Miniature Fiber Optic Gyro

Fizoptika model VG949P, VG103PT (analog differential output)
                                        

Summary

The sensing technology used in the construction of the gyros is described along with suggestions regarding use and installation.  Advice is also given regarding practices to avoid when handling these devices.  A Glossary of Terms is provided to aid readers in understanding the terms used.

1.0 OVERVIEW

Fizoptika is designer and manufacturer of miniature fiber optic gyros (FOGs) with over 20 years experience. It has facilities in Fizoptika Ltd (Arzamas, Russia) and Sentech Malta Services Ltd. (Malta, EU). Our products are used all over the world as embedded devices by original equipment manufacturers (OEMs) or as stand alone sensors for test and measurement to provide critical monitoring, feedback and control input. We are in the core of many industrial and defense products and provide a reliable link to the true physical data.  The “in-line splice-less” fiber optic technology used in our FOGs was originally invented in Fizoptika. Being a worldwide pioneer in high volume FOGs production Fizoptika has supplied more than 60,000 units to industrial and aerospace customers. We offer a unique variety of models to meet the widest spectrum of environmental, physical, and performance requirements.

2.0 WARRANTY

Fizoptika warrants its products to be free of defects in materials and workmanship for a period of 12 months from date of purchase. FIZOPTIKA adds an additional two (2) month grace period to the normal one (1) year product warranty to cover handling and shipping time. This ensures that our customers receive maximum coverage on each product. The liability of Seller under this warranty is limited to replacing or repairing any instrument or component thereof which is returned by Buyer, at his expense, during such period and which has not been subjected to misuse, neglect, improper installation, alteration, or accident. WARRANTY does not apply to defects resulting from any action of the purchaser, including but not limited to mishandling, improper interfacing, operation outside of design limits, improper repair, or unauthorized modification. The WARRANTY is VOID if the unit shows evidence of having been tampered with or shows evidence of having been damaged as a result of excessive corrosion; or voltage, current, heat, moisture or vibration; improper specification; misapplication; misuse or other operating conditions outside of Fizoptika control. Sensitive components like SLD which may be damaged by wrong interfacing are not warranted. Seller shall have the right to final determination as to the existence and cause of a defect. In no event shall Seller be liable for collateral or consequential, incidental or special damages.

Fizoptika is pleased to offer suggestions on the use of its various products. However, Fizoptika neither assumes responsibility for any omissions or errors nor assumes liability for any damages that result from the use of its products in accordance with information provided, either verbal or written. Fizoptika warrants ONLY that the parts manufactured by it will be as specified and free of defects.

2.1 Receiving Inspection

Every Fizoptika gyro is carefully inspected and is in perfect working condition at the time of shipment. Each gyro should be checked as soon as it is received. If the unit is damaged in any way, or fails to operate, a claim should immediately be filed with the transportation company.

3.0 VG949P CHARACTERISTICS AND USE
Manufactured from single length of polarization maintaining optical fiber, our spliceless in-line technology provides:

• endurance to highest levels of shock and vibration • flat frequency response, no resonance • outstanding “spliceless” reliability  • negligible off-axis sensitivity • low power consumption • low noise and drift  • no hysteresis or dead zones • no bias sensitivity to vibration or acceleration

The top and bottom plastic parts are screwed together via silicon washer. This protects the gyro against dust and short-term water (moisture) penetration. The Light Diode (SLD) is pigtailed by soldering at 115°C. This sets the destructive temperature limit.

3.1 Electrical Characteristics
Our gyros measure rotation using precise optical signal filtering and synchronous detection at 80kHz modulation frequency. This requires a clean and stable, low noise 5Vdc power source or battery supply. Excitation voltages greater than 5.5 volts (or reversing polarity) cause some components to heat and eventually fail. Short (ns-scale) pulses from power transients or from accidental shortening to grounds or to the objects under potential may bring irreversible damage to the Light Diode (SLD). Voltage pulses may reach SLD via capacitive bridges existing even for spatially separated objects.

3.2 FOG Installation
Only qualified, highly skilled and experienced personnel should be allowed to handle and connect the device. Connection or installation errors may damage the gyro or result in its accelerated degradation. Make sure that during connecting and installation all parts, tools and components are properly grounded and all contacts/wires are at zero potential. If any doubts on mounting method or application environments, please, contact info@fizoptika.com to avoid erroneous action.

