Joe's 1000x600x200 steel and aluminium router started

[AndyUK]

My solution is similar to Neale's. The UB1 board I use has a simple safety-relay circuit, so the first 10 or so inputs can be linked to close a relay. My driver's fault signals feed into this, alongside my VFD's fault signal and a signal from my e-stop circuit. The driver's enable line is linked to the relay output, so if one drive faults the drives remain powered but not enabled. Same happens if the VFD drops out, or I hit the e-Stop button (but that also has the effect of killing the power to the drivers directly).

[devmonkey]

Thanks guys, lots of inspiration here.

The DDCS has two estop inputs but one is difficult to access on the MPG port. I have an idea how to sort this now. Currently I use the output of the safety relay to switch the low side of the contactor coil and also signal low to the DDCS estop input which is configured to go into emergency stop if this signal is high. I was looking to integrate the alarms on the estop switch side of the safety relay but this is wrong, instead I can put them on the output side. Take a resistor to +24v then series the NC alarm outputs via the same safety relay contact to ground. Take the DDCS estop input from the resistor.

With this configuration the alarms will only estop the DDCS not the contactor, but when the safety relay is tripped both DDCS and contactor will estop. Million dollar question, do I need a diode to prevent current flowing from emitter to collector through the alarm NPNs into the DDCS input when the safety relay is open? My analogue electronics is extremely rusty.

[Kitwn]

Analogue is easy, just work out where the current will flow. Ohm's Law plus Superposition Theorem equals The Answer.

[devmonkey]

Good news machine is moving, very nicely indeed, I will post some video tomorrow.

I have a question about a potential issue I've discovered with the 24v dc smps I'm using for powering the controller and proximity switches. The unit is a high quality double insulated (no earth connection) TDK-lamda din rail SMPS and I've used them in automation projects before. However this is the first time I've noticed this issue.

Basically you get a slight tingle (barely noticeable electric shock) when touching both DC ground and mains earth.

I believe this is down to the 'Y' capacitor used in the SMPS to reduce EMI, it is a small capacity capacitor that couples input to output (when there is no earth connection) and is required by all SMPS to meet EMI regulations, however it has the nasty side effect of raising the DC output to around half mains voltage wrt mains earth. The capacitance is tiny so it isn't dangerous.

I noticed it whilst leaning on the earthed machine and holding the shield of my db37 cable which is connected to dc ground.

I've checked the unit with a megger to prove to myself it is properly isolated.

So do other people see this, does it matter? What do other people use for the 24v side of their systems? I might substitute it for a small power supply with an earth connection (class1??).

EDIT:
Just ordered a class1 meanwell unit MDR-60-24, this has an earth connection and the datasheet shows it's 'Y' capacitor is connected to it.
https://www.meanwell.co.uk/pub/media...DR-60-SPEC.PDF

Cheers, Joe

[Neale]

Good news machine is moving, very nicely indeed, I will post some video tomorrow.

I have a question about a potential issue I've discovered with the 24v dc smps I'm using for powering the controller and proximity switches. The unit is a high quality double insulated (no earth connection) TDK-lamda din rail SMPS and I've used them in automation projects before. However this is the first time I've noticed this issue.

Basically you get a slight tingle (barely noticeable electric shock) when touching both DC ground and mains earth.

I believe this is down to the 'Y' capacitor used in the SMPS to reduce EMI, it is a small capacity capacitor that couples input to output (when there is no earth connection) and is required by all SMPS to meet EMI regulations, however it has the nasty side effect of raising the DC output to around half mains voltage wrt mains earth. The capacitance is tiny so it isn't dangerous.

I noticed it whilst leaning on the earthed machine and holding the shield of my db37 cable which is connected to dc ground.

I've checked the unit with a megger to prove to myself it is properly isolated.

So do other people see this, does it matter? What do other people use for the 24v side of their systems? I might substitute it for a small power supply with an earth connection (class1??).

