The story begins at the Field Effect Lasers exhibition in Liverpool in August 2009. A researcher from Munich University approached the Metrolab team with a very specific requirement. His colleague was working on a permanent magnet system intended to focus a particle beam and needed a high-precision three-axis Hall teslameter able to map intense fields in a space... less than 5 mm in diameter! It took a few exchanges of emails and detailed discussions with its partner Senis, which manufactures the HF probes for the THM1176, before Metrolab was able to answer: "Yes, we can."

Stripping
The best ideas are often the simplest. In a nutshell, it was a question of “undressing” the existing HF: Senis replaced the standard case with a miniature printed-circuit board that provides the IC with some protection from mechanical damage. "Finally, the new sensor has a cross section of 0.5×2 mm², compared with 2.3×5 mm² for the standard version,” states Metrolab’s Philip Keller. “The length, on the other hand, has been increased – from 16 to 42 mm – to reach farther into magnet gaps.” By mid-December, Senis had finished the stripping act, the prices had been estimated and two prototypes of the new, ultra-compact probe were sent to Metrolab, which then got down to the serious business of finalising the calibration process. “This is the critical point in developing any new Hall probe,” stresses Philip Keller. “It has a fundamental effect on the accuracy of measurement… and is one of Metrolab’s greatest contributions to this type of project in terms of value added!” (see our article on the subject in this issue).

Handle with care
The positioning jig used for calibration had to be rethought for the new probe, baptised HFC. And this proved to be a challenge: “The jig needed to guarantee geometric precision to within about one tenth of a degree, while taking account of two specific features,” points out Metrolab’s Claude Thabuis: “the probe’s fragility – as a bare probe is sensitive to impacts – and its length, which meant the jig was designed to gently bend the sensor 90° over the entire length of the mini printed-circuit board, to allow calibration along the longitudinal axis.” In the end, the jig was machined from a small block of aluminium to which the probe is attached throughout the calibration process. This contrasts with the jig used for an HF probe for example, where three different positions of the probe on the jig are used to complete the calibration.

Between rotor and stator
By the end of April 2010, so just nine months after the first contact with the client, the brand new HFC probe was delivered to Munich University. But as you no doubt guessed, this new development, completed in record time by Metrolab, is going to benefit not just a group of physicists in Bavaria. It could render valuable service to a rapidly expanding sector of industry, that of the new-generation, energy-efficient motors and generators being developed for markets as varied as wind turbines, hybrid or electric cars, boats or even air-conditioning systems. “In all these fields, designers are now aiming at energy efficiency levels of 90-95%,” emphasises Philip Keller. And to achieve them, they need to understand in detail the magnetic interactions in the small space between rotor and stator, which is often less than 1 mm wide...
Interest from the industrial sector was confirmed recently at several conferences, “in particular the last CWIEME international coil-winding show in Berlin, where our HF and HFC probes were very well received,” concludes Philip Keller. We should have the pleasure of announcing the first firm industrial order for HFC probes soon.