Vibrating structure gyroscope

A vibrating structure gyroscope is a type of gyroscope that functions much like the halteres of an insect. The underlying physical principle is that a vibrating object tends to continue vibrating in the same plane as its support rotates. In the engineering literature, this type of device is also known as a Coriolis vibratory gyro because as the plane of oscillation is rotated, the response detected by the transducer results from the Coriolis term in its equations of motion ("Coriolis force").

Vibrating structure gyroscopes are simpler and cheaper than conventional rotating gyroscopes of similar accuracy. Miniaturized devices using this principle are a relatively inexpensive type of attitude indicator.

Theory of operation

Consider two proof masses vibrating in plane (as in the MEMS gyro) at frequency $\scriptstyle \omega _{r}$. Recall that the Coriolis effect induces an acceleration on the proof masses equal to $\scriptstyle a_{c}=-2(v\times \Omega )$, where $\scriptstyle v$ is a velocity and $\scriptstyle \Omega$ is an angular rate of rotation. The in-plane velocity of the proof masses is given by: $\scriptstyle X_{ip}\omega _{r}\cos(\omega _{r}t)$, if the in-plane position is given by $\scriptstyle X_{ip}\sin(\omega _{r}t)$. The out-of-plane motion $\scriptstyle y_{op}$, induced by rotation, is given by:

$y_{op}={\frac {F_{c}}{k_{op}}}={\frac {2m\Omega X_{ip}\omega _{r}\cos(\omega _{r}t)}{k_{op}}}$

where

$\scriptstyle m$ is a mass of the proof mass,

$\scriptstyle k_{op}$ is a spring constant in the out of plane direction,

$\scriptstyle \Omega$ is a magnitude of a rotation vector in the plane of and perpendicular to the driven proof mass motion.

In application to axi-symmetric thin-walled structures like beams and shells, the Coriolis forces cause a precession of vibration pattern about the axis of rotation. For such shells, it causes a slow precession of a standing wave about this axis with an angular rate which differs from input one. It is so-called "wave inertia effect" discovered in 1890 by British scientist George Hartley Bryan (1864–1928).

If we consider a polarization of a shear (transverse) elastic wave propagating along an acoustic axis in a solid, a polarization rotation effect due to rotation of the body as a whole (so-called, "polarization inertia effect") can be observed too (it was noted by Ukrainian scientist Sergii A. Sarapuloff in early 80th, as well as a corresponding modification of Green-Christoffel's tensors in Acoustics).

Implementations

Piezoelectric gyroscope

A piezoelectric material can be induced to vibrate, and lateral motion due to centrifugal force can be measured to produce a signal related to the rate of rotation.

Wine glass resonator

Also called a hemispherical resonator gyro or HRG, a wine glass resonator makes use of a hemisphere driven to resonance, the nodal points of which are measured to detect rotation. Almost a century after George H. Bryan's discovery of the underlying physical phenomenon, Dr. David D. Lynch, et al developed and patented several variants of such gyros for the Space & Launch divisions of Delco Electronics / General Motors Corp., Litton Guidance & Control Systems, and Northrop Grumman Corp. (CA, USA). There are two basic variants of the system, one based on a rate regime of operation and one based on an integrating regime of operation, usually in combination with a controlled parametric excitation. It is possible to use both regimes with the same hardware, which is a feature unique to this type of gyroscope. Engineers and researchers in several countries have been working on the technology.

Coriolis Vibratory Gyroscope (CVG)

This type of gyroscope was developed and patented by Innalabs uses alloys or metal based cylindrical design resonator, which can be designed as an axisymmetrical shape or not, but only axisymmetrical shapes lead to top of the range performances. This breakthrough technology allowed substantially increase product life of the gyroscopes (MTBF > 500,000 hours) and their shock resistance (>300g). The resonator is operated on its second order resonant modes. Standing waves are therefore elliptical shape oscillations with four antinodes and four nodes located circumferentially along the rim, angle between two adjacent antinode – node being 45 deg. One of the elliptical resonant modes is excited to a prescribed amplitude. When the device rotates about its sensitive axis (along its inner stem), the resulting Coriolis forces acting on the resonator’s vibrating mass elements excite the second resonant mode. Angle between major axis of the two modes is 45 deg. A closed-loop drives the second resonant mode to zero and the force required to null this mode is proportional to the input rotation rate. Corresponding control loop system is called force-rebalanced mode. In order to provide forces and to sense induced motions, piezo-electric elements placed on the resonator are used. This electromechanical system is particularly effective and leads to low output noise and large dynamic range as required in case of demanding applications.

