Pipeline IMU Detection Principle and Data Processing

 https://www.ericcointernational.com/application/pipeline-imu-detection-principle-and-data-processing.html

1.IMU measurement principle

IMU (Inertial Measurement Unit) is a device that can measure the angular velocity and acceleration of an object in three-dimensional space. Its core components usually include a three-axis gyroscope and a three-axis accelerometer. Gyroscopes are used to measure the angular velocity of an object about three orthogonal axes, while accelerometers are used to measure the acceleration of an object along three orthogonal axes. By integrating these measurements, the velocity, displacement and attitude information of the object can be obtained.

 

2.Pipe bending strain identification

In pipeline inspection, IMU can be used to identify the bending strain of the pipeline. When an IMU is installed on a pig or other mobile device and moves within a pipeline, it can sense changes in acceleration and angular velocity caused by pipeline bending. By analyzing this data, the degree and location of pipe bends can be identified.

 

3.Diameter measurement and pipe cleaning process

The diameter measuring and cleaning process is an important part of pipeline maintenance. In this process, a caliper pig equipped with an IMU is used to move along the pipeline, measure the inner diameter of the pipeline, and record the shape and size of the pipeline. This data can be used to assess the health of pipelines and predict possible maintenance needs.

 

4.Steel brush cleaning process

The steel brush pigging process is used to remove dirt and sediment from the inner walls of pipelines. In this process, a pig with a steel brush and an IMU moves along the pipeline, cleaning the inner wall of the pipeline through brushing and scouring. The IMU can record the geometric information and cleanliness of the pipeline during this process.

 

5.IMU detection process

The IMU inspection process is a key step in using IMU for data collection and measurement during pipeline maintenance. The IMU is installed on a pig or similar equipment and moves inside the pipeline while recording acceleration, angular velocity and other parameters. This data can be used to analyze the health of the pipeline, identify potential problems, and provide a basis for subsequent maintenance and management.

 

6.Data acquisition and post-processing

After completing the IMU detection process, the collected data need to be collected and post-processed. Data acquisition involves transferring raw data from the IMU device to a computer or other data processing device. Post-processing involves cleaning, calibrating, analyzing and visualizing the data. Through post-processing, useful information can be extracted from the original data, such as the shape, size, bending degree, etc. of the pipe.

 

7.Speed and attitude measurement

IMU can calculate the speed and attitude of an object by measuring acceleration and angular velocity. In pipeline inspection, measurement of speed and attitude is critical to assess the health of the pipeline and identify potential problems. By monitoring the speed and attitude changes of the pig in the pipeline, the shape, bending degree and possible obstacles of the pipeline can be inferred.

 

8.Pipe Curvature and Strain Assessment

Using the data measured by the IMU, the curvature and strain of the pipeline can be evaluated. By analyzing acceleration and angular velocity data, the radius of curvature and bending angle of the pipe at different locations can be calculated. At the same time, combined with the material properties and loading conditions of the pipe, the strain level and stress distribution of the pipe at the bend can also be evaluated. This information is important for predicting the life of pipelines, assessing safety, and developing maintenance plans.

 

Summarize

To sum up, IMU plays an important role in pipeline inspection. By measuring parameters such as acceleration and angular velocity, comprehensive assessment and maintenance of pipeline health can be achieved. With the continuous advancement of technology and the expansion of application fields, the application of IMU in pipeline inspection will become more and more extensive. The MEMS IMU independently developed by ERICCO has relatively high accuracy, such as ER-MIMU-01 and ER-MIMU-05, which are more accurate and are navigation-grade products. If you want to know more about IMU, please contact our professional technicians as soon as possible.

Application of Fiber Optic Gyroscope

 There are many ways to classify fiber optic gyroscopes. According to the working principle, it can be divided into three types: interference type, resonant type and stimulated Brillouin scattering fiber gyro. Among them, the interferometric fiber optic gyroscope is the first generation of fiber optic gyroscope, which uses multi-turn fiber coil to enhance the Sagnac effect. It is currently the most widely used; it can be divided into open-loop fiber gyroscope and closed loop according to different electric signal processing methods. Fiber optic gyroscopes, in general, closed-loop fiber optic gyroscopes have higher precision due to closed-loop control; they can be divided into single-axis fiber gyroscopes and multi-axis gyroscopes according to structure, among which three-axis fiber gyroscopes have volume Small, measurable spatial position and other advantages are an important development direction of fiber optic gyroscopes.

