在数控机床中，对工作台实际位移量进行自动检测，并与指令进行比较，用差值进行控制调节，这种控制系统称为（）。
A.模拟控制系统
B.数字控制系统
C.闭环控制系统
D.开环控制系统
参考答案：C
解析：本题考查的是机器的组成。在闭环控制系统中，操纵变量作用于被控对象的被控变量；而被控变量的变化又通过自动控制去影响操纵变量。从信息的传递关系来看，构成了一个闭合回路，所以称之为闭环控制系统。
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| Helen Keller was a very bright, beautiful girl. 1 the age of six months she could already say 2 words. But before she was two years old, she was badly ill. She could not see or hear, and soon she could not even talk. Since then Helen had to fight for _ 3 she wanted. When she was six, her parents invited a teacher for her. 4 _ the help of the teacher, she 5 to see and hear the world around her through her 6 .She learned to 7 the books for the blind. The teacher took Helen for long walks, and 8 her about all the beautiful sights. Helen 9 flowers, climbed the trees and smelled a rainstorm before _10 came. She also learned how to swim and _11_ a horse. After she _12 ,she became a famous writer in America. Her first and most famous _13 is The Story of My Life. Her story has _14 new hope to many blind and deaf people. It has given light to 15 in darkness and encouraged them to live and work. | |||
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