Automatic locking device for linkages subjected to undesirable mechanical stresses, applicable in particular to electrical switches

An automatic locking device for linkages subjected to undesirable mechanical stresses comprises a transducer with transmits an electrical signal as a function of the mechanical stress received, and feeds it to a coil which generates an electromagnetic field causing locking of the linkage.

BACKGROUND OF THE INVENTION 
This invention relates to an automatic locking device for linkages 
subjected to undesirable mechanical stresses, whichis particularly 
applicable for example to the opening release devices used in electrical 
switches. 
It is known that a linkage subjected to mechanical stresses such as impact, 
vibration, acceleration etc. of sufficient intensity and suitable 
direction is able to undergo movement. 
In many cases this movement is undesirable, and leads to the functioning of 
the system in which the linkage is inserted, the result being untimely and 
sometimes unacceptable operation. 
Electrical switches installed in structures susceptible to high stress are 
usually provided with locking devices for the release linkage, in order to 
prevent involuntary opening of the contacts due to undesirable mechanical 
stress. 
These locking devices are of mechanical type. One type comprises for 
example a ball elastically held between two jaws, to produce by the effect 
of impact a mechanical action which is able to lock the linkage for 
opening the switch contacts. In practice, on impact, the ball moves from 
its rest position and causes the two jaws to diverge, thus preventing 
movement of the release system and locking it. 
However in the particular case of an overcurrent release device, when in 
this locked position even if a release control signal due to an overload 
acts simultaneously with the impact stress, it is possible for the release 
device to be unable to operate and cause opening of the switch contacts. 
This represents a drawback. 
Furthermore, a negative characteristic typical of these known mechanical 
locking devices is their response time to mechanical stresses. This time 
is relatively high and is due to the need for the sensing mass (the ball) 
to undergo a finite displacement sufficient to move the mechanical locking 
elements (the two jaws). 
This can be particularly disadvantageous in those applications which use 
electromagnetic releases with a switch opening device in the form of a 
solenoid which is retained in position by a permanent magnet field, and is 
operable by an electrical demagnetisation pulse. 
Such opening solenoids are known to comprise an armature which operates the 
switch release lever. This armature is held in its "set" position by a 
permanent magnet and simultaneously loads an operating spring. A very 
small displacement of the armature from its "set" position is sufficient 
to enable the spring to prevail over the permanent magnet force, so as to 
cause the armature to move rapidly until it operates the switch release 
lever. In the absence of impact, such a displacement can be obtained 
merely by an electromagnetic pulse force acting against the permanent 
magnet force and generated by a coil energised by the output signal from 
overcurrent sensors. 
It therefore follows that even impacts of a not particularly high intensity 
are able to cause the release device to involuntarily operate. 
The intervention action of present-day mechanical locking devices is 
however not sufficiently rapid to anticipate the release device, because 
of the rapidity of the elastic snap-action of the armature in contrast to 
the high inertia of the mechanical locking device. 
The ideal solution would be a locking device which is so rapid as to not 
enable the armature to make even a minimum movement, because if this were 
not the case once the effect of the impact and thus the intervention of 
the locking device has ceased, the armature, under the action of the 
spring, would continue its movement until it operated the release lever, 
even if a signal from the overcurrent sensors were not present. 
SUMMARY OF THE INVENTION 
The object of the present inventin is to obviate the aforesaid drawbacks, 
and generally to provide an automatic locking device having a high 
response speed and a behaviour which is reliable and constant with time. 
This object is obtained according to the invention by an automatic locking 
device for linkages subjected to undesirable mechanical stresses, 
characterized by a transducer 10 rigid with the mechanical structures on 
which the linkage is disposed, to emit an electrical signal which is a 
function of the mechanical stress received, and a coil 13 fed by said 
transducer 10 to generate an electromagnetic field which causes locking of 
the linkage. 
