Design of a dynamic weighing scale

Design of a dynamic weighing scale

0 Introduction
A dynamic checkweigher is a device that performs real-time dynamic weighing of products under the operation of a fully automatic production line and automatically classifies the products based on the weighing results. This paper focuses on the planning and design of an online dynamic checkweigher for a 200 L lubricating oil filling line. The requirements are as follows:

The weight of a 200 L drum of finished lubricating oil is 185.3 kg, and the allowable deviation range set by the enterprise is (185.3 ± 0.3) kg. The deviation of the dynamic weighing scale designed in this paper should be controlled within ±0.1 kg.

(2) It can conduct real-time weight checking on the 200 L finished product barrels on the filling line. When the weight exceeds the upper and lower limit values allowed, it can automatically screen or remove them from the production line and simultaneously issue an audible and visual alarm signal.

The dynamic checkweigher should be capable of weighing at a speed of 120 barrels per hour.

(4) The weighing data can be promptly fed back to the controller of the filling machine, thereby adjusting the filling volume, saving raw materials, and providing technical support for cost control and refined management of enterprises [1].

1. Composition and Working Principle of Dynamic Weighing Scale
The dynamic weighing scale is composed of a conveyor, weighing sensors, weighing display controller, control system, and rejection device. The conveyor is placed on the weighing platform of the weighing sensor. The conveyor is made up of three parts: motor, reducer, and conveyor rollers [2]. The configuration of the dynamic weighing scale is shown in Figure 1, and the structure of the control system is shown in Figure 2.

Figure 1 Schematic Diagram of Dynamic Weighing Scale Configuration

Figure 1 Schematic Diagram of Dynamic Weighing Scale Configuration Download Original Image

Figure 2 Structure of the Dynamic Weighing Scale Control System

Figure 2 Structure of the Dynamic Weighing Scale Control System Download Original Image

2 Dynamic Checkweigher Hardware Design
The hardware design of this article includes hardware selection, electrical main circuit design, control circuit design, and PLC I/O point allocation.

2.1 Hardware Selection
This paper selects the equipment hardware based on the principles of meeting production needs, high cost performance, strong reliability, and leaving a certain margin, as shown in Table 1.

Table 1 Hardware Equipment Download Original Table

Table 1 Hardware Equipment

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Table 1 Hardware Equipment

2.2 Electrical Main Circuit Design
The main circuit of the dynamic weighing scale is shown in Figure 3, mainly including: circuit breaker, contactor, surge protector (SPD), isolation transformer, switching power supply, PLC, belt conveyor, weighing display controller, etc. The system is connected through the QF1 switch, and all the equipment in the main circuit is powered on. As it is supplied by three-phase AC power with a voltage of 380 V, but the PLC input control circuit, touch screen, and intermediate relay of the system require a DC 24 V power supply, a switching power supply is needed to provide a 24 V DC power supply. The power management of the PLC is supplied by 220 V AC after the isolation transformer and the switching power supply.

Figure 3 Electrical Main Circuit

Figure 3 Electrical Main Circuit Download Original Image

2.3 Control Loop and I/O Point Allocation of PLC
According to the control principle, the input, output (I/O) points and intermediate register addresses are reasonably allocated as shown in Tables 2 to 4. The control loop is composed of the PLC host, intermediate relays, etc. The entire control system can be automatically or manually controlled for the dynamic weighing scale through the switching switch. The two control modes serve as backups for each other, ensuring the safety of the control system. The wiring diagram of the PLC host control loop is shown in Figure 4.

Table 2 Digital Input Address Allocation Download Original Table

Table 2 Digital Input Address Allocation

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Table 2 Digital Input Address Allocation

Table 3 Digital Output Address Allocation Download Original Table

Table 3 Digital Output Address Allocation

Figure 4 Wiring of the PLC Host Control Loop

Figure 4 Wiring Diagram of PLC Host Control Circuit Download Original Image

Table 4 Intermediate Register Address Allocation Download Original Table

Table 4 Allocation of Intermediate Register Addresses

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Table 4 Allocation of Intermediate Register Addresses

3 Dynamic Weighing Scale Software Design
3.1 Touch Screen Program Design
Firstly, the Vijeo Designer programming software is used to configure the touch screen interface, including the process interface, manual interface, alarm interface, etc., as shown in Figures 5 to 7. Then, the completed "file" is downloaded to the touch screen processor via the communication interface of the notebook computer and Schneider GXU3512 touch screen, and the communication between the touch screen and PLC is established [2]. The configuration interface includes parameters such as current weight, system start/stop, cumulative bucket count, cumulative weight, and belt running time.

Figure 5 Editing the "Process Interface"

Figure 5 Editing the "Process Interface" Download the original image

Figure 6 Editing the "Manual Interface"

Figure 6 Editing the "Manual Interface" Download Original Image

Figure 7 Editing the "Alarm Interface"

Figure 7 Editing the "Alarm Interface" Download Original Image

3.2 PLC Program Design
3.2.1 Hardware Configuration and Main Program Design
The Schneider TM218LDA16DRN PLC serves as the core of the control system in this paper. Based on the control flow shown in Figure 2, the hardware configuration and the configuration settings for the weighing display controller are carried out using the So Machine M218 v2.0.31.45 programming software, as illustrated in Figures 8 and 9. The control ladder diagrams are written as shown in Figures 10 to 12 to achieve the following functions: start and stop of the belt conveyor and corresponding logic control; real-time weighing, deviation rejection, fault indication and alarm; and necessary protection control [3].

