Prototypes of a piezo-electric vibratory angular rate transducer (gyroscope) (PVG) have been constructed and evaluated. The construction is on the lines suggested by Burdess. The sensitive element is a cylinder of radially poled piezo-electric ceramic. The cylinder is metallized inside and out, and the outer metallization is divided into eight electrodes. The metallization on the inside is earthed. A phase locked loop, using pairs of the electrodes, causes the cylinder to vibrate in one of its two fundamental, degenerate modes. In the presence of rotation, some of the vibration is coupled into the outer mode. This can be detected, or suppressed with a closed-up technique and provides a measure of rotation rate. The gyroscope provides a number of advantages over rotating mass and optical instruments: low size and mass, lower power consumption, potentially high reliability, potentially good dormancy, low cost and high maximum rate

Nuttall, J. D.

English

NASA Technical Reports Server

N94- 29790
GEC FERRANTI PIEZO VIBRATORY GYROSCOPE
J. D. NUTTALL
GEC FERRANTI, Silverknowes, Edinburgh, EH4 4AD Scotland
Summary
Prototypes of a piezo-electric vibratory angular rate transducer (gyroscope)
(PVG) have been constructed and evaluated.
The construction is on the lines suggested by Burdess (reference 4). The
sensitive element is a cylinder of radially poled piezo-electric ceramic. The
cylinder is metallised inside and out, and the outer metallisation is divided into
eight electrodes. The metallisation on the inside is earthed.
A phase locked loop, using two pairs of the electrodes, causes the cylinder to
vibrate in one of its two fundamental, degenerate modes. In the presence of
rotation, some of the vibration is coupled into the other mode. This can be
detected, or suppressed with a closed-up technique and provides a measure of
rotation rate.
Thy gyroscope provides a number of advantages over rotating mass and optical
instruments: low size and mass, lower power consumption, potentially high
reliability, potentially good dormancy, low cost and high maximum rate.
1. Introduction
The measurement of angular rate (relative to inertial space) is often a significant
problem in spacecraft, (and other) system's development. Gyroscopes (angular
rate sensors) are perceived as large, heavy, expensive, unreliable or some
combination of these. Although the perception is not well-justified, the moves
towards lighter space craft suggest that gyroscope technology is ripe for
change.
There is a sizeable market (not confined to space craft) for gyros capable of
measuring high angular rates, with a null accuracy of around a degree per
second, and a scale factor accuracy of around one percent. Such instruments
have to be cheap, small and robust. The GEC-Ferranti Piezo-Vibratory Gyro
(PVG) is being developed for such a market. The target specification is
reproduced below.
265
https://ntrs.nasa.gov/search.jsp?R=19940025286 2020-06-16T14:20:08+00:00Z
Parameter
Full scale rate
Full scale output
Over-Range
Excitation
Power Consumption
Null
Temperature Range
S/O to S/O Re_eatabi=iity
Hysteresis
Resolution
Threshold
Line&rity _
Sca[e-F-a_or ',_
Start=-upTime
Bandwidth
Noise
Size (in single-axis pack)
Size of three axis pack
tt _
Design Aim
_-L_100deg/sec to + 5000 deg/sec*
..+..10 volts d.c.
25,000 deg/sec
> = == =
+ 15 Volts d.c.
1 Watt/axis
+ 1 deg/sec after temperature consumption
_ 40degt 0+80deg.C
<0.0S deg/sec
<0.05 de_g/sec
<0.05 degJse_
<0.05 deg./Sec
:0.2S % of%ll Scale "
compensation
100 mS - _........ :
25Hz to 200Hz*
<0.04 [deg./sec]/,,/Hz
25mm_x 25mm long
40mm cube max.
*Adjustable by change to electronics only.
Additionally, for a gyro of this type, low cost, high reliability, long operating time,
and the capability of working in severe environment, are regarded as Of
particular importance.
2. Choice of technology
T_aditionaiiy, iOtation Sensors have _=_een based--on Spinning wheelS, but
gyroscopes using either optical techniques and/or vibrating masses have
become practical in recent years .......
Existing spinning-mass gyros are unsuitable for this specification because of:-
(a) Cost: they are either expensive, or cost-cutting measures have
unacceptable effects On lffetime or perf0rtnanc&-_: _ =
=
(b) Maximum:rate: usually spinning-mass instruments cannot cope with
more than (say) 100 degrees/second, and/or there is-c0nSiderable
increase in heat dissipation with rate.
Optical gyros are unlikely to meet the requirement because oi both cost and
size, although they are expected to do well in more demanding applications
(e.g. better than 10 degrees/hour).
266
This leaves vibratory gyros, which have considerable potential, especially in
terms of cost, size, weight, maximum rate, lifetime, bandwidth, power
consumption and reliability.
3. Design Considerations in Vibratory Gyroscopes
There have been a number of attempts to develop vibratory gyros, dating back
over the last thirty years. Examples include the Sperry "Gyrotron', RAE's
experimental tuning-fork gyro, GE's vibrating beam gyro, and Honeywell's
vibrating wire-gyro. The major problems with these devices, as far as we can
tell, have been associated with the need to get the two resonant frequencies
