GB2415261A - Cable gripping current sensor - Google Patents

Abstract

A sensor 100 for detecting current flowing though an insulated conductor comprises a housing hinged 104 at one side, a portion of a ferrite structure 106 in each part of the housing, a Hall Effect device 108 magnetically coupled to the ferrite structure, securing means 118a, 118b to lock the housing closed around and engaged with the insulation of the conductor and a screened wire 126 to transfer sensed signals to remote conditioning circuitry. The Hall sensor may be disposed between the two portions of the ferrite core or may be located in a slot 120 in one of the portions. The conditioning circuit may reduce the dc component of the sensed signal and digitise it. A window comparator may be used in processing the signal and a warning may be issued if the output indicates a fault. The sensor may be used to detect current flow in the firing circuit of a vehicle airbag.

Description

241526 1

ELECTRONIC SENSOR

The invention relates to a sensor for engaging onto and around an insulated wire of a crash test vehicle, a sensor head for the measurement of a control signal in a circuit of an on-board electrical system of a crash-test vehicle and to a method of crash-testing a vehicle.

Crash testing may be carried out on many different types of vehicle as a key part of development to rate a vehicle's safety performance. One major need for crash testing is for cars and similar, because new models need to be assessed before release to the market. The invention may be used in crash testing of cars or the like, but may also be applied to other situations where there is a need to non-intrusively sense current flow.

When testing cars, data acquisition units in the car are used to record an accurate log of information relating to car conditions during the crash conditions. One specific, although no-limiting situation that needs to monitored is the airbag trigger circuitry. During a high-speed impact, it is the responsibility of the airbag control module to decide whether and when to deploy the airbag or airbags. The module is able to come to this decision based upon the information it receives from various accelerometers located around the vehicle.

A trigger signal generated by the control module and sent to the airbag must be such as to be able to burn out a resistor (the "squid") located within the airbag module, thereby igniting an explosive charge and deploying the airbag. During a crash test it is important to determine the point in time at which the signal is received by the airbag from the control module. If the airbag fails to deploy in the correct manner at the correct time during a crash test, it could be that the control module has failed to send a signal at the correct time or at all, or the harness is damaged and fails to convey the signal from the control module to the airbag module. It is also possible that the airbag itself could be at fault. It is therefore necessary to monitor the current at a predetermined point in the harness (often at, or close to, the electrical terminals of the airbag module) to determine when the signal is received with reference to the time of crash.

At present, one technique to use one or more current probes, as shown in Figure 1, to monitor the airbag harness and thereby determine the time at which the airbag firing signal is received.

Such probes 10 have a jaw portion 12 with an aperture 14 through which the wire or cable (not shown) to be monitored passes. The jaw portion 12 is openable by a jaw actuator 16 to allow positioning around the wire or cable.

When the jaw actuator 16 is released, the jaw portion 12 closes around the wire by virtue of a spring in the body portion. A current sensing transducer (not shown) is associated with the jaw portion 12 to sense current in the wire by detection of the magnetic field surrounding the wire due to current in the wire.

Conditioning circuitry in the probe "cleans up" the output of the transducer to provide an output indicative of current flow through the wire of concern.

The reliability of a probe 10 may be poor at high accelerations ("high g") and, as vehicle safety systems develop, the number of airbag/pretensioner channels is increasing with some car manufacturers already using two-stage devices thereby requiring an increase in the number of sensors required and the space required to accommodate them. As they are relatively large it is unlikely that they can be used in "tight" areas, such as behind vehicle trim. This may therefore undesirably constrain the test engineer.

Also, probes of this type are quite heavy, around 250 grams for the probe itself, and under high g force, their inertia is sufficient that they are prone to move significantly during crash conditions. Therefore, a substantial amount of time is required to securely fix the sensors in the desired position before a test to ensure that it does not come loose under crash conditions andlor interfere with any other test equipment. Even then, some movement is prone to occur.

Movement is undesirable due to the possibility of damage to other vehicle components, failure of the sensor, or merely poor results.

The cost of probes of this general type is high and this makes it important to retrieve the probe(s) after use. The act of retrieving the probe may however disturb the wiring, for example behind the dashboard, when it would be desirable for the wiring to be left undisturbed.