The gyro is designed to be operated from 5Vdc excitation and provide a ±2V full scale output with a 1Vdc bias voltage present on the output leads.

           (+OUTPUT)=1Vdc+SF/2*W, (-OUTPUT)=1Vdc-SF/2*W,

where SF is Scale Factor of the gyro and W is the rotation rate. The difference of outputs voltages represents input rate of rotation.

                                                  (+OUTPUT)-(-OUTPUT)=SF*W

The output is DC-coupled and can be used in either single-ended or differential mode. Differential mode offers the best performance since common mode errors and noise are minimized.  The positive and negative outputs of the unit should be connected to differential input amplifiers with an input impedance of at least 500kOhm referred to ground. The amplifier should also have good common mode rejection and a suitable bandwidth for the application. In situations where the signal-conditioning amplifier has a single-ended input, one of the outputs should be left unconnected (with the sensitivity reduced to the specified value). The unused output should not be tied to ground or any low impedance.

Factory contacts.
Some models incorporate spare (not used) contacts in the output connector. These contacts may be used for factory inspection of the unit and should not be connected to any input or shield. Do not attempt to hook-up these contacts (wires) to any instrumentation.

Diagnostics.
A bias monitoring voltmeter or/and current consumption milliamp-meter may be used for the purpose of indicating normal or abnormal sensor condition. When you have installed the sensor and checked out the DC bias of the output (with respect to ground) and consumption current and they all fall within the acceptable ranges (1V±10% for outputs and certified current  I_+5V ±10mA @25˚C), the sensor’s data are correct. Otherwise, the sensor is failed.
The sensor’s system has several feedbacks for conditioning the optical and electrical signals. Signal parameters along its path (from optical output to the sensor’s output) and parameters of controlled components (like SLD or PZT) are determined by design. If any failure occurs in the system, it is unable to reach its conditioned mode and all normalized electrical signals are changed. Those signals may be used for diagnostics. For example, SLD current, PZT excitation signal, optical preamplifier AC voltage, etc.

BIT (Built-In Test).
The BIT may be performed if diagnostic signal is delivered to the sensor’s output by external command (logic). On customer request VG949P may be equipped with a BIT option realized via command input. If sensor operates correctly – its output stands up to U_BIT~0.5V within 1ms from receiving BIT command. In the sensor does not operate properly – its output does not reply to BIT command.

3.3 Precautions
Our FOGs are designed to tolerate very harsh mechanical and temperature environments. However they may still be damaged by incorrect installation and operation.
• FOGs are designed to be mounted inside water protected equipment bays or instrument cubicles and should be kept at low humidity all time.
● Wet gyro condition must be avoided all time of keep and use
● Avoid any stress to gyro contacts. Use only thin flexible cables for gyro wiring.
● In a harsh mechanical environment ensure cable mechanical stability with respect to contacts to avoid stress in joints of wires and output contacts.
• Avoid even slight collision with heavy objects. High-g is easily achievable (gyro lightweight) and this may damage fragile interior of the device. Treat as delicate device and handle it carefully (for instance as you would handle high-end digital camera).

3.4 Mounting Guidelines
FOGs are typically screw mounted, and also may be adhesively mounted or clamped.

Screw Mounting Guidelines
• The mounting surface should be clean and free of any residue or foreign material.
• The mounting surface should be smooth and flat.
• Apply a thin (1mm max) rubber washers on the mating surface. Apply rubber washers on top of the flanges below normal washers. This will result in all flanges being clamped between elastic washers. This suppresses high frequency sound waves and temperature induced stress.
• Torque screws M2 (M3 max) to 3-7 Ncm limits. Make sure screws are not in contact with flanges. Use manual torque wrench (do not use electric tools).  

Adhesive Mounting Guidelines
• The mounting surface should be clean and free of any residue or foreign material.
• The mounting surface should be smooth, and flat.
• For best performance a silicon adhesive is recommended. Apply to both surfaces using activator according to manufacturer’s recommendations. Aim for an adhesive thickness in the range 0.1-0.2mm  
• Use blade to remove. Gently pull gyro apart from mating surface while gently sliding blade between surfaces. 
• There is an interest in using tape with gyros weighing less than 100 grams. For those applications where the sensing device needs to be removed safely and quickly, the use of Double Coated Tapes might be considered. The high tack adhesive provides relatively high initial adhesion and good shear holding power to a variety of surfaces.

