Interposed force sensor including amplifiers

An interposed sensor having a housing whose lateral extend is not greater than the lateral extend of the machine parts is to be installed between and having a plurality of measuring elements welded under preloading to the housing. Electronics including charge amplifiers are integral with the housing, either interior the lateral extend or in an annular compartment.

BACKGROUND AND SUMMARY OF THE INVENTION 
The present invention relates to interposed sensors having measuring 
elements paralleled forcewise, which are fitted in machine parts 
transmitting force and moment. A principal requirement for such sensors is 
that they should take the form of disks or plates as thin as possible, so 
that they alter the installation conditions as little as possible. 
The chief requirement for arrangements of this kind is that the subdivision 
of the total force transmitted through the plate into a smaller measuring 
force to the force sensors and a supplementary force bypassed must be 
constant in time. 
A sensor of this kind is shown in DE 34 40 670 C2. Glued in these measuring 
plates are commercial force measuring elements. Their very critical 
overall heights compared with the measuring plate surface must be adjusted 
exactly to within a few microns of adapter plates and thrust washers. One 
consequence of this series connection of a number of disks, of which there 
are usually already five in the commercial force measuring elements, is 
great uncertainty in the force transmission by paralleling these elements. 
This is because eight disks connected in series, for example, with the 
contact surfaces have ten air gap layers, which according to their 
preloading have different elasticity ratios. 
As a result, the individual force measuring elements in a measuring plate 
of this kind will give different force signals on account of the ten air 
gaps, because the size of these gaps is difficult to control. Moreover 
owing to the fitting of commercial force measuring elements and the 
necessary adapter and thrust washers, the plate thickness cannot be 
reduced below a minimum of 10 to 12 mm. 
The purpose of the invention is to provide force measuring elements and 
measuring plates with significantly fewer air gaps in the force measuring 
arrangement, requiring no additional adapter plates and thrust washers, 
and being much thinner. This object is attained by the measuring elements 
consisting of disks having not more than five air gap layers including the 
force introduction surfaces, whereby the measuring elements are welded in 
under high mechanical preload, by the sensor surface being ground to 
ensure flatness, and by the measuring elements forming an assembly unit 
together with the charge amplifier arrangement. With this combination 
according to the invention, new installation and monitoring are possible 
at economical costs, with unified and constant signal conditions of the 
individual force measuring elements. 
The invention relates primarily to piezoelectric force measuring 
arrangements operating together with charge amplifiers. Quartz (SiO.sub.2) 
is usually employed as piezo material, allowing measurements lasting up to 
15 minutes with only 1 to 2% signal loss, so that satisfactory static 
calibration is possible. The use of piezo-ceramics may be advisable in 
certain cases where stronger signals are needed and quasistatic measuring 
is not important. Nevertheless because quartz plates can be loaded both in 
compression and shear, depending on the crystal cut, quartz has proved to 
be the ideal material for multicomponent dynamometry. 
The invention can, however, make use of other known sensor techniques. Thus 
the application of thin and thick film processes and possibly capacitive 
principles also is conceivable. However, only piezoelectrics makes 
possible the necessary rigidity against a solid metal plate arranged in 
parallel. 
Other objects, advantages and novel features of the present invention will 
become apparent from the following detailed description of the invention 
when considered in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE DRAWINGS 
FIG. 1 shows an interposed sensor according to the invention having an 
annular form 1, built into a driving shaft of a boring machine for 
example. Measurement of the axial force Z and driving moment M is 
required. Other force components may also be selected and will be 
described later. Between driving shaft 2 and tool carrier shaft 4 is the 
interposed sensor 1, preloaded with screws 3 so that the force 
transmission corresponds nearly to that of a solid steel disk. To avoid 
altering the installation conditions on the machine as little as possible, 
the thickness H of the interposed sensor 1 is as small as possible, 
typically about 1/10th of D of the sensor or a few mm. 
In FIG. 4, the interposed sensor 1 consists of a metal disk 14 in which at 
least one disk-shaped measuring element 6 is fitted, though usually a 
number of disk-shaped measuring elements 6, 7 are spaced around the 
circumference. 
