A FBG sensor with a pivot is illustrated in the following picture. A fiber is fixed to an inertial frame on the left and on the right side of the frame.
Inside, a seismic mass can oscillate due to acceleration loads, which act on the sensor. The force is coupled into the fiber via a lever. The oscillating parts are connected to the inertial frame with a small pivot. Between the fixed point of the fiber and the fixed lever FBG are written into the fiber.
If acceleration is applied to the sensor in its sensitive axis, the FBGs change their reflected wavelength due to the force coupling of the mass to the fiber.
The functional principle and mechanical properties are similar to a FBG – Cantilever Sensor in combination with a spring-mass system. The mechanical complexity in comparison to the cantilever concept is increased. The influence of the pivot-cantilever system in comparison to the spring-mass system is significant bigger. Due to this in terms of simplification the performance is estimated like the performance of the FBG cantilever sensor. Also the linearity and cross sensitivity can reach the required values.
This sensor can be multiplexed with other sensors on one optical fiber.
With a size of about 60mm x 30mm x 10 mm and a mass of about 30 gram this sensor is big in comparison to other sensor concepts.
The production costs for this sensor are low and production technologies are available.
Trade criteria (properties and behavior of the sensor)
- Multiplexing ability: Yes
- Linearity: Good expectation (<1%).
- Cross sensitivity: Good expectation (<2%). This parameter depends on the mechanical layout and can be minimized for example by mechanical guidance of the moving parts.
- Resistance to shock load: No good expectation. A FBG with pivot concept isn´t able to survive the mechanical stress caused by the bending moments.
- Vulnerability to thermal drift: Low. The thermal drift is a problem for all optical sensor concepts. But this drift can be compensated mathematically.
- Saturation effects: None. Optical sensor concepts are not concerned about the problem of saturation effects, contrary to electrical acceleration sensors (see the page Saturation effects).
- Mechanical complexity, producibility: The FBG pivot is similar to the Cantilever Sensor. The mechanical complexity can be reduced to a spring-mass system. The producibility is possible and cheap.
- Size: Big, The outer dimensions of this sensor are 60mm * 30mm * 10mm.
- Weight: Medium, it weighs about 30g.
Sensor performance requirements
- Acceleration, which implies a Measurement range of 14g, a Sensor Bandwidth> 1000 Hz and a Resolution >= 9 Bit : Yes. The FBG with pivot sensor can afford this combination of parameters that the acceleration requires.
- Vibration 1, which implies a Measurement range of 50g, a Sensor Bandwidth> 1000 Hz and a Resolution >= 9 Bit : Yes. The FBG with pivot sensor can afford this combination of parameters that the vibration 1 requires.
- Vibration 2, which implies a Measurement range of 20g, a Sensor Bandwidth> 3000 Hz and a Resolution >= 9 Bit : No. The FBG with pivot sensor cannot afford this combination of parameters that the vibration 2 requires.
- Shock load, which implies a Measurement range of 5000g, a Sensor Bandwidth> 3000 Hz and a Resolution >= 9 Bit : No. The FBG with pivot sensor cannot afford this combination of parameters that the shock load requires.
You can make a comparison of the same criteria with other sensors using optic fiber.
- FBG based Fibersensing GS6500 – COTS
- FBG – Cantilever Sensor
- FBG tube sensor
- FBG with pivot coupling of the seismic mass