Further, the insulated wires of the airbag harness usually has a diameter of around 1- 3 mm and is therefore very much smaller than the diameter of the aperture 14 of the jaw portion 12 which is typically 20 millimetres. Referring to Figure 2, it has been found possible to modify a wine bottle cork 22 so that the wire 24 runs through the middle of the cork 22 whilst the jaw portion 12 of the probe 10 grips the cork in place. One problem with this is that the magnetic field produced by the current in the wire is most dense close to the wire and, as the sensor is necessarily somewhat removed from the wire, the response of the sensor may be of reduced magnitude, thereby requiring careful scrutiny of test results. Another is that it is often necessary to apply the current sensor probe in awkward or inaccessible locations, and the need to manipulate a cork may add to the difficulty.

Once the airbag harness is located, heavy duty tape (not shown) is used to clamp the jaws of the jaw portion 12 closed, to assist with the springbias of the jaws of the jaw portion 12. Despite this, the jaws can still rattle and vibrate during an impact especially under the g-forces arising out of crash conditions.

Such vibrations can create noise in the sensor which is apparent on the output signal of the sensor.

A purpose of monitoring the wire is to detect the airbag firing pulse. Figure 3, shows a comparison between a typical desired result in Figure 3a and the type of result which is sometimes obtained using the probe and arrangement of Figures 1 and 2. Figure 3a depicts a desired monitored signal 30 over a period of time, the detected airbag firing pulse 32 also being shown. Figure 3b shows an actual monitored signal 34 which, as can be seen, comprises a substantial noise component and is quite different from the desired signal of Figure 3a. It may still be possible to obtain some form of useful information the actual result of Figure 3b - closer inspection of section 36 of the signal 34 depicts a section 38 comprising a detected pulse 40 - but this is time consuming. It can also be difficult to assess the accuracy of the derived results.

As the requirement for monitoring, during crash test conditions, of an ever increasing number of channels in multiple safety systems of the crash-test vehicle comes about, so the importance of having smaller and more reliable current sensing devices increases. Further, it is also desirable to achieve a better-quality signal as desired and as illustrated in Figure 3a.

In an attempt to minimise the size of the sensor, miniature clamp-on ammeters have been developed. However as conditioning circuitry for the sensor signal is provided within the sensor head of these devices, they remain heavy - with the associated inertia under crash-test conditions and relatively expensive.

Furthermore, given the relative expense of these clamp-on ammeters, time is required to remove them from the vehicles after crash tests in order that they may be used again.

Aspects of the invention are set out in the independent claims. Some additional features of the invention are included in the dependent claims.

By providing a sensor having no on-board conditioning circuitry, the cost and the inertia of the sensor is significantly reduced. Indeed, as the expense of the sensor element has been significantly reduced, they can be considered "disposable"; there may be no need to spend time removing them from vehicles after the tests, and in turn the ability to leave the sensor in place reduces the impact of the sensor on the vehicle by reducing further disturbance In embodiments where the sensor head clamps onto the insulation of a wire of the circuit to be monitored, the sensor head can remain immobile on the wire during crash conditions in which the sensor head can be subjected to deceleration forces of more than 40g. This means that the location at which the test is performed can be determined.

In embodiments where the sensor head merely engages the insulation of a wire of the circuit to be monitored, the sensor head can move along the wire during crash conditions and thus the physical impact of its inertia on the wire and the circuit components is reduced.