1.What is a fiber optic gyroscope? What are its applications in life?

A fiber optic gyroscope (FOG) is an angular velocity sensor based on the Sagnac effect, which detects angular velocity by measuring the phase difference of light traveling through an optical fiber. Compared with the traditional mechanical gyroscope, fiber optic gyroscope has the advantages of small size, light weight, high precision and good reliability, so it has been widely used in many fields.
In life, fiber optic gyro is mainly used in the field of navigation, such as car navigation, ship navigation, aircraft navigation and so on. Fiber optic gyroscopes can provide high-precision angular velocity measurements to help navigation systems more accurately determine the position and direction of a vehicle, boat or aircraft. In addition, fiber optic gyro can also be used in industrial control fields, such as robot attitude control, motion control of industrial machine tools, etc. In these applications, fiber optic gyroscopes can help achieve high-precision motion control and improve production efficiency and quality.
In addition, fiber optic gyro has also been widely used in the military field, such as missile guidance, spacecraft attitude control and so on. In these applications, the high accuracy and reliability of fiber optic gyroscopes are crucial. In general, the fiber optic gyro is a very important sensor, its application field is constantly expanding, for our life and work to bring a lot of convenience.

2.How does a fiber optic gyroscope work?

The working principle of a fiber optic gyroscope is based on the Sagnac effect. In simple terms, the Sagnac effect means that when light travels in a closed optical path, the phase of the light changes if there is an angular velocity of rotation.
In a fiber optic gyroscope, light from a light source passes through a coil of fiber and forms two beams of light in the fiber, which travel in opposite directions. When a fiber optic gyroscope spins, the phase of the two beams of light differs due to the Sagnac effect. By detecting this phase difference, the rotation angular speed of the fiber optic gyroscope can be calculated.
In order to detect phase differences, interferometers are usually used in fiber optic gyroscopes. The interferometer can convert the phase difference of two beams of light into the change of light intensity, so as to measure the rotational angular velocity. In practical applications, the fiber optic gyroscope also needs to carry out temperature compensation, polarization control and other measures to improve the measurement accuracy and stability.
The working principle of fiber optic gyroscope is relatively complicated, but its high precision and high reliability make it widely used in many fields. In the future, with the continuous development of technology, the performance of fiber optic gyro will continue to improve, bringing convenience to more fields.

3.What factors will affect the use of fiber optic gyroscopes? How to improve its measurement accuracy?

The measurement accuracy of fiber optic gyroscope will be affected by many factors during its use. Here are some common contributing factors:
Temperature: Changes in temperature can cause changes in the length and refractive index of the fiber, which affects the speed and phase of light propagation. Therefore, the fiber optic gyroscope needs temperature compensation to reduce the influence of temperature on the measurement accuracy.
Vibration: Vibration will change the optical path of the fiber optic gyroscope, which will affect the measurement of the phase difference. In order to reduce the impact of vibration, vibration reduction measures can be adopted, such as using shock absorbers, increasing the stiffness of the structure, etc.
Magnetic field: The magnetic field will change the polarization state of the light in the fiber, which will affect the output of the interferometer. In order to reduce the influence of magnetic fields, magnetic shielding technology can be used or diamagnetic fiber can be used.
Light source stability: the power fluctuation and wavelength drift of the light source will affect the measurement accuracy of the fiber optic gyroscope. Therefore, it is necessary to select a stable light source and control the temperature and power of the light source.
Fiber loss: The loss of the fiber will lead to the attenuation of the optical signal, which will affect the measurement accuracy. In order to reduce the impact of fiber loss, it is necessary to select low-loss fiber, and optimize the fiber connection and packaging.
In order to improve the measurement accuracy of the fiber optic gyroscope, the following measures can be taken:
Optimizing the design of the fiber optic gyroscope: the sensitivity and stability of the fiber optic gyroscope can be improved by reasonably designing the fiber winding structure and optical path layout.
The use of high-precision detection technology: for example, the use of high-resolution photodetectors, advanced signal processing algorithms, etc., can improve the measurement accuracy of phase difference.
Error compensation: According to the specific use of the environment and conditions, the temperature, vibration, magnetic field and other factors for error compensation to improve the measurement accuracy.
Multi-axis measurement: Using multiple fiber optic gyroscopes for multi-axis measurement can improve the measurement accuracy and reliability of the system.
Regular calibration and maintenance: Regularly calibrate and maintain the fiber optic gyro to ensure that it is in good working condition, thereby improving the measurement accuracy.
In addition, with the continuous progress of technology, new optical fiber materials, light sources and detection technologies can be used to further improve the measurement accuracy and performance of the fiber optic gyroscope. At the same time, in practical applications, it is also necessary to choose the right fiber optic gyroscope according to the specific needs and environment, and carry out reasonable installation and use to give full play to its advantages.

