Sensors

SENSORS

A sensor is a device that measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument. For example, a mercury-in-glass thermometer converts the measured temperature into expansion and contraction of a liquid which can be read on a calibrated glass tube. A thermocouple converts temperature to an output voltage which can be read by a voltmeter. For accuracy, most sensors are calibrated against known standards.

1.  GYROSCOPE

A gyroscope is a device for measuring or maintaining orientation, based on the principles of conservation of angular momentum.

The first commercially available surface-micro-machined angular rate sensors with integrated electronics, they are smaller—with lower power consumption, and better immunity to shock and vibration—than any gyros having comparable functionality. This genuine breakthrough is possible only because of the Analog Devices proprietary integrated micro electro-mechanical system (iMEMS) process, proven by use in millions of automotive accelerometers.

PRODUCT DESCRIPTION

Gyroscopes are used to measure angular rate—how quickly an object turns. The rotation is typically measured in reference to one of three axes: yaw, pitch, or roll.

Figure shows a diagram representing each axis of sensitivity relative to a package mounted to a flat surface. A gyroscope with one axis of sensitivity can also be used to measure other axes by mounting the gyro differently, as shown in the right-hand diagram. Here, a yaw-axis gyro, such as the ADXRS150 or ADXRS300, is mounted on its side so that the yaw axis becomes the roll axis.

Axis of sensitivity

Depending on how a gyro normally sits, its primary axis of sensitivity can be one of the three axes of motion: yaw, pitch, or roll. The ADXRS150 and ADXRS300 are yaw-axis gyros, but they can measure rotation about other axes by appropriate mounting orientation. For example, at the right: a yaw-axis device is positioned to measure roll.

As an example of how a gyro could be used, a yaw-axis gyro mounted on a turntable rotating at 33 1/3 rpm (revolutions per minute) would measure a constant rotation of 360° times 33 1/3 rpm divided by 60 seconds, or 200°/s. The gyro would output a voltage proportional to the angular rate, as determined by its sensitivity, measured in millivolts per degree per second (mV/°/s). The full-scale voltage determines how much angular rate can be measured, so in the example of the turntable, a gyro would need to have a full-scale voltage corresponding to at least 200°/s. Full-scale is limited by the available voltage swing divided by the sensitivity. One practical application is to measure how quickly a car turns by mounting a gyro inside the vehicle; if the gyro senses that the car is spinning out of control, differential braking engages to bring it back into control. The angular rate can also be integrated over time to determine angular position—particularly useful for maintaining continuity of GPS-based navigation when the satellite signal is lost for short periods of time.

CORIOLIS ACCELERATION

Analog Devices’ ADXRS gyros measure angular rate by means of Coriolis acceleration. The Coriolis Effect can be explained as follows, starting with Figure 2. Consider yourself standing on a rotating platform, near the center. Your speed relative to the ground is shown as the blue arrow lengths in Figure 2. If you were to move to a point near the outer edge of the platform, your speed would increase relative to the ground, as indicated by the longer blue arrow. The rate of increase of your tangential speed, caused by your radial velocity, is the Coriolis acceleration.

If Ω is the angular rate and r the radius, the tangential velocity is Ωr. So, if r changes at speed, v, there will be a tangential acceleration Ωv. This is half of the Coriolis acceleration. There is another half from changing the direction of the radial velocity giving a total of 2Ωv (see the Appendix). If you have mass, M, the platform must apply a force, 2MΩv, to cause that acceleration, and the mass experiences a corresponding reaction force.

Coriolis acceleration

Figure shows Coriolis acceleration example. A person moving northward toward the outer edge of a rotating platform must increase the westward speed component (blue arrows) to maintain a northbound course. The acceleration required is the Coriolis acceleration. The ADXRS gyros take advantage of this effect by using a resonating mass analogous to the person moving out and in on a rotating platform. The mass is micromachined from polysilicon and is tethered to a polysilicon frame so that it can resonate only along one direction.

Coriolis effect on resonating mass

Figure shows that when the resonating mass moves toward the outer edge of the rotation, it is accelerated to the right and exerts on the frame a reaction force to the left. When it moves toward the center of the rotation, it exerts a force to the right, as indicated by the orange arrows.

To measure the Coriolis acceleration, the frame containing the resonating mass is tethered to the substrate by springs at 90° relative to the resonating motion, as shown in Figure 17. This figure also shows the

Coriolis sense fingers that are used to capacitively sense displacement of the frame in response to the force exerted by the mass, as described further on. If the springs have stiffness, K, then the displacement resulting from the reaction force will be 2 ΩvM/K.

Schematic of gyro mechanical sensor

It shows the complete structure, demonstrates that as the resonating mass moves, and as the surface to which the gyro is mounted rotates the mass and its frame experience the Coriolis acceleration and is translated 90° from the vibratory movement. As the rate of rotation increases, so does the displacement of the mass and the signal derived from the corresponding capacitance change.It should be noted that the gyro may be placed anywhere on the rotating object and at any angle, so long as its sensing axis is parallel to the axis of rotation. The above explanation is intended to give an intuitive sense of the function and has been simplified by the placement of the gyro.

