High Temperature Capacitive Proximity Systems - Gas Turbine and Aerospace Industry - Temperature Measurement

Principle of Operation - FM System

The FM Capacitive Clearance system works on the principal of measuring the capacitance change between the probe electrode and a passing blade. This blade-to-probe tip capacitance change forms a part of tuned oscillating circuit. Changes in capacitance at the probe tip due to the passing blades cause frequency modulation of the carrier signal. This FM change is demodulated to a change in output voltage each time a blade passes the probe tip.The two main factors which govern changes in probe tip capacitance are the probe sensor/blade tip geometry and blade tip clearance. During operation the probe sensor/ blade tip geometry usually remains constant and so any changes in capacitance at the probe tip are the direct result of variations in tip clearance.

Use of FM makes the system sensitive to carrier frequency variations and ignores ESD (Electro Static Discharge) or EM (Electro Magnetic) varying fields. The FM also operates via a low carrier voltage avoiding gas ionisation effects found in turbines. The oscillator electronics are designed to minimise cable and probe capacitance variations which may change as ambient temperatures surrounding the probe and cable increase to 2550°F (1400°C).

Clearance Measurement - FM System

The FM system comprises a demodulator, oscillator and optional signal conditioning module. All modules are 3U high and are housed in custom built 19 inch or half-width racks. The oscillator is designed to be engine-mounted and can be supplied with an air-cooled enclosure if required.

Clearance information is displayed on-line via the front panel LCD. Data output is via BNC connectors to the rear of each rack, with analogue data ports available as standard or direct input to a laptop or other remote data acquisition systems.

The FM system has a manual tuning facility to complete system setup and operation. System health monitoring is achieved by a carrier frequency to voltage conversion which converts carrier frequency to a 0-10 VDC output.

Clearance Measurement Systems

Clearance Measurement systems are available for engine development work packages where remotely sited channels of electronics are required.

These systems are modular in construction and can be supplied as single or multi-channel installations which may be sited up to 490 feet (150 metres) away from the engine.

Oscillator

An embedded microprocessor based frequency measurement and control system maintains the carrier frequency at a user selectable frequency. Any change in probe or cable characteristics is compensated for and the carrier frequency is kept constant.

The oscillator may be factory reconfigured for different cable lengths/probe types and the firmware in the embedded microprocessor may be upgraded/changed. This allows the possibility of custom frequency settings and future upgrades.

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FM Systems

Principle of Operation - FM System

Clearance Measurement - FM System