Multi-variable control loop systems

What is a multi-variable loop?

Multivariable loops are control loops in which a primary controller controls one process variable by sending signals to a controller of a different loop that impacts the process variable of the primary loop. This kind of system must have more than one input and more than one output. A disturbance in any input causes a change of response from at least one output.

Example loop:

  • The primary process variable may be the temperature of the fluid in a tank that is heated by a steam jacket.

  • To control the primary variable (temperature), the primary (master) controller signals the secondary (slave) controller that is controlling steam pressure.

  • The primary controller will manipulate the setpoint of the secondary controller to maintain the setpoint temperature of the primary process variable.

  • The primary channel can’t be changed and there is field device connected directly to the primary controller. As the process variable changes the primary controller reads and sent to the secondary controller.

  • In the secondary controller, the controller compares with primary value with the secondary value and produce a proper control action. This control action is changed according to both the variable change. Controller action occurs only with one flow channel.

  • In another way, the increased flow rate in the primary change will be compensated with the controlling the flow in the secondary channel.

  • When tuning a control loop, it is important to take into account the presence of multivariable loops.The standard procedure is to tune the secondary loop before tuning the primary loop because adjustments to the secondary loop impact the primary loop. Tuning the primary loop will not impact the secondary loop tuning.