Provision of in-line conditioning circuitry has advantages over the inhead circuit, since for example, it is possible to select a region of the vehicle which is "safe". Since the region may be an accessible region, the conditioning circuitry may include indicators to enable the test engineer to confirm that power is available, that the sensor is correctly installed etc. Embodiments of the invention will now be discussed, by way of example only, and with reference to the accompanying drawings in which: Figure l is an illustration of a known sensor head utilised in crash tests; Figure 2 is an illustration of the manner in which the device of Figure 1 may be secured around a wire to be monitored; Figure 3 is a diagram demonstrating a comparison of the desired results from the known sensor head and that which is sometimes obtained; Figure 4 is a diagram illustrating a sensor embodying the present invention; Figure 5 is perspective view of a housing for a sensor head embodying the present invention; Figure 6 is a schematic diagram of a conditioning circuit embodying the present invention; Figure 7 is a schematic diagram of a battery voltage monitor; Figure 8 is a circuit diagram of a second battery voltage monitor; Figure 9 is a schematic diagram of a warning system monitor useful with the present invention; Figure 10 is an oscilloscope trace illustrating the reaction of the probe of Figures 1 and 2 to an airbag fire signal; and Figure 11 is an oscilloscope trace illustrating the reaction of a probe embodying the invention to an airbag fire signal.

With reference to Figure 4 a sensor 100 has a generally cylindrical housing 102, here of plastics, having a diameter around three times the axial height of the cylinder. The housing 102 is divided generally diametrically into a first housing portion 102a and a second housing portion 102b, which are thus substantially half cylinders. In this embodiment the housing portions 102a, 102b are injection moulded and extend one into the other via a web 104 of plastics material interconnecting the portions along corresponding height parallel edges. The web 104 is of a thickness so as to provide flexibility and allow the housing portions 102a, 102b to open and close, acting as a hinge 104.

Cut-outs 114a, 114b are disposed so that when the two housing portions are closed to form a cylinder, the cut-outs form holes 112 in the opposing major faces of the cylinder, the holes 112 being concentric with the cylinder. The second housing portion 102b has a height-parallel slot 120 at a location remote from its ends, and a second slot 118b at a location near to the end distal to the hinge web 104. The first housing portion 102a has a hook portion 118a extending from the end distal to the hinge web 104. The hook 118a has a first portion that is a continuation of the curved wall of the housing portion, and a second inwardly-directed portion. The hook portion 118a and the second slot 118b together form a catch.

The housing 102 is generally hollow and defines a circular-cylindrical enclosure. The enclosure houses a ferrite core 106 that in this embodiment has an annular plan, and hence a central axial aperture 116, and a rectangular cross section. In other embodiments, a toroid or other shape may be substituted. The ferrite core 106 is used in conjunction with a Hall-effect sensor 108 to afford non-intrusive current sensing means for monitoring the current in the wire.

The core has two similarly-dimensioned parts in this embodiment. In the view of Figure 4, only one part is shown for the sake of clarity.

The Hall-effect sensor 108 is disposed in a slot (not shown) in the underside of the ferrite core 106 through the first slot 120 in the second housing portion 102b in the direction 122 as shown, to thereby become magnetically coupled with the core 106.

The Hall-effect sensor has three terminals 124 connected to a connecting cable 126 having individual conductors 128 and a screen and/or overall sheath 130.

In other embodiments of the invention, the individual wires 128 are have individual screens (not shown). The cable 126 may be chosen to be highly flexible where necessary. The wire has an end connector 132 for connection to a conditioning circuit, which will be described below.

The holes 112 in the opposing end walls and the aperture 116 of the core allow a conductor (not shown) to pass through the sensor 100 so that a current through the conductor can be sensed. The split structure of the sensor 100 allows positioning of around the conductor (not shown for the sake of clarity) after which the two halves are closed to engage onto and around the conductor.

In the vehicle art, the conductors are insulated, and the holes 112 in the present embodiment are dimensioned to lightly engage the insulation. It will seen that the conductor will, in use, be at least substantially surrounded by the ferrite core 106 through aperture 116 (half of which is shown) of the ferrite core 106.

In this embodiment, the cut out portions 114a and 114b engage onto the insulation of the wire sufficiently to allow the sensor to move by sliding along the wire.

The ferrite core 106 may alternatively have any suitable shape or form other than a toroid.

The catch 118a, 118b holds the distal regions of the two portions 102a, 102b in tight engagement so that the housing acts as though it were an integral body.

Other embodiments use other securing means, such as a clamping device around the periphery of the housing, a cable tie, glue.

In other embodiments, packing of a suitable material such as a rubber grommet (not shown) may be used to ensure a tight fit of the sensor log on the wire, if that is required.

