A TECHNICAL AID IN THE DESIGN OF WEIGHING SYSTEMS
What is a Weighing System?
In the past, a weighing system was made up of two components. These were simply a scale and a person to operate it. The operator would pick an object up and set it on the scale. They would then read the weight display and decide from the reading what to do with the object. This might involve modifying the object in some way (adding pieces to a box or cutting material off of the object) or simply sorting objects by weight. They then lifted the object from the scale and sent it on its way and moved on to the next piece.
Today's automated manufacturing and process control requirements demand a much more sophisticated (faster and more reliable) weighing system. This Technical Brief begins by describing the components that may go into a modern Electronic Weighing System.
Weighing System defined.
For this discussion, a weighing system is more than just a load-cell base and a meter. A system, as we will refer to it is:
• a weigh-cell base,
• a setpoint controller, (meter),
• some type of input mechanism which meters the item to be checked one part at a time,
• some sort of fixturing on the scale which could be robotics, or conveyors,
• some sort of sorting mechanism to get rid of "bad" product,
• and a programmable logic controller, (PLC).
The basic electronic weighing system consists of a weighing platform or "load cell" and some type of meter to report the weight of the object on the scale.
A full-blown Weighing System usually involves a variety of support equipment, such as Input and Output Equipment used to bring objects to the scale, and then remove and sort them by weight; conveyors, vibratory feeders, robots, gates, chutes, rollers, part sensors, switches, etc.
It goes on to point out a number of concerns that the engineer must be aware of when designing an accurate weighing system.
There are several factors that work together to degrade the accuracy and reliability of any weighing system. These notes explain what these are and how they are overcome by some unique design innovations.
THE MAJOR COMPONENTS OF AN ELECTRONIC WEIGHING SYSTEM
The Weigh Cell
The starting point in an electronic weighing system is the "load cell". This is a device that converts mechanical movement or pressure into an electrical signal or output.
When an electric current is passed through electronic resistors called strain gages a measurable signal is received. This signal, which is usually expressed in millivolts per volt of input, varies in direct proportion to mechanical pressure applied to the load cell.
Load cells are often marketed individually and built into weighing systems. A Weigh Cell is a more complete structure, which contains the load cell. It provides the mechanism by which an object creates movement or pressure on the load cell. A well-constructed weigh cell protects the load cell and transfers the "weight" in a consistent and linear manner.
Meters and other Electronics
The meter portion of the weighing system takes the millivolt output from the weigh cell and converts it into useful information. The signal is amplified and translated into digital segments, which are displayed for the user. The translated signal may also be fed directly to computers and process control devices. It is only when a weigh cell and meter are used together that the device is called a scale.
A complete weighing system includes a variety of support equipment. A major concern is the use of Input and Output Equipment to bring objects to the weigh cell and to then remove and sort them. Some of this peripheral equipment might be conveyors, vibratory feeders, robots, gates, chutes and rollers. There might also be such devices as switches and photo sensors to keep track of objects as they move through the system.
Fixturing refers to any equipment which is permanently fastened to the top of the weigh cell. The fixturing is used to hold objects for weighing or to move them across the weigh cell while they are being weighed. This fixturing might be as simple as a platter or bowl to catch the parts.
Or it might be a more complex conveyor or hopper to catch, move and dispense materials. The fixturing might even be a pick and place type of robot.
HOW FIXTURING DEGRADES SYSTEM ACCURACY
When fixturing is part of the weighing system, the weigh cell must accept the weight of the fixture as well as the part being weighed. This fixturing is commonly many times heavier than the part. The usual procedure is to use a scale with a high enough capacity to accommodate both the part and the fixturing.
The electronics is then programmed to ignore the fixture weight and "read" only the additional weight of the object. This means that only a fraction of the live range of the weigh cell is actually being used to weigh the object. We will see that this use of the weigh cell severely limits the reliability of the system.
There are certain factors that tend to limit the accuracy of every electronic scale. Among these are Electronic Noise, Temperature Drift and Vibration. These factors are discussed below where it will be shown that the ill effects that they introduce are greatly magnified by using a larger capacity weigh cell to make up for the weight of fixturing.
In order to better visualize the effects of these factors, we will create a typical example of a system that includes fixturing which is heavier than the object to be weighed. The accuracy's which may be achieved will be compared with those attained by using a weigh cell whose capacity matches the weight of the part.
Suppose that we have a high-speed application to weigh 5 pound machine parts. We also find that to keep the system up to speed, we must weigh the parts as they pass across a conveyor which will be mounted to the top of the weigh cell. The conveyor that we plan to use weighs 155 pounds.
This means that we must use a weigh cell which has a capacity of at least 160 pounds. Using a standard model weigh cell, probably one rated at a 200 pound capacity, would normally do this. So, simply put, we would be weighing a 5 pound part on a 200 pound scale. (This means using only 1/40 of the full live range of the weigh cell to weigh the object!).
FILTERING AS A PARTIAL SOLUTION
It is very common to use electronic filter or digital filter or both to fight these problems. Filtering is basically a means of electronically compensating for the effects of electronic noise and vibration.
The effects of these factors (on the output) may be pictured graphically as a series of valleys and mountains, where a correct output would be pictured as a plateau or relatively flat line.
