fbg – Cantilever Sensor
The FBG cantilever sensor is built as shown on the following picture.
The grating in the fiber is bounded to a known place of the cantilever. Near the clamping of the cantilever the bending effects are larger than at the free end. To enable an oscillation without damaging the fiber, the fiber has to be mounted with a tension release.
According to the requirements the sensor should be designed (if possible) for a measurement range up to 50g and a bandwidth up to 1 kHz. The lowest Eigen-frequency of the cantilever must be higher than the bandwidth of the sensor to avoid measurement errors due to resonance effects.
The thermal sensor drift cannot be unconsidered but can be compensated with a temperature sensor in the same housing of the acceleration sensor. This temperature sensor can be integrated on the same measurement fiber with the acceleration sensor.
A COTS version of this sensor named ACxx is available from the company Aither Engineering. The linearity of this sensor by the manufacturer is < 1% and the cross sensitivity < 2%.
The mechanical complexity of this sensor concept is simple. The costs are low and production technologies are available.
A disadvantage of this sensor is the big size and the weight.
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 cantilever based sensor 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 cantilever has a very simple mechanical design. Production technologies are available and cheap. Sensors which use this concept are available from the company Aither Engineering.
- Size: Big, due to the mechanical dimensions
- Weight: Heavy, due to the mechanical dimensions
Sensor performance requirements
- Acceleration, which implies a Measurement range of 14g, a Sensor Bandwidth > 1000 Hz and a Resolution >= 9 Bit: Yes. The cantilever 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 cantilever 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 cantilever 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 cantilever 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 based waveguide sensor
- FBG tube sensor
- FBG with pivot coupling of the seismic mass
Alexander Tindl
