Ladder Logic: Auto cycles and safety routines
Machine operators are prevented from changing modes if the system is faulted or entering manual mode if the machine is auto cycling (see Figure 1). The machine also is placed in manual mode upon a home request. The auto cycle command allows the auto cycle to start in auto mode if no faults are present and for the cycle to stop only when the auto sequence has not been started.
If a fault were to occur, this would ensure that the machine would hold its present state. Other things you might see include a setting that only allows a cycle start when the machine is at home or origin position. There might also be a three second hold-down requirement for the cycle start pushbutton (during which a horn might sound), or any number of special requirements as defined by a customer.
Other items such as an immediate stop button (allowing stopping of the machine even if in auto cycle) or a memory bit for cycle stop (allowing the cycle to complete without having to press the button again) are common.
A note on the format of the last rung: the first two rungs use latch or "set" bits for auto and manual mode. The last auto cycle rung also is latched by means of a "hold-in" contact, the parallel auto cycle bit with the same address as the coil. This is a common technique used in programmable logic controller (PLC) ladder programming and is really just a matter of preference. It also is important not to start machinery instantly. Rather, users should warn personnel around the machine that it is about to move. This is required for most machinery, with the exception of simpler machines such as test stations.
The logic in Figure 2 shows a method of pulsing a horn to warn people that the system is starting. It requires the person starting the machine to hold the button to create the cycle start request signal. The next step in the logic is to actually start the system (see Figure 3).
This is by no means the only way to do this, but it gets across the general idea. There are four things users should note:
- The operator must hold down the cycle start button for the entire time for the system to start. Letting go starts the timer again.
- The system won’t start unless in auto mode with no faults.
- The system will not stop if in the middle of an auto sequence; this should be modified to taste. It should allow sequences to come to rest in a natural position; a specific sequence step or position can replace the sequence active bit.
- A fault stops the auto cycle immediately. Again, this may not apply in every case.
The auto cycle bit or status is not really a mode in itself; it is more of a state within auto mode. Generally, it is used to allow auto sequences to start or proceed, but not to disable output energization.
Frank Lamb is the founder of Automation Consulting LLC, and is a member of the Control Engineering Editorial Advisory Board. This article originally appeared on Automation Primer’s blog in two parts. Automation Primer is a CFE Media content partner. Edited by Chris Vavra, production editor, Control Engineering, email@example.com.
- The auto cycle command allows the auto cycle to start in auto mode if no faults are present.
- There are safety parameters in place to warn others before a machine is supposed to start.
- Auto cycle bit or status is designed to allow auto sequences to start or proceed.
What other benefits can auto cycles provide for ladder logic?