Use case: formula one pitstop

This example is based on the DailyMail blog entry https://www.dailymail.co.uk/sport/formulaone/article-4401632/Formula-One-pit-stop-does-crew-work.html where a nice image shows 21 people changing the 4 tires of a Formula 1 Ferrari. In this example, only 16 out 21 people are represented. This notebook can be tested online at mybinder.org

from IPython.display import YouTubeVideo
YouTubeVideo('aHSUp7msCIE', width=800, height=300)

Imports

import processscheduler as ps
%config InlineBackend.figure_formats = ['svg']

Create the scheduling problem

The total horizon is not knwown, leave it empty and only set the problem name.

change_tires_problem = ps.SchedulingProblem('ChangeTires')

Create the 16 available resources

Each people in and around the car is represented as a worker.

nb_lifters = 2
nb_gunners = 4
nb_tyre_handlers = 8
nb_stabilizers = 2

# Lift tasks
lifters = [ps.Worker('JackOperator%i' % (i + 1)) for i in range(nb_lifters)]
gunners = [ps.Worker('Gunner%i' % (i + 1)) for i in range(nb_gunners)]
tyre_handlers = [ps.Worker('Handler%i' % (i + 1)) for i in range(nb_tyre_handlers)]
stabilizers = [ps.Worker('Stabilizer%i' % (i + 1)) for i in range(nb_stabilizers)]

Create tasks and assign resources

One period is mapped to one second. For example, if lifting the rear take 2sec then the duration will be set to 2.

# lift tasks and lifters
# both lift tasks can be processed by any one of the lifters
lift_rear_up = ps.FixedDurationTask('LiftRearUp', duration=2)
lift_front_up = ps.FixedDurationTask('LiftFrontUp', duration=2)
lift_rear_up.add_required_resource(lifters[0])
lift_front_up.add_required_resource(lifters[1])

lift_rear_down = ps.FixedDurationTask('LiftRearDown', duration=2)
lift_front_down = ps.FixedDurationTask('LiftFrontDown', duration=2)
lift_rear_down.add_required_resource(lifters[0])
lift_front_down.add_required_resource(lifters[1])

# unscrew tasks
unscrew_front_left_tyre = ps.FixedDurationTask('UnScrewFrontLeftTyre', duration=2)
unscrew_front_right_tyre = ps.FixedDurationTask('UnScrewFrontRightTyre', duration=2)
unscrew_rear_left_tyre = ps.FixedDurationTask('UnScrewRearLeftTyre', duration=2)
unscrew_rear_right_tyre = ps.FixedDurationTask('UnScrewRearRightTyre', duration=2)

gunner_unscrew_front_left_tyre = ps.SelectWorkers(gunners, 1)
unscrew_front_left_tyre.add_required_resource(gunner_unscrew_front_left_tyre)

gunner_unscrew_front_right_tyre = ps.SelectWorkers(gunners, 1)
unscrew_front_right_tyre.add_required_resource(gunner_unscrew_front_right_tyre)

gunner_unscrew_rear_left_tyre = ps.SelectWorkers(gunners, 1)
unscrew_rear_left_tyre.add_required_resource(gunner_unscrew_rear_left_tyre)

gunner_unscrew_rear_right_tyre = ps.SelectWorkers(gunners, 1)
unscrew_rear_right_tyre.add_required_resource(gunner_unscrew_rear_right_tyre)

# screw tasks and gunners
screw_front_left_tyre = ps.FixedDurationTask('ScrewFrontLeftTyre', duration=2)
screw_front_right_tyre = ps.FixedDurationTask('ScrewFrontRightTyre', duration=2)
screw_rear_left_tyre = ps.FixedDurationTask('ScrewRearLeftTyre', duration=2)
screw_rear_right_tyre = ps.FixedDurationTask('ScrewRearRightTyre', duration=2)

gunner_screw_front_left_tyre = ps.SelectWorkers(gunners)
screw_front_left_tyre.add_required_resource(gunner_screw_front_left_tyre)

gunner_screw_front_right_tyre = ps.SelectWorkers(gunners)
screw_front_right_tyre.add_required_resource(gunner_screw_front_right_tyre)

gunner_screw_rear_left_tyre = ps.SelectWorkers(gunners)
screw_rear_left_tyre.add_required_resource(gunner_screw_rear_left_tyre)

gunner_screw_rear_right_tyre = ps.SelectWorkers(gunners)
screw_rear_right_tyre.add_required_resource(gunner_screw_rear_right_tyre)

