Weighing Machine using PLC

sir's
I have a weighing machine needs to be retrofitted .the machine have 14 hoppers with a load cell attached to each one.
the signal from the load cell is connected to an amplifier with output signal 0-10v.Can I use a Plc with analog input modules in order to compare the weigh in each hopper(channel) with a preset value and produce an output signal to the corresponding solonoied valve to discharge this weigh in a packing machine.Is there any limitations for the plc time responce(scan time)?.Is this system will be an accurate & productive one?.
I am waiting for your valued responce

Most PLC brands have anaolg inupt cards that will accept the 0-10 VDC signal. You can then use four function math or built in scaling functions (depending on brand) to convert this input data to engineering units. Most PLCs have ">" and "<" functions to compare the actual weight to a set limit, and then open the hoppers. Unless you require incredibly fast (less than a few mili-seconds) response using a PLC is an excellent approach to this problem.

You should contacct a couple of local distributors to get assistaqnce in selecting the PLC hardware & programming software, and allow them to provide some help in getting started.

Tom
Thanks for your attention.
Acually each of the 14 hoppers is fed from linear vibrator in the same time the 14 linear vibrators are fed from one main radial vibrator.in this way it seems to be a contionous weighing process for 14 hoppers at the same time.this 14 value will be compared to a set value and only one hopper will be opened.I am afraid that the PLC will not be able to update the reading from the 14 Channel .so it may produce the stop signal for the linear vibrator and the open signal for the hopper with a little bit delayed (due to the scan time ) this will cause the weigh to be some grams more than the setting value.It will not be accurate.

I'm not 100% sure I undertand what your problem is, but the PLC CAN compensate for scan time, or "material-in-flight", or "time it takes to close a valve" by use of a pre-act.

Instead of stopping the process when setpoint is reached, the process is stopped when (setpoint minus pre-act) is reached. With a properly set preact, by the time "the dust settles", the exact setopoint amount of material has been transfereed.

You can even rig a system where the pre-act tunes itself, getting smaller if theres an undershoot, larger for an overshoot.

If accuracy is so critital, how are you planning to do it without a PLC?

It's not just the PLC scan time that will introduce inaccuracy into the systems. You will also need to consider the resolution and the update time for the analog input module(s).

In general, the higher the resolution of the analog module, the less frequently that each channel gets updated. You'll need to read the data sheets for the modules that you're cosidering. In some cases, you may find that individual channels get updated less often than once per I/O scan.

The key question is how often can you expect to get fresh load cell data into the PLC. Combine that with the fill rate of the hoppers to determine if you will be within an acceptable tolerance range for weight variation. If the hoppers are filling at 100 grams per second and the PLC is getting load cell updates every 50 milliseconds, then you'll be dealing in increments of 5 grams.

You also need to carefully size the load cell to the application. Don't use a 1000 Kg load cell to measure a 10 Kg load.

Several PLC manufacturers offer dedicated weighing modules. These modules accept the load cell directly and also integral high speed outputs. The weighing calculations are performed and outputs controlled independent of the processor scan time.

You can find information on the Schneider Electric offering at: http://www.schneiderautomation.com/Default.htm.

Allen
I am thinkigh to implement this system using an impedded system with digital and analog I/O cards .Nowadays there are many manufactures for such systems.do you think it will be more usfull.tacking into consideration the "material in fliegh time" and as "STEVE" mention the wrigh selection of the load cell and the time responce for the valves and vibrators
what are you suggest

I just keyed in on a word: vibrators. To me, that sounds like you are handling solids, probably powders. Those can be tricky to work with, especially to dispense accurately (been there, done that, have clothes that are permently covered with toner).

I'm trying to think of the major differences between "an embedded system with digital and analog I/O" and a PLC. It's a queston we get here a lot (and usually refuse to answer):
- An embedded system might have a faster responce time (if only because it's not busy handling other processes besides the time-critical one).
- An embedded system might be cheaper (at least in inital cost. By the time you add in "cost to debug", "cost to maintain/modify", "cost due to loss of production because there's only one guy who knows how to fix them, and he's on vacation", and it could come out even.
- An embedded system won't have on-line monitoring (let alone programming) which is an incredible debug tool.
- Presumably you'll be programming it yourself (which is the usual drawback - no way to make changes. The code stays as the OEM wrote it). But do you have the skill/experience to do it right? Or will you have to hire someone to do it for you? See "cost" above.

Regardless of which type of system you go with, you will still need to worry about your analog resolution (one of Steve's points), and system response time (my point).

How are you planning on cutting off the flow - butterfly valve? How fast will IT close? Will it always close that fast?

Lots of other questions: How do you plan on entering setpoints, or will the dispense amount be fixed? If "fixed", are you sure? How do you plan to start (and stop?) the dispense? Pushbuttons at each station? A central HMI?

You've got a lot more design thinking to do before deciding "To PLC, or not to PLC".

Allen
As I mentioned before the machine is already exist with a malfunction control system (old one it may by emdedded).All what i am going to do is to replace this malfunction system with a new one.
I will deal with freezed vegtables it will be alawys solid for example
beens, okra,potatos .....etc
Set point will be entered through a HMI It will never be constant they need to vary it according to the product .
The unique way to stop the flow in this existing system is to stop the vibrator cooresponding to the hopper which reach the setpoint
By the way the hopper will be opened to discharge the weigh penumatically(solonoied valve+Piston).

I have never go through embedded systems before and according to your notes it seems that I will go away from it
Thanks

I have 4 linear doser with weighing modul on itch connected with 4 weight controller to fill 4 dosing head and they all release in one funnel, this controllers gives digital signals that weight is ok,(one dosing head is for one bag of beans) wich function use to order when first weight is ok to release in funnel, then next. Not to go by order 1-2-3-4, but which one is first full that is released in hopper. I need to use delta plc?

OK the best way is to build a queue, for example a sort of shift register as it is only 4 weighers there is no need to use an actual shift register as it can be done with some simple logic, for example you have a sort of array of registers lets say reg1 - reg5
When a feeder gives a signal to say it is upto weight then put that feeder number into reg1, then move them down this is a little more tricky than a standard shift register. Attached, is a very simple one done on a Mitsubishi now this is in FBD but easy to do in ladder the functions may have slightly different names I would normally do this by indirect addressing so the code is compact but can be difficult to debug if your not used to indirect addressing & as it has only 4 feeders it's not a big program. For testing, I used an HMI simulation so there is no code for filling the conveyors I just set the relevant bits on the HMI to simulate the filling of the feeders as feeder ready, Hopper ready & hopper Dropped you will need to control or set these bits when the relevant state has been reached. for example: when a feeder conveyor has the correct weight, set the feeder ready bit, also the hopper for example when a pack to be filled is in place then set the hopper ready to drop & when dropped, set the hopper dropped bit.