diagnostics of a cylinder driven by a double solenoid valve

Bartech · Aug 6, 2012

Hello, I am a newbie.

congratulations for this very interesting forum!

My question:

to perform diagnostics of a cylinder controlled by single solenoid valve I can use this method:



|

|

|   valve    front sensor          +----------------+

|----] [---------]/[--------+------|     Timer      |

|                           |      +----------------+

|                           |

|                           |

|                           |

|                           |

|   valve    rear sensor    |

|----]/[---------]/[--------+

|

|

|

|

|

|   Timer                    Fault

|----] [--------------------( SET )

|

|

how to run diagnostics of a cylinder driven by a double solenoid valve?

What I mean is that once the cylinder has finished the forward run the solenoid is de-energized.
Now how do I know if the sensor is burnt out, or if the operator manually operates the valve and turn back the cylinder?

I had thought of using a latch bit (retentive) in this way:



|   valve forward                  +----------------+

|-------] [------------------------|     Keep       |

|                                  |   OP_LATCH     |

|                           +------|                |

|                           |      +----------------+

|                           |

|                           |

|                           |

|   valve backward          |

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

|

|

|

|

|     OP_LATCH    front sensor       +----------------+

|-------] [-----------]/[-------+----|     Timer      |

|                               |    +----------------+

|                               |

|                               |

|                               |

|                               |

|     OP_LATCH    rear sensor   |

|-------]/[-----------]/[-------+

|

|

|

|

|

|   Timer                    Fault

|----] [--------------------( SET )

|

|

|

|

|

is it correct this method, or is there a more clever way?

Thanks to all

Bartech

JesperMP · Aug 6, 2012

Your first example is OK. (*).
Only, it is not good enough to have one alarm covering very different conditions. The error message will be confusing to the operator.
So you can split into two error messages by using the timer and the valve and sensor states in series. Or use two timers if there are enough timers, and the cylinder moves at different speeds in the two directions.

Re "Now how do I know if the sensor is burnt out, or if the operator manually operates the valve and turn back the cylinder?". Then there is no way for the program to distinguish between the operator manipulating the hardware and a genuine hardware error. There should be no reason for anyone to manipulate the hardware in normal operation.

edit: Both examples are OK, in principle. The second example also covers that the cylinder moves after deenergizing the valves.
BUT, (assuming this is a pneumatic cylinder) normally at least one valve is kept energized, even when the limit switch is reached. This because to achieve smooth movements, you must keep the volume inside the cylinder pressurized at all time. If you dont, then there will be an initial jerk at the start of the movement.
So when you want to start a movement, then you switch from ON-OFF to OFF-ON for the two valves.
There are some applications where you dont have to worry about smooth movements, but in most cases it is the desired functionality.

Sergei Troizky · Aug 6, 2012

JesperMP, you misunderstood the topic starter. There are no valves, it is only a single two-position double-solenoid valve.
Both examples are correct in principle, but:
- Air pressure applied must be added to the timer conditions.
- Invalid combination of sensors (both sensors on) must be unconditionally deemed an error.
I see nothing wrong for a single error message to cover all mentioned errors, e.g. "Pusher error".

JesperMP · Aug 6, 2012

I dont think I misunderstood the OP.
It is two different examples. The 1st sample have only 1 valve. The 2nd sample has 2 valves.

Re single error message for several possible causes, each to his own. It takes just a little effort to get from "not so bad" to "as good as it gets".

Eric Nelson · Aug 6, 2012

JesperMP said:
Re single error message for several possible causes, each to his own. It takes just a little effort to get from "not so bad" to "as good as it gets".

Agreed. I would have (at least) 4 fault conditions here to narrow down the 'problem'. I use a different approach, but it looks like even the OP's example could easily be expanded to have a separate fault for each type of problem.

|

|

|   valve    front sensor          +----------------+

|----] [---+-----]/[--------+------|     Timer      |

|          |                |      +----------------+

|          | rear sensor    |

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

|                           |

|                           |

|   valve    rear sensor    |

|----]/[---+-----]/[--------+

|          |                |

|          | front sensor   |

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

|

|

|

| 

|   Timer    front sensor     Fault 1

|----] [---+-----]/[----------( SET ) "Front Sensor Failed to Turn ON"

|          |

|          |

|          |  rear sensor     Fault 2

|          +-----] [----------( SET ) "Rear Sensor Failed to Turn OFF"

|          |

|          |

|          |  rear sensor     Fault 3

|          +-----]/[----------( SET ) "Rear Sensor Failed to Turn ON"

|          |

|          |

|          |  front sensor    Fault 4

|          +-----] [----------( SET ) "Front Sensor Failed to Turn OFF"

|

|

-Eric

Bartech · Aug 7, 2012

Hi all,

I think Sergei had understood the meaning.

In our machines there are many air cylinders, driven by double solenoid valves.

These valves have 2 states that can be achieved by energizing the solenoid 1 or the solenoid 2 with a pulse.
Once the position forward or backward is reached (ie turns ON the corresponding sensor),the solenoid is de-energized.

Usually makes no sense to keep the solenoid energized, because the valve is designed to keep the pressure in the cylinder, even if the solenoid in the meantime has been de-energized, or in case of power failure.

For this reason I thought of a method to monitor the status of the sensors even if the solenoids are de-energized .. the only thing that comes to mind is to use a retentive flag, as in example 2.

I don't know if there is a smart way to solve the problem.

Thanks so much for your suggestions on how to evaluate all possible scenarios (eg both sensors ON).

Bartech

JesperMP · Aug 7, 2012

Bartech said:
Usually makes no sense to keep the solenoid energized, because the valve is designed to keep the pressure in the cylinder, even if the solenoid in the meantime has been de-energized, or in case of power failure.

If you have two valves, how does the circuit diagram look like ?
I have a hard time visualizing how the air gets out of the cylinder when activating a valve, if the valves work as you describe.

Bartech · Aug 7, 2012

The bistable valves have two equilibrium conditions in the absence of control signal, for change position is used a pulse (the duration of at least 200-300 ms) and when the pulse is lacking the valve remain in the last reached position, so the cylinder too remain in the last position.
They have two coils for command. The name is 5/2-way bi-stable

Bartech

JesperMP · Aug 7, 2012

So for correctness, it is a single bistable 5/2 valve with two coils. And then you probably have a flow control valve at each end of the cylinder, where air pressure is released automatically.
OK that makes sense. Yes, in that case you have to memorise that a coil has been actuated.

I would do like Eric, apart from I would use the memorised state of the valves, and I would add the alarm for both sensors activated.

edit: And Sergei was right, I did misundertood you, but to my defence there are so many possible ways to design a pneumatic system that a small diagram will be really helpful to visualise the actual case.

Bartech · Aug 7, 2012

Jesper, you're right

I'm sorry, I was not very clear from the beginning of the first post, next time I'll try to attach a simple diagram.
thanks again!

Bartech

HJTRBO · Aug 8, 2012

Maybe stick this in your library. Untested but looks good on paper.

Bartech · Aug 21, 2012

Thanks HJTRBO, looks very good!

diagnostics of a cylinder driven by a double solenoid valve

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