I am most interested in to what kind of milling machine this kit is connected. What changes did you have to make to the mill? Is it still capable of manual operation? You gave the cost of a turnkey kit, but how did that compare to new CNC mill? Did you do this just for the satisfaction or was it to increase productivity? Does it have 3D capability?
Shielding of signal leads is the way that I always start an installation. Also, in many instances, running an isolated ground connection for the sensor in the same cable as the sensor signal and power lines. While the standard practice for grounding shields is often the best choice, sometimes grounding the shield at a different point is the cure. My instructions have always been to carry the shield ungrounded until the signal conditioning input is reached, and if there is a problem I will ground it someplace else. That method avoids rework and allows the most flexability in grounding.
Note that these were custom designed and built production testing machines used in industrial factory enviroments. Sometimes the electrical noise could be read on an analog multimeter.
Cabe is busy making parts on his mill with his $12 solution, and everybody is arguing about how to properly shield something.
Why shield? The machine is running, the solution was much cheaper than purchasing wire that may or may not actually solve the problem. I have observed noise from spindle and servo drives that all the shielding, ferrite, and line reactors in the world was not able to solve.
Good job, Cabe. I think your solution is excellent, and appropriate.
Not knowing the particular construction of the motor you refer to - I would assume that the shield connection at the motor end is just continuing the shield connection through to parts on the motor not directly connected to electrical ground. This would keep the noise (pwm is really noisy..) from the motor cables from getting to the encoder/resolver wires in a parallel cable.
I should have been more specific - I was talking about the power cable for a motor. Quoting from one drive manufacturer's installation manual:
The Shield terminal provides a grounding point for the motor cable shield. It must be connected to an earth ground by a separate continuous lead. The motor cable shield should be connected to this terminal on the drive (drive end) and the motor frame (motor end). Use a shield terminating or EMI clamp to connect shield to this terminal.
Motor power cables are the only time I would do this; I would be curious if this might have eliminated the positioning motor and the spindle motor radiated noise.
Single ended encoders will work just fine. Plasma machines with 5V single ended encoders and proper shielding and machine grounding will work just fine. Because of the arc involved in Plasma cutting poor machine grounding can induce pulses in the various electrical circuits and cause damage. And grounding is at times a black art... Machines that run fine in one location with minimal attention to grounding practice won't function in another area without special grounding practice.
If you connect a shield at both ends it becomes a ground wire and can cause even more problems if and when there are gound currents. The purpose of a shield is to block and blead off any external electromagnetic garbage, not to act as a possible conductor. There should be a dedicated ground wire in the motor harness.
Another important thing to consider is GROUND LOOPS. If you have multiple ground paths on a machine you have the potential for different currents to flow in the various ground wires/paths. Then your grounds are no longer at the same reference and wierd and wonderful things can happen like solid state components turning on and off seemingly at random.
Just wondered if you considered using a 4-20-mA current loop in place of the voltage signals. Current loops are not susceptible to electrical noise that can scramble voltage signals. Avago Technologies has 8-pin DIP optically coupled 20-mA transmitters and receivers that simplify current-loop circuits and provide isolation, too. Texas Instruments has at least one 20-mA transmitter.
I have driven a teletypewriter (circa 1978) with a 20-mA current loop over almost a quarter of a mile of wire. Granted, at 110 baud. But in short-distance wires, you can get a high data rate.
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