DC solenoid with an almost horizontal force-stroke characteristic curve in the operating range and which can be controlled in proportion to voltage or current (Figure P 19). The latter is preferred in most cases because of the significantly shorter switching time.
Proportional solenoids may be controlled in respect of force, distance or position. As far as force control is concerned, the magnetic force is directly proportional to the current for a stroke of close to zero (used with pressure valves).
As far as distance control is concerned, the magnetic force acts against a spring with a working stroke of 2 to 3 mm. In order to reduce the relatively large hysteresis (3—6%), there is an option to create a dither signal. The solenoid can also be used, however, in a closed circuit position control system , which may significantly improve the response sensitivity and reversal error too (Figure P 20). Other aspects are the greater stiffness compared with other types of influences and a high level of repetition accuracy, repeatability.
Figure P 19: Schematic representation of a characteristic curve for a proportional solenoid and DC solenoid
Figure P 20: Proportional solenoid with an internal closed circuit position control system