Metal detector search head and aperture: sizing the opening of a balanced-coil head and the metal-free zone around it

01

Inside the head: one transmitter coil, two receiver coils wired against each other

Mettler-Toledo Safeline's patent application EP2544027 describes the standard head. A box-shaped metallic enclosure has an entrance and an exit aperture; inside it, three coils are wound on one shared hollow former of non-metallic material whose cross-section matches the apertures, so that former and apertures form the tunnel the product travels through. The centre coil is driven by a high-frequency oscillator; the two receiver coils on either side sit at equal distances from it and are connected in series with their windings in opposition, so the voltages induced in them cancel and the output is zero until metal disturbs the field. [1]

The enclosure keeps airborne signals and nearby metal from interfering, and gives the rigidity the head needs: the patent notes that even microscopic movement of the coils relative to each other or to the enclosure can disturb a detection system that is sensitive to signals in the nanovolt range. [1]

Bunting's training material for the milling industry describes the signal a contaminant produces as it passes: first receiver coil, zero at the transmitter coil, opposite half-wave at the second receiver, giving a sine-shaped signal at constant speed. It groups metals by the phase of that signal — ferrous about 80–100°, non-ferrous about 40–70° or 110–140°, and stainless steel about 0–40° or 140–180° — and names stainless steel as the hardest to detect. [3]

02

Aperture size against sensitivity: 2 in finds 0.4 mm ferrous, 6 in only 0.8 mm

Bunting states that sensitivity is always given for the dead centre of the coil aperture, which is the area of lowest sensitivity, and that as aperture size increases it directly affects sensitivity; its example is that a 2-inch diameter aperture may see 0.4 mm ferrous where a 6-inch aperture may see 0.8 mm. [3] Fortress Technology's Steve Gidman makes the same point from the other side: maximum sensitivity occurs when the product is closest to the edge of the portal, so the ratio of aperture to product size matters and a smaller aperture is the more fail-safe one. [4]

Editorial calculation: 2 in = 50.8 mm and 6 in = 152.4 mm, so in Bunting's example tripling the aperture doubled the smallest ferrous sphere detected, from 0.4 to 0.8 mm. Bunting lists what the aperture is “typically determined by”: product, package or bag size, flow rate and bulk density, flowability and feed-line size — and adds that products with a high signature may need a larger aperture. [3]

The metal type changes the sphere the head can see at the same aperture. Fortress says a stainless steel sphere in a dry product needs to be about 50% larger than a ferrous sphere to give a similar signal, and that the gap can rise to 300% in wet products such as ready meals, meat, fish, sauces and bread, because moisture creates a conductive product signal resembling stainless steel. [4] For a wet block of cheese about 75 mm high it quotes industry sphere guidelines of 2.0 mm ferrous, 2.5 mm non-ferrous and 3.5 mm stainless steel. [4]

Editorial note: give the supplier the largest pack's height and width with its orientation on the belt, and ask for the aperture they propose and the ferrous, non-ferrous and stainless spheres guaranteed at its centre with your product passing through.

03

The metal-free zone: about 1.5 aperture heights for stationary metal, 2 for moving metal

The field of the transmitter coil does not stop at the box. EP2544027 states that it extends outside the entrance and exit apertures to as far as two or three times the length of the detection zone, so a conveyor frame or other metal parts within that range produce unwanted signals; the area that must be kept clear is called the metal-free zone. The zone's length along the transport path is specified as a multiple of the aperture height h — about 1.5 h for stationary metal and about 2 h for moving metal, citing the earlier EP0536288. [1]

Editorial calculation: for a 150 mm aperture height, 1.5 h is 225 mm and 2 h is 300 mm on each side of the head, so a head whose surroundings include moving metal — rollers, reject arms — needs up to 600 mm of clear line beyond its own length.

Where the line is too short, the patent sets out the trade-offs. Turning the sensitivity down until outside metal no longer registers also reduces detection of contaminants inside the head. [1] Metallic flanges or collars at the aperture rims can act as short-circuit coils that cancel the field beyond the apertures and reduce the zone to zero, but the patent states that the full sensitivity of such a zero-metal-free-zone head is lower than that of a conventional head installed with its full zones. Its own asymmetric design needs essentially no zone at the exit and a standard zone of about 1.5 to 2.5 aperture heights at the entrance. [1]

A related application, EP2629121 by Mettler-Toledo Safeline and TNA Australia, addresses heads in a vertical flow path, such as a chute from a weigher to a bag-maker: a funnel-shaped chute in a cylindrical head leaves widening air space that is sub-optimal for sensitivity, so it proposes a head whose exit aperture is smaller than its entrance, with each coil progressively smaller in sequence, while keeping the balanced-coil behaviour. [2]

Bunting's installation list adds what to keep out of the zone in practice: power cables and conduit, high-voltage panels and lines, drive motors close to the coil, infeed and discharge conveyors touching the detector conveyor, coiled spare cable and bolted frames — it calls for welded frames. [3]

04

Spheres against wires and flakes: the orientation effect in the aperture

Heads are specified and tested with spheres because a sphere looks the same from every direction, but Fortress points out that contaminants are more likely to be swarf, flat flakes or wire than spheres. A stainless steel or non-ferrous wire is detected more easily passing through the aperture sideways or upright than lying along the conveyor, because of the low magnetic permeability of stainless steel. [4]

Fortress puts the gap in proportion: a 0.5 mm sphere could equal a wire contaminant 2.5 cm long, and a 3.5 mm stainless sphere could equal a 30 cm wire, depending on its diameter. A head calibrated to a 2 mm stainless sphere may miss a thinner stainless wire longer than 2 mm in an unfavourable orientation. [4] Its listed remedies are smaller apertures, more than one detector at different angles, or a head with a second field scanning in the other plane, as in its own Interceptor DF. [4]

05

Data for a search-head and aperture quotation

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Product: [largest pack height × width × length; orientation on the belt; wet or dry; packaging film]
Flow: [conveyor, gravity chute or pipeline; belt width and speed or pipe diameter]
Sensitivity: [ferrous, non-ferrous and stainless sphere sizes required at the aperture centre]
Aperture: [proposed height × width or diameter; clearance around the pack]
Installation space: [length available; metal within 2 aperture heights; moving parts nearby]
Head type: [standard, reduced or zero metal-free zone; asymmetric or funnel head]

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