ece-4960-speedbots

Lab report repository for ECE 4960: Fast Robots

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Lab 10

Summary

In this lab, I learned to use a simulation environment for a virtual robot. Using the simulator, I was able to demonstrate a basic control of my robot with both open and closed loop controllers. In addition, I plotted various robot data related to its position from the odometry and the expected pose.

Procedure

Setup

The lab required using Python for the plotter and interfacing with the robot in the virtual environment using the controller. For this, I installed a fresh venv of Python 3.10 with all the necessary libraries.

Simulator

The simulator represents a closed room space with the robot around the center of the room. The robot is represented by three pose values: x, y, and yaw (orientation). Using basic API calls in Python, the robot can change any of its three values, which can effectively simulate a rotation or translation.

In addition to the simulator, there exists a plotter that maps two data series:

  1. The odometry (sensor) data, which is measured by the “sensors” on the robot. These sensors are simulated.
  2. The ground truth pose of the robot. This is exact.

Open loop control

For a basic demonstration of open loop control of the virtual robot, I used a simple set of calls to the cmdr variable to execute a rough square trajectory. Here is the code I used to do this:

def plot():
    pose, gt_pose = cmdr.get_pose()

    cmdr.plot_odom(pose[0], pose[1])
    cmdr.plot_gt(gt_pose[0], gt_pose[1])

for i in range(4):
    cmdr.set_vel(1, 0)
    await asyncio.sleep(0.5)

    cmdr.set_vel(0, np.pi)
    await asyncio.sleep(0.5)

    cmdr.set_vel(0, 0)

    plot()

As evident from the animation, the ground truth (green) follows the exact path that the robot took in its trajectory. The odometry data (red) is far less accurate, but tries to follow the robot’s estimated pose using a virtual “IMU.” As the duration of the velocity command of the cmdr is forever, it must be explicitly changed to zero to stop the robot’s motion.

Another interesting thing of note is that the robot does not always execute the same shape. One possible reason for this is due to the asyncio delay, since the library is going to have some processing delay time depending on the conditions of the computer, the Jupyter server, etc. Another explanation is that the turns use np.pi which is going to have floating point approximation which may lead to observing nondeterministic behavior.

Closed loop control

With these tools for the simulator, I was able to design a closed loop controller for the robot. Here is a snippet of code that demonstrates how the robot is able to intelligently avoid obstacles (most of the time – more on this later):

def drive(duration):
    start = time.time()
    turn_threshold = 0.5
    max_speed = speed = 20  # The initial speed

    while (time.time() - start < duration):
        if (get_front_dist() > speed * 0.1 + turn_threshold):
            await move_forward(speed, 0.1)
            speed = min(speed * 2, max_speed)
        elif (get_front_dist() < turn_threshold):
            await turn(random.random() * 360)
        else:
            speed /= 2

drive(10)

The algorithm works as follows:

  • The initial parameters are set: the tolerance (front distance) for turning, the max (and starting) speed of the robot, and how long to run the algorithm.
  • While the robot is far away from the front wall, move forward at double the speed, up to the max speed.
  • If the robot is within the turn threshold, turn in a random direction.
  • If the robot is close to the wall but still can move before turning, reduce its speed by half.

Here is a demo of the robot in action with the corresponding plot:

For my closed loop controller, I was able to achieve what I wanted without a specific turn algorithm. Because of how often the controller recomputes the next action (as soon as the Python process can), using randomized turns worked well enough. I found that a reasonable speed was around 15-20. Any speed faster than that could sometimes lead to collisions due to processing delay. As evident in the video, the robot can move pretty quickly without colliding, and because I set the turn_threshold = 0.5, it can move as close as that distance. Reducing this distance further is possible, but I wasn’t able to get as good of a result as I did with 0.5.

Overall, the controller works well in most scenarios. Very seldomly, the robot does collide into the wall, but only if the random turn direction was really unlucky several times in a row, as the robot does not ever move backwards and it will keep moving forward even as it turns, albeit with reduced speed. To minimize crashes, an obvious solution is to reduce the speed even more. I’m sure with a better algorithm, crashes could be averted in even more cases, but with such a simple closed loop controller, I am pretty happy with the results.

Conclusion

This was a nifty lab to serve as an introduction to the simulator environment. I am looking forward to using this simulator in the next few labs for more complicated tasks!

References

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