WM16 Mill Arduino Power Feed

[Tom GullanParticipant @tomgullan59234]

Hello,

I recently designed a power feed on the X axis of my milling table using a wiper motor and a PWM. Everything works fine, however as yet I have not incorporated some sort of failsafe device like micro switches to stop the motor before the table reaches its maximum travel in either direction.

I am thinking of using an Arduino Uno micro computer along with some sort of sensor setup on the lead screw to count the amount of turns on the screw. I would then be able to write a program to count the turns and stop the wiper motor before the end of the travel.

I would be interested in hearing from anyone who has done this or who has any thoughts on how to do it.

Regards

Tom

[Tom GullanParticipant @tomgullan59234]

[John RuddParticipant @johnrudd16576]

Rather than count the number of turns, just use a switch positioned to detect the end of travel?

The switch could then be placed anywhere along the axis for smaller travel jobs?

[EmgeeParticipant @emgee]

Hi Tom

As John suggests use a switch or even 2 switches, set to each end of travel required, by using dpco switches or perhaps switched relays you can have auto feed fwd and rev.

Emgee

[duncan webster 1Participant @duncanwebster1]

Tom,

I've used an arduino to convert lead screw rotations into distance travelled, it was written up in either ME or MEW, You can find it online at

**LINK**

You'd need to write more code to enable end of travel. However if all you want is to set end of travel, then microswitches are a lot easier, My miling machine table is driven by arduino and stepper motor, but microswitch end of travel as I reckoned it more relaible.

[John HaineParticipant @johnhaine32865]

If you really want to use an arduino for this then why not use a stepper motor and driver instead of a wiper motor?

[I.M. OUTAHEREParticipant @i-m-outahere]

I would use a 555 timer set up as a one shot with a time base of 1 hour and use centrally located micro switch unit to trip the reset pin of the 555 timer . You can then use adjustable end stops like on a commercially available power feed to stop the table where you want , you may only want a 100mm of travel on some jobs and more on others . The stops also act as end stops for full table travel when set at each end of the table . The 555 simply activates a relay to power up the pwm controller .

I made a power feed controller for the z axis of my mill using a pwm and an arduino nano that reads the pulses froma rotary encoder to work out the feed rate in mm/ min and display it on an lcd screen , it uses a micro switch end stop and an adjustable stop on the quill feed rod so i can set it to stop where i want .

The sketch is just a slightly modidied tacho sketch i found on the internet and changed two lines of code .

[Speedy Builder5Participant @speedybuilder5]

If using a micro controller, then the failsafe should be fixed microswitches at each end of the traverse. If you had a blip in the power, all sorts of things could happen. If using a wiper motor, perhaps incorporate a fuse in the circuit to prevent burn out of the motor.
BobH

[Brian GParticipant @briang]

In order for the Arduino to stop the table by counting leadscrew revolutions it would need to know its starting position. This would either require storing the position between runs, manual setting, driving the table up to a sensor in the same way as a 3D printer, or reliably storing the position.

Personally I would be unhappy with storing the position as this could fail in the event of an uncontrolled shutdown, or if the table was moved whilst the Arduino was powered down, whilst manually setting the position each time would be a chore, which could lead to damage if forgotten. That leaves use of a microswitch, optical or magnetic sensor any of which could much more simply send a stop signal directly to the Arduino.

Brian

[Martin KyteParticipant @martinkyte99762]

Why not keep things simple and run the motor power through the limit switch(s). Or if you want to get fancy encorporate an emergency stop relay held on by your limit switches and estop switch wired in series.

It's not normal to have safety systems controlled by electronics, computers or uprocessors. You just keep things as simple as possible with contacts that just knock the power off.

regards Martin

[John HaineParticipant @johnhaine32865]

Or see here for a well thought out system of adding stops and adjustable power feed, from Mike Cox.

[Phil GrantParticipant @philgrant54580]

I know this is an oldish thread but I'm researching a similar topic for adding a power feed to my Warco mini lathe.

Having had a lot to do with 3D printers and making my own why not look at a non contact inductive sensor as a limit switch, they are used on industrial machinery all the time and don't wear out or get swarf in them or liquid.

Just a thought.

[not done it yetParticipant @notdoneityet]

Belt and braces, but I would still want a mechanical link to fail if the table managed to reach full travel at one end or the other. Switches are OK for control points, but a shear pin does not need anything other than itself to decouple the dive from the driven. Keep it simple.

