Analog Proportional Controller

An analog controller typically employs op-amps to provide the necessary gain and signal processing.

Let’s consider the flow control system below:

In the system above, the controller’s function is to maintain the flow of a liquid through a pipe at 6 gallons/minute.  

This system consists of:

• An electrically operated valve
• A flow sensor
• The analog controller

The flow valve is operated with a signal of 0 – 5 V, where 0 V corresponds to completely closed and 5 V is all the way open. The flow sensor provides an output signal of 0 – 5 V, which corresponds to 0 – 10 gallons/minute. The system is designed so that a sensor voltage swing of 2.5 V (i.e. 50 % of the range) will cause the flow valve to swing from full OFF to full ON. Thus, this system has what is termed to as a 50 % proportional band.

O_p = Controller output due to proportional control

K_P = Proportional constant for the system called gain

E – Error, the difference between where the controlled variable should be and where it is

The analog controller illustrated in the figure above, consists of three op-amps: The first op-amp (A₁) is acting as a differential amplifier with a gain of 1, subtracting the sensor feedback signal from the set point to create the error voltage. To maintain a flow rate of 6 gallons per minute, the set point must be 3 Vdc as calculated below using the flow sensor transfer function:

Set point = 6 gallons per minute x 5 V/ 10 gallons per minute = 3 Vdc

The output of A₁ (error signal) is fed into op-amp A₂, a simple (inverting) summing type amplifier whose purpose is to provide the proportional gain (k_p). To make the required gain of 2, the ratio of R_f/R_i (20 Kꭥ / 10 kꭥ) is set to 2. Note that, the pot R_B can add bias voltage to the error signal if necessary. The output of A₂ must be inverted to make the output positive; this is done with A₃, which is simple inverting amplifier with unity gain.

You can also read: Basic Features of Modern PID Controllers