chakorules
Member
Oh great and many wise ones of plcs.net... I have a safety question and wondered how most of you handle this situation.
Let's say you have a machine that is an automatic dial table on an indexer, and there are pneumatic pick and place heads on the dial. The dial has 16 stations let's say. And you have a bit shift register that tracks your parts etc around the dial table.
Your dial table is completely guarded with Lexan and has four access doors on each side. You want to design a safe system so naturally you would tie your door switches into your E-stop circuit that would blow the CRM (control master relay) and dump the system air at the same time when a door is open. This is the best and safest way always in my opinion. But makes restarts a nightmare depending on your valves (single or double) and generally if you drop parts have to clear your entire shift register on the table and throw away 16 parts on the dial which makes the customer unhappy…
Let's say that your customer asked you to design a “pausing” machine. I.E when you open the door, the machine pauses or holds motion until the doors are closed again. Obviously there are some safety issues involved here.
I know that you guys have had this situation come up, how have you handled the situation?
Do you decline totally from the customer request to design a pausing machine and say TOUGH you loose 16 parts every time you open the door instead of pushing cycle stop FIRST before you open the door?
Do you design a control system that will work?
Do you cheat and put your inputs into your PLC and program “DOOR_OK” bits in your program to pause the machine and get a signed waiver from the customer?
If you design a control system that will work, what is your method of safe controls for the above situation?
If your 16 station pick and place has single solenoid valves and you dump the air on an e-stop, upon restart if you have a gripper you will drop the part on a pick and place head. If you design all double valves in every motion and still dump the air, the chances are better for an auto recovery but who's to say the gripper won't still drop the part...or someone gets in there and pulls the part off the gripper.
Or do you guys NOT dump the air, and only kill the output power only?
Just curious what your methods are. Seems challenging in trying to design a system that is safe, make the customer happy, and follows the NFPA 79. I’ve always voted dump the air, but I wondered what your ideas are IE.
SAFE:
Door switches = E-STOP (CRM circuit)
Dump Valve releases = CRM
Output power shuts off = CRM
OR:
Door switches = E-STOP (CRM circuit)
Dump Valve = PLC controlled
Output power shuts off = CRM
OR:
Make up your own?
Let's say you have a machine that is an automatic dial table on an indexer, and there are pneumatic pick and place heads on the dial. The dial has 16 stations let's say. And you have a bit shift register that tracks your parts etc around the dial table.
Your dial table is completely guarded with Lexan and has four access doors on each side. You want to design a safe system so naturally you would tie your door switches into your E-stop circuit that would blow the CRM (control master relay) and dump the system air at the same time when a door is open. This is the best and safest way always in my opinion. But makes restarts a nightmare depending on your valves (single or double) and generally if you drop parts have to clear your entire shift register on the table and throw away 16 parts on the dial which makes the customer unhappy…
Let's say that your customer asked you to design a “pausing” machine. I.E when you open the door, the machine pauses or holds motion until the doors are closed again. Obviously there are some safety issues involved here.
I know that you guys have had this situation come up, how have you handled the situation?
Do you decline totally from the customer request to design a pausing machine and say TOUGH you loose 16 parts every time you open the door instead of pushing cycle stop FIRST before you open the door?
Do you design a control system that will work?
Do you cheat and put your inputs into your PLC and program “DOOR_OK” bits in your program to pause the machine and get a signed waiver from the customer?
If you design a control system that will work, what is your method of safe controls for the above situation?
If your 16 station pick and place has single solenoid valves and you dump the air on an e-stop, upon restart if you have a gripper you will drop the part on a pick and place head. If you design all double valves in every motion and still dump the air, the chances are better for an auto recovery but who's to say the gripper won't still drop the part...or someone gets in there and pulls the part off the gripper.
Or do you guys NOT dump the air, and only kill the output power only?
Just curious what your methods are. Seems challenging in trying to design a system that is safe, make the customer happy, and follows the NFPA 79. I’ve always voted dump the air, but I wondered what your ideas are IE.
SAFE:
Door switches = E-STOP (CRM circuit)
Dump Valve releases = CRM
Output power shuts off = CRM
OR:
Door switches = E-STOP (CRM circuit)
Dump Valve = PLC controlled
Output power shuts off = CRM
OR:
Make up your own?