The goal is to design an elevator control system so that the elevator will move from floor to floor rapidly and slop accurately at the selected floor (Figure DP12.7). The elevator will contain from one to three occupants. However, the weight of the elevator should be greater than the weight of the occupants; you may assume that the elevator weighs 1000 pounds and each occupant weighs 150 pounds. Design a system to accurately control the elevator to within one centimeter. Assume that the large DC motor is field-controlled. Also, assume that the time constant of the motor and load is one second, the time constant of the power amplifier driving the motor is one-half second, and the time constant of the field is negligible. We seek an overshoot of less than 6% and a settling time (with a 2% criterion) of less than 4 seconds.

FIGURE Elevator position control.
 

Drum Motor Belt Command from computer Cable Third floor Elevator Second floor 10 fn. Elevator First floor shaft