US 3929308 A
A motion sensor combined with a journal bearing thermal detector produces an output signal utilized to activate a train's brake system upon detection of a local wheel derailment or excessive bearing temperatures. Wheel impact with the roadbed and/or bearing temperatures exceeding a predetermined value causes the sensor to activate a percussion-initiated power source. The output signal triggers an electroexplosive brake venting mechanism, puncturing and venting the brake line for a full service brake application to stop the train.
Description (OCR text may contain errors)
tent 1191 Armstrong et a1.
[ 51 Dec. so, 1975 LOCAL DERAILMENT SENSOR AND BRAKE ACTUATOR SYSTEM  Inventors: John 11. Armstrong, Silver Spring; William W. Wassmann, Rockville, both of Md.
 Assignee: The United States of America as represented by the Secretary of the Navy, Washington, DC.
 Filed: Aug. 7, 1974  Appl. No.: 495,480
 U.S. Cl 246/169 A; 246/170; 340/57  Int. Cl. B61K 9/04  Field of Search 246/169 R, 169 A, 170,
246/172, 182 A, 182 B, 188; 116/114 AM, 114 S, 114 AB, 114 Y,101, 106, DIG. 38; 200/6108; 340/57; 102/702 GA; 180/103  References Cited UNITED STATES PATENTS 1,512,456 10/1924 Carman 246/169 A 3,625,178 12/1971 Prachar 180/103 Primary Examiner-Trygve M. Blix Assistant Examiner-Reinhard J. Eisenzopf Attorney, Agent, or Firm-R. S. Sciascia; J. A. Cooke; F. K. Yee
 ABSTRACT A motion sensor combined with a journal bearing thermal detector produces an output signal utilized to activate a trains brake system upon detection of a local wheel derailment or excessive bearing temperatures. Wheel impact with the roadbed and/or bearing temperatures exceeding a predetermined value causes the sensor to activate a percussion-initiated power source. The output signal triggers an electroexplosive brake venting mechanism, puncturing and venting the brake line for a full service brake application to stop the train.
21 Claims, 6 Drawing Figures US. Patent Dec. 30, 1975 Sheet 1 of3 3,929,308
US. Patent Dec. 30, 1975 Sheet 2 of3 3,929,308
US. Patent Dec. 30, 1975 Sheet 3 of3 3,929,308
FIG" 5 LOCAL DERAILMENT SENSOR AND BRAKE ACTUATOR SYSTEM BACKGROUND OF THE INVENTION The present invention relates generally to motion sensor systems and, more particularly, to a motion sensor system which can detect the derailment of wheels of railroad cars and generate a signal to apply the trains brake system.
In a certain proportion of train derailments, initial local derailment, in which one or more wheels drop off the rail head, will occur and the train will proceed some distance with little further disturbance and with no awareness of the derailment until some obstruction or further damage to the derailed equipment leads to a general derailment, the magnitude of which is most directly related to train speed at the time.
The most likely obstructions triggering the general derailment are turnouts or highway grade crossings. Since both of these are more common in villages or other developed areas, there appears to be a disproportionate tendency for pile-ups to occur in populated areas. Such pile-ups often involve, in addition to costly property damage and expensive loss time, explosions, fires or release of hazardous materials. While the actual number of such two-stage major derailments is not readily determined, even if as few as 2 to 5% of the train derailments should be in this category, the total potential for savings would be in the $2 to millionper-year range. In view of such potential savings and the concomitant safety improvements, the desirability and requirement for a reliable and economical means to detect local derailment and initiate corrective procedures are clearly evident.
Local derailments could occur in connection with almost any of the classifications of equipment and roadway failures or defects causing derailments. They could also arise from such forms of negligence as improperly secured loads or highway vehicles running into side of train and from a variety of causes classed as miscellaneous, including track obstructions. In these derailments, the wheels and axle drop off the rail head, once the flange on one side has passed beyond its outer edge, and accelerate downward until the flange strikes the roadbed. Dynamically, three different situations may occur. One wheel may drop from the rail head and hit the tie plates while its mate and the other axle of the truck remain on the rail. This type of local derailment results in the highest impact velocity. In most derailments, the second wheel on the axle must leave the rail head at about the same time as its mate because the typical overlap is less than inch. This case results in lower impact velocities. For two-axle derailments both wheels on one side of the truck derail at the same instant. For this to occur, both flanges on one side must travel some distance atop the rail head, crossing it at a rather small angle, so that both go over the brink at the same time. Once a wheel drops from the rail head, it hits the tie plates in approximately 30 to 60 milliseconds (3% to 7 feet travel distance at 60 mph). This situation is relatively rare in occurrence, and since the completely derailed truck which results is not stable in yaw, it is therefore not likely to represent one of the delayed pile-ups characteristic of two-stage derailments.
