Control Performance of Servo-Controlled Testing Machine with Asymmetrical Cylinder
In the previous papers, control performance of servo-controlled testing machines with a symmetrical cylinder was discussed analytically and numerically. An asymmetrical cylinder is widely used to reduce the cost and size of testing machines. However the asymmetrical cylinder has nonlinear characteristics, and the control performance of it is inferior to the symmetrical cylinder. In this paper, the control performance of a servo-controlled testing machine with asymmetrical cylinder is discussed analytically on basis of the experimental results.
Uniaxial compression tests using Sanjome-andesite, Akiyoshi-marble and Inada-granite were conducted in order to clarify the control characteristics. Actually, pressures of upper and lower chambers within the cylinder were measured. It was found that the pressures changed depending on the direction of piston movement. The pressure decreased in case of moving to the loading direction, and the amount of pressure decrease was large at high piston velocity. The pressures in the cylinder were analytically examined with aid of a simple model which consisted of a cylinder and a servo-valve. These theoretical results coincided with the experimental results.
Sometimes in the post-failure region, the piston tended to move reversely. At that time, the asymmetrical cylinder showed inferior control performance caused by the followings;
1) Larger amount of oil inflow is necessary.
2) Feedback gain changes depending on the direction of piston movement.
The control performance of asymmetrical cylinder can be possibly improved by adjusting the followings; 1) large oil-leakage at the cylinder, 2) variable flow-coefficient at orifice, 3) variable feedback gain.
KEY WORDS: Asymmetrical cylinder, Servo-controlled testing machine, Rock, Control performance, Loading rate