source return

Electric circuit supply return

Connection means to 5

Connection means to 5

Front impact sensor 1

Filter capacitor

Filter resistor

Front impact sensor 2

Filter capacitor

Filter resistor

Connection wiring for 27

Connection wiring for 27

Connection wiring for 30

Connection wiring for 30

Overvoltage protection resistor for 3

Overvoltage protection resistor for 3

Two terminal protection device for 3

Two terminal protection device for 3

Two terminal protection device for 3

Two terminal protection device for 3

Blocking diode for 1

Blocking diode for 45

Backup electric power source FIG. 6 Reference Number List

Electric power source

Connection means

Amplifier

Switch means

Electrically operated ignition device - squib

Conversion means

Logical means

Integrator low level setting resistor

Bias level setting resistor

Bias level setting resistor

Gain setting amplifier feedback resistor

Microswitch actuator

Filter resistor

Filter and blocking capacitor

Open circuit monitoring node for 12

Maximum power source potential for 3

Protected inverting input of 3

Output of 3

Protected non inverting input of 3

Switch means control input

Signal outputs from 7 applied as inputs to 6

Signal outputs from 6 applied as inputs to 7

Electric power source return

Electric circuit supply return

Connection wiring to 14

Connection wiring to 28

Microswitch actuator

Filter capacitor

Filter resistor

Connection wiring from 5 to 28 Supply return connection to 29

Overvoltage protection resistor for 3 Overvoltage protection resistor for 3 Two terminal protection device for 3 Two terminal protection device for 3 Two terminal protection device for 3 Two terminal protection device for 3

Connection wiring from 14 to 5