Around the servo drive and motor block is the servo controller that closes the position loop. A basic
servo controller generally contains both a trajectory generator and a P.I .D. controller. The trajectory
generator typically provides only position setpoint commands labeled in Fig.1 as q* (s). The P.I .D.
controller operates on the position error and outputs a torque command that is sometimes scaled by an
estimate of the motor's torque constant, t K . I f the motor's torque constant is not known, the P.I .D.
gains are simply re-scaled accordingly. Because the exact value of the motor's torque constant is
generally not known, the symbol "^ " is used to indicate it is an estimated value in the controller. In
general, equation (3) holds with sufficient accuracy so that the output of the servo controller (usually + / -
10 volts) will command the correct amount of current for a desired torque. t t K K (3)