To use, the ferrite core 106 is positioned into the slot 110 in the second housing portion 102b and placed in the desired position around the wire. The second part of the ferrite core 106 (not shown) is placed over the conductor so that the ferrite core in this embodiment at least substantially encircles the wire and the housing 102 is closed by fixing the catch 118a, 118b. The closing action provided by this catch together with the size of the holes 112 is sufficient to retain the sensor head on the wire under the g-forces arising out of crash conditions.

Further securing means may also be provided.

Referring now to Figure 5A, an alternative housing 150 for housing a core 206 generally similar to core 106 has first and second housing portions 152a, 152b having a circumferential surface 154. The surface 154 is bounded by two opposing circumferentially-extending flange lips 156 of the housing to define a pathway for retaining a web (not shown) or similar around the housing. The web may form a securing means, for example a cable tie disposed around the periphery of the housing 150 and pulled tight. The provision of such securing means ensures the sensor remains secure.

With continued reference to Figure 5A, the housing portions 152a, 152b are each generally semicircular in plan, and each has a respective pair of edges 152c, 152d that runs generally parallel to a diameter of the circle defined by the portions 152a, 152b when disposed to form a circle. The portions 152a, 152b are connected together via a hinge 160 having a pivot point 153 offset outwardly from the respective diameter-parallel edges 152c,152d so that when the sensor head 150 is closed until the edges are mutually parallel, they are spaced apart- see Fig 5B.

The ferrite core 206 of this embodiment is in two parts, and is disposed in the housing enclosure of the housing so that end portions 206a, 206b project circumferentially from the distal ends 152e, 152f of the housing. On the other hand, the disposition is such that in the pivot region the core end portions 206c, 206d are substantially level with the diameter-parallel edges 152c, 152d. The projecting end portions 206a, 206b extend such that when they are in mutual engagement, the edges 152c, 152d are parallel. Hence this leaves a gap at the pivot end between the ends 206c, 206d. This gap is selected to correspond to the size of the Hall-effect sensor (not shown), which is secured in this location during manufacture.

The edges 152c, 152d have cut-outs forming a hole 158 when closed corresponding with an aperture of the core 206 so that a conductor (not shown) can pass through the sensor housing 152 through aperture 158.

The output of the Hall-effect sensor includes do offsets. The output is processed by conditioning circuitry to remove the offset and to provide logic level outputs after window thresholding to reduce noise effects. The output of the conditioning circuit is fed to data acquisition circuitry, which may be of a convention type. Diagnostic devices may be provided to allow checking/monitoring of the operation of the elements of the system. An exemplary conditioning circuit 650 is illustrated in Figure 6.

Conditioning circuit 650 has a positive voltage rail 652 at +5 volts and a negative voltage rail 654 at-5 volts. The circuit 650 has three stages 658,668 and 672. An input terminal 656 connects the Hall sensor signal to the first stage 658 of the conditioning circuit 650 which is a buffer stage acting as voltage follower. The buffer is an operational amplifier 660, with direct connection of output to inverting input, thus providing a high impedance buffered signal 659. The buffered signal 159 forms the input to the second stage 668, the second stage 668 serving to remove do offset. The second stage has a shunt circuit 662 having a resistor 664 having one node receiving the signal 659, and having its other node connected via a capacitor 666 to the negative rail 654.

The common node of the resistor 664 and the capacitor 666 is connected via a resistor 667 to the inverting input of a second operational amplifier 670, receiving the signal 659 at its non-inverting input: negative feedback from the output 673 is provided by a further resistor 671.

The third stage 672 is a window-comparator stage 672 receiving the output 673 of the second operational amplifier, and applying it to the noninverting input of a third operational amplifier 674 and to the inverting input of a fourth operational amplifier 676. The inverting input of the third operational amplifier 674 and the non-inverting input of the fourth operational amplifier 676 are connected to taps of a potential divider 675 having three resistors 675a, 675b, 675c connected serially across the rails 652, 654. The first resistor 675a connected between the positive rail 652 and the inverting input of the third operational amplifier 674 and the third resistor 675c between the non-inverting input of the fourth operational amplifier 676 and the negative rail 654 are of like value. Typically the second resistor 675b has a value between 1/lOth and 1/20th the value of the first resistor 675a, so that the threshold window is around 1/20th to 1/40th the supply rail potential difference. For a 10 volt supply with R67sa = R67,c =15K and R67sb =1K, this gives a noise threshold of around 0.32 volts.

