Copycat stepper driver--which is better?

I'm converting all 3 of my benchtop mill's axes to CNC. For the stepper drivers, I originally selected USDigital's MD1. Looking
around, I've come across another that looks like a copy--Geckodrive's G201. The datasheets are almost identical, down to the resistor values for current control and the capacitor required for too-long power leads.
Geckodrive's price is $30 lower, but USDigital has a 2 year warranty and I've bought from them in the past.
Is one a ripoff? Or, are they the same device with different packaging? If they are, the $90 savings would sure come in handy elsewhere in the project...
USDigital MD1: http://www.usdigital.com/products/md1/ Geckodrive G201: http://www.geckodrive.com/item.htbml?order_id=&item_id=G201
Thanks, Chris respond to: snipped-for-privacy@NOrammspeedSPAM.com
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The USDigital drives are Gecko drives. I assume that USDigital has become a distributor for Gecko. BTW, Geckos are excellent drives.
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Chris wrote:

Gecko makes the drives for US Digital, under the US digital label. You DO get a warranty with the Geckos, but maybe not 2 years, however most problems show up in the first week of use.
Jon
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Thanks everyone.
You've talked me into it--I'm switching from steppers to servos. At the beginning 30ish ipm would be plenty, but it wouldn't be long before I wanted more, especsially for rapids.
I have one question: How do servo motors 'hold'? The coils in a stepper are energized and you can't move it until you overcome the holding torque. I'm guessing that it's a function of the G320--if it senses motion without a step input, it corrects it.
Thanks again, Chris
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On 16 Jul 2004 05:47:08 -0700, snipped-for-privacy@go.com (Chris) wrote:

AWRIGHT Chris! I talked you into spending more money! Actually, there was help. But, you asked about position holding. The servo driver gets a signal from the encoder. The driver wants the servo to be at a certain position and the encoder tells it where it actually is. A small voltage is associated with the encoder pulses. Each pulse off the desired location raises the voltage a bit. As the voltage rises more power is sent to the motor. So, the greater the position error the more power fed to the motor. So if the motor is off position the servo driver tries to put it back where it's supposed to be. If the driver is not adjusted correctly the motor will overshoot and then reverse direction, overshoot, and so on. The drive will be adjusted correctly when you buy it. When you have everything connected to the servos grab the shaft and try to turn it. It will be easy to move it a tiny bit. But the farther you try to turn it the harder it will be to turn. Until you turn it too far. After the maximum postioning error is reached and passed the driver will shut down. The driver counts the pulses from the encoder and turns this into the control voltage. Each pulse raises the voltage a fixed amount. So, that means that the driver only cares about how many pulses you are off, not the actual position. This means that an encoder with 500 lines will rotate 4 times as far as one with 2000 lines before an error is generated. Still, the gecko drives will allow you to get .0001 resoultion at pretty high speeds. Good Luck! Eric R Snow, E T Precision Machine
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Chris writes:

Same way they 'move'. Feedback control. If you try to move a stopped servo-driven handwheel by hand, it sort of feels like it is sitting in a large, V-shaped detent.
See my servo CNC conversion pages at:
http://www.truetex.com/machinery.htm
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On Sat, 17 Jul 2004 11:13:40 -0500, Richard J Kinch

Chuckle..the small servos will get into a wrestling match with you when you try to turn em by hand..<G>
Gunner
That rifle hanging on the wall of the working-class flat or labourer's cottage is the symbol of democracy. It is our job to see that it stays there.         - George Orwell
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Gunner writes:

Small or large. Not that many foot-pounds there even in large motors, compared to what your skeleton and muscles can apply.
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On Sun, 18 Jul 2004 03:35:01 -0500, Richard J Kinch

Chuckle..maybe Im getting weaker..as the big ones are now winning the match.
Gunner
That rifle hanging on the wall of the working-class flat or labourer's cottage is the symbol of democracy. It is our job to see that it stays there.         - George Orwell
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On 16 Jul 2004 05:47:08 -0700, snipped-for-privacy@go.com (Chris) wrote:

If you have a small machine, you may be interested in the DeskCNC servo drivers too. $65 per axis. 30 vdc, 5 amps.
http://www.imsrv.com/deskcnc /
Fred Smith - IMService Specials and discounts at: http://www.cadcamcadcam.com/hobby
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On 14 Jul 2004 12:00:01 -0700, snipped-for-privacy@go.com (Chris) wrote:

Chris,. Have you considered using servos? Look at the Gecko site at their servo drives. They are either the same price or less than the stepper drives. US Digital sells encoders for less than 50 bucks per axis. The Gecko drives will work with your stepper software. They convert step and direction to servo output. I built a stepper driven positioner to machine small, complex parts on a Bridgeport mill. I'd bolt the X-Y table to the mill and cut away. Losing steps was really frustraring. In the end the only way to prevent any loss of steps and still keep the .0001 resoultion I needed was to keep table travel to less than 35 inches per minute. This would not be a problem with servos. And if you look on the web for surplus servo motors it will be easy to find ones cheap that will work for your bench top mill. BTW, I recently bought 3 Gecko servo drives and encoders from US Digital but have not had the time to hook everything up yet. Eric R Snow, E T Precision Machine
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