Yet another Denford for the collection (Triac Retrofit thread)

[AndyGuid]

Rather than have this thread as just another "look at what I got/done" thread, I'm wanting to explain some of the reasoning behind the choices.

Thanks m_c, I am finding the reasoning and associated detail that you're providing particularly beneficial for my edification and ultimately future build.

Andy

[m_c]

Next problem was power for the steppers.
After finding a couple datasheets for the fitted stepper motors which listed the motor inductance, a reasonable accurate working voltage could be calculated.
The listed inductance is 3.2mH (milli Henries for those wondering what the H means) for the X and Y motors, and 5.5mH for the Z axis motor.
Now using the calculation courtesy of GeckoDrive's Stepper Motor Basics guide (worth a read for those wanting to know more about steppers and suitable power supplies), which is 32 * vL = VMAX.
L is the Inductance, and the little v is meant to be a square root symbol, so after a bid BODMAS I.e. work out the root before doing the multiplication, we get a max voltage of 57.24V for X and Y, and 75V for the Z.
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The original Parker SD rack ran of a transformer outputting +44,+18,0,-18 & -44V.
For those unfamiliar with how AC voltage is measured, it is commonly measured as RMS (Root Mean Square - google it if you really want to know the detail!) voltage, which in a very simplistic way can be considered the average voltage.
Taking a 44VAC voltage, in an ideal world, the peak voltage would be 44 times the square root of 2 (RMS nicely works out as root 2, or 1.414), which gives us 62V.
Off course, in the real world, that voltage will be higher. Due to mains voltage fluctuations (can be as high as 10%), and the fact the transformer voltages are at rated load meaning due to internal losses the unloaded voltage is higher (if you see voltage regulation figure for a transformer, this is what it relates to), you can end up with a peak voltage far higher than calculations.
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However, the original transformer is marginally small to make full use of the steppers. No official ratings could be found for it, but going by a rough guestimate of it's physical size, I'd put it around the 2-300VA size, which was perfectly adequate for the original drives, running 2A on the X and Y, and 3A on the Z. Plus there would be the problem the outputs are split between positive and negative, meaning I would have to in effect create two separate linear supplies to make full use of the transformers capability.
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So, a new supply was needed.
After a running a few calcs on commonly available toroidal voltages, I picked a 500VA 33VAC.
Given this is a mill, where every last ounce of performance is not needed (voltage only limits maximum speed), I decided somewhere around 50V would do fine. Combine that with running modern drives, and higher current, performance will be greatly increased anyway.
The 33VAC should in theory give me a peak voltage of 46.67V, and by using a higher rated transformer (a 300VA would most likely work good enough) there will be less voltage drop under load.
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Once again, I took a bit aluminium plate, arranged a few bits, drilled some holes, and added some bit wires, to end up with this-

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One tip here is for connecting up the transformer. As I'm paralleling the outputs, and the bridge rectifier uses faston/spade terminals, I need to get two wires into a suitable crimp. My personal preference is to use uninsulated crimps, and crimp the wires like this-

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Using a single wire, the top of the crimp should crimp the insulation, which gives better support to the wire and reduces the likeliness of the wire snapping due to fatigue, however you won't get the insulation of two wires into this kind of crimp. So the solution-

A couple bits of heatshrink. I shrink the first bit on so it only covers the crimps, as per the lower orange/black wires, then put a second bit to act as the insulator for the terminal as per the top red/yellow wires. A single bit would do the job, but I like to provide a bit extra support for the wire.
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One tip for all crimp terminals, is after you think you've crimped them, try pulling them of the wire. For uninsulated crimp terminals, the wire should snap before you're able to pull the connector off, however insulated crimp terminals, they're more likely to pull out.
For any size of reasonable wire, if you can pull the terminal off just by using your finger and thumb, then they're not crimped tight enough, and likely to give problems later.
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One thing not visible in the picture, is prior to final assembly the bridge rectifier gets some heat transfer paste put on it improve heat transfer to the aluminium plate, which brings me to the final bit of linear power supplies.
A bridge rectifier also introduces a voltage drop between the input and output, which is why it needs to be attached to some form of heat sink.
I think this one was rated at 1.1V drop at load, so if I was to manage to use the full 15A output of the transformer, I'd be looking at 16.5W of heat needing removed. In free air with no heatsink, the bridge rectifier would quickly be reduced to a smouldering blob.
Now that volt drop also needs to be taken into account for out final output voltage, so in theory with the peak AC voltage of 46.67V, I should get a DC voltage of around 45.5V at the capacitor.
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Now as I mentioned earlier about theory and real world, using a reasonably accurate multimeter, I get around 52VDC at the capacitor. That's a good 14% higher than our calculated figure. This will drop under load, but it may also rise above that under hard deceleration of motors, which is why you should always allow a reasonable safety margin between your power supply voltage and the maximum allowable stepper driver voltage.

