Ready Steady Eddy

**EddyCurrent** · 14-11-2013

Dean, you are correct in that every situation has to be risk assessed on it's own.
I also think the braking resistor is not required because of the fairly low inertia of the load.
I agree that for DIY either strategy is fine but maybe not everyone reading this is in a DIY situation, in which case they should be referring to the relevant regulations.
The main aim of this is to get a good emergency stop system using the gear we've got.

It would be useful if we could define some scenarios and strategies for stopping a cnc machine, your input is valuable here.

I've listed some ways to stop the machine and the strategy to adopt, I'm not saying it's comprehensive so please add or amend as you think.

*emergency stop button pressed - strategy 1 or 2
*limit switch activated (not home limits) - strategy 2
*spindle over temperature switch activated - strategy 3
*Stepper driver alarm relay activated (e.g. AM882) - strategy 3
*'Stop' button pressed - strategy 4
*Charge Pump error - strategy 1 or 2

Strategy 1
----------
* remove power to all devices immediately

Strategy 2
----------
* remove power to stepper drives immediately
* remove power to ancillary devices immediately e.g. air, water, dust extraction
* issue stop command to VFD then after delay remove power to VFD

Strategy 3
----------
* issue audio visual alarm before taking action (with time delay)
* controlled stopping of all machine actuators and ancillary devices
* leave power supply on to VFD

Strategy 4
----------
* controlled stopping of all machine actuators and ancillary devices
* leave power supply on to VFD

**JAZZCNC** · 15-11-2013

Originally Posted by EddyCurrent

I agree that for DIY either strategy is fine but maybe not everyone reading this is in a DIY situation, in which case they should be referring to the relevant regulations.
The main aim of this is to get a good emergency stop system using the gear we've got.

It would be useful if we could define some scenarios and strategies for stopping a cnc machine, your input is valuable here.

Yes Agree in industrial situation then machine needs to be safe and Regs followed to keep the HSE police happy but mostly folks here are DIY and there's world of difference between Reg compliant and being used safely in a shed.!! . . . Practical common sense is all that's needed IMO.

Regards Strategy's then here's my take.!! (which has changed over the years.! I used to kill everything regardless)

E-stop means Foooooooking Hell EVERYTHING STOP NOW.!! . . . So Strategy 1 only.!

Limit/s trip means machine is outside it's working parameters so do something about it.!! . . So disable the drives using the drives enable signal, Not killing power to drives so maintaining holding torque. At same time inform the Control software so it's halts program execution.
Send Stop signal to Spindle/VFD again leave power on has it's not an emergency, it's a machine position error so we just need it to come to a controlled stop.
Along side this you could if wanted turn off other things like vacuum etc either by informing control software or by locally controlling on/off thru Relays but all this should just be on the Limit circuit NOT The E-stop.
Same regards the Fault signals from drives, It's a machine Error not an emergency so same applies just stop the machine in a controlled manner.

If any of the above happens then it should be wired in such away that some form of Latch is dropped and not allowed to re-latch until fault is cleared and then only with Push of a Momentary button not Reset switch.

The E-stop should have ultimate control and Kill power to everything in the event of an emergency so all Latches are dropped and won't reset or start again without some input. Ie Pushing Reset momentary button.

Charge pump and over Temp etc are again machine errors not emergency conditions so just bring machine to stop in controlled manner.

Personally I wouldn't have Temperature control anything over than alert the user thru audible or Visual means. Problem comes from the manner in how the machine is stopped.? To safely stop the machine and not wreck the work piece or lose position requires a Feed hold not a Stop command.! If you stop the machine while it's moving, either thru E-stop, limit trip/machine error or just pushing Stop button on control screen then you MUST presume you have lost position thru inertia pushing, THE ONLY SAFE WAY to bring the machine to stop and not lose position is to use Feed hold and allow the control software to do a controlled stop.
Problem with this is it's doesn't do it instantly so there's always some delay until look ahead buffer is cleared or it's finished the current move. If you have an overtemp problem you won't want to wreck the job so a controlled stop is required so feed hold must be used.
It is possible to use an input and have Mach watch it then use an OEM trigger to activate the Feedhold if the state changes but personally I wouldn't and just have a Spindle siren scream at me.!!