Depending on the crystal cut, the measuring elements 6, 7 consist of 
pressure or Z force elements 6, or shear force elements 7, so that Z, X or 
Y forces or M moments can be measured, according to the orientation of the 
sensitivity axes of the crystals. Fitted in the bore of the disk 14 is an 
amplifier 8, to which the signal electrodes 15 of the measuring elements 
6, 7 are connected. As shown in FIG. 1, a central connection sleeve 9 
surrounds the power lines and signal lines 10, which lead in the cable 
channel 12 to the signal transmission not shown. 
FIG. 2 shows a detail of section E--E of FIG. 4. With this embodiment, the 
metal disk 14 has a blind hole in which a measuring element, as in FIG. 8, 
is fitted. The charge amplifier 8 is held in a groove 18 on the metal 
annulus 14. The signal electrode 15 between the crystal disks 16 leads to 
the corresponding connecting point 19 on the charge amplifier 8 directly 
without intervening wires or cables. 
FIG. 3 shows the plan to FIG. 2. The metal annulus 14 is welded to the 
metal cover disk 17 of the sensor measuring element. 
As already stated, FIG. 4 shows the assembled unit consisting of the metal 
annulus 14 and the built in charge amplifier 8, on which the signal 
electrodes 15 are connected directly. The entire arrangement is sealed 
airtight with a sealing compound 13, making it easy to fit because no 
highly insulating lines or connections have to be provided. The user is 
quite capable of fitting the highly insulating sensor himself. However, 
the charge amplifier 8 may also be connected in a different way. Preload 
bolts 3 may be led straight through openings 30 in the metal annulus 14 as 
illustrated in FIG. 1. 
FIG. 5 shows in a detail how the measuring cell of FIG. 8 is welded into 
the metal annulus 14. The measuring element 15, 16, 17 is placed under 
high mechanical load by a pressing ram 21, after which it is welded to the 
annulus 14 by a laser welding fiber 22. Other welding techniques may also 
be used. 
In FIGS. 6 and 7, the cover disk 17 and bottom cover disk 20 are slightly 
oversized compared to the disk 16. After welding in, both surfaces are 
ground over or lapped flat to vvv finish on a two-wheel lapping machine to 
have a common planar surface. 
Because the cover disks are of the same material as the metal annulus 14, 
the dimensions H1 and H2 of the annulus 14 are absolutely equal, i.e. the 
grinding or lapping operations on the annulus 14 and on the metal cover 
disks 17 remove exactly the same thickness of material. 
FIG. 7 shows a measuring element for through hole. This embodiment is used 
in FIG. 6. 
FIG. 8 shows a measuring element for a blind hole. This embodiment is used 
in FIG. 2. Usually two crystal or ceramic disks 16 are provided, with the 
signal electrode 15 between them. An arrangement with only one piezo disk 
is also possible. The signal electrode then lies between an insulating 
disk and the piezo disk. 
Either pressure or shear crystals may be used for both measuring element 
types. Crystal and metal parts of the measuring elements in FIGS. 7 and 8 
are fused into an assembled unit, ensuring minimal gap effect and simple 
fitting. Gluing may also be used. 
Whether the metal annulus 14 has a through hole as in FIG. 6 or a blind 
hole as in FIG. 2, or whether only one piezo disk is used, is a design 
choice. Every possibility follows the same main principle. 
Important according to the invention is that the measuring elements 6, 7 
are welded in under high mechanical preload. After this, they are ground 
or lapped on one or both sides, depending on whether elements according to 
FIG. 7 or FIG. 8 are used. In this way and using fused or glued measuring 
elements, it is possible to attain the objective of reducing the multiple 
gap effect. Reliable measuring results may be expected over a long period 
from such measuring elements. 
FIGS. 9 and 10 show a complete application example of an interposed sensor 
with integrated amplifier electronics and digital inductive signal pick 
off. Of course, other wireless transmission means may be employed also. 
