In the method and system for knowledge extraction of this invention, knowledge extraction is realized through acquiring an initial sentence group including one or more sentences, and then comparing the length of the initial sentence group with an expected length to determine the initial sentence group to be expanded according to the comparison result. Since the sentence groups are formed by consecutive sentences, it may be guaranteed that the sentence groups themselves have good coherence in logic, so that the final sentence groups obtained through expanding the initial sentence groups have good coherence in logic correspondingly. Thus, this invention may override the drawback of lacking logical coherence in extracted knowledge information in the prior art.
Please help me write a proper abstract based on the patent claims. CLAIM: 1. A knowledge extraction method, characterized in comprising the following steps: acquiring an initial sentence group, the initial sentence group including one or more sentences; expanding the initial sentence group, in which the length of the initial sentence group is compared with an expected length to determine the initial sentence group to be expanded according to the comparison result; extracting knowledge, in which the sentence group that is finally obtained after expansion is outputted to realize knowledge extraction. 2. The knowledge extraction method of claim 1, characterized in that the step of expanding the initial sentence group comprises: setting a weight threshold, in which the weight threshold is set for the initial sentence group according to the result of comparing the length of the initial sentence group with the expected length; expanding the sentence group, in which while expanding the initial sentence group weights of sentences to be expanded are compared with the weight threshold, and expanding the initial sentence group according to the comparison result. 3. The knowledge extraction method of claim 2, characterized in that the step of setting a weight threshold comprises: determining a comparison result F: determining the result F of comparing the length of an initial sentence group with the expected length, F=the expected length/(the length of the initial sentence group+a redundant value); determining a weight threshold: the weight threshold when F is greater than or equal to 1; and the weight threshold when F is less than 1. 4. The knowledge extraction method of claim 3, characterized in that, in the step of determining a weight threshold: when F is greater than or equal to 1, the weight threshold=(K/F)/G; when F is less than 1, the weight threshold=(K/F)*G; wherein, G is a threshold adjustment factor and G is a value greater than 1; K is a property weight density. 5. The knowledge extraction method of claim 4, characterized in that: the threshold adjustment factor G is in a range 5≦G≦30. 6. The knowledge extraction method of claim 1, characterized in further comprising: determining a set of properties, the set of properties including N property parameters αi and weights vi corresponding to the property parameters αi, wherein N is a positive integer, i is an integer and 1≦i≦N; acquiring a property weight density K using an equation K=Σvi/N. 7. The knowledge extraction method of claim 2, characterized in that the step of expanding the sentence group further comprises: selecting an initial sentence group, in which an initial sentence group is selected for expansion; obtaining a weight of a left sentence and/or a weight of a right sentence, in which a weight WL of the left sentence and/or a weight WR of the right sentence adjacent to the initial sentence group is obtained according to property parameters αi contained in a left sentence and/or a right sentence adjacent to the initial sentence group and corresponding weights vi; left expanding and/or right expanding the initial sentence group, in which if the weight WL of the left sentence and/or the weight WR of the right sentence adjacent to the initial sentence group is greater than or equal to the weight threshold, the left sentence and/or the right sentence is expanded into the initial sentence group to form a new sentence group; otherwise, no expansion is performed on the initial sentence group; obtaining a final sentence group, in which the new sentence group is used as an initial sentence group and the step of obtaining a weight of a left sentence and/or a weight of a right sentence and the step of left expanding and/or right expanding the initial sentence groups are repeated until the initial sentence group cannot be expanded anymore, so as to obtain the final sentence group; loop expansion, in which each initial sentence group is expanded through the step of selecting an initial sentence group to the step of obtaining a final sentence group, so as to obtain all final sentence groups. 8. The knowledge extraction method of claim 3, characterized in that in the step of determining the comparison result F: in the case of left expansion of the initial sentence group, the redundant value is set to half of the length of the left sentence adjacent to the initial sentence group; in the case of right expansion of the initial sentence group, the redundant value is set to half of the length of the right sentence adjacent to the initial sentence group. 9. The knowledge extraction method of claim 7, characterized in that the step of expanding the sentence group further comprises: setting a sentence number threshold for left and/or right expansion, in which the left-expansion sentence number threshold is L and the right-expansion sentence number threshold is R; in the step of left expanding and/or right expanding the initial sentence group and the step of obtaining a final sentence group, when the number of sentences for left expansion of the initial sentence group is greater than the left-expansion sentence number threshold L, no left expansion is performed on the initial sentence group anymore; when the number of sentences for right expansion of the initial sentence group is greater than the right-expansion sentence number threshold R, no right expansion is performed on the initial sentence group anymore. 10. The knowledge extraction method of claim 9, characterized in that: in the step of setting a sentence number threshold for left and/or right expansion, in the case of both left and right expansion of the initial sentence group, the left-expansion sentence number threshold L is set to 6 and the right-expansion sentence number threshold R is set to 6; in the case of only left expansion of the initial sentence group, the left-expansion sentence number threshold L is set to 12 and the right-expansion sentence number threshold R is set to 0; in the case of only right expansion of the initial sentence group, the left-expansion sentence number threshold L is set to 0 and the right-expansion sentence number threshold R is set to 12. 