New Machine Build- External encoder power in a Gecko 320 System

[bharbour]

I am working on a system using 4 Gecko 320's that needs to use external power to feed the encoders and differential Rx/Tx board pairs. I am concerned that tying the Gecko encoder ground leads together at the external power supply will create undesired paths for motor currents and maybe add a lot of noise to the encoder signals. I am wondering if I should put some resistance in the ground path between the Geckos and the differential Rx boards or something similar? Opto's would be my first choice, but a real pain to implement in this system.

Has anybody successfully used a single +5V power supply to run encoders in a multi servo system? Any suggestions?

Thanks,
Bob

[cjmerlincnc]

Hi, I've just finished my mill cnc drive using Gecko's. I kept all the low power 0v lines going direct to the low power board and the high power 0v lines to their respective power supply caps. I then ran a single line from each power supply 0v to a single bolt on the chassis, this is then connected to earth. I've had no problems with any interference as yet.

Dont know about the differential board though. I hope you can get help on that.

Regards
John

[Al_The_Man]

Why not use the PC 5v supply from a spare 4 pin P.S. connector?
Al.

[Dennis Bohlke]

I found it is not the best setup to connect the encoder grounds together.

The encoder ground changes dependent on the current being drawn by the Servo motor.

My solution was to optically isolate the encoders from the drives.

That way with differential amplifies all the wires can be in one bundle.

I used a DC-DC converter to drive all the encoders. Deriving the DC input from the Motor power supply. Otherwise the motors jumped on power on/off.

Dennis
www.super-tech.com

[Al_The_Man]

Originally Posted by Dennis Bohlke The encoder ground changes dependent on the current being drawn by the Servo motor.

To me, that would indicate poor grounding practice and a ground loop being present?
Drive current, even with a drive supply common grounded should not influence the encoder supply.
Al.

[Dennis Bohlke]

Al

The poor grounding practice is inherent with the Gecko G320. It is just the way it is.

The Encoder Ground is a logic ground to the drive unit. It actually is slightly above the module power ground.

When the servo motor is drawing big amperes the Encoder Ground voltage difference to the power ground will change from when there is little current drawn. The voltage differences can be high frequency spikes derived from the PWM to the servo motor armature.

If you connect all the Encoder Grounds together with all the drives you can get some really spurious faults. Course if the servo motor currents are low they will never haunt you.

The bigger the Servo motors the more important this becomes. You can tell when it is a problem when one motor is working hard, a motor that is not working will fault or change position. Seen it happen, took long time to figure out why. Separated the encoder grounds and problems go away.

I noticed it when one axis would go through a high speed change in direction. Kind of the worst case situation. A motor attached to another axis but not involved in the movement would change position, ergo jump, at peak currents to the driven motor.

Just striving for perfection in an imperfect world.

Dennis
www.super-tech.com

[mc-motorsports]

I'm not going to be much help here, but if you call the engineer over at Gecko, he will probably talk your ear off, and you guys can talk electrical engineering until one of you passes out, trust me, he can talk!

Gecko has the best customer service of any company I've ever bought from or delt with, 10/10 hands down!

After the servo electronics I bought blew up right in my face and the manufacture did nothing to help me, the Gecko engineer and customer service helped me identify the problem of mis-matched encoders (over-currnet draw, suppliers fault), told me where to go for the correct encoders and even replaced one of my burned up drives as a "one time **** happens policy"! I had to pay for the other two drivers, but they didn't even have to do that, I was so impressed! Gecko really eased the pain of buying from a shady supplier and did more than then expected to make the end user happy! And I am happy!

[mc-motorsports]

And for the original supplier (servo's and boxed plug and play driver system)... He did nothing, tried to claim he couldn't offer a refund or make right for the mistake because of financial difficulties and family problems... Unprofessional... Cost me $800 in repairs to get the system to work.

[bharbour]

[QUOTE=Dennis Bohlke;513734]Al

The poor grounding practice is inherent with the Gecko G320. It is just the way it is.

The Encoder Ground is a logic ground to the drive unit. It actually is slightly above the module power ground.

When the servo motor is drawing big amperes the Encoder Ground voltage difference to the power ground will change from when there is little current drawn. The voltage differences can be high frequency spikes derived from the PWM to the servo motor armature.

If you connect all the Encoder Grounds together with all the drives you can get some really spurious faults. Course if the servo motor currents are low they will never haunt you.

snip

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This is exactly the problem I am hoping to avoid. I currently have a star ground arrangement with the commons tied to the negative terminal on the power supply cap. The ground wiring is about 8" to about 18" (different lengths for different amps) of #14 wire between the cap and the Geckos.
If the encoder inputs were isolated, it would be easy to extend to the differential boards or just external encoder power. So much for being a lazy dog...

Thanks for the suggestions everybody!

Bob