Ward-Leonard characteristics from nonlinearly-controlled series-motor drives

Two methods are explored for improving the manoeuvrability of fork-lift trucks driven by chopper-controlled series motors. The first method attempts nonlinear feedforward cancellation of one system state (speed) but is shown to be unstable by simulation and analytical solution of the system equations. The instability results from neglect of field inductive effects in the feedforward model. A second method employs synthesised torque and transient-velocity estimates derived purely from armature current feedback. The method is shown by simulation to be highly tolerant of uncertainty in the vehicle and drive parameter estimates and to produce overall control characteristics resembling those of Ward-Leonard drives. The manual handling of the truck on a variable-gradient test track shows a considerable improvement on conventional linear current control systems.