Figure 8 Hardware Configuration of the Control System

Figure 8 Hardware Configuration of the Control System Download Original Image

Figure 9 Configuration Settings of Ind131 Weighing Display Controller

Figure 9 Configuration Settings of Ind131 Weighing Display Controller Download Original Image

Figure 1-0 Main Program

Figure 1.0 Main Program Download Original Image

Figure 1-1 Control Program 1

Figure 1-1 Control Program 1 Download Original Image

Figure 1-2 Control Program 2

Figure 1-2 Control Program 2 Download Original Image

3.2.2 Program Upload/Download and System Debugging
(1) Set Variables. Variables serve as the bridge between the functional areas of the touch screen and the I/O points of the Schneider PLC. Through variables, the touch screen can achieve functions such as parameter input to the PLC, functional control, and output of the current value of the PLC, as shown in Figure 13.

(2) The program is downloaded to the PLC. Set the communication port, establish communication between the PLC and the computer, download the completed PLC program to the PLC, and test the program through the program status function and simulation software. If any problems are found, modify the program in time. The Modbus master and slave station configurations are shown in Figures 14 and 15, and the serial line settings are shown in Figure 16.

Figure 1-3 Variable Settings

Figure 1 3 Variable Settings Download Original Image

Figure 1-4 Modbus Master Configuration

Figure 1 4 Modbus Master Configuration Download Original Image

Figure 1-5 Modbus Slave Configuration

Figure 1 5 Modbus Slave Configuration Download Original Image

Figure 1-6 Serial Line Settings

Figure 1 6 Serial Line Settings Download Original Image

(3) After the PLC software debugging is normal and the touch screen configuration software is completed, the entire system is jointly debugged. The on-site weighing signal, position detection signal, feedback signal and all motors are simulated to run according to the actual control requirements to detect whether the operation and performance of the entire system can meet the design requirements. After all tests are normal, the test of simulating fault alarm situations is also conducted. When all test data links are normal, the system debugging is completed.

4. Interference Resistance Analysis to Enhance the Accuracy of Weighing Scales
4.1 Main Factors Affecting the Accuracy of Dynamic Weighing Scales
(1) The volume of the product being weighed, the target weight, and the conveying speed, etc. In this case, the 200 L drums of lubricating oil being weighed have relatively little movement, and the conveying speed is 120 drums per hour, which is matched with the speed of the filling line and the weighing control instrument.

(2) The accuracy of the selected load cell and weighing control instrument. In this paper, Mettler Toledo's load cells and control instruments are chosen, with an accuracy of 0.1 kg.

(3) The temperature, humidity, ground vibration and air circulation of the surrounding environment. The dynamic weighing scale is installed on the first floor of the workshop, with a flat ground, stable temperature and humidity, and low ground vibration.

4.2 Anti-interference Analysis
In this paper, anti-interference is fully considered in the selection of equipment and circuit design. The main measures taken are:

(1) Electromagnetic shielding. This article implements electromagnetic shielding through three measures: control cables use shielded twisted-pair cables; they are laid in covered galvanized carbon steel trunking with good electrical connection between the trunking; and the parallel laying distance between control cables and power cables is kept greater than 600mm to prevent external electromagnetic interference.

(2) Grounding for anti-interference. In this paper, all signal circuits are grounded through wires to a common grounding point. Signal ground and power AC ground must be separated. The shielding ground of the signal cable and the ground of the PLC system equipment are shared. The length of the grounding wire is shortened, and the grounding wire and terminal are used in accordance with standard specifications.

(3) Lightning protection and interference resistance. This paper adopts three protective measures, namely, outer shielding of cables, reasonable wiring, and installation of surge protective devices (SPD), to achieve lightning protection and interference resistance [4].

5 Dynamic Weighing Scale Testing and Performance Evaluation
After the dynamic weighing scale was debugged, it was tested for two weeks using a 200 kg standard weight that had passed the verification in accordance with the "JJG539-2016 Verification Regulation for Digital Indicating Scales". A total of 10 groups and 40 measurement data were collected for statistical analysis, as shown in Table 5. The dynamic weighing scale has high measurement accuracy, a running speed that matches the filling line, strong anti-interference ability, fully meets the process technology requirements, and is easy to set and operate.

Table 5 Dynamic Weighing Scale Test Data Analysis Download Original Table

Table 5 Dynamic Weighing Scale Test Data Analysis

Starting from the design of dynamic weighing, this paper realizes the 100% online weighing of 200L finished barrels of lubricating oil through the application of Schneider PLC in dynamic weighing scales. Compared with the traditional timed manual spot check, the efficiency is increased by 10 times, and the labor cost is saved. Especially for large barrels, it is difficult for manual handling, and the advantage of online real-time weighing is obvious. The measurement data can be fed back to the filling machine to achieve automatic adjustment of the filling volume, which greatly saves costs. With the help of the network, the weight of the products on the production line can be remotely monitored, improving the informatization level of the enterprise. It is worthy of widespread promotion.