Upon local derailment, the truck and suspension geometry gives the springs, backed by the inertia of the 2 car body, a very significant leverage, driving the wheel, axle and journal downward with an initial acceleration of approximately 5g in the case of an empty car and 19g for a fully-loaded car. Vertical velocity of the wheel prior to impact upon the roadbed will be about l9ft/sec for a single-axle drop on a loaded car or 8 ft/sec on an empty car, which would be equivalent to free-fall drops of 66 and 13 inches, respectively.
It is theoretically possible for an integrating accelerometer to sense the velocity change involved in these downward accelerations during actual derailment and probably to differentiate them from low joints, frogs, flat wheels and other normal accelerations and impacts that involve higher accelerations but smaller velocity change. In practice, however, this approach would involve such sensitive devices that acceptable life in the face of the severe vibration environment is not likely, and direct readout in a simple device with the limited operating force available would be a problem. A much more practical event for detecting derailment is impact with the roadbed. In essentially all cases in a drop onto typical mainline roadbed (21-inch tie spacing), the wheel flange or tread will first hit on one or two tie-plates. This is a reasonably reproducible target in hardness and location for analytic studies from which impact parameter estimates may be made to identify and separate the derailment impact signal" from the noise" of normal impacts. The local derailment signals are of approximately half-sine wave shape and of 3 to S-millisecond duration.
From these data and estimates, the derailment roadbed impact in either a loaded car or an empty can be distinguished fairly reliably from normal service conditions by a sensor which actuates when appropriate threshold acceleration and velocity-change criteria are both met.
Presently there are no existing sensors that can distinguish the impact signal of local derailment from normal vibratory noise and produce a usable output to activate a general train anti-derailment system only when local derailment occurs.
SUMMARY OF THE INVENTION Accordingly it is an object of the present invention to provide a new and improved sensor-system which can detect the derailment of train wheels.
Another object of the present invention is to provide a new and improved sensor system which can detect train wheel derailment by distinguishing the particular shock signature associated with wheel derailment from normal shock and vibratory motions.
Another object of the present invention is to provide a new and improved sensor system capable of detecting the derailment of train wheels and generating a signal to automatically stop the train.
Yet another object of the invention is the provision of a new and improved sensor system capable of detecting the derailment of train wheels and generate a signal to automatically activate the trains air brake system.
Still another object of the invention is to provide a new and improved sensor system for detecting train wheel derailment that is reliable, economical and without an excessive false-alarm rate.
Yet still another object of the present invention is the provision of a new and improved sensor system for detecting train wheel derailment using only inexpensive components which are rugged and able to withstand the severe mechanical abuse and adverse environment encountered at the axle journal location in railroad servlce.
Briefly, these and other objects of the present invention are attained in a combined wheel roadbed impact and journal bearing thermal sensor positioned adjacent the wheels of each train axle. Excessive bearing temperatures and/or wheel impact with th attendant prerequisite acceleration forces and velocity changes triggers the sensor which in turn activates an electrical power source. Bearing temperatures higher than a predetermined value causes a heat-responsive release pin to foreshorten, releasing a cocked firing pin to impact upon a percussion-initiated electrical generator. Wheel impact with the road bed displaces an accelerometer mass, causing a ball retainer to free the spring-loaded release pin to separate from the cocked firing pin and activate the electrical generator. The resultant electric signal activates an electroexplosive brake venting mechanism, puncturing and venting the brake line for a full service brake application to stop the train.
BRIEF DESCRIPTION OF THE DRAWINGS A more complete understanding of the invention and many of the attendant advantages thereof will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:
FIG. 1 is a partial view of the sideframe, partly in section, showing the installation of the motion sensor subsystem of the present invention;
FIG. 2 is an alternative embodiment of the sensor of FIG. 1;
FIG. 3 is a partially-sectioned plan view of a train car embodying the anti-derailment system of the present invention;
FIG. 4 is an elevation view of the train car of FIG. 3;
FIG. 5 is a partially-sectioned view of the brake line venting mechanism; and
FIG. 6 shows the bellows actuator of FIG. 5.
DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings, and more particularly to FIG. 1 thereof, there is shown generally a combined roadbed impact and bearing thermal sensor 10 positioned within a bore 16 provided in a standard roller bearing adapter 12. A filler plug 18 of a suitable heattransmitting material, such as aluminum, fills the remaining portion of bore 16 after sensor installation, helps to position the sensor securely and serves to seal out dirt and moisture. Adapter 12 has the customary arcuate surface 20 to conform to the roller bearing. Sensor housing 14 is separated by partition 36 into a lower, larger chamber 22 to receive the impact sensing portion of sensor 10 and a smaller, upper chamber 24 to receive a spring loaded impact member.
The impact-sensing portion of sensor 10 is essentially an integrating accelerometer which requies a combination of a threshold acceleration and the prerequisite velocity change for actuation and is capable of detecting the impact of the wheel on its axle with the roadbed in a local derailment while ignoring normal shock and vibration conditions in service. For these particular conditions, a simple preloaded-spring, seismic-mass system arranged to sense both acceleration threshold and velocity change before actuation is of reasonable size. Escapements, clutter mechanisms or flywheels,
needed in mechanisms required to integrate over longer periods of time, can be eliminated by simply requiring a certain amount of spring compression by the mass, for example, Kr-inch, prior to actuation. The energy thus absorbed is made equal to that represented by the mass of the seismic element and its change in velocity.
The seismic element shown in FIG. 1 includes an upper impact sensor mass 26 and a lower impact sensor mass 28, suitably joined to be displaceable as a unit, such as by screw threads 30. The lower impact mass 28 is substantially a cylindrical shell, open at both ends with the lower portion having a greater thickness than the upper portion. The intermediate area joining the different-thickness portions forms an arcuate step 32, the purpose of which will be considered more fully below. The upper impact mass 26 is of substantially disc shape provided with a central aperture 38, attached to the upper edge of lower mass 28 by means of the screw threads 30, and under the force of accelerometer spring 34 abuts against the partition 36. Impact masses 26 and 28 are able to displace downwardly within chamber 22 under acceleration forces, and to this end a space is provided between the lower impact mass 28 and the base of chamber 22.
A ball release 40 extends through the lower, open end of the impact mass 28 to restrain movement of the accelerometer prior to the wheels undergoing the required acceleration forces necessary to activate the impact sensor 10. The ball release 40 includes an outer, cylindrical support sleeve 42 with a base 46 resting on the lower chamber 22 and an inner, concentric, cylindrical thermal pin collar 44, the bottom of which is closed and provided with a flared 'thro'ugh bore 48. Near the upper open end of support sleeve 42 are a plurality of holes 49 which receive balls 50. An annular recess 52 on the upper portion of collar 44 provides a flared opening of bore 48 so that the downward force of the compressed spring 34 translates into an upward force on the collar 44 which in turn tends to force the balls 50 outwardly against the lower impact mass 28 to immobilize the accelerometer.
The firing pin 58 is slidably positioned within the upper chamber 24 and is restrained against the force of spring 60 by the engagement of pin 54 with groove 56. Positioned in a bore 62 aligned with the axis of the firing pin 58 is a pulse-type thermal battery 64, secured in place by a threaded connector fitting 66. A stab-type percussion primer 68 is positioned in battery 64 directly in line with the firing pin 58. The battery includes alternating layers of a pyrotechnic material contained within the wafer-type cells of the heat pads 70 and an electrolyte, a salt such as lithium/potassium chloride, contained within a disk-shaped electrochemical cell 72. Terminals 74 provide the proper connection with the battery 64 and a resistor 76 joins the terminals 74 to facilitate electrical continuity check during testing of the system.
In the configuration shown in FIG. 1, the size and weight of the impact masses 26 and 28 may be suitably varied according to train car size and cargo loads. Simi larly; the length of spring34 may be varied according to the anticipated acceleration loads. Tests have indicated the adjustment range, as determined by existing car length and loads, to be between 100 to 150g for the preload on the accelerometer spring 34 and a mass travel equivalent to to ft/sec velocity change for the impact masses 26 and 28.
