Towards Industry 4.0 — #8 Safety in PLC, Latching & Interlock

Andi Sama
5 min readMay 8, 2023

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The Digital Transformation Journey to Industry 4.0 starts by understanding Industry 3.0

Andi Sama CIO, Sinergi Wahana Gemilang with Cahyati S. Sangaji

In Summary
- Implementation of Latching and Interlock, e.g., to change motor direction to Clockwise (CW) or Counterclockwise (CCW), through VFD (Variable Frequency Drive).
- PLC Hardware: Siemens S7-1200 DC/DC/DC.
- Software tools: Siemens TIA Portal on Windows.
- Language: Ladder Logic Diagram.

In our first series of “Towards Industry 4.0” (late 2022–early 2023), we discussed various things covering some of the technological developments on the journey towards Industry 4.0, the convergence of Information Technology (IT), and Operational Technology (OT).

In those five articles (six, including a summary article) — listed in the reference section in this article, we discussed the industrial automation technology driven primarily by Programmable Logic Controller (PLC) along with its surrounding technologies. Field devices such as various sensors and actuators like motors, pneumatic, and hydraulic are examples of that surrounding technologies.

A few working examples were also presented, such as the typical “Hello World” program in PLC, including the mixer, traffic light, and box sorting using conveyor use cases. It covered PLC programming with a ladder logic diagram and SFC (Sequential Function Chart), as well as visualizing PLC data by SCADA (Supervisory Control and Data Acquisition) using Open PLC Simulator software, ScadaBR software, Siemens PLC Simulator software, Siemens TIA (Total Integrated Automation) software, and the actual hardware of Siemens PLC S7–1200.

Latching & Interlock

Latching means when a tag (e.g., the start push button configured to energize an output coil) is pressed and released, the start push button keeps energizing the output coil as the start push button is latched with that coil output. The start push button is not energized, but the output tag configured in parallel to the start push button is the one that keeps the output coil energized.

The interlock mechanism ensures that the other should not be active when certain connections are active. This prevents dangerous operations such as forward and reverse commands simultaneously to a 3-phase industrial motor.

Latching and Interlock can also be implemented physically through physical and mechanical movement.

The Implementation in Siemens S7–1200 PLC

Start and Stop Push Buttons

  • The Start, Forward (CW), and Reverse (CCW) push buttons are buttons with NO (Normally Open) connections, meaning they are only energized when manually pressed. Both are configured as NO in the ladder diagram and the wiring.
  • The Stop push button is configured as NO in the ladder diagram but as NC (Normally Closed) in the wiring. When the Stop push button is pressed, it is de-energized.
Start Push Button configured as NO, parallel to the “Running” tag. The Stop Push Button is configured as NO in this ladder diagram but NC in the physical wiring for safety purposes.
When the Start Push button is pressed, the Running tag is set. The Running tag will be reset if the Stop Push button is pressed.

Latching

When the Start Push button is not pressed, the Running tag is not set. Network 2 is not active.
When the Start Push button is pressed, the Running tag is set. Network 2 is active.

The “Forward command” tag in parallel to the “Forward (CW) button” is for latching the “Forward (CW) button.” In the implementation, the “Forward command” coil, the output of the PLC, can be connected to a VFD (Variable Frequency Drive) ‘forward digital input’ port to set the rotation direction of a 3-phase AC induction motor clockwise.

The “Reverse command” tag in parallel to the “Reverse (CCW) button” is for latching the “Reverse (CCW) button.” In the implementation, the “Reverse command” coil, the output of the PLC, can be connected to a VFD ‘reverse digital input’ port to set the rotation direction of a 3-phase AC induction motor counterclockwise.

Interlock

The “Reverse command” tag (normally closed) in series after the “Forward (CW) button” is for the interlocking mechanism. It ensures that the “Forward command” coil can only be energized if the “Reverse command” tag is not energized.

When the “Forward (CW) button” is pressed and the “Forward command” coil is energized, the “Forward command” coil can only be de-energized by pressing the “Stop button.” To energize either the “Forward command” coil or the “Reverse command” coil, we need to press the “Start button,” followed by either the “Forward (CW) button” or “Reverse (CCW) button.”

When the “Forward (CW) button” is pressed, the “Forward command” coil is energized only if the “Reverse command” coil is not energized. At this moment, even if we press the “Reverse (CCW) button,” the “Reverse command” coil will not be energized. It is ensured by the “Forward command” tag configured as NC, which is in series with the “Reverse (CCW) button” (1 of 2).
When the “Forward (CW) button” is pressed, the “Forward command” coil is energized only if the “Reverse command” coil is not energized. At this moment, even if we press the “Reverse (CCW) button,” the “Reverse command” coil will not be energized. It is ensured by the “Forward command” tag configured as NC, which is in series with the “Reverse (CCW) button” (2 of 2).

The “Forward command” tag (normally closed) in series after the “Reverse (CCW) button” is for the interlocking mechanism. It ensures that the “Reverse command” coil can only be energized if the “Forward command” tag is not energized.

When the “Reverse (CCW) button” is pressed and the “Reverse command” coil is energized, the “Reverse command” coil can only be de-energized by pressing the “Stop button.” As also applied previously, to energize either the “Forward command” coil or the “Reverse command” coil, we need to press the “Start button” followed by either the “Forward (CW) button” or “Reverse (CCW) button.”

When the “Reverse (CCW) button” is pressed, the “Reverse command” coil is energized only if the “Forward command” coil is not energized. At this moment, even if we press the “Forward(CW) button,” the “Forward command” coil will not be energized. It is ensured by the “Reverse command” tag, configured as NC, which is in series with the “Forward (CW) button”

Summary and Moving Forward

In this article, we demonstrate and experiment with PLC's latching and interlock mechanism. The program runs on a Siemens compact PLC S7–1200.

The processing is done using a ladder diagram, part of supported programming languages in the PLC.

More experiments are being planned for the future.

References

  • Andi Sama, Cahyati S. Sangaji, 2023, “Embarking on a Digital Transformation Journey to Industry 4.0 — Automation & Control.”
  • Siemens AG, 2021, “S7–1200 Programmable controller.

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