# tires OFF and handlers
front_left_tyre_off = ps.FixedDurationTask('FrontLeftTyreOff', duration=2)
front_right_tyre_off = ps.FixedDurationTask('FrontRightTyreOff', duration=2)
rear_left_tyre_off = ps.FixedDurationTask('RearLeftTyreOff', duration=2)
rear_right_tyre_off = ps.FixedDurationTask('RearRightTyreOff', duration=2)

for tyre_off_task in [front_left_tyre_off, front_right_tyre_off,
                      rear_left_tyre_off, rear_right_tyre_off]:
    tyre_off_task.add_required_resource(ps.SelectWorkers(tyre_handlers))

# tires ON and handlers, same as above
front_left_tyre_on = ps.FixedDurationTask('FrontLeftTyreOn', duration=2)
front_right_tyre_on = ps.FixedDurationTask('FrontRightTyreOn', duration=2)
rear_left_tyre_on = ps.FixedDurationTask('RearLeftTyreOn', duration=2)
rear_right_tyre_on = ps.FixedDurationTask('RearRightTyreOn', duration=2)

for tyre_on_task in [front_left_tyre_on, front_right_tyre_on,
                     rear_left_tyre_on, rear_right_tyre_on]:
    tyre_on_task.add_required_resource(ps.SelectWorkers(tyre_handlers))

Stabilizers start their job as soon as the car is stopped until the end of the whole activity.

stabilize_left = ps.VariableDurationTask('StabilizeLeft')
stabilize_right = ps.VariableDurationTask('StabilizeRight')

stabilize_left.add_required_resource(stabilizers[0])
stabilize_right.add_required_resource(stabilizers[1])

ps.TaskStartAt(stabilize_left, 0)
ps.TaskStartAt(stabilize_right, 0)

ps.TaskEndAt(stabilize_left, change_tires_problem.horizon)
ps.TaskEndAt(stabilize_right, change_tires_problem.horizon)

TaskEndAt_16a5d204(<class 'processscheduler.task_constraint.TaskEndAt'>)
1 assertion(s):
StabilizeRight_end == horizon

Task precedences

# front left tyre operations
fr_left = [unscrew_front_left_tyre, front_left_tyre_off, front_left_tyre_on,
           screw_front_left_tyre]
for i in range(len(fr_left) - 1):
    ps.TaskPrecedence(fr_left[i], fr_left[i+1])
# front right tyre operations
fr_right = [unscrew_front_right_tyre, front_right_tyre_off, front_right_tyre_on,
           screw_front_right_tyre]
for i in range(len(fr_right) - 1):
    ps.TaskPrecedence(fr_right[i], fr_right[i+1])
# rear left tyre operations
re_left = [unscrew_rear_left_tyre, rear_left_tyre_off, rear_left_tyre_on,
           screw_rear_left_tyre]
for i in range(len(re_left) - 1):
    ps.TaskPrecedence(re_left[i], re_left[i+1])
# front left tyre operations
re_right = [unscrew_rear_right_tyre, rear_right_tyre_off, rear_right_tyre_on,
           screw_rear_right_tyre]
for i in range(len(re_right) - 1):
    ps.TaskPrecedence(re_right[i], re_right[i+1])

# all un screw operations must start after the car is lift by both front and rear jacks
for unscrew_tasks in [unscrew_front_left_tyre, unscrew_front_right_tyre,
                      unscrew_rear_left_tyre, unscrew_rear_right_tyre]:
    ps.TaskPrecedence(lift_rear_up, unscrew_tasks)
    ps.TaskPrecedence(lift_front_up, unscrew_tasks)

# lift down operations must occur after each screw task is completed
for screw_task in [screw_front_left_tyre, screw_front_right_tyre,
                      screw_rear_left_tyre, screw_rear_right_tyre]:
    ps.TaskPrecedence(screw_task, lift_rear_down)
    ps.TaskPrecedence(screw_task, lift_front_down)

First solution, plot the schedule

solver = ps.SchedulingSolver(change_tires_problem)
solution_1 = solver.solve()
solution_1.render_gantt_matplotlib(fig_size=(10,5), render_mode='Resource')

Solver type:
===========
        -> Standard SAT/SMT solver
Total computation time:
=====================
        ChangeTires satisfiability checked in 0.05s

Second solution: add a makespan objective

Obviously, the former solution is not the best solution, not sure Ferrari will win this race ! The whole “change tires” activity must be as short as possible, so let’s add a makespan objective, i.e. a constraint that minimizes the schedule horizon.

