Note: Descriptions are shown in the official language in which they were submitted.
33
J.C. Greenwood - 44
LOAD SENSORS
Background of the Invention
This invention relates to load sensors.
It has previously been proposed to provide an
- electrically excited mechanical resonator which has a
resonant frequency dependent on a mechanical loa~ applied
to the resonator and to utilise variations in the resonant
frequency as an indication of the applied load.
problem with this kind of device is to prcvide the
mechanical resonator with a high Q and with little
likelihood of spurious resonances either due to pick-up of
external vibrations or coupling with other masses or with
alternative modes of vibration.
Summary _f the Invention
~;, 25 An object of the invention is to provide a load
'i ~
sensor incorporating an electrically excited resonator in
which these problems are reduced.
According to the invention there is provided a
load sensor comprising a balanced vibratory system
including two lamina masses supported by filaments in
tension from a support structure and electrically coupled
to electrodes for causing angular vibration of the masses,
and for providing an output signal, wherein the resonant
frequency of the system can be sensed from the output
signal and serves as a measure of the load applied to the
sensor, the masses and the filaments having been
fabricated by a photolithographic process.
Provided the two vibratory masses are eqllal,
their absence of translational movement and the directly
opposed movement provide a balanced system which tends to
be insensitive to external vibrations and tends not to
couple in e~ternal masses in a spurious vibratory mode.
Preferably the masses vibrate abo~t a common axis
and the rotational stiffness of the sections of filament
extending beyond the two masses are less than the
stiffness of filament between t:he masses so that the
frequency at which the two masses would vibr~te in phase
is remote from the fre~uency at which they vibrate in
anti-phase. This low stiffness of resilient couplin~ to
(- an external mounting for the system tends further to
reduce the coupling to external masses or transmission of
spurious external vibration.
The use of electro-static co~pling through what
are in effect capacitor plates adjacent the vibrating
plates provides a system in which direct electrical
coupling between the plates is negligible.
The assembly of filaments and plates can
conveniently be formed by a selective etching process from
a silicon wafer. In the same etching process, the
support and mounting terminations for the filament can be
formed integrally from unetched portions of the silicon
wafer.
The moments of inertia of the two masses abo~t
the vibrational axis, or axes should preferably be equal
to each other.
In order that the invention can be easily
understood reference will now be made to the accompanying
drawings in which:-
Fig. 1 is a diagrammatic representation o~ a loadsensor according to an embodiment of the invention;
Fig. 2 shows diagrammatically a plan view of
another embodiment according to the invention;
Fig. 3 shows diagrammatically a plan view of a
third embodiment;
~L2~
Fig. 4 shows diagrammatically a plan view of a
fourth embodiment;
Fig. 5 shows diagrammatically a plan view of a
fifth embodiment; and
Fig. 6 shows in perspective a part of the Fig. 4
embodiment.
Description of the Preferre~ ~mbodiment
The load sensor shown in Fig. 1 incorporates a
filament f which extends between fixed mounting
terminations 1~ and 13 on a support S. The filament f
carries two transverse plates forming masses Ml, M2
which are of equal dimensions and formed of the same
material so that they both have the same moment of inertia
about the axis of the filament. The plates form
vibratory masses of a mechanical resonant system. The
resilience of the system is provided by torsional
resilience in the filament f and in particular by a
central resilient portion 16 of filament f joining the two
masses and two outer portions joining the masses to the
terminations 12 and 13. The intended vibratory mode of
the system is such that the two plates are deflected about
the axis of the filament in opposite rotational directions
as shown by the deflected positions of the plates and
~~ 25 indicated by the arrows 19 and 21. The primary
resilience controlling this vibratory motion is the
central section of the filament f which is twisted to an
angle equal to the sum of the deflections. The outer
sections of the filament are each twisted through an angle
equal to a single deflection. The reduced deflection of
the outer sections of the filament reduces their effect on
the resonant s~stem compared with the central section and
this effect may be reduced even further by making the
outer sections longer or narrower than the central
section. An alternative mode of resonance of the system
would be for the masses to be deflected angularly in
unison under the control of the torsional resilience of
-- 4
the outer sections of filament. The resonant frequency in
this mode should be made well outside the operating range
of frequencies in the intended mode of operation and this
can be achieved by ensuring a low torsional stiffness or
the outer sections of filament f.
It is convenient to form the support S, the
masses and the three sections of filament as a single unit
by a selective etching process on a silicon wafer. By
this technique a small mechanic21 system can be
manufactured accurately. With this construction, the
filament is
constituted by three separate sections rather than a
continuous filament but this is a practical detail which
is not fundamental to the invention.
The load sensor is provided with an electrical
drive for co~pling an input signal to the masses to cause
them to vibrate about the filament axis and is discussed
in greater detail in Figs. 4 and 6. The drive in this
example is constituted by four fixed electrodes El,
E2, E3 across which a sinusoidal electrical input
signal is applied. This input signal is coupled by
electrostatic attraction and repulsion to mechanical
oscillation of the masses. Oscillation of the masses
about the ~ilament axis varies the capacitances between
the masses and the electrodes. When suitable electrical
polarisation is applied between the silicon support S and
reference or earth an electrical output signal at the
frequency of oscillation is generated. Resonance occurs
over a very narrow frequency band so that the system has a
very high Q.
