Design and Application of Online Cosmetics Weighing Scale

Design and Application of Online Cosmetics Weighing Scale

1 Preface
The checkweigher, also known as the weight inspection scale, sorting scale, weight selection scale, inspection scale, and sorting scale, can classify different mass pre-packaged individual loads (items) into two or more categories based on the difference between their mass and the set reference point. It is a high-speed and high-precision online weight detection automation device. Integrated with various packaging production lines and conveying systems, the checkweigher can detect in real-time whether the products on the production line are overweight or underweight, as well as whether the packaging is missing components, etc. The checkweigher is widely used in the automatic weight detection of production lines in the pharmaceutical, food, chemical, beverage, plastic, rubber, and other industries, and is an indispensable part of the production process in the food and pharmaceutical industries.

In order to protect the legitimate rights and interests of consumers, producers and sellers, in accordance with the "Measurement Law of the People's Republic of China" and the "Measures for the Supervision and Administration of Quantitative Packaging Commodities", for quantitative packaging commodities, the actual content of each single piece of quantitative packaging commodity should accurately reflect its marked net content, and the difference between the marked net content and the actual content shall not be greater than the allowed shortage quantity as stipulated. The final inspection of product weight is to recheck the weight of the product at the final production stage, and remove the non-conforming products to ensure that the weight of the products leaving the factory meets the requirements. This is conducive to safeguarding the interests of both consumers and producers. Consumers will not suffer losses due to short weight, and producers will not be damaged in reputation due to customer complaints or even lawsuits.

The existing checkweighers are divided into online detection and offline detection. Online detection includes continuous and intermittent types, while offline detection is generally intermittent. Online continuous detection usually adopts conveyor mode and is integrated with medium and high-speed production lines. Online checkweighers include feeding conveyor, weighing conveyor and rejection conveyor. The system determines the speed of the feeding conveyor based on parameters such as the production line speed, product throughput, product length and weighing conveyor length, to separate the products on the production line, ensuring that only one product is weighed on the weighing conveyor at a time and reducing the impact of different speeds of the front and rear conveyors on the weighing process when products enter and exit the weighing conveyor. For products with a large length-to-diameter ratio, such as long and slender cylindrical products or short cylindrical products, due to their tendency to roll during transportation and their relatively light weight, the state of the products is unstable during passage, which affects the weighing and leads to inaccurate measurement results. Especially for cosmetics (such as eyebrow pencils, lipsticks, etc.), which are small in diameter, long and slender, and can only be transported along the length direction, using a weighing conveyor for weighing, they are prone to rolling during transportation, with poor stability and a significant impact on the weighing process. To overcome the shortcomings of the existing technology, for small-diameter and long slender cosmetics, in collaboration with Shanghai Chuangyuan Cosmetics Co., Ltd., an online checkweigher was installed on the cosmetics production line. It adopts V-shaped trough-shaped guard plates and dynamic high-speed weighing technology to prevent product rolling and maintain the stability of the product during transportation, achieving online high-speed dynamic stable weighing and ensuring the accuracy of online weight detection of the products. The developed online checkweigher for cosmetics has been put into application.

2. Basic Structure and Working Principle of Online Weighing Scale for Cosmetics
2.1 Working Principle
2.1.1 The online weighing scale for cosmetics is composed of a feeding conveyor, a weighing conveyor, a weighing module, a wind shield, a V-shaped trough-shaped guard plate, an unloading conveyor, a rejection device, a weighing controller, an electrical control system and a frame, etc., as shown in Figure 1.

Figure 1 Schematic Diagram of Online Weighing Scale for Cosmetics

Figure 1 Schematic Diagram of Online Weighing Scale for Cosmetics Download Original Image

1- Feeding conveyor
2- Weighing conveyor
3- Weighing module
4- Wind shield
5- V-shaped trough guard plate
6- Discharging conveyor
7- Rejecting device
8- Weighing controller
9- Electrical control system
10- Frame

