Take Out The Shake

Here's how to eliminate instability and oscillations in hydraulic positioncontrol systems. One issue that designers of hydraulic position-control equipment often face is the low damping inherent in such systems — which usually consist of a hydraulic actuator, servovalve, position sensor, and controller. When designers try to improve accuracy by increasing loop gain, low damping causes the system to vibrate. Fortunately, adding acceleration or pressure feedback, or using pole-placement techniques, can remedy the situation. Here's how. For insight into the dynamics, let's first look at the underlying equations that mathematically describe hydraulic systems. The following is a summary of the linearized math model for a system with a servovalve and hydraulic actuator moving a mass. The position of mass M is X, and flow Q from the servovalve is defined as: The equation relating flow to the actuator with a cylinder-piston area A, the actuator motion, and volume under compression is: Eliminating Q and Pl from the above equations and taking the Laplace transform of the resulting equation produces an open-loop transfer function of the system: The accompanying graphic shows a basic system block diagram. Using block-diagram algebra gives the relationship between the desired position The above transfer function shows that a thirdorder equation governs the system's dynamics. We can learn a lot from this equation. For example, if Kp is zero — which means servovalve flow is independent of load pressure — then the coefficient of s2 disappears, leading to severe instability. The most critical point of operation is around the valve's null position. For a closed-center valve, Kp is very low. This leads to extremely low damping and is a major source of instability and oscillations in hydraulic systems. Adding acceleration feedback to the control loop can solve this by influencing the coefficient of the s2 term in the transfer