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Dear oregonsam, Only disadvantage is that when both inputs are set, system collapses which function to perform. To prevent this only one input is taken for logic either to control Auto or manual at only one operation at a time. Auto operation will be programmed only the particular input is set to ensure safety.
When that input not there, manual / jogging operation will be perfromed. By the way you can spare one PLC input. I think I have cleared your doubt. Cheers, Gope IMPOSSIBLE - The word itself contains 'I M Possible'. I'm a fan of 3-position switches.
Auto is fine. Manual via PLC can allow limited control, while maintaining critical safety functions.
A center OFF position makes the equipment unusable by an operator, especially when maintenance is being performed. Assuming manual when there is no input may be acceptable if there are other safeties or lockouts external to the PLC. When maintenance is being performed, the motive power should be isolated and locked, no way depend on 'off' selected. If the auto/man input has failed on you could kill someone. I aggree with PeterW.
It is not OK to rely on a regular PLC function to safeguard personnel. For very short work in the dangerous area, it is OK to rely on a safety circuit, i.e. An inspection door with door contacts + safety relay + removal of power via the safety relay. Facedub Elite Serial.
Edit: If the work is repetitive, like inserting removing pieces in the machine, then consider a light curtain solution. For longer work, the safety circuit must be CAT4, or you have to remove power completely + you must lock and tag the power circuit. Going back to the original question, I have used both 2 and 3 position switches. Two postion hand/auto, hand = off and auto = on. Three position, hand = hand position, off = no input, auto = auto position.
If both inputs are on, then its a design decision on which takes preference or assume off. There should be an alarm/warning message on this state.
Off would normally be to disable the area/drive temporarily, on a conveyor it maybe to remove something on the conveyor for example, not to dismantle the conveyor, for that the isolator would need locking off. Hand - Off - Auto has been around for a long time for good reasons. Edirol Hyper Canvas Vsti. Acupuncture Correspondence Course In 31 Lessons Pdf. Auto = Allow automatic operation from PLC etc. Add a contact block to tell the PLC that AUTO mode is enabled.
'Not In Auto' works fine and only one PLC input is needed. Hand = Allow manual operation without PLC intervention. If the PLC fries, some functionality should be allowed for jogging, maintenance, setup etc. Having an input to the PLC that tells it Manual mode is enabled does not serve any purpose if you assume that the PLC is junk. Adding a contact block in Hand mode is handy for changing the speed reference for a drive - for example, changing speed control from the PLC to a speed pot. Off = Off period and no machine movement should be expected. This is not the same as lock out/tag out.
Lock out/tag out is the only dependable method of ensuring safety regardless of what the control system is doing. We use 3-position HOA switches with nothing fed back to the PLC. Auto allows PLC control, Hand engages the motor starter, and Off is off. I did one recent project where Hand & Auto fed back to inputs. The electricians disliked it so much; all new projects are being done the old way. Given the choice I would feed everything back to the PLC with spring return switches.
I don't like that to be honest. How does the PLC know that it is in Auto? If you start a sequence and it is selected in auto, does it Alarm as it sees no movement/feedback?? If it sits in auto thinking its OK, what happens when you select Auto?
Jump into life without a start?? No, I don't thinkl I would do that personally. I was wondering the same thing? How do you have an HOA and no input to the PLC? Y'all getting a little knee jerky with the SAFETY thing! OFF is OFF, Manual is manual, and AUTO is automatic, never shall they meet. The switches are for operator control, not maintenance.
Manual should be there for the operator to 'jog' products or 'manually' adjust the line as needed.IT is not meant as a safety feature and the operators should be TAUGHT that. Y'all get too KNEE JERKY with the SAFETY thing, an HOA should never be thought of as a safety issue, it is plain and simple machine control, nothing more. IF and when maintenance occurs, this could be an operator clearing a jam etc, then all forces should be de-energized and locked out before accessing the machine. The 'controls' have nothing to do with this. DO NOT get SILLY and confuse control with access and safety. TEACH, TEACH.do not talk about it, TEACH IT to those that NEED to know.