Checkweighers are used in many industries to help eliminate manufacturing production waste. Checkweighers can be manual for small batches or they can be automated for high-speed production lines. While most applications for checkweighers are at the end of a production line, there are applications such as baking where they are used early in the process.
A checkweigher does exactly what its name implies, it checks the weight of items as they go through that point in the production process. This is a relatively simple concept, however there can be a considerable difference between checkweighers depending on their application.
If you’ve ever used a self-service checkout line, you’ve probably been exposed to a manual checkweigher. As you fill the bags with the items you scan, the entire bag is weighed, and if the weight does not match what the system expects based on the last scan, a helpful assistant will likely be there to assist you and correct any errors. . This application for a checkweigher is for theft protection and inventory control.
In modern high-capacity production lines, a manual system like the one you have experimented with is not practical. So the challenge is, how can hundreds of items be weighed per minute as they go through a high-volume production line?
For automated checkweighing, a requirement is often that the production line cannot be stopped, so these systems use a special conveyor belt called a “weighing belt” that is mounted on a weight transducer of some kind. The transducer can be a typical automation force measuring instrument, such as a strain gauge or a load cell, depending on the design. For very high speed / high precision applications, there are also advanced load sensing devices.
checkweigher manufacturers get their alternate names from the fact that they are tape-based or that they measure things in motion. The most common names are “belt scales”, “moving scales”, “dynamic scales”, “in-line scales” and “conveyor scales”. The checkweigher configuration can have its own “input belt” that corrects the speed of the item to be weighed to the speed that the scale can accept and can position the product orientation or space to adjacent items to ensure a result. precise. Also part of the checkweighing system can be a “trigger” that uses an optical or ultrasonic sensor to determine the precise location of the item to be weighed. Having the item weighed in the correct place on the belt when the reading is taken can increase accuracy.
As you can imagine, this means that dirt, debris, or moisture can affect the optical sensor, as can the buildup of any of these in the floating system. Stray air currents, particularly updrafts and downdrafts, can also affect accuracy, as can a variety of other factors, including ambient temperature and magnetic fields from other equipment or equipment power lines.
Often there is a method to deal with rejected items that are out of tolerance, sometimes a “reject belt” redirects these items or there may be a drop gate to drop products or an actuator that pushes rejects to the side.