MEASURING HEAD FOR A MATERIAL TESTING DEVICE

Technical Field

[0001] The present invention relates to the technical field of material testing. More particularly, it relates to a measuring head for a material testing device capable of carrying out indentation, scratch and tribological testing.

State of the art

[0002] Various material testing devices are known in the art which are suitable for carrying out indentation, scratch and tribological tests. In an indentation test, a stylus held in a headstock is driven axially into the surface of a substrate, and measurements of normal force and axial position of the stylus with respect to the surface of the sample are measured. In a scratch test or a tribological test, the procedure is substantially similar, except that a relative displacement between the stylus and the substrate is also generated perpendicular to the axis of the stylus. In such a test, additional measurements such as tangential force generated between the sample and the stylus, audio emissions and so on can also be measured. In this context, “normal” and“tangential” refer to directions relative to the sample surface, which is presumed to be perpendicular to the axis of the stylus.

[0003] Often, the sample holder is movable in three linear directions, and the indenter stylus can additionally be displaced axially with respect to its headstock so as to apply a normal force to the sample without moving the headstock or the sample holder in the Z direction. In such a situation, all other movements are carried out by means of moving the sample holder relative to the headstock. An example of such an arrangement is disclosed in document EP2291635.

[0004] Document DE 10 2006 052 153 discloses a more complicated arrangement comprising MEMS comb actuators arranged to displace the stylus not only axially, but also perpendicular to its axis. As a result, it is possible to carry out a scratch test over short distances without moving the sample or the headstock.

[0005] Other arrangements are disclosed in US2016/370272 and in EP3171153.

[0006] However, such arrangements present certain limitations. For pure indentation, the stylus and its supporting arrangements should be as stiff as possible in the direction perpendicular to the axis of the stylus in order to avoid parasitic off-axis displacements, rotations and forces which can adversely affect the measurement. However, for a scratch test, a certain lateral displacement of at least part of the stylus is required with current technology in order to be able to sense the tangential force (i.e. perpendicular to the axis of the stylus) applied to the sample. This is typically measured using a capacitive displacement sensor in combination with an elastic element such that displacement of the stylus against the force supplied by the spring can be correlated with the force applied by the stylus according to Hooke’s Law. The comb actuator based system of DE 10 2006 052 153 is particularly problematic on this point, since comb actuators are subject to significant deformations at high driving voltages. These deformations reduce the stiffness of the system, and introduce significant measurement errors.

[0007] Hence, in prior art material testing devices intended to carry out both indentation and scratch tests, the lateral stiffness of the stylus is a compromise so as to give reasonably good accuracy of measurement in both modes of operation. The errors resulting from this compromise are typically either ignored, or are mathematically compensated for when processing the measured data.

[0008] An aim of the present invention is thus to overcome the above-mentioned disadvantages of the prior art and thus to propose an instrument head for a material testing device which can be used for high-precision measurements in both indentation and scratch modes.

Disclosure of the invention [0009] More specifically, the invention relates to a measuring head for a material testing device comprising a headstock and a sample holder adapted to hold a sample facing said headstock. The measuring head comprises at least one chassis adapted to be attached to the headstock and a stylus supported directly or indirectly by the chassis and arranged to contact a surface of the sample, the stylus extending along a first axis which is referred to as an“axial direction”.

[0010] The measuring head further comprises:

[0011] - an axial displacement sensor adapted to measure axial displacement of the stylus with respect to the headstock;

[0012] - a lateral displacement sensor arranged to measure a displacement of the stylus in a lateral direction (that is to say at an angle to the above-mentioned axial direction, typically perpendicular thereto) and a lateral force sensor (which may be integrated with the lateral displacement sensor into a single sensor unit) arranged to measure a tangential force applied by the stylus to the sample, said tangential force being perpendicular to said normal force;

[0013] - a lateral actuator arranged to apply a lateral force to the stylus with at least a component perpendicular to said axis; and

[0014] - a control unit adapted to control displacements of the stylus with respect to the chassis by means of the lateral actuator and any other optional actuators present (see below).

[0015] According to the invention, the control unit is adapted to control the lateral actuator based on output of the lateral displacement sensor so as to substantially maintain the lateral position of the stylus in its initial lateral position when said tangential force is applied. This is to say that the control module is not just capable of controlling the lateral actuator in the manner described, but is indeed arranged, e.g. programmed, to do so.

