Auto/Manual Selector switch... one input or two?

[oregonsam]

We use a two position selector to switch our machines from auto to manual mode. In manual, many parts of the machine can be jogged by the operator. The program only uses one input (AUTO) and assumes manual when the auto input is not set.

What are the advantages/disadvantages of using this method versus a separate input for both auto and manual?

thanks

 

[TimWilborne]

Some may say a slight safety margin but I disagree unless you are using a safety monitoring system

The only advantage I know of is some use a 3 position selector and the middle position is off.

As far as a two position selector I would save my input

 

[Contr_Conn]

I always use 3 position spring return to center.
This will allow to control mode programmatically and prevent putting machine in manual in the middle of the cycle.
This also will preven AUTO mode if machine is not ready for auto.

 

[jstolaruk]

If not a feature in the HMI I use a 3 position maintained switch to two inputs, auto and manual, because I prefer not to infer a manual state if auto is not selected. However, if the input count is tight then the MANUAL IS NOT AUTO method will free up an input.

 

[Gope_69]

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".

 

[tomalbright]

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.

 

[PeterW]

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.

 

[rPraveenkum]

auto/manual keys should be with Key lock of three position for small maintenance operation during production surely it will help

 

[PeterW]

auto/manual keys should be with Key lock of three position for small maintenance operation during production surely it will help

Absolutely not, the motive power i.e. the 415 supply to the drive should be locked off, not the control signal.

 

[rPraveenkum]

415 supply locked off may be possiblein offline machine is possible But t in the continuous Production line If have 24x7 function 415 supply locked off not possible to do maintenance

 

[JesperMP]

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.
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.

 

[PeterW]

In addition to Jespers examples, you can have local isolators, lockable motor starters/MCB's, plug and socket drives, where a lock on the clamp can prohibit re-inserting the plug.

In the UK, what rPraveenkum is suggesting would be illegal and if an accident occured the design engineer could find himself in prison.

 

[PeterW]

Going back to the original question, I have used both 2 and 3 position switches.

Two postion hand/auto, hand = off and auto = on. 1 input.

Three position, hand = hand position, off = no input, auto = auto position. 2 inputs.

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.

 

[rPraveenkum]

I also agree with the PeterW. there other safety system like light screens, saftey mats , gate pins which in turns locks motor power

 

[Norml]

Hand - Off - Auto has been around for a long time for good reasons.

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.