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2009-10-02 18:09

CNC Abene





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CNC - Abene VHF-3 - Electrical part 2

Choosing a servodrive


Before looking for servo drives I need to know what specification the drive had to live up to regarding voltage and current. The datasheet for the motors specifies the voltage constant as 0.47Vs/rad but what does that mean... Well, basically it means that it takes 0.47V for the motor to turn one radian in one second.

1 radian is roughly 57.3 so 57.3/0.47 = 121.9/V/s or expressed even easier 20.32rpm/V (121.9 * 60 / 360 = 20.32). The datasheet furter states that the maximum useful speed is 2000rpm so in order to reach that I'd need a supply voltage of 2000/20.32 = 98V.



Electrically the motor armature can be thought of as a resistor, an inductor and the actual armature in series. The resistor is representing the resistnace in the wire that the armature winding is made of and the inductor the inductance in the winding. To reach 2000rpm the armature needs to "see" 98V, not the connectors on the motor. When current is pushed thru the motor armature there will be a voltage drop across the resistance, how large this drop is depends on the armature resistnace and the amount of current.

My X and Y axis motors have an armature resistance of 0.24 ohm and the maximum continous current is 19A, that means that when we push 19A thru the windind we drop 4.6V across the resistance, this means that to be sure we can reach 2000rpm even at full continous load we need an aditional 4.6V on top of the previously calculated 98V. Now on these particular motors this is pretty marginal and can almost be neglected but on smaller motors the armature resistance can easily be 1ohm or more.

Next thing to account for is the maximum PWM dutycycle of the servodrive. A modern servo drive, using PWM technology, usually can't modulate to 100%. This is because the internal bootstrap capacitors for the highside MOSFET drivers need to be recharged. What this means is that if the drives maximum PWM dutycycle is 90% the maximum voltage that the motor will see wil be 90% of what you feed the drive plus, depending on drive design, you might drop another volt or two across the actual switching elements and current sense resisotor in the drive.

Different drives has different maximum PWM dutycycle but most seems to be in the 85-95% span so my choise was to err on the safe side and went for 85% for my calculations.


So to be sure I could reach 2000rpm I need a supply voltage of 98V + 15% + 4.6V = 117V



First drive that springs to mind is the G320 from Geckodrive, I've had great success with their step-motordrive so I wouldn't hesitiate but the 80V maximum voltage was way to low. Geckodrive doesn't specify the maximum PWM dutycycle but even if it's 95% I would only be able to reach 1550rpm no load - so the G320 was a no-go.


Next I looked at (and actually got one to try) the Mammut from CNCDrive. It's rated at 180V and 40A peak so it looked like a perfect match - unfortunately I had severe problems with their tuning software so I simply couldn't make the drive work. Support emails suggested I had malware in my computers which of course was a possibillity but not very likely. Lately it was discovered to be a compatibility issue between different versions of the .net framework but at that point I had already moved on. (I'd like to point out though that I have since tried some of CNCdrives newer product and have had zero problems hard- or software wise this far.)


I also looked at the Viper200 from Larken. This looked like a nice drive as well but at that time they claimed to have features IN the drive which wasn't actually available yet (feed forward control). As far as I can see feedforward is now availble and I've read a couple of success stories from people using them so I'd really like to try them out some time!


Another option obviously is Rutex, but due to very bad reputation regarding support and product specifiactions I didn't dare to go there - YMMV.


Then I found the Finish company Granite Devices and their VSD-A drive. This is drive speced at 200V and 10A continously, 15A peak for 0.5seconds. 10A was a little on the low side for my motors, rated 19A and 24A continously but 10A would still give me 4.7Nm of torque - continously and with the 2:1 belt reduction that's 9.4Nm at the screw. I actually bought three of these drives (now discontinued in favour of the VSD-E and VSD-XE) and let me tell you, these are some really nice drives. True torque control, differential encoder inteface capable of tracking encoder speeds up in the MHz range, pulsemultiplier setable to any arbitrary value, input smoothing filter, etc etc. I really liked these drives, unfortunately it turned out that although 10A continous proably would have been enough to run the machine I had trouble getting the motors to accelerate in at an acceptable rate even with the motors unloaded. The 15A peak of VSD-A simply wasn't enough.



So, what I finally ended up using was the HP-UHU drive. This is a kit based drive that started as project on the CNCZone. I decided to get three kits even though I was quite sceptical at the beginning but I thought that if I could make them work I could always fall back on the nice VSD-A's, living with the limited acceleration. Although I did have some problems once that was sorted the HP-UHU has actually been working quite nice, so far no I haven't done any machining with them but I have run them hours and hours with out problems.


More about the HP-UHU drive on next page.


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