[0016] As a result, there is substantially no deflection of the stylus with respect to the chassis (and hence with respect to the headstock to which it is affixed) during a scratch test. In a typical scratch test, this deflection can be quite substantial due to the need to measure lateral force by conventional means, and must either be accepted as a source of error or must be accounted for. However, with the present invention, the lateral stiffness of the stylus is extremely high, since the lateral actuator works to maintain its position and hence to resist any lateral displacement of the tip of the stylus. Also, this high effective stiffness means that accurate indentation tests without lateral movement of the sample are also possible. The measurement head is thus capable of carrying out highly accurate indentation and scratch or tribological tests in equal measure.

[0017] Advantageously, the measuring head also comprises an axial actuator arranged to displace the stylus relative to the chassis along the first axis and an axial force sensor arranged to measure a normal force applied by the stylus to the sample. By incorporating both of these features into the measuring head itself, this latter is entirely self-contained and does not rely on any axial actuators or sensors integrated into other parts of the material testing device upon which it is mounted. It should also be noted that it is also possible to have one or other of these features in the measuring head, and the other in the headstock, in the sample holder or in a movement stage associated with one of these elements. For instance, axial displacement can be effected by an XYZ movement stage which supports the sample being moved towards the headstock, or by the headstock being moved towards the sample. In either case, the axial force sensor can be situated in the headstock, sample holder, a movement stage, or any convenient part of the material testing device.

[0018] Advantageously, the chassis comprises an outer chassis integrated with or arranged to be fixed on said headstock, and an inner chassis suspended in the outer chassis. The stylus is suspended in the inner chassis. This suspension can be effected by any convenient means, such as by strip springs arranged in a cruciform configuration, circular springs, systems of articulations, bearings, or similar.

[0019] Advantageously, the axial actuator is arranged to displace the inner chassis in said axial direction with respect to the outer chassis. Since the stylus is suspended in the inner chassis, this causes the stylus to apply force to the surface of the sample. [0020] Advantageously, the lateral actuator is mounted to the inner chassis, which is thus self-contained and compact.

[0021] Advantageously, the stylus is suspended so as to incline in response to a lateral force, the lateral displacement sensor being arranged to measure this inclination. As a result, the lateral displacement of the stylus tip can be measured at a position remote from the tip, which simplifies manufacture and eliminates cluttering of components near the tip.

[0022] Advantageously, the lateral actuator is connected to the stylus by an elastic element such as a spring. This spring can thus form part of the lateral force sensor, which results in a simple system with a minimum number of parts.

[0023] Advantageously, a vertical sensor is provided. This vertical sensor is arranged to measure on the one hand the relative displacement of the stylus with respect to the outer chassis and thereby to act as said axial displacement sensor, and is arranged on the other hand to measure the relative displacement of the stylus with respect to the inner chassis so as to act as said axial force sensor in cooperation with at least one elastic element such as a spring suspending the stylus in the inner chassis. This vertical sensor can be capacitive, piezoelectric, optical, or any other convenient type. This arrangement is particularly simple and compact.

[0024] Advantageously, the lateral force sensor is arranged to measure a force applied by the lateral actuator to the stylus. This arrangement integrates the measurement of the tangential force with the lateral actuator, which is a particularly compact arrangement. Since, by conservation of force, the lateral force applied by the lateral actuator to maintain the stylus in substantially its initial lateral position is equal to the tangential force applied between the sample and the stylus, there is no need to measure this force with a separate sensor.

[0025] Advantageously, the control unit is adapted to maintain the initial position of the stylus by means of servo control, thereby maintaining the lateral position of the stylus in real time with an absolute minimum of lateral deflection.

[0026] The invention further relates to a method of making an indentation, scratch or tribological test, comprising steps of: [0027] - providing a material testing device comprising a measuring head as described above;

[0028] - providing a sample on the sample holder;

[0029] - driving at least one of the headstock and the sample holder so as to position the sample facing the stylus;

[0030] - placing the stylus into contact with the sample and applying a force thereto while substantially maintaining the initial lateral position of the stylus with respect to the headstock by applying a lateral force thereto by means of the lateral actuator. In the case of a scratch or tribological test, relative lateral movement is generated between the sample and the stylus while said force is applied therebetween.

