转:https://blog.csdn.net/qq_18661939/article/details/53574981
目前单目slam存在初始化的尺度问题和追踪的尺度漂移问题,而双目也存在精度不高和鲁棒性不好的问题。针对这些问题,提出了融合imu的想法。
那么imu的作用是什么呢?
单目
(1)解决初始化尺度问题
(2)追踪中提供较好的初始位姿。
(3)提供重力方向
(4)提供一个时间误差项以供优化
双目
(1)追踪中提供较好的初始位姿。
(2)提供重力方向
(3)提供一个时间误差项以供优化
目前做这方面融合论文很多,但开源的比较少,这里给出几个比较好的开源code和论文
开源code:
(1)imu和单目的数据融合开源代码(EKF)
https://github.com/ethz-asl/rovio
(2)imu和单目的数据融合开源代码
https://github.com/ethz-asl/okvis_ros(非线性优化)
(3)orbslam+imu(立体相机)
https://github.com/JzHuai0108/ORB_SLAM
论文:
(1)Keyframe-based visual–inertial odometry(okvis的论文)
(2) IMU Preintegration on Manifold for Efficient Visual-Inertial Maximum-a-Posteriori Estimation(预积分)
(3)Visual-Inertial Monocular SLAM with Map Reuse (orb+imu)
(4)Robust Visual Inertial Odometry Using a Direct EKF-Based Approach(eth的rovio)
(5)On-Manifold Preintegration for Real-Time Visual-Inertial Odometry(gtsam)
由于是初学比较详细看得就是以上5篇,而且自认为还不错的论文。
本人研究的是基于非线性优化的视觉和imu融合的算法研究,那么这里先引出融合的方式:
滤波方法:
(1)紧耦合
(2)松耦合
非线性优化:
(1)紧耦合(本人研究方向)
(2)松耦合
imu'和视觉是怎样融合的呢?
仅仅视觉的时候我们优化的只是重投影误差项:
以上的公式我就不解释了。
而imu+视觉优化的是重投影误差项+imu的时间误差项:
其中imu时间误差项:
其中为:
这里:imu时间误差项要求的主要有5个变量:eR,ev,ep,eb,W。即求(R ,v,p,b,W)
这里先给出一张非线性优化视觉+imu融合的图:
下面我们就开始用与积分的方式求以上的6个变量,下面给出预积分的code
Eigen::Matrix4d Tracking::propagate(const double time_frame)
{
bool is_meas_good=getObservation(time_frame);
assert(is_meas_good);
time_pair[0]=time_pair[1];
time_pair[1]=time_frame;
Eigen::Vector3d tempVs0inw;
Eigen::Matrix<double, 15,15>* holder=NULL;
if(bPredictCov)
holder= &P_;
predictStates(T_s1_to_w, speed_bias_1, time_pair,
measurement, imu_.gwomegaw, imu_.q_n_aw_babw,
&pred_T_s2_to_w, &tempVs0inw);
pred_speed_bias_2.head<3>()=tempVs0inw;//速度偏差
pred_speed_bias_2.tail<6>()=speed_bias_1.tail<6>(); //biases do not change in propagation
Eigen::Matrix4d pred_Tr_delta=pred_T_s2_to_w*imu_.T_imu_from_cam;//camera-imu-world(矩阵的乘法从左开始)
cam_to_w=pred_Tr_delta;
pred_Tr_delta=pred_Tr_delta.inverse()*(T_s1_to_w*imu_.T_imu_from_cam);//由imu计算(预测)上一帧-》当前帧的变换关
// T_s1_to_w=pred_T_s2_to_w;
speed_bias_1=pred_speed_bias_2;
return pred_Tr_delta;
}
void Tracking::predictStates(const Eigen::Matrix4d &T_sk_to_w, const Eigen::Matrix<double, 9,1>& speed_bias_k,
const double * time_pair,
std::vector<Eigen::Matrix<double, 7,1> >& measurements, const Eigen::Matrix<double, 6,1> & gwomegaw,
const Eigen::Matrix<double, 12, 1>& q_n_aw_babw,
Eigen::Matrix4d * pred_T_skp1_to_w, Eigen::Matrix<double, 3,1>* pred_speed_kp1,
Eigen::Matrix<double, 15,15> *covariance,
Eigen::Matrix<double, 15,15> *jacobian)
{
double time=time_pair[0],end = time_pair[1];
// the eventual covariance has little to do with this param as long as it remains small
Eigen::Matrix<double, 3,1> r_0(T_sk_to_w.topRightCorner<3, 1>());
Eigen::Matrix<double,3,3> C_WS_0(T_sk_to_w.topLeftCorner<3, 3>());
Eigen::Quaternion<double> q_WS_0(C_WS_0);
Eigen::Quaterniond Delta_q(1,0,0,0);
Eigen::Matrix3d C_integral = Eigen::Matrix3d::Zero();
Eigen::Matrix3d C_doubleintegral = Eigen::Matrix3d::Zero();
Eigen::Vector3d acc_integral = Eigen::Vector3d::Zero();
Eigen::Vector3d acc_doubleintegral = Eigen::Vector3d::Zero();
Eigen::Matrix3d cross = Eigen::Matrix3d::Zero();
// sub-Jacobians
Eigen::Matrix3d dalpha_db_g = Eigen::Matrix3d::Zero();
Eigen::Matrix3d dv_db_g = Eigen::Matrix3d::Zero();
Eigen::Matrix3d dp_db_g = Eigen::Matrix3d::Zero();
// the Jacobian of the increment (w/o biases)
Eigen::Matrix<double,15,15> P_delta = Eigen::Matrix<double,15,15>::Zero();
double Delta_t = 0;
bool hasStarted = false;
int i = 0;
for (int it=0;it<measurements.size();it++)
{
Eigen::Vector3d omega_S_0 =measurements[it].block<3,1>(4,0);//角速度
