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PLC, A smart brain for tea & coffee packaging equipment

plc in automatic equipment

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Keywords: PLC, Automatic packing, Tea packing equipment, Coffee packing machine.

At present, an excellent tea packaging machine could finish the task of packing tea products in batches. It can complete various packing processes in an orderly manner. Including measuring, feeding and filling, pulling the film, bag sealing, and so on. The automatic tea packing equipment runs efficiently without any unnecessary action.

As a die-hard fan of automated packaging, it’s all thanks to PLC. How frustrating it is to operate a packaging machine without PLC! Just like you have to use a cell phone made 10 years ago.

You might be curious about that, such as what is PLC? How PLC works? What are the benefits of automated packaging? And how to choose? Then in this article, you will get answers to all the above questions

What is PLC?

plc controller

PLC is called a Programmable Logic Controller. It refers to a specially designed industrial computer control system. As a reliable controller, the PLC can perform in harsh industrial environments. Such as damp, dusty, and even extreme temperature conditions.

When applied to the food packaging industry, the PLC is used to automate packing processes. It can monitor all variables in packaging production and input them into the CPU. By running in the stored program, the computer makes a decision. And then, it controls outputs to automate the packing machine in time.

Basic Structure of PLC

The structure of PLC is like that of office computers. A complete PLc with the following module.

Powder supply module

Usually, the PLC can run normally on a low voltage 24 V DC. The power module is used to convert the available external AC power into DC power required by the CPU, I/O module.

I/O modules

inputoutput module

Input module

Think about it, how does the packaging machine sense the weight of tea materials? We need to translate this specific data into a signal that the computer can understand. Of course, the input module A / D (analog to the digital module)handles the function of data conversion.

Usually, it is connected to external devices, such as various switches, valves, sensors, relays, and so on. The input module can monitor the data, such as the weight of tea material, bag length, and the temperature of the sealing device. What’s more, it can convert analog inputs into digital signals for CPU analysis and processing.

Output module

The output module work in the reverse process. That is to say, the output module D / A (digital to Analogue module) converts the received digital signal into the corresponding packaging instruction. So as to control different parts to complete the corresponding action.


The Central Processing Unit,can store programs and execute specified programs. Continuously evaluate the status of input, output, and other variables through a loop. Finally, the CPU will then send out the updated commands via the output module.

HMI module(Human-machine interface)

HMI is also known as Communication Module. It is an operator interface device that can simplify monitoring the packing progress. It can display All the states of the packaging machine.

Basic functions of PLC

As its name implies, Programmable Logic Controller performs three basic functions as follows:


The CPU Can store the packing program based on the actual requirements. And the stored program can be easily changed or reprogrammed.


The PLC can input a variety of variables, which came from the switches and sensors. For example, the range of voltage or current values is often used to describe the packaging process conditions. It can convert all kinds of analog data into electrical logic signals. Furthermore, the CPU executes the designed program. including adding, comparing, multiplying, subtracting, dividing, negation, etc.


The CPU sends the operation result to update the status of the outputs device. That means the PLC can control and regulate the packing process.

How PLC works in automatic packaging equipment.

PLC, the key to automatic Packaging

You may think the automated packaging process is no big deal. Before the PLC came about, this automated packaging process need to break into steps. This approach increases the loss of time and labor. It also brings uncertainties to product quality.

For instance, the weighing part only performs measuring. Another device plays the role of sealing them up properly. In this way, more operators are needed for finishing the packing tasks on time. And each task needs its particular equipment to be complete.

Moreover, prior to PLC, hard-wired technology is often used to control the packing process. Electricians must install the specified electrical components according to the designed circuit diagram. But, it consumes a lot of power and needs regular maintenance. That means it required extensive component changes and rewiring for a technical failure. it takes too much time and money to figure it out.

The PLC can integrate the subsystems with different purposes. As a result, a simplifies the packaging processes came about.

Working Principle of PLC

In a nutshell, the task of PLC is to repeatedly evaluate the relevant data and execute the stored program. There are 3 small cycling scans continually taking place in a repeating loop. We call it a scan cycle for PLC.

plc working principle

1. Input scan: Monitor the status of all input modules.

2. Program scan: The CPU executes the set logic program to get the operation result.

3. Output scan: Updates the output status and performs specific steps.

After completing the instructions, the system evaluates the entire packaging process and then goes back to the next loop. It takes only a short time to execute the program once, usually in milliseconds. It is the scan time of a PLC.

Advantages of PLC

There are some advantages to using PLC in food packaging equipment.

Modular design idea, easy to install and maintain. No need to wire many components.

Adopt a widely accepted programming language. Easy and fast to change the logic program.

Self-diagnostic is available. The fault can be easily judged by monitoring the status of the program.

