A servo is mostly used for steering, because it brings the steering axle or the rudder into the transmitted position. In contrast to a motor, a servo must be able to measure its angle of attack in order to steer accurately, but it cannot drive a model because it has a stop at the minimum and maximum position. The servo receives the signal as a pulse width modulation which defines a position between the minimum and maximum; digital servos are more precise than analogue ones. As technical specifications, the weight of the servo, the force, i.e. the torque, and the speed are important for model aircraft builders.