Automatic control of the loading process of the weighing scale

Automatic control of the loading process of the weighing scale

1. Overview
Some goods packaged in bags, bottles, cans, etc. need to be filled first and then weighed on a checkweigher to ensure that the net weight of the goods inside the package is consistent with the weight indicated on the package. However, due to changes in the mechanical characteristics of the filling machine, changes in the properties of the materials, and other reasons, the weight of the goods filled into each package may vary, resulting in overfilling or underfilling. Excessive overfilling affects the company's profits, while underfilling may lead to customer complaints and damage the company's reputation.

If the goods are in liquid, powder or small granular form during the filling process of the packaging, the filling process can also be automatically controlled through the real-time measurement values of the weighing scale to ensure that the weight of the goods marked on the packaging is accurate and not overly heavy.

2. Automatic Control of Loading on the Checkweigher
When the loading machine is under automatic control based on feedback signals, the detection data from the checkweigher is compared with the preset target weight value of the goods. A feedback signal is then sent to the controller, and the loading amount of the loading machine is adjusted through the actuator, thereby reducing the weight error of the goods and the variation in the loading amount of the goods caused by the drift of the loading machine's loading amount (as shown in Figure 1). Since it takes some time for the goods loaded by the loading machine to reach the checkweigher, the weighing display needs to delay before making the control calculation for the next loading amount adjustment of the loading machine.

Figure 1 Schematic Diagram of Automatic Control Principle for Loading of Weighing Scale

Figure 1 Schematic Diagram of Automatic Control Principle for Loading on the Weighing Scale Download Original Image

2.1 Basic Types of Loading Control
There are two basic types of loading control that can be operated on a checkweigher. They are:

Continuous loading control: This continuous control system can generate a control signal whose output is proportional to the difference between the weight of each package and the target weight set by the user. It is a regulating system with an extremely fast response time.

Average loading control: Average loading control requires setting a "duration" first, which can be determined by the number of packages passing through within that time. For example, when the throughput is 120 pcs/min, if 20 packages are determined to pass through, then the "duration" is 10 seconds. The control system calculates the average weight of the 20 packages within the 10-second duration, compares the difference between this value and the target weight set by the user, and generates a correction signal to adjust the filling volume of the filling heads. This system is mostly used in filling machines with multiple filling heads.

2.2 Types of Feedback Control Signals for the Charging Machine
There are three types of feedback control signals sent to the charging machine:

Analog modulation: Analog control signals, such as voltage or current loop control signals.

Pulse frequency: The control signal is a series of output pulses based on the direction and amplitude of the weight deviation.

Pulse duration: The control signal is a single pulse with a varying duration based on the direction and magnitude of the weight deviation.

2.3 Double-Check Weighing Scale Feedback Control System for Loading
When high packaging accuracy is required and the tare weight variation of the commodity packaging needs to be taken into account, a double-check weighing scale feedback control system for loading can be adopted as shown in Figure 2. In this system, the weight of the empty bottle is detected by weighing scale 1, and the signal is transmitted to the weighing display of weighing scale 2 for the detection of the net loading weight. The detection data of the net loading weight is input as a feedback signal to the loader, thereby achieving real-time control of the net loading weight.

Figure 2 Schematic Diagram of the Automatic Control Principle for Double-Check and Re-weighing of Loading Quantity

Figure 2 Schematic Diagram of Automatic Control Principle for Double-Check and Re-weighing Loading Quantity Download Original Image

3 Standard Automatic Control Process
During the feedback automatic control process of the loading machine, the weighing scale and the loading machine are constantly communicating. If a deviation in the weight of the goods is detected, the loading volume of the loading machine is adjusted before it has a significant negative impact on production to ensure that the impact of the deviation can be corrected. The following are several steps in the process where the weighing scale sends feedback signals to the loading machine and performs automatic control [90]:

Step 1 - Deviation occurs in the loading machine

Step 1 shows that the weighing scale detects a deviation in the loading quantity of the loading machine in the decreasing direction (as shown in Figure 3). If this trend continues, the deviation in the loading quantity will increase, and the goods may be underweight (the underweight goods are marked with a cross).

Step 2 - Send a feedback signal to the loading machine

Step 2 shows that the feedback signal from the weighing scale is sent to the loading machine to adjust the loading quantity (as shown in Figure 4). However, there is a lag time during which the weighing scale will no longer guide the loading machine to make adjustments.

Figure 3 Deviation of the loading machine

Figure 3 Deviation of the loading machine Download original image

Figure 4 The checkweigher sends a feedback signal to the loading machine.

Figure 4 Feedback signal sent from the weighing scale to the loading machine Download original image

Step 3 - After the lag time, the loading machine changes the loading volume.

Step 3 shows that the lag time is equal to the time it takes for the goods to reach the weighing scale after the loading machine changes the loading amount (as shown in Figure 5). Beyond the lag time, the loading machine has changed the loading amount, and the weight of the goods returns to normal (the normal goods are marked with a checkmark).

