Gyroscopes
As children, we all remember playing around with things that spin such as frisbees and spinning tops, that we would play for hours to see how long we could keep the thing spinning, or how fast we could get it to spin, but perhaps most common was the yo-yo.
Let's think about how bicycles stay upright, keep this in the back of the mind for a moment;
Perhaps at some point in time you have picked up a device that had some sort of spinning rotor or weight and it was at that moment, you felt the strange way it moved and responded as you moved it about.
I don't recommend do this to a good hard disk drive with important data, however if you get an older hard disk drive, power it up, then pick it up, as you move it about, you can feel it resisting your input movements . . . A strange feeling.
What you are feeling is the gyroscopic effect or motion, which applies to all spinning objects. Gyroscopic motion is the tendency of a rotating object to maintain the orientation of it's rotation or more precisely, its spin axis. Therefore gyroscopes are used for measuring or maintain orientation and angular velocity.
The most noteworthy effects demonstrated by gyroscopes is precession, which is the change in the orientation of the rotational (spin) axis of the spinning object in response to an applied input force, which actually occurs at a point that is 90° later in the direction of rotation, and gyroscopic procession is the key principles of physics as to why bicycles stay upright and how helicopters move in flight.
What is important to note about a (mechanical) gyroscope, is that of its construction. The spinning rotor, which is a heavy flywheel, is mounted into into a pivoting frame known as a gimbal. Typically, upto three gimbals may be used to provide three-axis of freedom, however, depending on the application, the outer gimbals may be fixed or omitted.
The applications of gyroscopes are fundamental in aviation such as artificial horizon (attitude) instruments, direction / heading indicators (Directional Gyro), turn and slip indicators and gyroscopic autopilots, and in navigation systems such as inertial guidance units.
Gyroscopes (or gyrometers) are now common place in consumer electronics, including gaming consoles, smartphones and image stabilisation in the form of Micro-Electro-Mechanical systems (MEMS).
On of the best demonstration gyroscopes that I have come across is made by Glenn Turner over at Gyroscope.com in the UK.
I originally purchased the original precision gyroscope in 2007, then in 2018 I purchased their new and improved super precision gyroscope with upgraded bearing and a balanced flywheel for a run-time of upto 25 min, and I have certainly tested this figure.
Going back to around 2005, a contact of mine knowing that I was interested in lasers, generously donate two surplus Ring Laser Gyroscopes (RLG) that were set of disposal. One of the units was dissembled prior to receiving it (which is pictured) and perhaps missing components, while I have kept the second unit completely intact, However, the hermetic seal is broken.
Ring Laser Gyroscopes of this nature are pretty much a rare item to come across and have the opportunity to study.
This unit was manufactured by the former company, Sperry Corporation, upto this point I have had very little information, however digging around at the time of writing, I have come across some references to a Sperry laser gyroscope used in "SLIC-7" and "SLIC-15" strapdown inertial measurement units (IMUs).
Apparently from some literature that I have found the complete IMU consists of three of the RLGs together with their associated accelerometers mounted on the orthagonal structual element (which positions each of the elements in their respective X-Y-Z planes). The RLGs measure angular rotation while the accelerometers measure linear acceleration.
The main optical block of the gyroscope is machined into a single block of CER-VIT, which is a family of ceramic-glass material comprised of Lithium, Aluminium and silicon oxides and melted down to form a glass substrate. CER-VIT is also commonly known by its trade name, ZERODUR®, which is a lithium-aluminosilicate glass-ceramic and trademark of Schott AG and produced since the 1960s.
The most important and significant properties of ZERODUR® optical glass-ceramic is the extremely small coefficient of linear thermal expansion as well as the homogeneity of this coefficient throughout the entire piece. For more information on this material, checkout the Schott Website.
The optical block is housed inside a series of three steel housings of increasing size which form part of the magnetic shielding, and finally the assembly housed in an evacuated, hermetically sealed solid steal enclosure.
A departure from Honeywell's design where the entire optical block is also the discharge tube, the laser discharge tube used in the Sperry RLGs were of a design implementation of a separate laser tube with Brewster windows to allow the tube to be replaceable in the field.
