High performance hot metal detectors -- A new technique

The capability of hot metal detectors (HMD`s) has been dramatically improved. Standard infrared detecting HMD`s often switch at a predetermined threshold and have a fixed narrow field of view. For consistent detection, the target must not wander from view, and should the IR signal become reduced by the product`s passage through steam or oxide scale, standard HMD performance is significantly reduced. A new HMD will be described that operates on the basis that the object to be detected will always be significantly hotter than its immediate surroundings. A comparison is made between the hot metal signal and its background level. Signals representative of both the target and background are derived from an electronically scanned photodetector array. Detection is determined by the comparison of the levels of the instantaneous signal and the running average. The scanning technique gives enhanced detection sensitivity in the presence of steam and scale. The wide field of view makes adequate allowance for target wander. A red LED bar indicator mimics the scan of the detector and, thus, shows the target position in the field of view. This assists with alignment and proving. Continuous health testing and remote signal testing are also described.