Most metal detectors use a balanced, three-coil, system to detect small particles of non-ferrous and stainless steel. The coils are wound on a non-metallic frame, each parallel with the other. The center coil is connected to a high frequency radio transmitter. Coils on either side of the transmitter coil are receivers. As these two coils are identical and placed the same distance from the transmitter, they receive the same signal and produce an identical output voltage. When the coils are connected in opposition, the output is cancelled, resulting in a zero value. A schematic of the coil configuration is shown in figure 1.

When a particle of metal passes through the coils of a metal detector, the high frequency field is disturbed under one coil, changing the voltage by a few microvolts. The state of balance is lost and the output from the coils is no longer zero. It is this phenomenon that is used to detect metal.

An important aspect of metal detector operation is the metal free zone, which is needed for proper operation. The zone includes a space on each side of the aperture that must be free from any metal structure such as rollers and supports. As a general rule, this needs to be approximately 1.5 times the aperture height for fixed structures and 2 times the aperture height for moving metal such as reject devices or rollers.

Food product conditions can have a major effect on metal detection. Electrical conductivity in foods such as cheese, fresh meat, warm bread, jam and pickles can generate a signal in a metal detector even though metal is not present. This phenomenon is known as the “product effect.” It is best to be aware of this phenomenon and to work with your metal detector supplier or manufacturer to determine the best means to compensate for product effect.

The purpose of rejection mechanisms is to remove the contaminants from the product/process stream. The mechanism must be designed to remove 100 percent of the detected contaminants along with a minimum amount of salable product. The contaminants are removed and stored in a manner that eliminates any possibility of their being reintroduced into the product or process stream.

Metal detectors are installed in three basic configurations: pipeline, conveyor and free-fall. Pipeline detectors are used for any product conveyed in tubes or pipe. An example of a pipeline detector is shown in figure 2. A diversion valve is normally used to redirect materials containing detected contaminants into a separate vessel or container.

Conveyor detectors are by far the most common; examples are given in figure 3. Many variations of manual, semi-automated and fully automated rejection mechanisms are possible, such as air blow (figure 3a), push arms, retractable conveyor bed (figure 3b), reversible conveyor (figure 3c), slider gate, ink marker or coder, diversion conveyors and robotic grippers. Free-fall detectors are often found on dry product fillers and are useful for any material that can be dropped through an aperture. A fast-acting valve or deflector plate is used to divert detected material to a separate container.

and other product contaminants. Line strainers are useful for removing foreign particles from liquid products and magnets have long been used to remove ferrous materials from particles and pumpable liquids.