Figure 5 shows that the loading machine changes the loading volume after the lag time.

Figure 5 The loading machine changes the loading volume after the lag time. Download original image

Step 4 - Correcting Loading Deviations

Step 4 shows that the deviation of the loading quantity of the loading machine in the direction of reduction is corrected by feedback control (as shown in Figure 6).

Figure 6 Correcting Loading Deviation

Figure 6 Correcting Loading Deviation Download Original Image

When the conveying speed of the production line remains constant, if the distance between the filling machine and the checkweigher increases, more packages will be located between the filling machine and the checkweigher, and the aforementioned lag time will be prolonged. Therefore, in an ideal situation, the checkweigher should be installed as close as possible to the filling machine to respond most directly and quickly to changes in filling weight.

Feedback control aims to show that the charging volume of the charger can be adjusted over time. If the adjustment cannot be made in one go, it can be achieved through successive approximation (as shown in Figure 7).

Figure 7 Feedback Control Trend Chart

Figure 7 Feedback Control Trend Chart Download Original Image

4. Application Examples of Feedback Control
4.1 Milk Filling
The domestic standard for the 500g liquid milk packaging of a certain group company is 508g with packaging, and the production process control index is 510g. Before the use of the weighing scale, the average filling weight was 515.231g, with a fluctuation range of 17.5g. The original process had no weighing equipment, and the filling machine was a Tetra Pak product. The filling volume was manually adjusted by a manual knob and pull wire to control the mechanical device's filling volume. An automatic control test was conducted using the CWM750 weighing scale from Bizerba Germany. First, the weighing scale was tested, and the obtained weight distribution curve data was close to the ideal normal distribution, with a standard deviation of approximately 1g. During the automatic control test, the target weight was set at 510.3g, the upper limit limit value TO1 was set at 512.6g, and the lower limit limit value TU1 was set at 508g. After the automatic control test of the filling machine's filling volume based on the feedback signal from the weighing scale was implemented, the automatic adjustment of the filling volume was achieved by the stepping motor driving the lead screw and adjusting the lever (as shown in Figure 8). The test results showed that the average filling weight decreased to 510.299g, and the fluctuation range decreased to 8g. On average, 4g of milk was saved per bag, resulting in considerable economic benefits.

Figure 8 Schematic Diagram of the Feedback Control System for Liquid Milk Filling

Figure 8 Schematic Diagram of the Feedback Control System for Liquid Milk Filling Download Original Image

1- Weighing scale
2- Weight display
3- Signal with different pulse counts based on weight deviation
4- Control system PLC + HMI
5- Stepper motor
6- Connecting rod
7- Filling volume adjustment lever
8- Tetra Pak packaging machine

4.2 Toothpaste Filling
Thermo Fisher Scientific installed a dual-channel checkweigher on Procter & Gamble's toothpaste filling production line in Guangzhou to detect and control the weight of toothpaste (as shown in Figure 9).

The weighing range of toothpaste is 10g to 250g, with the maximum length and width being 220mm and 40mm respectively. The throughput is 120 pieces per minute, and the accuracy is 0.1g. By adding a checkweigher device to the toothpaste filling line, not only can the upper and lower weight limits be controlled, but also a feedback control option can be added to control the filling volume of toothpaste, thereby significantly reducing the proportion of substandard products. The upper limit range of the feedback setting is between the upper limit of the allowable deviation of the standard value and the upper limit of rejection; the lower limit range of the feedback control is between the lower limit of the allowable deviation of the standard value and the lower limit of rejection (as shown in Figure 10). Based on the actual average weight value of the latest sampling queue data, the pulse width of the feedback error adjustment value is obtained.

Figure 9 Dual-channel checkweigher on the toothpaste filling production line

Figure 9 Dual-channel checkweigher on the toothpaste filling production line Download original image

Figure 1-0 Feedback Control Adjustment Range

Figure 10 Feedback Control Adjustment Range Download Original Image

The upper and lower limit feedback output adopts relay pulse output, which is connected to the switch input of the filling machine to control the servo motor and change the control pitch, thereby adjusting the toothpaste filling volume. For instance, when the toothpaste weight sampling value is detected to drift downward, the weighing scale sends an adjustment signal to the filling machine to increase the toothpaste filling volume. During the process of the filling machine adjusting the toothpaste filling volume, the weighing scale does not send any adjustment signals. Once the toothpaste filled by the filling machine after adjustment passes through the weighing scale, it can be observed that the downward drift trend of the toothpaste weight sampling value has been corrected. If the toothpaste weight sampling value still fails to meet the quality requirements, the weighing scale can send an adjustment signal to the filling machine again. Before the feedback control was added, Procter & Gamble controlled the filling volume by overfilling each tube of toothpaste by 2g to avoid underweight caused by filling system errors. With a production rate of 120 tubes per minute, the annual output is approximately 120 million tubes, and each tube weighs 100g. If the overfilling can be reduced to 1g, 120 tons of toothpaste filling volume can be saved annually, which means an additional 1.2 million tubes of toothpaste can be filled. Calculated at a cost of 2 yuan per tube, the annual loss reduction is 2.4 million yuan. The market price of a high-speed and high-precision weighing scale is less than 400,000 yuan, and the investment cost can be recovered within two months.

