Field of invention

The present invention relates to a force sensor which is designed to measure forces that occur along a single axis only.

The present invention also relates to a method for constraining motion which is designed to prevent movement along a single axis only.

The present invention also relates to a device that can simultaneously act as both a force sensor and a method for constraining motion. The device is designed to measure force and constrain motion along a single axis only.

Background to the invention

Strain gauge load cells are the most commonly used force transducer. These sensors convert force to an electrical signal by measuring strain or displacement of a relatively compliant element. In terms of type and shape, the basic sensing element categories are beam, proving ring, column and diaphragm.

An electromagnetic force transducer uses an electromagnetic induction-based displacement sensor such as a Linear Variable Differential Transformer (LVDT) and a force to displacement converter such as a linear spring. Using Hooks' law (F - Kx, where F is applied force, K is the spring constant and x is displacement) within the linear range of the spring, the applied force can be quantified by measirring the displacement. As the spring and LVDT are both linear, the output voltage of the

LVDT is directly proportional to the applied force F.

A piezoelectric force transducer works on the principle that electric charges that are proportional to the force are formed on the surface of a crystal used as part of the sensor. By using a charge amplifier, these charges can be amplified to a measureable signal which is proportional to the applied force.

A hydraulic force transducer operates on the principle that it is easier to measure the pressure when the applied force is distributed over a known surface area or applied to a fluid. Pressure can then be converted to force by integrating the pressure over the surface area. Sensors using this method to measure pressure are known as hydraulic force transducers. The applied force is transmitted as fluid pressure (usually held in the cavity of a fluid filled bellow) to the pressure sensor. The output of the pressure sensor is directly proportional to the applied force. This type of sensor is resistant to the ambient electric or magnetic fields that electromagnetic and strain gauge force transducers are prone to.

One known problem with existing force transducers is that the technology used senses variations in strain, displacement, pressure, and acceleration to quantify force. Force transducers generally function in pure axial tension and compression. In some cases, off axis loads are involved and can result in inaccurate measurements or could even damage the transducer.

Eccentric or off-axis loading on force transducers has long been recognized as a technical hurdle in obtaining accurate force calibrations and subsequent end use measurements. Current force transducers frequently suffer from inaccurate readings or damage to the transducers when subj ect to off-axis forces .

An ideal uniaxial force transducer records only the component of force acting along its measurement axis, being insensitive to all components of force orthogonal to this axis and all torques. Furthermore, it has infinite linear stiffness along its

measurement axis, zero linear stiffness orthogonal to this axis and zero rotational stiffness about all axes.

The most widely used force transducers are based upon strain gauges bonded to a stiff elastic substrate, typically metal. Although there are numerous designs, most exhibit non-ideal behaviour: load cells have relatively large linear and rotational stiffness about all axes; beam, diaphragm, and proving ring force transducers are relatively low linear stiffness along their measurement axis, have relatively large linear stiffness about other axes, and have relatively large rotational stiffness about some axes. Some of these drawbacks can be addressed by, for instance, using ball joints to reduce the rotational stiffness and off-axis linear stiffness. However, small ball joints concentrate forces over a small area, resulting in stress concentrations with possible yielding or fracturing of the j oint materials . Similar problems are found with actuator technology where operation solely along a single axis is desired, with any off-axis component minimised or effectively neutralised.

It is therefore an object of the present invention to provide a force sensor which goes some way towards addressing the problems outlined above, or which at least provides the public with a useful choice,

It is a further object of the present invention to provide a method for constraining motion which goes some way towards addressing the problems outlined above, or which at least provides the public with a useful choice.

It is a yet still further object of the present invention to provide a combined force sensor and method for constraining motion which goes some way towards addressing the problems outlined above, or which at least provides the public with a useful choice.

In this specification where reference has been made to patent specifications, other external documents, or other sources of information, this is generally for the purpose of providing a context for discussing the features of the invention. Unless specifically stated otherwise, reference to such external documents is not to be construed as an admission that such documents, or such sources of information, in any jurisdiction, are prior art, or form part of the common general knowledge in the art.