Eigen::Vector3d acc_S_0 = measurements[it].block<3,1>(1,0);//线加速度
Eigen::Vector3d omega_S_1 = measurements[it+1].block<3,1>(4,0);
Eigen::Vector3d acc_S_1 = measurements[it+1].block<3,1>(1,0);
ave_omega_S=ave_omega_S+omega_S_0;
ave_omega_S=ave_omega_S/(it+1);
// time delta
double nexttime;
if ((it + 1) == (measurements.size()-1)) {
nexttime = end;
}
else
nexttime =measurements [it + 1][0];
double dt = (nexttime - time);
if ( end < nexttime) {
double interval = (nexttime - measurements[it][0]);
nexttime = end;
dt = (nexttime - time);
const double r = dt / interval;
omega_S_1 = ((1.0 - r) * omega_S_0 + r * omega_S_1).eval();
acc_S_1 = ((1.0 - r) * acc_S_0 + r * acc_S_1).eval();
}
/* if ( it+1==measurements.size()) {
double interval = last_dt;
nexttime = end;
double dt = (nexttime - time);
const double r = dt / interval;
omega_S_1 = ((1.0 - r) * omega_S_0 + r * omega_S_1).eval();
acc_S_1 = ((1.0 - r) * acc_S_0 + r * acc_S_1).eval();
}
else
nexttime =measurements [it + 1][0];
double dt = (nexttime - time);*/
if (dt <= 0.0) {
continue;
}
Delta_t += dt;
if (!hasStarted) {
hasStarted = true;
const double r = dt / (nexttime -measurements[it][0]);
omega_S_0 = (r * omega_S_0 + (1.0 - r) * omega_S_1).eval();//求开始是加权的角速度和线加速度
acc_S_0 = (r * acc_S_0 + (1.0 - r) * acc_S_1).eval();
}
// ensure integrity
double sigma_g_c = q_n_aw_babw(3);//Gyroscope noise density.
double sigma_a_c = q_n_aw_babw(1);
// actual propagation
// orientation:
Eigen::Quaterniond dq;
const Eigen::Vector3d omega_S_true = (0.5*(omega_S_0+omega_S_1) - speed_bias_k.segment<3>(3));//w
const double theta_half = omega_S_true.norm() * 0.5 * dt;
const double sinc_theta_half = ode(theta_half);
const double cos_theta_half = cos(theta_half);
dq.vec() = sinc_theta_half * omega_S_true * 0.5 * dt;
dq.w() = cos_theta_half;
Eigen::Quaterniond Delta_q_1 = Delta_q * dq;
// rotation matrix integral:
const Eigen::Matrix3d C = Delta_q.toRotationMatrix();
const Eigen::Matrix3d C_1 = Delta_q_1.toRotationMatrix();
const Eigen::Vector3d acc_S_true = (0.5*(acc_S_0+acc_S_1) - speed_bias_k.segment<3>(6));
const Eigen::Matrix3d C_integral_1 = C_integral + 0.5*(C + C_1)*dt;
const Eigen::Vector3d acc_integral_1 = acc_integral + 0.5*(C + C_1)*acc_S_true*dt;
// rotation matrix double integral:
C_doubleintegral += C_integral*dt + 0.25*(C + C_1)*dt*dt;
acc_doubleintegral += acc_integral*dt + 0.25*(C + C_1)*acc_S_true*dt*dt;
// Jacobian parts
dalpha_db_g += dt*C_1;
const Eigen::Matrix3d cross_1 = dq.inverse().toRotationMatrix()*cross +
okvis::kinematics::rightJacobian(omega_S_true*dt)*dt;
const Eigen::Matrix3d acc_S_x = crossMx(acc_S_true);
Eigen::Matrix3d dv_db_g_1 = dv_db_g + 0.5*dt*(C*acc_S_x*cross + C_1*acc_S_x*cross_1);
dp_db_g += dt*dv_db_g + 0.25*dt*dt*(C*acc_S_x*cross + C_1*acc_S_x*cross_1);
// covariance propagation
if (covariance) {
Eigen::Matrix<double,15,15> F_delta = Eigen::Matrix<double,15,15>::Identity();
// transform
F_delta.block<3,3>(0,3) = -crossMx(acc_integral*dt + 0.25*(C + C_1)*acc_S_true*dt*dt);
F_delta.block<3,3>(0,6) = Eigen::Matrix3d::Identity()*dt;
F_delta.block<3,3>(0,9) = dt*dv_db_g + 0.25*dt*dt*(C*acc_S_x*cross + C_1*acc_S_x*cross_1);
F_delta.block<3,3>(0,12) = -C_integral*dt + 0.25*(C + C_1)*dt*dt;
F_delta.block<3,3>(3,9) = -dt*C_1;
F_delta.block<3,3>(6,3) = -crossMx(0.5*(C + C_1)*acc_S_true*dt);
F_delta.block<3,3>(6,9) = 0.5*dt*(C*acc_S_x*cross + C_1*acc_S_x*cross_1);
F_delta.block<3,3>(6,12) = -0.5*(C + C_1)*dt;
P_delta = F_delta*P_delta*F_delta.transpose();
// add noise. Note that transformations with rotation matrices can be ignored, since the noise is isotropic.
