Description

Field of the invention

This invention relates to the sector of inertial measurement units (IMU) and specifically relates to a method for configuring an insulation system capable of providing adequate protection against mechanical shock and vibration stresses to the electronic and mechanical systems used to make inertial measurement units (IMU).

Background of the invention

At the state of the art it is known that the inertial measurement units (IMU) generally consist of a set of three accelerometers and one set of gyroscopes rigidly mounted on a supporting structure.

This structure, together with the electronics, is assembled on a container which is rigidly fixed to the means through a mechanical interface.

From a functional point of view, the sensor must provide inertial data that, once processed by a computer, is translated into information for the control and navigation of the aforementioned means: acceleration, pitch, roll and yaw, etc.

Any external mechanical stress that induces acceleration (linear or angular) on the inertial unit causes an error in the measurement of these parameters.

To limit the influence of external mechanical stresses on the measurement performance of the sensor, elastic elements are used which, with various shapes, materials and installation methods, create the insulation system.

In the insulation methods normally used for sensors of this type it is tended to create an iso-centric configuration (coincidence between the Elastic Centre (EC) of the insulation system and Centre of Gravity (CoG) of the sensor) and iso-elastic (insulators of the system all the equal and stiffness of the single insulator equal along the three axes) so as to decouple the linear movements on the base (external stress) from those of rotation induced on the insulated system. This ideal configuration is complex to design (the accuracy in determining the CoG is directly proportional to the effectiveness of the system) and difficult to implement due to the extreme precision required both in the manufacture of details (structures and insulators) and in their assembly. Insulation systems employing methods of this type must therefore create a structure capable of providing the same response to external mechanical stresses applied by any whatsoever direction.

Figure 1A shows a perspective view of a known configuration of the sensor block (100) for inertial systems of a set of three accelerometers (101, 102, 103) and one set of three gyroscopes (104, 105, 106) rigidly mounted on a support structure (107).

Figure IB represents a complete IMU in perspective view; the unit of measurement (200) consists of the sensor block (100), of the electronics (108) and the container (109) which is fixed to the vehicle, plane or other structure by means of holes (110). The insulation system can be used to decouple mechanically the sensor block (100) from the base plate (111) of the container (109) or to decouple the entire inertial measurement unit (IMU) (200) from the support structure of the means. Hereinafter, it will be generally referred to the device to be protected against vibrations and shocks, meaning in a regardless way either the sensor block (100) or the entire measurement unit (200).

Figures 2A and 2B respectively represent in a side view and from above the device to be protected against vibrations and shocks (200) as a rectangle with the CoG (206) positioned in the intersection of the symmetry axes; the insulation system consists of four elastic elements (201, 202, 203, 204) interposed between the framework of the device (200) and the support structure (205). The position of the EC (207) depends on the position, the orientation and the stiffness ratio (in the three axes) of the elastic elements (201, 202, 203, 204) and may not coincide with the geometric intersection of the main axes of the elastic elements.

Figure 2C shows a side view of an insulation system wherein the position of the EC is different from that of the CoG and wherein the device (200) is free to rotate around the EC (207), urging both compression and cutting the elastic elements (201, 202, 203, 204).

Under these conditions, two possible effects can be induced on the measuring system (200):

- coning: caused by two simultaneous rotations along two orthogonal axes;

- sculling: caused by simultaneous acceleration and rotation along two orthogonal axes.

These errors induced by the vibrations and dependent on the insulation system, although compensated by the calculation algorithms of the IMU, limit the performance.

The insulation system must therefore combine opposing requirements:

- guarantee maximum damping of external mechanical stresses (large displacements/low stiffness);

- minimise parasitic effects (small displacements/high stiffness).

The techniques known to guarantee the damping of shocks and vibrations and minimising the parasitic effects, even with different manufacturing implementations are all based on the creation of an iso-centric system where the EC coincides with the CoG.

Figures 3A and 3B respectively represent in a side view and from above an implementation of the configuration of the device described in Figures 2A and 2B wherein the action lines of the insulators (301, 302, 303, 304) are contained in a plane passing through the CoG (305) of the device to be protected (300) and oriented so as to converge towards it. Figure 3C represents an implementation of the configuration described in Figures 3A and 3B wherein the insulation system consists of a mechanical structure (307) for fixing to the means and a resilient structure (308) connected to the support structure (307) and to the device to be protected (306). The intermediate plane of the insulation system (307, 308) passes through the CoG (309) of the device to be protected (306).

Figure 3D represents an iso-centric configuration usable when it is not possible to position the insulators in a plane passing through the CoG; the solution consists in inclining the elastic elements (311, 312, 313, 314) in such a way that their action lines converge towards the CoG (315) of the device to be protected against vibrations and shocks (310).

