题目内容
步进电动机是将电脉冲信号转变为角位移的执行机构。
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步进电动机受脉冲的控制,其转子的角位移量和转速分别与输入脉冲的数量和脉冲频率成正比。
伺服电动机又称执行电动机,在自动控制系统中,用做执行元件,把所接收到的电信号转换成电动机轴上的角位移或角速度输出。
伺服电动机的主要特点是,当信号电压为零时无自转现象,转速随着转矩的增加而匀速下降。
Cell phones provide instant access to people. They are creating a major 1 in the social experiences of both children and adolescents. In one recent US survey, about half the teens polled said that their cell phones had 2 their communication with friends. Almost all said that their cell phone was the way they stayed in touch with peers, one-third had used the cell phone to help a peer in need, and about 80% said the phone made them feel safer. Teenagers in Australia, 3 , said that their mobile phones provided numerous benefits and were an 4 part of their lives; some were so 5 to their phones that the researchers considered it an addiction. In Japan, too, researchers are concerned about cell phone addiction. Researchers in one study in Tokyo found that more than half of junior high school students used their phones to exchange e-mails with schoolmates more than 10 times a day.Cell phones 6 social connections with peers across time and space. They allow young people to exchange moment-by-moment experiences in their daily lives with special partners and thus to have a more 7 sense of connection with friends. Cell phones also can 8 social tolerance because they reduce children’s interactions with others who are different from them. In addition to connecting peers, cell phones connect children and parents. Researchers studying teenagers in Israel concluded that, in that 9 environment, mobile phones were regarded as “security objects” in parent-teen relationships—important because they provided the possibility of 10 and communication at all times.A) affiliatedB) attachedC) contact D)contendE)continuous F)diminish G)endurance H)foster I) hazardousJ) improvedK) instantaneousL) intrinsicM) relativelyN) shiftO) similarly
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