Checkweigher
A checkweigher or checkweigher is a balance that serves to monitor the manufacture / production of goods according to weight criteria. They can be divided into static and dynamic checkweighers. Due to the automation, automatic dynamic checkweighers are usually used in the production process, which measure their test object in motion.
Automatic dynamic checkweighers
Automatic checkweighers within the meaning of the definition of calibration law have been in use in Germany since around 1965. Even at this time, the automatic belt scales, also known as “checkweighers”, achieved respectable performance in terms of cycle rate and measuring accuracy. However, the handling was still very cumbersome, since neither modern digital technology nor fast, high-resolution load cells with satisfactory linearity were available. In the course of technical developments, checkweighers are nowadays controlled by fast processor systems; Several model principles are available as load cells.
The most important areas of application for checkweighers are:
- Fully automatic weight control with removal of excess weights
- Fill quantity optimization through continuous trend calculation and automatic adjustment of the upstream dosing, filling or cutting system
- Complete statistical weight data acquisition , especially when checking the filling quantity according to the prepackaging ordinance (FPVO)
For economic reasons (point 2), a scale is used to control and optimize the production process. Feedback systems and tendency monitoring report back to the filling systems in order to intervene. The focus here is on controlling and improving the cost of goods. The tendency control of a checkweigher constantly records the product weights and calculates a mean value cyclically. This is set in relation to the nominal value and based on the deviation, a correction signal is sent to the dosing / cutting device, which is proportional to the size of the deviation. This optimizes the product weight in the medium and long term in such a way that there is a quasi-correspondence between the actual value and the target value. The recording and documentation of the weights is important for quality control.
For legal reasons (point 3), a manufacturer of finished packaging with a uniform nominal weight must ensure that the products put into circulation do not fall below certain values on average (TU1) or absolute (TU2). The scales used here must be verified in order to guarantee accurate documentation of the products sold.
construction
The typical dynamic checkweigher consists of a system of conveyor belts in a row, one of which - the weighing belt - is built on a fast, high-resolution measuring cell.
Outline of the conveyor belts
For precise measurements, it is very important to perfectly separate the supplied products in front of the weighing unit so that only one product is fully positioned on the weighing belt during a measurement. If this is not possible due to the properties of the upstream units, a dynamic checkweigher can also be equipped with several acceleration belts in front of the actual weighing belt.
a) Infeed belt / removal belt from previous machine b) Optional acceleration belt to separate products c) Weighing belt d) Discharge belt / with sorting for "bad" -classified products
Load cells
The load cells used today - also called measuring cells - are usually full-range scales. The most important designs used in checkweighers are:
- Force-compensated measuring cell ("electromagnetic force compensation", EMK)
- Load cells based on strain gauge technology (DMS)
Individual evidence
- ↑ Prepackaging Ordinance (FPVO)
- ↑ EC-FS: Wipotec weighing and positioning systems GmbH. In: www.wipotec.com. Retrieved August 12, 2016 .
- ↑ FIT7A digital load cell: the heart of the checkweigher | HBM. (No longer available online.) In: www.hbm.com. Archived from the original on August 12, 2016 ; accessed on August 12, 2016 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
literature
- Lutz Krause: Dynamic weighing technology
- Philip M. Parker: The 2007-2012 World Outlook for Automatic Checkweigher and Bulkweigher Scales
- Béla G. Lipták: Instrument Engineers' Handbook, Volume 1, Fourth Edition: Process Measurement and Analysis