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Robin | Mechanical Engineer
Robin | Mechanical Engineer

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Designing Closed-Loop Hydraulic Test Bench Control Systems: Architecture and Safety

#engineering #automation #hydraulics #safety

Building a hydraulic pressure test bench for aerospace or defence qualification testing isn't just a plumbing exercise. The control system design determines whether you get ±0.1% pressure accuracy and EN 13849-compliant safety — or an unsafe rig that fails its first audit.

Control Architecture 

Pressure Setpoint (HMI/PLC)
        │
        ▼
PID Controller (in PLC)
        │
        ▼
Servo Proportional Valve ──→ Hydraulic Circuit ──→ DUT
        ▲                          │
        │                          │
        └────── Pressure Feedback (transducer, 0-10V or 4-20mA)
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For aerospace-grade accuracy: use a 16-bit ADC for pressure transducer input. A 700 bar range with 16-bit resolution gives 0.01 bar resolution — well within the ±0.1% target.

PID Tuning for Hydraulic Systems

Hydraulic systems are challenging for PID control due to nonlinear flow characteristics and fluid compressibility. Recommended starting parameters:

  • Proportional gain (Kp): start low, increase until oscillation begins, then halve
  • Integral gain (Ki): essential for eliminating steady-state error at hold pressure
  • Derivative gain (Kd): usually minimal — hydraulic systems have enough natural damping

Use feedforward control from the pressure ramp profile to improve tracking during pressure rise. Switch to pure PI control during hold phase.

Safety Interlock Architecture (EN 13849 PLd)

For aerospace test benches, target Performance Level d (PLd) for the overpressure protection function:

Overpressure Protection (redundant):
├── Software limit in PLC: if P > P_max_sw → close servo valve
├── Hardware pressure switch: if P > P_max_hw → kill HPU motor (direct relay)
└── Mechanical pressure relief valve: set at 1.1× P_max_hw (passive, always active)

Door/Guard Interlocks:
├── Safety relay monitors guard switches
├── If guard open during pressurisation → immediate pressure dump
└── Pressure dump valve: energise-to-close (fail-safe open)
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Redundant channels + diverse technology = PLd with CCF (common cause failure) analysis per EN 13849-1 Annex F.

Data Acquisition Requirements for AS9100

Every test record must capture:

  • DUT serial number, part number, drawing revision
  • Test bench ID and calibration status
  • Operator ID
  • Test profile: target pressure, ramp rate, hold time, acceptance criteria
  • Actual measured data: pressure vs time at minimum 10Hz sampling
  • Pass/fail result with specific failure mode if applicable

Store in SQL database. Generate PDF report automatically. Never allow manual editing of raw data files.

Fluid Cleanliness Monitoring

For NAS 1638 compliance, integrate an online particle counter in the return line:

  • ISO 4406 reporting: three particle size channels (4μm, 6μm, 14μm)
  • Log cleanliness level at start and end of each test
  • Automatically hold test if cleanliness exceeds specification

Reference the Neometrix hydraulic test bench for a production implementation of this architecture:
→ https://neometrixgroup.com/products/hydraulic-pressure-test-bench

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