In effect, what filtering does is take the fluctuations and average them over a period of time. This creates the appearance of a plateau and tends to compensate for the variations.
Note that the key word here is time. The major problem with this type of filtering is that it slows the system down considerably. This is especially true where filtering is used to compensate for mechanical vibration which generally occurs at a relatively low frequency cycle. Electronic Filtering or Digital Filtering is a valuable tool in many applications. However, this brief is primarily meant to deal with high-speed process control applications where filtering may not give us all of the speed and accuracy required.
There are a number of factors that tend to compromise the accuracy of all electronic weighing systems. Among these are Electronic Noise, Temperature Drift and Vibration. The problems these cause are generally kept at an acceptable level so long as the weigh cell is of a reliable design and is the same capacity as the part to be weighed. However, these problems are greatly magnified when we are forced to use a larger capacity weigh cell to accommodate fixturing as well as the part to be weighed. We looked at each of these three problems individually; but consider that all of these will be present together and will effectively magnify each other, making the already suspect weight readings even more unreliable.
Note that the hostile conditions chosen for the example were purposely kept to a minimum.
There are numerous sources of electronic noise in a manufacturing environment. The 100 micro-volt noise level we chose would certainly be much higher in an actual application. Levels of 1000 micro-volts or higher could well be expected.
Temperatures often vary greatly in a manufacturing environment. Changes in the season, furnaces operating, lights and machines going on and off, bay doors opening and closing; all of these cause fluctuations. We chose 5°C. (9°F.) for our example. Daily (or even hourly) temperature shifts of two or three times this much is common in many work places.
Likewise the level of external vibration we used, in the example, could be much more severe. Many weighing systems operate right next to large engines or punch presses.
As was briefly mentioned, Electronic Filtering is sometimes used to help compensate for these problems. However, this is done at considerable cost to the speed of the system. In many high-speed automated systems, this loss of speed is unacceptable.
At this point it would appear that our need to place fixturing on the weigh cell would make the readings so unreliable and inaccurate as to be virtually meaningless. How then, do we build the accurate, high speed weighing systems that are needed for today's automated, process control equipment?
To meet this need, Advance Weight Systems, Inc. produces an exclusive electronic weighing system that uses "Counterbalancing" to mechanically subtract out the effects of the mass of the fixturing. By developing Counterbalancing, Advance Weight Systems has made it possible to use the most accurate capacity of weigh cell even when heavier fixturing is needed. It is actually possible to weigh the 5 pound part (in the example) on a 5 pound weigh cell and still use the 155 pound conveyor.
HOW COUNTERBALANCE WORKS
Quite simply, the counterbalanced weigh cell works the way a child's see-saw works. The load cell is mounted in the weigh cell assembly in such a manner that it is possible to place mass on both ends of the assembly and mechanically maintain a zero output.
Take a look at the earlier example, where we needed the accuracy of a 5 pound weigh cell; but we also needed to use a 155 pound conveyor. By using an Advance Weight Systems 5 pound weigh cell, we start with an extremely reliable unit on which we can place the conveyor. Then, a mass equivalent to the 155 pounds can be added on the opposite side of the load cell. The sketch shows a simplified representation of the concept.
Now, we are using a 5 pound weigh cell to weigh a 5 pound part. We can achieve the finest available resolution and accuracy, as if the fixturing were not even there. The net result of using counterbalance is the elimination of the ill effects that were introduced by placing fixturing on the weigh cell.
It may be shocking to think of placing as much as 160 pounds on a 5 pound unit. Most weigh cells would be severely damaged by such treatment. Advance Weight Systems, Inc. builds its weigh cells with rugged overweight and underweight stops to protect the load cell from damage. These weigh cells commonly accept overloads of more than 1000% above capacity.
The design of most weigh cells is such that internal vibration or oscillation tends to slow response time, even if there is no outside source of vibration. This tendency is magnified by the use of extra mass on the weigh cell. The standard solution to this is the addition of a dampening device (damper). Most dampers work by one of three principals; air, oil or magnetic dampening.
Air dampers work by forcing air in and out of a cylinder similar to the way a shock absorber works. The uses of air dampers are limited by the fact that they introduce friction into the system. This friction will severely jeopardize the reliability of the output.
Oil dampers work by forcing a moving portion of the weigh cell to move within a cylinder of oil. This oil retards the movement or vibration, but does not create friction. A good oil damper should be adjustable so that it stops vibration quickly without slowing the response time by "over dampening". The proper oil level should be maintained and the oil should be kept free of debris.
Magnetic dampers create a magnetic field within which a moving portion of the weigh cell must pass. A conductive piece of metal is fastened to the weigh cell at this point. By the action of eddy currents, the magnetic field retards the movement of the weigh cell. This causes motion to settle out quickly. A magnetic damper is generally much more expensive than an oil damper and tends to collect metal chips from the surrounding area. These chips cause friction which destroys accuracy.
ADVANCE WEIGHT SYSTEMS, INC. manufactures its weigh cells with an oil damper. This damper is conveniently placed for adjustment or occasional refilling. It uses silicon base oil which is clean, clear and odorless. The dampening rate is extremely reliable, because the oil remains at a constant viscosity through any range of temperatures. AWSI's weigh cells come to full rest and provide a stable output in as little as 1/10 second.
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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