Advanced Sensor Technology Lecture 11 Jun. QIAN

A Review of Lecture 10 Functional components of Human nose An array of fast-response sensors and pattern-matching algorithm with a odor library Classical Techs: high precision yet big size Infrared spectroscopy Mass spectroscopy Chromatography Recent Techs: can be arrayed to certain extent Modified FET: ISFET Fiber Optical Chem Sensor Chemresistor

Basic Intent Overview the basic issues related to the detection of rotation. There are several approaches, with advantages and disadvantages. The goal of this overview is to become familiar with the issues and the relative characteristics of some gyros on the market today.

Gyroscope: types Mechanical Spinning (disk) mass Vibrational Laser-based, two main types of optical rotation sensors ring laser gyroscope (RLG), (most sensitive!) fiber optics gyroscope

A surprise! As forces are applied to the axle, the two points identified will attempt to move in the indicated directions. As the two points rotate, they continue their motion.

Gyro Equations Newton’s equation for gyro: SMx = Ixq" - Iy(f')2cosqsinq + Izf'sinq(f'cosq + y') SMy = Iy(f'q'cosq + f"sinq) - Izq'(f'cosq + y') + Ixf'q'cosq SMz = Iz(- f'q'sinq + f"cosq + y") - Ixf'q'sinq + Iyf'q'sinq

Mechanical/rotational: A special solution Set q' = 0 f" = 0 y" = 0 cos(900) = 0 sin(900) = 1 mgR = Izzf'y‘

Basic Gyro As viewed from an inertial reference frame, the angular momentum vector (aligned with the spin axis of the disk) will stay oriented in the same direction under most circumstances As viewed from the frame of the instrument, an applied rotation will lead to a torque on the spinning disk. If the torque is about the gimbal axis, the gimbal will rotate about this axis, allowing the orientation of the spinning disk to remain fixed in inertial frame. The main advantage of this approach is that a sophisticated mechanical design with low-friction bearings can achieve very good accuracy.

Measurement of rotation with an accelerometer An example The resolution of the ADXL05 is 0.5 mg/sqrt(Hz). If we set this equal to 2r, we have The resolution in this sensor : acceleration of 0.5 mg/sqrt(Hz), which is equivalent to a rotation error of 0.7 rad/sec*sqrt(Hz). An ADXL05, mounted 1 cm off axis.

Coriolis Force If you walk outwards from the center of a spinning merry-go-round (spinning counter-clockwise as viewed from above), there will be a force pushing you to the right which increases as you move away from the center. Since the earth is a rotating reference frame, this force exerts subtle forces to the right or left, depending on the hemisphere you happen to be in. This small force is the determinant factor in picking the direction of rotation of water in sink. dR/dt Coriolis Force

Mechanical: Coriolis Force Gyros Pick up Vibration A beam is driven into a lateral oscillation to move back and forth in a plane parallel with the paper surface of the drawing. When this oscillation is taking place, the points along the beam have a velocity vector which is in the plane of the drawing, but oscillating back and forth within that plane. Because of the Coriolis force, the beam will begin to oscillate in the direction normal to the plane of the drawing. If the resonant frequency of this beam for in-plane and out-of plane oscillation is the same, this oscillation will gradually build up an amplitude that is proportional to the rotation rate and inversely proportional to the damping rate in the beam. This is how a resonant coriolis gyro works. Measurement of the out-of-plane oscillation amplitude is used to measure the rotation rate of the platform. Coriolis F

Mechanical: Torsional resonator Gyro chip First made in Charles Stark Draper Labs, MIT A 2-layer torsional resonator is constructed using silicon micromachining techniques. The outer resonator is driven by electrostatic forces. The result of this torsional oscillation is to give the electroplated metal mass a velocity vector perpendicular to the torsional support beam. Then, if this device is rotated about the input axis, there will be a rotation vector and a velocity vector which are occasionally perpendicular to one another. The result of this situation is an oscillating force about the internal torsion element of this structure. If the designers and manufacturers are careful, the two torsional resonances match, and the internal resonator gradually builds up an oscillation amplitude that can be measured by capacitive sensing. V Input Axis:  Output axis Driven Oscillation 0.3x0.6 mm Gimbal structure No need of bearings Capacitive sensing

BEI GYROCHIP® Model QRS11 Micromachined Angular Rate Sensor A MEMS technology, solid-state "gyro on a chip." This DC input/output device, extremely small and lightweight. SPECIFICATIONS Standard Ranges  ±50, 100, 200, 500, 1000°/sec Threshold/Resolution 0.004°/sec Input Voltage   + and - 5 Vdc ±3% regulation Output Noise ( DC to 100 Hz)   0.01°/sec/Root(Hz)

Torsional gyro 1: Principle of Operation The GyroChip® uses vibrating quartz tuning tines to sense rate, acting as a Coriolis sensor, coupled to a similar fork as a pickup to produce the rate output signal. Each comprised of a pair of tuning forks, the GyroChip along with their support flexures and frames are fabricated from piezoelectric quartz The piezoelectric drive tines are driven by an oscillator to vibrate at a precise amplitude, causing the tines to move toward and away from one another at a high frequency. For vibrating tines ("arms"), an applied rotation rate causes a sine wave of torque to be produced, in turn causing the tines of the pickup fork to move up and down (not toward and away from one another) out of the plane of the fork assembly, causing electrical output signals to be produced by the Pickup Amplifier.