[m_c]

Wiring. Lots and lots of wiring.
And how do you wire a retrofit?
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Thankfully, Denford are quite happy to make available what wiring diagrams they have for older machines. Their forum (www.denfordata.com) contains lots of information, and also a few different sets of wiring diagrams for Triac's, and having already retrofitted a Cyclone lathe, I've learnt a good bit about how they do things.
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The first problem I knew I had to deal with was the homing sensors. Denford use NAMUR output sensors, which are a two wire sensor aimed at explosion risk applications. It has been asked on the Denford forum why they were chosen, but none of the current employees know.
They could be made to work, either through some expensive NAMUR barrier interfaces, cutting the old control board up to get the required circuit, or playing around with opamps, resistors and transistors, but swapping them out for new 3 wire sensors makes far more sense.
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Until you realise where the wiring runs for the X-axis sensor and limit switches, and the existing 4 core cable needs an extra core. All the X-axis switches sit at the front of the table underneath the front bellows, and upon initial inspection, I thought the wiring passed down through the front of table, and I was looking at a major strip down job to run new wiring.
After a bit studying of parts diagrams, a good bit umming and arring, a start was made, with me expecting to need to remove the complete Y assembly from the mill, and possibly even the mill from the enclosure. However, after reaching this point-

A deep breath of relief was had, upon realising the wire simply passed over the top of the assembly, before going down through a hole and being cable tied to the autolube pipes underneath the base-

With some use of various pliers, side cutters, and skin removal, I managed to get the old cable removed, and a new cable fed in. For the new cable, I opted for some high oil resistance chainflex cable from Igus (CF9.02.06 to be precise), which is a similar diameter to the old cable, but with 6 cores.
Something to consider when carrying out a retrofit, is the condition of the original wiring. Although this cable was still functional, underneath the table, the wire had gone brittle, so it would of likely failed at some point fairly soon. Spending a bit extra time during the retrofit to check things can save you lots of unplanned grief later.
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Now the biggest wiring headache on the machine has been dealt with, and the main new bits are mounted in the cabinet, it 'just' leaves this to sort out-

[m_c]

Wires. Wires. Wires everywhere.
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So where do you start with wiring a retrofit?
For me it's always the power supplies, and related circuitry.
First up was a switched mode 5V DIN rail mounted PSU to power the KFlop/Kanalog. Nice and simple, it gets a permanent live feed whenever the machine is switched on. It can just be seen on the lower DIN rail just to the left of the original cream coloured 24V supply.
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Next is the big linear supply for the steppers, however it relies on being controlled by the E-stop circuit. As is best practise when powering stepper drivers, you only ever switch the input side, so a suitable contactor was added, controlled via the E-stop circuit, which is at the very left of the lower DIN rail.
Something I struggled with while initially deciding on how to handle E-stops and enabling on earlier retrofits, was what should be hardware controlled, and what should be software controlled. The advice I got, was keep them distinctly separate. Two main reasons for this, first is it avoids a race situation, and two, should something go wrong, you should know if it's due to a hardware or software problem.
For those not familiar with a race situation, it's more a programming term, where due to two interdependent conditions, they both get stuck because of each other.
In terms of what I'm discussing here, if you were to rely on the control/software activating the E-stop circuit, how would you initially activate it, given the control would see it as tripped due to the fact the control hasn't activated it?
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This is where drive enable circuits come in.
The E-stop circuit cutting power should be seen as a final way to kill motion in the event of controller failure, as it will usually take a second or two for the power to drop low enough for motion to stop.
By using the enable signal to stop motion, motion should stop quicker, although by removing the enable, the motor will usually be left to freewheel to a stop. However on a stepper driven machine this is not usually a major issue.
Off course, as I'm using a KFlop, this gives me the ability to handle this aspect however I like. This gives me the option to implement a minor delay between stopping the step/dir pulse stream, prior to the enable signal being removed.
I end up with a flow like this-
1) E-stop circuit deactivated (I.e. E-stop button pressed, limit switch hit)
2) Power disabled and KFlop notified E-stop triggered simultaneously
3) KFlop immediately stops motion signals
4) 200ms later KFlop removes enables
5) Residual power drains
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You may be wondering why I'm happy to rely on the KFlop in this way, but it's because it will do exactly what it's programmed to do. It's not like a PC which may hang due to myriad of things. At this level of operation, it's programmed as a microcontroller, with no influence from the PC. Once the system has been initialised, you can unplug the PC, and the KFlop will still react in the same way. Even if it wasn't, the removal of power will still stop motion eventually.
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Off course, once you get into servo drives, you have other considerations, like do you really want to cut power instantly (cutting power will usually result in drives faulting and motors freewheeling to a halt), servo drives usually have the option of implementing a Stop signal, which when triggered will cause the drive to stop the motor as fast as possible.
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The only addition I did make to the original E-stop circuit, was the Kanalog board has an enable signal, which goes active once the KFlop and Kanalog board have fully powered on. The reason for this, is during initial power up, prior to the KFlop establishing communication with the Kanalog board, the Kanalog outputs will be in an indeterminate state (i.e. they are not guaranteed to remain deactivated). I simply use the signal, which is a relay driver capable output, to power a relay, which in turn is part of the E-stop circuit. For those familiar with parallel ports/Mach/LinuxCNC, it can be thought of as a charge pump circuit.

