The FBG tube sensor consists of a tube which is coated with Teflon. The seismic mass is a cylinder of heavy material and also coated with Teflon. The inner cylinder is fixed to the fiber as we can see on the following picture. The fiber also is fixed on both ends of the tube and the FBGs are between the fixed points.
Teflon on Teflon has the property, that the static friction is equal to the dynamic friction which avoids stick slip effects.
If acceleration is applied in the sensors sensitive axis, the seismic mass acts on the FBGs and the wavelength change can be measured by an interrogation unit. The acceleration is the chronological derivation of the position change of the seismic mass. Due to the small friction, this parameter is not recognized for the calculations of this sensor.
This sensor should not provide any cross sensitivity.
One problem of the tube sensor is the rotational sensitivity. This degree of freedom has to be disabled. The easiest way is to design the sensor not as a round tube but as an elliptical tube. In terms of simplicity the sensor is calculated as a round tube and rotational degrees of freedom are excluded.
Another problem, which has to be analyzed in further studies, is the dependency of friction effects between the seismic mass and the sensor housing while acceleration in different angle to the measurement axis is applied. In terms of simplification these effects are not considered for further performance calculations and the sensor design.
The sensor can be modeld by a spring-mass system, where the fiber acts as a spring.
The fiber acts as a spring when it is pulled. In the other direction, the fiber can be disregarded.
The maximum stress load before a damage of the fiber is 1019g. But it can be significantly increased by a mechanical stopper. This feature would protect the fiber from damages by mechanical overload.
This sensor concept has a further advantage in terms of temperature drift. The thermal elongation of the fibers is equal on both sides of the seismic mass and can be computational eliminated. Due to this effect the FBG tube sensor can also be used as a temperature sensor.
The size of this sensor is about 50 mm length and 16 mm diameter which results in a weight smaller than 30 gram.
The mechanical complexity is simple and the resulting costs for manufacturing, assembly and integration are low.
Trade criteria (properties and behavior of the sensor)
- Multiplexing ability: Yes
- Linearity: Good expectation (<1%). This sensor concept uses a spring mass system for the acquisition of acceleration.
- Cross sensitivity: Good expectation. Due to the mechanical design, especially the guidance of the seismic mass of the FBG tube sensor, its cross sensitivity should theoretically not exist. The real value should be inside the required range.
- Resistance to shock load: Good expectation. Among the other sensor concepts listed below, the FBG tube sensor is the only sensor concept that can manage a specified shock load of 5.000g.
- 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 tube sensor has a simple mechanical design. Production technologies for this sensor are available. The costs for this sensor are cheap.
- Size: Medium, the sensor has a diameter of 16mm and a length of 50mm.
- Weight: Medium, it weighs 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 tube 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 tube 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: Yes. The FBG tube sensor can 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 tube sensor cannot afford this combination of parameters that the shock load requires because this sensor can only reach a resolution of 8 Bit.
You can make a comparison of the same criteria with other sensors using optic fiber.
- FBG based Fibersensing GS6500 – COTS
- FBG – Cantilever Sensor
- FBG based waveguide sensor
- FBG with pivot coupling of the seismic mass