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Sequence Control Requirement
Sequence control technology is applied to each and every home appliance as well as such various machines and equipment as vender machines, plant machines, automation equipment, elevators, boilers and power plants. Its application varies from a simple ON-OFF control to a large scale and complicated signal control system. It is not an exaggeration to say that almost all production lines in the plants use this technology including all advanced automobile production plants.
About Sequence Control
When consulted with an ordinal dictionary, the brief meanings of “sequence”(si:kwens)may be summarized as (a) chronological order of events, (b) continuation of events, (c) having order and (d) causing a result. In JIS-C0401 (Japan Industry Standard), sequence control is defined as “Step-by-step process control based on the preset order”. In the sequence control, the succeeding process is programmed to take place upon completion of the preceding process.
Sequence Control Example
Full-Automatic Washing Machine
In a full-automatic washing machine, all operations of (1) Water supply, (2) Wash, (3) Rinse and (4) Dewatering are processed in the due order. This casually accepted machine is a very good example to understand “Sequence Control”. This full-automatic washing machine takes the typical step-by-step processes as shown here.
- Issues a start signal if the start switch is pressed (after supplying clothes and detergent).
- Opens a water supply valve.
- Closes the valve when water reaches to a preset height.
- Activates a washing motor for washing.
- Stops the washing motor after a preset period of time.
- Opens a water-drain valve.
- Closes the valve upon completion of drain.
- Opens the water-supply valve and closes it at a preset height.
- Activates the washing motor for still-water rinsing.
- Stops the washing motor after a preset period of time.
- Opens the water-drain valve.
- Closes the valve upon completion of drain.
- Activates the washing motor for over-flow rinsing while keeping the water-supply valve open.
- Stops the washing motor and close the water-supply valve. Then open the water-drain valve.
- Closes the water-drain valve.
- Activates a spin motor for dewatering.
- Stops the spin motor after a preset period of time.
- Activates buzzer alarm to announce the end of operations.
After a careful study of the operations processed in the full-automatic washing machine, it may become clear that a sequence control is applied to realize the convenient service of this machine.
Sequence Control Element
Let's look another example of sequence control to study its configuration and control methods. An elevator provided in building responds to the call buttons from each and every floor or the destination buttons inside of it so as to go up or down to the required floors, then open and close after a preset period of time.
As shwon here, the elevator performs a step-by-step operation in the programmed order thus clearly suggesting an application of sequence control. This sequence control of the elevator consists of the following control methods.
- When a call button at the 4th floor was pressed after that at the 1st floor while the elevator cage is at the third floor, the elevator responds first for the call from the 1st floor since it came first. This type control method is called “Sequential Control”.
- When the elevator cage stops at a floor, the elevator door opens by “Door Open” instruction and closes after a preset period of time. This type control method is called “Time Control”.
- Only when the elevator door is completely closed and a call or destination button is pressed, the elevator cage starts going up or down. Before the elevator cage starts to move, these conditions must be satisfied first. This type control method is called “Conditional Control”.
The elevator control is a combination of these three control methods: “Sequential Control”, “Time Control” and “Conditional Control”.
Let's check now what kinds of devices are required in each control method.
- Sequential Control:
- Uses sensors to detect the end of each process in the programmed order, and then starts the next process. For this purpose, sensors such as limit switches, level switches and the like are required to detect and transmit signals.
- Time Control:
- Uses timers to start the next process at a preset timing in the programmed order.
- Conditional Control:
- Uses signals from the sensors to determine the next process according to the programmed condition. Electromagnetic Relays are used to issue new branching instructions.
As shown above, a sequence control generally consists of “Command Unit”, “Processing Unit”, “Control Unit”, “Sensor Unit” and “Monitor Unit”.
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(C) 2004 ADWIN Corp.