[Phil GrantParticipant @philgrant54580]

You'll get no disagreement from me

I already added a mechanical stop to my manual autofeed engagement on my Warco super mini lathe as I didn't want to crash the chuck if I got distracted.

[Mike PooleParticipant @mikepoole82104]

Posted by Martin Kyte on 15/03/2018 09:19:46:

It's not normal to have safety systems controlled by electronics, computers or uprocessors. You just keep things as simple as possible with contacts that just knock the power off.

regards Martin

The motor industry has been using electronic safety devices for at least the last 35 years, safety PLCs and bus systems are now common.

Mike

[Tom GullanParticipant @tomgullan59234]

Gentlemen,

 

Thanks for your comments. I actually ended up settling for a window wiper motor and a PWM system. Here’s my attempt:

 

Regards

Tom

 

Edited By Tom Gullan on 10/10/2018 00:30:39

Edited By Tom Gullan on 10/10/2018 00:31:41

Edited By Tom Gullan on 10/10/2018 00:35:44

[Phil GrantParticipant @philgrant54580]

Looks good and it looks like you used inductive sensor for limit switches?

Edited By Phil Grant on 10/10/2018 06:36:05

[Martin KyteParticipant @martinkyte99762]

Posted by Mike Poole on 09/10/2018 21:15:32:

Posted by Martin Kyte on 15/03/2018 09:19:46:

It's not normal to have safety systems controlled by electronics, computers or uprocessors. You just keep things as simple as possible with contacts that just knock the power off.

regards Martin

The motor industry has been using electronic safety devices for at least the last 35 years, safety PLCs and bus systems are now common.

Mike

Cars don't generally have an emergency stop system or safety interlocks on guards. Machinery and system which are generally electronically controlled will have safety systems built in to that level in the form of watch dog timers, system health monitors etc. However the top level Emergency Stop system is generally made as simple as possible and is best when it overrides the electronics and just kills the power/closes the shutter on the laser/etc.

regards Martin

regards Martin

[Phil GrantParticipant @philgrant54580]

I think we're getting a bit over the top here, don't mix cars safety systems and machine interlocks up with each other, each have different requirements.

Early prototype vehicles do have E stops fitted to them but they will only cut out specific systems ..say fuel pump incase there's a fire or the E-Throttle, production vehicles don't have these because the systems have all met certain requirements.

A modern lathe or mill will already have an emergency cutout switch all I would need is a limit switch feeding into the processor, it could indeed drop the power to the driver module but I would not like to put a switch in series with the 240V motor as a limit switch, this would require having mains voltage wiring around the machine and cause a bigger problem than cutters in chucks.

[Martin KyteParticipant @martinkyte99762]

Thats kind of my point Phil I was really only talking about machine tool systems and the like.

There are two levels of control. Normal operation where all is working as it should which may include all manner of limit switches, home sensors etc all of which can happily be controlled by the process control element.
Emergency stops are there as a catch all and should not normally be triggered at all except in an emergency or when the process system has failed. They therefor need to be separate from the processor and act at the top level to cut the power, close the shutters drop the control rods or whatever is appropriate to the system.

regards Martin

[Clive FosterParticipant @clivefoster55965]

Posted by Martin Kyte on 10/10/2018 12:43:55:

Emergency stops are there as a catch all and should not normally be triggered at all except in an emergency or when the process system has failed. They therefor need to be separate from the processor and act at the top level to cut the power, close the shutters drop the control rods or whatever is appropriate to the system.

regards Martin

No argument about the need for separate, ideally brute simple, emergency stop systems. But not so sure about "not normally triggered at all except in an emergency". You really need some form of test procedure to verify that the emergency system is working on a regular basis.

I have personal experience of an emergency system not only not being in working order but also rendered incapable of working by ill thought out modifications. Horrifying thing is that I only checked everything because I didn't trust the folk who'd modified the system. Mostly thinking or reliability as, in my view, a rock solid system had been converted into a flaky prima donna. Good job I did look as the thing failed hard sometime later and needed its emergency stop. Worst thing was that clever clogs engineers had included an emergency system verification routine in the controller which returned a permanent OK status. Ooops!

An excellent lesson in the dangers of reworking a fairly complex, imperfectly documented and not completely understood system.