Further aspects and advantages of the present invention will become apparent from the ensuing description which is given by way of example only.

Summary of the invention

In one aspect the invention may be said to broadly consist in a force sensor comprising:

· a casing comprising a compliant material substantially defining and enclosing a core volume filled with relatively incompressible fluid or compliant solid; • a reinforcing structure associated with at least a portion of the compliant casing material such that a force indicative output is provided substantially only in response to a force exerted on the device in a single axis and substantially not to forces off the single axis.

In another aspect, the invention may broadly be said to consist in a device that acts as a combined force sensor and method for constraining motion comprising:

a casing of compliant material substantially defining and enclosing a core volume filled with relatively incompressible fluid or compliant solid;

a reinforcing structure associated with at least a portion of the compliant casing material such that a force indicative output is provided, or a force is exerted, or both, substantially only in response to a force exerted on and/or by the device in a single axis or direction and substantially not to forces off the single axis or direction. The combined force sensor and motion constraint is compliant to forces occumng or exerted off the single axis or direction so that substantially no force is registered or exerted off the single axis or direction.

In another aspect, the invention may broadly be said to consist in a device that acts as a force sensor, comprising:

• a casing of compliant material substantially defining and enclosing a core

volume filled with relatively incompressible fluid or compliant solid;

• a reinforcing structure associated with at least a portion of the compliant casing material such that a force indicative output is provided substantially only in response to a force exerted on the device in a single axis or direction and substantially not to forces off the single axis or direction, the force sensor is compliant to forces off the single axis or direction.

In another aspect, the invention may broadly be said to consist in a device that acts as method for constraining motion, comprising: a casing of compliant material substantially defining and enclosing a core volume filled with relatively incompressible fluid or compliant solid;

a reinforcing structure associated with at least a portion of the compliant casing material such that the device operates to exert force or produce motion substantially in a single axis or direction and is substantially compliant to forces off the single axis or direction so that substantially no force is exerted or motion produced off the single axis or direction.

In another aspect, the invention may broadly be said to consist in a device that acts as a combined force sensor and method for constraining motion comprising;

· substantially identical upper and lower sections formed from a compliant

material each having an open inner end and a closed outer end, the upper and lower sections connected, closed and sealed to one another around the perimeter of their open inner ends so that they are mirrored around a central axis of symmetry and enclose an internal fluid tillable volume, each section having a side wall that extends away from the central axis of symmetry when the device is relatively incompressible fluid or compliant solid, and an end portion that closes the outer end, the side walls having a reinforcing structure substantially spaced along their length or height.

In another aspect, the invention may broadly be said to consist in a device that acts as a combined force sensor and method for constraining motion comprising:

a central tube, formed from a transversely isotropic elastic material;

two stiff end caps at opposite ends of the central tube, each of the end caps formed so that internal fluid pressure does not significantly change the shape of the caps;

· a substantially incompressible fluid or gel filling the central tube;

the tube, stiff in the circumferential direction, and compliant in the radial and axial directions, such that a force indicative output is provided, or a force is exerted, or both, substantially only in response to a force exerted on and/or by the device in a single axis or direction and substantially not to forces off the single axis or direction, the combined force sensor and method for constraining motion is compliant to forces occurring or exerted off the single axis or direction so that substantially no force is registered or exerted off the single axis or direction.

Preferably the ends of the central tube are closed by the end caps to form a closed internal volume.

Preferably the tube is constructed from fibre reinforced elastomer.

Preferably the tube is formed from an elastomer that has a low stiffness, high strength, high fracture toughness, which can be formed into a film, and which can be bonded to the reinforcing fibres.

Preferably the elastomer is one of EVA, VHB, polyurethane, neoprene, silicone or butyl rubber.

Most preferably the elastomer is butyl rubber.

Preferably the fibre is stiff, strong, and able to be bonded to the elastomer.

Preferably the fibre reinforcement is carbon fibre.

