A belt scale system consists of 3 elements:
- the weigh frame, which measures instantaneous mass,
- a tacho which measures belt speed, and
- the belt weigher electronics which integrates both these inputs to determine flow rate and totalised weight.
The belt weigher system thus integrates conveyor belt loading with conveyor belt travel to calculate the total amount of material that has been carried past the weighing system and which also calculates the flow rate of material (almost) instantaneously. So with a belt scale on your conveyor you can see how much material has travelled this day, or this shift and you can also monitor your plant output in tonnes per hour, or some other convenient unit.
What do we mean when we talk about 'conveyor belt loading' and 'conveyor belt travel'?
To measure 'conveyor belt loading' we insert a weight sensitive frame into the conveyor structure which supports a section of the loaded conveyor. This 'Weigh Frame' weighs a specific length of the loaded conveyor which we call the 'Weigh Length' whose length might be measured in metres and the weigh frame is also calibrated to read weight in real units such as kilograms. As a result, the weigh frame is able to take a measurement of the kilograms per metre belt loading that happens to be the case at this instant on the loaded conveyor.
When we speak about 'conveyor belt travel' we mean how many metres of belt are travelling by. To measure belt travel we use a wheel or pulley in contact with the belt which is called a 'tachometer'. The tachometer has an electro-mechanical system such as metal tags and a proximity sensor which gives a set number of pulses per meter of belt travel. As a result, in any one time or distance interval, we are able to measure both the distance the belt has travelled, probably calibrated in metres, and how much the material on that belt weighed, in kilograms per metre. When multiplied together, these inputs yield just the weight that has passed in the interval. The final steps are to sum this weight to a totaliser (counter) and to calculate a flow rate of the material, in some appropriate units such as tonnes per hour.
It is important to understand the difference between general instrumentation systems, which generally require only a low level of accuracy, say 3%, and belt weighing systems, which can require accuracies up to .1%. An instrument 'indicates' the flow rate of some material of interest, whether it be a liquid in a pipe or solid on a conmveyor. Other instruments indicate temperatures, densities, liquid levels, speeds and so on. Such indications are typically only about 3% accurate and this is usually enough to control a process or alarm something which must be monitored. Belt Weighing systems (Belt Scales) are, however, much more accurate and must be maintained with much more care than a general instrument to achieve the accuracies which are necessary, typically 0.25% or 0.5%. As such, belt scales claim to be 'weighing systems' not general instruments and they do require quite a different mind set in their maintenance and care to achieve their promised accuracies.
How can accurate measurements be made through the belt?
One of the most significant challenges in belt weighing is to make an accurate measurement of material on a conveyor belt, through the conveyor belt. The weigh frame is located in the conveyor and it supports a length of the belt, the 'weigh length', but how can accurate measurements be made through the belt? The weigh frame supports the belt via idler rollers or in some cases via a slider bed. The conveyor belt rests in contact with these rolls and belt tension tends to resist the slight movements downward which the weigh frame must make to record the weight. Thus it can be understood that belt tension as it interacts with weigh frame alignment can have a dramatic effect on weigh frame accuracy. The answer to counteracting the 'lifting' effect of belt tension is to build a weigh frame which is robust enough to not move sgnificantly under the influence of belt load and which is long enough to support enough material to reduce the influence of belt tension errors. When a weigh frame is designed adequately, taking in to account and compensating for a variety of factors which work against accurate weighing of the material passing over the weigh frame, the belt scale is a true weighing system, not merely an 'indicator'.
To allow a belt scale to read accurately it must be calibrated.
To achieve this, a weight and length reference is used to calibrate the weigh frame and a length reference is used to calibrate the tachometer. When the weigh frame is strong enough and of sufficient length (Designed for an Accuracy) it can be calibrated accurately with static mass directly applied to the weigh frame. This static mass is our reference mass which is traceable to a National Weight Standard. The weigh length is measured accurately with a certified tape. As long as the weigh frame is well -aligned, we know have a calibrated weighing system able to accurately measure the linear density of kilograms per metre on the conveyor. To calibrate the tachometer the best way is to measure the total belt length with a certified tape and to also capture the number of tachometer pulses in one complete revolution of the belt. In this way we have a highly accurate independent calibration for the tachometer.
The calibration of a belt scale can be established by independently calibrating the weigh frame and the tachometer or the complete system can be calibrated with a test load of real material often termed a 'live load'.
Live Load/'Closed Loop' Calibration vs Static Load/'Open Loop' Calibration
The use of a live load for calibration is an absolute test, which tests every aspect of the weighing system; this can be termed a 'closed loop' calibration.
