With fast development of aerospace, aviation, automobile, electronic and advanced manufacturing, the demands of micro parts are dramatically increasing. Due to the shadow effects, limited probing space and force, the precision and measurable depth cannot satisfy the requirement of the measurement. To achieve the measurement of more tiny structure and enhance the measurable depth, high-aspect-ratio tactile probes are usually inserted into the micro parts and a trigger method is applied to get inner contact points. In general, the structures of a micro part are measured using a coordinate measuring machine (CMM) combining with a tactile probe. The CMM technology is relatively mature and a precise three-dimension motion is already equal to the measurement. It can be concluded that it is of great significance to develop a precise probing system for measuring structures of a micro part. FBG probes are not influenced by the shadow effect and the measurable depth is either not limited as a result of the separation between the probe and the sensing system. They also feature miniaturization and anti-interference. So FBG probes are very suitable for dimensional measurement of structures of a micro part.
Existing FBG probes include following designs:
Jiwen Cui and Fuling Yang proposed in 2011 a FBG bending probe for the measurement of micro holes. Their FBG probe is fabricated by common single mode fiber and the core comprising FBG is located in the center of the probe. Therefore, the FBG is in the neutral plane when the probe is used to achieve radial measurement. The radial resolution of this probe is low and it cannot be used to achieve the measurement of a micro hole. The structure of this FBG probe is not reasonable and its performance cannot further be improved.
Jiwen Cui and Kunpeng Feng proposed in 2014 a method and equipment based on multi-core FBG probe for measuring structures of a micro part. A multi-core FBG probe is designed and the FBG is subjected to the sufficient stress when the multi-core FBG probe gets contacted with the micro parts. In comparison to the FBG bending probe, the resolution, especially the radial resolution, is significantly improved, and decouple capacity of two-dimensional radial displacement and three-dimensional displacement can be achieved by designing the distribution of the fiber cores. The multi-core FBG probe has favourable characters. However, inscription of FBG on the fiber cores of multi-core fiber brings several problems. On the one hand, there is intense crosstalk among the inscription of FBGs on each fiber cores of multi-core fiber. On the other hand, parameters of the multi-core FBG probe cannot be designed as a result that the brought multi-core fiber is already determinate.
Above all, the multi-core FBG probe has a high resolution, especially the radial resolution, and a decouple capacity of multi-dimensional displacement. It attracts extensive attentions and has a good application prospect of all the existing FBG probes. However, the multi-core FBG probe has following drawbacks:
1. Considering the cost of the probe, the multi-core fiber cannot be customized and should be selected among the manufacturers' existing types. So the parameter of the multi-core FBG probe cannot be designed as the requirement of the measurement because of the limited multi-core fiber types.
2. The distance among fiber cores is several ten micrometers and inscription of FBGs on each fiber cores of multi-core fiber has intense crosstalk. The spectra of the FBGs on the fiber cores are distorted and their reflectivity is low, which influences the accuracy of the measurement.
3. The optical signals should access to every cores of the multi-core fiber to achieve the spectrum of the FBGs and a fan-out device should be configured. However, the fan-out device for the multi-core FBG probe has such disadvantages as expensive price, high insertion loss. So it is adverse to apply the multi-core FBG probe in the field of the dimensional measurement that the multi-core FBG probe is directly made of the brought multi-core fiber.