Alternatively the fibre reinforcement is para-aramid synthetic fiber (e.g. Kevlar). Alternatively the fibre reinforcement is glass.

Alternatively the fibre reinforcement is a mix of one or more of carbon fibre, kevlar and glass.

Alternatively the fibre reinforcement is a metal drawn into wire

Most preferably the metal wire is an alloy of one of tungsten, titanium, nickel, or steel.

Preferably the end caps are formed from a relatively stiff plastic, metal, or ceramic.

Preferably the caps are tightly bonded to the tube.

Alternatively, the ends caps can be formed to be relatively compliant and bonded to rigid plates oriented normal to the longitudinal axis. Preferably the relatively compliant end caps can be formed from stiff thermoplastic, such as acetyl, bonded to an aluminium plate.

Preferably the incompressible fluid or gel only supports hydrostatic stress and is relatively compliant to deviatoric stresses,

Most preferably the fluid or gel is one of water, oil, brake fluid, hydraulic fluid, elastomer, or grease.

Preferably the reinforcing structure restrains the sidewalls so that the sensor or actuator will only distend along the one axis, and is substantially compliant in both radial and axial directions.

Preferably the reinforcing structure is a continuous spiral, each of the turns of the spiral substantially equally spaced from the previous and subsequent turns, and each turn forming one of a series of ribs.

Alternatively the reinforcing structure is a set of concentric rings each of the turns substantially equally spaced from the previous and subsequent turns, and each turn forming one of a series of ribs .

Most preferably the reinforcing structure is connected to and around each of the sidewalls, and evenly spaced along the sidewalls.

Preferably the compliant material is an elastomer.

Preferably the compliant material has an expansion ratio of at least about 2:1. Preferably the inner ends are connected to one another by sandwiching a relatively inextensible rigid or flexible spacer between the edges of the upper and lower sections.

Preferably the edges and the relatively inextensible spacer are sandwiched between two outer layers of stiff polymer film.

Preferably the device also comprises a pair of end plates connected one each to an upper and lower part of the device. Most preferably the pan of end plates are connected to the outer surface of the end portions formed by the compliant material.

Alternatively one or both of the upper and lower end portions of the compliant material are open, and the end plates are connected to the end portions of the compliant material in such a manner as to close the opening or openings.

Most preferably the pair of end plates are aligned perpendicular to the axis of force measurement or transmission of the device.

Preferably the end plates are made from relatively stiff plastic, metal, or ceramic.

Preferably the device is filled with a relatively incompressible fluid or gel.

Preferably the structure further has an inlet.

Most preferably the inlet is located between the two outermost layers of stiff polymer film.

Preferably the device according to this aspect is substantially flexible and flat in its unfilled state.

Preferably the combined force sensor/actuator further has a pressure sensor.

Preferably the pressure sensor is placed in or next to the tube.

Alternatively the pressure sensor is located remotely from the combined force sensor/actuator, and is connected by a hydraulic hose.

In another aspect, the invention may broadly be said to consist in a Stewart platform formed using six of the combined force sensor/actuators of any one or more of the preceding statements to form the actuator legs of the platform.

This invention may also be said broadly to consist in the parts, elements and features referred to or indicated in the specification of the application, individually or collectively, and any or all combinations of any two or more said parts, elements or features, and where specific integers are mentioned herein which have known equivalents in the art to which this invention relates, such known equivalents are deemed to be incorporated herein as if individually set forth. The invention consists in the foregoing and also envisages constructions of which the following gives examples only.

The foregoing describes the invention including preferred forms thereof.

Modifications and alterations as would be readily apparent to a person skilled in this particular art are intended to be included within the spirit and scope of the invention described.

Brief description of the drawings

Preferred embodiments of the invention will be described by way of example only and with reference to the drawings, in which:

Figure 1 is shows an exploded isometric view of a preferred form of a device that acts as a combined force sensor and method for constraining motion.

Figure 2 is a diagrammatic isometric assembled view of the device of Figure 1.

Figure 3 is a further diagrammatic isometric view of the assembled apparatus of figure 2 illustrating the reinforcing structure in the central section of the device.