Torsional gyro 2: JPL’s design A cylindrical metal post is suspended by a flexural support structure Electrostatically drive 1 and 3 of the structure, inducing a torsional oscillation of the post between the 1 and 3 corners of the structure. When the structure is rotated about the axis perpendicular to the support structure, the Coriolis force induces some force into an oscillation in the 2-4 direction of this structure. By measuring the amplitude and phase of oscillation in the 2-4 direction, it is possible to determine the rotation rate. The main advantage of this design over the Draper design is that the mechanical symmetry of the structure makes it much easier to match the two resonant modes of the flexure. The main disadvantage of this design is common with the Draper design - the need for a tall metal structure to produce a measureable force. We should expect a significant research effort in these flexural Coriolis gyros in the coming years. These designs are easily miniaturized.

Uses for gyroscopes Science demo Computer pointing device Gyrocompasses Virtual Reality

Commercial example: Watson Industries, Inc.

Laser Gyros Laser gyroscopes have number of advantages over more conventional gyros such as : no moving parts simple design, generally less than 20 component parts very rugged, vibration g and g2 insensitive wide dynamic range (>109 ) output is inherently digital and TTL compatible fast update rate - less than 50ms to measure a rotation of 0.5o/hour long and reliable lifetime (>30,000 hours) low total cost of ownership . "fit and forget", no maintenance

Sagnac Effect

Sagnac Effect (cont.)

Ring Laser Gyro Light leaving the point P on the triangular path, which is rotating about O with angular velocity , and making one complete transit of the triangle. The time for the transit t = 3L / c =S/c where L is the side length When the gyro begins to rotate, however, during this time point P moves a distance d = ( r)t= ( r)S/c

Ring Laser Gyro (cont.) d = ( r)t= ( r)S/c path difference: S=dcos(60)=d/2 =3  L2/2c Area of the triangle: 3 L2/4, S=2A/c Beat freq. f=(2c/)S/S =4 A/S Example: calculate the frequency shift corresponding to a rotation rate of 0.1/ hr in a triangle with 0.1 m side length assuming  = 632.8 nm. 0.1/hr = 4.85 x 10- 7 rad s -1 The area of the triangle is 4.33x10-3 m2 f = 0.044 Hz.

Performance Limitations In principle laser gyros should be very sensitive and accurate with a fundamental limit of less than 10-6 degrees per hour. In practice the performance is less than this, the limits being set by the accuracy of fabrication, cleanliness and a few inherent operational difficulties. As mentioned above, the optical ring is in fact a resonator and steps must be taken to ensure that the ring is always exactly in resonance. This is important not only to maintain the lasing action but also to accurately stabilize the lasing frequency to maintain the long-term gyro performance. mounting one of the mirrors on a piezo-electric crystal; this enables small movements of the mirror to be made by voltage inputs (movements of 0.01m are required). The whole system can be thought of and treated as a servo-loop. the output frequency of fringes is linearly proportional to the angular rotation rate. We shall discuss only 2 main deviations

Performance Limitations - freq. bias The output of the RLG has remained linear but that the output is shifted up or down the vertical axis. Hence, even when the RLG is not rotating, a fringe count is acquired. This results from a number of mechanisms : Fresnel drag, forward scattering and, the predominant effect, Langmuir flow of the laser gas: ~ 1000/hr

Performance Limitations – “lock-in” A dead band: lock-in This occurs when the rotation rate becomes very small: the frequency difference between the clockwise and CCW beams being very small. The effect is due to interaction effects between the two counter-rotating beams, when on reflection, a small amount of energy is scattered from the mirror surface back into the oppositely travelling beam. If this difference becomes too small the counter propagating beams will lock together in the same way that coupled mechanical oscillators operating at slightly different frequencies lock together How solve this problem: mechanical dither, magnetic mirror

RLG: An Example Scale factor 0.79 period/arcs (Hz/deg/h) Zero shift, < 0.6 deg/h Zero drift stability with algorithmic correction < 0.01 deg/h Alignment time 2 min Mean lifetime 10,000 h Volume 2.5 dm3 Operating temperature range -20 ... +50 °C Power supply= 24 V Dither supply~ 10V, 77Hz Total power consumption10 ... 12 W

Fiber Optic Gyro The light from the laser is split and the two parts traverse a coiled optical fibre in opposite directions.

Fiber Optic Gyro (cont.)

Working Principle Virtually no moving parts and no inertial resistance to movement. It consists of a coil of as much as 5 km of optical fiber and uses the interference of light to detect mechanical rotation Extremely precise rotational rate information, due to its lack of cross-axis sensitivity to vibration, acceleration, and shock. Stabilized right after start up (does not require starting calibration), Small energy consumption

FOG with phase modulation

Closed-Loop Configuration

Set m=1.8 to max J(x):

Fiber Optic Gyro (cont.) The fiber coils may comprise several hundred turns and have a diameter of 0.1-0.2 m e.g. 200 turns, 0.1m: = corresponds to 738800 deg/hr The IMU 200 incorporates three FOG 200s and three accelerometers for highly accurate inertial sensing, achieving 0.1 deg/hr in-run drift stability. A version of this IMU is used as part of the National Missile Defense System. Northrop Grumman IMU 600 Particularly well suited for tactical missile and torpedo guidance/navigation

Low-cost IMU (inertial measurement unit)

What’s inside?

We are not alone in this business……35yrs experience! CalTech

Summary Two types of gyro: mechanical & laser based Measurement Rate of rotation Rotation angle Mechanical rotatary Coriolis force sensing torsional Laser-based Ring laser gyro (RLG) Fiberoptic gyro