as the carriage 129 moves across the bridge 128 each depositing one of the four colors sequentially on each dot position across the substrate 15 each of the heads 130 a d is moveably mounted to the carriage to individually move vertically or perpendicular to the plane of the substrate 15 the distance of each head 130 a d from the plane of the substrate 15 is controlled by a respective one of a set of servos 137 a d mounted to the carriage 129 to follow one behind the other over the same contour of the substrate 15 the servos 137 a d are responsive to signals from the controller 35 which control the positions of the heads 130 a d to maintain each a controlled distance from the surface of the substrate 15 where the surface 16 of the substrate 15 is contoured usually it is desirable to maintain the heads a fixed distance from the surface 16 on which they are to print this is achieved by providing optical sensors 138 a 138 b on the opposite transverse sides of the carriage 129 the printhead set 130 is bidirectional and prints whether moving to the right or to the left as the printhead carriage 129 moves on the bridge 128 the leading one of the sensors 138 a or 138 b measures the distance from the sensor 138 and the.