In previous articles, we gave examples of some of the most common safety instructions in a safety PLC. In this article, we will talk about another important safety instruction which is the Muting function, and we will give a simple PLC example code.
Contents:
A muting function is when a group of sensors (usually 2 or 4 sensors), are used to temporarily disable the safety function of a safety light curtain of a safety sensor when a workpiece or a conveying palette is passing through the guarded area of the safety function.
The challenge facing a muting function is to be able to differentiate between a passing workpiece and a passing worker through the hazard area.
There are two different cases of activating a muting function:
So, a muting function should only work when a workpiece is passing through the risk area, but if a worker or an operator passes through the risk area, the safety function should be activated and the machine should stop immediately.
See picture 1 for MUT_P or Parallel muting safety function in TIA Portal.
As you can see, The MUT_P safety function has a lot of parameters, let’s see what some of them mean:
There are many different approaches to muting functions, but the most common are:
In this approach, 4 sensors (MS_11, MS_12, MS_21, and MS_22) are used to provide detection of the moving workpieces or a conveying palette.
Each two sensors facing each other should be installed in such a way that the moving palette will be detected by the two sensors at the same time or within a discrepancy time. And also both of them should be deactivated at the same time.
Also, the 4 sensors should be installed in such a way that sensors MS_21 and MS_22 will get activated before MS_11 and MS_12 are de-activated, to enable continuity of the muting function.
The following steps show the sequence of achieving a muting function without any errors:
If reflection light barriers are used as muting sensors, then only 2 sensors MS_11 and MS_12 Can be used for a muting function, the 2 sensors will be arranged diagonally. See picture 6.
The sequence of the muting function is obtained according to the following steps:
Hardware configurations:
First, let’s define all needed inputs for our simulation. See picture 7.
Start with adding your MUT_P safety instruction, see picture 1, and then choose Multi-instance DB for your function. See picture 8.
Next, add your inputs to the MUT_P safety instruction, also define some static outputs and add them to the MUT_P outputs. See picture 9.
The coding part is done, compile your code and start a new simulation.
First, run your PLC and notice the MUT_P at start-up, see picture 10.
From picture 10 you can see that there is a fault in the instruction, because the FREE signal is FALSE, which means the light curtain is interrupted without muting, so a safety condition will be triggered.
Now, set the FREE signal to TRUE (to simulate the light curtain not being interrupted) and acknowledge the safety condition to reset the fault. See picture 11.
Now, see pictures 12 to 17 for a complete sequence of the muting function.
As the product keeps moving, the light curtain will eventually be interrupted, that should happen while MS11 and MS12 are still active, (see picture 3) to keep the muting function triggered. See picture 14.
After a while, MS21 and MS22 will also be activated (see picture 4), this should happen while the light curtain is still interrupted by the product and while MS11 and MS12 are still active see picture 15.
After a while, MS11 and MS12 will be deactivated, but the muting function will still be active because MS21 and MS22 are still activated. See picture 16.
When MS21 and MS22 are deactivated, the muting function will be stopped. See picture 17.
Note that, if the TIME_MAX of the safety instruction is elapsed before muting one complete safety muting function finished, the PLC will give a fault and the output will stop immediately.
We now finished the simulation and test of a 4 sensors muting function sequence.
Check the PLC software attached to the article and try to see if you can simulate the 2-sensor Muting function sequence.
Download the muting function logic and its project back up in the Siemens Tia Portal.
P.S. password for safety PLC software is 123
The muting function is a very important safety function for when you need to interrupt the safety light curtain without triggering a safety condition that will probably stop your process.
Again, safety PLC made the implementation of this function very easy due to the already built-in safety instructions that comply with the safety standards. If you would try to implement the same functionality with a normal PLC, you will need a large code to perform the same function as the MUT_P in safety PLCs.
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