Ericco's fiber optic gyroscopes have long life and medium accuracy, such as ER-FOG-50, ER-FOG-60 is our very hot selling fiber optic gyroscopes, if you are interested in our products, please feel free to contact us.

Methods to Improve the Orienting Efficiency of Gyro Theodolite

https://www.ericcointernational.com/application/methods-to-improve-the-orienting-efficiency-of-gyro-theodolite.html

The gyro theodolite is a measuring instrument used to determine the true north azimuth angle. It mainly consists of a gyroscope and a theodolite. The gyroscope uses its own physical properties, such as axiality and precession, to be sensitive to the horizontal component of the earth's rotation angular velocity, thereby determining the true north azimuth angle. This instrument is widely used in fields such as mine surveying, engineering surveying, and military surveying and mapping. It is also an important supporting equipment for radar antenna orientation, UAV flight orientation, artillery and remote weapon launch orientation. Next we will propose some methods to improve the directional efficiency of gyro theodolite.

 

1.Factors affecting the directional efficiency of gyro theodolite

1) It takes a long time to measure the instrument constants before and after orientation, which is even more troublesome when the weather is bad. According to the requirements of the regulations, it is necessary to compare with the known sides twice before and after orientation to determine the instrument constants. Normally, two walking and directional measurements take half a day.

2) Directional measurement requires multiple measurement rounds. For example, using a 15~20s instrument requires 4 measurement rounds. The purpose is to improve accuracy and avoid errors, which usually takes 2h.

3) The final directional measurement results cannot be obtained at this time. Because the final orientation result cannot be obtained until a series of in-house calculations such as post-measurement constants, meridian convergence angles, and coordinate azimuth angles are completed, it takes two visits to the site to complete the subsequent measurement work of the orientation, such as accurately specifying the orientation. .

4) Some gyro theodolite or automatic gyro theodolite require a long waiting time. Some instruments need to pre-suspend the gyroscope in order to obtain more stable readings and reduce rework. This time is from tens of minutes to 1 hour;

5) When the instrument comes from the cold ground to the well, it will fog up and it will take 0.5 hours for the water vapor to dry.

6) Windproof treatment at the directional measurement site or organize and coordinate the shutdown of other work on site. This is because manual gyro theodolite cannot work in wind and vibration environments. This is because the manual gyro theodolite cannot work in an environment with wind and vibration. The nutation of the gyro indicator line often causes orientation errors to exceed limits and rework. It takes more time to coordinate and solve this problem.

7) Gyro-theodolite or high-precision automatic gyro theodolite can only solve the orientation problem. Actual surveying work often requires the completion of other measurements such as positioning, orientation, and stakeout at the same time. Usually, other instruments and personnel are required, and even need to be carried out again. In this way, the total man-time consumption is even more.

It can be seen that under normal circumstances without rework, one orientation takes about 5 to 8 hours. What takes more time is constant measurement, waiting before testing, multiple test orientations, etc. To improve efficiency, these problems should mainly be solved.

 

2.Methods to improve the orientation efficiency of gyro theodolite

There are many ways to improve the directional efficiency of a gyro theodolite. Here are some suggestions:

 

