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The Cascade Loop

Most control loops try to keep whatever is measured at a value set by the operator (SetPoint, SP). Sometimes the operator lets something else decide the setpoint. This is called cascade.

In most cases this means two control loops work together. The output of the first decides the setpoint of the second.

Why would you do this?

As an example, we can look at a gas heater. The thing we care about is the temperature of the stuff that leaves the heater. We want this temperature to be as close to our target value as possible. Changing the temperature could lead to wasting energy, producing low quality products or even cause an unsafe condition.

To control the temperature, we alter the rate gas enters the furnace. If the temperature is too low, we can increase the rate of gas, which provides more heat and increases the temperature.

The problem is that our gas supply is not steady. There are lots of other process units that put gas into the system and lots more that use it. Changes to these othe precedes can cause a change in the supply pressure. If the supply pressure increases, and the control valve doesn't move, then more gas will enter the heater and this will increase the outlet temperature. At this point the control loop will see the higher temperature and will close the valve slightly to reduce the temperature. But by this point, we have had a disturbance. The temperature changed. We have wasted energy or changed our product quantities. It would have been nice to move the control valve before the temperature changes.

A cascade loop can help in this case. It takes a while for the change in pressure to cause a change in the outlet temperature, but the flow rate of gas would change quickly. If we were trying to keep the flow rate of gas the same, the controller would close the valve almost immediately, long before the outlet temperature is changed. The disturbance was rejected earlier.

Of course, other things can change the outlet temperature, such as the inlet temperature. If the temperature of the stuff going into the furnace goes down, then the temperature leaving the furnace will also go down. In a simple loop, the decrease in outlet temperature would cause controller to open the gas valve.

In a cascade loop, the decrease in outlet temperature would cause the first loop to ask the second loop for more gas. The second loop would then aim for more gas and as a result it would open the gas valve.

In this case, the cascade loop doesn't help any more than the simple loop. It may even make things slightly slower. The cascade only helps deal with changes in the gas supply. To be able to handle changes in many other things, a true advanced control system is needed such as multivariate predictive control. But that is for a different post.

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