The outputs of each of third 674 and fourth 676 operational amplifiers are connected via respective diodes 678, 680 to an output node 679. The output node 679 voltage is then halved via a potential divider 682 having resistors 684 and 686 to provide 5 volt output signal 688. The voltage output signal 688 is use, fed to data acquisition circuitry (not shown).

In this embodiment of the invention, the conditioning circuit has an indicator sub-circuit 690 which indicates the presence of battery power across the supply rails 652, 654, and also indicates a fault in the Halleffect circuit. The sub circuit 690 has a first green LED 689 connected between the rails 652, 654 to be illuminated so long as the 10 volts is present across the rails. A second window comparator 691, having a potential divider 693 across the rails, fifth and sixth operational amplifiers 692, 694 and respective diodes 695, 696 is connected so that the non-inverting input of the fifth operational amplifier 692 and the inverting input of the sixth operational amplifier 694 receive the signal at input node 656. The diodes 695, 696 are connected to a red LED 698 which is thus energised if the Hall effect sensor signal is other than close to O volts (midway between the supply rails 652, 654) -close to 0 Volts is the output voltage it should be if correctly connected and not faulty.

As shown in Figure 7, a bakery voltage monitoring circuit 200 can be connected to monitor the state of the crash-test vehicle bakery, or the instrumentation bakery. Such a circuit offers the advantage that, if the airbag system would not deploy correctly during a crash-test, this situation can be established prior to the test, thereby leading to less wasted time and cost.

The circuit 200 has a power rail 201 and a ground rail 202 powered at 12V from the vehicle battery (not shown). The circuit 200 has a potential divider 211 across the rails 202, with a tap 212 to the non-inverting input of an operational amplifier 203. The inverting input is derived from a second potential divider 205 connected across a regulated 5V supply 209 derived from the input power via a 5V regulator 207. The output of the operational amplifier 203 is connected to the anode of a green LED 204 having its cathode connected to ground, and to the cathode of a red LED 208 having its anode connected to the regulated supply 209. Hence, if the vehicle battery state is healthy, green LED 204 is illuminated by a positive going output from the operational amplifier 203. If the battery voltage falls below a certain value, (10.5 V in the present example), the operational amplifier 203 provides a low output and the red LED 206 is illuminated. If no battery is present (eg battery disconnected) neither LED is lit. The output of the operational amplifier 203 is fed to an opto isolator 210 to allow the monitor signal to be passed to a warning lamp control circuit (see later herein) Turning to Figure 8, a generally similar circuit 300 is supplemented by a second operational amplifier 303 having a non- inverting input connected to the tap of the potential divider 205. The inverting input is connected to the tap of a second potential divider 305 which has an LDR 302 connected to the 5V rail 209 in series with a variable resistor 306 connected to earth. When the LDR is unlit, it is high resistance and the inverting input of the second operational amplifier 303 is low, causing the output to go high. The output is coupled to the cathode of a second red LED 305 having its anode connected to the 5V rail 207, so that so long as the LDR is un-illuminated, the second red LED 305 is off. If however a light (eg a warning light of the vehicle) close to the LDR 302 becomes illuminated, the LDR 302 goes low resistance, the operational amplifier has a low output and the second LED 305 becomes on.

Referring to Figure 9 a warning light circuit, 250 receives inputs from monitoring circuits 200, 300 and provides an externally visible or audible warning when a fault occurs. Opto-isolators are used to avoid earth communing that could give rise to earth loops and hence noise. Even if the crash-test vehicle has been set up and all of the technicians are well away from the vehicle, (for example in a crash-test control room), the circuit indicates that a fault has occurred, giving ample opportunity of stopping the test. The warning may be via a lamp (400) or another signalling device such as siren.

