Lab 7 · Obstacle Avoidance

Crates on the way

The same field as Hello Racer. The waypoints are in a straight line, 8 m apart. Some gaps have a crate in them. The car has a lidar: 24 rays, each a distance to the first thing it hits.

You drive. Your code reads the rays and adds a pull to your steering, so the car leans toward open space. That is teleop assist: a pull, not a takeover.

Do this: drive the course in Play with assist on, gain 1.0. Hold the throttle and let the pull steer you around the crates. Watch the assist number in the readout change sign as a crate passes. Then set the gain to 0 and drive the same course raw. Count the hits both times.

The assist

The Code card has one function: assist(ranges, bearings, goal). It returns a pull in [-1, 1], + is left. The starter blends two pulls: away from the nearest ray when a crate is close, toward the waypoint when the way is clear.

Do this: set KP to 3.0, Apply, and drive. The push away from a crate is now three times harder, and the car swings wide. Then set NEAR to 1.0: the assist only wakes up when a crate is one meter away, too late at speed. Put both back.

Then do this: the ladder. P first, which is what the starter is. If the car oscillates, add a D term: subtract a share of how fast the pull is changing. If it hugs one side, add an I term: a share of the pull summed over time.

// All six waypoints with hits 0 finishes the lab.

Follow the White Rabbit

The controller ladder (P, then D, then I) is the oldest trick in control. It runs your thermostat, your drone, and every VESC.

PID controller - the three terms, what each one fixes, and why you tune them in that order.
Brian Douglas: PID control, a brief introduction - the ten-minute version with plots. The overshoot is your oscillation.

Play

loading the simulator

racer.drive(steer=+0.00, throttle=+0.00) assist +0.00 waypoints 0/6 hits 0

assist gain assist lidar camera

// Arrow keys or W A S D. The waypoints are in a line, 8 m apart; some gaps have a crate. The green rays are the lidar. While you throttle, your assist() from the Code card reads the rays and the waypoint and pulls the steering. Gain 0 is raw driving. All six waypoints with hits 0 finishes the lab.

Code

// Your controller, called 15 times a second while you throttle in the Play card. Apply it, then drive.

# Teleop assist: your code pulls the steering while you drive.
#
# ranges:   24 lidar distances in metres, one per ray, right to left
# bearings: each ray's angle in radians (0 = dead ahead, + = left)
# goal:     (distance, bearing) to the lit waypoint, bearing + = left
# return:   a steering pull in [-1, 1], added to your steer (+ = left)
#
# The ladder: P first. It oscillates? Add D. It hugs a side? Add I.

KP = 1.0     # push away from the nearest crate
KG = 0.8     # pull toward the waypoint
FAN = 0.8    # only listen to the forward fan (about +-45 deg)
NEAR = 2.5   # a crate closer than this (metres) is in the way

def assist(ranges, bearings, goal):
    distance, to_goal = goal
    # the nearest ray in the forward fan, and the side it is on
    near, at = NEAR, 0.0
    for r, b in zip(ranges, bearings):
        if abs(b) <= FAN and r < near:
            near, at = r, b
    if near >= NEAR:
        return KG * to_goal  # the way is clear: the waypoint pulls
    # a crate: push away from its side, harder the closer it is
    side = -1.0 if at >= 0 else 1.0
    push = KP * side * (1.0 - near / NEAR)
    return push + KG * to_goal * (near / NEAR)

Output