Patent Description:
Multimeters, also called digital multimeters or "DMMs", are adapted for measuring a number of parameters generally needed for service, troubleshooting, and maintenance applications. Such parameters typically include a. (alternating current) voltage and current, d. (direct current) voltage and current, and resistance or continuity. Other parameters such as frequency, capacitance, and temperature may also be measured to meet the requirements of the particular application. In order to measure current with a general purpose multimeter, an internal current shunt having a known resistance must be inserted in the current path, requiring a break in the current-carrying conductor. The voltage drop across the current shunt is then measured to determine the current flow.

General purpose multimeters employing internal current shunts are generally limited to ten amperes maximum because of the capacity of the multimeter test leads and circuitry to carry the current. Furthermore, the multimeter generally must be protected with an internal fuse to prevent excessive current levels from flowing through the multimeter, both for safety reasons and to prevent damage to the multimeter. The difficulty in removing a blown fuse, coupled with the time and cost necessary to procure a replacement fuse, make it desirable to obtain a non-contact current measuring instrument that requires no internal fuse.

Clamp-on multimeters provide improved capability for measuring current over general purpose multimeters by employing an integral current clamp which senses the current in the current-carrying conductor without having to cut the current-carrying conductor or break the circuit including the current-carrying conductor. A current clamp is typically provided in the same housing with a multimeter which measures other parameters such as voltage and resistance in the conventional manner using separate test probes. The current clamp is closed around the current-carrying conductor, which may include copper wires and buss bars, to sense the magnetic field created by the current flow. The current clamp provides a voltage signal for measurement by the multimeter which calculates and displays the measured current level. Because there is no current shunted from the current-carrying conductor through the clamp-on multimeter, the constraint on the maximum current that may be measured has largely been eliminated. Likewise, the internal fuse has been eliminated in clamp-on multimeters.

In order to obtain a valid current measurement, the magnetic core in the current clamp must completely encircle the current-carrying conductor so that the current clamp is completely closed. The current clamp must be mechanically actuated to open the jaws, the current-carrying conductor inserted, and the jaws then closed around the current-carrying conductor. In tight physical spaces such as an electrical cabinet, inserting the clamp-on multimeter and using this technique to make a current measurement is inconvenient and difficult. Moreover, the jaws must be aligned to complete the magnetic core for obtaining a valid current measurement. Clamp-on multimeters are therefore difficult to use in confined spaces and require a large physical space in which to open the jaws of the current clamp.

Clamp-on multimeters also tend to be physically heavy because of the substantial amount of iron used on the magnetic core. Furthermore, high levels of current may saturate the magnetic core. The current measuring capacity of the clamp-on multimeter is accordingly limited to current levels that do not saturate the magnetic core.

<CIT> relates to a current probe suitable for detecting a large current from DC to high frequency with first and second detecting means.

<CIT> relates to a clamp on multimeter with a display for indicating the results of a plurality of measurements. Further relevant examples of current measurement approaches are described in <CIT> and <CIT>.

Document "<NPL>, discloses an AC current probe composed of a flexible sensor and an electronic module to permit measurements on conductors where standard clamp-on probes cannot be used.

Document "<NPL>, discloses flexible AC current probes that allow current measurements on conductors that are hard to reach or inaccessible using typical clamp-on current probes.

There is provided a clamp-on multimeter as set out in claim <NUM>.

In accordance with the present invention, there is provided in one embodiment a clamp-on multimeter for measuring alternating current flow in an electrical conductor of an electrical circuit, comprising: a digital display configured to show a measurement of the alternating current flow in the electrical conductor; a housing including the digital display; a first sensor having a magnetizable material core configured to inductively sense the alternating current without breaking the electrical circuit, the first sensor includes a first clamp portion fixedly coupled to the housing and a second clamp portion pivotally coupled to the first clamp portion, wherein the electrical conductor passes through a gap between the first and second clamp portions in an open arrangement of the first sensor and the first and second clamp portions surround the electrical conductor in a closed arrangement of the first sensor; a second sensor including a Rogowski coil having a non-magnetic core and configured to inductively sense the alternating current without breaking the electrical circuit; a processor disposed in the housing and configured to receive an input signal in a first voltage range and to send an output signal to the digital display; and an integrator disposed in the housing and configured to scale a signal from the second sensor in a second voltage range to the first voltage range, wherein the housing includes the processor, the integrator, and a selector configured to select the input signal to the processor from one of (a) the first sensor and (b) the integrator.

