1. Unit III Class III
Servo valves

2. Servo valve Introduction
A servo valve is a directional control valve that has infinitely variable positioning capability.
It can control not only the direction of fluid flow but also the amount.
Servo valves are coupled with feedback-sensing devices, which allow for the very accurate control of position, velocity, and acceleration of an actuator.

3. Mechanical – Hydraulic Servo valve

4. It is a force amplifier used for positioning control.
In this design, a small input force shifts the spool of the servo valve to the right by a specified amount.
The oil flows through port P1 , retracting the hydraulic cylinder to the right.
The action of the feedback link shifts the sliding sleeve to the right until it blocks off the flow to the hydraulic cylinder.
Given input motion produces a specific and controlled amount of output motion.
Such a system where the output is fed back to modify the input is called a closed-loop system.
Application - hydraulic power steering system of automobiles and other transportation vehicles.

5. Electro Hydraulic Servo valve
The torque motor includes coils, pole pieces, magnets, and an armature.
The armature is supported for limited movement by a flexure tube.

6. The flexure tube also provides a fluid seal between the hydraulic and electromagnetic portions of the valve.
The flapper attaches to the center of the armature and extends down, inside the flexure tube.
A nozzle is located on each side of the flapper so that flapper motion varies the nozzle openings.
Pressurized hydraulic fluid is supplied to each nozzle through an inlet orifice located in the end of the spool.
The four-way valve spool directs flow from supply to either control port CI or C2 in an amount proportional to spool displacement.
The spool contains flow metering slots in the control lands that are uncovered by spool motion.
Spool movement deflects a feedback wire that applies a torque to the armature/flapper.

7. Questions What is servo system? What are servo valves?
How the servo valves are classified?
What are electrohydraulic servo valve?
Differentiate between hydromechanical and electrohydraulic servo valve?
Differentiate between single and two stage servo valves?

9. Summary
Servovalve, electrohydraulic - A servo-valve that is capable of continuously controlling hydraulic output as a function of an electrical input.
Servovalve, electrohydraulic, flow-control - An electrohydraulic servovalve whose primary function is control of output flow.
Servovalve hysteresis - The difference in the servovalve input currents required to produce the same output during a single cycle of valve input current when cycled at a rate below that at which dynamic effects are important.
Servovalve null leakage - The total internal leakage from the valve in the null position.
Servovalve, pressure-control - A hydraulic servo valve whose primary function is the control of output pressure.

10. MCQ & HOQ
1. Infinitely variable valves are for: A. hydraulic circuits only. B. pneumatic circuits only. C. both hydraulic and pneumatic circuits. 2. Proportional and servo directional control valves have___________flow response than conventional directional control valves. A. Slower B. the same C. faster 3. Always size infinitely variable directional control valves: A. with a high pressure drop. B. with a low pressure drop. C. it really does not matter what the pressure drop is. 4. A servo valve circuit for actuator positioning uses a/an: A. encoder. B. linear transducer. C. load cell. 5. Electro-mechanically controlled servo valves are: A. more contamination tolerant. B. less contamination tolerant. C. Have about the same contamination tolerance.

11. Answer
1. Infinitely variable valves are for: A. hydraulic circuits only. B. pneumatic circuits only. C. both hydraulic and pneumatic circuits. 2. Proportional and servo directional control valves have___________flow response than conventional directional control valves. A. Slower B. the same C. faster 3. Always size infinitely variable directional control valves: A. with a high pressure drop. B. with a low pressure drop. C. it really does not matter what the pressure drop is. 4. A servo valve circuit for actuator positioning uses a/an: A. encoder. B. linear transducer. C. load cell. 5. Electro-mechanically controlled servo valves are: A. more contamination tolerant. B. less contamination tolerant. C. Have about the same contamination tolerance.

12. Higher Order Question
Draw the automotive power steering application of mechanical-hydraulic servo system

13. SOLUTION