GPSS manufacturing shop, conditional transfer

A GPSS model of a manufacturing shop represents parts as transactions that move through machines (facilities or storages). Conditional transfer sends a part to different blocks depending on the state of the system or on chance, using the TRANSFER block in its modes.

Modes of TRANSFER

• Unconditional: TRANSFER ,PLACE always goes to PLACE.
• Statistical: TRANSFER .25,A,B sends 25% of the transactions to B and the rest to A (probabilities, for example for rework or inspection rejection).
• BOTH: TRANSFER BOTH,A,B tries A first; if A is blocked (for example the machine is busy), tries B; if both are blocked, the transaction waits.
• ALL/PICK/FN also exist, for choosing among many destinations.
• TEST/GATE: TEST L Q$LINE,5,OTHER or GATE NU DRILL,OTHER sends the transaction elsewhere if a condition is false.

Scenario

Parts arrive at the shop about every 6 min (6 ± 2). They need turning. There are two lathes: Lathe 1 is preferred. If it is busy, the part goes to Lathe 2 (conditional transfer). After turning, 10% of the parts fail inspection and go for rework (statistical transfer), the rest leave.

GENERATE 6,2
TRANSFER BOTH,LATH1,LATH2
LATH1 SEIZE L1
      ADVANCE 9,2
      RELEASE L1
      TRANSFER ,INSP
LATH2 SEIZE L2
      ADVANCE 10,2
      RELEASE L2
INSP  ADVANCE 2
      TRANSFER .10,,REWRK
      TERMINATE 1
REWRK ADVANCE 5
      TERMINATE 1
START 200

Flow

GENERATE -> TRANSFER BOTH -> L1 --+
                  |               +-> INSPECT -> TRANSFER .10
                  +-----------> L2 -+      |            |
                                       TERMINATE     REWORK

Explanation

• TRANSFER BOTH first tries the label LATH1; if the facility cannot be seized, the transaction goes to LATH2. This gives load balancing without queuing at the first lathe.
• Facilities L1 and L2 report utilisation and number of entries.
• The statistical transfer models random rejects; TERMINATE 1 with START 200 stops after 200 parts.
• Output: utilisation of each lathe, number of reworked parts, and total time in the shop (add MARK/TABULATE for this).