Abstract: A sensor assembly comprises a U-shaped support straddling an edge portion of a straight stretch of the abrasive belt of a sanding machine. Each leg of the U-shaped support has three air discharge nozzles spaced along its length, each nozzle on one leg being aligned with and opposing a nozzle on the other so that streams of air issuing from the opposing nozzles, unless intercepted, impinge upon one another. Low pressure air is blown out of the nozzles on one leg, high pressure air out of those on the other. Either of two pressure conditions can thus exist upstream of each low pressure nozzle, depending upon whether or not a part of the belt intervenes between it and its opposing high pressure nozzle. The center pair of nozzles senses normal tracking conditions, the outer pairs respond to edgewise excursion of the belt beyond the tracking zone.