In the last few decades, miniaturization has been in increasing demand in many applications. The miniature actuators should comprise high power-weight ratio while having a fast dynamic response, and high efficiency. In this thesis several approaches have been studied to develop a low pressure water hydraulic system driving a miniature rotary joint as the end-effecter. While the system was expected to have a cross sectional diameter of 15 mm, two prototypes were manufactured having cross sections of 12.5 mm × 13 mm (18 mm dia.) and 16 mm X 18 mm (24 mm dia.). The system involves a novel closed-circuit water hydraulic system in which a controlled volume of water supplied by a motor-cylinder pump drives the rotary joint cylinders (4 mm bore). The linear motion of these cylinders is converted to a rotational motion through a sliders-pulley mechanism. The rotation of an arm attached to this pulley is measured by a magnetic rotary encoder.

The dynamic behaviour of the system was studied and the most important parameters have been modelled. This model has been utilized in the design of a model based feedforward (FFWD) controller which was added to a PID controller to enhance the position control of the rotary arm. Numerous experiments were conducted with the horizontal rotation of the rotary arm. The maximum error ( emax) recorded for a mixed input (a combination of rising and falling cycloidals of 120° and 60° with pause periods of 1 and 0.5 s) with a PID controller was 8.3° while with the addition of the FFWD term this dropped to 6.2°. The maximum root mean square error (RMSE) for the same trajectory has been 1.7° and 0.9° for PID and FFWD+PID controllers respectively. The steady state error (SSE) which was measured during a eycloidal input of 120° was recorded to be in the range of ±0.2° for both of the controllers. The robustness of the controllers was evaluated by adding a mass of 8.5 g to the end of the rotary arm which produced an un-modelled extra large inertia to the system dynamic when the arm rotating horizontally. Robustness of both controllers was demonstrated as the change in the main numerical performance indicators (RMSE, ernax, and SSE) were not remarkable. Another set of experiments performed with the rotary joint positioned vertically introduced with an un-modelled changing force. The numerical performance indicators were almost unchanged again. A comparison between the results of this thesis with the ones from the previous work in our laboratory by R. Sindrey indicates a significant improvement.