Figure 3E is a representation of the mechanical and geometrical characteristics of an elastic element and of the iso-centric system of which it is part, wherein:

- Kc and Kr are the compression and radial stiffness, respectively;

- Q is the inclination that the elastic elements must have to make EC and CoG coincide.

In the implementation of the iso-centric and iso-elastic insulation system of Figure 3E, the effectiveness is as high as more accurate is the design step; the relative position of CoG and EC and the dynamic response of the insulation system (same natural frequencies for stresses from every direction) depend, in fact, on the design choices:

materials and geometries of the structures;

size of the parts;

- materials and geometries of the insulators;

positioning of the insulators;

orientation of the insulators.

To the approximations resulting from the project activities must be added those much wider that, due to the technological and assembly limitations, emerge during the manufacture of the devices: accuracy in the mechanical machining and assembly tolerances, as well as the invariance of the mechanical characteristics of the insulators (insulator screening) are factors that can significantly change the expected effectiveness of the insulation systems.

At the state of the art, in the sector of the isolation of inertial devices from vibrations the following documents are known:

- US 2013/241376 A1 that discloses an inertial platform comprising a housing and a suspended inertial sensor assembly (ECIS) furnished with rigidly inter- linked motion sensors, comprising:

- a first internal suspension assembly (ESI) inside the said sensor assembly and

- a second external suspension assembly (ES2) outside the said sensor assembly.

- US 2003/167863 A1 that discloses an apparatus comprising:

- an assembly having a center point;

- a plurality of isolators, each substantially axially symmetric about one of a plurality of axes and each operably coupled to the assembly such that each of the plurality of axes substantially converges at the center point.

- US 2013/111993 A1 that discloses a micro inertial measurement system including:

- a housing,

- a sensing module,

- a damper.

The sensing module includes a rigid sensing support, a measuring and controlling circuit board mounted on the rigid sensing support and an inertial sensor set on the measuring and controlling circuit board.

The inertial sensor includes a gyroscope and an accelerometer. The sensing module is mounted in the housing. The damper is mounted in the housing and set in the gap between the sensing module and the inside wall of the housing.

The isolation configurations described in US 2013/241376 Al, US 2003/167863 Al and US 2013/111993 Al, with different methods, materials and shapes, try to bring the elastic center to coincide with the gravity center by imposing design constraints both on geometry and materials.

None of said configurations offers a solution to improve the performance of inertial sensors initially designed for environments with low vibration levels (rigid assembly).

Disclosure of the invention

Object of this invention is to provide a method of insulation from mechanical shock and vibration stresses capable of providing adequate protection to the electronic and mechanical systems used to create inertial measurement units (IMU).

Another object of this invention is that of pursuing, by implementing the proposed method, design simplicity, compactness of the solution and ease of implementation (manufacture and assembly).

Another object of this invention is that of providing a method which can easily be used also on sensors not designed to be insulated from vibrations, by exploiting the surfaces of the mounting interfaces fixing the sensor on the housing.

Another object of this invention is that of providing a method to avoid applying shear and tensile stresses on insulators which would require a rigid fixation on the inertial sensor on one side and on the housing on the other side.

A further object of this invention is to provide a method capable of creating a configuration of the insulation system which allows optimising the machining conditions of the individual elastic elements. Last but not least, object of this invention is that of providing an insulation method which can be applied to units already manufactured.

Further characteristics and advantages of the invention will become apparent from the description of a preferred but not exclusive embodiment of a method of insulation from the mechanical shock and vibration stresses object of this patent application, illustrated by way of non-limiting example in the drawing units described below:

- Figure 3F is a schematic representation of the distribution of forces on the elastic elements arranged according to the proposed method;

- Figure 4 A is a schematic representation of an insulator type;

- Figure 4B is a perspective representation of the assembly mode of the insulation system;

- Figure 4C is a schematic representation of the assembly and adjustment of the insulation system;

- Figure 4D is a schematic representation of an assembly alternative of the insulation system;

- Figure 5A is a representation of the position of the elements constituting the single point of the insulation system;

- Figure 5B is a perspective representation of the fixing point of the insulation system;

- Figure 5C is a detail of the fixing point.

Detailed description of the invention

According to a preferred - but not limiting - embodiment, this invention relates to a method for configuring a shock and vibration insulation system of an inertial measurement unit for mounting said insulation system on an external structure, said system making elastic elements work along a single axis regardless of the direction of the external mechanical stress.

The application in the insulation systems of the method proposed in this invention has the objective of creating a configuration of the insulation system which optimises the work of the individual elastic elements, with the intent of simplifying the manufacture of the parts and their assembly.

The iso-centric configuration is used to reduce the rotations of the device to be protected around the elastic centre and thus have linear movements regardless the direction of the applied external mechanical stress.

In this way, once the geometry of the insulation system is fixed, the elastic elements must be designed in such a way as to have a relationship between axial stiffness and constant radial stiffness, such as to always counter the relative components of the reaction loads on the insulators thereof (Figure 3E).

