1. Introduction
A shaft boring machine (SBM), as a special type of shaft drilling equipment, can carry out mechanical rock breaking and support quickly. It has many advantages, such as a high degree of mechanization without blasting operation, fewer underground workers, high construction efficiency, good completion quality, and high security. In fact, it has become the major piece of technical equipment for infrastructure construction and resource development, as well as an important development direction of intelligence and automation for modern mines [
1,
2]. The SBM is composed of a control system, support system, propulsion system, driving system, and cutterhead [
3,
4]; the overall structure is shown in
Figure 1. The control system is the core part, and it is responsible for coordinating the normal work of other parts and completing the data interaction with a remote monitoring center. Because the SBM adopts the rolling impact and shear mode to break the complex rock structure, the vibration is strong, and it has a small operating space, high temperature, high dust concentration, and serious electromagnetic interference, and these factors have a serious influence on the control system [
5,
6]. In particular, the movement track of the SBM is easy to deviate from the shaft design axis, and the trajectory is out of alignment, which has a direct impact on construction efficiency and quality and even causes safety accidents. Therefore, attitude measurement units are the key part of the control system, and they are extremely important for the SBM working safely and stably.
At present, attitude measurement methods of tunnel boring machines applied in underground construction contain manual methods and automatic methods [
7]. The operation time is long, and the calculation is complicated for manual methods, but the manual methods are often used as supplementary means for an automatic measurement method because of their high detection accuracy. Automatic measurement methods mainly include a gyroscope guidance system, prism guidance system, laser target guidance system, and visual measurement guidance system, and these systems have some advantages and disadvantages. TMG-32B (gyroscope guidance system) is low cost but needs regular calibration due to the poor stability and low measurement accuracy. ROBOTEC, PPS, and RMS-D (prism guidance system) have a simple structure and low cost but have high requirements for the installation environment with poor real-time performance and low precision. ZED and TUnIS (laser target guidance system) have long measurement distance and high precision but easily cause errors and improper installation. The visual measurement guidance system has a large field of view and good real-time performance, but it is still in the research stage and has some limitations, so there is no typical engineering application [
8,
9,
10,
11,
12].
Obviously, different attitude measurement methods have their applicability and limitations, and the working environments of the SBM are complicated, including mechanical vibration, electromagnetic interference, temperature effect, pressure change, and so on. Consequently, there are no effective technical means for the attitude measurement of the SBM, and it is easy to generate cumulative and random errors only by a single attitude measurement method [
13,
14]. To avoid the above problems, multi-sensor fusion technology can be used to study attitude calculation and error compensation methods, and thus can improve the accuracy and reliability of the attitude measurement system.
Multi-sensor fusion technology is first applied in strategic early warning, precision guidance, and other military fields, and it is gradually extended to remote sensing, medical diagnosis, wireless communication, industrial control, fault diagnosis, and other civilian fields. Currently, the common data fusion algorithms mainly include the weighted average method, Bayesian estimation method, complementary filter method, fuzzy logic inference method, wavelet transform method, artificial neural network method, gradient descent method, and Kalman filter method [
15,
16]. Multi-sensor fusion technology can avoid lots of problems existing in the single sensor system, enhance the viability of related systems, expand time and space coverage, improve credibility, reduce the ambiguity of information, and enhance the robustness and reliability of total system [
17]. At the same time, the accuracy, applicability, and fast response of the multi-sensor data fusion method are important reference indexes, especially the implementation complexity and computational efficiency of different algorithms in software and hardware. In these aspects, the complementary filter and Kalman filter have certain advantages compared with other methods [
18,
19,
20], but the related theories and implementation processes still need further study.
The sensors of micro electro mechanical system (MEMS) have advantages of small size, low power consumption, low weight, low cost, and so on, and they have been widely used in fields of spacecraft, unmanned aerial vehicles (UAVs), mobile robots, pedestrian navigation, and vehicle attitude measurement and control [
21,
22]. Nevertheless, due to their manufacturing processes and working principles, inertial measurement units (IMU) based on MEMS have large systematic errors, such as deviation, scale factor, and drift. Moreover, the installation mode, temperature, vibration, impact, and noise also cause random errors [
23,
24], so the accurate attitude measurement can be realized only after signal processing. Nekrasov et al. [
25] and Lall et al. [
26] analyzed the failure mode of the MEMS gyroscope by thermal cycle and vibration tests and improved the performance of the gyroscope with modification tests. However, the method tends to have poor versatility only through hardware and mechanical structure improvements because there are differences between different MEMS sensors. Therefore, it is necessary to study the data fusion and error compensation method based on MEMS multi-sensors to effectively improve their applicability in the attitude measurement of the SBM.