The structure of such a device, which is based on the use of a terminal 
coil able to generate an electromagnetic force which is a function of the 
undesirable mechanical stress, makes is advantageously applicable to all 
release systems of electromagnetic type used in electrical switches.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The device of FIG. 1, indicated overall by 9, comprises a transducer 10 
rigid with the mechanical structures on which the linkage is disposed. It 
is able to supply an electrical output signal which is a function of the 
mechanical stress received. A transducer of piezoelectric type can for 
example be used. The output signal from the transducer 10 is fed to an 
amplifier 11, and from here to an electrical signal former 12 which 
suitably feeds a coil 13. When fed, the coil 13 generates an 
electromagnetic force which can be used in various ways for locking any 
linkage (not shown in FIG. 1). 
The device 9 also comprises a regulator 14 for setting the mechanical 
stress level at which the device is to activate, and a regulator 15 for 
setting the duration of the locking effect. 
The electronic component of the device 9, constituted by the amplifier 11, 
the former 12 and the two regulators 14 and 15, is indicated overall by 
16. 
FIG. 2 is a diagrammatic illustration of an important application of the 
device 9 in the electrical switch field, and relates to an overcurrent 
release device 17 of the demagnetisation type, already described in the 
introduction. 
As already stated, the release device 17 comprises an operating armature 18 
retained in its "set" position by a permanent magnet 19 against a thrust 
spring 20. If overcurrent occurs in the circuit in which the electrical 
switch is connected, a coil 13A is fed by way of suitable overcurrent 
sensors, to generate an electromagnetic force which unbalances the two 
forces acting on the armature 18 by an extent sufficient to induce a 
slight displacement of the armature from its "set" position, and thus 
cause the elastic force of the thrust spring 20 to prevail. The armature 
18 moves to operate a lever 21 for releasing the main switch contacts 21A 
(this operating feed for the armature 18 is shown diagrammatically by an 
arrow 70). 
In the illustrated embodiment, the coil which releases the armature 18 is 
the same coil, indicated by 13A, as that used as the locking element in 
the device according to the invention, as will be apparent hereinafter. 
FIG. 6 is a detailed illustration of the circuit diagram (electronic 
component 16) of the embodiment of FIG. 2. 
The electrical output signal from the transducer 10 is fed by way of a 
diode rectifier bridge 22 to a transistor 23 fed by a source of direct 
current S.sub.1. The rectifier bridge 22 applies a positive polarity to 
the base relative to the emitter of the transistor 23, independently of 
the sign of the signal from the transducer 10. A potentiometer 24 
adjustably limits the intensity of the signal applied to the base of the 
transistor 23. 
When this signal exceeds a determined value, the transistor 23 becomes 
conducting and triggers a controlled diode 25 (electronic switch) by way 
of the gate G. The current flowing between the anode A and cathode K of 
the controlled diode 25 causes two further transistors 26 and 27 to 
conduct. 
In this circuit situation in which the controlled diode 25 and the two 
transistors 26 and 27 conduct, a capacitor 28 discharges. In this respect, 
under normal conditions, ie when the circuit is not activated by a signal 
from the transducer 10, the capacitor 28 is charged and kept charged by 
the source S.sub.1. When the circuit is activated, the feed voltage to the 
capacitor 28 is reduced almost to zero by way of a branch 29 and the 
conducting transistor 26. The capacitor 28, which is no longer fed with 
electricity, thus discharges. 
The discharge current of the capacitor 28 keeps the controlled diode 25 and 
the two transistors 26 and 27 conducting, for the time during which its 
value is sufficient to keep these circuit elements activated. The duration 
of the discharge can be regulated by means of a potentiometer 30. 
For the entire conducting period of the transistor 27, the coil 13A is fed 
by the source S.sub.1 by way of a collector-emitter junction of said 
transistor 27. The direction of flow of the current through the coil 13A 
is such as to create an electron magnetic force which adds to the force of 
the permanent magnet 19. 
The coil 13A is also connected by way of a controlled diode 31 (electronic 
switch) to a second direct current source S.sub.2 with an output voltage 
greater than the source S.sub.1. The controlled diode 31 is triggered by a 
release signal indicated diagrammatically by an arrow 32 and fed by the 
overcurrent sensors, not shown, to its gate G. When the diode 31 is 
triggered, the feed voltage of the source S.sub.2 determines in the coil 
13A a current circulating in the opposite direction to that caused by the 
source S.sub.1, and thus generates an electromagnetic force which 
subtracts from the force of the permanent magnet 19. 
