SEQUENTIAL CONTROL SYSTEM
In many situation, various operations of a plant or process take place in a particular order. A sequential control system involve the sequential execution of well- defined operations that are performed in a prescribed order. Each operations or activity is called step. Each step may be an open or closed loop continues process or even a sequential sub- process. For example, while using automatic camera, the various basic steps in a sequence are switch on, battery check, auto- focus the image, auto flash on/off, taking the image, saving the image and then switching off the camera.
Each step of the prescribed sequence usually requires a switching of the equipment configuration and it may be triggered by time or an event(push of a button, completion of early task etc.). the sequential controller may be classified into two types: 1. Event-based and 2. Time -based. In event based controllers, the next event or step cannot be performed until the previous event or step is complete. In time-based controllers, the series of operations are sequenced with respect to time event-based controllers are more reliable than time-based controllers. Traditional such as control could be obtain by an electrical circuit with sets of cam-operate switches or relays which are wired up in such a way as to give the desired sequence .industrial sequence controllers may employ a relay or semiconductor logic. More complicated operations are handled by programmable logic controllers(PLCs).
Example of sequential control system:
Automatic Domestic Washing Machine System:
As a example of sequential controller, consider domestic washing machine system shown in fig. in which processes such as pre- wash cycle, main wash cycle and spin cycle are performed in a particular sequence as follows.
1. A pre wash cycle in which clothes in the drum are live wash in cold water.
2. A main wash cycle wherein clothes are washed in hot water.
3. A rinse cycle where clothes re rinsed with cold water a number of preset time, and
4. A spin cycle in which the spinning of drum takes place to drain the water from clothes and the drum
The various process of the washing machine as stated above were carried out using cam operated switches in early days.in modern automatic washing machine, the cam operated switches are replaced with the micro processor based controller where the software programs are fed to perform various sequential operations.
Step 1: Pre-Wash Cycle
In this cycle , an electrically operated valve opens to allow cold water into the drum for a period of time determined by output from the microprocessor. A level sensor is used to check whether the drum is filled to the present level. When the water reaches a preset level , the sensor ives output to microprocessor which in turn stops the water supply to the drum switching off the current to the valve.
Step 2: Main Wash Cycle
When the pre-wash is completed , the microprocessor activates an electrically operated valve to open and allow the cold water into the drum for a period of time. The level is sensed by a level sensor and the water is shut off when the required level reaches in the drum. Now, the microprocessor activates the switch to supply current to electric heater to heat the water for main wash.
Step 3: Rinse Cycle
When the main wash is completed, the microprocessor gives an output for rinse cycle, it opens the valve to allow cold water to drum and it closed when it reaches a preset level. A drum motor is operated to rotate the drum and it closed when it closed preset level.
Step 4: Spin Cycle
The microprocessor switches on the drum motor and it is signaled to rotate at a higher speed then rinsing cycle. Due to the centrifugal action, the water drains the from the clothes.