2.1.2 Working Principle of Online Weighing Scale for Cosmetics

The online cosmetic weighing scale is integrated into the user's cosmetic packaging production line or conveying system. Cosmetics enter through the feeding conveyor and are smoothly guided into the weighing conveyor by the V-shaped trough-shaped guard plate. When the weighing controller is in the external trigger mode, the weight detection starts when the input photoelectric switch detects the cosmetics, and ends when the output photoelectric switch detects the cosmetics, thus obtaining the detected weight value of the product. When the weighing controller is in the internal trigger mode, the internal weight trigger value and the internal weight end threshold are preset. The weight detection starts when the weighing conveyor detects that the weight of the cosmetics is greater than the internal weight trigger value, and ends when the weight of the cosmetics detected by the weighing conveyor is less than the internal weight end threshold, thus obtaining the detected weight value of the product. The weighing controller compares the detected weight value with the target weight value to determine whether the weight of the detected object is qualified. Unqualified products are removed by the rejection device. The speed of the feeding conveyor is adjusted to control the speed at which cosmetics enter the weighing conveyor, ensuring that only one object to be detected is on the weighing conveyor at a time, which guarantees the weighing accuracy of the weighing scale and also ensures the consistency of the speeds of the feeding conveyor, weighing conveyor and discharging conveyor.

2.2 Main Indicators
2.2.1 Dimensions of the inspected workpiece: 200mm × φ10 - 30mm;

2.2.2 The weight of the inspected workpiece is large: 300g;

2.2.3 Inspection capacity of workpieces: 80 pieces per minute;

2.2.4 Conveyor belt dimensions: The lengths of the input conveyor, weighing conveyor and rejection conveyor are all 300mm, and the width is 100mm. The height of the production line is 750 ± 50mm.

2.2.5 Conveyor speed: 0.7 m/s;

2.2.6 Accuracy grade: Grade Ⅲ;

2.2.7 Detection accuracy: ±0.5g

2.2.8 Load cell capacity: 5kg, safe overload: 150%, protection grade: IP65;

2.2.9 Maximum weighing capacity: 500g;

2.2.10 Measurement Method: Dynamic Measurement;

2.2.11 Ejection method: Air blow ejection;

2.2.12 Power Supply: 220V/50Hz

2.2.13 Compressed air: 0.4 - 0.7 MPa;

2.3 System Functions
2.3.1 The system is equipped with both local and remote control functions, and has a communication interface for centralized control as well as interlocking functions with the production line.

2.3.2 It has functions such as automatic zero adjustment, automatic zero tracking, and automatic calibration.

2.3.3 It features static and dynamic calibration to eliminate dynamic effects.

2.3.4 It has the functions of internal trigger and external trigger for reweighing.

2.3.5 It features multiple product settings and selection functions, allowing for easy switching.

2.3.6 It has 5 weight classification zones, and the screen displays them directly.

2.3.7 It has functions of class statistics, daily statistics, monthly statistics and long-term statistics, including the quantity of qualified products, unqualified products (underweight, overweight), qualified product rate, hourly output, etc. It can upload the statistics to the user management system in real time and display them in various graphical forms.

2.3.8 Provide feedback regulation signals to control the weight of product packaging, effectively saving costs.

2.3.9 It has information on non-conforming products and fault alarms, which are displayed by sound and light.

3 Design Calculation
3.1 Determination of the Speed of the Weighing Conveyor
3.1.1 Length of the Workpiece to be Inspected L1: 200mm

3.1.2 Length of the weighing conveyor L2: 300mm

3.1.3 The effective weighing running distance L3 of the inspected workpiece on the weighing conveyor is L2 - L1 = 100mm.

3.1.4 Inspection capacity of workpieces N: 80 pieces per minute

3.1.5 Time t required for a single workpiece to pass through the weighing conveyor: 60/N = 0.75 seconds

3.1.6 The operating speed v of the weighing conveyor: (L1 + L2) / t = 0.67 m/s

To maintain the consistency of the speeds of the inbound conveyor, the weighing conveyor and the rejection conveyor, the operating speed V of the conveyor is set at 0.7 m/s.

3.1.8 The actual weighing time T of the inspected workpiece on the weighing conveyor is: L3/V = 0.143 seconds.

3.1.9 Sampling frequency f of the weighing controller: 0.005 s

3.1.10 Sampling number n of the weighing controller: T/f = 28 (satisfying the requirement of n ≥ 20)

3.2 Weighing Sensor Selection
3.2.1 Weight of the Weighing Conveyor Platform G1: 3.7 kg

3.2.2 Number of load cells n1: 1

Load of the weighing sensor: G1/n1 = 3.7 kg

3.2.4 The HBM PW6KRC3 single-point load cell is selected. According to the sensor selection manual, the rated load (range) is set at 5 kg.

3.3.1 Determination of the Method for Rejecting Nonconforming Products
3.3.1.1 Quantity of inspected workpieces: large
3.3.1.2 Weight: 300g

3.3.2 Inspection capacity: 80 pieces per minute

3.3.3 When the weight of the inspected workpiece is relatively light (<500g) and the passing speed is high, the rejection method selected is: air-blow rejection.