A static weight calibration, however, is part of an 'open loop' calibration. What we mean by that is that when we install, align, commission and calibrate a belt weigher, we are making sure that all the individual parts of the calibration are good. We make sure that we do the 7 Essential steps for successful Static Load Calibration:
- The weigh frame is well designed for this application
- The weigh frame is located in the right location
- The weigh frame is installed straight and level and the idler sets are also square and level and aligned to tolerance
- The weigh length has been measured properly
- The weigh frame is calibrated correctly with a reference mass
- The tachometer is correctly calibrated
- The total system is tested and shown to be repeatable and demonstrated to produce the outputs we expect with the test loads we apply.
It is not possible to really know how well the total system works until we 'close the loop' by using live load testing. However, CST have had an excellent experience of 'open loop' calibration because we do notcompromise any one of the 7 steps outlined above. In some cases it may be necessary to perform a live load test to convince the customer that the system really is working well.
This dead weight vs live weight or Open Loop vs Closed Loop calibration question is what can make belt weighing a challenging technology and this is why CST is determined not to compromise any of the steps required to achieve a credible repeatable calibration.
These days, most instruments and weighing system use microprocessor power to gather data, perform calculations and display and transmit the results. Belt weighers are no exception, and modern belt scales have benefited dramatically from the introduction of modern processor based technology. The role of the electronics set is to accept weigh frame and tachometer data, to provide a means of calibration, to multiply the inputs together, to calculate a flow rate and update a totaliser, or tonnes counter. The electronics set has sometimes been called an integrator, but this is only a part description. In fact, originally integrators were used with all types of flow meters (which produce a tonnes/hour or litres /second etc output) to integrate this flow rate against time to work out the total tonnes, total litres or whatever has passed. Some older analog belt weighers were initially produced as flow meter instruments (little better than an indicator) and they were combined with an integrator to produce a total, and so the name has stuck.
The role of the modern belt weigher electronics set has grown considerably since the early days of belt weighing. Now one of the most important roles of the electronics set is to make calibration and maintenace of the belt weighing systems easier and more accurate.
One of the most interesting aspects of belt weighing is the way in which the Zero setting is achieved, and this is becoming so much easier with modern microprocessor based electronics systems. As can be appreciated, the weight of the weigh frame itself, the idler sets on the weigh frame and the conveyor belt resting on the idler sets on the weigh frame are not what what we are trying to measure. The zero setting is how we subtract off this ever present but irrelevant weight so that only the weight of actual travelling material is measured.
The problem is, however, that the weight of the conveyor belt itself is not constant, it varies surprisingly and considerably along its length and to have a proper accurate zero setting, it is necessary to take into account the weight of the belt along its entire length and to use the average weight of the belt in the zeroing process. As a result, a zero adjustment for a belt scale requires that the entire belt be circulated at least once so its average weight can be experienced. This can take a little time and makes belt scales more difficult to manage than static scale systems.
Modern electrronics systems make this process automatic, so it is often called 'auto zero' and they can also intitiate the process automatically when the flow rate is near zero and this is called 'zero tracking'. Now, with the increasing capacity of electronics sets it has become practical to actually remember a zero setting for each part of the belt and to zero off the pre-weighed belt weight for each weight calculation along the entire length of the belt. This use of a belt 'zero image' sometimes also called a 'footprint' is an exciting development which in many respects converts a belt scale into a static scale. This is really only possible in this era of computer technology, as you can imagine, rather than remembering just one 'zero constant' a belt scale needs to remember hundreds of zero constants, in fact in our system we have capacity to remember over 2500 zero constants.
So a modern belt weigher electronics is now much more than a simple calculator or integrator or display device, it has become a specialised computer dedicated to getting the most out of the information inputs and dedicated also to making the task of living with a belt scale as easy as possible. Now with the internet available for the connection of smart instruments such as belt scales it is possible to monitor these devices remotely, to gather data about cargo deliveries, process throughputs and material movements, to check for correct operation and to carry out some remote maintenance.
In summary then, what makes a belt scale a system? Although designed to make some relatively simple measurements and using such fundamentally simple input devices as a weigh frame and a tachometer system, a belt scale is a sophisticated microprocessor controllled measuring device which is constantly monitoring many inputs , in motion, in real time. If taken seriously a belt weigher system can produce reliable management data for sustained periods between maintenance and can provide output and analysis directly to a computer in front of the process operator or stakeholder, anywhere in the world.