[0031] - recording measurements of parameters relating to the test by means of at least one of said sensors. Such parameters can include tangential force, normal force, displacement of the stylus normal to the sample, displacement of the sample in the X and/or Y directions, and so on. This method results in an effectively infinite stiffness of the stylus perpendicular to its axis, which dramatically reduces the lateral displacement of its tip during a test. Hence, improved accuracy of measurements is attained in both indentation and scratch/tribological tests.

[0032] Advantageously, the lateral position of the stylus with respect to the headstock is maintained by means of servo control based on output of the lateral sensor.

[0033] Advantageously, the measured parameters include measurements of force applied to maintain the initial lateral position of the stylus. This force is equal to the tangential force applied between the stylus and the sample, so no further force measurements are required.

[0034] These methods can be commanded by a computer program product comprising instructions which, when the program is executed by a computer arranged as a control unit for a material testing device as described above, cause said material testing device to carry out one of the methods as described above. Brief description of the drawings

[0035] Further details of the invention will appear more clearly upon reading the description below, in connection with the following figures which illustrate:

- Figure 1 : a schematic cross section parallel to the XZ plane of a material testing device comprising a measuring head according to the invention;

- Figure 2: a schematic cross-section of a detail of the vertical sensor of the measuring head of the invention;

- Figure 3: a schematic cross-section parallel to the XY plane of an arrangement of lateral actuators; and

- Figure 4: a generic flow-chart describing the method of the invention in its most generic form.

Embodiments of the invention

[0036] Figure 1 illustrates schematically a lateral cross-sectional view of a material testing device 1 in which a measuring head 3 according to the invention has been integrated. This measuring head 3 may be used for indentation and/or scratch and/or tribological testing at the millimetre, micrometre, nanometre, or even sub-nanometre scale.

[0037] Material testing device 1 comprises a headstock 5 attached to a stand (not illustrated), and a sample holder 7 situated beneath the headstock 5 and arranged to support a sample 9 facing the headstock 5. Sample holder 7 and headstock 5 may be of any known type, and are arranged to permit relative movement in at least the X and Z directions with respect to each other by means of a movement stage (not illustrated). Relative movement in the Y direction (perpendicular to the page) can also be provided if desired, and should be understood as always being a possibility in the forgoing, should three-dimensional movement and/or measurements be required. To this end, headstock 5 may be fixed and sample holder 7 may be movable in at least the X and Z directions, or vice-versa. Alternatively, headstock 5 may be movable in the Z direction and sample holder 7 in the X direction, or vice versa. Such movements are controlled by a control unit 24 as is generally known.

[0038] The measuring head 3 according to the invention is attached to, or integrated with, the headstock 5. Advantageously, it is a self-contained module that can be utilised with any existing material testing device, and such a modular construction is illustrated here.

[0039] In this embodiment, measuring head 3 comprises an outer chassis 11 rigidly attached to the headstock 5. An indenter tip 13 of any convenient type (e.g. Vickers, Brinell, pyramidal, conical, etc) carried by a stylus 14 protrudes through an opening 15 on the underside of the outer chassis 11 , so as to be able to be brought into contact with the upper surface of the sample 9 provided on an upper surface of the sample holder 7.

[0040] An inner chassis 17 is suspended inside the outer chassis 11 by means of an axial actuator 19, situated between an upper portion of the inner chassis 17 and an upper portion of the outer chassis 11 , as well as by a set of suspension springs 21a, 21 b. These latter are represented on the figure by means of dotted lines with circular dots. The suspension springs 21a, 21b are attached to the outer chassis 11 and to convenient points on the inner chassis 17. This suspension arrangement permits a certain degree of relative movement in the Z direction between the two chassis 11 , 17, but substantially prevents relative movement in the X and Y directions between these elements. Examples of suitable springs for this role are pairs of strip springs arranged in a cruciform configuration, or systems of suitably stiff flexible articulations arranged to permit the desired relative movement and restoring forces. The term“spring” should be understood in this context as a generic term encompassing all elastic elements which provide a restoring force.

