CURRENT PROBE

Abstract: A current probe is provided which comprises a handle (1) and a ring (7,8) of magnetisable material for positioning around an electrical conductor, the ring being split into two portions (13,14) one being fixed relative to the handle means and the other being slidable therewithin between a position in which the ring is closed and a position in which the ring is open. The line of split of the ring is at an angle from the line of sliding movement of the linearly slidable portion. Damping means (27,28,29) are provided to prevent the slidable portion (14) of the ring from too rapidly moving from a position in which the ring is open to a position in which the ring is closed.

Description: This invention relates to a current probe of the type used for remotely detecting electrical current flowing through a conductor which may, for example, form part of a wiring loom of an automobile. One conventional type of current probe utilises the Hall-effect and is disclosed in U.K. patent 2099159B. The probe has a body portion adapted to be gripped by the hand of an operator and at one end of the body portion is hingeably mounted a pair of jaws which together define a ring of magnetisable material. The jaws are movable between an open position in which they can be placed over an electrical conductor and a closed position in which the jaws meet around a conductor and the ring is substantially complete. Whilst this type of probe is perfectly satisfactory for measuring current flowing in an electrical conductor which is readily accessible, it will be appreciated that where the electrical conductor is disposed in a confined space or is amongst several other conductors, it can be difficult, if not impossible, to attach the probe satisfactorily since the opened jaws of the probe can become snagged. A further problem arises where the electrical conductor is in such a position that in order to take a satisfactory measurement, the conductor must be pulled towards the operator. With a probe having hinged jaws this can lead to slippage and although this can be overcome to some extent by e.g. incorporating a strong spring within the probe to permanently bias the jaws into a closed position, nevertheless it will be appreciated that this arrangement is far from satisfactory since it then makes the jaws difficult to open unless a mechanical advantage is incorporated into the trigger mechanism. The above problems are overcome to a limited extent by another probe known in the art which has, instead of hinged jaws, an arrangement in which a rectangular ring of magnetisable material is mounted on the end of a handle-shaped body. One side of the ring is split from the remaining three sides and is slidable within the handle of the probe to thereby permit access into the ring of a conductor. The slidable side of the ring is urged back to a position in which the ring is complete by means of a compression spring held within the handle. One problem with this second kind of probe known in the art is that the biassing spring acts only along the line of sliding movement because the line of the split is parallel to the line of sliding movement. This in turn means that when the ring is complete additional spring means must be provided to ensure that a good contact is made between the sliding part of the ring and the fixed part. A similar probe is disclosed in DE-C-822570. A disadvantage of such a probe is that unwanted space is taken up by the presence of the sliding part of the ring and the presence of the spring means biassing the sliding part of the ring into good contact with the remaining part of the ring. The problem is not as severe as with a probe of the type in which the jaws are hingeable, but on the other hand the problem is not negligible either. Yet a further probe is disclosed in FR-A-2170303, having a rectangular ring of magnetisable material split in two parts along a transverse line. One part of the ring is fixed to a handle body and lies at a distal end of the probe while the other part is linearly slidable within the body in a direction perpendicular to the line of split. Another probe with a rectangular ring is shown in FR-A-2538556. In this case both a fixed ring portion and a slidable ring portion have end faces which lie along an oblique line of split. It is an object of the invention to provide a current probe which is compact and convenient to use in confined spaces, yet which is also accurate. According to this invention, a current probe comprises handle means, a first ring portion of magnetisable material fixed relative to the handle means and a second ring portion of magnetisable material linearly slidable relative to the first ring portion between a position in which it engages the first ring portion to form a closed ring of magnetisable material for encircling an electrical conductor and a position in which the ring is open, the first and second ring portions each having end faces, the faces of one portion engaging the faces of the other portion along a line of split which is oblique to the direction of the linear sliding movement of the second ring portion, characterised in that the probe further comprises damping means operable to damp the said movement of the second ring portion from the position in which the ring is open to the position in which the ring is closed. For best accuracy, the ring is preferably circular such that magnetic asymmetry and difficulties in manufacture are kept to a minimum. The prior art rectangular shape tends to introduce into the probe signal errors as a result of the asymmetry in the magnetic circuit. The circular configuration has