In the field of underground space exploration, there are few studies on the data fusion and attitude estimation of MEMS sensors. Because of the complexity and particularity of the SBM, there is no reliable solution, and yet, some research results in other fields are worthy of reference. In the application of aircraft navigation, the noise and drift of MEMS inertial sensors will produce large errors in the attitude and heading reference system (AHRS), which can be improved by some sensor fusion methods, such as the Kalman filter [
27]. The adaptive Kalman filter [
28] and extended Kalman filter (EKF) [
29] with a linear model and adaptive gain were adopted to avoid non-linear problems of conventional methods and the influence of dynamic acceleration on filter performance. Furthermore, the information fusion and attitude estimation based on the non-linear observer of a quaternion is a further improvement for the Kalman filter [
30]. Zhu et al. [
31] carried out real-time measurement and state estimation for the attitude and stability of articulated heavy vehicles by MEMS sensors and the complementary filter, which improved the safety and stability of the vehicle. Actually, the complementary filter method has advantages of having simple principles and small computation, but its disadvantage is that there is no reliable method to identify the boundary between high-pass filtering and low-pass filtering. The Kalman filter has a limited ability to adjust the changes of acceleration and vibration, but MEMS sensor fusion can restrain abnormal changes of external parameters. Generally, the accuracy is not ideal only through the simple integration of a gyroscope, accelerometer, and magnetometer [
32], but it is feasible to compensate the drift errors of the gyroscope with data from the accelerometer and magnetometer.
At the same time, to reduce the data acquisition and calculation burden of remote control centers and improve the ability of autonomous attitude measurements and rapid response for the SBM, it is necessary to implement the algorithm in attitude measurement units and the embedded processor of the monitoring terminal, but the implementation complexity and computational efficiency of the hardware and software must be considered. Due to the speed limit, the 8-bit and 16-bit microcontrollers only support fixed-point operation, have some difficulties in algorithm implementation, and can barely run some simple algorithms [
33,
34]. However, the flexibility of the algorithm is obviously improved on advanced RISC (reduced instruction set computing) machine (ARM), field programmable gate array (FPGA) and other high-speed chips. For instance, the STM32F3 evaluation board can process the gradient descent method, nonlinear complementary filter, and standard EKF method quickly [
35]; the STM32F103 controller can implement EKF based on quaternion, which meets the accuracy requirement for the attitude measurement of UAVs in dynamic environments [
36]; Sabatelli et al. [
37] proposed an IMU sensor fusion method based on a double stage Kalman filter to reduce the complexity of the algorithm, which was verified on FPGA.
To solve the above problems, based on hardware design and multi-sensor data acquisition, this paper presents a dual coordinate method suitable for the SBM. The method reflects the working characteristics of the SBM and external environment and can be used to calculate the attitude angle and displacement. Combining the complementary filter with EKF, a fusion method of multi-sensor data is put forward to improve the accuracy of attitude estimation, and the method can effectively compensate estimation errors under working conditions. Meanwhile, the theoretical analysis and simulation experiments are adopted to validate related methods. Finally, the reliable attitude information of the SBM is obtained. The block diagram of multi-sensor fusion and error compensation is shown in
Figure 2.
Thus, the main contributions of the presented work are (i) a dual coordinate system structure and analysis method suitable for the attitude measurement of the SBM, (ii) a fusion method of attitude estimation and error compensation based on the complementary filter and EKF, (iii) the complexity and computational efficiency analysis of different algorithms to ensure the real-time performance and reliability of data fusion results.
The rest of this paper is structured as follows. Firstly,
Section 2 introduces the hardware composition and design of the attitude measurement system for the SBM.
Section 3 presents the dual coordinate method based on angle and displacement and then analyzes the attitude characteristics of the SBM, matrix transformation, and the relationship between the attitude changes and coordinate values. The implementation processes of the improved attitude estimation algorithm are explained in
Section 4. Next, experimental results are presented and discussed in
Section 5. Finally, the conclusions and future work are proposed in
Section 6.
