The proportional solenoid
The proportional solenoid is frequently used for the purpose of converting an electric control signal into a proportional mechanical force. Unlike the pure on-off solenoid, where only the start and end positions are relevant, here all intermediate stages of the armature movement are important.
The solenoid must have a horizontal to slightly falling characteristic curve that is, above all, as linear as possible. To this end, the pole is shaped as a cone in the area of the power stroke, which mostly tapers against the stroke direction.
In contrast to the on-off solenoid, with the proportional solenoid the return movement is also modulated, i.e. the exciting current is reduced, instead of being simply switched off. An equilibrium between magnetic force and counter-force should thus prevail at all times.
In keeping with the role of the electronics, which must regulate the current very precisely instead of simply switching it on and off again, there are a number of additional parameters which must be taken into account in the design of proportional solenoids.
The reference voltage generates the limiting current at steady-state temperature and must be constantly available as supply voltage.
The limiting current is the maximum current at which the solenoid can be continuously operated at reference temperature without thermal overloading.
The cold resistance is the ohmic resistance of the coil at 20 degrees Celsius ambient temperature, while the heat resistance occurs in operation with limiting current and reference temperature.
When a ferromagnetic material is introduced to a magnetic field, for instance when the armature is introduced into the field of the coil in the solenoid, it is magnetized. If the external field is now deactivated, the magnetization gradually decreases. It does not normally reach the unmagnetized initial state again after the external field is deactivated – the residual magnetization, known as remanence, remains.
In addition to the described magnetic components, the mechanical hysteresis components are also contained in these curves. Friction must be overcome during the attraction movement, thus reducing the magnetic force; during the return movement it works together with magnetic force and remanence against resetting.
The control of the coil can contribute to this, by setting the armature into micro-vibrations through slight oscillation of the exciter current, so that the armature never quite comes to rest. It is then only subject to kinetic friction and not the more than double static friction.
The control forms "dither signal" and "pulse width modulation" should be mentioned in this connection.
Application example 1
Proportional solenoid for a twin-clutch transmission
Twin-clutch transmissions, thanks to the higher precision and dynamics of the shifting operations...
Application example 2
Proportional solenoid for camshaft phasing
The proportional speed-dependent phasing of the inlet/outlet camshaft in car engines...