How does a proportional valve differ from a servo valve in a hydraulic system, and what are their respective applications?

Proportional valves and servo valves are both types of electro-hydraulic valves used to control flow or pressure in a hydraulic system proportionally to an electrical input signal, but they differ in precision, response time, and complexity. A proportional valve typically uses a solenoid or torque motor to position a spool, which controls the flow path. The spool position is proportional to the input current or voltage. Proportional valves generally offer good control and are suitable for applications where moderate precision and response time are sufficient. They often have open-loop control, meaning there's no feedback mechanism to verify the actual flow or pressure achieved. A servo valve, on the other hand, provides higher precision and faster response times. Servo valves typically use a more sophisticated two-stage design. The first stage is a small, highly responsive valve (often a flapper-nozzle or jet pipe design) that controls the pilot pressure to the second stage, which is a larger spool valve that controls the main flow. Servo valves also incorporate closed-loop feedback, using sensors to measure the actual flow or pressure and compare it to the desired value. This feedback signal is used to correct any errors, ensuring highly accurate and repeatable control. Due to their higher precision and faster response, servo valves are used in applications requiring precise positioning or force control, such as flight control systems, high-performance machine tools, and robotic systems. Proportional valves are commonly used in applications like hydraulic lift systems, agricultural machinery, and industrial automation where high precision is not essential, but proportional control is desired. For example, a proportional valve might control the speed of a hydraulic motor driving a conveyor belt, while a servo valve would be used to precisely control the position of a robotic welding arm.