Cascade Control

[vignesh]

A simplified system to control the temperature of a process employing a gas burner using two Cascade Control digital controllers is shown in Fig. 21.21.

Each controller has a number of digital inputs and outputs, and also a number of and outputs, the actual inputs and outputs are limited to those used to control the system, although the inputs and outputs are also available to the user in a practical system.

In the case of the master controller, the remote set points enable input which is a digital input. If the controller has a logic 0 applied to it, this disables the analog ‘remote set point’ input of the master controller. The temperature of the process being controlled is sensed by a Thermocouple (TC). This signal, after amplification, is applied to the process variable analog input of the controller.

This signal is compared with the set point of the master controller, and the resulting deviation produces an output at the analog 3-term output.

This signal becomes the analog remote set point of the slave controller (The remote set point of the slave controller is enabled by the digital ‘remote / ratio’ status terminal of the master controller), hence the master controller controls the set point of the slave. The analog 3-term control output signal of the slave controller changes the relative rates of the flow of air and gas via a ratio controller which could also be a digital controller. A signal proportional to the flow rate of the air-gas mixture is used as the analog process variable input of the slave controller.

A copy of this signal appears at the process variable output terminal and is used as a track signal for the master controller. This ensures that, during start up, the 3-term output of the master is forced to follow the process variable signal of the slave, so that the remote set point of the slave is equal to its own process variable signal. The object of this arrangement is to ensure a bumpless’ transfer from any operating mode into cascade. The track operation of the master controller is enabled by a logic signal applied to thetrack’ enable digital input of the master controller.

As the temperature of the process varies, the master controller causes the slave controller to alter the flow rate of the air-gas mixture to result in a stable operating temperature.

[Mike]

One distinctive advantage of PID controllers is that two PID controllers can be used together to yield better dynamic performance. This is called cascaded PID control. In cascade control there are two PIDs arranged with one PID controlling the setpoint of another. A PID controller acts as outer loop controller, which controls the primary physical parameter, such as fluid level or velocity.

The other controller acts as inner loop controller, which reads the output of outer loop controller as setpoint, usually controlling a more rapid changing parameter, flowrate or acceleration. It can be mathematically proven[citation needed] that the working frequency of the controller is increased and the time constant of the object is reduced by using cascaded PID controllers.[vague].

For example, a temperature-controlled circulating bath has two PID controllers in cascade, each with its own thermocouple temperature sensor. The outer controller controls the temperature of the water using a thermocouple located far from the heater where it accurately reads the temperature of the bulk of the water.

The error term of this PID controller is the difference between the desired bath temperature and measured temperature. Instead of controlling the heater directly, the outer PID controller sets a heater temperature goal for the inner PID controller. The inner PID controller controls the temperature of the heater using a thermocouple attached to the heater. The inner controller’s error term is the difference between this heater temperature setpoint and the measured temperature of the heater. Its output controls the actual heater to stay near this setpoint.

The proportional, integral, and differential terms of the two controllers will be very different. The outer PID controller has a long time constant – all the water in the tank needs to heat up or cool down. The inner loop responds much more quickly. Each controller can be tuned to match the physics of the system it controls – heat transfer and thermal mass of the whole tank or of just the heater – giving better total response.