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1. Any system of program control consists of the following main components: a control program unit, an actuating mechanism and a feedback system. In numerically controlled machines, the program carrier may be punched tapes, magnetic drums and other devices. To put a program into effect, the machine must read that is encoded on the program carrier, reading device being provided for this purpose. As orders are fulfilled, the program carrier must be continuously or intermittently moved relative to the reading device by tape drive mechanisms and intermittent-action devices serving this purpose.
2. The program unit centralizes all the information, which is necessary and sufficient for performing a specified manufacturing operation. This unit converts the information into electric signals, which are fed to the control unit where signals from the program unit are decoded and processed into working orders controlling the sources of power and actuating mechanisms.
3. The actuating mechanism is a unit, which combines mechanisms and devices that put into effect a specified program. Therefore, a program control system includes a program carrier, a program input device, a readout device, a conversion device which converts signals into operating instructions to drive the machine operative members, a feedback system.
4. Control program systems fall into closed and open, analog and pulse, coded and non-coded. The closed program control system, unlike the open system, includes a feedback.
5. A particularity of open program control systems is that they lack devices for checking the execution of instructions and so are less accurate as compared to closed program control systems. However, a substantial advantage of the open control systems as compared to closed systems is their simplicity.
6. The closed program control systems are more complex but are more accurate in the execution of commands because of the introduction of an additional unit for checking the operation of the machine through feedback. In analog system, a signal that determines the motion of a working member is set as a physical parameter, e.g., a voltage which is compared to an analog feedback signal which varies as the working member moves. In pulse counting system, the motion of the working member is set as a sum of pulses: each pulse correlates with motion of the working member through a small distance.
Упражнение 6. Укажите номер абзаца, в котором выражена следующая мысль: «Существенное преимущество систем открытого управления по сравнению с закрытыми – это их простота».
Упражнение 7. В первом абзаце прочитайте предложение с независимым причаст-ным оборотом, а во втором – со страдательным залогом. Предложения переведите.
Упражнение 8. Составьте описание системы программного управления.
Упражнение 9. Озаглавьте абзацы своими собственными предложениями.
Упражнение 10. Составьте план текста.
Упражнение 11. Прочитайте текст еще раз и выделите то новое, что вы узнали из него.
Text 2
Упражнение 1. Прочитайте текст со словарем и ответьте на следующие вопросы:
1. How did man learn to obtain metal from stone?
2. What do we call the age which followed the Bronze Age?
3. Why do scientists consider metals to be the most useful in the form of alloys?
4. Can an alloy have properties which no metal in nature has?
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