11. The knowledge extraction method of claim 7, characterized in that: In the step of obtaining a weight of a left sentence and/or a weight of a right sentence,: the weight WL is the sum of weights vi corresponding to all property parameters αi contained in the left sentence adjacent to the initial sentence group; the weight WR is the sum of weights vi corresponding to all property parameters αi contained in the right sentence adjacent to the initial sentence group. 12. The knowledge extraction method of claim 1, characterized in that: the step of acquiring an initial sentence group comprises: dividing text into sentences; forming an initial sentence group by I consecutive sentences, wherein I is an integer greater than or equal to 1. 13. (canceled) 14. The knowledge extraction method of claim 1, characterized in further comprising: acquiring a final sentence group weight, in which a final sentence group weight is obtained according to property parameters αi contained in the final sentence group and corresponding weights Vi, the final sentence group weight being the sum of corresponding weights Vi of all property parameters αi contained in each sentence in the final sentence group; acquiring a final sentence group weight density according to the final sentence group weight, in which a final sentence group weight density K′=the final sentence group weight/the length of the final sentence group. 15. The knowledge extraction method of claim 1, characterized in that the step of extracting knowledge further comprises: deduplicating and outputting the final sentence group, in which the final sentence group is deduplicated and then outputted; removing and outputting the final sentence group, in which a minimum length is set for the final sentence group and the final sentence group having a length less than the minimum length is removed; sorting and outputting the final sentence group, in which the final sentence group is sorted according to each weight density K′ of the final sentence group and then outputted. 16. (canceled) 17. (canceled) 18. A knowledge extraction system, characterized in comprising: an initial sentence group acquisition module (1) for acquiring an initial sentence group, the sentence group including one or more sentences; an initial sentence group expansion module (2) for comparing the length of the initial sentence group obtained by the initial sentence group acquisition module (1) with an expected length to determine the initial sentence group to be expanded according to the comparison result; a knowledge extraction module (3) for outputting a final sentence group that is finally obtained by the initial sentence group expansion module (2) to realize knowledge extraction. 19. The knowledge extraction system of claim 18, characterized in that: the initial sentence group expansion module (2) comprises: a weight threshold setting unit (21) for setting a weight threshold for the initial sentence group according to the result of comparing the length of the initial sentence grous with the expected length; a sentence group expansion unit (22) for, in expansion of the initial sentence group, comparing weights of sentences to be expanded with the weight threshold, and expanding the initial sentence group according to the comparison result. 20. The knowledge extraction system of claim 19, characterized in that: the weight threshold setting unit (21) comprises: a comparison result determination subunit (211) for determining the result F of comparing the length of an initial sentence group with the expected length: F=the expected length/(the length of the initial sentence group+a redundant value); a weight threshold determination subunit (212) for determining a weight threshold, a weight threshold when F is greater than or equal to 1, the weight threshold being less than a weight threshold when F is less than 1. 21. The knowledge extraction system of claim 20, characterized in that: the weight threshold determination subunit (212) comprises: a threshold adjustment factor setting device (212a) for setting and outputting a threshold adjustment factor G, wherein G is a value greater than 1; a property weight density acquisition device (212b) for obtaining and outputting a property weight density K; a weight threshold acquisition device (212c) for obtaining and outputting a weight threshold according to outputs of the threshold adjustment factor setting device (212a), the property weight density acquisition device (212b) and the comparison result determination unit (211); when F is greater than or equal to 1, the weight threshold=(K/F)/G; when F is less than 1, the weight threshold=(K/F)*G, wherein, G is a threshold adjustment factor and G is a value greater than 1; K is a property weight density. 22. (canceled) 23. The knowledge extraction system of claim 18, characterized in further comprising: a property set module (4) for storing a set of properties including N property parameters αi and weights vi corresponding to the property parameters αi, wherein N is a positive integer, i is an integer and 1≦i≦N; wherein the property weight density acquisition device (212b) obtains a property weight density K using an equation K=Σvi/N. 24. The knowledge extraction system of claim 19, characterized in further comprising: the sentence group expansion unit (22) further comprises: an initial sentence group selection subunit (221) for selecting an initial sentence group for expansion from the initial sentence group acquisition module 1; a sentence weight acquisition subunit (222) for obtaining a weight WL of the left sentence and/or a weight WR of the right sentence adjacent to the initial sentence group according to property parameters αi contained in a left sentence and/or a right sentence adjacent to the initial sentence group and corresponding weights vi; a comparison subunit (223) for comparing the weight WL of the left sentence and/or the weight WR of the right sentence adjacent to the initial sentence group with the weight threshold; a new sentence group acquisition subunit (224) for, if the weight WL of the left sentence and/or the weight WR of the right sentence adjacent to the initial sentence group is greater than or equal to the weight threshold, expanding the left sentence and/or the right sentence into the initial sentence group to form a new sentence group and outputting it to the sentence weight acquisition subunit (222) as an initial sentence group, until no expansion is performed on the initial sentence group anymore, so as to obtain a final sentence group, the final sentence group being outputted to the knowledge extraction module (3); a loop expansion subunit (225) for, after the new sentence group acquisition subunit (224) obtains a final sentence group, controlling the initial sentence group selection subunit (221) to select another initial sentence group for expansion from the initial sentence group acquisition module (1). 