Thermal release pin 54 may be made from 55-nitinol, the generic name for a series of nickel-titanium intermetallic compound alloy having a unique memory property developed by the Naval Ordnance Laboratory. Nitinol alloys have chemical composition in the range from about 53 to 57 weight percent nickel and the balance titanium. The memory properties are such that, given the proper conditions, Nitinol objects can be restored to their original shape even after being permanently deformed out of that shape. The return to the original shape is triggered by heating the alloy to a moderate temperature. The use of Nitinol in a thermal release pin for a temperature detector is disclosed more fully in the copending application of J. H. Armstrong and F. C. Kluge, Ser. No. 495,478, filed Aug. 7, 1974. The composition and properties of Nitinol are described more fully in US. Pat. No. 3,174,851, issued March 23, 1965.
Briefly, the steps in imparting a shape memory to a Nitinol article include: forming the alloy into the shape that it will be called upon to remember, i.e., its memory configuration; heat treating the Nitinol shape while it is constrained in a fixture and subsequently cooling it below the transformation temperature range; and then straining the part to an intermediate shape, which is the shape that the part is to retain until it is heated to restore it to the memory configurations. The temperature to which the part must be heated in order to return it to the memory configuration depends upon the chemical composition of the alloy. This is described more fully in US. Pat. No. 3,558,369, issued Jan. 26, 1971. Thus the Nitinol release pin 54 would have a heat-initiated, shrunken memory length sufficient to release the firing pin 58, and from this length the pin would be strained longitudinallyto be just long enough to restrain the firing pin in the cocked position of FIG. I.
In operation, the combined thermal detector and wheel impact sensor 10 of FIG. 1 is actuated either when the journal bearing overheats and the temperature detected exceeds that determined by the characteristics of the Nitinol thermal release pin 54 or when the acceleration threshold and velocity change of the accelerometer are exceeded by a wheel striking the roadbed. If the journal bearing temperature exceeds the limit of the Nitinol pin 54, it physically shrinks, disengaging itself from groove 56 in the firing pin 58. The cocked firing pin 58 impacts upon the stab primer 68 in the thermal pulse battery 64 to produce a flame which ignites the heat pads '70. Burning of this material produces sufficient heat to melt the electrolyte within the electrochemical cells 72. Once the electrolyte is melted and its ions released, normal electro-chemical action generates a voltage until the active materials are depleted or cooling resolidifies the electrolyte.
Upon impact with the roadbed of a derailed wheel, upward acceleration of the sensor 10 above a value corresponding to the preload of the accelerometer spring 34, results in movement of the upper and lower impact sensor masses 26, 28 as a unit downward with respect to the balls 50 which are locking the thermal pin collar 44 in position. After travel corresponding to the threshold velocity change, the step 32 inside the lower mass frees the balls to move outwardly, releasing the thermal pin collar 44. The accelerometer spring 34 than pushes the thermal pin 54 and its collar downward, releasing the firing pin 58 and initiating the same action as the thermal sensor function of the thermal pin 54'.
The arrangement shown in FIG. l is as it might be built for experimental service to determine the proper settings for detecting the largest practical proportion of local derailments without encountering an excessive false-alarm rate. The two-piece sensor mass allows adjustment of the velocity change (AV) threshold independentiy of the acceleration threshold. For mass production, a one-piece mass as shown in FIG. 2 might be used.
The embodiment of FIG. 2. uses a Nitinol thermal release pin 54 having a flat, enlarged head 78 received in a recess provided in the one-piece impact mass 80. A spring 34 biases upwardly both the release pin 54 and the impact mass 80, with the mass 80 being received in a bore 82 provided in the adaptor 12 and the release pin 54 extending into a firing pin-receiving-bore 84 to restrain the firing pin 58 against spring 60. The open end of bore 82 is closed by a cover 86 and a filler plug 18 seals the bore. The operation of the release pin 54' and the one-piece mass 80 is identical to the corresponding elements of FIG. I.
