November 3, 2015

The Tiny Lathe is Working!

After great struggles with the lathe electronics, the lathe is finally working.

One of the main challenges was form factor.  I wanted all the electronics to fit into the base of the lathe, and I wanted the base to be thin (~25mm thick)  so I needed a 600 watt, 45-50 volt power supply that fit in a 25 mm thick package.  Bayley suggested using a Vicor DC-DC converter to generate 48 volts at 600 watts.  Vicor makes some extremely compact converters perfectly suited for this, and they can be had pretty cheaply on ebay.  Only downside is that they need around 300 volts in to turn on.

The basic electronics system would then be wall to doubler for 340 VDC, doubler to Vicor converter for 48 V, and 48 V into an AMC servo drive donated by Dane to drive the spindle motor.

This seemed like a good idea, so I got an appropriately sized Vicor brick on ebay and built up the rest of the system:

And so began the the second challenge: regen.

When the spindle rotates, kinetic energy is stored in the rotating inertia.  When the reference spindle speed is suddenly turned down quickly, the extra kinetic energy in the spindle is converted back into electrical energy and shoved onto the DC bus.  This is all well and good when your power system is supplied by a rechargeable battery, but is much less okay when your source is the wall.  This problem manifests itself as skyrocketing bus voltage when the spindle reference is set to zero from full speed.  The spikes were on the order of 20-30 volts despite the huge bus caps used, which was enough to cause the servo drive's over voltage protection to kick in and shut it down.

One solution is a shunt regulator.  The basic idea is to have a circuit which looks at the bus voltage, and if it sees the bus voltage rising too high, short the bus through a resistor to burn that energy.  Sounds easy enough, should just be  comparator,  FET, big resistor, and some passives, right?

Version 1.  The TO-247 is a big diode to make sure any voltage spike doesn't make it back to the power supply:

Version 1 was derp because the comparator sometimes couldn't pull down the mosfet gate hard enough by itself, or something like that.  The symptom was that occasionally the FET would latch on.

Versions 2/3 had real gate drivers, and actually worked:

At this point, my electronics (minus Vicor brick) looked like this.  Getting less attractive, as I had to shove in an extra 10 V supply to run the shunt regulator.

More problems.  The Vicor converter I got on ebay was DOA.  I returned it, and ordered another identical one from a different seller.  New one shows up and is also DOA.....

Time for something different.  Bayley pointed me to another Vicor product, Vichip BCM converters like this one.  Normally these cost ~$75 a pop on Digikey, but can be found on ebay for $10.  These are unregulated, fixed ratio DC-DC converters good for 300(!) watts each in about a square inch of chip.  They output 1/8 of their input voltage, so like the other Vicor converters I tried, they needed >300 volts in to get useful voltage out.  They also come in a funny surface mount package.

I ordered a few of them, and figured while I was at it I should lay out a PCB for the doubler, Vichips, and shunt regulator:

The shunt regulator is the top left corner.  The large D2-Pak devices are the fet, power resistors, and diode.  In the middle are 3 Vichips, and to the right the rectifier and caps for the doubler.  For gate drive and logic power there's a TI LM2594 12V buck IC.

Through the magic of 3pcb, boards appeared:


The board required a little fixing to get working.  I left the comparator ground pin unconnected accidentally (the net in the eagle schematic looked connected but actually wasn't).  More importantly, I didn't read the Vichip datasheet carefully enough, and missed two important details.

First, they really don't like output capacitance.  I had to remove all the output capacitors to get the thing to turn on.  Instead you should put the caps on the input.

Second, the Vichips can be expected to draw 6.5-13 W idle.  They got seriously hot just sitting there doing nothing.  A kill-a-watt confirmed that the board consumed 22W idle.

I didn't design the board for easy heat-sinking, but I was able to make it work by machining an aluminum spacer which gets sandwiched between the Vichips and lathe base with some thermal pads.  I forgot to take any pictures of the final electronics assembly, but here's the first chips generated under the lathe's own power (not a bench supply):

And a beauty shot:

I'lll do another post with more shots of the whole thing soon.  The lathe is currently functional, but there are a number of things I'd still like to do, such as:

  • Take a finishing pass of the inside of the ER-32 taper to guarantee its concentricity with the spindle.
  • Finish the new tailstock riser.  I've machined it, but mismeasured somewhere and it's a millimeter too tall.
  • Add closed-loop spindle speed control.  
  • Make chuck backplates so I can use real chucks, not just collets.
  • Add a leadscrew and motor for power feed.

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