The interposed sensor 1 is clamped by preload bolt 3 between driving shaft 
2 and tool carrier shaft 4. For example both the force components X, Y, Z 
and the drive moments M are transmitted to the interposed sensor 1. For 
each of the four measured values, two symmetrically disposed measuring 
elements are provided, making altogether two for compressive forces Z and 
six for shear forces and moments X, Y and M. Accordingly, four charge 
amplifiers 8 are needed. They are arranged tangentially and mounted and 
sealed in the annular electronics housing 25. 
This electronics housing 25 may be equipped with the rotating signal 
induction coil 23 and other transmission electronics parts. The stationary 
signal induction coil 24 is mounted on the machine guideway and contains 
the voltage supply as well as the signal pick off. However it is also 
possible to provide the signal pick off and voltage supply for the charge 
amplifiers 8 centrally at the end of the driving shaft 2 by means of a 
special transmission unit. 
According to the invention, the charge amplifiers 8 are arranged so that 
the signal electrodes 15 can be connected straight to the ceramic plates 
of the amplifiers 8. However, other interesting connections are possible 
as well. 
FIG. 11 shows a rectangular interposed sensor 1 with the charge amplifiers 
8 placed inside the force contact surface. 
The example according to the invention in FIGS. 11 and 12 shows six 
measuring elements, two each for the space coordinates X, Y and Z. Again 
according to the invention, the measuring elements 7 are welded into the 
metal plate 28 under high mechanical pressing (shown in FIG. 5). The plate 
28 is then ground and lapped on both sides. 
For certain applications it may be advantageous to load some measuring 
elements more than others, especially if small changes of force are to be 
measured. 
In such cases, either with a stencil or by photolithography the cover disk 
17 of measuring elements 6, 7 is given a vapor-deposited layer 27 of 
thickness `S` as illustrated in FIG. 13. The thickness is exactly a 
function of the deposition time. The layer 27 is of a hard material such 
as metal. In this way, on the basis of the ground and lapped overall 
surface, very accurately defined additional stresses can be set up in the 
measuring elements 6, 7, without requiring extra disks or films, which 
would introduce further air gap effects into the measuring elements. 
A power pack 29 is connected to advantage before the three charge 
amplifiers 8 (-X, -Y, -Z). It may be connected straight to the 
standardized 24 to 36 V network of the machine. Charge amplifiers and 
power pack may be hybrid types. 
Sensors of any form may be chosen instead of disk-shaped, annular or 
rectangular interposed sensors. Such forms can be adapted to the design 
configuration of the machine. 
The invention thus makes possible a new category of interposed sensors, 
which are especially suited for measuring and especially monitoring 
cutting forces, riveting forces and welding forces on machine tools and 
robots. Machines having such monitoring sensors are capable of operating 
fully automatically. Piezoelectric metrology is eminently suited for 
multicomponent dynamometry. Disadvantages are that it measures 
quasistatically, and that highly insulating cables are needed between 
sensor and charge amplifier. Thanks to newly developed charge amplifiers 
which are factors smaller than prior commercial ones, it has become 
possible according to the invention to combine piezoelectric sensors and 
charge amplifiers into one unit, so that no connecting cables are now 
needed. With that, the quasistatic measuring capability using quartz as 
piezo material has been improved decisively. 
Measurements on such combinations have revealed that a jump signal of 10% 
FS (full scale) produces less than 2% drift after 5 minutes force 
exposure. 
Since in machining operations 95% of the working cycles last less than 5 
minutes, a wide application scope has been opened up in industrial 
metrology by this sensor technique according to the invention. 
By virtue of the very low height H of typically 6 to 8 mm, the interposed 
sensors according to the invention are easily fitted and are adaptable to 
the geometric requirements of the machine in simple fashion. Furthermore, 
they alter its stability structure only imperceptibly. The use of 
standardized disk measuring elements 6, 7 and welding these in under high 
mechanical pressure, also the avoidance of air gaps connected in series 
and the possibility of directly connecting the measuring elements to 
charge amplifiers while avoiding cable connections, yields an interposed 
sensor concept opening up new possibilities in machining technology and 
robotics.