Types of controllers

Most control valve systems in the past were implemented using mechanical systems or solid state electronics. Pneumatics were often utilized to transmit information and control using pressure. However, most modern control systems in industrial settings now rely on computers for the controller. Obviously it is much easier to implement complex control algorithms on a computer than using a mechanical system.

For feedback controllers there are a few simple types. The most simple is like the thermostat that just turns the heat on if the temperature falls below a certain value and off it exceeds a certain value (on-off control).

Another simple type of controller is a proportional controller. With this type of controller, the controller output (control action) is proportional to the error in the measured variable.

The error is defined as the difference between the current value (measured) and the desired value (setpoint). If the error is large, then the control action is large. Mathematically:

c(t) = K_c * e(t) + c_s

In the above equation, e(t) represents the error, K_c represents the controller's gain, and c_s represents the steady state control action necessary to maintain the variable at the steady state when there is no error.

The gain K_c will be positive if an increase in the input variable requires a decrease in the output variable (direct-acting control), and it will be negative if an increase in the input variable requires an increase in the output variable (reverse-acting control). A typical example of a reverse-acting system is controlling flow of cooling water - if the temperature increases, the flow must be increased to maintain the desired temperature. Conversely, a typical example of a direct-acting system is controlling flow of steam for heating - if the temperature increases, the flow must be decreased to maintain the desired temperature.

Although proportional control is simple to understand, it has drawbacks. The largest problem is that for most systems it will never entirely remove error. This is because when error is 0 the controller only provides the steady state control action so the system will settle back to the original steady state (which is probably not the new set point that we want the system to be at). To get the system to operate near the new steady state, the controller gain, Kc, must be very large so the controller will produce the required output when only a very small error is present. Having large gains can lead to system instability or can require physical impossibilities like infinitely large valves.

An open-loop control systemutilizes a controller or control actuator to obtain the desired response, as shown in Figure 4. An open-loop system is a system without feedback. Its control algorithm is based on the established system functional algorithm and isn’t connected with other factors – disturbance input signals or output values of controlled process. A system error (the deviation from the desired position) is defined by its parameters and control algorithm.