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Mobile cobots: machine tending

A mobile cobot is a vehicle with an arm: it carries a part to a machine, puts it in, and later takes it out and carries it on. Use it to answer "how many machines can one cobot tend": the machine waits for the cobot, and the cobot waits for the machine.

The fleet

Give a fleet an arm:

Field Meaning
arm the name of a record of motion_profiles; empty: an ordinary fleet
exchange the cobot has a second holding place (double gripper or tray); default off

Everything else about the fleet (speeds, dispatch, park, battery, schedules, failures) works as for any vehicle. An arm fleet carries one part, or two during an exchange; it cannot be a worker or gantry fleet.

The station

Tick Tending on a STATION block (file key tending):

Field Meaning
place the arm cycle that puts a part into the station, written as MOTION segments
pick the arm cycle that takes a part out
time_unit unit of the timed segments, default seconds
hrc, zone human-robot collaboration, as on a MOTION block
cobot leave (default) or stay

A missing cycle uses the fleet's flat unload or load time. Tending applies only when an arm fleet serves a station that has it: a cobot at a plain station, or a forklift at a tending station, behaves like any vehicle.

What happens

"The machine" is the first block after the STATION that holds a resource (PROCESS, SEIZE, MOTION) or has finite space (a BUFFER, a queue_capacity).

  • Pick. A finished part stays in the machine, which cannot start another, until a cobot has arrived and finished the pick cycle.
  • Place. A cobot arriving with a part waits, holding it, until the machine accepts it. Then the place cycle runs, and the machine starts when it ends.
  • Leave. After placing, the cobot is free for other jobs; any cobot of the fleet comes back for the pick.
  • Stay. After placing, the cobot waits at the station and picks the part itself, with no dispatch. It counts as busy the whole time and drains its battery at the idle rate. If the part goes on with another fleet or leaves the model (a DISPOSE, a balk or renege routed to one, a BATCH that absorbs it), the cobot is freed at that instant.
  • Exchange. A cobot with exchange arriving with a new part at a machine holding a finished one takes the finished part out, puts the new one in, and carries the finished part on, saving a trip.

One cobot and no exchange

A single cobot holding a new part for a machine that waits to be emptied can never finish. Validation warns beforehand (W_TENDING_DEADLOCK), and the run stops with a deadlock error, naming the fleet and the stations, at the instant every cobot of the fleet waits loaded at a machine whose finished part waits for a cobot of that fleet. Turn on exchange, add a second cobot, put a BUFFER before the machine, or let the cobot stay.

The error is raised only when the stall is certain: the machine is one PROCESS or MOTION with no queue places, balking, reneging or preemption, on a plain resource with no capacity schedule. Other stalls (a full BUFFER before the machine, a SEIZE ... RELEASE machine) still show as Picks far below Places.

Results

The tending_summary frame gives, per station and fleet: picks, places, exchanges, the mean pick and place cycle, the mean time a cobot waited loaded for the machine, the mean time a finished part waited in the machine for a pick to start, and the mean stay. Scenario and optimizer keys read tending/<station>/<fleet>/<column>.

Not modelled

Working on a part on board or while driving, more than two holding places, and multi-load tours for arm fleets. If the first block after the station holds nothing (a DELAY), it starts while the place cycle is still running.