near identical, the need to prevent the vibration leaking away through the
support, and various problems associated with the piezo-electric transducers
which were bonded to metal vibrators.
The motion of any vibrating body is, in general, affected by the presence of
rotation, through the Coriolis coupling [Reference 1]. The Coriolis effect
depends on rotation rate, and therefore, in principle, any vibrating body can be
used to detect rotation, i.e. as the basis of a gyroscope.
As an example, for educational purposes only, consider the vibrating
cantilevered beam shown in Figure 1. The effect of the rotation is to introduce
an out-of-plane vibration, as shown. This induced ['secondary'] vibration is at
the same frequency as the primary vibration. Thus, to build such a gyro one
would need a method of sustaining the primary vibration of the beam, and of
detecting its out-of-plane [secondary] motion which is a direct measure of
rotation rate. Both of these requirements can be met by pairs of piezo-electric
transducers: one pair arranged to bend the beam and one pair to detect the
strain in the other plane.
It is clear that to get a primary vibration of reasonable amplitude from the rather
weak piezo-electric effect, one needs to make use of a mechanical resonance
in the primary direction. To avoid difficulties with the resonant frequency
variations [from component manufacturing tolerances or temperature, for
example], it is necessary to incorporate the beam itself into the oscillator as the
frequency-determining component. Similarly, it is clear that the secondary must
be operated on resonance, and thus ideally one should have the two
resonances at the same frequency. In practice, the frequency split between the
two resonances needs to be rather smaller than the width of the resonances.
As the mechanical resonator is the frequency-determining part of the oscillator,
it is desirable to have a resonator with a high Q factor. This situation gives a
rapid change of phase with frequency, and thus a stable oscillator with little
phase noise. Of course, to get a high Q factor, it is necessary to minimise the
losses: in this case the power lost as vibration leaking out of the gyro. [A further
reason for minimising the energy loss from the gyro is to reduce the risk of the
cross-talk between the gyros].
267
In the educational example given above, one would expect therefore a gyro
with that particular configuration would be unsuccessful. However, there are
several better possibilities, including tuning forks, H-shapes and cylinder.
. Description of the GEC-Ferranti Piezo-Electric
Vibfatory_ Gyro ...._......
GEC-Ferranti at Silverknowes, EdinbUrgh in Scotland, has been developing a
vibratory gyro for around 3 years. The initial phase included a study of possible
configurations and a tolerancing study.
.... = -: : z:" _i ;
An early decision was to avoid problems with bonding piezo-electric
transducers to a metal resonator, by making the resonator itself from piezo-
electric material, and by plating metal electrodes onto it. For each configuration
considered, the possible materials and the directions of the piezo-eiectric
properties, were considered. The outcome of this study was the decision to
work primarily on a cylindrical c0nfigurationl withone open end and one closed
end. The support was to be at the node at the centre of the closed end. As the
cylinder's piezo-electric properties have to be cylindrically symmetric materi_a/s
which are naturally piez0'-e_c cannot be=0sedl - instead, :we have-been
working with the sintered piezo-electric ceramic, which can be radially poled.
[The material is sold under various names including "PZT"]
Once the ceramic cylinder has been ground to the correct dimensions, it is
plated with metal on both the inside and outside, The outside plating is divided
up into eight electrodes, which cover virtually all the area, but Which are divided
by small earthed strips. The metal on the inside is earthed. Figure 2[a]
illustrates the configuration. '
r .... i
Several authors have described how such a Configuration can be used asa
gyro [reference 2]. A cylinder has, of course, a large number of natural modes,
but most of these are unsuitable for use in the gyro. Figure 2[b] illustrates the
primary mode excited, and the secondary mode detected, in the GEC-Ferranti
gyro. -_ _ :
Before we started work on a cylinder gyro, Burdess had already patented
[Reference 3] and published [Reference 4] the idea of the cylindrical vibratory
gyroscope in piezo-electric ceramic and the ownership of the patent was with
the British Technology Group [BTG]. GEC-Ferranti has made a licensing
arrangement with BTG.
This cylinder gyroscope has much in common with the one described by Harris
[Reference 5]. The major difference between that device and the Ferranti one is
that in the former one the piezo-electric transducers are bonded onto the
resonator, but in the GEC-Ferranti device the piezo-electric material is the
resonator, ': ,
268
There are a number of advantages with the GEC-Ferranti arrangement, mainly
to do with its simplicity, and hence its reliability. If a metal cylinder is used,