EDIT:
Just ordered a class1 meanwell unit MDR-60-24, this has an earth connection and the datasheet shows it's 'Y' capacitor is connected to it.
https://www.meanwell.co.uk/pub/media...DR-60-SPEC.PDF

Cheers, Joe

Why leave the 24V supply floating wrt earth? Take the 0V connection to the control box star point and all will be well. Can't see any reason not to ground it. That's what would normally be done - my control box has a 24V smps and a 5V+12V smps and all the 0V connections go straight to the star earth.

[devmonkey]

Why leave the 24V supply floating wrt earth? Take the 0V connection to the control box star point and all will be well. Can't see any reason not to ground it. That's what would normally be done - my control box has a 24V smps and a 5V+12V smps and all the 0V connections go straight to the star earth.

Is that really recommended? Doesn't it defeat the purpose of the galvanic isolation?

[devmonkey]

Should add the noise is coming from the stepper drivers as it only happens when the stepper is moving, so basically the EMI filter in the 24v smps is filtering what comes into it and coupling it to the 24v DC output which causes the DC output to float relative to mains earth because the neutral and earth are coupled at the electricity company's box in the street. I will fit an EMI filter in front of the toroidal stepper PSU to suppress noise returning back up the supply and into the 24v SMPS (and the rest of the house).

[Neale]

Not quite sure what the issue is here. If the output of a psu - any psu - is left floating them you can guarantee what volts you will see between the terminals but volts wrt ground is undetermined. It will be somewhere between earth and full mains depending on leakage, transformer internal coupling, EMI filler leakage and so on. Take one terminal to ground and everything is now properly referenced to that - no more tingles!

I don't see that the EMI filter effectiveness or anything else will be affected. In fact, electrically, you are in a much better position because at the moment I would be worrying about the effect of stray voltages on sensitive components. If that "tingle" happens to occur around any high-impedance inputs...

The reason for leaving the outputs floating is that someone might want a -24V supply so in that case they would ground the +'ve output pin. There might be other reasons but that's one good one.

[devmonkey]

I was thinking that the galvanic isolation (floating DC) is there to prevent a current path between AC live and DC, and that when using a probe you temporarily defeat it. I think also galvanic isolation prevents ground loops should a secondary piece of equipment grounded equipment like a PC be connected, not a consideration here.

But as you say you might get a tingle.

[Neale]

There are a few reasons for the galvanic isolation. That's actually just a posh name for saying that there are no connections from the live/neutral terminals to the output terminals. I mentioned one reason for this earlier - so that the same smps could be used as a +24V source or a -24V source with one output terminal grounded. Another reason (and someone else mentioned this in a post a little while back) for industrial equipment is that if the 24V supply rails are floating with respect to ground, then you can put in fault detection mechanisms that check if either rail does get shorted to ground under fault conditions; the machine can then be stopped before any magic smoke is emitted. Or, maybe, you have a motor speed controller that needs a 24V supply, but is connected to the mains supply to the motor with no isolation. However, if neither smps output terminal is grounded, you really need to make sure that none of the control electronics wiring or terminals are touchable (due to the whole thing floating at some undetermined but possibly dangerous voltage with respect to ground).

On a practical note, this also means that a simple electrical touch-off system for tool height setting is not going to be easy as this depends on the touchplate being connected to one controller input and being shorted to ground by the tool tip via spindle/machine frame, etc. If the controller is not referenced to ground, then this ain't going to work, or you will need to isolate the spindle from the machine frame and provide an additional connection to it - which will all be floating at "it tingles a bit" voltages.

All these issues disappear if you just take the SMPS 0V connection to ground. I cannot see any downside to this.It is what just about any other home-built machine does.

External, e.g. PC, connections? Valid point, and might be an issue in an industrial environment where different bits of kit are connected to different supplies. In a domestic situation where all the equipment is probably connected to the same ring main, this is a non-issue. In addition, an RJ45 ethernet connection will be galvanically isolated and I believe that the USB connection spec calls for galvanic isolation as well.