Tuning fork gyroscope

This type of gyroscope uses a pair of test masses which are driven to resonance. Their displacement from the plane of oscillation is then measured to produce a signal related to the system's rate of rotation.

F.W. Meredith registered a patent for such a device in 1942 while working at the Royal Aircraft Establishment. Further development was carried out at the RAE in the late 1950s by G.H. Hunt and A.E.W. Hobbs, who demonstrated drift of less than 1 °/h (3.6×10⁻⁴ °/s).

Modern variants of tactical gyros use doubled tuning forks such as those produced by SAGEM Défence Securité / Safran Group (France).

Vibrating wheel gyroscope

A wheel is driven to rotate a fraction of a full turn about its axis. The tilt of the wheel is measured to produce a signal related to the rate of rotation.

MEMS gyroscope

Inexpensive (as of 2010, approximately US$5 per part in quantity) vibrating structure gyroscopes manufactured with MEMS technology have become widely available. These are packaged similarly to other integrated circuits and may provide either analog or digital outputs. In many cases, a single part includes gyroscopic sensors for multiple axes. Some parts incorporate both a gyroscope and an accelerometer, in which case the output has six full degrees of freedom. Panasonic, Robert Bosch GmbH, InvenSense, Seiko Epson, STMicroelectronics, and Analog Devices are major manufacturers.

Internally, MEMS gyroscopes use lithographically constructed versions of one or more of the mechanisms outlined above (tuning forks, vibrating wheels, or resonant solids of various designs).

Applications

Spacecraft orientation

The oscillation can also be induced and controlled in the vibrating structure gyroscope for the positioning of spacecraft such as Cassini-Huygens. These small Hemispherical Resonator Gyroscopes made of quartz glass operate in vacuum. There are also prototypes of Cylindrical Resonator Gyroscopes (CRG) made from high-purity single-crystalline sapphire. They provide accurate 3 axis positioning of the spacecraft and are highly reliable over the years as they don't have any moving parts.

Automotive

Automotive yaw sensors can be built around vibrating structure gyroscopes. These are used to detect error states in yaw compared to a predicted response when connected as an input to electronic stability control systems in conjunction with a steering wheel sensor. Advanced systems could conceivably offer rollover detection based on a second VSG but it is cheaper to add longitudinal and vertical accelerometers to the existing lateral one to this end.

Entertainment

The Nintendo Game Boy Advance game WarioWare: Twisted! uses a piezoelectric gyroscope to detect rotational movement. The Sony SIXAXIS PS3 controller uses a single MEMS gyroscope to measure the sixth axis (yaw). The Nintendo Wii MotionPlus accessory uses multi-axis MEMS gyroscopes provided by InvenSense to augment the motion sensing capabilities of the Wii Remote. The iPhone 4, iPad2, iPod Touch, Nintendo 3DS, Nexus S and HTC Titan also feature gyroscopes.

Photography

Many Image stabilization systems on video and still cameras employ vibrating structure gyroscopes.

Hobbies

Vibrating structure gyroscopes are commonly used in radio-controlled helicopters to help control the helicopter's tail rotor or in radio-controlled airplanes to help keep the tail steady during take-off or (especially with discus launched gliders) hand launch.

Other

The Segway Human Transporter employs a vibrating structure gyroscope made by Silicon Sensing Systems to maintain stability of the operator platform.

External links

• Proceedings of Anniversary Workshop on Solid-State Gyroscopy (19–21 May 2008. Yalta, Ukraine). - Kyiv-Kharkiv. ATS of Ukraine. 2009. - ISBN 978-976-0-25248-6 . See also the next meetings at: International Workshops on Solid-State Gyroscopy [1].
• Silicon Sensing - Case Study: Segway HT
• Apostolyuk V. Theory and Design of Micromechanical Vibratory Gyroscopes
• Prandi L., Antonello R., Oboe R., and Biganzoli F. Automatic Mode-Matching in MEMS Vibrating Gyroscopes Using Extremum Seeking Control //IEEE Transactions on Industrial Electronics. 2009. Vol.56. - P.3880-3891.. [2]
• Prandi L., Antonello R., Oboe R., Caminada C., and Biganzoli F. Open-Loop Compensation of the Quadrature Error in MEMS Vibrating Gyroscopes //Proceedings of 35th Annual Conference of the IEEE Industrial Electronics Society - IECON-2009. 2009. [3]