piezo-electric elements have to be bonded onto the cylinder, and wires taken
from them, near the point of maximum vibration. By contrast, there is no
bonding to the vibrating part of the piezo-electric cylinder: the electrical
connections can be made near the node.
Further advantages come from the fact that, in the piezo-electric cylinder, the
electrodes are positioned by dividing up the plating, and this position can be
trimmed later by removing more metal. This process is potentially more
accurate than ones involving bonding the piezo-electric elements in place with
adhesives.
The motion of the cylinder is too small and too fast to be seen by normal
techniques, but it can be investigated by interferometric holography [Reference
6]. Such a technique gives a photograph of a fringe pattern, with the fringes
being contours of equal vibration amplitude. Figure 3 is a line drawing traces
from such an interferogram, showing the vibration node, and antinodes of
around 1200 nm peak-to-peak.
5. Eleqtronics
A block diagram of the gyro electronics is given in Figure 4. It can be divided
into two areas
(a)
(b)
Primary drive circuit
Secondary detection and output circuit
The primary drive circuit is required to establish and sustain the cylinder's
fundamental mode of vibration. This is achieved by using the resonant
characteristics of the cylinder as the frequency selective parameters of a phase-
locked, amplitude-stabilised oscillator. This configuration ensures fundamental-
mode operation over a wide temperature range, since the cylinder's
temperature-sensitive parameters are automatically compensated for.
As the rotation-induced secondary vibration is in phase with the primary
vibration, it is possible simply to demodulate the output from any secondary
electrode pair, with respect to the primary vibration, and to use this as the gyro
output. However this 'open-loop' detection system is inherently non-linear.
To give an output which is linear with applied rate, a closed-loop secondary
system is used. This system acts as a null-seeking servo, which suppresses the
secondary vibration by applying a feedback-derived voltage to the secondary
drive electrodes. This voltage is a linear function of applied rate.
6. Some results obtained with orototyoes
To date, prototype PVGs have been characterised over + 1000 degrees/second
[scale factor of 10mV/[degree/second]], or over + 100 degree/second [scale
269
factor 100mY [degree/second]]. Examples of the results of rate tests are given
in Figures 5 and 6. in each case the upper figure gives the output voltage asa
function of input rate, and the lower figure the deviation of the data from the best
straight line. The lower plots are over the range +0.5% of full rate. For the +100
degree/second data, the worst deviation is 0.1% full scale, and the errors seem
randomly distributed. For .+_1000 degree/second data, the worst deviation is
0.3% full scale. There is a systematic element to this error, and we believe we
understand the nature of the constructional inaccuracies which lead to this.
Even so, the r.m.s, non-iinearity is only 0.15% full scale.
Figure 7 shows the in-run drift of the gyro on a static overnight run. A simple
model has been used to eliminate temperature effects. The data all lies within
+0.02 o/sec (_+..72C/hour) : _ _: _
Figure 8 shows the variation of scale factor with temperature. Although the
actual variation is rather large, the effect has good linearity, and there is every
prospect that the temperature effect can be cancelled out.
Reference_
i 'Dynamics' by F.P. Beer and EIR. Johnston
McGraw-Hill, pp 6_2!!6.I2 .......
2. C.H.J. Fox
DGON Gyro Symposium 1988, Paper 5
,
4.
UK Patent BG 2 154 739 B
5
6.
J.S. Burdess, Proc I Mech E Vol 200 (C4), pp 271-280 and UK Patent GB
2154739B
D_G.- Harris, DGON Gyro Symposium lg88, Paper 6
R.J. Parker and D.G. Jones, Optical Engineering Vol 27 [1], January 1988
270 _r_
/_SEN_13V_E AX;S OF"
./ GYROSCOPE
r-7
L.-J
ROTA110N- INDUCED
'_IBRAnON
Fiquse 1
A hypothetical vibrating-beam gyroscope illustrating the principles of
all vibratory gyros. The beam is caused to vibrate at its resonant
frequency. The figure shows two views of the system with the solid and
hatched beams representing the two extremes of the vibration. Rotation
about the axis shown induces an out-of-plane vibration as indicated in the
diagram.
271
Fi.Eigu_re___
The GEC-Ferranli Piezo-Electric Vibratory Gyro, showing the vibrating
cylinder, made from piezo-electric material, and also the plated
electrodes whose position is denoted by the hatched areas.
The modes in a cylinder gyroscope. The primary mode comprises
oscillation between the two solid ellipses. The secondary [rotation
induced] mode is an oscillation between the two dashed ellipses.
272
Laserinterferogramof the vibratorycylinder. The fringes are lines of
constant amplitude. [This is a line drawing produced from the
interferogram].
273
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275
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Results of a scale factor temperature sensitivity test on a
GEC Ferranti Piezo Vibratory Gyro
278


GEC Ferranti piezo vibratory gyroscope

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