Type in the number, hit Xy, type in 0. They were not trying to figure out which stars could host a human habitable planet. They were trying to figure out which stars could host a planet that was not so hideously uninhabitable that no possible form of life could live there.
The theoretical understanding and application dates from the s, and they are implemented in nearly all analogue control systems; originally in mechanical controllers, and then using discrete electronics and latterly in industrial process computers.
Sequential control and logical sequence or system state control[ edit ] Sequential control may be either to a fixed sequence or to a logical one that will perform different actions depending on various system states.
An example of an adjustable but otherwise fixed sequence is a timer on a lawn sprinkler.
A computerized accounting system brings with it many advantages that are unavailable to analog accounting systems. This article does not tackle the use of spreadsheets that are often used instead of proper accounting software to process financial data. An online invoicing solution for freelancers, small business owners, and companies of all sizes. The effort spent in creating invoices and chasing payment takes away from opportunities to focus on those actions that make money. Manual Accounting Systems - advantages, disadvantages and overview of Manual accounts proceedures Manual accounting systems, even in todays world where computerised accounting systems seem to triumph over paper based systems, can still provide an effective and useful way of of recording business transactions and can deliver an efficient accounting information system for the small business owner.
State Abstraction This state diagram shows how UML can be used for designing a door system that can only be opened and closed States refer to the various conditions that can occur in a use or sequence scenario of the system.
An example is an elevator, which uses logic based on the system state to perform certain actions in response to its state and operator input. For example, if the operator presses the floor n button, the system will respond depending on whether the elevator is stopped or moving, going up or down, or if the door is open or closed, and other conditions.
Relays were first used in telegraph networks before being developed for controlling other devices, such as when starting and stopping industrial-sized electric motors or opening and closing solenoid valves. Using relays for control purposes allowed event-driven control, where actions could be triggered out of sequence, in response to external events.
These were more flexible in their response than the rigid single-sequence cam timers. More complicated examples involved maintaining safe sequences for devices such as swing bridge controls, where a lock bolt needed to be disengaged before the bridge could be moved, and the lock bolt could not be released until the safety gates had already been closed.
The total number of relays, cam timers and drum sequencers can number into the hundreds or even thousands in some factories. Early programming techniques and languages were needed to make such systems manageable, one of the first being ladder logicwhere diagrams of the interconnected relays resembled the rungs of a ladder.
Special computers called programmable logic controllers were later designed to replace these collections of hardware with a single, more easily re-programmed unit. In a typical hard wired motor start and stop circuit called a control circuit a motor is started by pushing a "Start" or "Run" button that activates a pair of electrical relays.
The "lock-in" relay locks in contacts that keep the control circuit energized when the push button is released. The start button is a normally open contact and the stop button is normally closed contact. Another relay energizes a switch that powers the device that throws the motor starter switch three sets of contacts for three phase industrial power in the main power circuit.
Large motors use high voltage and experience high in-rush current, making speed important in making and breaking contact. This can be dangerous for personnel and property with manual switches. The "lock in" contacts in the start circuit and the main power contacts for the motor are held engaged by their respective electromagnets until a "stop" or "off" button is pressed, which de-energizes the lock in relay.
Suppose that the motor in the example is powering machinery that has a critical need for lubrication. In this case an interlock could be added to insure that the oil pump is running before the motor starts. Timers, limit switches and electric eyes are other common elements in control circuits.
Solenoid valves are widely used on compressed air or hydraulic fluid for powering actuators on mechanical components. While motors are used to supply continuous rotary motion, actuators are typically a better choice for intermittently creating a limited range of movement for a mechanical component, such as moving various mechanical arms, opening or closing valves, raising heavy press rolls, applying pressure to presses.
Computer control[ edit ] Computers can perform both sequential control and feedback control, and typically a single computer will do both in an industrial application.
Programmable logic controllers PLCs are a type of special purpose microprocessor that replaced many hardware components such as timers and drum sequencers used in relay logic type systems. General purpose process control computers have increasingly replaced stand alone controllers, with a single computer able to perform the operations of hundreds of controllers.
Process control computers can process data from a network of PLCs, instruments and controllers in order to implement typical such as PID control of many individual variables or, in some cases, to implement complex control algorithms using multiple inputs and mathematical manipulations. They can also analyze data and create real time graphical displays for operators and run reports for operators, engineers and management.
Control of an automated teller machine ATM is an example of an interactive process in which a computer will perform a logic derived response to a user selection based on information retrieved from a networked database. The ATM process has similarities with other online transaction processes. The different logical responses are called scenarios.
Such processes are typically designed with the aid of use cases and flowchartswhich guide the writing of the software code. The earliest feedback control mechanism was the water clock invented by Greek engineer Ctesibius — BC Early history[ edit ] Ctesibius's clepsydra 3rd century BC. It was a preoccupation of the Greeks and Arabs in the period between about BC and about AD to keep accurate track of time.
In Ptolemaic Egyptabout BC, Ctesibius described a float regulator for a water clocka device not unlike the ball and cock in a modern flush toilet. This was the earliest feedback controlled mechanism.Hydroelectricity is electricity produced from plombier-nemours.com , hydropower generated % of the world's total electricity and 70% of all renewable electricity, and was expected to increase about % each year for the next 25 years..
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