Limit switch

Seb___01

Member
S
Join Date
Oct 2016
Location
perth
Posts
5
Gate hits limit switch
limit switch signal does not act directly on contactor signal wire but goes into PLC through Ethernet and PLC switches motor OFF.

Problem - gate running past limit switch because of processing time.

My fix is to have the limit switch interrupt supply to contactor and to also supply feedback to PLC via a double pole relay.

My Boss (Dad) thinks the control should go through the PLC.
Digital signal in for the limit and
Digital signal out to stop motor

If anyone can help clarify this issue would be greatly appreciated.:)
 
Welcome to the forum, your way is my prefered way. I never use PLC processes to position critical or important equipment, as you have found the scan time of the processor can have a marked effect on the position Although it's not so critical these days as scan times are remarkably fast compared to older and lower end of market processors).

Steve
 
But wait there's more

Thanks Steve

Also the current limit switches are cam and arm type switches, mechanical essentially.

Would replacing them (current mechanical switches are not broken, maybe 1 was a little sticky) with proximity switches, benefit in any way, more than what is currently installed and working.
 
Can you provide us with brand and model of PLC

How fast does the system operate as plc,'s scan time is measured in milliseconds
or tens of milliseconds which for most uses is very fast.

Seeing as you use the limit switch to operate the contactor to stop the motor
I don't think the scan time is the issue which means the boss maybe right.

Or maybe you are in the situation that the boss is never wrong ..... DAD .....
 
Last edited:
Seb___01 said:
Gate hits limit switch
limit switch signal does not act directly on contactor signal wire but goes into PLC through Ethernet and PLC switches motor OFF.

Problem - gate running past limit switch because of processing time.

My fix is to have the limit switch interrupt supply to contactor and to also supply feedback to PLC via a double pole relay.

My Boss (Dad) thinks the control should go through the PLC.
Digital signal in for the limit and
Digital signal out to stop motor

If anyone can help clarify this issue would be greatly appreciated.:)
Click to expand...

Hi
By your description does not seems like gate is so critical to worry about the exact stop position.... or maybe it is.... it depends
In my personal opinion I would wire the motor contactor coil trough the limit switch and a feedback signal to the PLC through the contactor aux pole.
If you have high and low limit switches do for both.

Regards
 
In addition to the relevant observations about safety-related applications, I would try to modify the mechanical setup so that the "Gate Opened" and "Gate Closed" limit switches remain actuated all the way to the physical end of travel of the gate.
As currently configured, when a switch is open, you don't know whether the gate is between the two limit switches or beyond one or the other of them. Too much ambiguity that can be reduced by a simple mechanical modification.
 
Steve Bailey said:
In addition to the relevant observations about safety-related applications, I would try to modify the mechanical setup so that the "Gate Opened" and "Gate Closed" limit switches remain actuated all the way to the physical end of travel of the gate.
As currently configured, when a switch is open, you don't know whether the gate is between the two limit switches or beyond one or the other of them. Too much ambiguity that can be reduced by a simple mechanical modification.
Click to expand...

Totally agree
 
Seb___01 said:
Thanks Steve

Also the current limit switches are cam and arm type switches, mechanical essentially.

Would replacing them (current mechanical switches are not broken, maybe 1 was a little sticky) with proximity switches, benefit in any way, more than what is currently installed and working.
Click to expand...

The only benefit to using a prox is non contact and no wear to parts contacting each other. Otherwise there is no control advantage...prox is really no more precise given its field of range and switching hysterisis.
 
from somewhere like http://www.schneider-electric.us/en/faqs/FA102359/
you can see that the time to drop out a contactor is typically 20-50ms.
if your plc is not picking up the signal, even taking into account that it is currently going through an ethernet I/O adapter of some support, then your contactor may not either.

Do some math to work out how long the door switch is on for, then compare that to your maximum system response delay.

I agree that the answer here is to redesign the system mechanically. Find a way of sensing whether the gate is past the sensor or not.

If this is safety related, please make sure that the performance level of the system of components used are appropriate for the risk you assessed.
 
PLC type

Sixnet M2M with a cycle of 2 milliseconds,
so you think that it will process it perfectly fine, but it isn't

Maybe the size of the tab(50mm) - it may only be ON for 500mS

I found most TOL tripped, and don't have great trust in these PLCs, so I directly wired the contactor signal wire in series with the limit switch, I would think that is problem solvered


Gil47 said:
Can you provide us with brand and model of PLC

How fast does the system operate as plc,'s scan time is measured in milliseconds
or tens of milliseconds which for most uses is very fast.

Seeing as you use the limit switch to operate the contactor to stop the motor
I don't think the scan time is the issue which means the boss maybe right.

Or maybe you are in the situation that the boss is never wrong ..... DAD .....
Click to expand...
 
Thanks every1, Issue is still under rectification, I will let u know how I go,
I highly rate the keep it simple method by using auxilary contact on contactor for feedback,
also enlarging the tab, but this may affect critical position
Still unsure of how critical the position is (although there were comment it was passing grain) so I guess critical, how critical will need to be calculated.
I will do some killer math on site when im there for a week, doing what im told...sigh

Thanks again will post in a week or so
 
You may want to make sure that the flag/tab for the switch stay's in contact with the switch. Give the switch some lead time to make up for gearbox/ motor over run will help.
If its on a weighing system, you might thing about a 2 stage closing. wide open for most of the flow then close down to a trickle to complete.
 

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