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CNC Noise Cures

After backlash, the second biggest source of frustration seems to be noise problems on homebrew CNC systems. Noise can manifest itself in a variety of ways including uncommanded movements, servo encoder problems, and limit switches tripping unexpectedly. There are many noise sources, but motors and VFD's appear to be the worst. Any source of electrical sparks or arcing is going to make noise. In fact, the first radio transmissions were nothing more than great big sparks!

Noise suppression is often a matter of surrounding the noise producers and/or the noise victims with a grounded metal shield. With enough foil and grounding, you can shield anything from noise:

With enough foil and grounding, you can shield anything from noise!

Fortunately, there are easier ways, so put away your aluminum foil and read on. Below is a laundary list of tips I've come across that you can consider and try if you think you're having a noise problem.

Important Preventitive Tips

These are the things you should always do as you're building the system to avoid having to deal with noise later on. They're either madatory or so cheap and easy you may as well do them anyway.

Use an Opto-Isolated Breakout Board

Your CNC electronics and your PC are each a source of noise as well as being vulnerable to noise from the other. Putting each into its own grounded chassis will help keep them isolated and the opto-isolators help keep them isolated.

Use Shielded or Twisted Pair Cables

Shielded cabling for the stepper/servo motor wiring, encoder wiring, and limit switch wiring is ideal for noise resistance.

I bought a 500' spool of 6 conductor shield 22 AWG cable from Action Electronics. For connectors, I'm using some microphone connectors, also from Action Electronics :

6 conductors and a foil shield: this is the right cable for my step motors!

For others who may read this, you want to use shielded cables or noise will make your system unreliable. FWIW, when I first got my limited switches going, I did not ground the cable shields to the CNC cabinet. As a result, I had a bogus limit fire about every 2 to 3 minutes. Grounding the shields made that go away completely. If you didn't think you had noise in your system, maybe it was because you didn't have a way to check?

Other thoughts:

- You can use CAT-5 cable for encoders or limit switches, just be sure you use the twisted pair feature and pair each signal with it's ground to provide noise immunity. Apparently this idea has been blessed by none other than Mariss F., Mr Gecko, himself.

- DIN cables such as are used as MIDI patch cables in the music business and older keyboard cables (before the PS/2 and USB styles) in the PC business are another possibility. Even Radio Shack carries some DIN cable componentry. DIN for MIDI applications have 5 conductors plus a ground/shield and are supposed to be rated to 2 amps @ 100V. What's nice is you can get premade DIN cables in a variety of colors and lengths. If they'll serve the power needs of your system, they could be quite convenient. Mini-DIN is the PS/2 keyboard connector, and is also shielded. It might be useful for limits and encoders. Cables are available already made up. Note that Homann sells a mini-DIN breakout board that would make it easy to connect to your mini-DIN cables with point to point wiring.

MIDI DIN patch cable...

- Cinch connectors are another common type of connector used for motors. They will carry a bit more current than DINs or microphone connectors.

Cinch connectors...

Higher Voltage Control Signals Have Better Noise Immunity

3-5V DC is pretty marginal from a noise immunity standpoint. If you can run 12V or more to your limit switches you will find it is much less susceptible to noise. Unfortunately, this may be difficult to change as your breakout board will likely have made the voltage decision for you. My GRex uses 12V.

Sometimes you can use a Zener diode to run a higher voltage and convert it back to 5V for your breakout board. Run a 24V circuit and have the switch ground that. Then use a resistor in series something like 1K to the digital input, with a 5V1 zenner diode across the input and ground. (Bar of diode to input). The Zener diode will clamp the digital voltage at 5volts.

There are also opto-isolators that allow differential voltages so you could have +5V to your breakout (actually may as well go to the parallel if you are using an opto-isolator--don't need 2 of them!) and 24V to your switch circuits.

Higher Current Helps Too!

If you can not up the voltage try putting some 100 ohm pull up resistors to +5 on the switch, this will give you 50 milliamps of current through the
switch and may help increase reliability. This is .250 watts so get ½ watt resistors or they will get warm.

Wire Home and Limit Circuits Normally Closed

Noise spikes are usually positive, so a normally open circuit can see a noise spike as circuit closure. Wire the circuits normally closed and they're less susceptible. Most of the time normally closed is a good idea from a reliability standpoint too as any damage to the circuit usually opens it and trips the circuit.

Keep "Noisy" Wires Aways from "Quiet" Wires

For example, don't run the limit wiring in the same conduit as the cable from VFD to motor! When noisy wires must be near quiet wires, try to cross them at right angles rather than running them parallel. Parallel wires are good at passing noise to each other.

Separate Chassis for PC, Driver Electronics, and VFD

Put each of these components into its own chassis (don't just stick your VFD in a box next to a bare PC motherboard, next to a bare breakout board, next to bare servo drives), and then run a common ground to each chassis.

