How to choose load sensor for automatic checkweigher

Abstract: At present, whether it is for the inspection of food, daily chemical, pharmaceutical, petrochemical industries, or for the inspection of batteries and auto parts, companies use checkweighers as a key means to inspect the quality and quantity of products. This weighing instrument has become more and more popular. The more it is used in many manufacturing industries. With the continuous improvement of production efficiency, all production is over

At present, whether it is for the inspection of food, daily chemical, pharmaceutical, petrochemical industries, or for the inspection of batteries and auto parts, companies use checkweighers as a key means to inspect the quality and quantity of products. This weighing instrument has been increasing. It is widely used in many manufacturing industries. With the continuous improvement of production efficiency, the stability of the whole production process has been challenged.

How can companies ensure that high-speed packaging production lines can still achieve high-precision detection at low speeds? Can the enterprise automatically detect the machining error of small parts on the conveyor line under the vibration condition of the stamping machine tool? Can the company dynamically weigh the precise value of the automatically loaded carbon ring and the extremely small error of the electrolyte filling weight on the high-speed automatic battery production line? Consistency and reliability are new topics that manufacturers pay attention to, and breakthroughs in new online detection technologies are the best way to solve this problem.

Development history of automatic checkweigher

Through the study of history and the exploration of many ways, the oldest and richest historical data of automatic checkweigher (also known as sorting scale, sorting machine or automatic sorting weighing instrument) is in Thermo Fisher Scientific (Thermo Fisher). Fisher Scientific) was discovered. Thermo Fisher Scientific is the first company to launch a checkweigher on the market through multiple mergers and acquisitions.

As early as 1933, after many attempts, the use of mechanical equipment to automatically transport the products "as in motion" to the weighing platform for weighing was finally realized. Founded in 1945, Illumatronic, California, USA, successfully applied dynamic weighing technology to commercial scales for the first time in 1953.

In the mid-1960s, Illumatronic changed its name to Icore. Since then, Icore has gradually become synonymous with "checkweigher" all over the world. Today's food manufacturers adopt checkweighers as a result of increasing attention to product quality and production efficiency. Icore entered the fast lane of corporate development through the acquisition of Acurex. In 1985, Icore was managed and controlled by Ramsey Engineering Company (Ramsey) in Minnesota, USA, and gradually grew into a leading checkweigher business unit in the US and Japanese markets, with offices and agents around the world Equipment manufacturing center.

Types of load cells on the checkweigher

Currently, there are two main types of load cells used in automatic checkweighers: strain resistance sensors and electromagnetic compensation sensors.

The factor that affects the dynamic accuracy of the checkweigher does not lie in which type of sensor is selected. Even if the accuracy of the load cell is very high, if the motor vibration of the equipment itself, the vibration of the scale on the product and the basic vibration cannot be overcome, the desired dynamic accuracy cannot be achieved, even if a very high-precision balance sensor is selected.

In addition to the mechanical vibration of the machine itself, the factors that affect the dynamic weighing accuracy of the checkweigher, the length of the product, the internal shaking of the product, and environmental factors will all affect the accuracy. Under the same weighing platform length condition, the longer the product length and the shorter the weighing time, the worse the accuracy will be. The way to overcome this is to lengthen the length of the weighing section.

When the checkweigher uses an electromagnetic compensation weighing sensor (that is, the sensor form used by the electronic balance), if the repeatability of the checkweigher is poor, there will be high precision and low repeatability. The final result is poor dynamic accuracy, and it is difficult to adjust dynamic weighing accuracy through parameters.

When the checkweigher selects the strain resistance sensor (that is, the sensor used in industrial scales, automatic quantitative scales and batching control, etc.), if the checkweigher has a high repeatability, low accuracy and high repeatability will occur. However, finally by adjusting the dynamic and static ratio coefficients, the dynamic accuracy of the checkweigher can reach a very high level. The core component of the checkweigher is the load cell. Currently, two types of load cells are generally used internationally: strain resistance sensors and electromagnetic compensation sensors.

The working principle of the electromagnetic compensation weighing sensor is the principle of electric conversion: when the current passes through the wire between the two poles of the permanent magnet, the electromagnetic force is generated by the action of the electromagnetic field. In order to balance this system, it is necessary to add some auxiliary devices, such as a scale that needs to place the measured object and a displacement sensor that distinguishes the position of the wire in the magnetic field. In addition, a current regulator and a measuring loop for measuring current are required. When no load is applied, a certain current controlled by the current regulator flows in the wire, so that the two pointers of the indicator at the same horizontal position remain stationary, and the system reaches a balanced state. At this time, the current indicated on the measuring instrument corresponds to the zero point, that is, the zero point of the balance. Electromagnetic compensation weighing sensors are widely used in electronic balances in laboratories, and are characterized by high sensor accuracy, generally 0.001%. Taking into account the influence of the conveyor on the weighing platform, the overall static weighing accuracy of the general weighing platform is 0.01% of the range. Electromagnetic balance process is needed inside the sensor, the weighing stability time is longer, and the dynamic response time is slower. Therefore, a checkweigher using an electromagnetic compensation type load cell cannot break through the dynamic accuracy bottleneck of 400 pieces/minute and a positive/negative deviation of 300mg.

The strain resistance load cell is composed of four resistance strain gauges attached to metal aluminum or stainless steel elastomer to form a Wheatstone bridge. When the sensor is under force, its shape changes. This forces the resistance values of the four strain gauges attached to the sensor to change accordingly. The resistance value of the four strain gauges attached to the sensor becomes smaller, and the resistance value of the strain gauge becomes larger. Through the Wheatstone bridge measurement circuit, the resistance change can be measured more accurately, and the force can be converted. Strain resistance load cells are widely used in industrial scales and special weighing control in various industrial situations. They are characterized by being suitable for the harsh environment of industrial scenes, with rapid dynamic response and a sensor accuracy of 0.01%. Taking into account the influence of the conveyor on the weighing platform, the overall static weighing accuracy of the general weighing platform is 0.03% of the range.