The Most Preferred PLC Programming Methods by Electrical Engineers: A Typical Case of Drill Control Using Universal PLC Programs

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Task Introduction

In the field of industrial control, sequential control is one of the core skills of PLC programming, especially suitable for equipment that operates in a fixed process, such as machine tool processing and assembly lines. The Siemens S7-1200, as a main controller for small to medium automation scenarios, can perfectly implement complex sequential logic with its flexible programming methods. This article takes the dual-head drill as a typical case, guiding you step-by-step from task analysis to functional diagram design and then to ladder diagram programming, helping you master the core logic of sequential control.

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Task Assignment

Device Background

A certain drill uses two drill heads to process two holes in a workpiece simultaneously, achieving a fully automated processing flow through PLC control, involving actions such as workpiece clamping, drill head downward/upward movement, and limit detection.

Control Requirements

1. Initial State: Both drill heads are at the top position, with limit switches SQ1 (Drill Head 1) and SQ3 (Drill Head 2) both “ON”; the workpiece is in an unclamped state.
2. Start Process: After the operator places the workpiece, they press the start button I0.0, and the system begins automatic operation.
3. Operation Steps:

• Step 1: The workpiece is automatically clamped (proceed to the next step after clamping is complete);
• Step 2: Both drill heads move downward simultaneously (drilling);
• Step 3: When Drill Head 1 moves down to limit switch SQ2 (“ON”), it stops moving down and starts moving up; when Drill Head 2 moves down to limit switch SQ4 (“ON”), it stops moving down and starts moving up;
• Step 4: Drill Head 1 stops moving up when it reaches SQ1 (“ON”); Drill Head 2 stops moving up when it reaches SQ3 (“ON”);
• Step 5: When both drill heads return to the upper limit (SQ1 and SQ3 are both “ON”), the workpiece is automatically released;
• Step 6: After the workpiece is released, the system returns to the initial state, waiting for the next start.

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Task Implementation

1. Draw Hardware Circuit Diagram

Define PLC Inputs (Detection Signals) and Outputs (Execution Signals) Based on Control Requirements:

2. Functional Diagram (SFC) Design

The functional diagram is the “skeleton” of sequential control, intuitively expressing logic with steps (S), transition conditions (X), and actions (Y), dividing the drilling process into 6 key steps (sequences), clarifying the “execution actions” and “transition conditions” for each step:

Initial StepS0:Q0.0 (clamping) = OFF, Q0.1~Q0.4 (drill head actions) = OFF; only when I0.0 (start) = ON, transition to S1.

StepS1:Q0.0 = ON (workpiece clamped); set clamping delay T37 (e.g., 2 seconds), transition to S2 when T37 = ON.

StepS2:Q0.1 = ON (Drill Head 1 downward), Q0.3 = ON (Drill Head 2 downward); when I0.2 (SQ2) = ON, Drill Head 1 prepares to move up; when I0.4 (SQ4) = ON, Drill Head 2 prepares to move up.

StepS3:Q0.1 = OFF, Q0.2 = ON (Drill Head 1 upward); Q0.3 = OFF, Q0.4 = ON (Drill Head 2 upward); when I0.1 (SQ1) = ON and I0.3 (SQ3) = ON, transition to S4.

StepS4:Q0.0 = OFF (workpiece released), Q0.2 = OFF, Q0.4 = OFF; set release delay T38 (e.g., 2 seconds), return to S0 when T38 = ON.

Based on the working steps of the drill, the following sequential functional diagram is drawn.

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Task Implementation

First, complete the device configuration, select to add a new device—controller—choose the CPU 1214C DC/DC/DC PLC, and select version V4.5 to match the physical device.

Create PLC variable table

Based on the functional diagram, implement using Siemens S7-1200 ladder diagram (LAD), with the key logic as follows:

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Task Acceptance

Testing Steps

• Simulation Testing: Use PLC simulation software (such as TIA Portal’s PLCSIM) to simulate input signals and verify whether each step transition is correct:

• Press I0.0, observe whether Q0.0 is powered (clamping);
• After the delay, check whether Q0.1 and Q0.3 are powered (drill heads moving down);
• Trigger I0.2 and I0.4, check whether Q0.1 and Q0.3 lose power, and whether Q0.2 and Q0.4 are powered (moving up);
• Trigger I0.1 and I0.3, confirm that Q0.2 and Q0.4 lose power, and Q0.0 loses power (release);
• After release, check if it returns to the initial state.

• On-site Debugging: Connect to the actual device, focusing on testing the reliability and continuity of limit switches to avoid logical errors due to mechanical delays.

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