Weigh Belt Conveyor

My company will be doing some installtion work for Tobacco processing factory in Africa. (The equipment had been dismantled earlier from a running factory).

We need to install 2 weigh band conveyors,
- each for different types of Tobacco leaves (Lamina and Stem),
to be mixed in different percentages.
- the plant would be running in different speeds as well.

The user will be required to input the percentage of each type of tobacco, The gross Capacity KGs/Hour.

Now lets get to what I'll be handling,
Company wants me to do some good ONLINE WEIGHING
only I've selected the components.
Since our company will be fabricating the conveyor,
Design can be modified to accomodate any newer type of technology.

I think I'll be measuring the linear speed of the conveyor using a encoder, and ofcourse will be controlling the speed thru the VFD.
secondly, the weighing has me confused
the models I've heard used in our industry so far,
were simple belt conveyors, with a platform just under the belt,
of 1/3 side of the conveyor length.

The friction of the running belt will force the platform and may generate errors, this type of conveyor is using a single loadcell in the center.

Can someone share his practical experience to this type or any other type of WEIGH BAND CONVEYOR?

I was thinking of using a SIWAREX with a S7-2xx plc, coupled with a MAPLES HMI, any comments? recommendation?

There are several companies that make weigh-belt conveyors that can interface to your PLC in almost any way imaginable. I would be looking at those. Because PLC's don't integrate nicely, it's a fairly difficult process to automate with any accuracy. Personally, I wouldn't even consider trying to do this in the PLC. I'd use one of them. Thayer, Milltronics, and KTron are three that quickly come to mind.

Several points onm nmotion weighing systems that you will run into.

1) you will need to make sure that the weighing conveyer is long enough to stabilize the product weight as it transfers from one conveyer to the other.

2) Use a photoeye to start the weighing application this will allow you to place it at that stable location.

3) many weighing conveyers take multiple examples of the weights then calculate an average.

4) single or multiple load cells can be used most scales I've come across for conveyers will typically use multiple load cells mounted in the direction of travel and sized at least twice the full scale limit.

5) the smoother the conveyer belting the faster the stabilization.

for indicators several options come to mind the Rice Lake 920I indicator has lots of flexibility However I've also used GSE preference leans to the 920I more bang for the cost. (also has optional I/O cards)

I would also recommend that you talk to scale companies that specialize inmotion weighing as this is one of the more difficult weighing applications and if I understand your process there isn't much room for error. Typically a good weighing system inmotion will have an error twice that of static systems.

We installed a new K-Tron Weigh Belt Feeder last year, that can operate as a proportional feeder, or just as a totalizing and current-flow weigh belt.

We've had absolutely zero problems with it, which cannot be said for our older weighbelt feeders.

Project - Update

Thanks for the feedback, here are some more details to my earlier query.


The equipment is in place, except its electrics which seems to be out of order. it was installed with a single LOADCELL holding a small PLATFORM, just underneat the belt.

Here is what I intend to do.

1) Calculate the weight of PRODUCT on the Platform.

2) Recalculate the weight after the old PRODUCT has moved and new one has moved in. (or averaging it out per HALF PLATFORM length - in direction of flow)

3) compare the weight of PRODUCT (per linear motion) required, for instance 2500KGS per hour. this depends on the width and length of the Platform (which I have not seen so far).
The linear motion will be calculated by an ENCODER.
This way I will be obtain the current flow rate.

4) Compare the CURRENT FLOWRATE with the REQUIRED FLOW RATE.

5) adjust the Speed of the Conveyor (using VFD)

6) ..... keep doing all of the above.


I know This all sounds quite simple. but would appreciate any feedback, as I have never done an online weighing application like this before. (Just measuring the flowrate and displaying is easier, than to do it in the exactly opposite sequence, like above)

Also I'm intending to use SIWAREX on this application, and recommendation (pros /cons?)

I think the wighing part and your calculations will work. The problem that you may face though is that if controlling your flowrate to within acceptable limits. You need to think of some way of buffering the tobacco before it gets to the weghing system.

Thanks for boosting my morale! i really needed it.

You are right just before this Conveyor,
we have a what we call GFP (Gravity feed pipe)
which is a pipe with a big dia (standig vertically just over the weigh conveyor)
It maintains its low and high lever by a photocell,
so the product flow is even

As I am interested to try out the SIWAREX in this application (1st time), can some1 shed some light, the problem I have is
the PLC existing on this environment is a Siemens S7-2xx PLC.
and as far as I know, the SIWAREX MS connects to S7-2xx, but is not recomended for online weighing? and comments?

It's probably not recommended for online weighing for all of the reasons already discussed here. That looks like it's probably a great system for batch weighing. Online weighing is a totally different beast -- accurately integrating with a PLC is nearly impossible.

I'd go along with other suggestions and use an instrument designed fir the job, it will cut a lot if heartache at the end of the day.

As suggested, the K-Tron is an industry standard, the encoder and loadcells go into that, and it controls the VFD.

You just send a 4-20mA signal for target load and receive a 4-20mA signal on actual.

Job done...

Remember as well, take great care during installation that no-one fixes anything solid to the conveyor, cables etc need to be connected via flexible joints.

some clarification

I would appreciate some more help on basics,
as I've never used SIWAREX MS, or Siwatool before.

1) Since siwarex should link up pretty well, I think this would be better than using a Weigh.controller from other brand, and setup the communication via Analog. (I'm asking this because the existing unit has an old weigh.controller - but the company has given me the option to totally replace the old system)

2) Once configure/tuned, thru the SIWATOOL/wizards (whichever applicable), would I still need to do scalling as in ANALOG, or would I be quite close to a more simpler version?

3) Would I need any extra libraries for microwin?, or the SIWATOOL is enuf?

4) I have Microwin 4 (sp3) does the new sp6 version have anything on the SIWAREX, a wizard perhaps, I've tried installing the SP6, but doesnt work, after trying all sorts of tricks!
had to reinstall my system altogether.

Siwatool MS Library

Where can I find the Siwatool MS Library?