A TECHNICAL AID IN THE DESIGN OF WEIGHING SYSTEMS
We have seen how the problems caused by Electronic Noise, Temperature Drift and Vibration are greatly magnified by using fixturing on a standard weigh cell. We have also seen how using counterbalance will eliminate most of the problems. However, there are some applications where unusually extreme resolution and accuracy are required. In such applications these factors cause problems even when there is no fixturing; or when the fixturing is counterbalanced. Even a 5 pound weigh cell may not supply the accuracies required on our 5 pound part.
Mass Balancing a Weigh-Cell
Mass Balance, a step beyond counter balance
The Mass Balanced weigh cell not only subtracts out the ill effects of fixturing. It also balances the weight of the part, allowing for the use of an even lighter capacity weigh cell. The weighing system reports the weight as it varies from an ideal or master weight. The system is then shooting for a target weight of zero and measures ± about zero. This provides the finest degree of accuracy available.
An Example of a Mass Balance Application
Suppose that unusually extreme accuracy is required when checking parts that should weigh exactly 5 pounds. The cradle that the part rests in while on the weigh cell weighs 10 pounds. We know that we can use counterbalancing to subtract out the effects of the cradle.
However, in this example, we are looking for even greater accuracy and resolution than a 5 pound weigh cell can deliver. Even with the fixture counterbalanced out, the normal problems (electronic noise, etc.) that work to limit weighing accuracy may not let us achieve the extreme accuracy we are looking for.
Once the 10 pounds is counterbalanced out, we can still add counterweight equal to the weight of the "ideal" part. The additional counterweight will be exactly equal to the precise 5 pound part weight that we are shooting for. Then, the output from the meter will be calibrated to show us how much the weight of the part varies above or below the ideal target weight of zero.
This allows us to use a one half (.5) pound weigh cell to weigh the 5 pound part. With the half pound weigh cell, we can achieve a much greater relative resolution and accuracy. As is explained below, the problems of noise, temperature and vibration are reduced to an even greater degree than we achieved with simple counterbalancing.
Increased Output Level, Decreased Electronic Noise and Temperature Drift.
Note that we are using a weigh cell that is 10 times more sensitive than the norm. This means that we have a 10 times greater signal output for the electronics to read and manipulate.
We have already seen that using counterbalance enables us to greatly reduce the effects of noise and temperature drift. Now, by using Mass Balance, we have again reduced their effects. This time by a factor of ten.
Many industrial environments are subject to a great deal of machine induced vibration. Presses, engines and passing vehicles are a few of the sources of vibration. The outputs of most weigh cells are greatly affected by vertical vibrations. The vibration is transferred into vertical motion of the weigh cell platform, the fixturing and the part being weighed. The electronics reads this motion as meaningless weight variations.
The Mass Balanced weigh cell is virtually immune to such effects of vibration. Since the mass of the fixturing, part and counter-weight are balanced on both sides of the load cell, vibration affects both sides equally. This causes a net zero effect on the load cell and negates the effects of vibration.
The cartoons below may help to visualize this effect. Suppose that we are looking at a city playground scene where passing subways periodically cause the ground to shake. Child A is standing on the diving board and the twins B and C are sitting balanced on the see-saw just as a train passes.
Child A has not started his dive yet, but when the train passes, the ground begins to shake. The shaking, of course starts the board and the child in motion. However, there is a considerable mass suspended out at the end of the board. We can visualize that the vertical motion will be magnified at this point.
Since the twins are equally balanced on the see-saw, the shaking of the ground affects them both just the same. So while the whole see-saw may be moving up and down, the twins remain in exactly the same vertical position relative to each other. This also means that there is no rocking of the see-saw.
Now, if these pictures represented weighing systems, the diving board would be a standard model weigh cell. The load cell would be mounted at the base of the board where it would register all of the rocking motion taking place out at the end.
The see-saw, on the other hand, would represent a mass balanced system. Here, the load cell would be mounted at the center (fulcrum) where counterbalance keeps the rocking motion from even occurring.
Here is a simple experiment you can do to help visualize how mass balancing negates the effects of vibration:
Take a ruler from your desk and hold it at one end. Your fingers now represent the load cell where weight is "felt" by the system. Now, move your arm up and down repeatedly to simulate vibration. Notice how, even though the weight of the ruler has not changed, your fingers (the load cell) still feel a change (rocking). This helps you visualize how a conventional scale measures, (even a force restoration scale).
Now, hold the ruler at the midpoint. Again, your two fingers are the load cell. This time, when you move your arm up and down, your fingers do not feel a change. This is because the weight of the system is balanced around the "load cell".
These examples should help you to see how Mass Balancing makes the Advance Weight System Weigh Cell virtually immune to outside vibration. This immunity and extreme accuracy make it the most reliable and versatile product available for industrial weighing.
Modern Process Control requirements create the need for reliable, high speed weighing systems. This often requires the use of material handling type fixtures on top of the weighing platform. To accomplish this, industry has typically used higher capacity weigh cells to accommodate the added load. We have seen that doing so severely limits the reliability and speed of the system.
Advance Weight Systems, Inc. has created a weigh cell design which mechanically negates those problems that arise from the traditional use of the technology. This counterbalanced weigh cell provides unexcelled resolution and accuracy, even in harsh industrial environments. The additional refinement of Mass Balance takes the concept even further and provides unmatched accuracy even in high speed, high vibration applications.
You can use other load cells and scales, but in an industrial environment, you do not want to start with something that needs to have its basic design forced to be compensated for even before you start your project, start with reliable measurements that need no compensation.
This Technical Brief is intended to help you design your weighing system. The products and technology described are available from Advance Weight Systems, Inc.
Our sales department is available to make suggestions and to help you select the equipment which best suits your needs. We also have the facilities to design and build the complete system, if you so desire. We will be happy to look at your plans and supply a quotation.
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