//F_tot = F_delta*F_tot;
const double sigma2_dalpha = dt * sigma_g_c * sigma_g_c;
P_delta(3,3) += sigma2_dalpha;
P_delta(4,4) += sigma2_dalpha;
P_delta(5,5) += sigma2_dalpha;
const double sigma2_v = dt * sigma_a_c * q_n_aw_babw(1);
P_delta(6,6) += sigma2_v;
P_delta(7,7) += sigma2_v;
P_delta(8,8) += sigma2_v;
const double sigma2_p = 0.5*dt*dt*sigma2_v;
P_delta(0,0) += sigma2_p;
P_delta(1,1) += sigma2_p;
P_delta(2,2) += sigma2_p;
const double sigma2_b_g = dt * q_n_aw_babw(9) * q_n_aw_babw(9);
P_delta(9,9) += sigma2_b_g;
P_delta(10,10) += sigma2_b_g;
P_delta(11,11) += sigma2_b_g;
const double sigma2_b_a = dt * q_n_aw_babw(6) * q_n_aw_babw(6);
P_delta(12,12) += sigma2_b_a;
P_delta(13,13) += sigma2_b_a;
P_delta(14,14) += sigma2_b_a;
}
// memory shift
Delta_q = Delta_q_1;
C_integral = C_integral_1;
acc_integral = acc_integral_1;
cross = cross_1;
dv_db_g = dv_db_g_1;
time = nexttime;
interval_time=Delta_t;
last_dt=dt;
++i;
if (nexttime == end)
break;
}
// actual propagation output:
const Eigen::Vector3d g_W = gwomegaw.head<3>();
const Eigen::Vector3d t=r_0+speed_bias_k.head<3>()*Delta_t+ C_WS_0*(acc_doubleintegral)+0.5*g_W*Delta_t*Delta_t;
const Eigen::Quaterniond q=q_WS_0*Delta_q;
(*pred_T_skp1_to_w)=rt_to_T(t,q.toRotationMatrix());
(*pred_speed_kp1)=speed_bias_k.head<3>() + C_WS_0*(acc_integral)+g_W*Delta_t;//???语法曾有错误
if (jacobian) {
Eigen::Matrix<double,15,15> & F = *jacobian;
F.setIdentity(); // holds for all states, including d/dalpha, d/db_g, d/db_a
F.block<3,3>(0,3) = -okvis::kinematics::crossMx(C_WS_0*acc_doubleintegral);
F.block<3,3>(0,6) = Eigen::Matrix3d::Identity()*Delta_t;
F.block<3,3>(0,9) = C_WS_0*dp_db_g;
F.block<3,3>(0,12) = -C_WS_0*C_doubleintegral;
F.block<3,3>(3,9) = -C_WS_0*dalpha_db_g;
F.block<3,3>(6,3) = -okvis::kinematics::crossMx(C_WS_0*acc_integral);
F.block<3,3>(6,9) = C_WS_0*dv_db_g;
F.block<3,3>(6,12) = -C_WS_0*C_integral;
}
// overall covariance, if requested
if (covariance) {
Eigen::Matrix<double,15,15> & P = *covariance;
// transform from local increments to actual states
Eigen::Matrix<double,15,15> T = Eigen::Matrix<double,15,15>::Identity();
T.topLeftCorner<3,3>() = C_WS_0;
T.block<3,3>(3,3) = C_WS_0;
T.block<3,3>(6,6) = C_WS_0;
P = T * P_delta * T.transpose();
}
}
以上code来自以下公式:
其中认为角速度w和加速度a在连续两次imu测量之间是匀速的,因此上式可写成:
其中上式的角速度和加速度:
因此最终公式:
上面公式给出5个变量(R,V,P,b,W)中的3个最重要的变量:R,V,P。
而偏差变量b我们可以初始化的时候可以设为0(其实最好是要求出来的,这里就不给出推倒公式了)。
下面的们就是有关W(权重)的公式了。
其中
是有关R,P,V,b的协方差矩阵
到此为止已经把imu时间误差项求完。
下一篇将是怎样把时间误差项融合到目标函数里(主要是局部地图的优化)
--------------------- 本文来自 金木炎 的CSDN 博客 ,全文地址请点击:https://blog.csdn.net/qq_18661939/article/details/53574981?utm_source=copy