Solenoid with armature biased towards the rest position with two springs

A solenoid having an armature movable with respect to a stator from a rest position to an energized position when a coil is energized. Two return devices, for instance helical compression springs, urge the armature to return to the rest position. Two slidable sleeves are provided between stator and the armature. Ingress of dirt into the armature may jam either sleeve with respect to the stator or the armature without jamming the solenoid, since the other return devices/sleeve combination will still permit movement of the armature.

BACKGROUND OF THE INVENTION 
The present invention relates to a solenoid. Such a solenoid may be used as 
an actuator in various applications. 
A known type of solenoid comprises a coil or winding of a conductor on a 
ferromagnetic stator. The stator is hollow and contains a ferromagnetic 
armature which is movable rectilinearly inside the stator. Once sufficient 
electric current is supplied to the coil, the armature moves axially of 
the coil. A return spring is provided to return the armature to a rest 
position when current to the coil is interrupted. 
Solenoid actuators of this type are in widespread use and generally 
function satisfactorily. However, in hostile environments and/or in 
critical applications where failsafe operation is required, problems can 
arise in ensuring that the armature returns to its rest position when the 
coil current is interrupted. For instance, if the return spring breaks, 
then the restoring force is lost and the armature may not return to its 
rest position. Also, if the armature becomes bent or if contaminants such 
as particles of dirt enter the gap between the armature and the stator, 
the armature can become locked in the actuated position and the return 
spring may be incapable of returning the armature to the rest position. 
OBJECTS AND SUMMARY OF THE INVENTION 
According to the present invention, there is provided a solenoid comprising 
a stator including an electromagnetic coil, an armature movable with 
respect to the stator from a rest position to an energised position when 
the coil is energised, and first and second return means, each of which 
urges the armature towards the rest position. 
The armature may be arranged to perform substantially rectilinear motion 
with respect to the stator when moving between the rest position and the 
energised position. 
Preferably one or each of the first and second return means comprises a 
spring, such as a helical compression spring. 
Preferably the first return means acts between the stator and a first 
sleeve which is movable with respect to the stator and the armature and 
which is urged by the first return means against the armature, for 
instance against a first shoulder of the armature. 
Preferably the second return means acts between the armature, for instance 
a second shoulder thereof, and a second sleeve which is movable with 
respect to the armature and the stator and which is urged by the second 
return means against the stator. 
Preferably the first and second sleeves are made of non-ferromagnetic 
material. Preferably the second sleeve abuts against a non-ferromagnetic 
part of the stator. 
It is thus possible to provide a solenoid actuator which cannot be 
prevented from returning to its rest position by a single failure when the 
electromagnetic coil is de-energised. The reliability of the solenoid 
actuator is thus greatly improved, allowing it to be used in critical 
applications and in hostile environments where failure to return to the 
rest position would have undesirable or unacceptable results.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The solenoid actuator comprises a stator composed of a non-ferromagnetic 
front plate 1 and a non-ferromagnetic rear plate 2 fixed to opposite ends 
of a ferromagnetic pole piece 3. An electromagnetic coil 4 is wound on an 
electrically insulating former 5, for instance of plastics material, and 
is fixed inside the pole piece 3. 
An armature comprises a ferromagnetic member 6 attached to a 
non-ferromagnetic rod 7 which passes through the centre thereof. An end 8 
of the rod is chamfered and extends through an opening in the front end 
plate 1 so as to provide an output member of the solenoid. 
A sleeve 9 is mounted on the rod 7 adjacent the end 8 so as to be slidable 
with respect to the rod and with respect to the stator. Thus, the rod 
forms a clearance fit inside the sleeve 9 and the sleeve 9 is a clearance 
fit in the aperture in the front plate 1. A helical spring 10 is held in 
compression between a region of the front plate 1 surrounding the aperture 
and a shoulder 11 formed at an inner end of the sleeve 9. The spring 10 
thus urges the sleeve 9 against the armature which, in turn, is urged 
towards an end stop, such as the rear end plate 2. 
Another cylindrical sleeve 12 surrounds the member 6, which is a clearance 
fit within the sleeve 12. The sleeve 12 is a clearance fit within the 
stator, and is therefore slidable with respect to the stator and with 
respect to the armature. Another coil spring 13 is held in compression 
between a shoulder 14 of the member 6 and a shoulder 15 formed at one end 
of the sleeve 12. Movement of the sleeve 12 to the right in the drawing is 
limited by abutment on a ring 16 which is fixed to the pole piece 3. The 
spring 13 thus urges the armature to the left in the drawing. 
The sleeve 9 and the ring 16 are made of non-ferromagnetic material. The 
sleeve 12 is mainly made of non-ferromagnetic material but has an end 
portion 12a made of ferromagnetic material so as to reduce the effective 
width of the air gap between the pole piece 3 and the member 6. 
In use, in the absence of electric current through the coil 4, the springs 
10 and 13 hold the armature in its rest position against the end plate 2. 
When the coil 4 is energised, it attracts the ferromagnetic member 6 such 
that an end face 17 of the member 6 is urged towards an inner end face 18 
of the pole piece 3 and the end 8 of the rod 7 moves to the right in the 
drawing. This movement is limited by abutment of the end face 17 of the 
member 6 against the inner end face 18 of the pole piece 3. When the coil 
4 is de-energised, the springs 10 and 13 return the armature to its rest 
position. 
If one of the springs 10 and 13 fails, the other is still capable of 
returning the armature to its rest position. If the sleeve 12 fouls 
against the stator, operation of the solenoid actuator is not affected as 
movement of the sleeve 12 is not required for correct operation. If the 
sleeve 12 becomes fixed to the member 6, for instance because of the 
ingress of a particle of foreign material therebetween, the spring 13 
ceases to act but the spring 10 continues to urge the armature towards its 
rest position. 
If the sleeve 9 becomes fixed to the stator, for instance because of the 
ingress of a particle of foreign material between the sleeve 9 and the end 
plate 1, the spring 10 ceases to provide a restoring force for the 
armature. However, the spring 13 continues to urge the armature towards 
its rest position. If the sleeve 9 becomes fixed to the rod 7, for 
instance because of the ingress of a particle of foreign material or 
because of bending of the rod so as to foul the sleeve 9, the spring 10 
continues to provide a restoring force. 
The solenoid actuator is thus immune to the effects of a single failure in 
the restoring force system. Further, the actuator is immune to some double 
failures, such as fouling of the sleeve 9 by the rod 7 and fouling of the 
sleeve 12 on the stator. In this particular example, the actuator is 
immune to three failures, since failure of either spring in these 
circumstances will not prevent the other spring from providing a restoring 
force. 
The reliability of the solenoid actuator is therefore greatly improved 
compared with actuators of known type. Further, the construction and 
manufacture of the actuator are not significantly more complicated than 
for known types of actuators. The solenoid actuator is therefore suitable 
for use in critical applications where failure of the armature to return 
to its rest position when the coil is de-energised must be avoided for 
single failures within the solenoid. Further, the actuator may be used 
with improved reliability in hostile environments where the chances of 
contaminants entering the actuator are significant. 
Various modifications may be made within the scope of the invention. For 
instance, one or more force sensors may be provided to monitor the 
restoring force on the armature provided by the springs 10 and 13. Such a 
sensor arrangement can be used to detect a reduced restoring force in 
order to provide an indication that a fault or failure has occurred so as 
to prevent a "hidden" failure from going undetected.