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. The proportional control concept is more complex than an on–off control system such as a bi-metallic domestic thermostat, but simpler than a proportional–integral–derivative (PID) control system used in something like an automobile cruise control. 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.
Theory 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. In other words, the output of a proportional controller is the multiplication product of the error signal and the proportional gain. This can be mathematically expressed as
P o u t = K p e ( t ) + p 0 {\displaystyle P_{\mathrm {out} }=K_{p}\,{e(t)+p0}}
where
p 0 {\displaystyle p0} : Controller output with zero error.
P o u t {\displaystyle P_{\mathrm {out} }} : Output of the proportional controller
K p {\displaystyle K_{p}} : Proportional gain
e ( t ) {\displaystyle e(t)} : Instantaneous process error at time t. e ( t ) = S P − P V {\displaystyle e(t)=SP-PV}
S P {\displaystyle SP} : Set point
P V {\displaystyle PV} : Process variable Constraints: In a real plant, actuators have physical limitations that can be expressed as constraints on P o u t {\displaystyle P_{\mathrm {out} }} . For example, P o u t {\displaystyle P_{\mathrm {out} }} may be bounded between −1 and +1 if those are the maximum output limits. Qualifications: It is preferable to express K p {\displaystyle K_{p}} as a unitless number. To do this, we can express e ( t ) {\displaystyle e(t)} as a ratio with the span of the instrument. This span is in the same units as error (e.g. C degrees) so the ratio has no units.
Development of control block diagrams
Proportional control dictates g c = k c {\displaystyle {\mathit {g_{c}=k_{c}}}} . 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. g p , {\displaystyle {\mathit {g_{p}}},} is process transfer function; the input into the block is flow rate and output is tank level. The output as a function of the setpoint, r, is known as the closed-loop transfer function.
g c l = g p g c 1 + g p g c , {\displaystyle {\mathit {g_{cl}}}={\frac {\mathit {g_{p}g_{c}}}{1+g_{p}g_{c}}},} If the poles of g c l , {\displaystyle {\mathit {g_{cl}}},} are stable, then the closed-loop system is stable.
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