Figure 4 is a diagrammatic partial cross-sectional detail of an example of one construction through a wall of a fibre reinforced elastomer material according to the invention.

Figure 5 is a graph of pressure sensor voltage as a function of applied forces (N) for the device of the preceding figures

Figure 6 shows eighteen examples of linear and/or rotational constraint that can be achieved using the combined force sensor and method for constraining motion of the various embodiments described and shown with reference to the preceding figures, in different configurations.

Figure 7 shows an isometric view of one of the two end plates used to construct a Stewart platform force/torque transducer.

Figure 8 shows a view of a Stewart platform force/torque transducer that uses two of the end plates illustrated in Figure 7 plus six of the combined force sensor and method for constraining motion devices described with reference to the preceding Figures.

Detailed description of preferred embodiments

The device of the present invention can be used in two preferred main ways (although other nses are not excluded): firstly as a single-axis force sensor; and secondly as a method for constraining motion. The structure for both uses is substantially the same, The two can also co-exist in a single device (i.e. a single device can be used both as a force sensor and method for constraining motion simultaneously). The intention in both cases is that forces will either be measured (in the sensor configuration) or transmitted (in the motion consteaining configuration) along a single axis only. The device of the present invention is substantially compliant to forces or force components which are not directly along this axis. The device measures force applied along an axis, by recording changes in fluid pressure, while remaining relatively immune to off-axis forces and torques. The device can also be used to constrain linear motion along a single axis, while leaving other translation and rotation degrees of freedom relatively unrestricted.

Figure 1 shows an exploded view of a preferred form of a device 1 that acts as a combined force sensor and method for constraining motion, the device having a casing constructed in this example from three main parts: a central cylindrical section 2, and two end caps 3 and 4. It will be appreciated that the central section 2 may be described as tubular, but is not necessarily limited to a circular cross section. Also, the central section 2 may be a container in its own right, and have end plates attached to each end in place of the end caps 3 and 4.

In one embodiment the central section 2 is constructed from a transversely isotropic elastic material, significantly stiffer in the circumferential direction (indicated by arrow 5 in Figure 1) than the radial and axial directions (indicated by arrows 6 and 7 respectively in Figure 1). The central section 2 in the example shown in Figure 1 is connected to the end caps 3, 4 as shown in Figures 2 and 3 to provide an enclosed core volume which in use is filled with relatively incompressible fluid or compliant solid.

The fluid or compliant solid used to fill the sensor could be either a relatively incompressible fluid or a compliant solid. The essential characteristic is that it supports hydrostatic stress (pressure) and is relatively compliant to deviatoric stresses (shear). Examples of suitable materials include any relatively incompressible fluid (e.g. water, oil, brake fluid, hydraulic fluid) or compliant solid (e.g. elastomer, grease, gel). Other properties, such as viscosity and temperature coefficient of expansion, should only have a minor effect on the behaviour of the device unless boiling or freezing occurs. Gels have the advantage that they can make the device relatively immune to buckling when internal pressure drops below ambient.

As show in Figure 2 when assembled the end caps 3 and 4 provide a close sealed fit with the central section 2. It will be appreciated that the end cap 3 and 4 may take a variety of different forms, and appropriate sealing mechanisms, for example O rings may be required, depending upon the materials from which the device is constructed, and depending upon the fluid or compliant solid which is used to fill the core volume. In one embodiment the end caps are constructed from the acetal homopolymer, DELRM. Grooves or channels 8 may be provided to allow the central section 2 to be located and appropriately secured to the end caps 3, 4.

As can be seen from Figure 3, a reinforcing structure in the form of a series of rings of stiff material 11 is associated with the central section 2, to provide the result in transversely isotropic elastic material from which the central section 2 is constixicted. As will be described further below, the reinforcing structure maybe bonded to the otherwise compliant material from which section 2 is constructed, or may be part of a composite laminate material. When used as a force sensor, axial forces applied to the device raise the pressure of the fluid in proportion to the applied force. Because the fluid is incompressible, and because the enclosing elastic material is stiff in the circumferential direction, axial forces will not result in significant radial or axial deformations. The device is thus relatively stiff to forces applied in the axial direction. By contrast, axial torques and transverse forces and torques are constrained neither by the fluid incompressibility nor circumferentially oriented transversely isotropic elastic material. The device is thus relatively compliant to all forces and torques except axially applied forces.