a further advantage, particularly in a Hall-effect probe, in that it is generally easier to manufacture than a rectangular ring, since, if used, the windings that make up the rectangular ring must have a discontinuity at each respective corner of the ring. Preferably also, the linearly slidable second portion of the ring is biassed by means which also act to bias the respective faces, formed by the split, of the second portion against the corresponding faces of the first portion of the ring. The biassing means may comprise one or more compression springs disposed within the handle means which serve to bias a slidable support towards the ring end of the handle means, to thereby urge the linearly slidable portion of the ring into contact with the fixed portion. The slidable portion of the ring may be permanently fixed to the slidable support and embedded in a jaw element. The fixed portion may be similarly embedded in a jaw element, the two jaw elements together defining openable jaws. The slidable support may be movable within the body of the handle means by means of a thumb-actuated trigger connected to the support and slidable along a slot in the handle means. The damping means prevent the second portion of the ring from moving too rapidly from a position in which the ring is open to a position in which the ring is closed. This is an important feature preventing damage of the contacting faces of the respective rings and also preventing or inhibiting signal errors in the magnetic circuit when the probe is in use, the errors being otherwise possible as a result of a disturbance in the magnetic field due to the shock of violent closure of the ring. The damping means may conveniently be connected to the slidable support, which support may be provided with a toothed rack geared to a spur gear and dashpot mounted on a dashpot support plate fixed to the handle means. The invention will now be described, by way of example only, with reference to the accompanying drawings in which:- Figure 1 is a front view of a probe in which the jaws are in a closed position. Figure 2 is an enlarged sectional view of the probe of Figure 1 in which the jaws are in an open position. Referring firstly to Figure 1, the probe comprises handle means in the form of a handle body 1 and a cover plate 2 secured to the handle body by four screws 3, 4, 5 and 6. At one end of the body 1 are a pair of jaws 7, 8 which each include a respective portion of a ring of magnetisable material, to be described with reference to Figure 2. Jaws 7 and 8 are shown in a closed position but are openable by slidable retracting inner jaw 8 into the handle body 1 by means of thumb pressure on a trigger 9 slidable within a slot 10 in the cover plate 2. At the end of the handle body remote from the jaws 7 and 8 is a flexible strain release member 11 of conventional type which reduces strain upon an electrical cable 12 only part of which is shown. Referring now to Figure 2 of the drawings, an enlarged sectional view of the probe of Figure 1 is shown, but in which it will be seen that the inner jaw 8 has been retracted within the handle body 1 to form an opening within which a cable, not shown, can be inserted. Embedded in the outer jaw 7 is a circular portion 13 of a ring of magnetisable material. A correspondingly shaped portion 14 is embedded in the inner jaw 8 and portions 13, 14 together define a complete ring of magnetisable material when the inner jaw 8 is in abutment with the outer jaw 7, as shown in Figure 1. The inner jaw 8 is integral with one end of a slidable support 15 having a pair of elongate webs 16, 17 which slidingly engage respective inner sides of the handle body 1. At the end of each web 16, 17 is a respective foot portion 18, 19, each foot portion having an aperture for receiving a respective spindle 20, 21. Each spindle 20, 21 is fixed into a respective support block 22, 23 integral with the handle body 1 adjacent the strain release member 11. Compression springs 24, 25 are provided around the spindles 20, 21 between each respective support block 22, 23 and each respective foot 18, 19. Compression springs 24, 25 bias the slidable support 15 and hence inner jaw 8 to a closed position as shown in Figure 1. Damping means are incorporated into the probe in order to prevent or inhibit violent closure of the jaws 7, 8 as a consequence of the spring bias. The damping means includes a toothed rack 26 fixed to the upper web 16 of the slidable support 15, the rack 26 being geared to a spur gear 27 mounted on a pinion 28 secured for rotation on a support plate 29 which is fixed to the inner side of the handle body 1 by screws. The pinion 28 includes a dashpot (not shown) of conventional type which limits the speed of rotation of the spur gear 27 such that sliding movement of the slidable support 15 within the handle body 1 is achieved in a damped, controlled manner and hence closure of the inner jaw 8 with respect to the outer jaw 7 is also achieved in a damped, controlled manner. This ensures that the respective end faces 13a, 13b and 14a and 14b of the respective portions 13, 14 of the ring of magnetisable material do not violently collide when the ring is closed, which would otherwise be the case in the absence of the damping means. A further advantage is that since closure is achieved in a controlled manner little or no shock is imparted to the respective portions 13, 14 making up the magnetic circuit and this in turn reduces signal errors in the probe. This feature is important in Hall-effect probes because they normally incorporate electronic zeroing means which compensate for inherent signal errors in the