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Magneto-inductive sensors keep blades sharp: a manufacturer of knife and blade sharpness testing machines is using non-contact magneto-inductive displacement sensors from Micro-Epsilon to calibrate the length and speed of the blade stroke.

Blade stroke enables the test machines to objectively compare the sharpness of various knife blades. The sensors were selected for their cost effectiveness, ease of set up, simplicity of use, robustness and non-contact wear-free operation.

The Cutlery and Allied Trades Research Association (CATRA) was established in Sheffield in 1952 to carry out R&D for the local cutlery industry. CATRA now specialises in cooperative investigations into materials, stainless steels, manufacturing methods, machinery, automation, robotics, product and performance testing.

One primary area of R&D is sharpness testing of knives and blades. CATRA designs, builds and supplies test machines for this purpose to knife and blade manufacturers.

The CATRA tester is an automatic and semi-automatic knife sharpness and life testing machine that measures sharpness and edge life (durability) of a wide range of blades and knives for guality control, R&D and competitor product evaluation. This machine is the only proprietary cutting test machine on the market that fully conforms to the International Cutting Test Standard (BS EN ISO 8442-5.2005). The lower capital cost, semi-automatic benchtop version of this machine utilises magneto-inductive sensors from Micro-Epsilon.


The blade is positioned with the edge vertical and a pack of specially developed synthetic paper is lowered onto it. By oscillating the blade back and forth, it cuts into the paper, the depth of the cut being the measurement of sharpness. The test media is loaded with 5 per cent silica, which has a mild wearing effect on the blade edge. By repeating the back and forth motion, further cuts are made, which wears the cutting edge. Cut depth at each subseguent stroke can then be plotted to produce a wear curve for each blade tested.

As CATRA Senior Development Engineer Simon O'Leary states: "We considered linear transducers [LVDTs] but these are contact sensors and so are prone to wear and are difficult to mount. We preferred the ease of set up of magneto-inductive sensors as these operate on a non-contact basis, are reliable, robust and easy to use."


In the Mainsensor range of displacement sensors, the sensor coil is supplied with alternating current, resulting in a primary magnetic field. This magnetic field induces eddy currents in the electrically conductive material arranged opposite the coil. In the field of the eddy current sensor is a special film, which attenuates the eddy current sensor depending on the strength of the magnetic field. Using this method, a linear relationship between the distance to the magnet and the output signal can be established. As printed coils can be used inside the sensor, manufacturing does not rely on any semiconductor processes.

Although the Mainsensor uses a permanent magnet as the target, unlike Hall Effect sensors, it does not suffer from relatively low sensor accuracy or high-temperature drift. The Mainsensor uses a patented measuring method, which is based on a linear relationship between the position of the permanent magnet and the output signal. As well as providing a simplified displacement measurement technigue, the design offers extremely high basic sensitivity and excellent temperature stability (low thermal drift), and is largely maintenance- and wear-free.

In the CATRA benchtop knife sharpness testing machine, the Mainsensor is set up to measure the distance (and speed) of each blade stroke (max stroke 40mm). The cutting speed is 50mm/s and a typical test time is 15 minutes for 60 cutting strokes. "As the machine is running, I am able to collect the measurement data as a complete curve," says O'Leary. "The magnet is positioned on the moving slide, while the sensor is fixed into position at a gap of 50mm. The voltage level tells us the distance the blade has moved for each stroke and the average velocity."
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Title Annotation:DESIGN IDEAS
Publication:Environmental Engineering
Date:Aug 1, 2016
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