Clamping Guidelines
• The gyro may be clamped between two surfaces with a force directed preferably along gyro sensitivity axis.
• The clamping should be gentle enough not to produce housing deformation. Since of the gyro is lightweight even low force is sufficient to fix gyro firmly.  

3.6 Cable Routing
Use high flexibility cables with low weight per length. It is strongly recommended that the cable be secured by fastening it at some point in the vicinity to output pins. This may be accomplished in a variety of ways such as by the use of a cable clamp, tie wrap, tape, etc. The initial attachment should be within 1-3 cm of the contacts. Top cover of the gyro may be use for gentle fixing near output pins. Take care that cable bending does not result in contacts stress. Avoid routing cables near high-voltage wires and also ground the shield at the signal conditioner to minimize ground loops.

3.7 Environmental ratings
MAXIMUM VIBRATION                20Hz-2kHz/ 16grms/ sine 20g     
MAXIMUM SHOCK                      half-sine 3ms/ 250g
TEMPERATURE LIMITS                -60C/ +100C

3.8 Maintenance and repair
The sealed construction and miniature size of the VG949 precludes any field repair.

4.0 Open Loop Fiber Optic Gyro

4.1 Principle of operation
A fiber optic gyroscope is a gyroscope that uses the interference of light to detect  mechanical rotation. The sensor is a coil of as much as 100m of optical fiber. Two light beams travel along the fiber in oppositedirections. Due to  the Sagnac effect, the beam travelling against the rotation experiences a slightly shorter path than the other beam. The resulting phase shift affects how the beams interfere with each other when they are combined. The intensity of the combined beam then depends on the angular rate of the device. The broadband laser diode (SLD) together with beam splitting components launch the diode light so that photons travel simultaneously in clockwise and counterclockwise directions through a cylindrical coil consisting of many loops of optical fiber. The effective area of the closed optical path is thus multiplied by the number of loops in the coil. A FOG provides extremely precise rotational rate information, in part because of its lack of cross-axis sensitivity to vibration, acceleration, and shock. Unlike the classic spinning-mass gyroscope, the FOG has virtually no moving parts and no inertial resistance to movement. Hence, FOG technology is considered to be one of the most reliable gyroscope technologies.
The input light beam passes through a polarizing/spatial filter to insure the reciprocity of the fiber coil (loop) for counterpropagating light beams. The fused coupler splits the two light beams into the fiber loop where they pass through a phase modulator (PZT) that is used to generate a time-varying output signal indicative of rotation. The modulator is offset from the center of the coil to impress a relative phase difference between the counterpropagating beams. After passing through the fiber coil, the two beams recombine and pass back through the PS-filter and are directed onto the photodetector. When the fiber gyro is rotated the phase difference between the two beams is proportional to the rotation rate. By including a phase modulator loop offset from the fiber coil center, a time difference in the arrival of the two light beams is introduced, and an optimized demodulation of the signal can be realized. An open-loop fiber optic gyro has predominantly even order harmonics in the absence of rotation. Upon rotation, the open loop fiber optic gyro has an odd harmonic output whose amplitude indicates the magnitude of the rotation rate and the phase indicates direction. The result is that the first or a higher order odd harmonic can be used as a rotation rate output and an improved dynamic range and linearity are realized. Synchronous demodulation behind the detector converts the rotationally-induced first harmonic signal into a corresponding output voltage.

4.1 Embedded Design, Method of Construction
The Fizoptika gyro is a complete gyro system which comprises fiber optic “minimum configuration” sensing assembly and advanced analog processing electronics.  The VG949P is designed to be embedded into protected equipment for monitoring, angular stabilization, short-term or GPS aided navigation, dynamic testing, etc. Key features of this FOG include:

● DC-1kHz response ● Differential (balanced) output ● Wide measurement range ● Highly integrated electronics with fastest 10ms start-up ● Solder pins configuration ● High resolution ● Outstanding shock/vibration survivability

