please suggest me automation suggestions

by nithin
(hyderabad)

This is the continuation of the question i asked previously i.e http://www.industrial-automation-hands-on.com/help-regarding-position-sensors.html
A sheet of rectangular paper moves continously on a conveyor belt.on top of this conveyor there is horizontal rail along width of paper on which a carriage moves.the carriage has a outlet which pours the liquid on the moving sheet of paper by moving to an fro on rail..the liquid spreads on sheet and later gets solidified and cardboard is formed (foam board). due to difference in speeds of the sheet and carriage th liquid will be poured in slightly zig zag pattern..ultimately the liquid gets spread uniformly and foam board of required thickness is formed.we want to automise this process using PLCs
the problems we are facing are
As the carriage is moving towards the end positions and stopping ,due to inertia more liquid is getting poured at corners resulting in increase in thickness of the foam board at edges
we are planning to overcome this problem by using position sensors which keeps track of the position and also the direction of the carriage and gives feedback to PLC...PLC would inturn control flowrate and speed of carriage according to position and direction of the carriage(i.e as carriage is nearing end points PLC would give signals to reduce the speed of carriage and also flow rate and vice versa when carriage is moving away from end points)
for position sensors to keep track of carriage you have suggested me two solutions as that was my previous question
now
1)please give me solutions for controlling flow rate when carriage receives signal from PLC
2)can you please suggest me how to change the speed of motor operating carriage in order to change the speed of carriage drastically at any given instant
3)there is i pre mixer which has three pipes as inlet which mixes the three different fluids coming in in a predetermined fixed ratio and the outlet pipe would go into the carriage..when we are controlling the flow rate suddenly there will be back pressure in the pre mixer so please suggest me a solution to this problem
4)for position sensing you have suggested me a GEL293 from Lenord+Bauer for example, see www.lenord.de. Plus a measuring wheel, SP500 from L+B...is it possible with this set up to vary the width of the cardboard(i.e operating length of the carriage on rail) by simply setting limits on PLC and having to adjust the length mechanically