The resonant frequency varies with tension in the
filament and this tension is changed by the load L applied
to the sensor.
The load sensor can be operated to detect whether
or not the load L is at a desired value by comparing the
output frequency with a preset frequency, or
al-ternatively, the output frequency range can be
calibrated in terms o~ the load L either directly or as
some other parameter dependent on load L e.g. pressure,
etc. One form of electrical circuit is described in Fig.
6 and is applicable to this and all the other embodiments.
Because of the balanced nature of the vibratory mode
used, spurious external vibrations tend not to be
transmitted to the system and this makes spurious
response of the system unlikely. ~160, as previously
explained, any other vibratory mode about ~he filament
axis is kept to a frequency outside the range of
operation of the system.
Referring to Fig. 2 asminar masses Ml and M2 are
supported by filaments fl, f2, f3, f4, and y
filaments f5, f6, f7, f8 respectively. The
restoring force provided by the filament is a combination
of torsional and tensile forces. The support s~ructure S
preferably encircles the masses Ml and M2 and their
associated filaments and the whole is fabricated by
selectively etching a single silicon wafer. The lower
surface of the wafer would coincide with the plane of the
filaments and the masses and the whole would be mounted
via a closed loop gasket of e.g. Mylar (brand) tape about
12 microns thick, on an insulating substrate SUB of e.g.
glass with metallised portions serving as electrodes
El, E2, E 3 shown in broken line in the drawing.
The masses Ml and M2 and their associated
filaments would have a thickness of about 5 microns.
Fig. 3 shows an embodiment in which the masses M
and M2, similar to Fig. 2, are supported from support
S2 by filaments fg~ fl0 and fl4 on one side and
nts fll~ fl2 and fl3 on the other side
F'ilaments fl3 and fl4 provide a restoring force which
is mainly tensile while the other filamen-ts provide a
combination of mainly ten~ile and torsional restraining
forces. Otherwise the sensor is constructed in the same
manner as Figs. l and 2.
.": . ~'1',
33
-- 6
Fig. 4 shows an embodiment in which the masses
Ml and ~2 have a common axis of angular vibration.
15~ fl6 on the one hand and fl7 and f
on the other support the respective masses M1 and M2
from the support structure S3 and there is an
intermediate filament flg joining the two masses Ml,
M2 .
A perspective view of part of Fig. 4 which has
been sectional through M2 at x-x is shown in Fig. 6, and
shows the right hand mass M2 s~pported by filaments
fl8~ fl9 from support structure S3. It also shows
as an example part of the ~lass substrate SUB supporting
two of the electrodes El, E2, and schematically the
basic electrical connections and circuit components: these
comprise a bias source 30 connected between the silicon
support S3 and earth and an amplifier 31 providing
feedback from one
electrode El to the other E2 in order to maintain
anaular vibration of the mass M2. Likewise mass M
will be vibrating angularly in anti-phase because
electrodes El and E3 would be connected together.
This will be the same for all embodiments described
here. The resonating masses ~1' M2 are thus driven
by electrostatic forces and the output signal OP provides
~- 25 an output frequency determined by the resonance of the
system and which, in turn, is determined by the applied
load in the direction of the arrow A parallel to the axis
or axes of angular vibration of the masses. There is an
a.c. change in capacitance between the tips of the masses
and the adjacent electrodes as vibration takes place. In
the drawing mass M2 is shown in one extreme angular
position i.e. close to electrode E2 and far from
electrode El. Arrow Al shows the movement of the tip
of mass ~2
3~ The electrical arrangement described operates the
sensor in a self-exciting mode by correct choice of the
phase relationship between the imput and output of the
~ - ~20~
amplifier 31. Oscillation can be initiated by any chance
effect; for example noise in the amplifier will start the
oscillation.
Fig. 5 shows another embodiment in which masses
Ml and M2 are supported by the filaments f20, f21
and the restoring force is mainly tensile in the
filament. Otherwise the manner of construction and
operation is the same as previously described except that
the force being sensed or measured would be in a direction
parallel to the filament and thus normal to the axes of
angular vibration of masses Ml, M2.
In all the embodiments described air damping can
be significant and in some applications the resonant
system will be sealed in a vacuum enclosure.
In all the embodiments described the thickness of
the masses Ml, M2 and the filaments is the same and of
the order of ten microns. It could be anywhere in the
range three to thirty microns. Each of the filaments is
in fact a ribbon i.e. wider than it is thick, anc the
width is about
thirty microns; it could however be anything in the range
ten to a hyndred microns.
The resource frequency of the embodiments
described is around ten KHz, but in other embodiments
(~ 25 could lie anywhere in the range one to twenty K~z.
