Thursday, 7 June 2012

Liquid Transfer Automation - Part 2


For any control we need measurements, effectors and some control algorithm - lets call it control logic. Letss look how there apply to a liquids transfer.

Typical Measurements are:

Source level or weight,
Destination level or weight,
Flow path confirmation – from limit switches on valves, flowplate switches etc
Pump status, flow meter readings etc

Typical Effectors are:

Discharge valve at source, inlet valve at destination
Flow path valve actuators
Pumps, control valves etc

Control Logic

To control a transfer, there are devices at each end, such as a discharge valve and a fill valve, pumps, pressurisation systems etc that should be controlled simultaneously.
The control logic needs to sets the devices in the route, monitor them, and determine the start and end of the transfer.
Here is a very simple State Transition diagram for a transfer.

We may also need consider such things as
Need to prime pumps before transfer
Need to drain, or blow line empty after transfer
In the case where the transfer is for dosing a specific quantity there may be need to allow for the 'in flight' material that will flow after the transfer is told to stop.
And we should always consider Exception handling 

Exception handling 

There are many things that can go wrong during a transfer, such as instrumentation, pumps or valves failing, control systems failure such as IO card failure, and even communications errors between the source and destination controllers. The control logic should be designed to handle this. Typically this would involve some logic that detects the failure and then sets the equipment to some fail state, this may well be the same as the Ready State.

Source and  destination coordination:

Now, it is very likely and good practise to have a separate program module to control the source tank and another to control the destination tank. (I will cover this more in future post.) 
It is also possible that the source is controlled by a different control (PLC or DCS) to the destination. Either way it means that we need to have an interface between the source and destination modules.
And I don’t think it right to have PC’s over a network having any real time involvement in this at all, I consider that this should be the domain of purpose designed real time controllers. PLC’s or DCS controllers for example. So this interface should flow on the control network controller to controller and not depend on communications with for example a batch manager.

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