1. Automation and intelligence: With the development of technology, realizing the automation and intelligence of gyro theodolite is the key to improving the orientation efficiency. By introducing automated control systems and artificial intelligence technology, manual intervention can be reduced and measurement accuracy and efficiency improved.
2. Optimize data processing algorithms: Improve data processing algorithms, reduce data processing time and errors, and improve orientation efficiency. For example, introduce efficient filtering algorithms, optimize data fitting methods, etc.
3. Strengthen instrument maintenance and calibration: Regularly maintain and calibrate the gyro-theodolite to ensure that the instrument is in good working condition. This can reduce instrument errors and improve measurement accuracy and efficiency.
4. Improve the skill level of operators: Provide training and assessment to operators to improve their operating skills and familiarity with the instrument. This can reduce operating errors and improve orientation efficiency.
5. Optimize the observation plan: Choose an appropriate observation plan based on the actual situation, such as selecting the appropriate observation time, optimizing the observation location, etc. This can improve the accuracy and efficiency of observations.
6. Utilize modern communication technology: The introduction of modern communication technology, such as remote data transmission, real-time data processing, etc., can reduce data transmission and processing time and improve orientation efficiency.
7. Integrated multi-sensor technology: Integrating the gyro theodolite with other sensors (such as GPS, accelerometer, etc.) can further improve orientation accuracy and efficiency.
8. Develop new gyro theodolite: Continuously develop new gyro theodolite to improve its measurement accuracy, stability and automation, thereby further improving orientation efficiency.

 

Summarize

Improving the orientation efficiency of gyro theodolite can be achieved through automation and intelligence, as well as optimizing data processing methods. The gyro theodolite developed by our company not only uses the above-mentioned technology to improve its directional accuracy in the directional navigation function, but more importantly, our company's gyro theodolite has higher directional accuracy. For example, ER-GT-02 is ultra-high precision. Gyro theodolite has the following features:

 

1. Orientation accuracy ≤3.6" (1σ);
2. Strong pit interference capability, integrated body design, compact structure and stable performance;
3. It has the functions of low position locking, automatic zero adjustment and observation, etc.

If you want to know more product knowledge about gyro theodolite, please contact our professional technical staff.

Precision Analysis of Fiber Optic Gyro Engineering Structure Deformation Detection

 

1 Method of engineering structure deformation detection based on fiber optic gyroscope

The principle of the engineering structure deformation detection method based on fiber optic gyro is to fix the fiber optic gyro to the detection device, measure the angular velocity of the detection system when running on the measured surface of the engineering structure, measure the operating distance of the detection device, and calculate the operating trajectory of the detection device to realize the detection of engineering structure deformation. This method is referred to as the trajectory method in this paper. This method can be described as "two-dimensional plane navigation", that is, the position of the carrier is solved in the plumb surface of the measured structure surface, and the trajectory of the carrier along the measured structure surface is finally obtained.

According to the principle of trajectory method, its main error sources include reference error, distance measurement error and Angle measurement error. The reference error refers to the measurement error of the initial inclination Angle θ0, the distance measurement error refers to the measurement error of ΔLi, and the Angle measurement error refers to the measurement error of Δθi, which is mainly caused by the measurement error of the angular velocity of the fiber optic gyroscope. This paper does not consider the influence of reference error and distance measurement error on the deformation detection error, only the deformation detection error caused by the fiber optic gyroscope error is analyzed.

2 Analysis of deformation detection accuracy based on fiber optic gyroscope

2.1 Error modeling of fiber optic gyroscope in deformation detection applications

Fiber optic gyro is a sensor for measuring angular velocity based on Sagnac effect. After the light emitted by the light source passes through the Y-waveguide, two beams of light rotating in opposite directions in the fiber ring are formed. When the carrier rotates relative to the inertial space, there is an optical path difference between the two beams of light, and the optical interference signal related to the rotational angular speed can be detected at the detector end, so as to measure the diagonal speed.
The mathematical expression of the fiber optic gyro output signal is: F=Kw+B0+V. Where F is the gyro output, K is the scale factor, and ω is the gyro
The angular velocity input on the sensitive axis, B0 is the gyroscopic zero bias, υ is the integrated error term, including white noise and slowly varying components caused by various noises with long correlation time, υ can also be regarded as the error of zero bias.
The sources of measurement error of fiber optic gyroscope include scale factor error and zero deviation error. At present, the scale factor error of the fiber optic gyroscope applied in engineering is 10-5~10-6. In the application of deformation detection, the angular velocity input is small, and the measurement error caused by the scale factor error is much smaller than that caused by the zero deviation error, which can be ignored. The DC component of the zero-bias error is characterized by the zero-bias repeatability Br, which is the standard deviation of the zero-bias value in multiple tests. The AC component is characterized by zero bias stability Bs, which is the standard deviation of the gyroscope output value from its mean in one test, and its value is related to the sampling time of the gyroscope.

