Project 5 - Extended Kalman Filter / Sensor Fusion

Overview

In this project, I utilized an Extended Kalman Filter and Sensor Fusion to estimate the state of a moving object of interest with noisy lidar and radar measurements. The project involved utilzing lidar data (Point Cloud) for position and radar data (Doppler) for radial velocity. Sensor Fusion was utilized to accurately predict where the pedestrian on a bicycle is obtaining RMSE values that are lower than the tolerance outlined in the project rubric.

Background - RADAR - RAdio Detection And Ranging

• Radar has been used in automotive for years, usually placed behind the bumpers on light-duty vehicles.
• Radar utilizes the Doppler effect to measure speed (velocity).
• Can be used for localization, generating radar maps of the environment. Can provide distance to objects without a direct line of sight. Can also see under vehicles & buildings, & other objects that could otherwise be obsecured.
• Least affected by rain or fog, such as Lidar or Cameras
• Wide filed of view ~ 150 degrees
• Range is 200 + metres
• Resultion in vertical dimension is limited
• Debris such as pop cans can have high clutter, high radar reflection, because of this auto manufacturers avoid static objects.

Background - LIDAR - LIght Detection And Ranging

• LIDAR uses infrared light laser beam to determine the distance between the sensor and nearby object.
• Use light in 900 nano-metre wavelength range (Infrared)
• Some LIDAR's use longer wavelengths which perform betterin rain or fog. Can get dirty though and are "bulky!".
• Data is returned as a point-cloud / high resolution
• Higher spatial resolution than Radar.
• Better at vertical scanning versus Radar.

Dependencies

This project requires C++ and the following C++ libraries installed:

Eigen Library - git clone

File Structure

• main.cpp - communicates with a simulator that recieves Lidar and Radar data from our vehicle, calls a function to run the kalman filter and calls a function to calculate the error RMSE. Sends a sensor measurement to FusionEKF.cpp
• FusionEKF.cpp - initializes the filter, calls the predict function, calls the update function. takes the sensor data and initializes variables and updates variables. The Kalman filter equations are not in this file. FusionEKF.cpp has a variable called ekf_, which is an instance of a KalmanFilter class. The ekf_ will hold the matrix and vector values. Uses the ekf_ instance to call the predict and update equations.
• kalman_filter.cpp* - defines the predict function, the update function for lidar, and the update function for radar.
• tools.cpp- function to calculate RMSE and the JacobianMatrix.

Process Flow

• We have a pedestrian on a bicycle that is represented by a 2-D position P_x, P_y as well as 2-D velocity v_x, v_y. Therefore x = P_x, P_y,v_x, v_y
• Each time we receive new sensor measurement data the estimation function process measurement is triggered.

• For the first iteration we just initalize the state x and P covariance matrix.

• After we will call predict and update

• Before predict we need to compute the elapsed time Δt between the current and previous observation.

• Based on the elapsed time we calculate the new predict x' state transition and P' process covariance matrices.

• The measurement update step depends on the the sensor type RADAR or LIDAR

• If sensor = RADAR then we have to compute the new JacobianMatrix using the non-linear measurement function H_j to project the predicted state h(x) and call the measurement update.

• If sensor = LIDAR then we just set up the H and R and then call the measurement update.

• then the car will receive another sensor measurement after a time period Δt. The algorithm then does another predict and update step in a continous loop.

Algorithim

The Kalman Filter algorithm will go through the following steps:

first measurement - the filter will receive initial measurements of the bicycle's position relative to the car. These measurements will come from a radar or lidar sensor.

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initialize state and covariance matrices - the filter will initialize the bicycle's position based on the first measurement. Then the car will receive another sensor measurement after a time period Δt.

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predict (state prediction) - the algorithm will predict where the bicycle will be after time Δt. One basic way to predict the bicycle location after Δt is to assume the bicycle's velocity is constant; thus the bicycle will have moved velocity Δt. We will assume the velocity is constant.

Let’s say we know an object’s current position and velocity , which we keep in the x variable. Now one second has passed. We can predict where the object will be one second later because we knew the object position and velocity one second ago; we’ll just assume the object kept going at the same velocity.

But maybe the object didn’t maintain the exact same velocity. Maybe the object changed direction, accelerated or decelerated. So when we predict the position one second later, our uncertainty increases.

x is the mean state vector. For an extended Kalman filter, the mean state vector contains information about the object’s position and velocity that you are tracking. It is called the “mean” state vector because position and velocity are represented by a gaussian distribution with mean x.