Model VG949P is a miniature fiber optic gyroscope which utilizes an open-loop configuration to generate voltage proportional to input angular rate of rotation along sensitivity axis. Signal and power are conducted over the pins soldered to connecting PCB that is attached to the sensor cover. Model VG949P has nominal sensitivity of about 6 mV/deg/s and is supplied with a performance certificate.  The optical sensing assembly (open-loop minimum configuration) is fabricated along the single length of optical fiber by fusion-tapering technique. Industrial silicon compounds are used to mount optical components on quartz substrates. The substrates are placed into a miniature plastic container filled with soft silicon gel for protection and mechanical stabilization.  Dissipating parts (PCB and SLD) are mounted on the inner side of the top cover. A small connecting board is used to connect processing PCB OE141-55 to external power sources and instrumentation via pins extending outside cover. The connecting board may optionally include a TS (temperature sensor) or a BIT (built-in test) input. When the gyro experiences mechanical, thermal or electrical shock that exceeds its specifications, the resultant failure is most often traced the SLD electrical/mechanical damage or to fiber and wires brake. To ensure the gyros are in good working order prior to leaving factory, each gyro runs at elevated temperature while scale factor, bias and current consumption are measured. Every unit is shipped with a certificate specifying major parameters.

 4.2 Optical components
Fiber coil is 100 m of the birefringent fiber wound on a bobbin to form a quadrupole pattern to suppress effect of vibration and temperature transients. The fiber is designed for the gyro application.
PZT phase modulator – about 0.5 m fiber length wound on the side of a piezoelectric cylinder. The PZT is mounted on the holder with a soft interlayer to weaken mechanical link to the sensor main frame.
Fiber optic fused coupler (C1,2) – an evanescent wave optical device to split light beam. Both fibers shaped as biconical tapers are placed and fused in close proximity to one another. The coupler is mounted on a quartz substrate to ensue mechanical and thermal stability. It is covered with silicon gel to reduce vibration sensitivity.
Fiber-crystal polarizer (PSF polarizing spatial filter)– an evanescent wave optical device to allow only the wave with certain polarization to  propagate inside the fiber. It is made as biconical taper cladded by birefringent mono-crystal. The fiber near crystal is twisted to adjust fiber birefringence axis with respect to the crystal birefringence axis. This is to reduce magnetic response and conditions optical loss. Since the device is based on tapered fiber, it may be fabricated in spliceless technique. The polarizer is mounted on a quartz substrate and covered with silicon gel to ensure mechanical and thermal stability.
SLD module – is made in soldering technique when light emitting SLD chip is mounted on miniature copper block and fiber is preliminary soldered to another bock. Blocks are aligned and soldered together to achieve maximum and stable coupling. The low-coherence source brings to the sensor the reduction of noise and drift.

4.3 Analog electronics type oe141-55
The open-loop FOG requires electronics to control SLD current and PZT excitation voltage for optical output conditioning and for precise demodulation of the interferometric signal after its conversion to the receiver voltage. The top level scheme for implementing the electronics is illustrated by block-diagram in section 4.1.

PHOTOAMPLIFIER is a broadband low-noise converter of the optical signal to the voltage.
SLD CONTROLLER is to provide DC drive current to the SLD. Operates in DC signal servo by using the photoamplifier output.
LOCK-IN DETECTOR (f) demodulates the first harmonic with the amplitude proportional to the rotation rate.
LPF (1000Hz) is an active third order Bessel filter damps satellite harmonics of switching frequency in the output signal of the lock-in detector. It forms the output bandwidth of the sensor.
SELF-EXCITED OSCILLATOR (VCO) uses PZT as a part of the feedback circuit to set oscillation frequency close to PZT resonance frequency. Feedback gain and oscillation amplitude are voltage controlled.
VOLTAGE COMPARATOR (VC) transforms a sine signal of the oscillator to the rectangular reference pulses of lock-in detector.
BIT option may be realized by using PZT current as diagnostic signal (patented).

