Position Tracking Control of Miniature Low Pressure Water Hydraulic Cylinders

Over the past 20 years research in the field of miniature actuators has increased substantially thanks to advances in smart material fabrication, semi-conductor chip technology and computer processing capability. Hydraulic cylinders offer many potential benefits as miniature actuators including: high power-to-weight ratio, mechanical stiffness, smooth motion and the potential for high positional accuracy. Despite their benefits the control of hydraulic cylinders with bore diameters under 10 mm has not been previously studied. The most significant obstacle to implementing the use of miniature cylinders is the unavailability of off-the-shelf proportional valves that are compatible with hydraulic fluid and precise enough for the position control task. In this paper two nonlinear control strategies are presented for the position control of two 4 mm bore diameter cylinders. Four off-the-shelf, low cost, 2/2 on-off miniature solenoid valves were used to control the flow of water to and from the cylinder chambers. A novel valve modeling technique is also presented, which allowed the on-off valves to approximate the behaviour of proportional solenoid valves, increasing their control capability. The tracking performance of each controller was experimentally tested and both controllers were found to achieve steady state positioning accuracies within ± 0.07 mm. The robustness of the controllers to changes in payload mass and vertical orientation were also tested. Results from several experiments are presented and compared.