Abstract
This thesis investigates the driver design and nonlinear control of DC motors for two engineering applications. The first application is on the use of the DC motor to drive a positioning platform via the worm-gear transmission. In this part, firstly static and dynamic analyses are conducted to investigate the characteristics of the worm-gear transmission. The static analysis reveals not only the non-backdrivability but also the dependency of break-in torques on the loading torque, direction of motion as well as crucial system parameters. The dynamic analysis generates four linear equations of motion, of which, at any particular instant, only one applies. The modeling results from the analyses motive us to design a sliding controller to deal with the uncertain loading torque and variations of system parameters caused by the speed-dependent nature of the coefficient of friction. The second application is on the drive-by-wire and electrical steering control of an electrical vehicle (EV) actuated by two in-wheel brushed DC motors. Due to the characteristics of DC motors and the special mechanical feature of the proposed EV, to enhance the system performance, two issues associated with motor control are investigated. Firstly, to perform the driving and steering functions both smoothly and efficiently, the motor driver should be switched to an appropriate mode of operation so that the associated motor can precisely generate the torque/current demanded without inducing torque discontinuity. Secondly, the electrical steering depends on the differential torque of two wheels. Whenever torque/current saturation occurs in either one of the motors, the steering performance can be degraded. Therefore, care should be taken that the steerability as well as the stability of the vehicle are maintained during control saturation. In the thesis, controllers of two levels are proposed to respectively address these two issues. For the low-level controller, it is capable of operating the motor automatically in forward (reverse) motoring mode during forward (reverse) acceleration and in forward (reverse) regeneration mode during forward (reverse) deceleration. A control input transformation is especially introduced to cancel the nonlinearity in the current dynamics so that the motor torque can be easily and precisely controlled regardless of mode switching. For the high-level controller, in addition to generating appropriate reference current commands, it also provides compensating action to avoid integrator windup and can properly re-distribute the control signals to maintain consistent steering performance during control saturation. The controllers are implemented on a prototype EV and the performance associated with driving and steering is experimentally validated.