5.0 PRODUCTION TECHNIQUE
The fiber optic sensing assembly is fabricated in specialized in-line technique. The fundamental of that technique is the fiber with a number of peculiar optical and mechanical characteristics. The fiber maintains its optical guiding ability under high elastic and plastic deformations. This makes possible the fabrication of various fiber optic components directly on a fiber length by shaping it at high temperatures when quartz glass becomes soft. The sequent fabrication of the interferometer optical components (couplers, polarizer, SLD module) on a single fiber length makes them naturally connected without optical loss. To shape the fiber special fusion-tapering technique was developed. During fabrication process the two fiber leads are installed together and held by two moveable holders. A stabilized high-frequency arc discharge is applied to the fibers so that they melt together. Simultaneously, the two fiber holders are moved apart so that a fused tapered region is formed. The speed of separation and heating length control the shape of the resulting taper and this also has a significant influence on the resulting loss. The arc-flame is of particular careful consideration. It is necessary to use an optimal arc length and arc current not to disturb the taper. The quality of the single-mode fiber is extremely important. The core and cladding must be highly circular and concentric with one another. Inferior quality fibers can result in high losses in the resulting coupler. It is also possible to monitor the coupler’s power-splitting ratio during fabrication and to make a coupler with any required splitting ratio at a given wavelength. The fabrication of the polarizer begins with a similar tapering process to achieve the waist diameter of 5µm. The waist is then placed into melt material from which the crystal is grown to form birefringent cladding. The taper length and size of the crystal determine the polarizer extinction ratio. To build SLD-module the soldering process is used. SLD crystal and fiber lead are soldered each to the separate copper blocks. The blocks are soldered one to another at lower temperature after precise mutual alignment. Both major techniques (fusion and soldering) produce temperature and mechanically stable components that bring to the sensor reliable and stable performances in a wide range of environments. Optical components are mounted inside the sensor’s case and covered with protecting silicon gel. Electronics is mounted on the sensor’s top cover.

6.0 ANALOG OUTPUT
At a normal operating state the sensor’s output voltage is a function of angular rate slightly dependent on temperature. The simple but quite general model of the output:


U = SF·W + U₀         SF(t°,W) = SF₀·k_t·k_W 
U₀ = U₀(t°,t,H,….)     k_W = 1 - K₂(W/W_m)² -K₄(W/W_m)⁴…     k_t  = 1 + T₁·t°+…


k_W - the term describing the deflection of the output characteristics from the linear curve. Such intrinsic (for the open-loop FOGs) nonlinearity is larger at faster rotation and may reaches 15% at high rates (K₂, K₄ ≈ 0.05–0.1). Nonlinearity error may be modeled by a polynomial of odd degree. k_t reflects temperature dependence of the scale factor due to SLD spectrum temperature induced shift. k_t - is well repeatable quasi-linear function T₁ ≈ - 0.05% / °C.

The initial voltage U₀ contains DC and AC components:

- “electronic bias” – the bias of operational amplifiers, dynamic detection error, interference of detection and oscillator circuits. The bias is characterized by repeatable quasi-linear dependence on temperature. It is also slightly sensitive to the supply voltage (≈ 0.5 µV/V).

- “quadrature bias” – PZT modulator may modulate intensity of light together with phase modulation and this generates detectable erroneous signal at the frequency of modulation. This bias looks like quasi-sine faintly repeatable function of temperature with a period of 0.5-5°C.

- “optical bias” - as spatial/polarizing filter does not operate perfectly a secondary nonreciprocal loops may exist and opposite waves acquire residual phase shift independent on rotation. It may also be characterized as a quasi-sine random function of temperature.

- “magnetic bias” – an external magnetic field may generate bias due to magneto-optical Faraday effect in the quartz material of fiber. It’s determined by fiber twisting rate and slightly depends on temperature.

- “temperature transient bias” occurs when the temperature of the sensor varies (Shupe effect).

- “output noise” results from light quantum fluctuations and thermal noise of electronic components. It appears as the scatter of data. The noise power spectrum density (PSD) is uniform within the working frequency range. Vibration may bring an extra noise via fiber coil structural dynamic deformations.

7.0 FREQUENTLY ASKED QUESTIONS

QUESTION: What is the key factor for the sensor lifetime?
ANSWER: Humidity. The gyros are designed for embedded use. With silicon sealing they are not fully protected from water penetration. The water inside sensor results in accelerated degradation and damage of quartz optical components. At all time keep in dry condition.

QUESTION: What is the highest input range you can reach?
ANSWER:  Using special winding pattern the input range may be adjusted up to 20,000 deg/s.

QUESTION: How much start-up time is required?
ANSWER: About 15 ms from power-on.

QUESTION: How much warm-up time is required?
ANSWER: 0.02s to achieve 99% of scale factor and 0.05-0.1s to pass bias transient. 

