A bit of history

Herbie at the head of the column

In «The Goal» the troop of scouts stretched out along the trail because Herbie walked slower than everyone, while the others now ran ahead, now waited. Alex did a simple thing: he put Herbie at the head of the column. Now nobody could get ahead, the column walked closely, and the whole troop marched at Herbie's rhythm. And to make the troop go faster, they lightened Herbie's backpack.

You cannot do that in a factory: you cannot move a machine to the start of the line. The bottleneck stands where it stands, and parts are released into the line at the very start.

«The Race», 1986: drum, buffer, rope

Two years after «The Goal», Goldratt and Robert Fox published the book «The Race». In it he proposed a way to run a shop whose bottleneck is somewhere in the middle:

  • drum — the bottleneck sets the rhythm for the whole shop;
  • buffer — a small stock of work is kept in front of the bottleneck so that it never stands, even if someone before it stumbles;
  • rope — a signal from the buffer to the start of the line: new work is released only when the stock before the bottleneck is running low.

On the trail it would look like this: Herbie walks anywhere, and the leader of the column is tied to him with a rope. The leader cannot get further ahead than the rope allows — so there will never be too many scouts between him and Herbie.

ropeHerbie — drumthe leaderbuffer: the scouts between Herbie and the leader
«Drum — buffer — rope» on the trail: Herbie sets the pace of the whole column, the leader is tied to him and cannot get further ahead than the rope allows.
M1M2M3M4M5drum — Herbiebuffer — work waiting for Herbiebuffer counterrope: buffer full — the source waits
The same in a factory: the bottleneck beats the rhythm, a small stock of work waits before it, and a signal from that stock controls the release at the start of the line.

Push or pull

In the 1970s factories began planning production on computers: a program worked out from a forecast how much to start and when, and pushed work into the shop by a schedule. If the real shop fell behind the plan, work piled up on the shelves.

Toyota with its kanban, which we met in FL-01, did the opposite: the next station itself pulls from the previous one as much as it has used up. «Drum — buffer — rope» is pulling too, only it is not every station that pulls, but the one bottleneck: only it has to watch its stock.

Electronics on the line, 1968

And who in a factory decides whether to release a part? For a long time it was cabinets with hundreds of relays: the rule «if the sensor sees a part and the counter is below the setting — open the gate» was built out of wires. For every new car model the cabinets had to be rewired for weeks.

In 1968 a General Motors transmission plant asked for a device in which the rule could be changed without touching the wires. The engineer Dick Morley and his colleagues built the programmable logic controller — the Modicon 084. Today such controllers stand on every production line in the world: they count parts by sensors, compare them with a setting and switch mechanisms on. The counter, adder and comparator you will build in this kit are exactly what a controller does on a conveyor.

relay cabinet: the rule = wires1968controllerPLC: the rule = a program
How electronics came to run production lines: relay cabinets, rewired for every new model, gave way to the programmable controller.