Reading up books: Digital control systems: Current texts on continuous and digital control

In a computer-controlled system, the output of a given plant is compared to the desired output; the resulting error signal is sampled and fed in to a digital computer. For example, consider the case of the elevation control of an antenna designed to track a satellite. The tracking error (the difference between the desired antenna position and the actual antenna position) is fed into a digital computer which generates a control signal in digital form. The digital control signal is converted into analog form by a D/A converter, and is amplified and fed to the motor drive of the antenna to move the antenna so as to decrease the tracking error. The azimuth of the antenna can also be controlled in a similar manner.