Cascade Control
Cascade control is a multiple loop control configuration, like the override control which involves more than one measurement and one manipulation. In the cascade configuration, there are two control loops to manipulate one process variable.
The outer loop that measures the controlled variable is the major one and its controller uses the set point given by the operator. The outer loop is also called as primary or master loop. The inner loop measures an intermediate process variable (usually a major disturbance) and uses the output of primary controller as its set point. The inner loop can also be known as secondary or slave loop. The secondary loop helps in the effective control of primary process variable.
Cascade control is a multiple loop control configuration, like the override control which involves more than one measurement and one manipulation. In the cascade configuration, there are two control loops to manipulate one process variable.
The outer loop that measures the controlled variable is the major one and its controller uses the set point given by the operator. The outer loop is also called as primary or master loop. The inner loop measures an intermediate process variable (usually a major disturbance) and uses the output of primary controller as its set point. The inner loop can also be known as secondary or slave loop. The secondary loop helps in the effective control of primary process variable.
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Block Diagram of Cascade control
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| Feedback Control (Left) and Cascade Control (Right) of Process Fluid Outlet Temperature |
The
disturbances can be nullified by cascading the temperature controller to a flow
controller (fuel gas inlet). The temperature control is the primary control and
its controller output is the set point for flow controller. The flow control
will be the secondary control and corrects the disturbances that affect the
fuel gas flow rate before it affects the outlet temperature.
Key rules for cascade control
- The cascade control has an advantage of disturbance rejection. The secondary controller corrects the variations occurring in secondary variable before it affects the primary variable.
- The secondary loop response must be at least three times faster than that of primary loop.
- Secondary variable has to be chosen in such a way that the process dynamics (time lags) are split equally. If most of the process dynamics comes under secondary loop, then the addition of cascade control leads to the instability of primary loop.
- The most common secondary variables are valve position control, flow control and temperature control (in chemical reactions).
- The common mode of secondary control is proportional controller. The proportional and integral control mode is also used at times. Integral control is said to slow down the secondary loop response. Moreover, in cascade control we have primary controller to eliminate the offset of proportional controller. Derivative action is not used in secondary control.
- In the process of tuning, the secondary loop is first tuned and then the primary loop.
Example for Cascade
control
Speed control of a DC
motor
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Cascade control in speed control of a DC motor
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The speed control of the DC motor is done controlling the armature voltage by a thyristor unit. The thyristor adjusts the armature voltage which gets reflected upon armature current and speed of DC motor. The armature current is the secondary variable because it has lots of chances to go high and low during start-up, shut down and loading the DC motor. To limit the maximum and minimum value of current, saturation is applied to the current set point. The secondary control loop takes care of these changes and helps in primary controller to control speed.
Also read other control schemes
Very good and useful blog.it's a request ,please post more blogs regarding instrumnentation and control
ReplyDeleteThank you. Sure i would post more. Keep following.
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