The device may be supplied with a mechanism to measure the pressure of the internal fluid or compliant solid. Internal fluid or compliant solid pressure is directly related to the applied axial force, and therefore the applied force may be inferred from the output of a pressure transducer, such as transducer 10 shown in Figure 2. We have found that the Honeywell 26PCGNM6G 0-250psi pressure transducer, or its metric equivalent 26PCGNH6G, are suitable for use with the apparatus.

For simplicity, Figure 3 omits the optional pressure transducer 10 which is shown in Figure 2 within the sealed core volume 8. As described above, the pressure sensor 10 allows the pressure of the fluid or compliant solid which is present in the core volume to be measured, which in turn provides a direct measurement of the axial force being applied to the device 1 between the top and bottom plates 3 and 4.

The pressure transducer 10 can be located directly within the fluid as shown in Figure 2, or located remotely using a fluid-filled catheter connected between the internal fluid in core volume 8 of the device and a port of the pressure transducer. Locating the pressure transducer remotely potentially offers several advantages over local placement:

• very small force transducers can be constructed; • The force sensing device might need to be located in an environment unsuitable for pressure transducers (e.g. in areas subject to extreme temperature, large electromagnetic fields, and high radiation). By separating the passive force sensor component from the pressure transducer, the pressure sensor can be moved to a less harsh environment;

Several force sensors might be multiplexed with a single pressure transducer. This might be an advantage where many force sensors are required (e.g. an array of force sensors) but it is desirable to use many fewer pressure transducers (e.g. because of the high cost).

Turning now to Figure 4, a dichiOmatic cross-sectional detail of a construction for the wall of the fibre reinforced elastomer tube 2 is shown. Those skilled in the art to which the invention relates will appreciate that there may be other materials or combinations of materials which provide the required transversally isotropic elastic material. As shown in Figure 4, longitudinal fibres 14 are bonded together or laminated by one or more layers of compliant material 16 such as an elastomeric film. The manufacturing process may involve a mould spool which is rotated at a required speed relative to a linearly moveable fibre thread distribution tool so that the required pitch (i.e. axial turn spacing of the fibres) can be controlled.

In Figure 4, four layers of elastomeric film 16 are shown, in accordance with one possible embodiment of the invention. In one embodiment, the film 16 comprises polyurethane film which is adhered to itself with the fibres 14 constrained within the polyurethane layers. In one example, the tube 2 can be manufactured using three layers of 0.25mm evlar fibre, bound with four layers of 0.05mm polyurethane film. The tube is heated at approximately 170 degrees Celsius for approximately 10 minutes and presses against the spool by the application of a Kevlar top layer to ensure good adhesion. The Kevlar top layer is then removed after bonding, With this arrangement the polyurethane film has sufficient surface area to bond with adjacent layers under the application of heat, while also securely retaining the fibres. In another embodiment tube 2 is constructed using butyl rubber rather than polyurethane. Butyl rubber has appropriate properties including impermeability, toughness, and resistance to plastic deformation. The tube 2 can be formed from a tube of butyl rubber which is then wound externally with thin Kevlar thread to provide ribs 11, and bonded to the tube with a vulcanising glue. Various methods may be used to bond the reinforced tube to the end caps 3, 4. In one embodiment (as indicated in Figure 2) the tube is glued into a circular slot 8 milled into the end caps 3, 4. In another embodiment, the reinforced tube is glued directly to the outer surface of a circular disk-shaped end cap such as cap 3 or 4, then further secured in place using a clamp such as a hose clamp tightened around the outside of the tube to persist hard against the outer perimeter of each end cap.