In the absence of impact or overcurrent in the switch comprising the device 
of FIG. 2, the armature 18 is retained in its "set" position by the force 
of the permanent magnet 19 which acts against and prevails over the force 
of the spring 20. 
In the case of overcurrent in the switch, the overcurrent sensors feed the 
release signal 32, which triggers the controlled diode 31. The 
electromagnetic force created by the coil 13A supplied by the source 
S.sub.2 opposes the magnetic field of the permanent magnet 19 to enable 
the force of the spring 20 to prevail over the force of the permanent 
magnet, to release the armature 18, which moves under the action of the 
spring 20 until it operates the lever 21 and opens the main switch 
contacts. 
In the case of impact against the switch and thus against the transducer 10 
rigid therewith, this latter emits an electrical output signal and thus, 
as is apparent from the aforegoing explanation, the armature is retained 
in its "set" position by a supplementary magnetic force which is added to 
that due to the permanent magnet 19, so as to prevent even small movements 
of the armature 18 taking place, or to return it to its "set" position in 
the case of a particularly large stress. 
If an impact and overcurrent are simultaneously present in the switch, the 
coil 13A is traversed in one direction by the current determined by the 
voltage of the source S.sub.1 (impact) and in the other direction by the 
current determined by the voltage of the source S.sub.2 (overload), this 
latter voltage being greater than the preceding. There is thus a resultant 
electromagnetic force in the reverse direction to the force determined by 
the permanent magnet 19, so that the armature 18 is released and the main 
switch contacts open. 
The priority requirement of opening the main switch contacts n the case of 
overcurrent, independently of whether the swtich is or is not subjected to 
stress, explains the need for an output voltage from the source S.sub.2 
which is always greater than the output voltage from the source S.sub.1. 
The purpose of the potentiometer 24 of the circuit of FIG. 6 is to adjust 
the mechanical stress level at which the device is to activate (regulator 
14 of FIG. 1). The purpose of the potentiometer 30 is to adjust the 
duration of the locking effect on the armature 18 (regulator 15 of FIG. 
1). 
Single transistors are used in the proposed circuit diagram. It is however 
obviously possible to use more complex transistor functions such as 
Darlington transistors or equialent integrated circuits. 
FIG. 3 is a diagrammatic illustration of an application of the device 9 
relative to another type of electromagnetic overcurrent release system 
which, as in the preceding system of FIG. 2, comprises an armature 33 
which an overcurrent operates a lever 34 for releasing the main switch 
contacts. However in contrast to the preceding release device of FIG. 2, 
the armature 33 is kept in its rest position by a return spring 35 and is 
driven against the lever 34 by the electromagnetic force created by a coil 
36 which is fed under the control of the overcurrent sensors (this 
operating feed for the armature 33 is shown diagrammatically by an arrow 
71). 
In this application, the coil of the device according to the invention, 
indicated by 13B, creates on impact an electromagnetic force which adds to 
the elastic force of the spring 35 in retaining the armature 33 in its 
rest position. 
The electromagnetic force created by the coil 36 (ie only in the presence 
of an overload) must always have a value greater than the forces created 
by the coil 13B (impact) and by the spring 35, and must be sufficient to 
drive the armature 33 in opposition to these two forces whenever 
mechanical stresses and overcurrent are simultaneously present in the 
switch, as already seen for the release device of FIG. 2. 
The simple circuit of FIG. 6 can be used for the electronic component 16, 
but obviously the supply line relative to the source S.sub.1 which has to 
serve the coil 13B, has to be separated from the supply line relative to 
the source S.sub.2 which has to serve the coil 36, in contrast to the 
preceding application where a single coil (coil 13A) had to be fed. 
