To prevent local under- or over-application of chemicals, a spray boom must be kept parallel to the soil. On a slanted field, the operator, driving a sprayer equipped with a passive boom suspension, has to turn the boom manually in a skewed position. This paper presents the development of a slow active controller to do this automatically and to add damping to the suspension. A controller, designed for a cable suspension, captures the signals of two ultrasonic sensors, measuring the distance between the boom and the soil, and activates a low-power electrical DC motor which positions the boom automatically. Different control strategies have been tried out in simulation and in a laboratory experiment on a six-post electro-hydraulic shaking table. The best controller was implemented on a real spraying machine and validated under field conditions. (C) 2002 Silsoe Research Institute. Published by Elsevier Science Ltd. All rights reserved.