PLC Ladder Logic

Question

I am trying to use PLC's to monitor a race track. I will be using 3 Photo sensors to show which car crossed the finish line first. Each sensor will have be OTL (Latched) instruction. Each lane will have a light to indicate which car was in first place. The car not in first lights will not come. There will be 3 judges. At the completion of each race, once the winner has been recorded, the 3 judges will use their respective switches to reset the indicator lamps to an off state (Unlatched) in preparation for the next race. To inhibit any attempt at cheating by the race judges , the judges switches are programmed so that all 3 judges must agree to reset and a reset can only occur after all 3 cars have passed the finish line. The judges will be programmed with a One Shot Rising (OSR) instruction. It should be noted that we're working with a SLC 500 PLC. I fail to mention that I am using Rockwell Automation software RS Logics, so please use Rockwell's instructions on your ladder logic.

Answer 1

This is rather straight forward. I'm answering generically for the PLC make isn't defined.

// You need 3 of these sets, one for each Lane, prefixed 1, 2, 3, accordingly

|---|Photo1|----------------|-----------(OTL1)----|
|---|OTL1|-----|/ResetCmd|--|

|--|OTL1|---|/OTL2|---|/ OTL3|----------(Light1)--|


// We have two work booleans, used in relation with the reset logic.
//  All3In is an AND of all 3 OTLs indicating that all 3 cars have passed the finish line
//  SomeOTLON is an OR of the 3 OTLs indicating that one or more OTL latches hasn't been reset

|--|OTL1|---|OTL2|----|OTL3|----------------------(All3In)--|

|--|OTL1|----|---------------------------------(SomeOTLON)--|
|            |
|--|OTL2|----|
|            |
|--|OTL3|----|

// We need 3 latches like the following, one per judge
// Essentially the Latch comes on when all 3 cars are in and the judge presses his button
// The latch comes off after all 3 OTLs dropped
|--|Judge1|---|All3In|----------|-------------(Judge1Latch)--|
|                               |
|--|Judge1Latch|---|SomeOTLON|--|


// Finally the  Reset Command
|--|Judge1Latch|---|Judge2Latch|---|Judge3Latch|--|------(ResetCmd)--|
|                                                 |
|--|ResetCmd|------|SomeOTLON|--------------------|

Et voilà. Only possible trouble is if the "JudgeN" switches are not One Shot Rising (but they seem to be as per the problem text), it would be possible for a judge to make his/her push button stick and still have the ResetCmd latch on when all 3 cars arrive.