An alternative power source is also shown in FIG. 2 wherein a piezoelectric crystal. 8% replaces the thermal pulse battery 64. It should be noted that both the thermal battery and the piezoelectric crystal are known to those skilled in the art and are briefly summarized for completeness of description. Crystal materials, such as lead zirconate/lead titanate sintered elements, electrically polarized to obtain the proper stress-output axis, are crushed by the explosive force of a detonator 90 to produce a relatively high voltage of short duration. In a typical low-resistance output circuit the current is approximately 35 amperes. Since there is a race between generation of a large electrical output from the extremely high explosive-generated pressures on the crystal and its termination by destruction of the electrical continuity of the output, the aluminum waveshaper 92 is used to strike a balance between these opposed events.
The power source shown in FIGS. 11 and 2 may be used interchangeably with the accelerometer/thermal sensor combination of these figures. Additionally, if the thermal sensor function is not desired, the Nitinol thermal release pin may be replaced with an ordinary release pin insensitive to temperature.
FIGS. 3 and 41 show the plan view and elevation view, respectively, of a train car 941 provided with" the brake actuation subsystem of the present invention. The impact/thermal sensors 10, each with the associated power source 64, are electrically connected by a shielded conductor system 96, such as sheathed or ,armored cable, to the brake line venting mechanism 108. The trains brake line 98 extend the length of car 94 and terminate in end couplings 100. Connected to the train line 98 are the brake valve 102, brake cylinder 1 and brake reservoirs 1106, elementscommon to a trains brake system and known in the art.
Positioned on the brake pipe 110 joining the brake valve 102 to the train line 98 is the brake line venting mechanism 108, shown more fully in FIG. 5.. The venting mechanism includes shield I12 surrounding a diaphragm cutter having a cylindrical housing ll4g an explosively-driven bellows motor actuator 116 connected to the electrical conductors 96 positioned at one end of housing 114; a slidably-mounted cutter 118 disposed adjacent the actuator 116; a shearable diaphragm 120 positioned adjacent the other end of housing 114 to separate the housing from the internal passage of brake pipe 110; an annular passage 122 provided in the housing 114 to permit passage of air from the brake pipe 110 after diaphragm rupture; and a calibrated venting orifice structure 124 to vent the released air. Also visible in FIG. is the dirt chamber 126 and the cut-out cock 128, elements common to train brake systems. The shield 112 around the diaphragm cutter, as well as the shielding around conductor 96, serves to prevent interference from stray electromagnetic radiation and to provide mechanical protection against the severe environment existing beneath the train car. The diaphragm cutter may be similar to that disclosed in copending application Ser. No. 465,400, filed Apr. 29, 1974, and the explosive-type cutter actuator described therein may be used in place of the bellows motor actuator 116. i
Details of the bridge-wire bellows motor actuator 1 16 may be seen in FIG. 6, wherein the wires 130 of the shielded conductor 96 are positioned against a propellant 132 contained in cup 134, the ends of wires 130 being joined by a fine bridge wire 136 embedded in the propellant. Wires 130 are suitably insulated with insulating material 138, and cup 134 is sealed with a plug 140 of glass, plastic or other suitable material. Bellows 142 is pleated from suitable malleable, ductile metal, such as copper, with the forward end formed into a blunt nose 144 and the edge of the aft, open end crimped over the seal plug 140. Approximate this open edge, the bellows 142 is provided with an outwardlyextending ridge 146, which receives a similarly-shaped ridge formed on the propellant cup 134 to properly position the cup.
The operation of the bellows motor actuator and the venting mechanism 108 can be readily seen from the foregoing description. Briefly, the propellant 132 is ignited by the signal generated by the thermal battery 64 or the piezoelectric element 88, as set forth above, the expanding gases forcibly extending the bellows 142, causing the blunt nose 144 to contact and displace cutter 118, which in turn severs diaphragm 120 to release the air from brake pipe 110, this slowing and eventually stopping the train. The escaping air flows out through passage 122 and the venting orifice 124. As the air flows through orifice 124, a distinct, audible sound is produced to help the train crew locate the car which has been braked and to determine what possible derailment condition has actuated the system. This permits remedial action prior to any actual derailment. Additionally, the actuation of the brake system can be monitored from a central location, such as the engine cab.
Bellows 142 a sufficiently rigid after expansion to prevent cutter 118 from being forced by air pressure back through the ruptured diaphragm and possibly obstructing the flow. To further assure free air flow, cutter 118 may be hollow with an opening 148 therein to permit unobstructed flow between brake pipe 110 and annular passage 122.