This also answers your "Stop Button" option.! . .Pressing stop will only stop the machine and other attached devices in a semi controlled manner but it won't be controlled and can't reliably be resumed from without first homing the machine to get back into position.

Also noted you said Home limit's.? Home switches are not limits of any kind neither are they part of any system, e-stop or limit etc. They solely define the Machine coordinate Zero position and are purely input's to the Control system and nothing else..!! . . . .(Eddy I know you probably know this but others may not)

**Jonathan** · 15-11-2013

Looks like we're all largely saying the same things now.

Originally Posted by JAZZCNC

This also answers your "Stop Button" option.! . .Pressing stop will only stop the machine and other attached devices in a semi controlled manner but it won't be controlled and can't reliably be resumed from without first homing the machine to get back into position.

For the non-emergency stop button, you could just wire it to an input set to activate the feed hold, so the machine does in a controlled manner. It's still likely to leave a mark on the work, but prevent something worse.

**JAZZCNC** · 15-11-2013

Originally Posted by Jonathan

Looks like we're all largely saying the same things now.

Didn't see your post until after I'd posted that.

Originally Posted by Jonathan

For the non-emergency stop button, you could just wire it to an input set to activate the feed hold, so the machine does in a controlled manner. It's still likely to leave a mark on the work, but prevent something worse.

Erm.!! . . . Well that would just be a Feed hold so why not Call it Feed Hold.??

**cncJim** · 15-11-2013

Originally Posted by EddyCurrent

.....
The main aim of this is to get a good emergency stop system using the gear we've got.

It would be useful if we could define some scenarios and strategies for stopping a cnc machine, your input is valuable here.

I've listed some ways to stop the machine and the strategy to adopt, I'm not saying it's comprehensive so please add or amend as you think.
.....

This is a great idea Eddy, great for noobs like me. Might be nice to add example circuits for each strategy (might be getting carried away now...

)?
I have made some changes based on some of the feedback and tried to make clear which things should be done in hardware and software, hope that's ok. Maybe it could make a good sticky?

List of scenarios and possible strategies for dealing with them.

Scenarios

Emergency stop button pressed - Strategy 1, 2 or 2.5
Limit switch activated (not home limits) - Strategy 2 or 2.5
Spindle over temperature switch activated - Strategy 3 or 3.5
Stepper driver alarm relay activated (e.g. AM882) - Strategy 3 or 3.5
'Stop' button pressed - Strategy 4
Charge Pump error - Strategy 1, 2 or 2.5

Strategy 1

remove power to all devices immediately (Hardware)
Inform control software of Estop condition (Software)

Strategy 2

remove power to stepper drives immediately (Hardware)
remove power to ancillary devices immediately e.g. air, water, dust extraction (Hardware)
issue stop command to VFD then after delay remove power to VFD (Hardware)
Inform control software of Estop condition (Software)

Strategy 2.5

Issue stop to stepper drives then after delay remove power to stepper drives (Hardware)
issue stop command to VFD then after delay remove power to VFD (Hardware)
remove power to ancillary devices immediately e.g. air, water, dust extraction (Hardware)
Inform control software of Estop condition (Software)

Strategy 3

issue audio visual alarm before taking action (with time delay) (Hardware/Software)
After delay:-
Stopping of all machine actuators and ancillary devices (Software - "Stop" Command)
leave power supply on to VFD

Strategy 3.5

issue audio visual alarm before taking action (with time delay) (Hardware/Software)
After delay:-
inform control software to activate Feedhold (Software)
Move to safe Z (Software)
Issue stop command to spindle (Software)
leave power supply on to everything