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MUST HAVE. SHOULD HAVE. COULD HAVE. Module # 100. Batch Control Software. PAM & CPM Services. FBM. Fieldbus. Ethernet TCP/IP. 1. Measurement Devices Scales & Flowmeters. Qi Performance – the calculations and theory. Objectives:

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Presentation Transcript







Module # 100










Ethernet TCP/IP





& Flowmeters

qi performance the calculations and theory
Qi Performance – the calculations and theory
  • Objectives:
  • To be able to Calculate the predicted performance of a Qi based system
  • To be able to recognize good and bad applications
qi performance the calculations and theory1
Faster is Better - (at least in the batching world!)

In order to control at higher Flow rates - You need Speed !

Qi > 333 A/D conversions/s (Digital is SLOWER!!)

Qi Comparator responds within 20 mS (Flowmeter < 5 mS)

For Example:

We have a Constant Material Flow rate of 100 kg/s

Our Comparator updates every 20mS or 50 x per second

Our weight will change by 2kg each time the comparator “compares”

Our theoretical error limits will then be within plus, minus 2kg



At Feed Flow rate of 100 kg/s


Qi Performance – the calculations and theory
effect of flow rate variations
With Higher Flow rates all variations in the process are magnified !

Imagine a Spill value of 200 kg

ie the material takes 2 seconds to travel from the Control Valve to the Scale

If the Flow rate changes between feeds by +20%

We can expect be over feed by roughly 40 kg! The High Flow rates have magnified the errors due to Flow rate variations.

This is why we have had to use two speed feeds in the past.

  • The Qi Solution !
  • Calculate the Flow rate of the material during the feed
  • Use this information to vary the point at which we close the Control Valve, thus greatly reducing the errors caused by the change in Flow rate.
    • Note: there’s more to this story

Control Valve


Effect of Flow rate variations
now for the real world
Now for the REAL World
  • Due to various imperfections, and random variation in the Flow rate and response characteristics of the Control Valve we don’t normally achieve our theoretical performance.
  • As a result we ‘de-tune the calculations’
  • Rather than divide the Flow rate by 50 we use a factor of between20 and 30
  • For a “Good system” such as a gravity fed liquid, a value of 30 and sometimes 35 can be used
  • For a “More problematic” system we would use a factor of around 10 to 20
    • Vibratory feeders, solids, Screw feeders etc
  • Practical trials will obviously be the only way of providing you with hard data
  • Looking at our previous system we can now re-calculate our expected error limits using the following formula:
    • 100kg/s ÷ by 30 = 3.33 kg
    • Our expected error limits are now + - 3.33 kg
factors effecting accuracy
Factors effecting accuracy
  • The following factors will reduce the performance and accuracy
    • Wild Flow rate variations, especially near the end of the feed can be problematic
    • Pulsating or fast, erratic flow rate variations
    • Solid with lumps
    • “Sticky” or erratic Final Control elements
    • Binding or problematic scale





review questions
Review Questions
  • If we estimate a theoretical accuracy of +- 1 kg
    • Would most feeds be off target by 1kg ?
    • Or Most feeds within 1kg of target ?
  • Is a Vibratory Feeder a good or bad application ?
  • Is a viscous liquid a good or bad application ?