electro-hydraulic

name	electro-hydraulic
description	Electro-hydraulic systems — servo valves, proportional valves, hydraulic servo loops, dynamic response, valve frequency response, electrohydraulic actuator (EHA), flight control, ISO 10770, MIL-H-5440.
metadata	{"priority":7,"promptSignals":{"phrases":["electro-hydraulic","servo valve","proportional valve","electrohydraulic actuator","EHA","hydraulic servo"],"minScore":3}}

Electro-Hydraulic Systems — Complete Skill

Servo Valve Fundamentals

Servo valve: converts electrical signal (mA or V) → hydraulic flow proportional to signal; fast, precise Types:

1. Flapper-nozzle (2-stage): torque motor → flapper deflects between two nozzles → control spool; most common
2. Jet pipe: torque motor deflects jet → pressure differential on spool; robust, tolerates contamination better
3. Direct-drive valve (DDV): linear motor directly on spool (no 1st stage); simple; lower bandwidth

Two-Stage Flapper Nozzle (Moog Type)

1st stage: torque motor drives flapper; nozzle-flapper amplifier provides pilot pressure to spool ends 2nd stage: main spool; controls flow to actuator Bandwidth: 30–200 Hz (-3 dB) depending on model and supply pressure

Flow through valve (saturated spool): Q = Cd × w × x_v × √(2ΔP / ρ) [m³/s; Cd = discharge coeff; w = port width; x_v = spool displacement; ΔP = supply-load ΔP] Linear range: Q ≈ K_q × x_v + K_c × (P_L) [K_q = flow gain; K_c = pressure-flow coefficient]

Key parameters:

• Rated flow: Q_R at ΔP_rated [L/min at 70 bar (or 3000 psi)]
• Threshold: minimum current to initiate spool motion [< 1% rated signal; typical 0.5–0.8%]
• Hysteresis: < 1–3% (servo valve); < 5% (proportional)
• Null leakage: flow at zero signal (< 1–3% rated flow)
• Bandwidth: f (-3dB) [Hz]; typically 30–200 Hz for aircraft; 5–50 Hz for industrial

Proportional Valve (Less Precise, Lower Cost)

Solenoid-operated: direct solenoid force on spool (no torque motor) Accuracy: hysteresis 2–5%; linearity ±2%; bandwidth 5–50 Hz Contamination tolerance: better than servo valves (larger gaps); ISO 4406 cleanliness requirement: 16/14/11 Servo valve requirement: ISO 4406: 14/12/9 (much cleaner)

Electrohydraulic Actuator (EHA)

Architecture: electric motor + fixed-displacement pump + hydraulic actuator (self-contained unit) Advantages: no central hydraulic supply required; fly-by-wire; no hydraulic lines to other aircraft zones Disadvantages: heat generated internally; requires motor drive electronics; weight

Power flow: P_electric = I × V → P_hydraulic = P_pump × Q = P_motor_shaft × η_pump = P_hydraulic_to_actuator × η_total η_total = η_motor × η_pump × η_actuator ≈ 0.80–0.90 (combined)

EHA dynamic model (position control): Transfer function (linear, actuator position / valve signal): G(s) = K_h / (s × (s² + 2ζ_h ω_h s + ω_h²)) [hydraulic natural frequency ω_h, damping ζ_h] ω_h = √(4β_e × A_p² / (V_t × m)) [β_e = bulk modulus; A_p = piston area; V_t = trapped volume; m = load mass] ζ_h = B_v/(4A_p) × √(V_t / (β_e × m)) + K_ce/(A_p) × √(β_e × m / V_t) [combined damping]

Typical: ω_h = 50–300 rad/s; ζ_h = 0.1–0.3 (hydraulic only)

Hydraulic Servo Loop Design

Open-Loop Transfer Function

Complete position servo (valve + actuator): G_OL(s) = K_v / s × G_valve(s) [K_v = velocity constant; G_valve = valve TF] G_valve(s) ≈ 1 / (s/ω_v + 1)² [2nd order approximation; ω_v = valve bandwidth in rad/s]

Full 4th-order model: G_OL = K_v / s × [1 / (2ζ_h ω_h / ω_h² × s² + 2ζ_h/ω_h s + 1)] × [1/(s/ω_v + 1)²]

Velocity constant K_v: K_v = K_q × K_servoamp / A_p [K_q = valve flow gain; K_servoamp = amplifier gain; A_p = piston area] K_v determines steady-state velocity error: e_v = v_cmd / K_v

Stability and Bandwidth

Gain margin: GM ≥ 6 dB; Phase margin: PM ≥ 35–45° Typical achievable bandwidth: 0.1–1.0 × ω_h (hydraulic natural frequency limits performance)

Rule of thumb: servo bandwidth f_BW ≤ f_h / 3 (to avoid hydraulic resonance)

Load stiffness: K_load = 4β_e × A_p² / V_t [effective hydraulic stiffness N/m] Position error under load: x_error = F_load / K_load (at DC)

Supply Pressure and Flow Sizing

Actuator force: F_max = P_supply × A_p × η_actuator [N; η ≈ 0.95 for cylinder] Actuator velocity: v_max = Q_valve_rated / A_p

Power requirement: P_hydraulic = F × v = Q × P [kW] Pump sizing: Q_pump ≥ Q_max_actuator × (1 + 0.10 for leakage) P_supply: select based on force requirement and cylinder area

Standard pressure levels: Aerospace: 3000 psi (207 bar) — classic; 5000 psi (345 bar) — modern (F-35, A380) Industrial: 200–350 bar

Fluid Contamination Control

ISO 4406 contamination code: Particles > 4 μm / > 6 μm / > 14 μm per mL → three-number code Servo valve: ISO 16/14/11 minimum; Class 14/12/9 preferred Proportional: ISO 18/16/13

Filtration: High-pressure filter: 3–6 μm absolute (β₃ > 200) Return filter: 10–25 μm Case drain filter: 10 μm

Flight Control Application

Primary flight control (elevator, aileron, rudder): Requirements: bandwidth 10–30 Hz; LVDT position feedback; dual-redundant actuator; MFDA (failure mode) Fly-by-wire: digital FCC → analog current signal → servo valve → surface Position accuracy: ± 0.1° (flight control surface)

Force-fight (dual actuators on same surface): Active/active: both powered; force-fight limiter detects differential pressure Active/standby: one off; engages on primary failure

Standards

Standard	Scope
ISO 10770-1	Hydraulic servo valve performance testing
MIL-H-5440	Hydraulic systems, aircraft (US military)
MIL-H-8775	Servo valves, aircraft
ISO 4406	Hydraulic fluid contamination classification
SAE AS5440	Hydraulic system design for aerospace

Output

Provide: servo valve type (flapper-nozzle/jet-pipe/DDV), rated flow Q_R [L/min] at supply pressure [bar], bandwidth f [Hz] at -3 dB, threshold [% rated signal], hysteresis [%], actuator area A_p [cm²], hydraulic natural frequency ω_h [rad/s] and damping ζ_h, servo loop velocity constant K_v [s⁻¹], servo bandwidth [Hz] vs. limit (ω_h/3), gain margin [dB] and phase margin [°], supply pressure [bar], ISO 4406 cleanliness required, EHA efficiency η_total [%] (if EHA), load stiffness K_load [N/mm], and applicable standard (ISO 10770, MIL-H-5440, SAE AS5440).