[Neale]

I read your description of your e-stop, etc, wiring with interest. Always good to see someone else's approach. Couple of questions come to mind.

Did you consider use of a safety relay? I managed to pick up one cheap on eBay, partly because it lets me switch a number of circuits from the e-stop switch, convenient way to configure "momentary contact" standby switch, etc. Does your e-stop connect to a latching relay or similar?

You mention limit switch triggering as equivalent to e-stop. I can see why you might want to do this, but will it give problems in separating limit and home switch operation? I use drive fault from my digital drives to trigger e-stop but limit/home switches go direct to CSMIO (similar argument to yours re dedicated firmware - no PC involvement).

[m_c]

Did you consider use of a safety relay? I managed to pick up one cheap on eBay, partly because it lets me switch a number of circuits from the e-stop switch, convenient way to configure "momentary contact" standby switch, etc. Does your e-stop connect to a latching relay or similar?

Technically to meet the latest regs, I should use a proper E-stop relay, however it's something I've not bothered with yet, and as I'll be the only person using the machine, I technically don't need to conform to any regulations.
However, the main reason is I'd need to add an enable button, and the control panel is still a sketch in a notepad at the moment. It's something I will likely add at a later date, once I have fully functional control panel.
The real benefit of a E-stop relay is the contact monitoring, whereby should a contact stick/weld shut, it won't enable.

You mention limit switch triggering as equivalent to e-stop. I can see why you might want to do this, but will it give problems in separating limit and home switch operation? I use drive fault from my digital drives to trigger e-stop but limit/home switches go direct to CSMIO (similar argument to yours re dedicated firmware - no PC involvement).

I always use separate home switches, so homing is not a problem. I personally think it's a waste of inputs wiring the limits directly to the controller, as there is little benefit. If you can't tell what switch you've just ran into, you're doing something wrong!
What I do have though, is all the limit switches and everything else in the E-stop circuit, pass through a row of DIN rail terminal blocks, so should something fail, 30 seconds with a multimeter lets me know where the problem is. I do take the Drive fault signals to the controller, as it lets me know what drive has failed, for the reason I need to turn the cabinet power of before I can open the cabinet at which point the drives get reset.

As always, there are several ways to achieve this. The main thing to consider with any system, is what would happen in the worst case scenario, should any/multiple parts of the system fail.

[JAZZCNC]

I don't class Limits as Emergency stop condition. They are positional errors which if talking directly to Controller and not relying on software can be handled by controller/drives safely with out any need to kill power. Soon becomes pain in the arse reseting if when approaching travel limits you accidentally trip limit.

[m_c]

I don't class Limits as Emergency stop condition. They are positional errors which if talking directly to Controller and not relying on software can be handled by controller/drives safely with out any need to kill power. Soon becomes pain in the arse reseting if when approaching travel limits you accidentally trip limit.

You can argue both ways, but Denfords have the limit switches as part of the E-stop circuit as standard, and I don't see any point in changing it.
Provided you have soft limits working correctly, you should never hit a limit switch anyway, so if you do, it's because something has gone wrong.

[Tom J]

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Now as I mentioned earlier about theory and real world, using a reasonably accurate multimeter, I get around 52VDC at the capacitor. That's a good 14% higher than our calculated figure. This will drop under load, but it may also rise above that under hard deceleration of motors, which is why you should always allow a reasonable safety margin between your power supply voltage and the maximum allowable stepper driver voltage.

Your driver EM806 has operating voltage up to +80 VDC so why 52VDC satisfy you?

[m_c]

Your driver EM806 has operating voltage up to +80 VDC so why 52VDC satisfy you?

Because theoretically the x and y motors are only good for 57V, and running them higher could result in overheating them.
Off course you could avoid motor overheating by reducing current, but then you reduce torque at all speeds, which is counter productive to getting the best performance.