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U1.7 Project: write the data sheet

The robot as a system · University · about 45 min

BugBotLab

What this lesson is about

Characterise the robot you were given and print the figures the rest of the course will use.

Questions 6 marks in all

  1. [1 mark]Put the steps of one data sheet experiment in order.

    Number the lines 1 to 5 to put them in the right order.

    1. Wait past the transient, about 3 tau
    2. Stop, and let the robot settle
    3. Take several readings and average them
    4. Stop, and let it settle again before the next experiment
    5. Apply the input
    Answer:
    Stop, and let the robot settle
Apply the input
Wait past the transient, about 3 tau
Take several readings and average them
Stop, and let it settle again before the next experiment

    The same discipline every time. Skipping the last step is the usual mistake.

  2. [1 mark]Why must the robot settle again after each experiment before the next one starts?

    1. AOtherwise the next measurement is taken while the robot is still coasting, so it measures the previous experiment
    2. BThe deadman needs half a second to reset
    3. CThe flow sensor has to re-zero between experiments
    4. DThe battery needs time to recover
    Answer: A. A reading taken while the robot still carries motion from the last input is contaminated by that input.

  3. [1 mark]Why does each student measure their own data sheet rather than use a shared one?

    1. AEach BugBot is built with its own gain on each axis, gyro bias and flow scale, fixed for the run
    2. BThe values change continuously during a run, so a shared sheet would be out of date within seconds
    3. CThe shared data sheet is in different units
    4. DThe simulator only reports speeds that have been measured in the same run
    Answer: A. The figures belong to your robot and stay fixed for the run. That is why measuring beats looking up.

  4. [1 mark]These are the settled readings from two experiments: forward speed from flow() and turn rate from imu(). What does this print?

    readings = [19.4, 18.9, 19.6, 19.1, 19.0]
rates = [-112.0, -115.5, -113.0, -114.5, -112.5]
v_max = sum(readings) / len(readings)
w_max = sum(rates) / len(rates)
print("v_max:", round(v_max, 1))
print("w_max:", round(abs(w_max), 1))
    Answer:
    v_max: 19.2
w_max: 113.5

    The speeds sum to 96.0, so the mean is 19.2. The rates sum to -567.5, a mean of -113.5, and abs() reports it as a size: 113.5.

  5. [1 mark]A command is walked up in steps of 5, recording the settled flow() speed at each. Anything above 1 cm/s counts as moving. What does this print?

    table = [(5, 0.0), (10, 0.1), (15, 2.4), (20, 3.6), (25, 4.9)]
for power, v in table:
    if v > 1.0:
        print("dead band:", power)
        break
    Answer:
    dead band: 15

    0.1 cm/s at command 10 is noise, not motion. The first command that clearly moves the robot is 15.

  6. [1 mark]The settled() helper waits 1.0 s after applying an input before reading. With a time constant of about 0.25 s, is that long enough?

    1. AYes: 1.0 s is 4 tau, past the 3 tau at which the response is 95 percent settled
    2. BNo: a response needs about 10 tau to settle
    3. CNo: 1.0 s is shorter than one time constant
    4. DYes, but only because the deadman stops the robot after half a second anyway
    Answer: A. 1.0 / 0.25 = 4 time constants, which leaves the response about 98 percent settled. A longer hold wastes the ninety second budget.

The task: the data sheet

Measure all four and print them, one per line, exactly like this: The whole run has ninety seconds, which is plenty for four experiments and nowhere near enough to be careless.

from bugbot import *
connect()

# measure, do not assume

The hint students can ask for: Four measurements, one after another, each one a short drive and a read. Work up from a small command in steps to find where the robot first moves. Keep the whole run under ninety seconds.

A solution

from bugbot import *
connect()

# top forward speed
forward(100)
wait(1.0)
fwd = [flow()[1] for i in range(10) if wait(0.1) is None]
stop()
wait(0.5)
v_max = sum(fwd) / len(fwd)

# top turn rate
drive(0, 0, 100)
wait(1.0)
turns = [imu()[1] for i in range(10) if wait(0.1) is None]
stop()
wait(0.5)
w_max = sum(turns) / len(turns)

# the dead band: work up until it actually moves
dead = 100
for power in range(5, 60, 5):
    forward(power)
    wait(0.8)
    if abs(flow()[1]) > 2.0:
        dead = power
        stop()
        break
    stop()
    wait(0.3)

ox, oy, oh = odometry()
tx, ty = position()
drift = ((ox - tx) ** 2 + (oy - ty) ** 2) ** 0.5

print("v_max:", round(v_max, 1))
print("w_max:", round(abs(w_max), 1))
print("dead band:", dead)
print("drift:", round(drift, 1))

Any program that meets the task's checks is marked correct in the simulator; this is one way, not the only way.