P is the state covariance matrix, which contains information about the uncertainty of the object’s position and velocity. You can think of it as containing standard deviations.

Q is the Process Covariance Matrix. It is a covariance matrix associated with the noise in states.

F is the Transition Matrix (the one that deals with time steps and constant velocities)

u is the motion noise. Motion noise & process noise refer to the same case: uncertainty in the object's position when predicting location. The model assumes velocity is constant between Δt intervals, but in reality we know that an object's velocity can change due to acceleration. The model includes this uncertainty via the process noise.

As an example, a pedestrian randomly changes her velocity (accelerating) between time steps, the overall mean though is zero. This is process noise!

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update (measurement update) - the filter compares the "predicted" location with what the sensor measurement says. The predicted location and the measured location are combined to give an updated location. The Kalman filter will put more weight on either the predicted location or the measured location depending on the uncertainty of each value. Then the car will receive another sensor measurement after a time period Δt. The algorithm then does another predict and update step.

z is the measurement vector.

• For a lidar sensor, the z vector contains the position−x and position−y measurements.
• For a radar sensor, the z vector contains the range(rho), bearing(phi) and radial velocity(rho_dot).

H is the matrix that projects your belief about the object’s current state into the measurement space of the sensor.

• For lidar, this is a fancy way of saying that we discard velocity information from the state variable since the lidar sensor only measures position: The state vector x contains information about P_x, P_y,v_x, v_y whereas the z vector will only contain P_x, P_y. Multiplying Hx allows us to compare x, our belief, with z, the sensor measurement.
• For radar, there is no H matrix that will map the state vector "x" into polar cordinates; instead you need to calculate the mapping manually to convert from cartesian coordinates to polar coordinates.

The H matrix from the Lidar and h(x) equations from Radar are accomplishing the same thing; they are both need to solve for the measurement update error y y = Z - H*x'

R is the covariance matrix. For a radar sensor this matrix represents uncertainty in our sensor measurments. The dimensions of the R matrix is squared and each side of its matrix is the same length as the number of measurement parameters.

• For laser sensors, we have a 2D measurement vector. Each location component Px, PY are affected by a random noise. So our noise vector "w" has the same dimension as "z". And it is a distribution with zero mean and a 2x2 covariance matrix which comes from the product of the vertical vector "w" and its transpose.
• These parameters are provided by the sensor manufacturer

Extended Kalman Filter / Sensor Fusion

Extended Kalman Filters(EKF) linearize the distribution around the mean of the current estimate and then use this linearization in the predict and update states of the Kalman Filter algorithm. An existing Kalman Filter cannot be applied to a non-linear distribution, common with Radar data. The key to be able to solve the kalman filter update equations is to linearize the h(x) function.

As the h(x) function is multi-dimensional the EKF will need to utilize a method called a multivariable Taylor Series Expansion to make a linear approximation of the h(x) function.

where Df(a) is called the JacobianMatrix & D²f(a) is called the Hessan matrix. These represent the first and second order derivatives of multi-dimensional equations.

Jacobian Matrix - To derive a linear approximations for h(x) function, we will only keep the expansion up to the JacobianMatrix Df(a). We will ignore the Hessan matrix D²f(a) and other higher order terms. Assuming (x-a) is small, (x-a)² or the multi-dimensional equivalent (x-a)^T(x-a) will be even smaller; the EKF we'll be using assumes that higher order terms beyond the Jacobian are negligible.

The Extended Kalman Filter Equations then become.

Results

My algorithm was run against Dataset 1 in the simulator which is the same as "data/obj_pose-laser-radar-synthetic-input.txt" in the repository. I collected the positions that the algorithm outputs and compare them to ground truth data. My P_x, P_y,v_x and v_y RMSE are less than or equal to the values [.11, .11, 0.52, 0.52]. Lidar measurements are red circles, radar measurements are blue circles with an arrow pointing in the direction of the observed angle, and estimation markers are green triangles.

Remarks

Sensor fusion is the ability to bring together inputs from multiple radars, lidars and cameras to form a single model or image of the environment around a vehicle. The resulting model is more accurate because it balances the strengths of the different sensors. Working with Lidar and Radar data was a great experience to accurately detect a pedestrian with accuracy. I really enjoyed going through the understanding of how a Kalmin Filter is implemented and updated. What would be really interesting to work with would be camera data, or acoustic data. Overall, though it would also be interesting to work with data from multiple sensors with measurement data being recieved at the same time and weighting the measurements. I really enjoyed this project and see it as an applicable skill as I progress towards becoming a self-driving vehicle engineer.