2.2 Calculation of deformation error based on fiber optic gyroscope

Taking the simple supported beam model as an example, the error of deformation detection is calculated, and the theoretical model of structural deformation is established. On this basis, the detection is set
Based on the operating speed and sampling time of the system, the theoretical angular velocity of the fiber optic gyro can be obtained. Then the angular velocity measurement error of the fiber optic gyro can be simulated according to the zero deviation error model of the fiber optic gyro established above.

2.3 Example simulation calculation

The simulation setting of running speed and sampling time adopts a range-varying mode, that is, the ΔLi passed by each sampling time is fixed, and the sampling time of the same line segment is changed by changing the running speed. For example, when the ΔLi is 1 mm, such as the running speed is 2 m/s, the sampling time is 0.5 ms. If the operating speed is 0.1 m/s, the sampling time is 10 ms.

3 Relationship between fiber optic gyroscope performance and deformation measurement error

Firstly, the effect of zero-bias repeatability error is analyzed. When there is no zero bias stability error, the angular velocity measurement error caused by zero bias error is fixed, such as the faster the motion speed, the shorter the total measurement time, the smaller the impact of zero bias error, the smaller the deformation measurement error. When the running speed is fast, the zero bias stability error is the main factor causing the system measurement error. When the running speed is low, the zero bias repeatability error becomes the main source of the system measurement error.
Using typical medium precision fiber optic gyro index, that is, zero bias stability is 0.5 °/h when sampling time is 1 s, Zero repeatability is 0.05 °/h. Compare the system measurement errors at the operating speed of 2 m/s, 1 m/s, 0.2 m/s, 0.1 m/s, 0.02 m/s, 0.01 m/s, 0.002 m/s and 0.001 m/s. When the operating speed is 2 m/s, The measurement error is 8.514μm (RMS), when the measurement speed is reduced to 0.2m /s, the measurement error is 34.089μm (RMS), when the measurement speed is reduced to 0.002m /s, the measurement error is 2246.222μm (RMS), as can be seen from the comparison results. The faster the running speed, the smaller the measuring error. Considering the convenience of engineering operation, the running speed of 2 m/s can achieve better than 10 μm measurement accuracy.

4 Summary

Based on the simulation analysis of the engineering structure deformation measurement based on fiber optic gyro, the error model of fiber optic gyro is established, and the relationship between the deformation measurement error and the performance of fiber optic gyro is obtained by using the simple supported beam model as an example. The simulation results show that the faster the system runs, that is, the shorter the sampling time of the fiber optic gyroscope, the higher the deformation measurement accuracy of the system when the sampling number is unchanged and the distance detection accuracy is guaranteed. With the typical medium precision fiber optic gyro index and the running speed of 2 m/s, the deformation measurement accuracy of better than 10 μm can be achieved.
Ericco's ER-FOG-851 has a diameter of 78.5mm and an accuracy of ≤0.05 ~ 0.1º/h. ER-FOG-910 precision 0.02º/h, belongs to the high tactical level of the fiber optic gyroscope, our company produced gyroscope with small size, light weight, low power consumption, fast start, simple operation, easy to use and other characteristics, widely used in INS, IMU, positioning system, north finding system, platform stability and other fields. If you are interested in our fiber optic gyro, please feel free to contact us.

Analysis of Main Performance Parameters of Fiber Optic Gyroscope

 

1.Fiber Optic Gyroscope

When measuring the rotation and direction of aircraft and other moving objects, the accuracy of fiber optic gyroscopes is inherently limited by using ordinary classical optical methods.
The phase difference measurement accuracy of fiber optic gyroscope determines the overall precision of rotation measurement. The accuracy of fiber optic gyro is limited by many noise sources, and the main influence factor is shot noise. The quantization of photons produces shot noise. When a single photon passes through a device, its discrete nature means that the flow is not perfectly smooth, resulting in white noise. While it is possible to reduce shot noise by increasing the power (the rate at which photons pass through), the greater the power, the greater the other noise, so there is a trade-off.
At present, due to the low power of the detectors used, the new fiber optic gyroscopes do not yet pose a threat to commercial (classical) fiber optic gyros. The researchers expect that as detector technology advances and photon source brightness increases, entangled photon fiber gyroscopes will be commercially available in the near future. Overall, physicists hope that the current results represent a first step toward pushing the ultimate limits of sensitivity in fiber-optic gyroscopes.
The realization of the fiber optic gyroscope is mainly based on the theory of Segnick: when the light beam travels in a circular channel, if the circular channel itself has a rotational speed, then the time required for the light to travel along the direction of rotation of the channel is more than the time required to travel along the opposite direction of the channel.