Obviously, numerous modifications and variations of the present invention are possible in the 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.
What is claimed as new and desired to be secured by Letters Patent of the United States is:
1. A motion and temperature sensor system for detecting train wheel local derailment and actuating the brake system comprising:
a motion sensor activated by wheel impact with the roadbed;
a thermal sensor responsive to journal bearing temperature exceeding a predetermined value;
an output signal generator operatively coupled with and activated by said motion sensor and said thermal sensor; and
an electroexplosive means coupled to said signal generator for venting the brake system,
whereby the train brake system is applied to stop the train upon detection of wheel impact with the roadbed or journal bearing temperature exceeding a predetermined value.
2. The sensor system of claim 1 further comprising an impact member and a biasing spring, said impact member being released by said motion sensor and said thermal sensor to activate said signal generator.
3. The sensor system of claim 2 wherein said thermal sensor comprises a heat responsive element which physically deforms at temperatures above a predetermined value.
4. The sensor system of claim 3 wherein said heatresponsive element comprises an intermetallic alloy of nickel and titanium.
5. The sensor system of claim 4 wherein said heatresponsive element comprises an elongated member extending adjacent said impact member to restrain said impact member against said biasing spring.
6. The sensor system of claim 5 wherein said motion sensor comprises:
a displaceable mass; and
a spring for maintaining said displaceable mass in a first position, said displaceable mass being moveable from said first position for wheel motions above a predetermined acceleration threshold and velocity change.
7. The sensor system of claim 6 wherein said displaceable mass is provided with an opening, said elongated heat-responsive member extends through said opening, and said spring maintains said displaceable mass in said first position and said heat-responsive member against said impact member.
8. The sensor system of claim 7 wherein said electroexplosive means comprises an explosively-actuated diaphragm cutter to puncture and vent the train brake line.
9. The sensor system of claim 8 wherein said diaphragm cutter is actuated by an explosively-extended bellows motor.
10. The sensor system of claim 9 further comprising a calibrated venting means connected to said diaphragm cutter capable of producing an audible signal upon brake line venting.
11. The sensor system of claim 8 wherein said outpu signal generator comprises a thermal battery activated by said impact member.
12. The sensor system of claim 11 wherein said thermal battery comprises:
a pyrotechnic reactant;
an-electrochemical reactant; and
9 a percussion primer triggered by said impact member to initiate an electric current producing reaction between said pyrotechnic reactant and said electrochemical reactant.
13. The sensor system of claim 8 wherein said output signal generator comprises a piezoelectric element initiated by said impact member.
14. The sensor system of claim 13 wherein said piezoelectric element comprises a detonator initiated by said impact member and piezoelectric crystals crushed by pressure from said detonator to produce an electric current.
15. The sensor system of claim 8 further comprising releasable restraining means for maintaining said displaceable mass in said first position.
16. The sensor system of claim 15 wherein said releasable restraining means comprises a ball release whereby a plurality of balls are movable between a restraining position and a release position.
17. The sensor system of claim 16 where said ball release comprises:
a first cylindrical sleeve;
a plurality of apertures provided in said first sleeve;
a plurality of balls positioned in said apertures and contacting said displaceablemass;
a concentric, inner cylindrical sleeve in contact with said plurality of balls and supporting said spring to exert an outward force on said balls; and
a receiving recess in said displaceable mass located adjacent said plurality of balls,
whereby said displaceable mass is maintained in said first position by the force of said plurality of balls, and upon acceleration of said displaceable mass by wheel impact with the roadbed, said mass is displaced downwardly to align said receiving recess with said balls, releasing said balls from the restraining position and permitting said heat-responsive member to release said impact member.
18. The sensor system of claim 17 wherein said output signal generator comprises a thermal battery activated by said impact member.
19. The sensor system of claim 18 wherein said thermal battery comprises a pyrotechnic reactant;
an electrochemical reactants; and
a percussion primer triggered by said impact member to initiate an electric current producing reaction between said pyrotechnic reactant and said electrochemical reactant.
20. The sensor system of claim 17 wherein said output signal generator comprises a piezoelectric element initiated by said impact member.
21. The sensor system of claim 20 wherein said piezoelectric element comprises a detonator initiated by said impact member and piezoelectric crystals crushed by pressure from said detonator to produce an electric current.