For a large antenna, the torque required to move the antenna is high, hence the large ratio gear reducers are used to meet the required torque demand. It is almost impossible in practice to manufacture a gear box which can deliver a large power with no back-lash. Any effort to reduce back-lash by tight coupling of pinions increases the friction of the gear box which reduces its efficiency. With the use of large gear ratios the backlash, hysteresis, and between the motor shaft and the load shaft increases. With the increase in these parameters the nonlinearity in the position loop increases, which leads to position loop instability. There are various ways to reduce the back-lash mechanically but they are inefficient and are unsuitable for a giant antennas like those at the GMRT. Instead one uses a dual drive. Here a pair of motors, gearbox and pinion are used to drive the common load.
Two amplifiers individually drive the motors. When the load is to be held at some position, the torque produced by two motors are equal and opposite, thereby eliminating the backlash. The net torque on the load is zero hence it does not move. For a slight movement of the load in a given direction, one motor increases its torque in that direction while the other reduces its torque. The load will be subjected to a net torque which causes small movement of the load.