CAPACITIVE SENSING

ADXRS gyros measure the displacement of the resonating mass and its frame due to the Coriolis effect through capacitive sensing elements attached to the resonator, as shown in Figures 4, 5, and 6. These elements are silicon beams inter-digitated with two sets of stationary silicon beams attached to the substrate, thus forming two nominally equal capacitors. Displacement due to angular rate induces a differential capacitance in this system. If the total capacitance is C and the spacing of the beams is g, then the differential capacitance is 2 ΩvMC/gK, and isdirectly proportional to the angular rate. The fidelity of this relationship is excellent in practice, with nonlinearity less than 0.1%.

The ADXRS gyro electronics can resolve capacitance changes as small as 12 × 10–21 farads (12 zeptofarads) from beam deflections as small as 0.00016 Angstroms (16 femtometers). The only way this can be utilized in a practical device is by situating the electronics, including amplifiers and filters, on the same die as the mechanical sensor. The differential signal alternates at the resonator frequency and can be extracted from the noise by correlation. 

Capacitive sensors with resonating mass

The frame and resonating mass are displaced laterally in response to the Coriolis effect. The displacement is determined from the change in capacitance between the Coriolis sense fingers on the frame and those attached to the substrate.

2.  MAGNETOMETER

A magnetometer is a scientific instrument used to measure the strength or direction of the magnetic field, either produced in the laboratory or existing in nature. The Earth's magnetic field (the magnetosphere) varies from place to place, for various reasons such as inhomogeneity of rocks and the interaction between charged particles from the Sun and the magnetosphere. Magnetometers are a frequent component instrument on spacecraft that explore planets.

USES

Magnetometers are used in ground-based electromagnetic geophysical surveys (such as magnetotellurics and magnetic surveys) to assist with detecting mineralization and corresponding geological structures. Airborne geophysical surveys use magnetometers that can detect magnetic field variations caused by mineralization, using airplanes like the Shrike Commander.[1] Magnetometers are also used to detect archaeological sites, shipwrecks and other buried or submerged objects, and in metal detectors to detect metal objects, such as guns in security screening. Magnetic anomaly detectors detect submarines for military purposes.

They are used in directional drilling for oil or gas to detect the azimuth of the drilling tools near the drill bit. They are most often paired up with accelerometers in drilling tools so that both the inclination and azimuth of the drill bit can be found.Magnetometers are very sensitive, and can give an indication of possible auroral activity before one can see the light from the aurora. A grid of magnetometers around the world constantly measures the effect of the solar wind on the Earth's magnetic field, which is published on the K-index.

A three-axis fluxgate magnetometer was part of the Mariner 2 and Mariner 10 missions.[3] A dual technique magnetometer is part of the Cassini-Huygens mission to explore Saturn.[4] This system is composed of a vector helium and fluxgate magnetometers.[5] Magnetometers are also a component instrument on the Mercury MESSENGER mission. A magnetometer can also be used by satellites like GOES to measure both the magnitude and direction of a planet's or moon's magnetic field.

FEATURES

·         Precision 3-axis Capability.

·         Factory Calibrated Analog Outputs.

·         40 micro-gauss to 2 gauss Dynamic Range.

·         Analog Output at 1 Volt/gauss (2.5V @ 0 gauss)

·         On-board +2.5 Volt Reference.

·         +6 to +15 Volt DC Single Supply Operation.

·         Very Low Magnetic Material Content.

·         -40° to 85°C Operating Temperature Range.

Magneto resistive sensors in a magnetometer

3. ACCELEROMETER

An accelerometer is a device that measures the proper acceleration of the device. This is not necessarily the same as the coordinate acceleration (change of velocity of the device in space), but is rather the type of acceleration associated with the phenomenon of weight experienced by a test mass that resides in the frame of reference of the accelerometer device. For an example of where these types of acceleration differ, an accelerometer will measure a value when sitting on the ground, because masses there have weights, even though they do not change velocity. However, an accelerometer in gravitational free fall toward the center of the Earth will measure a value of zero because, even though its speed is increasing, it is in an inertial frame of reference, in which it is weightless.

PHYSICAL PRINCIPLES

An accelerometer measures proper acceleration, which is the acceleration it experiences relative to free-fall and is the acceleration felt by people and objects. Put another way, at any point in space-time the equivalence principle guarantees the existence of a local inertial frame, and an accelerometer measures the acceleration relative to that frame. Such accelerations are popularly measured in terms of g-force.

STRUCTURE

Conceptually, an accelerometer behaves as a damped mass on a spring. When the accelerometer experiences acceleration, the mass is displaced to the point that the spring is able to accelerate the mass at the same rate as the casing. The displacement is then measured to give the acceleration.

Working of accelerometer

Capacitive accelerometers typically use a silicon micro-machined sensing element. Their performance is superior in the low frequency range and they can be operated in servo mode to achieve high stability and linearity.

SENSOR LIST (and functional description)