4. Main Structure and Technical Features
4.1 The weighing conveyor is composed of main and driven rollers, belts, servo motors, frames, etc. The main and driven rollers adopt a barrel-shaped structure design, effectively preventing the belt from deviating. At the same time, the main and driven rollers need to undergo dynamic balance tests, with a grade of 6.3G (deviation 0.3g), to avoid the impact of dynamic imbalance of the main and driven rollers on weighing and measurement. The weighing conveyor is driven by a servo motor, which can adjust the running speed of the conveyor in a timely manner according to the length and throughput of the inspected workpiece, ensuring that only one product is on the weighing conveyor table for weighing. The servo motor and the main roller are connected by a synchronous toothed belt, which ensures smooth transmission and no noise. The weighing conveyor belt is equipped with V-shaped groove-shaped guard plates, effectively preventing cylindrical products from rolling during the conveying process, ensuring the stability and accuracy of weighing and measurement. The weighing conveyor is equipped with a wind shield to prevent the influence of external wind on weighing and measurement, and also to prevent personnel from touching the weighing conveyor and affecting weighing and measurement. The weighing conveyor and the frame are designed with a tenon structure, which can be quickly released by latching, facilitating the cleaning and maintenance of the conveyor. Photoelectric detection switches are installed at the main and driven rollers of the weighing conveyor to detect whether the product has completely entered the weighing conveyor and whether the product is about to leave the weighing conveyor, ensuring that the entire product is weighed and measured on the weighing conveyor and guaranteeing the accuracy of weighing.

4.2 The structure of the feeding conveyor, discharging conveyor and weighing conveyor is the same, but the main and driven rollers do not undergo dynamic balance tests.

4.3 The weighing module adopts a fully enclosed structure design, consisting of a base shell, a weighing sensor, a connecting seat, etc. The base shell is equipped with an overload protection bolt below the connecting seat and the weighing sensor. After the weighing module is assembled, a ballast test needs to be conducted. When the load is increased to the rated load, the output of the weighing sensor is 2 mV. By adjusting the overload protection bolt, if the load is further increased beyond the rated load, the output millivolt value of the weighing sensor remains unchanged. The weighing sensor selected is the HBM PW6KRC3 single-point weighing sensor, with a maximum scale size of 300mm × 300mm.

4.4 The rejection method can be selected as air-blowing type or cylinder push-rod type based on product weight and throughput, etc. For products weighing less than 500g, the air-blowing type rejection can be adopted. The air-blowing type rejection has a simple structure and is highly efficient. The rejection device is fixed on the discharge conveyor and classifies the non-conforming products (underweight or overweight) by weight. Multiple rejection devices can be used to ensure that non-conforming products are respectively sent to the corresponding collection boxes, as shown in Figure 2. The collection boxes for non-conforming products are designed with a fully enclosed structure and are equipped with a material access door and lock. They are managed by designated personnel to ensure effective management of non-conforming products.

In the electrical control system, the Siemens S7-200SMART (6ES7288-1ST60-0AA0) programmable controller is selected. Upon receiving the operation signal from the user's production line, the electrical control system starts to operate automatically. Meanwhile, if the online weight checking scale for cosmetics issues a fault alarm, the fault information will be fed back to the user's production line control system. When the photoelectric switch of the feeding conveyor detects the product, the online weight checking scale for cosmetics operates to weigh and measure the product online. Unqualified products are removed from the production line through the rejection device. The real-time weight detection and statistics results provide feedback regulation signals to control the weight of the product packaging.

Figure 2 Pneumatic Schematic Diagram

Figure 2 Pneumatic Schematic Diagram Download Original Image

5 Conclusion
Based on the dynamic weighing technology of belt conveyors and controlled by PLC, cosmetics on the production line are conveyed into the weighing conveyor through the feeding conveyor. The weighing controller adopts either the external trigger mode or the internal trigger mode to conduct online weighing and measurement of the products. The obtained weight values are compared with the pre-set target weight values to determine whether the weight of the inspected items is qualified. Non-conforming products are removed by the rejection device, achieving the entire process without human intervention and real-time online weight detection. Meanwhile, the statistical results of weight detection are provided in real time as feedback regulation signals to control the weight of product packaging, effectively reducing costs. This technical product is applicable to the online weight detection of product production lines in industries such as pharmaceuticals, food, chemicals, beverages, cosmetics, plastics, and rubber.