2. Main performance parameters of fiber optic gyroscope

Through the above introduction, we must have a preliminary understanding of the fiber optic gyro. In this part, we mainly understand some of the main performance parameters of fiber optic gyroscopes.

2.1 Zero bias and zero drift

Zero bias is the output of the gyroscope when the input angular velocity is zero (i.e. the gyroscope is at rest), expressed as the equivalent input angular velocity corresponding to the average output value measured within a specified time, ideally the component of the Earth's rotation angular velocity. Zero drift is zero bias stability, indicating the degree of dispersion of the gyroscope output around its zero bias mean when the input angular rate is zero, expressed by the equivalent input angular rate corresponding to the standard deviation of the output in a specified time. Zero drift is the most important and basic index to measure the accuracy of FOG(fiber optic gyro). The main factor of zero drift is the non-reciprocal phase shift error introduced in the fiber coil by the ambient temperature variation. In order to stabilize zero drift, temperature control or temperature compensation of IFOG is often required. In addition, polarization will also have a certain influence on zero drift. In IFOG, polarization filtering and polarization-maintaining fiber are often used to eliminate the influence of polarization on zero drift.

2.2 Scale factor

Scale factor is the ratio of the output of the gyroscope to the input angular rate, which can be expressed by a specific linear slope on the coordinate axis. It is an index reflecting the sensitivity of the gyroscope, and its stability and accuracy are an important index of the gyroscope, comprehensively reflecting the test and fitting accuracy of the fiber optic gyro. The stability of the scale factor is dimensionless and is usually expressed in parts per million (ppm). The error of the scale factor mainly comes from the temperature change and the instability of the polarization state of the fiber.

2.3 Random walk coefficient

A technical index to characterize the white noise of the angular velocity output in a fiber optic gyro, it reflects the uncertainty of the angular velocity integral of the fiber optic gyro output over time, so it can also be called an angular random walk. The random walk coefficient reflects the development level of the gyroscope, and also reflects the minimum detectable angular rate of the gyroscope. The error is mainly due to random spontaneous emission of photons, noise and mechanical jitter introduced by photodetector and digital circuit.

2.4 Threshold and resolution

The threshold indicates the minimum input rate that a fiber optic gyro can sense. Resolution represents the minimum input rate increment that a gyroscope can sense at a specified input angular rate. Both threshold and resolution characterize the sensitivity of a fiber optic gyroscope.

2.5 Maximum input angular speed

Represents the maximum input rate of the gyroscope in the positive and negative directions, and represents the dynamic range of the gyroscope, that is, the rate range of the fiber optic gyro can be induced.

3.Summary

When choosing a fiber optic gyros, we mainly look at its zero bias stability, zero bias repeatability, measurement range, etc., such as our Ericco fiber optic gyro ER-FOG-50, its measurement range is -500~500, zero bias stability is 0.2~2.0º/h, zero bias repeatability and zero bias stability are consistent. ER-FOG-60, its measuring range is -1000~+1000, zero bias stability is 0.06~0.5º/h, compared with the ER-FOG-60 measurement range is large, the accuracy is relatively high, of course, we have a variety of medium precision fiber optic gyro models, mainly according to your application scenario to decide. If you are interested in our fiber optic gyro, please feel free to contact us.

Application of Gyro Theodolite in Precision Measurement of large-scale Aerospace Products

For precision measurement of spacecraft equipment installation, a theodolite is usually used to measure the reference cubic mirror on the equipment (hereinafter referred to as the "reference mirror").

 

To establish a coordinate system through alignment, the fixed-position theodolite is used to aim at each other with multiple other theodolites to achieve datum transfer, and finally the installation attitude relationship between the two devices is obtained. In the complete docking state of the "Tiangong-1" large spacecraft, it is necessary to accurately measure the installation posture of the equipment when replacing equipment in the final assembly platforms at different heights and levels of 9 to 11 m. In order to ensure the accuracy of measurement, the platform must be used to separate humans and machines. This results in the theodolite being unable to aim at each other between different platforms during the measurement process to complete attitude measurement between devices. In order to solve this kind of problem, this paper conducts research on the measurement principle of gyro-theodolite, and cleverly uses the geodetic coordinate system as the intermediate transfer coordinate system to solve the problem of conventional theodolite testing being unable to establish the relationship between the reference mirror coordinate systems due to mutual sighting being blocked.

