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Gyroscope Types, Selection

Image Source: Tricky Shark/Stock.adobe.com

By Lucas Costa for Mouser Electronics

Published January 25, 2022

Gyroscopes (also called gyros or angular velocity sensors) are a type of velocity sensor used to detect and measure an object's angular velocity—the orientation and rotation rate of a spinning object in relation to its frame of reference. Gyroscopes are used as stand-alone devices and in combination with other sensors in navigation and stabilization systems:

Non-magnetic compasses (gyrocompasses), where true north is maintained by a continuously-driven gyro and an axis parallel to the Earth's axis. Such compasses are often found on ships because they use true north (as opposed to magnetic north) and are unaffected by the ship's ferromagnetic materials.

Inertial Guidance Systems (IGSs), where gyros are used along with accelerometers and possibly magnetometers, which measure acceleration rate and magnetic force, respectively. These are used in applications where external reference points are unavailable or difficult to measure, such as in aircraft, ships, submarines, guided missiles, robotics, unmanned aerial vehicles (UAVs), and medical exoskeletons.

Inertial Measurement Units (IMUs), where the gyro provides the angular rate, which is used along with a craft's force and orientation, to guide craft where GPS data isn't either available or consistently available. IMUs are used in applications like aircraft, UAVs, spacecraft, satellites, autonomous vehicles, and cell phones.

Stabilizers, where gyros are used to offset movement or motion of cameras, video equipment, and even vehicles, where outside torque needs to be stabilized.

Attitude and Heading Reference Systems (AHRSs), where gyros provide roll, pitch, and yaw data that, along with accelerometer and magnetometer data, is used to determine a craft's attitude.

Gyroscope Types

Gyroscopes can be categorized according to several characteristics, including operating principles, output type, sensing range, number of axes, interface type, and more. As summarized in Table 1, this article categorizes gyros according to their underlying technology: Mechanical, optical, and vibratory.

Table 1: Categories of gyroscopes: Mechanical, Optical, and Vibratory

Mechanical Gyroscopes

The mechanical type is what most people think of when they think of gyroscopes. Mechanical gyros are comprised of a spinning rotor that's mounted on gimbals that enable measurements on up to six axes. Gimbals isolate the rotor from rotation-changing forces and, thereby, keep the motion of the rotor centered regardless of external forces imposed on it. When rotational force is applied, the gimbals resist the force in a coordinated way to offset the disturbance from multiple directions.

Mechanical gyros work according to three gyroscopic principles:

• Rigidity in space, which refers to the principle that gyroscopes remain in a fixed position within their plane and are unaffected by outside forces.
• Conservation of angular momentum, which refers to the principle that the total angular momentum of a closed system remains constant.
• Precession, which refers to what happens when an outside force is applied to a spinning rotor. Rather than affecting the rotor where the force is applied, the force manifests 90° later in the direction of the spin. The measurements of angular orientation and velocity are determined by the deviation of an object from its desired orientation.

Although dynamically-tuned mechanical gyros (DTGs) are still used in north-finding and navigation applications where extreme precision is needed, mechanical gyros have been supplanted in recent decades by other types that are smaller, lighter, and easier to manufacture, as described in the next sections.

Optical Gyroscopes

Optical gyroscopes feature small size, light weight, and an absence of moving parts. In these gyros, two separate beams of light are sent through an optical fiber in opposing directions within a closed platform. When each beam returns to the detector, its travel time is recorded. In cases of no rotation, the beams return to the sensor at the same time. Where rotation is present, the distance each beam must travel changes; the light traveling against the rotation direction arrives first, and the light traveling with the rotation direction arrives later. The phase shift between the two light beams indicates the rotational change.

Today's engineering designs commonly use two types of optical gyros: Ring-laser gyros and fiber-optic gyros.

Ring-Laser Gyros (RLGs)

Ring-laser gyros (RLGs) are a type of active sensor that uses a closed-loop optical path that houses the optical source. In RLGs, the two divergent light beams move around a cavity by reflecting off mirrors in each corner of the enclosure. RGLs work according to the Sagnac effect, where two optical beams counter-propagating in a ring structure change their relative phase if the ring is rotating. Therefore, the phase shift is proportional to the rotation and can be measured. RLGs are most often used in IGSs in military and commercial aircraft, ships, and spacecraft.

Fiber-Optic Gyros (FOGs)

Fiber-optic gyros (FOGs) are a passive sensor that uses an optical source that's external to the closed-loop path, which is a coil of optical fiber that can be up to 5km in length. Here, the light is split into two beams, and its operation is based on the interference of light as it passes through the coil. Like with RLGs, FOGs work according to the Sagnac effect; however, the strength of the effect depends on both the geometric area of the loop coupled with the number of turns in the coil. FOGs are most often used in high-shock and high-performance applications: IGSs for guided missiles, underwater vehicles, and surveying equipment.

Vibrating Structure Gyroscopes

Vibrating structure gyroscopes are solid-state vibrating structures used to determine the rate of rotation. Generally speaking, these are one-axis gyros comprised of a support structure, a resonator, an electromagnetic drive that causes the resonator to vibrate, and an electromagnetic sensor that senses resonator movement. These gyros work according to Newton's law of inertia, which includes the fact that vibrating objects tend to continue vibrating in the same plane even as its support rotates. Here, the Coriolis effect causes force to be exerted on its support, which causes a precession of vibration patterns about the axis. Angular velocity can be determined by the deviation of the object from its desired orientation.

