Speed And Accuracy: The Checkweigher

Tuesday, September 19th, 2017

How do you ensure products are packaged properly in each and every box that goes out of the processing factory? Checkweighers are an easy answer. By Santhosh Ramakrishnan, business development manager, Precia Molen Asia Pacific

It is quite amazing to see how technology has advanced. A problem I often pondered on for years as a youth was: how is it that each and every time I buy a carton of drinks, the number of bottles within is always the same? Are they checked individually by people? How many people are employed to do this on an enormous scale for accuracy?

Now, I had the answer: it is not a large number of people, but just a manmade machine called the checkweigher.

From Past To Present

At the very beginning of the Industrial revolution, people were employed to manually check each carton passing out of the facility, leading to low accuracy and long check times, eventually costing the enterprises big sums.

With the advancement in technology over time, static checkweighers were born. These are simple weighing platform scales attached with a display that people could use to confirm their visual interpretation with the weight indicated on the checkweigher’s display. This led to a drastic improvement in the level of accuracy and reliability in the packaging industry. But, this didn’t mean clear selling as well. The rapid growth of the industry meant larger volumes, speed and heavier weight.

It became increasingly difficult to manually check the cartons against the static checkweigher to match output efficiency. This was simply dubbed as “the speed, the volume and the weight problem.” This problem led to fewer companies actually using the static weigher; that is, until the invention of the dynamic checkweigher.

A dynamic checkweigher is a completely automated conveyor based checkweigher. This effectively solved the issue of speed and volume since the product passes through the dynamic conveyor based checkweigher, which meant it could handle high volumes and speed while maintaining the same accuracy of the static weigher.

Today, dynamic checkweighers can automatically capture, store, sort, reject and generate reports based on weight. They can communicate with supporting machines such as X-Ray or metal detectors, or with the most advanced PLC through various communication protocols like the RS 232/485 or Ethernet, and have internal storage options such as USB or Micro SD.

Types And Components Of A Checkweigher

Dynamic (or in-motion) checkweighers consist of two types: belt-based or roller conveyor-based. Regardless, the components of a typical checkweigher are detailed as follows.

The in-feed conveyor comprises a stopping mechanism for slow speed applications or spacing conveyor for high speed applications. These increase or decrease the speed of the package to bring it in line with the speed of the weighing conveyor. The stopper and the spacing conveyor ensure that there is enough space between the products so that only one product is passed over the weighing component at a time.

The weighing conveyor is mounted with a transducer to measure and check the weight.

The reject conveyor consists of a mechanism to remove the out-of-tolerance parcels or sort the parcels based on weight. There are different types of rejection and sorting mechanisms depending on the nature and size of the product:

  • • air blast: an air jet arrangement which blows out the parcel that is out-of-tolerance
  • • flip bar: a flip bar that rejects the parcels out of tolerance and can also be used to sort the products
  • • a pusher on belt or roller: a pneumatic cylinder which rejects the parcels out of tolerance
  • • drop-belt: rejected parcels are passed to a downstream conveyor where the pneumatic cylinder controls the up-down movement of the conveyor

Different types of sensors detect the parcel and send a signal to the controller that the object is ready to be weighed. They also help to avoid the overcrowding of the packages over the in-feed as well as out feed line to the micro controller.

A tower alarm light usually signals and warns the out of tolerance parcels or overcrowding of the parcels to the line operator.

The controller is the brain of the checkweigher. It is generally a highly capable computer that can capture and store weight data, generate a report and send this data to the PLC and other complementing machines in the production line through series of protocols.

It is also equally important to understand the factors affecting the measurement of weight using the dynamic checkweigher. A normal weighing scale under ideal conditions (an isolated chamber with pressurised dry nitrogen) can weigh up to +/- 0.01 g where ambient air pressure is also an influencing factor.

External factors can also affect readings. These include noise and vibration generated from moving parts, air conditioning or refrigerating effect which can cause draft, environmental noises playing a major role in determining the accuracy.

It is therefore important that the dynamic checkweigher takes samples of the packages on the weighing conveyor over a time period to analyse and accurately determine the weight.

Sequence Of Operation Of A General Checkweigher

The parcel is fed either manually or automatically on the infeed conveyor. The sensors present on the infeed conveyor detect the presence of the product and inform the controller; it now sets a time-delay in preparation for weighing. Another sensor at the weighing conveyor typically informs the controller to capture the sampled weight as the product reaches the sweet spot on the weighing conveyor.

Generally, a tolerance level is set on the controller to reject the parcel with upper or lower weight extremities. In some complicated cases with advanced controllers, there can be multiple batches of products coming in from different packaging lines which combine and enter the checkweigher line. The checkweigher checks the weight and later automatically sorts the products based on the weight back to the corresponding packaging out-feed lines.

The out-feed conveyor houses different types of rejecters with sorting mechanisms depending on the nature and size of the product. Today’s advanced controllers can generate reports and graphs in different formats having the number of rejected or sorted package details, then transfer the data to any PLC or computer through a series of protocols.

Versatile Applications

The dynamic checkweigher has its applications in almost every industry. The most common is the simple carton weigher at the end of the packaging line to identify and reject parcels with upper and lower tolerance extremities. They can also detect missing pieces of parcel such as missing chocolates from assorted chocolate gift packs.

They are also used to count the packs and totalise the weight of all the packages in a pallet moving for shipment, as well as read the dimension and weight of each package. An advanced controller can embark a shipping label with the details of carton ready for shipment, which can be confirmed by another checkweigher at the destination by scanning the bar code.

Additionally, checkweighers can collaborate and work along with metal detectors, x-ray machines, bar code scanners, a wide array of sensors and vision inspectors. They are also being widely used for quality management in many industries to ensure high reliability and consistency in the end-products packaging.