 

1.Application of gyro theodolite

1.1 Measuring principle of gyro-theodolite

The gyro-theodolite is an autonomous directional measuring instrument, which is an organic combination of a gyroscope and a theodolite. It uses the fixed axis and precession of the gyroscope to be sensitive to the Earth's rotation, thereby measuring the normal direction of the Earth's rotation, commonly known as the true north direction. It does not require any external information and can measure the true north direction within a range of 75° north and south, thereby obtaining the required orientation. The geodetic coordinate system can be defined by the true north direction and the geodetic horizontal plane. The relationship between the geodetic coordinate system and the theodolite absolute coordinate system is determined based on the reading of the code disk in the horizontal rotation plane of the theodolite in the true north direction.

 

1.2 Measurement algorithm of gyro-theodolite

The joint construction of the gyro-theodolite and theodolite is a new method. Its principle is to find the north through the gyro-theodolite and determine the true north datum. Use a theodolite to measure the normal pitch angle of the two mutually perpendicular horizontal reflection end surfaces of the measured reference mirror and the azimuth angle of the normal horizontal projection line. Using the relationship between spherical and solid trigonometric functions, the angles between the normals of each end surface of the reference mirror and the three coordinate axes of the geodetic coordinate system are calculated.

 

After measuring the attitude of the reference mirrors in the same coordinate system, the attitude between any two reference mirrors can be obtained through calculation, which is the attitude relationship between the reference mirrors.

 

The specific algorithm is as follows: use theodolite and gyro-theodolite to collimate and measure two arbitrarily placed reference mirrors, and perform mutual aiming between instruments after collimation. As shown in Figure 1, assume that the gyro-theodolite has observational quantities T1 (horizontal angle), V1 (elevation angle), a1 (azimuth angle) for the collimation of the cubic mirror, and the theodolite has the observational quantities T4, V2 and longitude for the collimation of the reference mirror. The transferred azimuth angle a2, the mutual aiming measurements of the gyro-theodolite and theodolite are T2 and T3

Figure 1 Reference mirror azimuth transfer diagram

 

The azimuth angle of the theodolite collimation after mutual aiming transfer can be obtained by the following formula

Based on the azimuth angle and vertical angle of the collimation line on each surface of the reference mirror, the direction cosine of each collimation direction is obtained:

1.3 Analysis of the accuracy error of the north position of the reference mirror measured by the gyro-theodolite

This mainly includes two items: one is the comprehensive error of the north-seeking orientation of the gyro; the other is the error of the theodolite, that is, the theodolite mutual aiming error, theodolite collimation error, and the gyro-theodolite collimation error. The attitude calibration process between cubic mirrors is equivalent to the alignment and mutual aiming measurement of two single reference mirrors. Each mutual aiming and collimation is regarded as the sighting of the theodolite, and the mutual aiming transfers the azimuth angle obtained by the reference mirror.

 

Summarize

According to the measurement principle of the gyro theodolite, the geodetic coordinate system is used as the intermediate transfer coordinate system, which solves the problem that the theodolite cannot establish the reference mirror coordinate system because the mutual sighting is blocked. The gyro-theodolite developed by ERICCO, such as: ER-GT-02, is a super High-precision gyro theodolite, orientation accuracy ≤3.6" (1σ); has strong pit interference capability, integrated body design, compact structure, and stable performance; most importantly, it can serve large spacecrafts and can be compared For accurate measurements, Ericco also has multiple gyroscopic theodolites.

If you would like to know more, please contact us.

Ripple Free Minimum Beat Control of Closed-loop FOG Gyro

 The interferometric FOG gyro is a kind of angular velocity sensor based on Sagnac effect, which has the advantages of all-solid state, shock resistance, low cost, small size, etc., and has been widely used in aerospace, aviation, navigation, oil exploration well and other fields. Digital closed-loop IFOG carries out phase modulation according to the feedback signal, eliminates the phase shift caused by the speed, improves the dynamic range and scale factor linearity of IFOG, and is currently the mainstream scheme of medium-high precision IFOG. In the digital closed-loop fiber optic gyroscope, it is necessary to control according to the intrinsic frequency. The intrinsic frequency of different fiber optic gyroscopes is different, and the intrinsic frequency is usually generated by phase-locked loop technology. However, the system clock will have random fluctuations, and the frequency generated by the PLL will also fluctuate, which will lead to ripple interference in the output of the control system, and seriously affect the precision of the digital closed-loop fiber optic gyro.

