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2009-10-02 18:09

CNC Abene





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UHU-servo chip


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CNC - Abene VHF-3 - Electrical part 3

The HP-UHU servo drive.



I got 3 HP-UHU kits from Paul over at CNCZone, he and Irfan have made a splendid job of putting the kits together at a very reasonable price, they arrived nicely packed and labeled, something I didn't really expect:

What's not included in the kit is the actual servo processor, the UHU-chip. That had to be ordered from Uli Huber in Germany together with the 24Mhz crystal that goes with the chip. Otherwise it's all there except mounting hardware.


I quickly started assembling my first drive which went pretty smoothly, a couple of capactiors was a little bigger than footprint on the PCB but that's no problem. The two capacitors in the output filter thoguh, was both too big and had way to large legs to fit the holes in the PCB so exchanged those smaller ones of the same type.

The MOSFETS are attached to the heatsink underneath the PCB but unfortunately there's no holes thru the PCB to gain access to the screw that holds them to the heatsink. It's very easy to assmeble the first time but when you need to take it appart, like had to do several times when troubleshooting it was a complete PITA so I made some brackets out of thick FR4-like material with threaded inserts. This way I could clamp the MOSFETS to the heatsink with screws from the back instead of under the PCB:



When I started the testing it turned out that the motor delivered torque in only one direction. It took a little while to figure out why but once found it was pretty obvious:

See the trace going from U7-pin 6 (that's the MOSFET driver) up to R17 (that's the resistor in series with the MOSFET gate)? Right where the trace "enters" the ground-plane there's a tiny short. Once that was removed and the drive put together I got torque in both directions.





I tested the drive with voltage up to 160VDC and it seemed to work just fine. Here's a final shot of the test-setup with a powersupply voltage of 130VDC which is what will be used on the machine:


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