It has great computational capabilities and a fast operating time.

There is no doubt that PLC brings excellent packaging performance. So What are the common PLC types? What is their feature? Keep on reading.

Types for PLC in automatic packaging equipment

Types of PLC Base on Hardware setup

When it comes to the types of PLC, there are two major types of PLC.

Compact PLC

omron compact plc

It is also called fix plc. All the functional components are integrated into a single case. It has a fixed number of input and output modules. The power supply, micro-controller, and communication cards are determined by the manufacturer. So there is no redundancy to expand extra input and output modules for compact modules. You can get some idea from the below image.

The compact PLC is a good choice for small-scale automatic packing operations. For example, Model XH-68 is a double chamber tea bag packaging machine. It is a well-designed and tried-and-tested packing equipment. We have adopted the compact PLC of the Omron brand. It brings sufficient ability to perform packaging control well.

Modular PLC

siemens modular plc

This is a PLC that can be easily expanded by adding input and output units. It brings you more flexibility. The independent modular structure design makes it easier to use.

In modular PLC, the power supply, communication module, and input/output module are all separated to the microcontroller. To create a PLC, the electricians need to connect the wires and combine the various modules. Depending on its high scalability and more customized options, Modular PLC can handle a higher number of devices. We prefer to use it in large-scale packaging equipment with complex processes.

Comparison between compact PLC and modular PLC

1. Extensibility

In general, The compact PLC has a fixed and compact shape. A fixed number of input and output modules limit its ability to perform the control. If the control operations are not onerous, fewer devices need to be connected to the PLC. Then the integrated PLC is competent and reduces the cost.

Modular PLC is not the case. You can add relevant input/output modules based on the actual needs. For example, you can add similar output modules to perform the same packaging task. In this way, you can expand an extra packaging line. You can even extend and match different modules to achieve more customized goals.

2. Troubleshooting and downtime

Due to its highly integrated design, compact PLC can be used in more small packaging machines. When it comes to troubleshooting, it is trapped. Once one of the modules fails, the compact PLC stops working. Troubleshooting requires proficient PLC inspection and maintenance personnel. In many cases, the whole PLC needs to be dismantled and sent back to the original factory for maintenance. It takes a lot of time.

In Modular PLC, each functional module works independently. That means the probability of multiple modules’ failure at the same time is very low. When a fault occurs, the electrician who has the knowledge of the proprietary circuitry of the PLc could troubleshoot it easily. The simple fact is that less downtime for the packing machine with a modular PLC.

Of course, PLC also can be classified in other criteria.

Type of PLC according to Output

In the automatic control system, the output module is related to the load types.

Relay output

Both DC and AC are applicable. The PLC controls the switching of the relay through the electromagnetic principle. But, the mechanical wear caused by frequent switching will reduce the service life of the relay. So, the relay type output is more suitable for operations that do not need to switch frequently. For example, activate a motor that requires a continuous operation.

Transistor output

The transistor output is a semiconductor device, which can only handle DC power output.

When a transistor output works, it responds faster than a relay, usually at 0.2 milliseconds. Besides, there are no moving mechanical parts in the transistor. PLC can output high-frequency action without considering mechanical wear. So the transistor output is also called a solid-state device.

Triac Output

TRIAC output is a semiconductor device that allows current to flow in both directions. It can be used in controlling the switch in AC conditions. It is also a kind of solid-state device with long service life, high sensitivity, and fast conversion.

Analog Output

As mentioned above, the output modules have only two states: ON and OFF. They are used to ON or OFF any control operations in the packing machine. They work in a binary (0 or 1) output from PLC, also known as digital outputs.

In fact, the output device sometimes needs to carry out continuous dynamic control. For example, the analog output PLC can control the running speed of devices, such as motors.

Types of PLC according to physical size

The size of the PLC is related to the number of input and output units connected. Base on the number of I/O features that a PLC can handle, The PLC can be further divided into 3 groups.

Small PLC is used in applications controlling less than 128 I/O Devices.

Medium PLC can control 64 to 1024 I/O devices.

Large PLC controls 512 to 4096 I/O devices.

In general, the program memory size is positively correlated with the number of input/output modules.

Small PLC has a compact structure and a lightweight. It can be installed in the control panel of the packaging machine. So as to replace the original logic control completed by wired relays, counters, timers, and so on.

Conclusion/Final Thoughts

There is no doubt that PLC is the heart of automatic packaging equipment. With a brief introduction of PLC, users can further understand the performance of automatic packaging equipment. In the future, we will share more knowledge of automatic packaging with you.

Picture of Eric Lu

Eric Lu

Xhteapack Marketing Sales Manager, I am responsible for the company's social media campaigns and blog posts.

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