With the implementation of the proposed method, the fact that the Elastic Centre (EC) and the Centre of Gravity (CoG) are not coincident is exploited, so that regardless the direction of application of the external load, the reaction pair that is generated tends to rotate the device. This rotation by small movements stresses the elastic elements with forces parallel to the axis thereof, thus decoupling the external load direction from that of the stress on the elastic elements.

The implementation of the proposed method in this patent application allows creating a configuration such as to stress all the elastic elements of the insulation system along a single direction (that of the main axis of the elastic element thereof) regardless the direction of the external mechanical stress application, pursuing at the same time design simplicity, ease of implementation and compactness of the system.

Therefore, with this insulation method it is desired to aim at optimising the configuration of the single elastic element rather than pursuing the optimisation of the system configuration (tending to an iso-centric and iso- elastic system).

The elastic elements described in this invention are only an example and do not cover all possible manufacture and assembly alternatives. Figure 4 A represents an elastic element (400) wherein:

- a resilient material (401) is co-moulded on two metal disks (402, 403);

- the main machining axis is parallel to the geometric axis and has an axial/radial stiffness ratio which is different from 1.

Figure 4B represents an implementation of the proposed method wherein:

- elastic elements (405, 406, 407, 408, 409, 410, 411, 412) are positioned in pairs on fixing flanges (413, 414, 415, 416) of a device to be protected (404);

- special screws (417, 418, 419, 420) are used to hold in position the pairs of elastic elements (405, 406), (407, 408), (409, 410), (411, 412) and at the same time fix the device (404) on the medium of the means.

Figure 4C shows how distance A between the EC (422) and the CoG (423) of the device (424) can be reduced up to distance B. When the mechanical constraints allow it, the distance between the couple insulators (405, 406), (407, 408), (409, 410) and (411, 412) of Figure 4B can be increased by increasing the size of the flange (425) up to the size (426).

In Figure 4C it can be seen that, by increasing the sizes of the flanges from (427) (size X) to (428) (size Y>X), the residual distance between the EC (422) and the CoG (423) is reduced from the value (429) (A) to the value (430) (B<A). In the same way it is possible to move the position of the EC on a horizontal plane by varying the relative position of the insulators in order to bring the EC (422) and the CoG (423) closer together.

Figure 4D shows an alternative implementation of the proposed method wherein: the flange (431) of the device (432) rests by means of the elastic element (433) on the ashlar (434) of the support (435). The plane (436) containing the EC (437) of the device (432) will position itself at a residual distance (C) (438) from the CoG (439) which, with the same Y size (440), will be lower than B ( Figures 4C). In Figure 5A the configuration of the insulation system referred to one of the flanges of the device to be protected is described in greater detail. The device to be protected (501) is insulated from the support structure (502) using a pair of elastic elements (503, 504) and a special screw (506). The element (503) is clamped between the surface (507) of the flange (505) and the surface (508) of the screw (506), while the element (504) is clamped between the surface (509) of the support structure (502) and the surface (510) of the flange (505). The screw (506), once tightened and secured onto the surface (509) of the support (502), it automatically applies the established pre-load level to the elastic elements (503, 504).

The elastic elements, during the assembly step, are held in position by means of a suitable seat made on the contact surfaces during their assembly and an appropriate design of the special screw.

Figure 5B shows a possible embodiment of the housing (509) on the support (502) where to position the elastic element (504) during the assembly step; a similar machining must also be performed on both faces of the fixing flange (505) of the device (501).

Figure 5C shows in detail the fixing method of the insulation system. The load applied to the elastic element (504) during the tightening of the screw (506) causes expansion of the resilient part (511) thereof. During this step the resilient element (511) touches the ring (510) of the screw (506) and, before it reaches the abutment (509) of the support (502), the elastic element (504) tends to become self- centred.

The method described in the Figures above can be summarised in the following steps:

- arrange the elastic elements (even non iso-elastic) in pairs on each fixing point; - define the distance among the elastic elements of the pair such as to reduce the distance between the Elastic Centre (EC) and the Centre of Gravity (CoG);

- direct the main axis of the orthogonal elastic element to the support plane of the device to be protected;

- position the pair of elastic elements on parallel surfaces;

- check the position of the pair of elastic elements by means of a seat made on the installation interfaces between the device to be protected and the external support;

- fix the pair of elastic elements by arranging the action axis of the screw coinciding with the main elastic element axis;

- compress a pair of elastic elements by applying a preload defined by controlled crushing;

- use the deformation of the elastic element during the preload step to self-centre the device along the main axis.

The materials and sizes of the invention as described above, illustrated in the accompanying drawings and hereinafter claimed, may be any according to the requirements. Furthermore, all the details can be replaced with any other technically equivalent, without thereby abandoning the protective scope of this patent application.