In order to improve the reliability and adaptability of digital closed-loop IFOG, we established a theoretical model of digital closed-loop IFOG based on the principle of four-state square wave modulation and demodulation, analyzed the stable operating conditions and dynamic performance of digital closed-loop IFOG, and pointed out the shortcomings of the original IFOG digital closed-loop FOG gyro control system. An optimal control scheme of minimum beat without ripple is proposed. The performance of the digital closed-loop FOG gyro before and after optimization is compared by experiments. The results show that the zero bias stability and other performance indexes of the gyroscope after optimization are obviously improved.

1 Theoretical model

The digital closed-loop IFOG structure mainly includes light source, coupler, phase modulator (y-waveguide), optical fiber ring, photodetector, preamplifier, analog/digital converter (ADC), digital logic processor, digital/analog converter (DAC) and output buffer amplifier. The Sagnac interferometer consists of a light source, a coupler, a phase modulator and a fiber ring. When the fiber ring rotates, the phase difference generated by the two light waves traveling opposite each other in the loop is proportional to the rotation rate Ω.

2 Performance analysis and optimization

2.1 Control system performance analysis

The digital closed-loop IFOG frequency bandwidth increases with the increase of open-loop gain K, when the open-loop gain is greater than 0.2, in the high band, the gain is greater than 1. In order to obtain large bandwidth and stable control, 0.2 open loop gain is the best open loop gain.

2.2 Optimization of closed-loop control system

The performance analysis of the IFOG control system shows that the control system is a type I system with steady-state error under the input of angular acceleration. The sampler introduced in the design of integrator and step wave increases the overshoot and decreases the stability of the system. In order to improve the stability of the digital closed-loop FOG gyro, two samplers can be eliminated by output from the front stage of the cache during integral control and step wave generation. According to the control design method of second-order ripple free minimum beat system, the original IFOG controller is modified to eliminate the static error under the ramp input, and the optimized digital closed-loop IFOG ripple free minimum beat control system is obtained.

3. Experiment and result analysis

According to the block diagram of the digital closed-loop IFOG non-ripple minimum beat control system, the input-output relationship of the controller can be obtained as
Y (k) = y (k - 1) + [y (k - 1) - (k - 2)] y + x (k - 1) + [x (k - 1) - x (k - 2)], type: x (k) for k times the amount of error; y(k) is the output at time k. The above analysis shows that the output error and output can be delayed by two control cycles respectively through registers in FPGA, and the change of error and output can be obtained by subtracter and used for integral control, so as to realize the control of ripple free minimum beat system in FPGA.

According to the GJB2426A-2004 test method of FOG gyro, we evaluate the advantages and disadvantages of the non-ripple minimum beat control method by comparing the difference between the original control and non-ripple minimum beat control in the main indexes of FOG gyro, such as zero bias stability, Angle random walk and scale factor nonlinearity. The specific steps are as follows: the original control method and the non-ripple minimum beat control method are used to test the performance index of type 70 fiber optic gyro at room temperature respectively for 1 h. During the experiment, the length of the fiber ring in the fiber optic gyro is 800 m, the average diameter of the fiber ring is 70 mm, and the light source is the superradiation light-emitting diode. The optical wavelength is 1 310 nm, the ADC bit width is 12 bit, and the DAC bit width is 16 bit. The test results show that the output noise of the fiber optic gyro is obviously reduced under the control of the minimum beat without ripple, which indicates that the original control system results in a large output noise due to the influence of the control ripple. The analysis results show that the performance of fiber optic gyro is obviously improved under the control of non-ripple and minimum beat.

4 Summary

The theoretical model of IFOG digital closed loop under four-state square wave modulation is analyzed theoretically. It is pointed out that the original control system is a type I system, and the two-stage delay generated in the process of integral control and step wave generation reduces the reliability of the system, and the steady state error is proportional to the acceleration and inversely proportional to the open-loop control gain under the input of angular acceleration. In order to eliminate the influence of steady-state error, a digital closed-loop FOG gyro control system without ripple minimum beat is designed to eliminate the ripple error caused by the frequency of digital closed-loop IFOG in principle. The experimental results show that the ripple free minimum beat control can effectively improve the performance of digital closed-loop IFOG.

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