probe just prior to positioning the probe around a conductor to be measured. This zeroing cannot be done when the probe is actually in position around the conductor since any current flowing therein would interfere with the electronics carrying out the zeroing function. It will therefore be appreciated that if after zeroing the ring is subjected to a severe shock, the magnetic circuit can be altered and an error can therefore arise in the measurement being taken. Such an error is reduced or eliminated by the damping means because the shock of closure of the jaws 7, 8 is reduced or eliminated. In operation, the jaws 7, 8 of the probe are moved to the open position shown in Figure 2 by sliding the trigger 9 within the slot 10 to a position in which it is nearest the strain release member 11. The open jaws are then placed around a conductor to be measured and the trigger 9 is then released. The compression springs 24, 25 then urge the slidable support 15 and hence the inner jaw 8 to a closed position, corresponding to that of Figure 1, and the damping means ensures that the closure is achieved in a controlled manner. The arrangement thus provides a probe which is extremely convenient to use, especially in confined spaces, and which also allows a conductor to be "hooked" by the outer jaw 7, if necessary, and pulled to a position in which satisfactory measurement can be obtained. It will also be seen that the compression springs 24, 25, in addition to providing the bias for sliding the inner jaw 8 into a closed position, also serve to bias the respective faces 14a, 14b of the slidable portion 14 against the corresponding faces 13a, 13b of the fixed portion 13 of the ring of magnetisable material so that a good contact is achieved.. It will be appreciated that the above description of the embodiment of the invention relates mainly to the design of the probe, rather than the electronic circuitry used in its operation. This electronic circuitry is conventional and for convenience of illustration only is omitted. In practice, however, the fixed portion 13 of the ring of magnetisable material could include a Hall chip which would be connected to a suitable circuit board disposed within the body 1 between the webs 16 and 17. However, the invention also extends to other types of current probe which do not utilize the Hall-effect for example, when required for use in measuring AC current, in which case the electronic circuity would not include a Hall chip. The invention is not limited to a purely circular ring of magnetisable material and other shapes or configurations can be chosen, although it will be appreciated that the further one departs from a circular shape, the advantages referred to in the preamble become lessened.

Claims: A current probe comprising handle means (1,2), a first ring portion (13) of magnetisable material fixed relative to the handle means (1,2), and a second ring portion (14) of magnetisable material linearly slidable relative to the first ring portion (13) between a position in which it engages the first ring portion (13) to form a closed ring of magnetisable material for encircling an electrical conductor and a position in which the ring is open, the first and second ring portions (13, 14) each having end faces, the faces of one portion engaging the faces of the other portion along a line of split which is oblique to the direction of the linear sliding movement of the second ring portion (14), characterised in that the probe further comprises damping means (26, 27, 28, 29) operable to damp the said movement of the second ring portion (14) from the position in which the ring (13, 14) is open to the position in which the ring (13, 14) is closed. A probe according to claim 1, characterised in that the damping means comprise a toothed rack (26) and a gear (27) engaging the rack (26). A probe according to claim 2, characterised in that the second ring portion (14) is fixed to a support (15) linearly slidable with respect to the handle means (1, 2), and in that the toothed rack (26) is connected to the slidable support (15). A probe according to claim 2 or claim 3, including a dashpot arranged to limit the speed of rotation of the gear (27). A probe according to any preceding claim characterised in that the ring (13, 14) is circular. A probe according to any preceding claim, characterised by means (24, 25) biassing the second ring portion (14) against the first ring portion (13). A probe according to claim 6, characterised in that the second ring portion (14) is arranged to be retracted in the handle means (1, 2) when in the position in which the ring (13, 14) is open, and in that the biassing means comprises one or more compression springs (24, 25) disposed within the handle means and arranged to urge the second ring portion (14) into contact with the first ring portion (13). A probe according to claim 3 or claim 4, characterised in that the second ring portion (14) is permanently fixed to the slidable support (15) and is embedded in a jaw element (8). A probe according to claim 8, characterised in that the first ring portion (13) is embedded in another jaw element, the said jaw elements (7, 8) together defining openable jaws. A probe according to any of claims 3, 4, 8 and 9, characterised by including a thumb-actuable trigger (9) connected to the slidable support (15) and slidable along a slot (10) in the handle means (1, 2), the slidable support (15) being located within the handle means (1, 2).

Inventors: CROSS, GEOFFREY, HARGREAVES, BRIAN
Applicants: H.E.M.E. INTERNATIONAL LIMITED
Application year: 1987
Main classification: G01R15
IPC codes: H01F38, G01R1