Back in the “good old days,” controls were all analog and mechanical, which simply means that they acted in a directly connected and variable manner based on a change in force. Both pneumatic (air pressure) or hydraulic (fluid pressure) systems are examples of mechanical, analog controls. When pressure increased or decreased on a particular device it signaled a change in action on another device like a pump/valve etc.
In the HVACR industry, we still see these types of controls, with a TXV being a common example. The TXV is controlled by pressures in the suction line, bulb, and spring to set the outlet superheat. These forces are all mechanical without electrical inputs or specific data points. The feedback from these forces is in constant tension to output the proper amount of refrigerant to properly feed the evaporator coil.