Class #7Servo-Hydraulic Systems

ME 4232:FLUID POWER CONTROLS LAB

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Notes• Class Next Week

– Data Acquisition & Controls Hardware• System Dynamics Review Assignment

– Due Wed/Thurs lab after break (3/21-22)– Counts as a lab assignment

• Upcoming Labs:– Lab 14: Integrated Lab– Make-Up & Help Session

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Agenda

• Feedback: Course• Servo Valves

– Function– Characteristics– Experimental System– Open-Loop System Identification– Force Analysis

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Feedback: CourseLike• Interesting / Hands-on

Labs• Seeing Components in

Class• Explanations in Class• Discussion of Research

Dislike• Grading Inconsistency• Hard to Absorb all Info• Want Well Defined Labs• Hard to See Board in

Video• Condense lectures

Suggestions• “Can we visit the hybrid vehicle lab?”

− Tuesday, 10 am

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Previously Discussed: Directional Control Valves

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Single Stage Proportional ValvesSolenoids

• Solenoids Create Spool Displacement• LVDT Spool Position Feedback• Spring (sometimes) for Safety

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Single Stage Proportional Valves• Advantages:

– Simple design– Reliable– Cost effective

• Disadvantages:– Poor dynamic performance (bandwidth)– At high flow rates and bandwidths, large stroking

force is needed– Large (and expensive) solenoids / torque motors

needed.– Low end market …..

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Multi-stage valves

• Use hydraulic force to drive the spool …..

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Electrohydraulic servo-valvePilot stage

Main stage

For a fun place to learn how a servo-valve works:R. Dolid “Electrohydraulic Valve Coloring Book”http://www.lulu.com/items/volume_67/7563000/7563085/3/print/Servovalve_Book__091207.pdf

• Multi-stage valve• Typically uses a flapper-nozzle pilot stage• Built-in feedback via feedback wire• Very high dynamic performance• Bandwidth = 100-200+Hz

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Servovalves

Servo-Valve Diagrams from:Dolid, R., 2010, “The Electrohydraulic Servovalve Coloring Book,” (http://www.lulu.com/items/volume_67/7563000/7563085/3/print/Servovalve_Book__091207.pdf)

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Servo Valve Function

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First Stage – Torque-motor-armature

A. Permanent Magnet (2) - yellowB. Wire Coil - pinkC. Armature Assembly – light blueD. Upper Pole Piece – light greenE. Lower Pole Piece – dark green

Transforms electrical current into torque, which changes relative nozzle pressures

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First Stage - Armature rotation

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First Stage – Nozzle and Flapper

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Second StageJ. Feedback wire – orangeK. Nozzles – redL. Spool – dark blueM. Filter – brownN. Fixed Orifice – light greenO. Variable orifice – dark green

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Null Position – Oil Flow path1. Supply pressure - red2. Filter pressure – light blue3. Spool press. – orange4. Return press – light green

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Activated Position – Oil Flow Path1. Supply - red2. Filter – light blue3. Spool hi-side – pink4. Spool lo-side – yellow5. C1 – orchid/magenta6. C2 – dark blue7. Return press – light green

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Servo Valve Parts

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Torque Motor Parts

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Three Stage Valve

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MTS Systems Corp – 4000 gpm Valve

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Nozzle & Flapper vs. Jet Tube

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Typical Flow vs. Pressure Curve

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Servo Valve Benefits / Characteristics

• Feedback from 2nd Stage to 1st

• Low Mass Torque Motor• Large ΔP in 1st Stage• Frictionless / Isolated 1st Stage• Mechanically / Hydraulically Symmetric• 300-1000 Hz Natural Frequency• Good Linearity

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Negatives

• Contamination Susceptible– < 3 micron filtration

• High Cost– Torque motor– Critically Lapped

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Servovalve vs. Proportional Valve