25. The knowledge extraction system of claim 20, characterized in that the comparison result determination unit (211) comprises: a redundant value setting device (211a) for setting a redundant value, wherein in the case of left expansion of the initial sentence group, the redundant value is set to half of the length of the left sentence adjacent to the initial sentence group; in the case of right expansion of the initial sentence group, the redundant value is set to half of the length of the right sentence adjacent to the initial sentence group. 26. The knowledge extraction system of claim 24, characterized in that the sentence group expansion unit (22) further comprises: a threshold setting subunit (226) for setting a left-expansion sentence number threshold L for the initial sentence group and/or a right-expansion sentence number threshold R for the initial sentence group; a first counting subunit (227a) for counting and outputting a number of sentences that have been left expanded into the initial sentence group; a second counting subunit (227b) for counting and outputting a number of sentences that have been right expanded into the initial sentence group; wherein the comparison subunit (223) is further used for comparing the number of sentences that have been left expanded into the initial sentence group with the left-expansion sentence number threshold L, and comparing the number of sentences that have been right expanded into the initial sentence group with the right-expansion sentence number threshold R; the new sentence group acquisition subunit (224) is further used for, if the number of sentences that have been left expanded into the initial sentence group is less than or equal to L and/or the number of sentences that have been right expanded into the initial sentence group is less than or equal to R, and if the weight WL of the left sentence and/or the weight WR of the right sentence adjacent to the initial sentence group are greater than or equal to the weight threshold, expanding the left sentence and/or the right sentence to the initial sentence group to form a new sentence group and outputting it to the sentence weight acquisition subunit (222) as an initial sentence group, until no expansion is performed on the initial sentence group anymore, so as to obtain a final sentence group, the final sentence group being outputted to the knowledge extraction module (3). 27. The knowledge extraction system of claim 26, characterized in that: in the case of both left and right expanding the initial sentence group, the threshold setting subunit (226) sets the left-expansion sentence number threshold L to 6 and sets the right-expansion sentence number threshold R to 6; in the case of only left expanding the initial sentence group, sets the left-expansion sentence number threshold L to 12 and sets the right-expansion sentence number threshold R to 0; in the case of only right expanding the initial sentence group, sets the left-expansion sentence number threshold L to 0 and sets the right-expansion sentence number threshold R to 12. 28. The knowledge extraction system of claim 24, characterized in that the sentence weight acquisition subunit (222) comprises: a first weight acquisition device (222a) for adding weights vi corresponding to all property parameters αi contained in the left sentence adjacent to the initial sentence group together to obtain a weight WL of the left sentence; a second weight acquisition device (222b) for adding weights vi corresponding to all property parameters αi contained in the right sentence adjacent to the initial sentence group together to obtain a weight WR of the right sentence. 29. The knowledge extraction system of claim 18, characterized in that the initial sentence group acquisition module (1) comprises: a sentence dividing unit (11) for dividing a document into sentences; an extraction unit (12) for constructing the initial sentence group with I consecutive sentences, wherein I is an integer larger than or equal to 1. 30. (canceled) 31. The knowledge extraction system of claim 24, characterized in that the sentence group expansion unit (22) further comprises: a sentence group weight acquisition subunit (228a) for acquiring a final sentence group weight according to property parameters αi contained in the final sentence group and corresponding weights Vi, the final sentence group weight being the sum of corresponding weights Vi of all property parameters αi contained in each sentence in the final sentence group; a sentence group length acquisition subunit (228b) for obtaining a length of the final sentence group; a weight density acquisition subunit (228c) for acquiring a final sentence group weight density according to the final sentence group weight, in which the final sentence group weight density K′=the final sentence group weight/the length of the final sentence group. 32. The knowledge extraction system of claim 18, characterized in that the knowledge extraction module (3) comprises: a final sentence group deduplicating and outputting unit (31) for deduplicating the final sentence group and then outputting the final sentence group; a final sentence group removing and outputting unit (32) for setting a minimum length for the final sentence group and outputting the final sentence group after removing those final sentence groups having a length less than the minimum length; a final sentence group sorting and outputting unit (33) for sorting and outputting final sentence groups, in which final sentence groups are sorted and then outputted according to the weight density K′ of each final sentence group. 33. (canceled) 34. (canceled) 35. One or more computer readable mediums having stored thereon computer-executable instructions that when executed by a computer perform a knowledge extraction method, the method comprising: acquiring an initial sentence group, the initial sentence group including one or more sentences; expanding the initial sentence group in which the length of the initial sentence group is compared with an expected length to determine an initial sentence group to be expanded according to the comparison result; extracting knowledge in which a final sentence group that is finally obtained after expansion is outputted to realize knowledge extraction.