1. Field of the Invention
The present invention relates generally to the field of solenoid actuators and more specifically to an improvement in positional accuracy for a solenoid actuator.
2. Discussion of Prior Art
Electrically operated solenoids are well known and comprise a slug of metal and an electrical coil whereby the metal moves into closer proximity to the flux field of the coil when the coil is electrically energized. Such solenoids have been utilized in many different applications for years.
More recently, there have been developments, such as rotary and linear proportional actuators, which permit rotary and linear operation at intermediate positions, i.e. positions between the coil's deenergized and completely energized position. Generally, such actuators utilize a spring of some sort to urge the actuators armature towards the deenergized position. In such devices, when the coil is only partially energized, the actuator moves part way between the deenergized position and the fully energized position. By controlling the energizing coil current, the position of the actuator can be changed.
FIGS. 1 and 2 illustrate a typical prior art rotary proportional actuator similar to that disclosed in U.S. Pat. No. 3,435,394 issued to Egger on Mar. 25, 1969. In FIG. 1, the armature 10 is mounted on output shaft 12 which in turn is mounted for rotation in bearings 14. An energizing coil 16 operates in conjunction with stators 18 such that when the coil is energized, an electromagnetic field is formed between the stators 18 in an axial direction. The armature 10 tends to move into the flux field created and, as shown in FIG. 2, would rotate in a counter clockwise direction when the energizing coil is supplied with current. Stop 20 serves to locate the armature position in the deenergized position and it is retained there by coil spring 22 (not shown in FIG. 2 for clarity of illustration).
The coil spring 22 continually urges the armature towards the deenergized position in contact with stop 20, but can be overcome by the torque applied to armature 10 through energization of the coil. It is beneficial to maintain a certain amount of overlap between armature 10 and stator 18 (in the vicinity of the lead line 10 in FIG. 2) to insure that upon coil energization the armature begins rotation in the proper direction.
Prior art solenoids, whether rotary or linear, which operate in intermediate positions, have a relatively "soft" feel at the intermediate position. This reason for this soft feel can perhaps be better understood by reference to diagram included as FIG. 3. FIG. 3 illustrates various positions of a rotary proportional actuator which rotates in a clockwise direction from the deenergized position on the left of the Figure towards the energized position on the right.
It can be seen that the spring torque tends to force the solenoid towards the deenergized position and the desired intermediate position is that position where the spring torque balances the electromagnetic torque from the coil's energization. Although illustrated as half way between the energized and deenergized position, the "desired" position could be anywhere between the deenergized and energized limits.
If the armature is manually deflected away from the desired position, a return force will be present tending to move the armature back to the desired position when the armature is released. If the deflection is towards the deenergized position, the armature movement is in the direction of the spring torque and thus the counter clockwise torque caused by the spring will be less. However, most actuators designed for intermediate operation (such as the rotary and linear proportional actuators) have a relatively uniform rotational torque created by the electromagnetic coil which does not vary significantly with the rotary position of the armature (within the normal limits of operation).
As a consequence, the only restoring force tending to move the armature from the deflected position back towards the desired position is the decrease in spring torque which does not now fully equal the electromagnetic torque of the actuator. Similarly, if the armature is deflected towards the energized position, the electromagnetic torque will not change but the restoring force caused by the spring will increase and, therefore, the difference in spring torque and the countering electromagnetic torque again will be the only restoring force.
The consequence of the above is that, in effect, there is only the change in spring torque caused by the rotation which serves to restore the armature to the desired position. This relatively weak restoring force results in a actuator which has a relatively "soft" feel to it. Where an actuator operates in a high vibration environment or where there are external loads applied to the output shaft (by a valve or other structure being controlled by the actuator), it is desirable to increase the restoring torque available in such an actuator. It would also be desirable to be able to command the actuator to a specific predetermined position and have the actuator move to that exact position.