Ring Laser Gyroscope - Basic Theory
A ring laser gyroscope is a special type of ring interferometer that employs the Sagnac effect, also referred to Sagnac interference that was discovered by French physicist, Georges Sagnac.
Sagnac interference is observed when a laser beam is split into two beam paths of equal length and travel in opposite directions in the ring. The two beams exit the ring and are recombined were they interfere with each other to setup an interference pattern also known as a beat pattern.
When the apparatus is subjected to a rotating motion, this causes a relative phase shift (frequency separation) of the two counter-rotating beams existing the ring in proportion to the angular velocity of the rotation if the platform in the plane of the ring.
At rest, the beams of light complete one a circuit of the ring in the same time frame, as the interferometer is spun, one of the paths of light become longer in the direction of rotation which results in the phase shift at the detector.
One effect that RLG suffer is that of "lock-in". In simple terms, at very low rotational speeds, the frequency between the two counter-rotating reach the same value (or very close to), in which may cause the internal moving standing wave to "lock-in" a particular phase, and become non-responsive to rotational changes (not dissimilar to gimbal lock on a mechanical gyroscope).
An approach to mitigate a lock-in situation is the implementation of "dithering", where the ring cavity is resonated back and forth at a relatively high-frequency as to keep above the lock-in threshold.
The optical path forms an equilateral triangle with the length of each leg approximatly 124mm and defined by the three mirrors: Path Length Control Mirror, Bias (kerr-effect) Mirror and the output mirror / combination prism. The laser discharge tube is mounted on one leg of the triangle.
Overall view of the RLG assembly. The He-Ne laser tube is located inside the metal covers, on the left is the Path Length Control Mirror (PLCM), the single mode etalon is located inside the retained cavity (top left, but not noted in the above diagram), which ensure a single laser mode is resonating, the Kerr-effect mirror at the top, then down to the output prism and detector.
The following image of the Kerr-effect magnetic (bias) mirror which were used on this design to overcome a lock-in condition. With this approach, there are no moving parts that are associated with alternate mechanical dithering methods (which can also introduce other errors into the system).
The bias mirror is coated with a special ferromagnetic material in which its refractive index can be changed with an applied electric field. The change in refractive index introduces a phase shift (or equivalent path length difference) on the two counter-rotating polarised beams. This phase shift varies with the square of the applied voltage on the mirror.
The path length control mirror (PLCM) (Left photo) is an essential component for high precision ring laser gyros. This mirror is used to precisely control the length of the laser resonator to ensure its length is maintained at the precise integral number of wavelengths which for a Helium-Neon Laser is 632.8nm**. The mirror is used to actively and continuously control the cavity length to maximise laser intensity and compensate for any temperature differentials.
The PLCM is mounted on a piezoelectric element and is deflected based on the applied voltage across the piezo element. This voltage is typically around 70 to 100 volts per wavelength. The mirror is typically pulsed to induced a fluctuation in the intensity of the laser at the detector due to changes in legth of the resonator where maximum gain equals maximum intensity.
The photo on the right is of the output mirror, combination prism and the detector (which s mounted behind the metal bracket / assembly.
Finally, at the heart of the Ring Laser Gyroscope, the laser source itself. This is a very interesting construction of a laser plasma tube, nothing like I have seen before nor have anything remotely similar.
The entire assembly together forms the complete resonant laser cavity with the output of the system at the prism and detector. Two separate beams are generated from either side of the laser, the counter-clockwise rotating beam path is reflected through the prism back into the cavity to travel around the ring while the clock-wise rotating beam path travels around the ring to the output prism where it exists the cavity and recombined with the counter-rotating beam.
The cathode is the central cylinder at the back of the ceramic block, while the anodes are at each end which is terminated by the Brewster windows.
I have attempted to fire the laser quite some time ago and did light up, however, have not yet made any further effort to put into an external cavity to see if can get it to lase.
The output of this laser would not be much more than 0.5 to 1mW if configured inside a conventional Fabry-Perot resonator with an output coupler, however the circulating energy within the resonator cavity (and the ring) would be close to 1W of power.
** Note: At the time of writing, it is apparent that the wavelength of the laser in this Sperry RLG is actually 1150nm in the infrared part of the spectrum.
Legacy Archive:
Sperry Laser Gyroscope
- Flavio Spedalieri -
Written: 16 August 2021
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