BACKGROUND OF THE INVENTION The present invention relates generally to a signal processing method and apparatus for compensating low frequency variations in a sensor output, and more particularly to a method and apparatus for compensating source and transmission path variations in intensity-modulated sensors, such as used in automotive crash detection systems for sensing the acceleration profile of a moving vehicle, wherein the sensor utilizes two optical fibers (transmit and receive) and the measured quantity varies (modulates) the coupling (transmission) between the two fibers.

Intensity modulated fiber optic sensors operate to modulate the absolute level of light "coupled by," i.e., transmitted through the sensor by a constant percentage per unit of excitation. A measured parameter, such as acceleration input on the sensor, will vary the percentage of light transmitted through the sensor. Receiver circuitry detects these variations in the percentage of transmission to quantify the measured parameter. Additionally, the percentage of modulation can be varied by transmission source and path variations, such as caused by fiber bend loss, LED aging, connector loss and sensor variations. Transmission source and path variations present undesirable corruptions of the sensor output signal. The modulation due to detected acceleration remains relatively the same and occurs rapidly, while the variations caused by source and path perturbations are of lower frequency than the acceleration signal. Typically, compensation of transmission source and path variations involves the generation of a signal proportional to the percentage modulation of the acceleration signal. One known method provides for separating the rapidly varying AC and relatively steady state DC voltage components of the sensor signal, and then computing a ratio of the two to arrive at a percent modulation of the DC level by the AC component. With this method, the AC voltage component tracks

the measured parameter, and the DC voltage component tracks the low frequency variations. The generation of a ratio automatically provides low frequency variation compensation for the measured parameter. This method of compensation is generally referred to as ratiometric compensation of the signal.

Generally, AC/DC filters are used to separate the sensor signal into the respective AC and DC components. These components are then applied to an analog divider circuit to generate the compensated signal. However, when a measured pulse length or vibration is equal to an appreciable amount of time relative to the inherent time constant of the AC/DC filter, an exponential decay, or drop in pulse magnitude, will occur in the compensated output pulse due to the filter time constant effect on the AC voltage component when a positive magnitude input pulse is measured. In a crash detection system, this exponential decay in the compensated output pulse can lead to considerable error in deploying an airbag or passenger restraint device. SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide an improved interface circuit for compensating source and path variations in a sensor output signal which is not effected by a filter time constant. It is also an object of the present invention to provide an improved method of ratiometric compensation which tracks slow changes in a sensor output signal without being effected by a filter time constant when rapidly changing or largely varying outputs are generated by the sensor. It is further an object of the present invention to provide an improved interface circuit which achieves ratiometric compensation of transmission source and path variations in an intensity modulated sensor output signal without a filter time constant effect on the compensated signal.

In accordance with the present invention, an interface circuit for compensating low frequency variations in

a sensor output signal, such as occurs from transmission source and path variations in an intensity-modulated output signal of an optical accelerometer, comprises a means for receiving the sensor output signal, a filter for separating a DC signal component from the received sensor output signal, and a means for subtracting the DC signal component from the received sensor output signal to generate an AC signal component. A compensating means is responsive to the AC signal component and the DC signal component for compensating the low frequency variations by generating an AC signal component to DC signal component. A switch is connected to the receiving means and the filter. A comparing means is provided for comparing the ratio signal to a predetermined threshold value. A control means is responsive to the comparing means for opening the switch if the ratio signal exceeds a predetermined threshold value, thereby controlling the filter to track the low frequency variations when the switch is closed, and to provide a constant level DC component when the switch is opened.

In further accordance with the present invention, a voltage divider circuit is provided for generating a fixed percentage of the DC component supplied to the compensating means by the filter. The compensating means and the control means are implemented as both digital circuitry and analog circuitry. The present invention further provides a method for ratiometric compensation of low frequency variations in a sensor output comprising the steps of receiving an output signal from the sensor, separating the received sensor output signal into AC and DC signal components, and compensating for the low frequency variations by generating an AC component to DC component ratio signal. The ratio signal is compared to a predetermined threshold value, and the DC signal component is held constant if the ratio signal exceeds the predetermined threshold value. The method of ratiometric compensation further includes the steps of discriminating the ratio signal for controlling the actuation or deployment of a vehicle passenger

safety restraint, such as an airbag, and scaling the DC signal component utilized in the compensation step to a fixed percentage so as to simplify the A/D conversion, thereby providing more rapid restraint response. The present invention will be more fully understood upon reading the following detailed description of the preferred embodiment in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of a digital interface embodiment in accordance with the present invention;