Switches are good being simple and pretty much foolproof in break on failure mode but do make sure there is enough over travel to accommodate slow down and stop. Screw drive should be OK but I have seen micro-switches knocked off on less positively braked systems in high speed tests. Sprung roller actuators are provided for a reason. Ramming straight up to the button may be OK in theory but margin of error is small. The major weakness of a break to actuate microswitch based system is that if the switch is knocked off odds are it will promptly make contact again and drive will resume. Embarrassing.

Clive.

[Martin KyteParticipant @martinkyte99762]

"not normally triggered at all except in an emergency" Ok I'll clarify that. Not triggered as part of normal operation except in an emergency or as a result of a system failure. Emergency stops may be tested periodically. However the philosophy in design should be that the emergency stop system should be Energised for normal operation and that a failure of the emergency stop system will itself cause a power down so the estop sytem has to be working in order for the machine to run. Switches are generally reliable but contacts can fail in closed state and the way to protect this is running two pairs of contacts in series.

'engineers had included an emergency system verification routine in the controller ' Thats why you dont include the controller in the estop system unless it is simply monitoring the state of things. You certainly don't let it have a say.

We are probably boring people now with detail when the OP only wanted to create a simple fail safe.

regards Martin

[SillyOldDufferModerator @sillyoldduffer]

Posted by Martin Kyte on 12/10/2018 08:55:40:

…

'engineers had included an emergency system verification routine in the controller ' Thats why you dont include the controller in the estop system unless it is simply monitoring the state of things. You certainly don't let it have a say.

We are probably boring people now with detail when the OP only wanted to create a simple fail safe.

regards Martin

Well a bit of gentle drift off-topic never hurt anyone. Let's hope we're not being too boring.

Are controllers used in safety critical systems? Yes! Not so long ago proposals to use fly-by-wire systems in aircraft and submarines were highly controversial. Now they are commonplace. Many other examples – railway signalling systems, air traffic control, power generation, hospital equipment, lifts, process control, and industrial robots etc etc.

Electronic controllers are used because they can manage multiple sensor inputs, react very quickly to complex events, and are able to apply some intelligence. For example, the sequence in which an industrial process is stopped might depend on where in the process the error occurred. Slamming the brakes on causes skids. Controllers are also light-weight and can be duplicated as necessary to provide resilience. Plus, by enforcing programmed rules, they can be made to resist human mistakes. Most accidents are caused by people. Driverless cars could be much safer than conventional cars.

Simplicity always being a virtue in design certainly recommends straightforward methods in simple cases. But once a machine or process gets above a certain level of complexity, controllers shine. I think the switch to electronics would have been faster had their reliability been better at the outset, but new technologies are always buggy. True that mechanical methods were once far more reliable than electronics. Now technology has marched on electronics have the advantage.

Designing a safety systems is not for amateurs whatever technology is used. 'Fail-safe systems fail by failing to fail safely.'

Dave

[Martin KyteParticipant @martinkyte99762]

As I say there is a fundemental difference between normal operation and failure/emergency mode.

Lets take your example of aircraft. The use of fly by wire is made possible when the failure rate can be made lower than the system it replaces. This has clearly happened. The avionics will have been made more robust by the use of multiple systems and a lot of health checking processing. All that however does not comprise what I would call the top level safety system. For an aircraft I would see the top level as in the event of a total systems failure, power outage or other catastrophic failure how is the aircraft intrinsicall safe. This would be things like does it glide, does can it maintain level orientation, will it float, is it hard to ignite, do the exits open freely without external assitance without power and does the seating protect as much as possible impact damage.

In the case of the power feed to the OP's mill the safety system could be that the motor stalls without damage to the mill (intrinsically safe), a shear pin (virtually intrinsically safe) or an eStop that cuts the power (irrespective of the control systems.)

All the systems I have designed have addressed the questions

Is it as simple as possible.

Does it function when all control systems malfunction.

Is it intrinsically safe.

Does any modification I may make cause it to be more intrinsically reliable rather than less.

regards Martin

[Mike PooleParticipant @mikepoole82104]

Siemens make a version of the S7 PLC that has the safety controller built into it, for many years we used Pilz safety PLCs on all the car production lines, the sheer quantity of safety devices on a robot production line make a PLC system essential, it would also be difficult to meet the highest category of safety without the massive amount of monitoring and self checking that these systems perform. A safety buyoff is still performed on a regular basis to verify that the system is performing as designed. This level of safety is way beyond what we use in the home workshop which is probably nearer none than the top standard. Death is probably unlikely in the home workshop but severe injury is a definite possibility. Industrial safety has to be the level above idiot proof that begins with a C and ends in a T.

Mike

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