[0041] In the illustrated arrangement, an upper suspension spring 21a is attached between the axial actuator 19 and the top of the inner chassis 17, and a lower suspension spring 21b is attached to an interior tubular section 17a of the inner chassis 17, this section being situated above a vertical sensor 23. This latter will be described in further detail below. This arrangement requires that the lower suspension spring 21b passes through adequately-sized openings 25 in the outer walls of the inner chassis 17 provided to this effect and represented here highly schematically. The illustrated arrangement of suspension springs 21a, 21b is not, however, intended to be limiting, and a large number of alternative arrangements are also possible. For instance, the suspension springs 21a, 21b could be attached at different points to the two chassis, or a more complex arrangement incorporating flexible articulations or bearings combined with appropriate elastic elements could be used.

[0042] It should be noted however that axial actuator 19 is optional, and axial displacement of the tip 13 with respect to the sample 9 can simply be carried out by moving the sample holder 7 and/or the headstock 5 in the Z direction in the case in which the axial actuator 19 is not present.

[0043] Axial actuator 19 can thus displace the inner chassis 17 in the Z direction with respect to the outer chassis 11 , without the inner chassis tipping or displacing in the X or Y directions.

[0044] The indenter stylus 14 is suspended inside the inner chassis 17 by means of an upper indenter spring 27a which is attached to the interior wall of the inner chassis 17 and to the stylus 14 at an intermediate point thereof, as well as by a lower indenter spring 27b, attached again to the inner chassis 17 and to a point on the stylus 14 proximate to the tip 13. To this end, lower indenter spring 27b passes through further openings 25 provided in an upper surface of a tube 11a which forms part of outer chassis 11 and surrounds the stylus 14. The indenter springs 27a, 27b are represented by dotted lines formed of square dots so as to better distinguish them from the suspension springs 21a, 21 b.

[0045] Both of the indenter springs 27a, 27b are implicated in measuring forces applied by the indenter tip 13 to the sample 9, as will be explained further below.

[0046] Upper indenter spring 27a is implicated in measuring normal force Fn, that is to say force in the Z direction, in cooperation with vertical sensor 23. This latter is operatively connected to a control unit 24 by wires, fibre optics, or any other convenient connecting means. Such connections are illustrated in figure 1 by means of chain lines. The sensor 23 can be of any type, but in the present embodiment is capacitive, illustrated at a larger scale in figure 2 in the absence of other features such as the lower indenter spring 27b.

[0047] Although the vertical sensor 23 looks at a glance similar to a differential capacitor, it is in fact formed of pairs of simple capacitive sensors each in an annular configuration. First capacitive sensor 23a comprises a pair of annular electrodes facing one another. These electrodes are distributed between a lower surface of a flange 14a of the stylus 14 and an upper surface of tube 11a. This first capacitive sensor 23a hence measures the relative position of the stylus 14 with respect to the outer chassis 11 , and as such forms an axial displacement sensor. Since the outer chassis 11 is fixed rigidly to the headstock 5, first capacitive sensor 23a thus measures the vertical displacement of the stylus 14 during an indentation test or a scratch test.

[0048] The second capacitive sensor 23b comprises a further pair of annular electrodes again facing one another, one being situated on an upper surface of the flange 14a, the other on the lower face of the interior tubular section 17a of the inner chassis 17. This sensor 23b measures the relative displacement between the stylus 14 and the inner chassis 17, and as a result acts as an axial (i.e. normal) force sensor in cooperation with the indenter springs 27a, 27b. Since the stylus 14 is suspended in the inner chassis 17 by the upper and lower indenter springs 27a, 27b, the normal force Fn applied by the indenter tip 13 to the sample 9 can simply be calculated by Hooke’s Law F=k.Dz, wherein k is the effective spring constant of the two springs 27a, 27b in the Z direction, and Dz is the relative vertical displacement between the stylus 14 and the inner chassis 17 as measured by the second sensor 23b (assuming linear spring response). In the case of non-linear springs, a non-linear variant of Hooke’s Law can be used in a similar fashion. It can thus be seen that the combination of vertical sensor 23b and the two springs 27a, 27b acts as an axial force sensor.

[0049] Although the capacitive sensor 23 illustrated comprises a pair of annular simple capacitors as described above, other arrangements are possible such as pairs of differential capacitors, or capacitors in a comb configuration. Further alternatives include piezoelectric sensors, optical sensors and so on.

