Figure 1. A fibre optic gyroscope.
L'Agence patronne la mise au point d'un gyroscope à fibres optiques et d'un gyroscope vibrant à haute fiabilit‚ pour l'usage spatial. Le gyroscope vibrant, peu co–teux mais de performances réduites, est destiné aux situations d'urgence ou à certaines phases critiques d'une mission. Le gyroscope à fibres optiques offre en revanche des performances élevées et satisfait à des exigences de pointage trŠs rigoureuses pour un coûon;t inférieur à celui d'un dispositif mécanique classique.
The measurement of the angular motion of a satellite in space is an essential for the control and stabilisation of its attitude. Systems employing a gyroscope supplemented by electronics provide the most direct method for sensing inertial angular velocity. Indeed they are the only means of obtaining a rate- signal whose quality is sufficient for the high pointing and stabilisation requirements of current and future ESA and European satellites. However, for satellite attitude acquisition, failure detection and operational back-up modes, low performance gyroscopes will suffice if their cost is low.
European spacecraft have traditionally used mechanical gyroscopes with spinning rotors. Although very many of these systems have been flown successfully, a significant number have suffered partial or total failure, with serious degradation of pointing performance or loss of mission [Preparing for the Future Vol.4, No.2, June 1994].
In the recent past months gyro system malfunctions on the satellites SPOT-3, SAX- BEPPO and ERS-2 have again highlighted the problem of in- orbit failures and the urgent need for accelerated development of more reliable space gyroscope systems in Europe.
Fibre optic gyros (FOG) and solid state gyros have long been used for terrestrial applications. Both systems are extremely robust and ESA has now started development of these devices for use in space with emphasis on high reliability and cost effectiveness. Developments follow two complementary directions. The first is a high performance FOG, under development by the EuroFog consortium (Photonetics and SFIM), which meets demanding pointing requirements, but is cheaper than a conventional mechanical gyro. The second is a low cost solid-state gyroscope, under development by Litef, which offers reduced performance suitable for emergencies or certain critical phases of a mission.
A fibre optic gyro (FOG) (Figure 1) is a purely opto- electronic device.
Figure 1. A fibre optic gyroscope.
For that reason it is of great interest to some high performance missions which cannot tolerate any mechanical disturbance. A few experimental FOG devices have recently been flown on American spacecraft. In a FOG, light is fed simultaneously into both ends of a long fibre optic coil (Figure 2).
Figure 2. The main elements
of a fibre optic gyroscope.
When the coil rotates about its center point, the counterrotating beams travel slightly different distances before they reach the detector, and their phases differ in direct proportion to the input rate. This is the Sagnac effect, first discovered in 1913 and also used in the ring laser gyro.The resulting interference, measured as a variation in power output, gives the input angular rate. The baseline high-performance gyro uses an erbium-doped fibre source with output in the 1550 nm window, and has a coil length of approximately 1000 metres.
The purpose of the contract awarded to Eurofog is to modify, as necessary, the electro-optical portions of FOG originally developed for non-space use, to enable it to withstand conditions encountered in launch and in space, especially space radiation. Components and one full gyro optical head will undergo radiation tests and vibration tests. The adaptation of the electronics for use in space will be part of a follow-up programme. Target specifications for space FOG are shown in Table 1.
Table 1.
| . | FOG | CVG | Unit |
| Bias stability | 0.005 | 5.0 | deg/hour |
| Random walk | 0.001 | 0.5 | deg/root(hour) |
| Scale factor tolerance | 50 | 10000 | ppm |
| Volume | 2 | 0.1 | liter |
| Mass | 0.2 | 0.1 | kilo |
| Power consumption | 12 | 0.2 | Watt |
| Temperature range | 0 ... 50 | -20 ... +80 | deg. C |
The Coriolis force experienced by an object moving with a velocity relative to a rotating reference frame can be used to sense this angular motion. This force is proportional to the relative velocity and to the angular rate and is perpendicular to both. This principle was demonstrated in the 19th century by Foucault with his pendulum and by Bryan with his ringing wine- glass experiment. Gyroscopes employing this principle are called Coriolis vibrating gyroscopes or CVGs.
Figure 3. A solid state gyroscope.
CVGs have three features in common: a mechanical resonator, an oscillator and a force-displacement sensor. One mode of the resonator is maintained in resonance by an oscillator loop while oscillations are induced in a second vibration mode. The amplitude of the oscillations gives a signal proportional to the input angular rate.
A wide variety of resonators, actuation and sensing schemes are possible. The simplest resonator is a tuning fork. CVGs can be made for a wide variety of applications, in space and elsewhere, and they make interesting replacements for conventional gyroscopes used on satellites. While CVGs have been used for many years in industrial and military applications, demand from the automobile market is now driving the development of low-cost, mass production methods using the latest micro-fabrication technology. The performance of this devices is not sufficient for many space applications and the space market is not large enough to pioneer new directions in their technology. Nevertheless existing devices appear to be useful for a limited range of space applications such as attitude control safe- mode, failure detection and pointing stabilisation.
ESA has awarded a contract to Litef for the development of a micro-CVG for space applications. In a first step, a selected commercial product will be tested thoroughly to identify further developments and prepare a space qualification program. The target performance of a micro- CVG is shown in Table 1.
Preparing for the Future Vol. 8 No. 1