The multimeter may included the housing having a port configured to electrically couple and decouple the second sensor relative to the integrator.

In accordance with the present invention, there is provided in another embodiment a clamp-on multimeter for measuring alternating current flow in an electrical conductor of an electrical circuit, comprising: a digital display configured to show a measurement of the alternating current flow in the electrical conductor; a housing including the digital display; a first sensor having a magnetizable material core configured to inductively sense the alternating current without breaking the electrical circuit, the first sensor includes a first clamp portion fixedly coupled to the housing and a second clamp portion pivotally coupled to the first clamp portion, wherein the electrical conductor passes through a gap between the first and second clamp portions in an open arrangement of the first sensor and the first and second clamp portions surround the electrical conductor in a closed arrangement of the first sensor; a processor disposed in the housing and configured to receive an input signal in a first voltage range and to send an output signal to the digital display; and a port configured to electrically couple a Rogowski coil with the processor; wherein the housing includes the processor, the port, and a selector configured to select the input signal to the processor from one of (a) the first sensor and (b) the port.

The clamp-on multimeter may further comprise an integrator electrically coupling the port to the processor, wherein the integrator is disposed in the housing and configured to scale a signal from the Rogowski coil in a second voltage range to the first voltage range.

In accordance with the present invention, there is provided in another embodiment a multimeter for measuring alternating current flow in an electrical conductor of an electrical circuit, comprising: a first sensor configured to inductively sense the alternating current without breaking the electrical circuit, wherein the first sensor has a magnetizable material core; and a second sensor configured to inductively sense the alternating current without breaking the electrical circuit, wherein the second sensor has a non-magnetic core.

The following optional features of the multimeter may be included:.

In accordance with the present invention, there is provided in another embodiment a kit for measuring alternating current flow in an electrical conductor of an electrical circuit, comprising:.

The kit may have the Rogowski coil including a mating feature configured to couple and decouple with the port. The kit may further comprise a carrying case configured to receive the multimeter and the Rogowski coil.

Embodiments of the invention are described hereinafter with reference to the accompanying drawings, in which.

Specific details of embodiments according to the present disclosure are described below with reference to electrical circuits including a conductor. Other embodiments of the disclosure can have configurations, components, features or procedures different than those described in this section. A person of ordinary skill in the art, therefore, will accordingly understand that the disclosure may have other embodiments with additional elements, or the disclosure may have other embodiments without several of the elements shown and described below with reference to <FIG>.

<FIG> is an exploded view of a kit <NUM> for measuring alternating current. The kit <NUM> includes a multimeter <NUM>, a Rogowski coil <NUM>, and a carrying case <NUM>. The multimeter <NUM> and the Rogowski coil <NUM> are shown extracted from the carrying case <NUM> in <FIG>. The multimeter <NUM> and/or the Rogowski coil <NUM> may also be inserted into the carrying case <NUM> such that the kit <NUM> may transported as a single unit (not shown).

<FIG> is a schematic illustration of the multimeter <NUM> coupled to the Rogowski coil <NUM>. The multimeter <NUM> includes a housing <NUM> having a slender shape whereby a user (not shown) is able to comfortably hold the housing <NUM>.

A clamp <NUM> is provided on the housing <NUM>. The clamp <NUM> includes a pair of clamp portions 142a and 142b having cores 144a and 144b, respectively. The clamp cores 144a and 144b can include windings (not shown) around a core made of a magnetizable material, e.g., iron. The first clamp portion 142a is movably attached, e.g., pivotally attached, to the housing <NUM> and capable of moving to an arrangement shown with one-dot-chain lines. The second clamp portion 142b can be fixed with respect to the housing <NUM>. The clamp portions 142a and 142b include ends 146a and 146b, respectively, and are accordingly capable of being positioned in an open arrangement with the ends 146a and 146b separated by a gap. The clamp portions 142a and 142b are also capable of being positioned in a closed arrangement with the ends 146a and 146b being contiguously engaged. Thus, the closed arrangement of the clamp <NUM> shown in solid lines in <FIG> has an approximately ring-like shape. A lever <NUM> fixed to the first clamp portion 142a can be used to move the first clamp portion 142a relative to the second clamp portion 142b and/or the housing <NUM>. The clamp <NUM> is configured to inductively sense a flow of alternating current in a conductor C surrounded by the clamp <NUM> in the closed arrangement. The clamp <NUM> may include a Hall effect current sensor (not shown) for, e.g., sensing a direct current flow in the conductor C. The clamp <NUM> can sense the flow of current without breaking an electrical circuit (not shown in <FIG>) that includes the conductor C. The clamp <NUM> may produce a first signal in a first voltage range corresponding to the current flow.

