Basic methods and requirements for ensuring the accuracy of dynamic checkweighers

(1) Guarantee the accuracy by ensuring the passing state of the tested product

Due to the diversity of the tested products, the state of the tested objects passing through the weighing part of the dynamic checkweigher has a great influence on the accuracy of the checkweigher. Before a batch of detected objects passes through the checkweigher, there must be a process of being equidistantly separated, and the distance must be large enough to a certain value. To ensure that the next product can touch the weighing table after the previous product completely leaves the weighing table, that is to say, to ensure that only one detected object enters the weighing table at a time. This can increase the conveying speed of the checkweigher, form a speed difference with the previous conveying equipment, increase the distance between products to a certain extent, and ensure the weighing requirements of the checkweigher. The stability of the tested product on the weighing table is also an important factor affecting the accuracy. When the inspected product enters the weighing section through the introduction conveyor, a smooth transition must be ensured. First of all, the speed of the introduction conveyor should be adjusted to be the same as the speed of the weighing table conveyor, and the speed difference should be minimized as much as possible to ensure that the tested products can transition smoothly when passing through the connecting position. In addition, due to the various shapes of the detected objects, if the center of gravity is too high or the area of the bottom contacting the table is too small, it may tilt, jump, etc. when passing through the connection position, causing the weighing table to vibrate and affect the accuracy. In this case, transition devices, such as transition slides, ball transition plates, etc., can be added at the connecting position of the conveyor.

Another important factor that affects the accuracy of a dynamic checkweigher is the conveying speed of the product being inspected. From the perspective of production operations, in order to improve production efficiency, it is always desirable to speed up the production line as much as possible. However, for dynamic checkweighers, the contradiction between speed and accuracy has always existed. The faster the speed, the more difficult it is to guarantee the accuracy. The residence time (effective weighing time) of the tested product in the weighing part should reach a certain length, and this time must meet the requirements of the instrument controller for the number of sampling points of the load cell signal. If the signal sampling time is too short and the number of sampling points is too low, the instrument controller cannot calculate the precise arithmetic mean value, and thus cannot guarantee the accuracy of the weighing result. By appropriately reducing the conveying speed, or appropriately increasing the length of the weighing table within the allowable range, the weighing time can be increased to ensure that the minimum number of sampling points of the instrument controller can be met. To a certain extent, the weighing accuracy is guaranteed.

(2) Ensure accuracy by improving the operating environment of the equipment

The dynamic checkweigher is a high-precision re-inspection equipment. Usually, the weight value of the detected object is very small, and the weighing accuracy will be more sensitive in the state of high-speed operation, which is affected by many environmental factors. For example, the temperature, humidity, electrostatic induction, radio frequency interference, ground vibration, air circulation, etc. of the working environment. Therefore, the use environment of dynamic checkweighers is also a key factor to ensure accuracy. First of all, try to put the dynamic checkweigher in a working environment with suitable and constant temperature and humidity to avoid unstable and irregular changes in various indicators. If necessary, an electric heating device can be added to the module of the load cell, which can directly control the working environment of the load cell.

Electrically, the equipment should be grounded to reduce electrostatic induction, stay away from strong radiation, and avoid wireless electromagnetic interference. The air flow will also have a certain impact on the accuracy of the dynamic checkweigher. It is best to install the equipment in a production workshop that is sheltered from wind, and add a draft shield to the weighing position, which can effectively improve the stability of the equipment operation.

(3) Ensure accuracy by improving production technology

The working state of the dynamic weighing scale is to weigh at high speed, which puts forward higher stability requirements for the conveying equipment participating in the weighing. Therefore, the conveying mechanism of the weighing part must minimize the noise and vibration during operation, which requires the static balance and dynamic balance experiments of the driving roller and the driven roller of the conveyor before assembly. The experiment needs to be repeated, and the weight of the drum is modified until the deviation value is reduced to within the range of design requirements. This is also a key factor to ensure weighing accuracy.

The load cell is the core component of the dynamic checkweigher, and the most common one is the strain gauge sensor in industrial applications. This is a sensor that uses resistance strain gauges to convert strain into resistance changes, and it is widely used in various fields of weighing. However, with the development and progress of science and technology, new types of equipment with higher scientific and technological content such as electromagnetic compensation sensors have also appeared in the sensors used in dynamic checkweighers. The working principle of the electromagnetic compensation sensor: when the weight is placed on the weighing pan, the balance state of the system will be broken. At this time, the displacement sensor will output a position change signal to the current regulator, and the current regulator adjusts the output current in time to increase the current flowing through the wire until the electromagnetic force generated by it and the gravity of the weight cancel each other out. Electromagnetic compensation sensors are characterized by faster response and higher accuracy.

The rejection part and the sorting part are also an important part of the dynamic checkweigher. In the process of online work, it is necessary to respond accurately and quickly in the shortest time when unqualified products or products of different weight classes are detected. If the mechanical action is too fast or the force is too large, it will cause a certain vibration, which will affect the weighing accuracy of the checkweigher. We can separate the frame of the rejecting or sorting part from the overall frame of the checkweigher, and fix the distance between it and the checkweigher through a soft connection, so as to minimize the impact caused by mechanical movement. Impact of vibration on accuracy.