Programming Application - Anti Tie Down Logic
Anti-tie down logic is normally a good design practice to include in your PLC logic when you have an operator standing in front of a semi-automatic machine. Operators have a lot of things on their minds when they are working on your machine. Using anti-tie down insures safety by making sure the operator's hands are in a safe position during a potentially hazardous motion.
You should always evaluate your machine safety requirements and make sure you are allowed to use two-handed inputs directly before implementing PLC logic for two-hand controls. If your machine safety evaluation finds that you are not allowed or your conditions are too hazardous, you should always use a safety rated two-hand control module instead of PLC logic or use two-hand control PLC logic in conjunction with a light curtain to protect the front of the machine.
The idea behind anti-tie down is to make sure the operator places both hands on a set of input feedback such as a set of Opto Touch buttons. If your application will not allow for PLC logic as the anti-tie down method, then there are safety modules available such as a DUO-TOUCH module that will accomplish the same things as this PLC logic.
Anti-tie down logic requires the operator to place their hands on the input device at the same time and not release until the condition is safe to remove the hands. In the case of a rotary index table, the operator hands must remain on the Opto Touch buttons until the index table has cycled one complete index and stopped motion. If this operator releases their hands before the motion is complete, the index table should stop.
Photo Courtesy of Banner Engineering Corp
Operators' placing their hands on the Opto Touch buttons at the same time is a safety issue. If a false input on one of the buttons were sensed, then it would only take the other input to start the machine cycle. Some past causes of a false input conditions could be a person laying a rag on top of one of the opto buttons. Granted, opto technology has become far more advanced and this situation is not likely. However these types of false input considerations must be taking into account so your logic can be written to prevent false cycle starts and possibility prevent bodily harm.
If you are in doubt about your machine's safety requirements, chances are best if you choose to install a safety module instead of PLC logic or use a light curtain with your Opto Touch buttons.