Runway debris scanner uses low-profile electric motor

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Clean runways are vital for air safety. Faced with the need for a compact, high accuracy motor to drive its runway scanning radar system, UK consultancy Plextek chose the XR15-05 direct drive torque motor from Printed Motor Works.

The millimetre wave radar was developed to locate the precise position of small foreign objects, such as debris lying on an airport runway. PMW’s external rotor motor is well suited for driving scanning devices: it allows the scanner to be mounted directly to its outer rotor without the need for a gearbox, thus saving cost, complexity and weight. It also avoids any possibility of backlash sometimes associated with gearboxes and thus helps to deliver greater positional accuracy.

The Plextek devices need to scan slowly over a 90° or 180° sweep with rock steady smoothness, and then get back to the start point quickly for the next scan. The design specification therefore called for a rotor with a low moment of inertia and enough torque for the fast sweep, deceleration to stop, reverse and fly back. It also needed to have minimal torque ripple, or cogging, so that it could sweep smoothly, with no disturbance to the positional accuracy. The angular positional accuracy required by the scanner was 0.1° or better, which the XR15-05 motor delivers using an internal optical encoder with 8288 pulses per revolution. The encoder feedback sent to the motor controller then regulates the current to each of the three motor phases so that any residual torque ripple is virtually eliminated.

The motor is compact, at 134mm in diameter and only 49mm in depth. This profile was required to allow the overall height of the scanner system to be sufficiently low that vehicles equipped with the radar could use airport underpasses. When driven along a runway between aircraft movements the mobile scanner can be used for the automated detection of foreign objects on the runway. Alternatively, larger airports can install multiple fixed units to provide continuous foreign object detection of a full length of runway or taxiway. The systems also include an electro-optical camera module for visual identification.

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