Proportional control
Proportional control, in engineering and process control, is a type of linear feedback control system in which a correction is applied to the controlled variable, and the size of the correction is proportional to the difference between the desired value (setpoint, SP) and the measured value (process variable, PV).
Two classic mechanical examples are the toilet bowl float proportioning valve and the fly-ball governor.
On–off control will work where the overall system has a relatively long response time, but can result in instability if the system being controlled has a rapid response time.
Proportional control overcomes this by modulating the output to the controlling device, such as a control valve at a level which avoids instability, but applies correction as fast as practicable by applying the optimum quantity of proportional gain.
A drawback of proportional control is that it cannot eliminate the residual SP − PV error in processes with compensation e.g. temperature control, as it requires an error to generate a proportional output.
To overcome this the PI controller was devised, which uses a proportional term (P) to remove the gross error, and an integral term (I) to eliminate the residual offset error by integrating the error over time to produce an "I" component for the controller output.
In the proportional control algorithm, the controller output is proportional to the error signal, which is the difference between the setpoint and the process variable.
This can be mathematically expressed as where Constraints: In a real plant, actuators have physical limitations that can be expressed as constraints on
From the block diagram shown, assume that r, the setpoint, is the flowrate into a tank and e is error, which is the difference between setpoint and measured process output.
is process transfer function; the input into the block is flow rate and output is tank level.
For a first-order process, a general transfer function is
Combining this with the closed-loop transfer function above returns
Introducing a step change to the system gives the output response of
For an integrating process, a general transfer function is
Introducing a step change to the system gives the output response of
Over a range of operating conditions, proportional control alone is unable to eliminate offset error, as it requires an error to generate an output adjustment.
[1] While a proportional controller may be tuned (via p0 adjustment, if possible) to eliminate offset error for expected conditions, when a disturbance (deviation from existing state or setpoint adjustment) occurs in the process, corrective control action, based purely on proportional control, will result in an offset error.
Consider an object suspended by a spring as a simple proportional control.
The spring will attempt to maintain the object in a certain location despite disturbances that may temporarily displace it.
Hooke's law tells us that the spring applies a corrective force that is proportional to the object's displacement.
While this will tend to hold the object in a particular location, the absolute resting location of the object will vary if its mass is changed.
This difference in resting location is the offset error.
, the proportional gain, is very high, the proportional band is very small, which means that the band of controller output over which the final control element will go from minimum to maximum (or vice versa) is very small.
is very high and hence, for even a small error, the controller output is driven from one extreme to another.
An analogy to on–off control is driving a car by applying either full power or no power and varying the duty cycle, to control speed.
When the speed falls below the target, with a certain hysteresis, full power would again be applied.
It can be seen that this would obviously result in poor control and large variations in speed.
Stability may be expressed as correlating to the power-to-weight ratio of the vehicle.
In proportional control, the power output is always proportional to the (actual versus target speed) error.
If the car is at target speed and the speed increases slightly due to a falling gradient, the power is reduced slightly, or in proportion to the change in error, so that the car reduces speed gradually and reaches the new target point with very little, if any, "overshoot", which is much smoother control than on–off control.
