Electronic – Legal for trade weighing using a PLC

industrialplc

We have a project where we need to weigh product material for selling to customers. This is a project installed in Europe.

This is an industrial product line performing material manufacturing, preparation & packaging. At the final step of the packaging, the product is weight using a load cell scale, information is read by PLC and then the PLC produces a label which is printed out and then applied to the packaged product.

INTRODUCTION

Our general understanding is that: "In all commercial application where product is sold by weight, it is a legal requirement that weighing equipment must be verified as legal-for-trade".

To go through a legal-for-trade process, the weighing system has to be approved by an "approved qualifier".

To make sure to obtain that approval, care must be done to have proper certifiable components for a trade application. Components that detect tilting for instance or that verify standstill, etc.
If a trade approved unit undergoes any alteration or repair, it does require re-verification for the approval to remain valid.
One additionnal concerns is tampering, so that the "approved qualifier" can ensure no tampering happens.

PROBLEM AT HAND

The weighing electronics can be locked-down with a password set by approver or with a seal.

Given how the PLC is involved in the weighing loop (getting data and printing the label), the PLC software cannot be locked-down as it will prevent troubleshooting and adjustment of the entire manufacturing line. How can an "approved qualifier" ensures and certifies legal-for-trade of the system?

What are common strategies employed in such industrial legal-for-trade application?

Would it be required to have dedicated hardware from weighing to printer without involvment of the PLC in the loop? This would require interfacing multiple components from various manufacturers together, the most difficult beeing the interface between the weighing electronics and the label printer; that seems most likely to fail!

Is there something I am missing?

Best Answer

I think you're going to need to seal the signal pathway from the weighing platform to the printer such any physical tampering will be evident, this means some sort of cover over the moving parts and the wiring

And also restrict operator customisiation such that they cannot print false labels. If necessary move the printing code out of the PLC that's controlling the line, into a controller that is only responsible for putting marks on paper and can be locked down. (possibly with an auxiliar output only signal path to the controlling PLC so the controller can see the weight, and some sort in input to command it to weigh and print.

Related Solutions

Electronic – Ladder Logic for PLC Program

Without knowing what PLC you are using, I'll give you an example with a CLICK PLC. The CLICK PLC makes this trivial as it has built in I/O on the processor and I'm using model C0-02DR-D $139 which has

(4) 24VDC inputs
(4) Relay outputs
(2) Analog inputs (configurable as 4-20mA or 0-5V)
(2) Analog outputs (configurable as 4-20mA or 0-5V)

The first thing is to scale the input. Some PLCs such as Allen Bradley's have instructions for doing this, but the CLICK makes this easier using a graphical editor. Here it is;

The temp input signal is connected to input AD1 (analog to digital). We select 0-5V option. For scaling we set 0V = 150deg F and 5V = 350deg F (since 25mV = 1 degree).

5V = 5000mV   
5000mV/25mV = 200
150 + 200 = 350

The scaled input is saved in memory register DF1 which is a 32 bit floating value address.

Now we scale the output;

The output fan speed output is connected to output DA1 (digital to analog). Again, we select 0-5V option. 206deg F = 0V and 215deg F = 5V. For this output, we assign memory register DF3, a 32 bit floating value address. Notice on both Input and Output scaling, we selected the option Enable Range Limiter which tells the CLICK to ignore values outside of the limits.

Here is the Ladder logic. I included comments for explanation

As you can see, it took only 7 lines of PLC code. I do this for a living, so I find this trivial. Good luck!

CORRECTION: On comment of line 3 in the ladder, I meant to say copy value of DF1 into DF3 not DF2.

Electronic – Using PLC for more over than 200 relays application

I suggested the PLC approach in my answer to your other question.

Figure 1. A remote I/O (input/output) block. From left to right: network ports, power connector, input / output terminals.

Remote I/O comes in many forms from panel / DIN-rail mount to modular, snap-together, etc. The units are "dumb" in that there is no program in them. They read the status of the input signals and pass them back to the controller on request. They receive output commands from the controller and set the outputs as instructed.

Remote I/O blocks are available for

Digital signals - typically 24 V DC, 110 or 230 V AC.
Analog signals - typically 0 to 10 V or 4 to 20 mA.
Thermocouple and Pt100 temperature sensors.

Some output blocks can switch high currents (maybe 10 A or so) and this may avoid the use of additional relays. Sometimes the replaceable relay is preferred for long-term ease and cost of maintenance.

You might find some suitable software to control the remote I/O directly from a PC. As I suggested in the answer to the other question, keep it simple, scaleable, reliable, long-term vendor support, and, above all, maintainable by someone other than you!

Best Answer

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Electronic – Ladder Logic for PLC Program

Electronic – Using PLC for more over than 200 relays application

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