Monday, February 19, 2018

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Automated processes can be controlled by human operators, by computers, or by a combination of the two. If a human operator is available to monitor and control a manufacturing process, open loop control may be acceptable. If a manufacturing process is automated, then it requires closed loop control.

Figure 0.2 shows examples of open loop control and closed loop control. One major difference is the presence of the sensor in the closed loop control system. The motor speed controller uses the feedback it receives from this sensor to verify that the speed is correct, and drives the actuator harder or softer until the correct speed is achieved. In the open loop control system, the operator uses his/her built-in sensors (eyes, ears, etc.) and adjusts the actuator (via dials, switches, etc.) until the output is correct. Since the operator provides the sensors and the intelligent control functions, these elements do not need to be built into an open loop manufacturing system.

Human operators are more inconsistent than properly programmed computers. Anybody who has ever shared the road with other drivers is familiar with the disadvantages of human control. Computerized controls, however, can also make mistakes, when programmed to do so. Programming a computer to control a complex process is very difficult (which is why human automobile operators have not yet been replaced by computers).

The recent development of affordable digital computers has made automation control possible. Process control has been around a little longer. The difference in the meanings of these two terms is rapidly disappearing.

Process control usually implies that the product is produced in a continuous stream. Often, it is a liquid that is being processed. Early process control systems consisted of specially-designed analog control circuitry that measured a system's output (e.g., the temperature of liquid leaving a tank), and changed that output (e.g., changing the amount of cool liquid mixed in) to force the output to stay at a preset value.

Fig. 0.2 Open loop and closed loop speed controlFig. 0.2 Open loop and closed loop speed control

Automation control usually implies a sequence of mechanical steps. A camshaft is an automation controller because it mechanically sequences the steps in the operation of an internal combustion engine. Manufacturing processes are often sequenced by special digital computers, known as programmable logic controllers (PLCs), which can detect and can switch electrical signals on and off. Digital computers are ideally suited for "automation control" type tasks, because they consist of circuits each of which can only he either "on" or "off."

Process control is now usually accomplished using digital computers. Digital controllers may he built into cases with dials and displays which make them look like their analog ancestors. PLCs can also be programmed to operate as analog process controllers. They offer features which allow them to measure and change analog values. Robots and NC equipment use digital computers and a mixture of analog and digital circuit components to control "continuous" variables such as position and speed.

Advances in automation and process control have been rapid since the start of the silicon revolution. Before modern silicon devices, controllers were built for specific purposes and could not be altered easily. A camshaft sequencer would have to have its camshaft replaced to change its control "program." Early analog process controllers had to be rewired to be reprogrammed. Automation systems of these types are called hard automation. They do what they are designed and built to do, quickly and precisely perhaps, but with little adaptability for change (beyond minor adjustments). Modification of hard automation is time-consuming and expensive, since modifications can only be performed while the equipment sits idle.

As digital computers and software improve, they are replacing hard automation. Digital computer control gives us soft automation. Modem digital computers are reprogrammable. It is even possible to reprogram them and test the changes while they work!

Even if hardware changes are required to a soft automation system, the lost time during changeover is less than for hard automation. Even a soft automation system has to be stopped to be retro-fitted with additional sensors or actuators, but the controller doesn't have to be rebuilt to use these added pieces.

Digital computers are cheap, powerful, fast and compact. They offer several new advantages to the automation user. A single digital controller can control several manufacturing processes. The designer only needs to ensure the computer can monitor and control all processes quickly enough, and has some excess capacity for future changes. Using digital computers, the equipment that is to be controlled can be built to be more "flexible." A computer controlled milling machine, for example, can come equipped with several milling cutters and a device to change them. The computer controller can include programs that exchange cutters between machining operations.

Soft automation systems can be programmed to detect and to adapt to changes in the work environment or to changes in demand. An NC lathe can. For example, modify its own cutting speed if it detects a sudden change in the hardness of a raw material being cut. It may also change its own programming in response to a signal from another automated machine requesting a modification in a machined dimension.




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