Another type of PID is in analog form. There are certain reasons to choose the analog form.One of the reason is it can implement continuous signal. Compared to digital control, the signal for analog is smooth and not discrete.
The controller was implemented in this box where it contains two controller. It has few potentiometer to implement the controller where the gain could be adjusted. By the way, the PID was implemented using operational amplifiers (op-amps). Op amp has all kind of application like linear gain, derivator, integrator, summing junction and the likes. Therefore, op-amps could be easily used to implement the analog controller.
This control box was used to control motors. The signal coming out of the op-amp are not strong enough to actuate motors. In other words, the current from the op-amp is not high enough. Therefore additional circuits need to be used to actuate the motors. Some of the simple circuit is the on-off relay circuit, or a B-Class amplifier.