An in motion checkweigher’s main purpose is to weigh and reject out-of-spec products, but that doesn’t mean that it can’t serve other purposes as well. It can also work well as part of a larger system by feeding back important information to machines up the line, allowing them to do their job better.
A good example of this would be in a plant producing chubs of ground meat product.
When chubs are packaged, they are filled by a pressurized machine which is set to a certain amount of pressure and filling time in order to get the right amount of ground meat. From there, you can pass all of these chubs over a in motion checkweigher to ensure that they meet a minimum fill level.
The rate of the machine output is often roughly two per second, so we make sure that our in motion checkweigher can match that. As the chubs pass over the in motion checkweigher, the scale verifies that they are at an acceptable weight and then rejects them if they are not, but it can also gather information that could benefit the filling machine.
With special diagnostic software that we offer, this process can be optimized for minimum product waste and giveaway. When linked with the checkweigher data, this software can take a sample of a number of chubs crossing the checkweigher and take notice of the filling trends.
For example, the software allows you to specify levels that you have to meet (i.e. one pound of ground meat product) and acceptable levels of overfilling.
The checkweigher can report weights to the software which determines how many packages were overfilled or underfilled. Given this information, the in motion checkweigher can then send a signal (voltage) to the filling machine to adjust its filling parameters. If the in motion checkweigher is rejecting too many products for being underweight, it can tell the filling machine to increase the pressure or fill time to bring the packages to an acceptable level.
In addition, the software can provide daily reports to help the company know their profits and whether they need to perform any maintenance to bring the filling machine up to speed.
An in motion checkweigher is a necessity for its main functions, but its secondary functions shouldn’t be ignored either! There is a lot of money spent on product giveaway and reworking to be saved by using a combination like this.
If you have questions and want to hear more about our in motion checkweighers, contact us today!
In-Motion Checkweighers all perform the same function of checking products to see that they fall into an acceptable weight range, but not all in motion checkweighers are exactly alike or follow the same principles. In the industry, there are actually two typical ways of making a in motion checkweigher that function off of different concepts of weighing and loading. The first one, which we use for all our checkweighing systems, is the concept of mounting a powered conveyor on a load cell based scale device, motor gear box and all.
This is a challenging proposition because the powered conveyor must be light enough to allow proper scale sensitivity, yet large enough to convey the heaviest product the checkweigher will see. The other method that gets used by some companies is to drag a conveyor belt across a weigh bed. This method allows use of heavy duty motors but generally yields poor repeatability results and requires engineering equations which can relate deflection back to weight.
So while they still reject products in a similar fashion, the method we use will give more repeatable results.
Both methods have their ups and downs, but they accomplish the same purpose.
Using the actual scale conveyor mounted onto the load cell helps us to achieve a great level of accuracy and repeatability, which is advantageous when seeking NTEP Approval.
There are not many people who produce NTEP approved in motion checkweighers, but we choose to go that extra step.
In-Motion Checkweighers are not required to be NTEP approved, but it is a great benefit to have.
There are many companies making in motion checkweighers these days, showing that they are becoming more and more of a necessity for certain plants, but we go the extra mile to ensure our product quality and accuracy.
Don’t buy just any old In-motion Checkweigher.
Get in touch with us for the best possible checkweighing experience!
Anyone who works on in motion checkweigher and digital weighing equipment (such as a bench scale, laboratory balance, etc.) knows the relatively simple task of weight testing the scale with known test weights.
It is simple right? You turn off automatic zero tracking (AZT), verify that the scale is at center of zero, then apply known test weights and compare the displayed value against the actual weight applied. You check it at several different weight values and provided you have a good static scale, bingo you are done! Simple right?
Now, put the test weight in-motion across the scale and suddenly things get really confusing. You run the weight once and you are done right? WRONG!!!! This is where statistics rears it’s ugly head. Often when you run the weight across the in-motion checkweigher, it weighs different! What should you do!?
Steps To Success
- Same as the static scale, check zero both static and in-motion.
- Get a product (one single example) that represents the product that you intend to check weigh in-motion or dynamically. Then verify the exact weight of that product on a very accurate static scale. You now have what is known in the industry as a “test puck.”
- Turn the in-motion checkweigher on (run the belt) at the speed that you intend to use the in-motion checkweigher in production.
- Apply your test puck to the infeed belt well ahead of the weighing section of the in-motion checkweigher to allow relative motion of the test puck-to-belt motion to stabilize before the test puck arrives on the in-motion checkweigher’s live section. Then collect the test puck after the in-motion checkweigher’s live section once weighing is complete.
- Record the in-motion checkweighers in-motion value.
- Repeat steps 4 and 5 at least 32-100 times. (no I am not kidding!)