[0050] On the basis of the foregoing features, an indentation test can be carried out by manoeuvring the headstock 5 and/or the sample holder 7 such that the tip 13 is in contact with the surface of the sample 9 or is just out of contact with said surface. Outer chassis 11 is then left in a stable axial position, and the axial actuator 19 is commanded by the control unit 24 to drive the inner chassis 17 downwards towards the sample in the Z direction so as to drive the tip 13 into the surface of the sample 9. The control unit 24 measures the normal force Fn and the displacement of the tip Dz in the Z direction based on the output of vertical sensor 23. The test can be servo controlled so as to apply a desired displacement profile, or so as to apply a desired force profile, as is generally known.

[0051] In order to measure tangential force Ft, that is to say the force that is developed in the X direction during a scratch test carried out with relative movement between the headstock 5 and the sample holder 7 in the same direction, the measuring head 3 comprises appropriate means, namely a lateral force sensor, as will be explained in more detail below.

[0052] Contrary to known scratch testers, the measuring head 3 of the present invention does not measure tangential force Ft by means of simply allowing the stylus 14 to be displaced laterally against a restoring force provided by a spring. Control unit 24 is adapted, in contrast, to substantially maintain the position of the stylus in the X direction with respect to the outer chassis 11 , and hence with respect to the headstock 3, during a scratch test by means of servo control, while measuring the force required to do so. As a result, the initial lateral position of the stylus 14, particularly of its tip 13, is substantially maintained to within a narrow tolerance.

[0053] In the illustrated embodiment, the lower indenter spring 27b is illustrated as having a kink, which is schematic for the spring having a lower stiffness in the X direction than, for instance, upper indenter spring 27a. Lower indenter spring 27b can, for example, be formed as strip springs in cruciform configuration, as a circular diaphragm with a corrugated form, or any other convenient arrangement or shape permitting the desired stiffnesses in the X and Z directions.

[0054] In order to measure the position of the stylus 14 in the X direction, a horizontal displacement sensor 29 is provided. This sensor may be of any type, such as capacitive, piezoelectric, optical or similar, and may be arranged in any convenient manner. In the illustrated embodiment, horizontal displacement sensor 29 is a simple capacitive sensor arranged to measure the inclination of the stylus 14 caused by the tangential force Ft, from which can be deduced the horizontal displacement of the stylus 14. It should however be noted that the horizontal displacement sensor 29 can be of any convenient type (whether involving tilting of the stylus 14 or not), and can be situated at any convenient place on the stylus 14, such as for instance between the upper and lower indenter springs 27a, 27b.

[0055] To this effect, stylus 14 is suspended at a mid-point by means of upper indenter spring 27a as mentioned above. Since this indenter spring 27a is configured to be extremely stiff in the X (and optionally also Y) direction, the stylus 14 substantially cannot translate in the plane of the indenter spring 27a in question. However, since the upper indenter spring 27a can bend, stylus 14 can incline by rotating about a central point in the plane where the upper indenter spring 27a intersects the stylus 14.

[0056] Horizontal displacement sensor 29 is arranged at the upper extremity of the stylus 14 so as to measure this inclination. On the basis of signals received by the horizontal displacement sensor 29, the control unit 24 commands a horizontal actuator 31 to apply a horizontal force via lower indenter spring 27b to the lower portion of the stylus 14 so as to keep the stylus 14 substantially vertical by servo control, and thus to retain its initial position in the X and Y directions. To this end, the horizontal actuator 31 is situated between the inner chassis 17 and the outer end of one branch of the lower indenter spring 27b.

[0057] The displacement of the outer end of the lower spring 27b with respect to the inner chassis 17 is measured by a further horizontal sensor 33 of any convenient type (capacitive, piezoelectric, optical, and so on). Since the spring constant k of the lower spring 27b in the X direction is known, the horizontal force applied to the stylus 14 to keep it vertical is again known by applying Hooke’s Law, F=k.Dx wherein Dx is known from the output of the further horizontal sensor 33 (assuming linear spring response). In the case of non-linear springs, a non-linear variant of Hooke’s Law can be used in a similar fashion. This force corresponds to the tangential force Ft developed between the tip 13 and the surface of the sample 9, which can thus be measured without having to allow the indenter tip 13 to displace in the tangential direction. It can thus be seen that the combination of the further horizontal sensor 33 and the lower spring 27b constitutes a lateral force sensor.