Vibrating structure gyros come in many forms, including piezoelectric gyros, wine-glass resonators, tuning-fork gyros, and others. Micro-electromechanical system (MEMS) gyroscopes in particular have become widely available in electronic devices because of their tiny size, low cost, and low power requirements. Rather than having rotating parts, bearings, or light beams, these tiny devices are printed onto circuit boards and measure angular velocity by transferring vibration from a drive mode to a sense mode. As the angular velocity of the gyro varies, a small resonant mass is displaced; this movement is transformed into very low-current electrical impulses that a host microcontroller can amplify and read. MEMS gyros are very sensitive and can detect slight degrees of rotation, which enables absolute position to be determined.

A newer type of MEMS gyro, called Precision-shell integrating (PSI) gyroscopes, claims to be 10,000 times more accurate than those in today's cell phones, but only 10 times more expensive, according to its developers at the University of Michigan. A type of IMU, the PSI uses a mm-scale with a quality factor of more than 5 million. These gyros promise to provide highly-precise indoor and outdoor location awareness, as well as long-term autonomy for mobile devices and autonomous vehicles.

Gyroscope Integration and Implementation

For all types of gyros, sensitivity, resolution, and stability are key performance indicators. Maximizing gyro performance begins with understanding application requirements and key gyro parameters. The following sections discuss integration and implementation parameters as they pertain to integrating a MEMS gyro. For other types of gyros, specifics would vary; however, these considerations are ones that can be applied more broadly as needed.

Mounting and Alignment

The location, orientation, mounting, and proximity of the MEMS gyro to other PCB components affects the gyro's performance and lifespan. Installed improperly, the gyro might lose sensitivity, generate incorrect readings, and degrade in performance over time. In general, the gyro should be mounted on a rigid board or substrate. Other general recommendations include placing insertion components at least 2mm from the gyro and isolating the gyro from temperature fluctuations and sources of vibration.

Alignment—or, rather, minimizing misalignment—of the MEMS gyro is also key. Misalignment refers to the angular difference between the gyro's rotational axis and the system's inertial reference frame. Several factors contribute to misalignment, including integration tolerances, imperfections of materials, device packaging, and the number of axes (because misalignment of one axis can influence alignment of other axes), among other sources. Misalignment can require significant workarounds such as corrective matrixes, special packaging, or special testing to ensure the gyro is accurate.

Mounting and misalignment mitigation during integration or during the design phase include knowing the error budget of the system (accepting that there will be an error margin) and isolating the mounting platform into which the gyroscope will be integrated. Defining the explicit application and how the gyroscope will be used will help address concerns in errors during gyroscope integration.

Operating Range, Noise, and Temperature

The measurement range, also known as the full-scale range, is the gyro's maximum angular velocity reading range, usually given in degrees per second (DPS or °/s). The smaller the range, the more sensitive the gyro will be to smaller input; the larger the range, the less precision and the more noise will be found. For this reason, operating range, precision, and noise are common tradeoffs in engineering design. Today's gyros are programmable from ±125 to ±2000, depending on the manufacturer.

Noise refers to variations in the gyro's signal output that stem from internal or external factors. Misalignment errors, discussed previously in terms of mounting and alignment, is one type of noise. Other major sources of noise include inherent sensor noise, which are variations in the gyro's output in static conditions, and noise that comes from the gyro's response to linear vibration. Understanding how noise impacts the gyro's behavior is important. Further, measuring the gyro's noise density is a useful metric for understanding the tradeoffs between noise and bandwidth, as well as determining noise filtering needs, including the use of a Kalman filter to reduce noise in gyroscope arrays.

Finally, temperature is related to noise and sensitivity. An increase in temperature can increase signal noise, and either increases or decreases in temperature can affect the gyro's sensitivity. Here, choosing a gyro that accommodates variations in operating temperatures is important. Gyros with built-in temperature sensors are widely available, too; with these, absolute accuracy of the temperature sensor is less important than closely coupling the temperature sensor to the gyro's temperature. In some cases, techniques such as temperature compensation and calibration can be applied to compensate for temperature-related issues.

Bias and Stability

All gyroscopes suffer from internal errors such as drift (bias instability), random angle walk, sensitivity variances, and lock in (optical gyroscopes). When selecting a gyroscope, minimizing the largest error is paramount. For different applications, this error will vary and thus it is important to define the application of the gyroscope explicitly to minimize errors within the gyroscope—e.g., if vibration is an issue, minimizing vibration errors from the gyroscope should be the most important concern.

However, a key parameter often cited when selecting a gyroscope is the bias instability. The bias instability of a gyroscope results in a drift of measurements over time and with an accumulation of angle errors. As time goes on, without recalibrating, the gyroscope errors (due to drift) accumulate and errors worsen. Bias stability represents the resolution floor of the gyroscope and is an important parameter for gyroscopes. Several methods to address bias instability include the addition of other sensors such as accelerometers (an IMU) or redundant gyroscopes to average out the drift. A common technique is to also calibrate the gyroscope at given time intervals to prevent drifts from getting too large.

Conclusion

Gyroscopes can be based on a wide range of mechanisms and can thus be easily adapted to different requirements. Indeed, gyroscope applications have spread to many industries, from aviation to communications. Mechanical gyros are often used for their reliability, while optical gyroscopes are very accurate but can be relatively large, complex, and costly to design and manufacturer. MEMS provide a sensible solution to a wide variety of needs.

Lucas has a Masters in Machine Learning and Robotics, and is an Artificial Intelligence developer, researcher, and practitioner as well as an Open Source Software developer, with experience in machine vision and automation for smart machinery in Precision Agriculture in his work as a researcher at the University of Florida and developer at his co-founded company Agriculture Intelligence. He has authored many papers as well as cloud software (Agroview) and data fusion systems.