Update your settings

I'm retro-fitting a system with following, and would appreciate comments,
SCENARIO:
A system to control Two conveyors carring two different products, being mixed upstream.
(It is necessary to remember that the accurancy is resonably tolerable, because the mixed product is not consumed from the line, but FED toa Rectangular SILO layer by layer from left to right and right to left, has to be cured for sometime before it can be consumed. the product inside the SILO is resting on Conveyor whice moves to one side after the required duration, the product is chipped from the side, so the blend of these two products is eventually mixed in several places, specially the delivery method of chipping the +3feets of product from side)
Two such conveyors, consisting ofa SINGLE SIWAREX R SB loadcell, Incremental Encoder on drive shaft, and separate Micromaster VFD for each conveyor, all these are connected to a CPU-S7226, with two SIWAREX MS modules, and aTouchscreen display.
REQUIREMENTS:
It is required that the Customer will enter the required flow rate for each conveyor in the HMI.
the PLC has to do everything else, such as,
1) Maintain the Product height (this is to be done by sensors stopping the conveyor if feeding pipe is near empty. Stopping the UPSTREAM equipment if FEED PIPE is full (almost choked!!)
2) One single Loadcell holding a Platform, just beneath the Conveyor Belt. which measures the product above it.
(I intend to do this by measuring the values from the LOADCELL, for say every Centimeter or INCH of displacement of Belt, trigered by the Incremental Shaft encoder. SAY if platform is 20inches long, I will measure 20 such instances, these 20 values store in a TABLE {RE: my recent postings for BLKMOV}, anything that moves out of this range is equal to what is FED to the next equipment.)
3) I'll be using 3 wires from the VDF (Terminal Mode), configuring it to multi-speed "BINARY" (instead of programming it to ANALOG speed controls)
once the FLOWRATE is received from HMI, PLC decides which of the available setpoint is to be used (first step being 14%, 28%, 42% ......100%)
to control the flow, the PLC will just STOP and RUN the Conveyor and the decided speed.
I think this text is enuf for now, I've attached a sketch of the conveyor for better understanding.
Comments???