[0058] Such servo control does imply the existence of small lateral displacements of the stylus 14, however these are extremely small in comparison to the displacements required by conventional direct Ft measurements, and are of the order of nanometres, compared to the 20-30pm which is typical of conventional systems. This represents an improvement of 3-4 orders of magnitude. As a result, the measurements of displacement in the X direction gathered from the movements of the headstock 5 and/or the sample holder 7 do not need to be corrected for the substantial horizontal displacement of the stylus 14 that occurs during a scratch test with a conventional measurement head. Any servo jitter that occurs with the arrangement of the present invention is at least an order of magnitude less than the error caused by lateral displacement of the stylus 14 during a scratch test using a conventional arrangement, and can thus effectively be ignored.

[0059] The proposed arrangement thus allows scratch and tribological testing with an indenter that has an extremely high effective stiffness in the tangential direction.

[0060] During an indentation test, the horizontal actuator 31 can be powered down, and does not influence the test in any way e.g. by servo jitter. However, it is also possible to also operate the horizontal actuator to actively maintain the horizontal position of the indenter tip 13 and thus to exhibit the extremely high effective stiffness mentioned above. [0061] Should it be desirable to carry out two-dimensional scratch tests incorporating displacements and force measurements not only in the X but also in the Y direction, a horizontal sensor 29 can be arranged to also measure inclination of the stylus 14 in the YZ plane, and a yet further horizontal actuator 31a and force sensor 33 can be arranged at right-angles to that illustrated in figure 1 , so as to apply force in the Y direction. This configuration is illustrated in figure 3, which also shows clearly the typical cruciform configuration of the lower spring 27b, which it shares in common with the other springs 27a, 21a, 21 b. However, it should be reiterated that the illustrated shapes of the springs 27a, 27b, 21a, 21 b are not to be construed as limiting, and any convenient types of springs can be used, including leaf springs, coil springs, diaphragm-shaped springs, systems of flexible articulations, and variations and combinations thereof.

[0062] In view of the foregoing, figure 4 illustrates a flow chart which describes a method of carrying out an indentation, scratch or tribological test by means of a material testing device 1 comprising a measuring head 3 according to the invention.

[0063] In step 101 , the material testing device 1 comprising the measuring head 3 according to the invention, and a sample 9 is placed on the sample holder 7 facing the headstock in step 102.

[0064] In step 103, the sample 9 is positioned with respect to the tip 13 of the stylus 14 by means of translating the sample holder 7 and/or the headstock 3 so as to position the tip 13 just out of contact, or in light contact, with the surface of the sample 9.

[0065] In step 104, the stylus 14 is driven in the Z direction so as to apply a normal force to the sample 9, according to any desired force or depth profile. In the case of a scratch or tribological test, lateral relative movement is generated between the tip 13 and the sample 9 by translating either the headstock 3 with respect to the sample holder 7 or vice-versa. In the case of an indentation test, no such relative lateral movement is generated.

[0066] While this is taking place, in step 105 the lateral position of the stylus 14 is maintained with respect to the headstock 3 as described above. [0067] Again, while this is taking place, in step 106, parameters relating to the scratch or tribological test are measured, including at least one of tangential force Ft, normal force Fn, penetration depth Dz, horizontal relative displacement between the headstock 3 and the sample holder 7, acoustic emissions, and so on.

[0068] It should be noted that the measuring head 3 of the invention is entirely compatible with various known indentation, scratch and tribological tests as known in the prior art, such as pre- and post-scanning methods for measuring residual depth after an indentation or a scratch test, integration with a microscope, combination with heating or cooling stages, and so on.

[0069] Finally, it is noted that the method of the invention can be controlled by means of a computer program product comprising instructions which, when the program is executed by a computer arranged as control unit 24, cause material testing device 1 to carry out at least one of the methods described above. Such a computer program product can be provided as code on a non- volatile computer-readable storage medium such as a hard drive, CD-ROM, flash drive, or any other form of storage medium.

[0070] Although the invention has been described in terms of specific embodiments, variations thereto are possible without departing from the scope of the invention as defined in the appended claims. It is particularly noted that wide variations in the structural forms, the arrangements of sensors and so on, can be envisioned, as can an arrangement without an inner chassis. Furthermore, the various sensors (force, displacement etc.) can be of any suitable type (capacitive, piezoelectric, optic, electrostatic, and so on), many types of which are well-known to the skilled person.