Design and application of combination scale control system

Abstract:

This article provides an overview of the weighing process of the combination scale, describing its hardware and software components, and analyzing the performance and features of the system.

Keywords: Multihead weigher, Combination scale, Step motor, Weighing system

Introduction

Since its introduction in the 1970s, the combination scale has become an essential tool in industries such as food, pharmaceuticals, and chemicals. It addresses the challenge of accurate metering and packaging of irregularly shaped materials, offering high measurement precision, fast packaging speeds, and high reliability. Specifically, for high-value granular or block products, combination scales enable fast, accurate, and quantitative packaging, with precision controlled within ±1 gram and the fastest weighing speed reaching 240 packs/min. Despite its benefits, imported combination scales are expensive, limiting their widespread use.

1. Weighing Process of the Combination Weigher

The combination scale consists of two primary components: the weighing mechanism and the control system. One of Kenwei's flagship products, the JW-A waterproof 10-head combination scale, is designed to handle a maximum weight of 3000g with a packaging speed of ≥60 packs/min, and a precision tolerance within ±1.5g.

The process begins by collecting materials from the collecting hopper. These materials are then transferred through a vibrating distributor and radial feeder into temporary storage in preparation hoppers. Once in the preparation hopper, materials are dispensed into weighing hoppers for measurement. The microcomputer controls the system, performing calculations to find the optimal combination from a set of 1023 possible combinations. The best combination is selected, and the corresponding weighing hopper discharges the material into the packaging area.

The system continues to cycle until manually stopped. If a hopper is not selected after 30 consecutive cycles, the material is discharged from that hopper, and the system re-calculates the best combination. The weighing hopper doors are operated by step motors, which fine-tune the opening and closing speeds based on the material's characteristics. Material levels in the collecting hopper are monitored by photoelectric sensors.

2. Hardware Design of the System

Amplifier:
The AD620 is a precision instrumentation amplifier with low power consumption and small size, ideal for accurate data acquisition in multihead weighers and systems with multiple channels.

A/D Converter:
The Successive Analog-to-Digital Converter is highly accurate, offers low power consumption, and has a fast conversion time of just 9.4µs. This component converts analog data from the load cells into digital signals, which are processed by the system for accurate weighing.

Step Motor Drive Circuit:
Each hopper in the weighing mechanism is controlled by a step motor, ensuring smooth door operation and minimizing material damage or blockages. This system is critical for maintaining efficient, precise material flow.

3. Software Design of the System

To meet packaging speeds of 70 packs/min or more, the system must process weight detection, combination calculations, and the selection of the best combination within 1 second. High-performance microcontrollers (MCU) or PLCs are used to handle the real-time calculations and control outputs. The control system communicates with the multi-way switch, selecting load cells for signal amplification and A/D conversion.

The MCU or PLC then performs the combination calculation, selecting the best hopper combination, and controls the step motor to open the corresponding hopper door. The material is discharged, and the hopper door is closed. Photoelectric sensors are used to detect material levels in the storage hopper. Additional circuits for display, alarms, and keyboard inputs enhance system functionality.

4. Conclusion

Combination weighing technology represents the pinnacle of modern weighing and packaging. Historically, China has relied heavily on imported combination scales, which were prohibitively expensive. However, with the rapid growth of China’s packaging industry and the increasing demand for high-speed, high-precision packaging solutions, Kenwei's combination scales are poised to revolutionize the industry. By offering affordable alternatives without compromising performance, Kenwei is making this technology accessible to a broader range of businesses.

Kenwei’s ability to manufacture large-scale, cost-effective combination scales has positioned it as a leading player in the industry. In 2017, Kenwei achieved a significant milestone by securing more than 5,000 units of sales across 87 countries. With the introduction of a semi-automatic production model, Kenwei has further increased its production capacity to 7,000 units annually, accelerating production cycles and solidifying its position as a trusted brand in the packaging machinery sector.

As the packaging industry continues to evolve, Kenwei remains committed to advancing weighing technology, contributing to the growth of the packaging industry while simultaneously enhancing the precision, efficiency, and affordability of its products.

Kenwei: Leading the Charge in Intelligent Weighing and Packaging Technology

Guangdong Kenwei Intellectualized Machinery Co., Ltd. is proud to be at the forefront of the weighing and packaging industry, offering high-quality combination scales and intelligent solutions that meet the demands of modern industries. Our commitment to innovation, quality, and customer satisfaction has made us the go-to partner for companies seeking to optimize their packaging processes.