The single axis force sensor has undergone several design iterations, and validated by testing. Figure 5 is a graph of pressure sensor voltage as a function of applied forces (N).

It can be seen that a single device will act (constrain motion and/or measure force) in the axial direction. A number of force sensors/actuators can be combined or arranged to constrain motion, actuate, and/or measure forces and/or torques in more than one axis. Some examples are outlined below:

• Two devices arranged so their axes meet at a single common point will

simultaneously act along both axes. In this case linear motion will be constrained and/or forces measured simultaneously along the two axes, allowing linear motion only along a direction normal to both axes, and rotation about any axis passing through the common point;

• Two devices arranged so their axes are parallel will also simultaneously act along both axes. In this case linear motion will be constrained and/or forces measured simultaneously along the two axes, allowing linear motions along directions normal to both axes, constraining rotation about an axis orthogonal to the device axes and the line joining them, but allowing rotation about any axis orthogonal to this line;

• Three devices arranged so their axes meet at a single common point will

simultaneously act along all three axes. In this case linear motion will be constrained and/or forces measured simultaneously along the three axes, preventing linear motion along any axis, but allowing rotation about any axis passing through the common point. This arrangement effectively acts as a ball joint with no surface-to-surface rubbing contact. It is able to resolve the 3-vector of forces applied about the common point. Such an arrangement could be used where 3 -axis force measurement is required, but torques are not;

• Three devices arranged so their axes are parallel will also simultaneously act along all three axes. In this case linear motion will be constrained and/or forces measured along the common axis, allowing linear motions along directions normal to both axes, allowing rotation about any axis parallel to the device axes, but constraining all other rotations;

• Six devices arranged along connected edges of a hexahedron will

simultaneously act along all six axes. In this case all three axes of linear motion and all three axes of rotational motion will be constrained, preventing any rigid body motion. If pressure transducers are incorporated into all devices, all six components of force and torque will be able to be resolved. This arrangement corresponds to that of a Stewart platform.

Referring to Figure 6, the six degrees of freedom which correspond to movement in the x, y, and z directions, and pitch yaw and roll are shown diagrammatically as a set of axes with arches therebetween. Each set of axes has alongside it six lines each line denoting the use of a force sensor/motion constraint device 1 as described above. The weighted (i.e. dark) lines indicate the degree(s) of freedom and associated force sensor(s)/ motion constraining device(s) for examples of linear and/or rotational sensing constraint that can be achieved using the apparatus described herein. Eighteen examples (numbered 1 to 18) of linear and/or rotational sensing/ constraint that can be achieved are shown in Figure 6.

A Stewart platform can be formed using six of the combined force sensor/motion constraint devices 1 as described above to form the "actuator" legs of the platform. Referring to Figure 7, an isometric view of one of the two identical end plates 12 used to construct a Stewart platform force/torque transducer is shown. In use, two end plates 12 are provided, and the ends (e.g. the end caps 3) of six devices 1 are connected to mounting regions 13 of one plate 12, and the other ends (e.g. end caps 4) are connected to mounting regions 13 of the other plate 12. The transducer/motion constraint device 14 resulting from the assembled arrangement is shown in Figure 8. The device 14 constrains all three axes of linear motion and all three axes of rotational motion, preventing any rigid body motion. If pressure transducers such as transducer 10 described above are incorporated into all devices, all six components of force and torque will be able to be resolved.

As mentioned above, the device can be used to selectively remove or constrain motion, one degree of fi'eedom at a time. This is in contrast to conventional bearings and hinges which selectively add one degree of freedom at a time. Furthermore, this constraining of motion is accomplished without any surface-to-surface rubbing. Avoiding surface-to-surface rubbing could reduce or eliminate wear that is characteristic of conventional bearings and hinges.

The directional terms as used above (e.g. upper, lower, horizontal, etc.) are indicative terms only and are not intended to limit the nse to one specific or even general orientation.

Unless the context clearly requires otherwise, throughout the description and the claims, the words "comprise", "comprising", and the like, are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense, that is to say, in the sense of "including, but not limited to".