It is a new platform technology similar to the VibroFibre™ Sensor working on the principle of interference between two wavelength specific mirrors. In VibroFibre™ Sensor, laser light is trapped between two wavelength specific mirrors: as the incoming beam reflects from the far end mirror, it goes through a 180 degrees phase change and starts to interfere with the incoming light. As a result, fringes are formed. QPS has developed various algorithms that operate on these fringes and result in vibration measurement, vibration + temperature measurement, dynamic strain and temperature and even the presence of chloride and moisture as a precursor to the corrosion process, forming the basis of our structural health monitoring solution.

This new innovation involves breaking the interference cavity into two; one side of cavity is still formed by a fiber Bragg gratings, the other side is formed by a target surface. As the length of the cavity varies, so will the fringe pattern: By counting the fringes, we form a Fiber optics air gap sensor. The advantage of this structure is that the signal remains in the optical domain and conversion from optical to electrical will only take place far away (up to 100 meters) at the interrogation system. There will be no electrical noise pickup nor voltages induced by the strong electromagnetic field inside the generator. It is therefore perfectly safe. This fiber optics air gap sensor will replace all the eddy current probes and also the capacitive proximity sensors currently in the market. By slightly enlarging the diameter of the sensor head up to 8mm, we make sure there will always be some reflected light coupled back to the cavity. The operation of the sensor will not be affected by the rough target surface. Various packaging methods would allow such a sensor through an air vent or a flat package that can be installed inside the stator core. Such a device will measure the static and dynamic distance between the rotor and the stator. The fiber optics air-gap sensor (FOAP) can measure shaft vibration, radial and axial shaft position and differential expansion between the rotor and the stator. Since it is built with fiber optics technology previously qualified by the Telecom industry and has no moving parts, it is expected to work for 25 years. One single FOAP can measure a range of air gap distances of 4 to 12 mm and could be used for all kinds of generators. Such a technology is independent of intensity and fringe counting becomes a very reliable method to measure distance between the rotor and the stator. FOAP would work with an interrogation system called the Rotor Guard.

Imagine a target surface which is vibrating, the vibration surface will create a variable cavity length, giving a periodically changing fringe counts. We can measure these low frequency vibrations like we measure the vibration found in the stator end windings, these vibration amplitude might very well correlate with the air gap measurement. As uneven rotor protrusions occur, they will give rise to uneven air gaps creating imbalanced electromagnetic forces, fueling more acute vibrations.

Finally, if the Air gap has been compromised and there exists rubbing, acoustic emission will occur, the Fiber optics proximity sensor being a wide band device would be able to catch these high frequency signals which is estimated to range between 100Khz to 500KHz. QPS has performed feasibility experiment to verify the performance of these fiber optics air gap sensors, fiber optics rotor vibration sensor and finally the fiber optics acoustic sensors. Different detection circuit would be required to deal with such high frequency acoustic emission signal.

The following diagrams shows the time waveform of the response and also the FFT of the signal.

QPS is looking for well established industrial partners with deep experience in high temperature packaging and mechanical design to help commercialize this new technology platform. Interested parties should contact Peter Kung at peter@qpscom.com to arrange a visit at our factory to see these proximity sensors catching vibration from 0.7Hz to 440 KHz. Appointment will be given only to mature industrial partners linking to the power, industrial processing, transport and aerospace industry. Ideas about new applications would be welcome.

QPS is in the business of giving peace of mind to customers stuck with difficult condition monitoring problems. The company focuses on extending the life of customer’s critical assets or getting them out of harm’s way.