4.3 Bagel Dough Slicing
People love the bagels produced by the British Bagel Factory. These are economical and easy-to-eat snacks that suit people's fast-paced lifestyles. The factory is eager to produce bagels that meet customer standards, so it would rather have the bagels be slightly overweight than underweight, which could affect customer satisfaction. However, overweight bagels affect the factory's profit margin, and bagels that are too light or too heavy have an unattractive circular shape. Therefore, it is necessary to control the dough slicing process, but the challenge is complex because the dough tends to expand in volume during slicing, reducing its density and thus the weight of the sliced dough. Although the slicer can cut dough pieces of almost the same volume, the dough pieces cut at the beginning of a batch are still heavier than those cut at the end of the batch.

Mettler-Toledo's high-speed company provides a solution for checkweighers to manufacturers: there is a cutting machine under the hopper that stores dough, which cuts the dough into pieces of similar size and shape, and then distributes them to four production lines. Each production line runs 75 bagels per minute, with a total throughput of 300 pcs/min. The multi-channel high-speed XS series checkweigher is integrated into the production line, and the operator can monitor and control the four production lines through a touch screen user interface of the weighing display. The data from the high-speed checkweigher can communicate with the cutting machine in real time and adjust the weight of the cut dough. If the checkweigher detects an overweight signal of the dough, it will reduce the weight of the cut dough by the cutting machine, and if it detects an underweight signal, it will increase the weight of the cut dough. The feedback information from the checkweigher is sent to the dough cutting machine, so that the dough cutting machine is not affected by fermentation and always maintains the consistency of the weight of the cut dough. The new system can reduce overfilling, reduce substandard products, and improve efficiency. The high-speed multi-channel checkweigher adopts a fully stainless steel structure, meeting and exceeding the strict cleaning and hygiene requirements of American certification. It also complies with the strict IP-69K standard for high-pressure and high-temperature flushing.

4.4 Beef Slice Tray Packaging
Customers in the Pacific Northwest region of the United States use beef processing machinery to slice the beef, and then the packaging machine places the beef slices onto foam plastic trays. The trays are then sent to the outer packaging machine for packaging. The weight of the beef slices in each tray ranges from 1.00 lbs to 1.05 lbs. If the tray weight is qualified, it will be labeled and boxed for shipment. If the tray weight is unqualified, it will not be labeled, and this unlabeled tray will be automatically removed. Staff must manually open the removed packaging, discard the foam plastic trays, and return the beef slices. Due to equipment configuration issues, the distance between the beef slice packaging machine and the weight and price label machine is quite large, approximately 100 trays in between. It is difficult to establish a feedback loop between the two machines over such a large distance, resulting in a tray rejection rate of over 20%!

To reduce the time needed for rework, the customer set the weight of the beef slices in the tray at over 1.05 lbs, which increased the overfilling quantity of the tray. In cases where packaging time was tight, the customer would even raise the upper limit of the weight setting to 1.10 lbs to ensure a reduction in the number of packages to be discarded. As a result, the number of overfilled goods increased significantly, causing the customer to suffer losses due to waste of packaging, excessive labor consumption, rework of overweight packages and overfilling of goods. They have been constantly seeking better solutions.

The customer installed a VBS checkweigher from the United States directly behind the beef slice packaging machine. Compared with the original distance of 100 pallets between the beef slice packaging machine and the weight-price label machine, the distance after installing the checkweigher was only 7 pallets. By providing feedback on the packaging weight trend, the weight of the beef slices on each pallet could be quickly adjusted. This enabled the customer to promptly eliminate those pallets that were overweight before packaging, significantly reducing the number of packages with weight nonconformities.

After installing the checkweigher, less than 2% of the pallets exceeded the weight range. The rework volume decreased by 95%! As a result, the customer saved $67,000 annually! This does not include the labor costs for removing the goods from the packaging. Additionally, they adjusted the weight range of the beef slices in the pallets to 1.00 lbs to 1.04 lbs, reducing overfilling by 28%. The average weight of each pallet was 1.014 lbs to 1.025 lbs. This seemingly minor weight difference led to an annual revenue of $102,000. This customer achieved a total annual revenue of nearly $170,000 (excluding labor costs), recovering the investment in the checkweigher within a few weeks.

5 Conclusion
The automatic control of the loading process of the weighing scale not only ensures that the net weight of the goods meets the requirements, but also minimizes the overloading of the goods as much as possible to maximize the enterprise's benefits. And various measures taken according to the actual conditions of the production site can enable users to achieve this goal.