The housing <NUM> includes a port <NUM>, e.g., three socket terminals are shown in <FIG>, for sensing other electrical parameters. Plugs on the ends of lead wires (not shown) may be connected to the port <NUM> to introduce to the multimeter <NUM> various signals that indicate voltage, resistance and/or temperature.

The housing can include a display <NUM>, for example, a liquid crystal display (LCD). The display <NUM> shows measured parameters such alternating current or alternating current frequencies that are inductively sensed by the clamp <NUM>. In particular, a signal is induced in the clamp <NUM> by a flow of current in the conductor C that is surrounded by the clamp <NUM>. The display <NUM> also shows the electrical parameters sensed by signals received via the port <NUM>, including those produced by the Rogowski coil <NUM>.

The housing <NUM> includes one or more selectors <NUM>, e.g., push-buttons <NUM> and/or a rotary switch <NUM>. The selectors <NUM> may turn on and off a power source (not shown) for the multimeter <NUM> and/or change the measuring modes of the multimeter <NUM>. For example, the rotary switch <NUM> can be turned to select a mode for measuring alternating current with the clamp <NUM>. Other modes for measuring voltage, resistance, temperature, etc. can be selected with the selectors <NUM>. According to embodiments of the present disclosure, the selectors <NUM> can also be used to select a mode for measuring alternating current with the Rogowski coil <NUM>.

The Rogowski coil <NUM> includes a loop <NUM>, a pendant <NUM>, a signal cable <NUM>, and a coupler <NUM>. Additionally referring to <FIG>, the loop <NUM> includes a toroidal coil <NUM> wound around a flexible, non-magnetic core <NUM> and sheathed in a flexible covering <NUM>. According to one embodiment of the present disclosure, the non-magnetic core <NUM> includes air. The covering <NUM> can be sufficiently rigid to protect the form of the toroidal coil <NUM> and still be sufficiently flexible to allow the loop <NUM> to be adjusted in length and/or shape. The length of the loop <NUM> can be adjusted with the pendant <NUM>. A start turn of the toroidal coil 212a and an end turn of the toroidal coil 212b are electrically connected by the signal cable <NUM> to the coupler <NUM> (<FIG>). The coupler <NUM> can include an arrangement for electrically coupling and decoupling the Rogowski coil <NUM> with respect to the multimeter <NUM>. For example, the coupler <NUM> can include one or more plugs arranged to cooperatively engage one or more socket terminals of the port <NUM>.

The clamp <NUM> and the Rogowski coil <NUM> are both able to sense alternating current flowing through a conductor surrounded by the clamp <NUM> or the loop <NUM>. There are, however, a number of differences between the Rogowski coil <NUM> and the clamp <NUM>. For example, the loop <NUM> is more flexible and has a smaller cross-section than the substantially rigid clamp portions 142a and <NUM>. The Rogowski coil <NUM> can accordingly be used in confined spaces that are too tight and/or too small for the clamp <NUM>. Further, the loop <NUM> can be reshaped to surround conductors having cross-sections that the clamp <NUM> cannot close around. Another difference is the greater current measuring capability of the Rogowski coil <NUM> as compared to the clamp <NUM>. Specifically, an air core does not become saturated at levels of current that saturate the magnetic material of the cores 144a and 144b. Yet another difference is the Rogowski coil <NUM> is spaced from the multimeter <NUM> by the signal cable <NUM> whereas the relative position of the clamp <NUM> is fixed with respect to the multimeter <NUM>. Thus, the clamp <NUM> and the multimeter <NUM> can be handled as a single unit whereas the Rogowski coil <NUM> allows the user to position the display <NUM> in an orientation that is convenient for viewing.

