LVDT Test Bench
Automated Acceptance Test System for Seven Types of Linear Variable Displacement Transducers Used in Electro-Hydraulic Actuators
The LVDT Test Bench is an automated test rig built to perform formal acceptance testing on Linear Variable Displacement Transducers used across electro-hydraulic actuator systems in defence and aerospace platforms. It supports seven LVDT types spanning stroke lengths from ±0.05mm to ±160mm, in both quadruplex and duplex channel configurations, and automates the acceptance test sequence across scale factor, linearity, null voltage, tracking accuracy and cross coupling, replacing manual test procedures with a PC controlled, repeatable testing process.
What is the LVDT Test Bench?
The LVDT Test Bench is an integrated test system combining a precision mechanical rig, a PC controlled 5-phase stepper motor drive, a high resolution linear scale with digital readout, and dedicated LVDT excitation and demodulation electronics in a single platform. Each LVDT unit is mounted on the bench with its corresponding electrical and mechanical interfaces. Mechanical positioning of the LVDT core is achieved through a rotary to linear conversion using LM guides, with position measured independently by a linear scale to confirm the bench is moving the core to the exact location the test sequence commands. Excitation, demodulation and conditioned output across multiple channels are acquired simultaneously through a digital multimeter and multiplexer system.
| Parameter | Specification |
|---|---|
| LVDT Types Supported | 7 (LVDT-1 through LVDT-7) |
| Stroke Length Range | ±0.05mm to ±160mm |
| Quadruplex Channel Units | LVDT-1, LVDT-2, LVDT-3 |
| Duplex Channel Units | LVDT-4, LVDT-5, LVDT-6, LVDT-7 |
| Mechanical Rig | |
| Stroke Length | 0.5mm to 400mm |
| Position Accuracy | 0.005mm |
| Drive Speed | 5mm/sec |
| Guide System | LM Guide or Linear Guide Ball Bush Bearing |
| Limit Protection | End Limit Switch |
| Ball Screw | 500mm |
| Motor and Slide Coupling | Flexible Coupling |
| Stepper Motor and Drive Electronics | |
| Motor Type | 5-Phase Stepper Motor |
| Readout Resolution | Better than 0.005mm |
| Input | Pulse and Direction |
| Control | PC Based |
| Linear Scale with Digital Readout | |
| Linear Scale Stroke Length | 400mm (max) |
| Linear Scale Resolution | 0.005mm |
| Display | Alphanumeric Display with Reset Facility |
| Remote Data | Available at Remote for Data Acquisition |
| LVDT Excitation and Signal Electronics | |
| Excitation Voltage Case 1 | 7.07 ±0.5 Volt RMS (Four Channels) |
| Excitation Voltage Case 2 | 7.00 ±0.5 Volt RMS (Four Channels) |
| Channel 1 Frequency | 3012 ±33Hz |
| Channel 2 Frequency | 2782 ±33Hz |
| Channel 3 Frequency | 3128 ±33Hz |
| Channel 4 Frequency | 2897 ±33Hz |
| Demodulation Channels | 4 (Identical Signal Conditioning Circuits) |
| DMM | Agilent 6.5 Digit Digital Multimeter |
| Multiplexer | 16 Channel with Toggle Switch – PC Based Control |
Testing Five Distinct Failure Modes, Not Just One
An LVDT can pass a basic continuity check and still be unsuitable for service. Scale factor confirms the LVDT produces the correct voltage for a given displacement. Linearity confirms that relationship holds consistently across the full stroke rather than only near the center. Null voltage checks for residual output at the mechanical zero point, which exposes winding asymmetry invisible to a casual inspection. Tracking accuracy confirms the output follows actual core movement without lag. Cross coupling checks whether the LVDT picks up false signal from forces acting outside its intended measurement axis. Each of these represents a different way an LVDT can fail in service, and a unit can pass four of these tests while failing the fifth. The bench runs all five as independent, dedicated tests rather than treating LVDT verification as a single pass or fail check.
Independent Position Verification Through the Linear Scale
Knowing how many steps a stepper motor has commanded is not the same as knowing where the core physically is. Mechanical backlash, coupling flexibility and cumulative step error can all introduce a gap between commanded position and actual position over a long test sequence. The LVDT Test Bench addresses this by reading actual position through an independent linear scale with 0.005mm resolution, separate from the motor's own step count. This gives the test operator a position reference that reflects where the core has actually traveled, which is the reference value every scale factor, linearity and tracking accuracy measurement on the bench is calculated against.
A 5-Phase Stepper Motor for Smoother Reference Motion
The accuracy of any acceptance test is bounded by how precisely the test bench itself can position the LVDT core. A standard 2-phase stepper motor moves in larger discrete steps, which introduces more velocity ripple at low speed. The 5-phase motor used in this bench has a finer step angle, producing smoother motion under micro-stepping control. This matters directly for tracking accuracy testing, where the bench needs to move the core through a continuous, even motion profile so that any lag or irregularity recorded in the LVDT output can be attributed to the transducer being tested, not to uneven motion from the bench's own drive system.
Simultaneous Multi-Channel Acquisition Across Excitation and Output Parameters
LVDT performance parameters interact with each other. Excitation voltage and frequency directly affect the AC output level at any given position, so a meaningful acceptance test needs to capture command position, LVDT position, AC output, DC output, excitation voltage and excitation frequency all at the same instant, not sequentially. The bench's DMM and 16 channel multiplexer combination acquires all of these simultaneously at each test point. This is what allows the test results to reflect how the LVDT actually behaves under real operating conditions, where excitation and output are never independent of each other.
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A Linear Variable Displacement Transducer converts mechanical linear motion into a proportional electrical signal, providing the position feedback electro-hydraulic actuators depend on in flight control and guidance systems. Since the control system treats the LVDT's output as ground truth for actuator position, any deviation in performance translates directly into a position error at the actuator. Acceptance testing catches this before the unit is integrated into a system, rather than after.
Scale factor describes the ratio between output voltage and physical displacement, essentially how many volts the LVDT produces per unit of movement. Linearity describes how consistently that ratio holds across the full stroke. An LVDT can have correct scale factor at the center of its range while still failing linearity if the output curve bends near the ends of travel, which is why both are tested as separate, independent parameters.
In practice no two LVDT windings are perfectly identical, so a small residual voltage almost always exists at the mechanical center position. The test measures this residual against a defined tolerance, because an excessive null voltage usually points to winding asymmetry or assembly defects that will also affect performance elsewhere on the stroke.
Cross coupling is unwanted electrical output caused by forces or movement that should not affect the LVDT's reading at all, such as side loading. In a multi-channel actuator feedback loop, cross coupling can introduce a false position signal that the control system mistakes for genuine displacement, leading to incorrect actuator response. The cross coupling test confirms the LVDT reports only the displacement it is meant to measure.
LVDT parameters are not fully independent. Excitation voltage and frequency influence the AC output level at any given core position, so capturing these together at the same point produces test data that reflects how the LVDT actually behaves when operating continuously, rather than separate snapshots taken under slightly different conditions each time.
Yes. The bench supports seven LVDT types, three configured as quadruplex channel units and four as duplex channel units, covering the redundancy requirements typically specified for flight-critical and mission-critical electro-hydraulic actuator systems.
