Designing and specifying clamp-on probes for accurate current measurement - Embedded.com

Designing and specifying clamp-on probes for accurate current measurement

Of the fundamental electrical measurements that have to be made for monitoring or troubleshooting purposes, current has always been the more problematic parameter. In a laboratory or testbench context, it may be simple to break a circuit and insert an ammeter to measure current directly.

In a production-test environment, in field service, or for monitoring and diagnostic measurements on systems while they are operating, that approach is often inconvenient or impossible. Placing a low-value resistive element in the current path and measuring the voltage developed across it presents similar problems of access, and in the case of high-voltage systems such as industrial drives or inverters, of safety.

A familiar tool to address this problem is the clamp-on current probe. From basic electromagnetic principles we know that a current flowing in a conductor forms a magnetic field around it; the clamp-on probe uses that field to indirectly measure the current. Some engineers view the clamp-on current probe as a useful accessory but with limited accuracy. New designs overcome any such limitations and deliver range, accuracy and stability performance that match the demands of today’s production and maintenance test scenarios. Probes can interface to a multimeter, or can be configured as waveform capture devices to connect to an oscilloscope.

Automotive system testing brings together many measurement challenges. It requires high dynamic range; a current probe must measure several hundred amps when the engine starter motor is engaged, while at the other end of the scale it must resolve currents of milliamps or even microamps – and the increasing pace of development of electric vehicles and hybrids will add further demands. A typical test case in modern automotive systems is to monitor the standby currents drawn when a car’s systems are notionally turned off.

With vehicle designs that incorporate many electronic control units (ECUs), all interacting via bus-based interconnects, the behaviour of the overall system as it progressively shuts down towards its quiescent state can be extremely complex, and can take many minutes or even hours. This leads directly to the need for measurement tools that not only resolve very low currents, but do so in a stable and completely repeatable way over long periods.

At the heart of the clamp-on probe is a detailed understanding and meticulous design of the magnetic circuit. While superficially similar, three distinct measurement principles are employed in different probe designs. In each one, the jaws of the probe, when closed around the conductor, form a magnetic loop.

One variant uses a Hall Effect sensor placed within a gap in the magnetic loop; the loop concentrates the magnetic field around the conductor into the gap, and the Hall sensor develops a proportionate voltage. This is referred to as an open-loop device: an alternative closed-loop configuration adds a winding around the magnetic core, through which a servo-amplifier drives a current to null the magnetic field at the Hall sensor. As with all such balanced measurement techniques, this closed-loop arrangement can increase range and linearity, in part by keeping the magnetic component well away from its saturation region.

A second basic principle, the flux-gate sensor, uses the full B-H magnetization characteristic of the magnetic loop. An AC signal drives the magnetic core in and out of its saturated region: the presence of the additional magnetic field due to the conductor under test influences the excursion of the magnetic material around its B-H loop, and from that the value of the test current can be derived.

A further technique measures AC currents only, whereas those already mentioned can handle both DC and AC; this is the Rogowskicoil method in which the magnetic loop is an air-cored winding. The magnetic field due to the current of interest induces a voltage in the coil that is the derivative (di/dt) of the current; signal conditioning then integrates that voltage to return to a measure of current.

Hall sensor-based probes can measure down to a few milliamps, with 1 percent accuracy and 1mA resolution; flux-gate techniques can extend this to measuring 1mA with a resolution of 100mA. Frequency response of high-performance probes extends beyond 100kHz, enabling  harmonic analysis of current flows in industrial drives and inverters, with pulse response times under 1ms.

Fig. 1: Clamp-on current probes must provide immunity against stray fields while achieving high accuracy.    

As noted above, magnetic design is key to performance. Environments such as under-bonnet measurements in the automotive sector, or close proximity to motor drives in industrial settings, are extremely hostile for precision systems. At the most basic level, the mechanical design of current probes must be robust and accurate, so that the magnetic loop is properly closed and identical conditions established each time the probe is applied. Achieving resolutions in the microamp region demands a high level of screening against external magnetic fields at the same time as providing high sensitivity to the field of interest.

The potential for stray fields (and other electrical noise) in the industrial-drive context is obvious; likewise, in the automotive environment, there will be multiple motors and other systems in close proximity. Even the Earth’s magnetic field has to be considered; the magnetic field strength generated by a current of 1mA at a radius of 2cm is 10 nanoTesla; the Earth’s field is some tens of microTesla. The magnetic design expertise needed to build a high-performance clamp-on probe goes well beyond screening, however. Probe accuracy is conventionally specified for a conductor in the middle of the probe aperture, with the wire normal to the plane of the magnetic loop.

In real test scenarios, this is unlikely, so the added error due to off-centre conductor placement becomes relevant. This can increase uncertainty by as little as 1 percent – but only by the most careful magnetic design to ensure symmetry and uniformity of the probe’s field patterns.

Fig. 2: Under-the-bonnet measurements in the automotive sector represent an extremely hostile environment for current probes.

About the authors:

Hahmet Selcuk is the Business development director, and Desmond Ebenezer is the technical director of GMC-I Prosys Ltd. (Skelmersdale, UK),  supplier of standard and customized current probes to high-profile OEM test, measurement and tool suppliers serving industrial and automotive markets.

This article appeared in Electronic Engineering Times Europe – July 2010

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