<FIG> is a block diagram of components of the multimeter <NUM> according to an embodiment of the present disclosure. The multimeter <NUM> includes the display <NUM>, the clamp <NUM>, the port <NUM>, the selectors <NUM> and a processor <NUM>. The clamp <NUM> includes a winding that inductively senses alternating current without breaking the electrical circuit and produces a first signal in a first voltage range corresponding to the alternating current. For example, the first voltage range may be measured in terms of millivolts. The first signal is received by the processor <NUM> which outputs a signal to the display <NUM>. Similarly, other signals from the port <NUM> are also received by the processor <NUM>. According to certain embodiments, the selector <NUM> selects the signal(s) that the processor <NUM> uses to present the desired measurements on the display <NUM>. The Rogowski coil <NUM> inductively senses alternating current without breaking the electrical circuit and produces a second signal in a second voltage range corresponding to the alternating current. For example, the second voltage range may be measured in terms of microvolts. The multimeter <NUM> also includes an integrator <NUM> to scale the second signal from the Rogowski coil <NUM> to the first voltage range, e.g., to scale a microvolt output of the Rogowski coil <NUM> up to a millivolt signal that the processor <NUM> can process. The housing <NUM> of the multimeter <NUM> includes the processor <NUM>, the integrator <NUM>, the display <NUM>, the clamp <NUM>, the port <NUM>, and the selectors <NUM>. According to certain embodiments, only the Rogowski coil <NUM> is separable from the housing <NUM>.

Specific details of the embodiments of the present disclosure are set forth in the description and in the figures to provide a thorough understanding of these embodiments. A person skilled in the art, however, will understand that the invention may be practiced without several of these details or additional details can be added to the invention. Well-known structures and functions have not been shown or described in detail to avoid unnecessarily obscuring the description of the embodiments of the present disclosure.

Unless the context clearly requires otherwise, throughout the description and the claims, the words "comprise", "comprising", and the like are to be construed in an inclusive sense, as opposed to an exclusive or exhaustive sense; that is to say, in the sense of including, but not limited to. Additionally, the words "herein", "above", "below", and words of similar connotation, when used in the present disclosure, shall refer to the present disclosure as a whole and not to any particular portions of the present disclosure. Where the context permits, words in the above Detailed Description using the singular or plural number may also include the plural or singular number respectively. The word "or", in reference to a list of two or more items, covers all of the following interpretations of the word: any of the items in the list, all of the items in the list, and any combination of the items in the list.

Claim 1:
A clamp-on multimeter (<NUM>) for measuring alternating current in an electrical conductor of an electrical circuit, comprising:
a first sensor (<NUM>) comprising a clamp (142a, 142b) configured to inductively sense the alternating current without breaking the electrical circuit to generate a first signal in a first voltage range, wherein the first sensor (<NUM>) has a magnetizable material core, wherein the clamp is movable between an open arrangement and a closed arrangement, wherein the open arrangement is configured to permit the electrical conductor through a gap between the first (142a) and second (142b) clamp portions, and the closed arrangement is configured to close the gap and thereby surround the electrical conductor with the first (142a) and second (142b) clamp portions;
wherein the multimeter is characterized by:
a second sensor (<NUM>) comprising a Rogowski coil configured to inductively sense the alternating current without breaking the electrical circuit to generate a second signal in a second voltage range, wherein the second sensor (<NUM>) has a non-magnetic core (<NUM>) in a loop having a smaller cross-section than the first and second clamp portions (142a, 142b) of the first sensor (<NUM>);
an integrator (<NUM>) configured to scale a signal from the second sensor (<NUM>) in the second voltage range to the first voltage range of a signal from the first sensor (<NUM>), wherein the second voltage range of the signal from the second sensor is different from the first voltage range of the signal from the first sensor; and
a selector (<NUM>) configured to select the first signal or the second signal as an input signal for measuring the alternating current in the electrical conductor
a housing (<NUM>), wherein the integrator (<NUM>) is included in the housing (<NUM>).