- Verify that zero did not change or you will need to start over.
- Verify that the test puck static weight is still the weight value when you started or you will need to start over.
- Analyze the results by determining the highest and lowest values, the difference is the spread, then average all the in-motion values and compare against the actual static weight of the test puck, oh yes don’t forget standard deviation calculations!
- Ask yourself, ARE YOU SATISFIED? If so, congratulations!! You are done (for now) as soon as you file the results to compare against future testing results.
Following the above steps, as tedious as they seem, will give you a clear picture of the true accuracy and repeatability of your in-motion checkweigher. Future testing with similar results will prove the performance is staying constant while variations with trends can indicate problems with the checkweigher.
Obviously, if any kind of change is made, it is time to test again. Be very careful to never change more than one thing at a time, as it becomes impossible to tell performance changes based on the “thing.”
Many in motion checkweigher problems may be similar, there are several ways to solve them. It’s been said of some people that they are generous to a fault. The same thing can be said about companies that do not exercise proper Quality Control by checking product weights and piece counts following packaging.
Every manufacturer knows that selling underweight products or products with a lower piece count than what is stated on the label is a quick route to legal trouble. However, attempting to avoid legal trouble by over-packing is usually the wrong answer for two reasons.
The first and most obvious reason is the cost to the bottom line. This type of generosity squeezes profits by forgoing revenue that is due to the company and its stakeholders.
But a less obvious reason is that generosity in product give-away may adversely affect your customers. This point was driven home to me recently by an engineer with a telecommunications company. He had a customer complaint when a package said it contained 10 connectors when, in fact, it had 11.
After that customer manufactured its product and found it had a left-over connector, it searched in vain to find its error, but it was the telecommunications company that had been generous to a fault. The result was a very frustrated customer.
We showed the telecommunications engineer that we can “count” the pieces in each box on our in-motion checkweigher at speeds that won’t put his company back in the Stone Age.
Speed is just what you get with in-motion check weighing. An in-motion checkweigher is a conveyor scale (meaning that product does not need to stop to be weighed) with one additional feature: a divert mechanism. A divert mechanism can be an air blast, a linear thrust device, a drop-down, or any other means of removing products that are not within a specified tolerance range.
If your product inspection needs to be Legal For Trade, we can design an in-motion checkweigher with division sizes as low as 0.005 lbs or for a capacity as high as 200 lbs. The rule for a Legal For Trade in-motion checkweigher is 2,000 divisions of the load cell’s capacity.
When purchasing an in motion checkweigher, one of the main things that has to be considered is product throughput. How many weighments an hour will have to be done to keep up with production? And how long are the products being weighed?
Let’s say you have a product that is 7 inches long and production is turning out 75 products per minute. Simple math will tell you that is 525 inches of product per minute or 43.75 feet per minute.
Now, that would be running the packages across the scale nose to tail with no space between the packages at all. This would result in two different packages being on the scale at the same time
, and this will not give accurate weighments.
To correct for this, we will need to know how long the weighing section of the scale is. For the purpose of this blog, we are not going to go into how to determine the scale length that will work best.
We are going to say that we are working with a weighing section that is 22 inches long. The weighing process for a product will stop just prior to the product’s leading edge reaching the end of the weighing section. At this point, the next product can begin to enter the weighing section of the checkweigher. Note that the first product does not have to clear the weighing section before the next product can begin to enter the scale as the weighing process does not begin until the next product is all the way on the scale and the scale has had a little time to settle.
So, with a 22 inch scale, the product should be spaced 22 inches from leading edge of product to the leading edge of the next product. Let’s also give ourselves a little cushion to be sure the weighing process has ended before we send the next product across by making that spacing 22.7 inches.
Now, this would all mean that we have 22.7 inches for each product that goes across the scale, and we are doing 75 products per minute. 75 x 22.7 = 1702.5 inches of belt speed per minute. Divide that by 12, and we know that we need 141.875 feet per minute.
We could use a shorter scale and go with a much slower belt speed, and that would be great because a slower belt speed would give us more accurate weights. For instance, an 18 inch weighing section would result in a minimum belt speed of about 117 feet per minute.
So, if your only product was 7 inches long, by all means an 18 inch weighing section would be for you. But, if you also wanted the ability to run a 13.75 inch-long product across the checkweigher, the longer 22 inch weighing section would be needed.
Another consideration is whether this 13.75 inch product also runs at 75 products per minute. If so, an even longer weighing section would be needed to allow for the product to be on the scale for enough time for the scale to determine an accurate weight. The most throughput you could expect on a 22 inch weighing section with a 13.75 inch product would be about 45 products per minute.
As you can see there are many things to take into consideration when selecting an in motion checkweigher.