The circuit 250 as shown has four inputs 252-5 from battery monitor circuits 200, 300. As noted above some of these 252, 253 are opto-isolated The inputs 252-255 are fed to a common line 256 that forms the inverting input of an operational amplifier 260 whose non-inverting input is derived from the tap of a potential divider 261 across input supply rails 270,271. The common line 256 is connected via a pull-up 262 to the positive rail to ensure that the default state is "fail" The output of the amplifier 260 drives a power transistor 263 operating the lamp 400 in this embodiment. The lamp is typically mounted on the roof of the vehicle so as to be highly visible.

A sensor embodying the present invention was found to provide a significant improvement over the sensor 10 of Figure 1. For example, and as illustrated in Figure 10, there is shown an oscilloscope trace of the reaction to an airbag firing signal by the probe of Figures 1 and 2. As can be seen, the reaction time of the probe of Figures 1 and 2 is more than 0.5 mS from the point 300 of the airbag firing pulse and the magnitude of the response is relatively small. On the other hand, and as shown in Figure 10, the reaction time of a sensor according to the present invention, is better than 0.3 mS from the point 302 of the airbag firing pulse with a much more discernible response magnitude.

It will be understood that the present invention has been described purely by way of example and that modifications of detail can be made within the scope of the invention. Features or aspects of the invention may be provided in any appropriate combination.

Claims (14)

1. Claims 1. A sensor for engaging around an insulated conductor of a crash

   test vehicle, the sensor comprising: a ferrite structure disposed in a housing; a Hall-effect device having output terminals and being magnetically coupled with the ferrite structure, wherein the housing and the ferrite structure define a through hole for a said conductor, and are connected at a hinge location to allow portions thereof distal to the hinge location to be separated to allow a said conductor to be introduced into the through hole; a securing device operable to secure the said distal portions together to maintain the housing closed about the said conductor; and a screened wire connected to terminals of the Hall-effect device for feeding a signal from the Hall-effect device to remote conditioning circuitry,
2. 2. A sensor according to claim 1, wherein the housing is adapted to engage around the insulation of the said conductor, whereby during high g conditions the sensor can slide along the conductor.
3. 3. A sensor according to claim 1, wherein the housing is adapted to lockingly engage onto the insulation of the said conductor, whereby during high g conditions the sensor remains immobile with respect to the conductor.
4. 4. A sensor according to any preceding claim, wherein the housing has an outer surface for a circumferential securing means.
5. 5. A sensor head according to any preceding claim, wherein the housing is made from plastics material
6. 6. A sensor according to any preceding claim, wherein the ferrite structure and the housing each have two respective portions.
7. 7. A sensor according to any preceding claim, wherein the Hall-effect sensor is disposed within a slot in the ferrite core.
8. 8. A sensor according to claim 6, wherein the Hall-effect sensor is disposed between the two portions of the ferrite structure
9. 9. A conditioning circuit for a sensor according to any preceding claim, the circuit comprising an input arranged to receive an output signal of the Hall effect device, and operable to provide an intermediate signal with reduced do component, and to digitise the intermediate signal.
10. 10. A conditioning circuit according to claim 9, having a window comparator receiving the intermediate signal.
11. 11. A conditioning circuit according to claim 9 or 10, and having circuitry for receiving the output of the Hall-effect device and for providing a warning if the said output is at an incorrect level
12. 12. A conditioning circuit according to claim 1 1, wherein the said circuitry comprises a second window comparator driving an LED
13. 1 3. A conditioning circuit according to any of claims 9 to 11, having power rails for receiving power, and an indicator to show that power is being received thereon. r
14. 14. A method of crash-testing a vehicle using a sensor as claimed in any of claims 1 to 8 together with a conditioning circuit according to any of claims 9 to 13, comprising engaging the sensor onto and around an insulated conductor of the crash test vehicle by separating the said distal portions to allow the said conductor to be introduced into the through hole; closing the said distal portions together so that the conductor becomes tapped in the through hole, and using the securing device, securing the said distal portions together to maintain the housing closed about the said conductor; connecting a distal end of the screened wire to the conditioning circuit; disposing the conditioning circuit remote from the sensor; and connecting the output of the conditioning circuit to signal acquisition circuitry.