FIG. 4 diagrammatically illustrates a further application of the device 9, 
for locking a linearly mobile slider 37 which has to maintain its position 
constant even under impact. For this application, the device 9 also 
comprises a pin-shaped armature 38 held in its rest position by a return 
spring 39. 
In the case of mechanical stress, the coil of the device 9, indicated by 
13C, generates an electromagnetic force which causes the pin 38 to snap 
into a cavity 40 in the slider 37, so as to lock it. 
It is also possible for this application to use a circuit analogous to the 
circuit of FIG. 6, but in which only one direct current source operates to 
energise the coil 13C at the appropriate moment, such as S.sub.1. 
Finally, FIG. 5 diagrammatically illustrates an application of the device 9 
for locking a rotating element 41 of an operating member, eg for 
electrical switches, which as in the preceding case has to maintain its 
operating position constant under all operating conditions. 
For this purpose, an electromagnetic brake is constructed in which the coil 
of the device according to the invention, indicated by 13D, generates a 
magnetic field which keeps the rotating element 41 at rest during the 
mechanical stress. 
With regard to the electronic component 16, that stated generally for the 
application of FIG. 4 is applicable. 
FIG. 7 shows a preferred embodiment of the transducer 10, indicated by 10A. 
The transducer 10A comprises a hollow support 50 for mounting rigid with 
the mechanical structures on which the linkage to be locked is disposed. 
The support 50 houses a ball 51 retained on one side by a losure ring nut 
52 screwed into one end of the support 50. On the other side of a ball 51 
there acts a main piston 53 slidable in the support 50. The ball 51 is 
enclosed between two frusto-conical surfaces 65 and 66 respectively of the 
ring nut 52 and main piston 53, and inclined to the axial sliding 
direction of this latter. On the main piston 53 there acts a setting 
spring 54 which interacts with a secondary piston 55 slidable in the 
support 50 coaxially to the main piston 53. The secondary piston 55 
comprises an axial stem-shaped piston 56 guided in a corresponding seat 57 
of the main piston 53 and surrounded by the spring 54. An axial head 
portion 58 of the secondary piston 55, covered by an insulating cap 59, 
exerts a pressure on a piezoelectric element 60. The piezoelectric element 
60 is mounted on an insulating support 61 fixed rigidly to the support 50 
in such a manner as to close this latter at its other end. The 
piezoelectric element 60 is held between a first contact strip 62 fixed to 
the support 61 and a conducting rivet 63 which externally fixes a second 
contact strip 64. 
During assembly, the described transducer is set by loading the spring 54 
to a predetermined value using the ring nut 52, which by being screwed to 
a greater or lesser depth into the support 50 determines a greater or 
lesser compression of the spring 54 by way of the ball 51 and the main 
piston 53. The loading of the spring 54 results in a pressure on the 
piezoelectric element 60 by way of the secondary piston 55. 
In the case of mechanical stress in the structure to which the transducer 
10A is rigidly fixed, the free masses, constituted by the ball 51 and the 
main and secondary pistons 53 and 55, move from their rest position of 
FIG. 7 to cause an increase or decrease in the pressure pre-existing on 
the piezoelectric element 60. This pressure variation generates in known 
manner a potential difference between the two surfaces of the 
piezoelectric element 60 in electrical contact with the two external 
strips 62 and 64. 
The potential difference is then sensed and suitably used by the electronic 
component 16 of the device 9 in order to feed the coil 13, as heretofore 
described. 
The transducer 10A is sensitive to mechanical stresses in any direction. 
This is due to the presence of the ball and to the particular inclination 
of the two surfaces 65 and 66 which enclose it. For any movement of the 
ball 51, the interaction between it and the surface 65 and 66 thus 
develops a longitudinal thrust component on the main piston 53, which is 
transmitted by the spring 54 to the secondary piston 55. 
The response speed of the transducer 10A is very high, as a minimum 
movement of the free masses is sufficient to create a potential difference 
across the piezoelectric element 60. 
For analogous reasons, mechanical-electrical transducers are generally of 
very rapid response, thus being clearly advantageous to the device 
according to the invention by virtue of this characteristic.