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My question is based on my understanding of wiring requirements. If my understanding of said requirements is wrong, the question is useless. My understanding is that wires leading INTO your Geckos are "ordinary" while wires BETWEEN the Geckos and the steppers must be shielded. If this is so--is there any reason not to mount the Geckos within inches of the motor (like on the gantry) so that more sophisticated shielded cable will only be needed in very short lengths while cheaper non-shielded wire can be used to get there?
You could do that. The reason for the sheilding on the drive wires is to reduce EMI (ElectroMagenticInterference) that could cause problems with other sensitive electronic equipment. The rapid switching rate of the drives to the motor can produce very high frequency radio interferance. Not good to have the wife complaing that the TV goes on the fritz every time you run your machine. Or worse yet, having the FCC knocking on yer door due to a complaint that the neighbor's heart monitor throws an alarm when your gantry moves. I have seen drive electronics as an integral part of the motor so mounting position of the drive is up to you. Keep in mind that you will be running power and control lines to the drive and the control lines may need to be sheilded to keep other things (flourescent lights for instance) from causing interferance with the drive.
Just the $0.02 of a 30 year electronics tech.
Protecting CNC drives from internal wiring effects.
There are two fields of thought on this subject. I will present my thoughts.
Originally Posted by IQChallenged
The Factory stepper cables that I have seen are NOT shielded. I have built many systems and have NEVER used shielded cable except on the servo systems where shielded wire was used from the encoders to the drives, and on the step and direction lines. (use 2 conductor with shield. Run step and direction signals for same drive in same cable.)
If I were to use shielded wire anywhere in a stepper system, it would be on the step and direction signals from the printer port breakout board to the Gecko drives. This is where electrical magnetic interference (EMI), Radiated Frequency interference (RFI), voltage spikes, and so forth could cause the most damage which is the Gecko drive input signals.
The stepper motors when slowing down from a higher speed to a lower speed, or to a stop, will also act as electrical generators in that they will generate voltages and currents of all phase angles in opposition to the applied voltages. (This is one of the reasons why filter capacitors are used on the outputs of the power supply.) Shielding the stepper motor cables will only keep this reverse emf from being radiated into the air inside the enclosure. Remember however, that this emf is present at this point in time in the steppers, the wires, the Gecko drive(s), in the capacitor on the drive voltage inputs, in the power supply wires to the Gecko's and also at the filter capacitors for the power supply. So, what has one accomplished with shielded motor cables? Not much.
An approach that works is this.
Run a seperate (+) and (-) wire from the power supply to each Gecko. Keep the power supply cables as short as possible, hopefully 30 inches or less.
Run a seperate wire for the 5 Volts from a common point. (depends upon Gecko model.) Do not run the 5 volts with the other power cables. Take a different route.
Keep the drives step and direction signal cables approximately two inches away from the power cables if shielded cable is not used.This air gap should not allow the mutual induction of the emi, rfi, and other trashy signals into the inputs. This is why I would only shield the step and direction cables. The cable shield should only be connected on the breakout board end. (NO connection to the drive end.)
Hope this helps.
Last edited by CJL5585; 04-15-2007 at 12:05 PM.
Reason: My Consideration was only for internal emi & rfi - not external effects.
Follow up question
After reading those two very impressive responses it seems there is controversy as to whether these interferences actually occur. I just want to simplify my life. If I need shielded I want to get it, If there's significant money to save in not getting it and if I don't need it-- then I don't want to get it. Generally speaking, all things being equal, what's the difference in cost? 48" x 48"router table, 3-1/4 Bosch router for the spindle, 3 Geckos, about 40volts. Thank you so much for your time.
Router Table Wiring
Ok, I tend to err of the side of caution sometimes... I would ensure that all the sensitive lines are sheilded. Those would be the inputs to the Gecko drives. You can use normal wire to the stepper motors and probably get away with it without interfering with other equipment. Speaker wire would work. Good advice on the power supply wires... seperate lines to each Gecko. If you absolutely had to run the power leads and step/direction leads a long distance to the Geckos (>30") you would want capacitors near the Geckos across the power, and sheilded (coax, not twisted pair) for the step and direction.
When a direct current runs through a wire it creates a constant magnetic field surrounding the wire. With and alternating or changing current, the magnetic field alternates or changes right along with the current. A magnetic feild passing across a wire will induce an electric current in the wire. The larger the current, the stronger the magnetic feild and vice versa. This is why transformers work. Take two wires running parallel to one another, change the current in one of the wires and a current will be induced in the other.
The current to the motors is relatively large, and it changes. That curent is coming from the power supply through the Gecko.....
CJL5585 is right on with his reccomendations.
Make your life simple, place your Geckos close to power and control, and run regular wire to the motors.
You may radiate some RFI but your machine will run fine (The router will generate RFI as well). Take an AM radio tuned between stations and put it anywhere near the running stepper motor leads for an interesting sound...
For the first time I think I'm beginning to get it. You've helped me a lot. I have a full day today but later I'm going to ask for more details. I'll use some drawings to make my questions more precise. I hope that's okay.