4. Attitude Estimation Algorithms
In the attitude measurement system of the SBM, the relative displacement, rotation angle velocity, acceleration, and magnetic strength can be obtained through the PSD, barometer, gyroscope, accelerometer, and magnetometer. However, the attitude angle of the SBM cannot be directly acquired by these sensors. The attitude angle, displacement, and speed must be solved by the estimation algorithm, and then they can be used for attitude control. MEMS sensors are the key data acquisition units, and they inevitably have noises because of the manufacturing process, temperature, vibration, and other factors [
42]. The error models should be established in the attitude calculation and corrected in the process of multi-sensor data fusion. The complementary filter is an analysis method applied in the frequency domain, whereas the Kalman filter is used to deal with the signals in the time domain. Under normal circumstances, useful signals and interfering noises overlap in the frequency and time domains, which leads to useful signals with a degree of randomness. Regarding the switching algorithm between the complementary filter and Kalman filter, the robust adaptive control [
43] and multi-model robust control [
44] are of some reference significance. However, the proposed algorithm is the combination of the two methods, and it is feasible in theory by combining the complementary filter with the EKF. Moreover, according to the characteristics of signals and noises, the error compensation models can be established to recover useful information and effectively improve the measurement accuracy and dynamic performance.
4.1. Complementary Filter
Due to the good dynamic response characteristics of gyroscopes, the attitude angle obtained by the integration is relatively accurate in a short period. However, the drift errors accumulate continuously over time and the accuracy decreases, so gyroscopes are not suitable for long-term measurement. At the same time, there are no time accumulation errors for accelerometers and magnetometers; they have good long-term stability, but they are not suitable for short-term measurement because of slow dynamic response, low short-time measurement accuracy, and are easily affected by the motion acceleration and the external environment. Obviously, the characteristics of the gyroscope, accelerometer, and magnetometer are complementary in the frequency domain, and the complementary filter can distinguish noise from the frequency domain, so the complementary characteristics can be used to obtain an accurate attitude angle by information fusion [
35]. The transfer function of the complementary filter can be written as:
where
is the real attitude matrix,
represents the attitude matrix of the complementary filter estimation, and the low-pass filter
is designed to remove the high-frequency noise
of the accelerometer and magnetometer. In addition, the observation data matrix is
, and the high-pass filter
is designed to remove the low-frequency noise
of the gyroscope; the observation data matrix is
at this time.
shows that there is no attenuation of the attitude signal.
To eliminate the influence of static errors, PI feedback control can be added on the basis of the complementary filter; thus,
will be expressed as
[
45], and
determines the cut-off frequency
of the filters (
. When the noise frequency
, the gyroscope plays a major role in the calculation results; as
, the results come from accelerometers and magnetometers.
mainly affects the dynamic performance and stability of the complementary filter; a large
value makes the adjustment time too long, which reduces the real-time performance of the algorithm, and the small
value increases output error. Moreover,
determines the time of the filter eliminating static errors, generally
,
,
in this paper.
The complementary filter is adopted to correct the roll angle and pitch angle data of the gyroscope by the accelerometer as well as compensate the yaw angle data by the magnetometer. The output values of the accelerometer and magnetometer in the angle coordinate system are respectively:
,
. When the accelerometer is stationary or moving at a constant speed relative to the reference coordinate system, the value of gravity acceleration is
, the unit vector is
[
38]. The roll angle
and pitch angle
solved by the accelerometer and the yaw angle
obtained from magnetometer can be derived as:
The value of the magnetometer is in the reference coordinate system, is the north component of the geomagnetic field in the reference coordinate system, and is the vertical component; thus, the unit vector is . The gravitational field measured by the accelerometer is converted to the angle coordinate system.
The geomagnetic field measured by the magnetometer is converted to the angle coordinate system.
However, the error divergence occurs when the data status is updated, which will reduce the reliability of angle estimation, and the attitude transformation matrix of quaternions no longer satisfies the normalization after a long time, so the attitude transformation matrix should be normalized when the quaternions are updated every time.
,
,
Q′ are, respectively, are the normalization form of the accelerometer measurement value
, the magnetometer measurement value
and the quaternion
Q; they are given as:
where
,
can, respectively, carry out the cross-product operation with
,
, and then
,
can be obtained,
is the measurement error of roll angle
α, and pitch angle
β,
is the measurement error of yaw angle
γ. Furthermore, the overall measurement error
e can be expressed as:
The compensation value
η of the gyroscope drift can be obtained by the overall measurement error
e by using PI feedback control.