FIG. 2 is a block diagram of an analog interface embodiment in accordance with the present invention;

FIG. 3 is a circuit diagram for the divider circuit of FIG. 2; and

FIG. 4 is a timing diagram for the divider circuit of FIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT (S)

Referring to FIG. 1, there is shown a block diagram for a first interface embodiment 10 of the present invention implemented using digital circuitry. An intensity-modulated signal is received by amplifier 12 from an optical sensor system 14 such as used in a vehicle crash detection system.

After amplification, the received sensor signal passes through an analog track-and-hold switch 16 to a low pass DC filter 18.

The output of the low pass DC filter 18 is then subtracted from the received sensor signal in differential amplifier 20 to provide the AC component of the received sensor signal. This

AC component is then supplied as an input voltage V _in to analog-to-digital (A/D) converter 22.

In accordance with the present invention, ratiometric compensation of the received sensor signal is accomplished by applying a fixed fraction of the DC voltage level V _oc output by filter 18 to a reference voltage V _ref input of the A/D converter 22. In the preferred embodiment, the fixed fraction is equal to 20%. A voltage divider resister network 24 is utilized to apply 0.2 V _DC from the output of the DC filter 18

to V _ref of the A/D converter 22. The reference voltage V _ref fixes the full scale span of the A/D converter 22 to 20% of the DC level independent of the actual level. Other full scale spans can be obtained with the present invention by adjusting the voltage divider R,, R ₂ accordingly.

Overall control for the digital interface 10 is provided by microprocessor 26. The microprocessor 26 provides timing signals to the A/D converter 22, and monitors the A/D output (Data Out) . Microprocessor 26 also controls the operation of the track-and-hold switch 16. When the track-and-hold switch 16 is closed, the filter 18 output will track the DC voltage level of the received sensor signal, and when switch 16 is open, the DC voltage level output by filter 18 is held constant, as described hereinbelow. The microprocessor 26 further processes the digitally-converted sensor signal for determining whether to actuate a vehicle passenger safety restraint, such as an airbag (not shown) in response to a vehicle crash.

In operation, the microprocessor 26 normally closes track-and-hold switch 16. This allows the low pass DC filter 18 to track slow changes, i.e., low frequency variations, in the received sensor signal caused by source/detector variations and transmission path variations. However, when microprocessor 26 detects that Data Out exceeds a predetermined threshold, thereby indicating a rapidly rising crash pulse detected by sensor 14 such as caused by a vehicle acceleration exceeding 3 g's of force, track-and-hold switch 16 is then opened, thereby forcing the DC voltage level to be held constant on the filter capacitor. The remainder of the received sensor signal is then digitally converted relative to this constant DC voltage level.

With the above-described operation of the tracking and holding of the DC level, the digitally-converted data is not effected by a slowly changing DC voltage reference level. Because the DC voltage level is held constant on the filter capacitor, the effect of the low pass time constant of filter 18 on a measured positive magnitude AC pulse can be

avoided. Thus, the present invention provides increased accuracy over the prior art by avoiding any exponential decay, or droop effect, in the converted output in response to the inherent filter time constant. When the microprocessor 26 detects the received sensor signal has returned below the predetermined threshold, the track-and-hold switch 16 is then closed again, thereby allowing filter 18 to return to the tracking mode.

The digital interface 10 of the present invention also employs a variable gain circuit 30 controlled by the microprocessor 26. In the preferred embodiment, variable gain for the received sensor signal is utilized to prevent V _ref from falling below a minimum voltage level which might otherwise promote signal processing errors due to input noise. The variable gain circuit 30 also allows for an AC to DC level input range of at least 10:1. In the preferred embodiment, the microprocessor controlled variable gain circuit 30 is implemented using four steps of gain adjustment, thereby allowing the digital interface 10 to accommodate a 16:1 input level range.

