Hello WJansen,

I have manuals for belt scales back to the Merrick E310 ( circa 1912) and also various other manufactures including Ramsey Technical manuals covering the model 40-15 (circa 1950's) through the current. The Merrick system was pure mechanical with the electronics added for output signals. The Ramsey systems originally started with LVDT's rather than loadcells, and the electronics evolving along the way. I have some flow diagrams, but it depends on the era you are trying research. ■

Hi WJansen,

The most basic form of belt weighing is as follows:

1. Generate a dc signal proportional to belt speed.

2. Use this signal, usually 0-10v to excite the load cell

3. Amplify the resultant signal which now represents weight x speed. Usually 0-5v.

4. Feed this signal into a 4-20mA generator to obtain rate

5. Feed the same signal into a voltage to frequency converter to obtain weight pulses.

As far as filtering and other gismos are concerned, this is usually done for display purposes and analogue outputs. Generally the integration is done using unfiltered signals.

Certainly in my Ramsey days, systems using loadcells used this technique until they introduced the Micro-tech. 40-15's are different because of the LVDT's.

As far as micro based systems are concerned, there are several different ways of approaching the problem, but the principles are the same.

Hope this helps

Jon Scarrott

(Thank you for your kind commment) ■

WJansen

I think you have a very interesting project.

I have personally designed and built a belt weighing system which we have been able to promote sucessfully to major mining companies for over 20 years. We have trade certification for our system in Australia, Canada, USA and the EU.

You have asked about loadcell signal acquisition. Most systems these days use a fully integrating ADC system. We like many others use a Sigma Delta converter. These units are fully integrating, ie even though you get a new number at say 60Hz, each number represents the true average of the weight since the last number. We dont use the AD7712 any more, but you could look up the data sheet to get an idea.

But, weigh frame design is where the accuracy of the system lives and dies. We have thought a lot about this. We could correspond some more privately about this if you wanted. ■

Dear L. Ian Burrell,

I think its fascinating that you still use the Sigma Delta converter as your integration method. I would question whether "many others " use this design, as the Sigma Delta technique was developed before micro-processors were thought of and only analogue system were available. That is not to say that it does not work, but there are better and simpler design solutions that can be used with higher resolution and greater accuracy in integration techniques.

Jon ■

John,

Sigma Delta is pretty remarkable. Our system is able to do true 20 bit conversions.

I am aware of other systems such including quite simple VCO's which are still fully integrating but we like the speed and precision of Sigma Delta as it makes our system equally as useful for static scale applications. ■

I agree with your remarks and you were very complete with your ''Black Art'' information.

You mention the tracking of the belt and use the word “probably”

I am sure that, if a belt does not track well, every idlerstation within the influence area of the belt weigher will have an negative effect with respect to the accuracy

A belt, that does not run in the centre of the troughing idler station, will ,as long as it does not have this excentric position over a very long way, not run in the direction in right angles with the idler station. In this situation the belt will also be trained by the weighing idlers.

While training the belt there are not only horzontal forces but also vertical forces due to the friction and the sliding contact of the idlers with the belt which are not equal for both wing idlers for the left and the right side of the weigh idler station.

Of coarse tracking idlers are not a solution and cause extra problems because the belt will try to take in the position it would have without the training idlers. There is a great chance that the centre line of the belt again is not in right angled with the idlerstation

There als is a proposal to rise the weighing idlerstations and the two lead in and lead out idlers with 10 mm. What is the advantage of doing so?

A bit further you write about the stiffnes of the mechnical stiffness of the troughed belt.

You understand that lowering the idlers introduces a bending moment in the stiff belt.

Those bending moments will give a wave effect over several idlersets away from this point.

The same problem occurs close to the pulleys where the extra tension in the belt edges is the reason that a weiging close to this position is not a good solution.

What is the reason for rising the weighing area?

There is absolutely no influence from a lifting force from higher positioned idlers out of this area as long as they are not several mm higher.

With respect to the tensioning of the belt I can say that in some situation a spindle tension will give a better performance than a gravity take up.

For this statement I refer to my article in Bulk Solids Handling last month.

Waiting for your reply

Johan Brands ■

raising it allows for a true weight at the time as the belt weight and ore wieght are known and the tension is constant at a raised height due to the altititude of the trough. ■

Dear H Wjansen,

Keep it your research on this subject its very important in process engineering. I would like to share few things in way of guide you.

Primary Sensors:

1.Speed measuring sensor.

2.Load or force measuring sensor.

These two primary sensors are playing main role in delivering good result, ofcource the mechanism too have a great impact on realiablity of the system.

The belt scale are using in the invironment where heavy field impacts like emf or esf are influencing alot on the system. To give good results you should mainly concentrate on the design side of the electronics as well mechaism. At electronics avoid of field hurmonics are very important so avoid passive components as posible as u can in desing, so the there should not be any field hurmonics impact on the system.

Secondary is you should select good senors and its sensing method in general all manufacturers taken speed feedback from return belt by wheel rest on the return belt, while rotaing the belt the wheels cuts the torget so it give the speed data to integrator.

Design engineer should concentrate on avoiding the slip effect of elt. Next is force measuring the load applying on the sensor must be reliable, i mean the mechanism of transfering force passing on the belt to loadcell.

If these areas concentrated more then sure the product out put will be efficient.

We are design the electronics with all these featurs so that it runs years together with out any discrimination to user.

Hope you got the thing in proper way.

You can mail or contact me through mail or phone

for Kehkashan Automation Pvt Ltd.

Mohammed Khasim

Mobile:09440022329

automationengineers@yahoo.co.in ■

My thanks to IZAHARIS for answering the reason for raising the belt in the weigh area. Nothing to much add to that other than two idler distance from the weigh idler nullifies most of the bending effects of the belt transition. I know this isn't a constant from empty to full, but generally the effect is so small as to make little affect on the weighing. Depending on the conveyor survey and belt alignment, we quite often don't raise the belt in the weighing area.

Belt tracking is a much more complex issue. I have seen belts tracking badly and yet still giving acceptable weighing results and of course the opposite. There are so many variables involved that each installation has to be judged on its own merits. The obvious advice is to ensure that the belt tracks correctly.

When customers say to me it would be expensive to get the belt to track correctly due to poor belt joints, twisted conveyor, badly aligned idlers and all the rest of the reasons, I always ask, how important is the weighing information? If it's not important don't have a weigher, if it is, spend the money.

Like any other plant transducer if the measured results are of value, look after it, don't leave it to an employee who has no knowledge of what it does nor its significance to the plant operation, let a competant person look after it.

I bow to your better knowledge on the merits of gravity versus spinle tensioners. From the weighing point of view, as long as the belt tension is kept as constant as possble, that's fine by me.

Jon Scarrott ■