That is the reason why most PLCs with generic (though industrial grade) 16-bit ADCs can connect to a standard 0-10V (or 4-20mA) output of a Summing amp (Passive, semi-Active OR Active) and yet give quite a good result. I have repaired quite a few Siemens S7-300 & -200 series CPUs and Analog modules, not to mention Allen Bradely, and i have seen quite dumb parts that one would not expect in such widely used systems.

My point on all this was that if a PLC can use generic ADCs and an analog front-end, primarily designed with the highest concern on input protection against electrical shorts, over-voltages and such, Then why cant an embedded system be built in the same fashion, though, not SO GENERIC. ?

I know that it is dumb to HAVE the opportunity of developing something from scratch and yet not doing the best possible way as indicated by all (one compact unit with tight integration and possibly no IAs in-between the Loadcell and ADC to reduce errors of those devices added on top of the load-cell internal error and line losses etc.). However, in a less than perfect world, we sometimes need flexibility as a trade-off.

Right now, what i am considering is to develop a Batching control system that is not involved or concerned with the load-cell correction and calculations etc.. Rather, a generic system which should expect a real-world quantity to be scaled to a 0-5V or 4-20mA (low-impedance) signal by a separate device (Summing amp) and just measure it precisely with a good quality ADC and scale the digital reading back to the real-world units for measurement and reporting to the master computer. WHy you may ask ?

1) Well, even though the present application is for weighing a batch of granular material and delivering that to production machine, i do not intend to let my efforts go to waste in the sense maybe a next project would involve batching of liquids where volumetric quantities are more desirable. If i develop a completely integrated control system with AFE designed specially for weighing with load-cells, i will have to do considerable changes in the analog design, AND get the PCBs fabbed again for the other applications. All this translates into re-work and a lot of time.

2) Moreover, not every batching & Weighing system needs such high resolutions as this one. There are other factors that count more in some systems. if i develop a high precision tightly integrated system, it might be too costly and overly sized for simpler applications. For example, A plant producing bricks with each batch of around 2000 KGs per 2 hours, can easily tollerate a precision of 11 to 12 bits. However, a Pharmaceutical plant producing quality dry-syrups or suspensions of mixed powders with batch sizes of 100 KGs would need atleast 16 bits or higher for controlled dosage of final product to a consumer.


So i am of the view to tackle problems piece-wise and generally break a whole system into small re-useable blocks, just like you develop smaller reuseable pieces of code for a larger application.


Now with all the valuable suggestions and feedbacks, there can be a few possibilities that i am open to and considering.

A) I go with the 2-part system, develop the control system that is part-1, and use a good quality Summing amp OR Indicator (as a signal conditioner) from the markets.

B) Again 2-part system, but i design a summing amp with an on-board uC and AD7730 that connects directly to the load-cells. The ADC can compensate for a lot of errors due to very low self-noise and AC excitation that too ratiometric. For transferring the conditioned data to the Control system, i either use a DAC for 0-5V output OR a serial (RS-485) link.

C) A single-unit system, but that will be tooo specific for this particular application. However, i am considering this as well though as a last resort.





For the question on whether the scale (Hopper) is really empty after discharging or not, well there are 2 main types - 1) which actually keep checking the weight going down in the hopper and keep the doors open till then. 2) most Mass or Bulk material Batching systems are based on time. They keep the hopper door open for a (programmable) time and then close it. Once the doors are closed, a new (stable) reading is taken and considered as new empty weight and tared to give ZERO. process resumes from there.

This last option, based on time, is especially used in places where the material(s) are sticky or of high-viscosity in nature, for example Cement / concrete weighing Batching system for brick making plant. If it is purely based on hopper discharging completely, then due to sticky or viscous material, the hopper may never close automatically and process will be put in a waiting state, unless someone manually scrapes off the material from the hopper for every batch load.

Some new systems use both weight and time. Once the weights have gone down to a certain tollerance, they start a timer, the expiry of which closes the doors and re-starts the cycle.


Load cells deteriorate over tie, specially in open / outdoor environments. carbon-steel ones become rusty and hence develop a lot of fatigue and shear. Such applications require Stainless steel cells, particularly of a SS-308 or SS-316 material.


Even slight changes in the components make a world of difference. I have seen changing simple 4-wire load-cells to 6-wire types of relatively the same precision grade giving quite better performance, specially in open atmosphere or environments. The effect of heat from Sun, RH, etc. play a very critical role. Wires elongate / expand under high temperatures and contract in lower temperatures... The elongation adds to the Effective-Series-Resistance, while contraction lowers it. the extraction and contraction also forms cracks and hence ghost resistance in the wires.

So thee are a lot of factors. Not all can be catered for. There is no such thing as an IDEAL system ! having said that, it would be foolish to ignore the ones we CAN correct or compensate for.

Regards,
Zeeshan