Referring now to FIG. 2, there is shown a block diagram for a second embodiment 100 of the present invention utilizing an analog divider circuit. The analog interface 100 performs ratiometric compensation which is unaffected by the low pass filter time constant in a manner similar to digital interface 10. Also note, elements that are the same as in FIG. 1 have like reference numbers and have been described hereinabove in context with FIG. 1. In FIG. 2, the A/D converter 22 and the microprocessor 26 have been replaced with divider circuit 110 and a threshold comparator 118. The divider circuit 110 is formed from a voltage-to-time converter stage 112 connected to receive the V _AC component at input X, a V _AC chopper stage 114 connected to receive the V _AC component at input Y, and a low pass filter 116 connected to the chopper stage 114 to provide the desired compensated output, i.e., X divided by Y, V _ouf . One particular implementation of divider circuit 110 is shown in FIG. 3 and discussed hereinbelow.

Divider circuit 110 could alternatively be implemented using a monolithic circuit chip.

Referring to FIG. 3, the voltage-to-time converter stage 210 utilizes an integrator to produce rising and falling ramps V _c which are fed into a first comparator 214. A high voltage reference V _w is generated by summing a low voltage reference V^ with the scaled DC voltage output V _DC of the voltage divider network 24. The high voltage reference is then supplied to the other input of the first comparator 214. The first comparator 214 output V _cs is connected to the set input of an SR flip-flop. A second comparator 216 receives V _c and the low voltage reference V _L . The output V _c? of the second comparator 216 is connected to the reset input of the SR flip-flop. The SR flip-flop controls the V _c ramp direction (i.e., rising or falling) by connecting either V _cc via a switch 217, or ground potential via a switch 219, in response to a set or reset input. Control for the V _AC chopper stage 212 is provided by a third comparator 218. The third comparator 218 receives V _c and a reference voltage V _r which is slightly higher than V^, and outputs a signal V _p which controls a pair of switches 220 and 222 for the V _AC chopper stage 212.

The operation of the voltage-to-time converter circuit 210 and the chopper circuit 212 will now be described in conjunction with the timing chart shown in FIG. 4. As shown, when ramp V _c reaches V _w , the first comparator 214 generates the output V _cs at T, to trigger the Q output of the SR flip-flop to go high and activate switch 217, thereby reversing the ramp voltage V _c . When V _c goes below the threshold V _r at t ₂ , the third comparator 218 generates the output V _p , thereby closing the switches 220 and 222 to allow V _AC to pass through the circuit. When V _c reaches V _L at t ₃ , the second comparator 216 generates the output V _c/? which causes the SR flip-flop to again reverse the ramp voltage V _c . When V _c exceeds V _τ at time t ₄ , the output V _p goes low, thereby cutting off V _AC . As described above, the output of the divider circuit 110 provides a ratio of V _AC /V _DC by using a timing signal

controlled by the DC voltage component V _oc for chopping the AC voltage component V _AC . The chopped M _AC is time averaged by a low pass filter 116, such as a 5 pole Butterworth filter, to provide an output effectively equivalent to the AC component of the received sensor signal divided by a scaled DC component of the received sensor signal. The V _C /V _DC ratio signal is independent of the chopping frequency, however the chopping frequency must be high enough to permit the low pass filter 116 to have a cutoff frequency that allows a pulse response sufficient for V _AC . The output of the divider circuit 110 is supplied to a suitable discrimination circuit (not shown) for controlling actuation of a vehicle passenger safety restraint.

The analog embodiment 100 further utilizes a threshold comparator 118 for controlling the track-and-hold switch 16. Comparator 118 monitors the divider 110 output, and if the output exceeds a predetermined threshold, e.g., a sensor pulse indicating an acceleration exceeding 3 g's of force, the comparator 118 will open the switch 16 to hold the V _oc constant. As previously described hereinabove, the use of the track-and-hold switch 16 prevents the time constant used in filter 18 from causing any exponential decay, or droop, in V _olΛ .

Although the present embodiments have been described in context with an optical intensity modulated sensor, the present invention is generally applicable to any compensation circuit using ratiometric measurement to compensate a transient measured quantity having a slowly drifting input. For example, such slowly drifting inputs can result from temperature changes, or stress release, causing low frequency variations in the output of a mechanical transducer, such as the silicon, micromachined accelerometer taught by White et al in commonly assigned U.S. Patent No. 5,060,504. It will also be understood that the foregoing description of the preferred embodiments of the present invention is for illustrative purposes only, and that the various structural and operational features herein disclosed are susceptible to a number of modifications none of which departs from the spirit and scope of the present invention as defined in the appended claims.