QUESTION: What is the most common problem for sensor failure?
ANSWER: User experience. Majority of problems are cleared up with some application support.

QUESTION: Are your sensors CE compliant?
ANSWER: Yes.

QUESTION: Why do you have so many different designs?
ANSWER: To meet various applications requirements. As the accuracy of FOG is usually a key factor, its environmental performance and mechanical and electrical interfacing are the subjects of specific claims. 

QUESTION: What is sensitivity of bias to vibration (SINE or random)?
ANSWER: Normally the sensitivity is negligible. However at extremely high levels of vibration the increasing output noise may reach levels of electrical saturations. Due to rectification this will appear at the output as vibration induced bias.

QUESTION: Does FOG sense external magnetic field?
ANSWER: FOG does not have magnetic shield and due to optical nonreciprocal effects it has residual magnetically induced bias. Even though Fizoptika assembling techniques suppress the response to magnetic field, for the applications where highest accuracy is required we recommend magnetic shielding. 

QUESTION: Do you happen to have any more detail for the connection of the cable to the gyro output pins? It says on your data sheet that the soldering should be done at low temperatures.
ANSWER: The reason for this caution is the potential risk of moving the pin if it is overheated. We caution against this since we have seen the loss of electrical connection and sealing. For our soldering we utilize a solder temperature of 183°C. 

QUESTION: Can Fizoptika provide lower temperature version of Model VG949P?
ANSWER:  Yes, we can make a lower temperature version which operates from -60°C to +60°C.

8.0 GLOSSARY

GYRO AXIS ORIENTATION
The sensitive axis direction is indicated by an arrow on the drawings in the Product Specifications & Drawings. When a gyro is mounted on a surface that rotates clockwise the gyro produces a positive (+) change in output signal.

SPECIFIED TEMPERATURE RANGE.
The temperature range in which the sensor meets all specification parameters. The sensor continues to function within the Operating Temperature Range; however, the specifications may gradually deviate from the data sheet.

SCALE FACTOR (SF)
If the gyro is exposed to rotation its output voltage changes. Scale Factor is defined as coefficient between the voltage change and angular rate of rotation (mV per deg/s). For open loop FOG the SF depends slightly on rotation rate (SF nonlinearity) decreasing at increasing rate.

INPUT RANGE (IR)
Input Range of the sensor is defined as the negative range limit or the positive range limit which is the lowest in absolute value. Values given on datasheets are approximate values and may vary with each sensor. The actual measured SF is provided for each gyro on the test certificate and referenced to its individual serial number.

MEASUREMENT RANGE (Rate Range).
Within IR the measurement (conversion) error may vary. Rate Range is defined as IR where measurement error does not exceed certain value (usually in %). Gyro has 4^th class of accuracy in RR if its error does not exceed 4%.

OUTPUT IMPEDANCE.
The resistance measured between the (+ or -) output line and the common line is the Output Impedance. Each output line has 1kOhm impedance with respect to the ground line. For best results, instrumentation used to monitor the sensor output should have an input impedance of at least 500 kOhm. Instrumentation with a low input impedance may reduce the sensitivity of the sensor by loading the output (typically a 1% reduction with an input impedance of 100 times the output impedance of the sensor). 

NON-LINEARITY
Non-Linearity is the deviation of the sensor output signal from a theoretical straight line which has been fitted to the data points of an actual calibration at low rates. It is expressed as a percentage of reading.

NON-REPEATABILITY
Non-repeatability is the deviation in sensor output signal levels when a specific input is applied in consecutive cycles of short time duration under the same conditions, such as temperature. It can be determined by performing two consecutive short time duration calibration cycles and can be expressed in deg/s or % what is applicable.

OPERATING TEMPERATURE RANGE
The temperature range in which the sensor functions without damage from thermal effects is the Operating Temperature Range. Exposure to temperatures above or below the Operating Temperature Range may cause permanent damage to the sensor.

TRANSVERSE SENSITIVITY
Transverse Sensitivity is the sensitivity to input in the non­sensitive, cross-axis direction, and it is a potential source of measurement error in a user’s application. Negligible for the fiber optic gyro. 

BIAS or ZERO OFFSET
The electrical output of the sensor when there is no applied input is the Zero Offset. It is sometimes referred to as the ”baseline”.