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Hydraulic Servo Valves in Automatic Gauge Control (AGC) Systems

 

Hydraulic servo valves are a critical component in automatic gauge control (AGC) systems and offer benefits such as, greater dynamic response capability, high accuracy, low hysteresis, a compact design and low power consumption; they are primarily applied due to their ability to control large fluid flows. While their central importance in AGC systems is the subject of this article, it merits noting that hydraulic servo valves have broad applications in equipment as varied as plastic injection molding & blow molding systems, die casting machines, hydraulic press brakes, roll bend/roll balance systems, among others.

Construction

In general, hydraulic servo valves are constructed in single-stage, two-stage or three-stage configurations. Single-stage valves are directly connected to an electric torque motor, which directs the position of the flapper plate of the orifice that sends hydraulic fluid flow to the hydraulic actuator of the mill. However, single stage, servo valves do not have good dynamic performance and are typically used in low flow applications. Two-stage servo valves are the most common type. They are constructed with pilot and main valves. The pilot valve is driven by an electric torque motor, which receives an electrical signal from an actuator position transducer. The motor drives the flapper plate of pilot valve to control the pressure differential of the valve orifices that send fluid flow to the main valve stage. Three-stage servo valves are similar to two-stage valves with one modification: the first or pilot stage is really a two-stage valve. This modification allows better dynamic performance and is typically used in high flow applications.

There are four types of pilot valves: spool, single flapper, double flapper or jet pipe. In the spool type, the fluid flow orifices change at the same time with one increasing as the other decreases to cause fluid flow to the actuator or the next stage. The single flapper type uses a fixed and variable fluid orifice, which is controlled by the flapper. The double flapper is a 4-way flow configuration, similar to a bridge circuit. It has improved pressure again over the single flapper type and can compensate for environmental variations.

Some servo valves are integrated into the control blocks and mounted on the hydraulic actuator. They are made of hardened stainless steel to minimize wear and erosion. The pilot stage generally has a filter to reduce oil or moisture contamination. Some vendors pressurize the servo valve to prevent dust contamination from entering the valve via the piston rod of the actuator. And still other vendors build redundancy into their servo valve assembly by including a reserve valve.

Detailed Operation

The hydraulic servo system of a rolling mill typically consists of a hydraulic servo valve, sensor/transducer, servo-amplifier/programmable controller and actuator (ram). The hydraulic servo valve receives an input from the position transducer, which senses the position of the cylinder and converts the position into an electrical signal. This signal is sent to the amplifier/controller where it is conditioned and amplified to drive the electric torque motor of the servo valve in a closed loop control system. Servo valves, used in conjunction with a hydraulic cylinder, position transducer and controller can provide cylinder position accuracy to within 1 percent, depending on factors such as, stroke length, load characteristics, steel slab hardness and irregularities (changes due to wear and tear) of the mill.