What Is Sequence Control

What Is Sequence Control?

Sequence control (Sequence Control) is a control method that executes each stage in a fixed order according to predetermined sequences and conditions. It is widely used in industrial fields such as factory automation, but sequence control itself is not a program — it is a "concept and approach to control."

The behavior is programmed in advance, and equipment is operated according to that program. Control that is programmed to execute a fixed series of actions in a specific order is called sequence control. It can be implemented using general-purpose programming languages such as C, or — as in the past — through mechanical means like cam mechanisms.

Understanding Sequence Control with a Washing Machine

The most familiar example of sequence control is a washing machine.

When you press the Start button, the washing machine follows its internal program and operates in this sequence:

1. Water Fill: Fill water to the set level
2. Wash: Run the motor to agitate the laundry
3. Drain: Discharge the dirty water
4. Rinse: Rinse the laundry with clean water
5. Spin: Remove water by high-speed rotation

The fact that this entire series of operations — Water Fill → Wash → Drain → Rinse → Spin — is executed automatically in a predetermined order is the essence of sequence control. Without any individual instructions from the user, pressing a single button triggers all the steps to run automatically.

Applying Sequence Control to Factory Robot Arms

The same type of control is used on factory conveyor lines.

When a product arrives on the conveyor, the robot arm operates in this sequence:

1. The sensor detects the product
2. The hand moves above the product
3. The hand descends
4. The hand closes to grip the product
5. The hand rises and moves to the designated position
6. The product is placed on the table (pallet)

This entire series of actions is also pre-programmed. Using the sensor signal as a trigger, actuators are operated in a defined order — this is sequence control.

Sequence Control in Elevators: A Real-World Example

A more everyday example is an elevator.

An elevator operates in this sequence:

1. Press the call button on the landing
2. The elevator moves to the specified floor and the door opens
3. Passengers board and press the destination floor button
4. The door closes and the elevator moves to the destination floor
5. Upon arrival, the door opens

Based on the button input condition, multiple actions are executed in a defined order. This is also a classic example of sequence control.

The Relationship Between PLC and Sequence Control

There are various ways to implement sequence control, but the most widely used in modern manufacturing environments is the PLC (Programmable Logic Controller).

PLCs primarily describe control logic using a graphical language called Ladder Diagram (Ladder Program). Because its notation resembles electrical circuit diagrams, it is relatively easy for electrical engineers to understand.

If you are involved in the design or maintenance of automated equipment or automated production lines, mastering PLC-based sequence control is essential. The next page explains the mechanism of PLCs in more detail.

Next Page: What Is PLC

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