Finally, is used to compensate the angle velocity of the gyroscope, and is substituted into Equation (12) to update quaternions iteratively; thus, the attitude angle is calculated preliminarily.
4.2. Extended Kalman Filter
The Kalman filter is an optimal linear estimation, and its principle is that the optimal value of the current state is estimated based on the statistical error at the previous moment and the error of the measured value at this moment; the attitude angle and displacement at the next moment are obtained through the prediction and update process. However, the filter model of the SBM is nonlinear, and the filtering algorithm must be suitable for a nonlinear system. The EKF is established on the basis of the Kalman filter, the specific process of the EKF is that the nonlinear part of the state equation is expanded into the Taylor series, after omitting the parts of the second order and above, the approximate linearized model is obtained and applied in the state estimation with a linear Kalman filter method.
To design the Kalman filter, the state equation and observation equation of the system need to be established.
is the state model variable,
is the attitude angle vector, and
is calculated by the complementary filter method.
is the displacement vector,
,
are the horizontal displacements from PSD, and
is the vertical displacement solved by the barometer. Moreover,
is the speed vector. According to the kinematics theory, the relations of different vectors are expressed as:
where
is the motion acceleration vector, and
is the transfer matrix from the angle coordinate system to the reference coordinate system.
is the noise vector of the accelerometer in the axial direction, and its value is available from the accelerometer manual. The measurement value
of the accelerometer is the vector sum of gravitational acceleration and motion acceleration, and it is necessary to compensate the gravitational acceleration when the motion acceleration is calculated. In practical application, the attitude angle, displacement, and vertical speed are the key indexes; other parameters are only process calculation variables. The interval time of system status update is
T, on the basis of the differential equation of the attitude angle and kinematics analysis, the state equation of Kalman filter is obtained as:
where
is the system noise vector. At the current moment,
is the attitude angle calculated by accelerometer and magnetometer, and the speed and displacement are solved by the integral of motion acceleration. Based on these observational variables, the observation equation is expressed as:
where
is the observed noise vector. By using the EKF, the nonlinear part of the state equation is expanded by the Taylor series and approximated by the first order, and the approximate linearized model is obtained; thus, the Kalman filter can be applied. After the state equation linearization, the Jacobian matrix
can be derived as:
The prediction and update process of the EKF algorithm is divided into five parts; they are as follows:
State prediction equation:
is the estimation of the status variable , is the state transition matrix, i.e., the Jacobian matrix.
Covariance prediction equation:
is the estimation of the state covariance matrix , is the covariance matrix of the system noise.
Kalman filter gain update:
is the covariance matrix of the observed noise.
State covariance matrix update:
In conclusion, based on the multi-sensor data of the gyroscope, accelerometer, magnetometer, PSD, and barometer, the fusion method of the complementary filter and EKF is used to calculate the displacement, attitude angle, and vertical movement speed, and the flowchart of the proposed method is shown in
Figure 9.
6. Conclusions and Future Work
This paper presents a dual coordinate method suitable for the attitude measurement of the SBM, and the key parts of coordinate analysis are the attitude matrix transformation and state update, and they are in the form of quaternions to improve the efficiency of the calculation process. The dual coordinate method makes full use of the data from the gyroscope, accelerometer, magnetometer, PSD, and barometer to calculate the angle, displacement, and speed, which can accurately describe the attitude characteristics of the SBM in the restricted space of the shaft. Meanwhile, to improve the environmental adaptability of the attitude measurement system, we proposed the improved attitude estimation and error compensation method and deduced the method implementation process theoretically.
Through simulation experiments of the attitude estimation, this paper analyzes the steady-state performance and dynamic response performance of different algorithms. The mechanical vibration, electromagnetic interference, and random noise have significant effects on attitude estimation, but the air pressure effect is small. As a method of combining the time domain and frequency domain, the proposed filter can give full play to the advantages of the complementary filter and EKF, and it can ensure the effective acquisition of real attitude information. Furthermore, the time consumption experiments indicate that the complexity of different algorithms have a certain requirements for the hardware configuration, thus the experimental results can provide a reference for the follow-up research.
All experiments in this paper are performed at room temperature. Actually, the influence of temperature change on attitude estimation cannot be ignored, so it is necessary to study the temperature estimation and error compensation method. Moreover, due to the limitations of experimental conditions, there are still some differences between the simulation experiment platform and the SBM; thus, the method proposed in this paper still needs to be improved by the actual working test of the SBM.