Anti-tie down


Thread Starter

Stephen Wright

I have recently reviewed an old machine we have had in our plant for some time. I had assumed that the capacitive palm buttons contained some
anti-tie down circuitry. When they failed and I replaced them I started a risk analysis to ensure I had proper safeguarding. The PLC controlling the machine has something like anti-tie down logic for the button operation but I am unsure as to the proper control to install and have not found any conclusive design direction. Can anyone relate some of their experiences or direct me to better information?
in ladder logic :

Rung 1. using one of the buttons PB1 set internal relay R1.

Rung 2. R1=NO starts on delay timer000 T000=NO resets R1.

Rung 3.PB2 starts timer001 T001=NC R1=NO set internal relay R2.

Internal relay R2 is the start process relay and should be reset by the end of process.

This works for me using a Keyence Visual series PLC.
pb1 n 0.2 sec.
pb2 |

pb1 pb2 timer n
---][----][---]\[-------( )----

<P>However, in general you should not implement anti-tie down controls in a PLC where a failure of the PLC output hardware of software in either the on or off state will allow and unsafe condition to exist. You will need to evaluate that risk for yourself. Anti-tie down controllers implement redundant fault checked relays for outputs, something most PLCs don't do. A number of companies such as Pilz, Banner,
AB, STI, Guardmaster, offer these kinds of devices. AB, Pilz, and a few others have a safety PLC, which does have reduncancy and fault checking on the physical hardware and redundant processing to check for faults in solving the program. They do not check for bugs in your program however. Do a thourough safety and risk evaluation for your process.</P>