Pneumatic Valve Questions

Ive had some PLC's but don't have much experience at wiring them up. I'm working on an assembly line now and looking around trying to figure things out and see if I can write a program. They have a double acting air ram laid horizontal that pulls a small pallet from one assembly line to the other. So Im thinking that it works like this: When Ouput-1 is on, that retracts the ram; Output-2 is on, that extends it; neither output on, the valve shifts to a closed position ???? So in my program when it retracts and triggers a sensor it turns off Out-1 and stops retracting. In the next rung the sensor turns on Out-2 and extends the ram. But this will cause the valve to get two signals at once I think. One to stop retracting the other to extend. Can a valve do this or do I need to add some delay?

On another part of the line they have a single action ram ( I think) connected to a spring loaded stop linkage that stops the pallets as the move along the line. When the ram is retracted the spring pulls the stop up. When it is extended it pulls the stop down. So I assume Out-3 off, ram retracted; Out-3 on, ram extended. I don't think there's any sensors there so this would mean to hold the stop down the valve holds pressure on the ram and keeps it extended. Can you do that or do you need a valve that shifts to a closed position??? Usually the stop stays down max for a couple minutes, but its possible if the line stopped that it could stay down for hours. Thanks for any help!

I will try to explain

When you create the rungs for retract and extend what you do is use a NOT (XIO) of the output in series with the opposing output...ie if you are retracting then put an XIO |/| of the extend output in series in the rung with the retract output. You do this on both rungs, this prevents either rung activating if the other is ON.

It can get more detailed if you want to stop an action with an action..ie if you are retracting then want it to extend while in motion.

Develop this part and you will be able to understand that as time goes by.

It is difficult to program these without knowing exactly what type of valve you are using. A single coil valve, 5 port 3 way allows you to provide power to the coil and keep it there as long as you need to keep the ram extended, or whatever else you need to do. Remove power, and the spring on the opposite end of the valve returns it to it's "normal" position. That position, de-energized, will position the ram either in or out, depending on the plumbing. I would look at the application so far as what happens when power is lost, estop, ect as to the non-hazardous position physically.
If you have a 2 coil valve, 5/3, there are three major types: 1) can be detented (pulse on one side and it will stay shifted until the other side is pulsed), 2)center closed or 3)center exhaust. Both of these need continuous power to a coil for an action to occur, extend or retract. The center closed valve will stop the cylinder in that position, wherever it is as it closes both ports, almost acting like a brake when power is removed. The center exhaust, when no power is applied to either side will exhaust all air from the cylinder, allowing a free movement of the cylinder by hand if required.

Hope this helps from a pnuematic standpoint.- use suppresion on the coils, MOV for AC, diode for DC. These little coils have quite an inductive kick when power is removed and will shorten the life of your output device.

-tommy23t

valve

Yes, it is hard to program unless you know which valve you are using. Also you need to know your press, some presses are mechanical shift, some Hyd., some air. You need more information to write a program plus do not forget about your safeties, perhaps a light barrier, palm buttons so both buttons pushed at same time and circuitry so both palms must be pressed and not just one. This keeps hands out of press, ram up upon loosing power, barrier invasion, estop,etc.

Can you do that or do you need a valve that shifts to a closed position???

Click to expand...

To answer this question, you need to know if the solenoid coil on the valve is rated for continuous duty. If it is then, theoretically, you can leave it on forever without worrying.

I find writing the controls for 2 position single acting valves is easier if you write two separate rungs for the 2 states of the device and then use those to drive the output. Otherwise, if you try to put everying on the output rung, half of your conditions are negations and it can become confusing. Something like this is what I'm talking about:

  AUTO    STOP_CMD  REL_CMD    PERMISSIVES     EXT_STOP_CTRL

--] [-------] [--------]/[---+-----] [----------------( )

                             |

                             |

MANUAL  EXT_ST_PB            |

--] [-------] [--------------+









  AUTO    EXT_STOP_CTRL         PERMISSIVES     RET_STOP_CTRL

--] [-------]/[--------------+-----] [----------------( )

                             |

                             |

MANUAL  RET_ST_PB            |

--] [-------] [--------------+





EXT_STOP_CTRL   RET_STOP_CTRL  PERMISSIVES         EXT_STOP_SOL

----] [-------+-----]/[------------] [---------------( )

              |

              |

EXT_STOP_SOL  |

----] [-------+