PDA

View Full Version : E-Stop,Limit Switches - Car door open switches ?



TrickyCNC
04-18-2012, 05:52 AM
Has this been tried ?

The switches fitted to most car door pillars are push to break, so N/C (the ones that make the interior light come on)
They are cheap
The are available almost everywhere
They are rugged
They have fixing/mounting points
They could easily be used with a hinged cover for e-stops

As for limit/Home switches, it depends on how repeatable the contact point is.

Is it worth spending a little time experimenting , or has it been tried and failed ?

I'll be making up an e-stop with one 1st


Just another of the ideas that came to me whilst trying to sleep :)

Rich

HorridHenry
04-18-2012, 06:01 AM
They're open contact when in closed operation...ie no good,remember these only turn on the light when the car door is open.

Blimey Tricky I can buy the proper switches for 99p on ebay.

TrickyCNC
04-18-2012, 07:58 AM
Think again Double H :)

wildwestpat
04-18-2012, 10:00 AM
Hi Rich

Those car door light switches could be used for limit switches but would be very poor as homing switches. This is because they are a simple push direct on the contacts to open the electrical contacts and there is nothing in those switches to ensure they break contact at exactly the same point each and every time. The mechanical microswitch uses an over centre butterfly construction to give a consistent and switch action for a given mechanical input change this is usually amplified further by an operating lever to provide over shoot protection for the switch. There are electronic switches of various types. The final choice is down to operating environment as much as financial cost.

The limit switches can be arranged to make use of either normally open OR normally closed. Normally open are wired in parallel so that any one closing activates the motion limit feature which the motion control will interpret to stop that particular axis traverse. Normally closed are then connected in series. Also don't forget Mach3 also has the ability to set software switches to limit traverse action.

Regards - Pat

keebler303
04-18-2012, 10:28 AM
Pat

Little typo in your second paragraph.

NO should be parallel
NC should be series

Matt

TrickyCNC
04-18-2012, 01:25 PM
N/C sounds a better idea to me, as you can then tell if a connection comes loose, or open.

As for not having a precise contact point, I'll check on I have in the garage, but I'm pretty sure they make contact in the fully open (sprung) postition.
I guess they might not be .001 tolerant though.

I think for an e-stop they should work fine.

wildwestpat
04-18-2012, 02:39 PM
Hi Matt

Thanks for pointing out my typo which I have now corrected.

Regards - Pat

brtech
04-19-2012, 01:51 PM
No they are not good for E-Stop. E-Stop is push on, push and twist off. When you activate E-Stop you want it to stay activated until you reset. It's not momentary contact. It is NC when unactivated.

TrickyCNC
04-19-2012, 01:54 PM
No they are not good for E-Stop. E-Stop is push on, push and twist off. When you activate E-Stop you want it to stay activated until you reset. It's not momentary contact. It is NC when unactivated.

Ahh .. I didn't know that. I thought e-stop needed just a pulse to 'lock' the system until reset.

Thanks

IBBruin
04-19-2012, 01:59 PM
May have spoken too soon. I'll check

brtech
04-19-2012, 01:59 PM
In most systems, if you momentarily trigger E-Stop, it won't start again by itself, but this is a safety system. You want a mechanical interlock from restart, not just an electronic one.

TrickyCNC
04-19-2012, 03:19 PM
I'll admit now, I'm new to this DIY CNC.

If I was designing a safety emergency stop system, it would be pulse stop.... then manual reset - after fault has been found.

If the system relies on the stop button remaining 'pressed', then IMHO this is unsafe. if the button popped out, when an operator was clearing/fixing the problem, the machine will start up again and possibly Kill the operator !"

I'll read some more on this, but if this is not the case, I might degign my own e-stop circuit

Rich

Al_The_Man
04-19-2012, 03:37 PM
The type of E-stop buttons that do not use the retaining contact across a start P.B. use a mechanical latch, push to latch - twist to release.
Although these are allowed, I myself prefer the start button reset circuit with momentary E-stop P.B.
Group Schnieder/Telemecanique have a PDF of sample control circuits on their site.
Al.

brtech
04-19-2012, 05:18 PM
If you have a positive "Start" button that must be pressed before any restart could occur after E-Stop, then a momentary contact E-Start is fine. Presumably you have to press START for every sequence, not just after an E-Stop.

All the CNCs I've seen will not automatically restart after E-Stop, even if it's a momentary action. They all stop, and you have to restart them. On a common system using Mach and a G540, Mach stops and the G540 completely resets (does not drive the motors). You have to reset the E-Stop (push and twist), and then you have to re-enable Mach, and then you restart the program.

The mechanical "push and twist to reset" is a mechanical backstop. If YOU pressed stop, YOU have to reset it, mechanically. Pressing "START" is fine. Push and twist to undo STOP is fine. It's a positive mechanical action that disallows any restart independent of the software.

Al_The_Man
04-19-2012, 05:55 PM
The bottom line is that the E-stop circuit and Reset is a hardwired function and Reset should not be a function of, or initiated by, the software HMI, apart from the HMI or controller E-stop OK/enable only.
The customary way is now moving toward the use of safety relays to carry out the different functions and options that may be required of a Emergency stop.
Al.