# add makespan objective
change_tires_problem.add_objective_makespan()

solver_2 = ps.SchedulingSolver(change_tires_problem)
solution_2 = solver_2.solve()
solution_2.render_gantt_matplotlib(fig_size=(9,5), render_mode='Task')

Solver type:
===========
        -> Standard SAT/SMT solver
Incremental optimizer:
======================
        Found value: 24 elapsed time:0.048s
        Checking better value < 24
        Found value: 23 elapsed time:0.163s
        Checking better value < 23
        Found value: 22 elapsed time:0.171s
        Checking better value < 22
        Found value: 21 elapsed time:0.197s
        Checking better value < 21
        Found value: 20 elapsed time:0.202s
        Checking better value < 20
        Found value: 19 elapsed time:0.208s
        Checking better value < 19
        Found value: 18 elapsed time:0.213s
        Checking better value < 18
        Found value: 17 elapsed time:0.233s
        Checking better value < 17
        Found value: 16 elapsed time:0.239s
        Checking better value < 16
        Found value: 15 elapsed time:0.255s
        Checking better value < 15
        Found value: 14 elapsed time:0.260s
        Checking better value < 14
        Found value: 13 elapsed time:0.281s
        Checking better value < 13
        Found value: 12 elapsed time:0.286s
        Checking better value < 12
        Can't find a better solution for problem ChangeTires.

        Found optimum 12. Stopping iteration.
        total number of iterations: 14
        value: 12
        ChangeTires satisfiability checked in 0.29s

Third solution: constraint workers

This is not the best possible solution. Indeed, we can notice that the Gunner2 unscrews the RearRightTyre and screw the RearLeft tyre. We cannot imagine that a solution where gunners turn around the car is acceptable. There are two solutions to fix the schedule: - let the gunner be able to turn around the car, and add a “Move” task with a duration that represent the time necessary to move from one tyre to the other, - constraint the worker to screw the same tyre he unscrewed. Let’s go this way

ps.SameWorkers(gunner_unscrew_front_left_tyre, gunner_screw_front_left_tyre)
ps.SameWorkers(gunner_unscrew_front_right_tyre, gunner_screw_front_right_tyre)
ps.SameWorkers(gunner_unscrew_rear_left_tyre, gunner_screw_rear_left_tyre)
ps.SameWorkers(gunner_unscrew_rear_right_tyre, gunner_screw_rear_right_tyre)

solver_3 = ps.SchedulingSolver(change_tires_problem)
solution_3 = solver_3.solve()
solution_3.render_gantt_matplotlib(fig_size=(9,5), render_mode='Task')

Solver type:
===========
        -> Standard SAT/SMT solver
Incremental optimizer:
======================
        Found value: 28 elapsed time:0.082s
        Checking better value < 28
        Found value: 24 elapsed time:0.089s
        Checking better value < 24
        Found value: 23 elapsed time:0.162s
        Checking better value < 23
        Found value: 22 elapsed time:0.165s
        Checking better value < 22
        Found value: 21 elapsed time:0.169s
        Checking better value < 21
        Found value: 20 elapsed time:0.173s
        Checking better value < 20
        Found value: 19 elapsed time:0.177s
        Checking better value < 19
        Found value: 18 elapsed time:0.180s
        Checking better value < 18
        Found value: 17 elapsed time:0.184s
        Checking better value < 17
        Found value: 16 elapsed time:0.188s
        Checking better value < 16
        Found value: 15 elapsed time:0.192s
        Checking better value < 15
        Found value: 14 elapsed time:0.195s
        Checking better value < 14
        Found value: 13 elapsed time:0.199s
        Checking better value < 13
        Found value: 12 elapsed time:0.203s
        Checking better value < 12
        Can't find a better solution for problem ChangeTires.

        Found optimum 12. Stopping iteration.
        total number of iterations: 15
        value: 12
        ChangeTires satisfiability checked in 0.20s

This is much better !