VFD Noise

If you were asked to produce a noise generator in order to sabotage a CNC or other sensitive electronic system, you couldn't ask for a more efficient one than a VFD!

A grounded flexible metal conduit is very desirable with the VFD connection to the motor. T&B SuperFlex also has a small copper wire wound with the metallic sheath for ground continuity. Use of twisted conductors inside the flex conduit will also help reduce the noise emissions from the VFD.

Keep the wiring length from VFD to motor as short as possible. Long lines can radiate more noise.

VFD manufacturers like Hitachi sell special filters called EMI, RFI, or EMC filters that cut out noise. They're installed on the AC mains side of the VFD. They also sell line reactors that go on the line between the VFD and the motor. A line reactor reduces noise by cutting out harmonics, and it will also increase the performance of the system since power is being lost through the harmonics. Outfits like AutomationDirect sell them, but they aren't cheap so I wouldn't necessarily add one unless you were running out of noise reduction steps to try.

Avoid Ground Loops

If you're familiar with ground loops and their impact on stereos and hi-fi equipment, you know they are a source of noise. Ground loops occur when all grounds are not quite a the same zero potential. The best way to avoid them is to use a common ground for all equipment. The exception to this rule is when using a plasma cutter you will want to keep the grounds for the plasma cutter separate from the rest of the equipment all the way back to the mains box if possible.

Diagnosis Tips

Your machine is behaving as though haunted--limits trip unexpectedly, motors don't move right, or your servos act as if the encoders aren't connected properly. What to do?

Diagnosis is a bit of trial and error. If you can figure out where the noise is coming from through a process of elimination, you will be that much closer to deciding what to do about it.

Try Disconnecting the VFD

Pull the plug on your VFD and see whether the noise problems continue.

Disconnect your DC supply

If the shutting down the VFD didn't help, next try shutting down the DC supply to your steppers. This one is hard with servos because the encoders will fault the system out if the shafts don't turn. With steppers, things will move merrily along juts as though everything was working. If you're seeing false limits hit with the DC shut off, you know the noise isn't due to your step motors.

Now try some combinations

If you've disconnected various motors until the noise problem stops, congratulations, you're on a good track. Now try different combinations of reconnection to see where the problem lies. Each individual motor is a possibility. Perhaps the shielded ground connection for a particular motor is not making a good connection and vibration has shaken it loose. When you find your bad actor, check that sort of thing very carefully.

Other Tips

If you've taken the preventative steps described above and still have noise problems, consider investigating the tips from this section.

Use Mach 3 Debounce

I put this one first because it is easy to try. Limits and similar inputs can be debounced in software using Mach 3. This can help to reduce noise sensitivity. Most sources recommend trying a debounce value of 1000 to see if it improves the situation. Values as high as 2000 are sometimes recommended.

Sometimes this debounce option can cause problems in homing circuits, such as strange reversals of motion, so be on the lookout. It is not a panacea. Lower values of debounce may make your switches more accurate and repeatable. It's worth experimenting to find the optimum lowest value that eliminates false hits from noise.

Ferrite Beads

Ferrite beads can really help on noise problems. Pop a bead onto each power line--AC to power supplies, DC to each Gecko, +5V logic supply, etc. The beads eat the noise. Ferrites are particularly good for VFD noise and should be one of the first things tried after shielding the cables.

They come in two forms:

Raw ferrite beads: thead the wire through them:

Wire threaded through a ferrite bead...

Clamp-on ferrite: ideal for power cords and adding after the fact...

Add a Low Pass Noise Filter

Low pass noise filters: These will primarily be of use on limit switches and other connections that don't need to respond blindingly quickly. A simple resistor and capacitor may be used to create the filter. Values of 3K ohms and 0.1 microfarad yield an upper cutoff frequency of 500 Hz. So, signals changing faster than 500 Hz will tend to be cut out by the filter. I wouldn't bother with one proactively, but if you are finding noise on your limit switch circuits, you should try one. Position it as close to the breakout board end of the line as possible.

Source for Professional Noise Filters

Try Vortex Technologies. They make a variety of devices that may be helpful. These include line filters as well as more innovative devices like their Ground Transient Eliminator, which can be used to clear up noise on the ground lines.

Another source of line filters is Ajax CNC which has them for $20:

EMI Line Filter stops noise from coming in your AC lines...

These filters are generally used to keep noise from coming in the AC line. For example, if noise is glitching your PC, put one of these between the AC outlet and the PC power supply.

Use Suppressor Diodes With Mechanical Relays and Solenoids

Mechanical relays and solenoids (are you using solenoid switches to operate pneumatic systems or turn coolant on and off?) can create noise as the magnetic field collapses when power is cut to these devices. A simple 12V relay can generate up to 1500V in this way when current to its coil is cut!

A number of sources recommend placing a rectifier diode in parallel with the relay coil and it will eliminate these transients to the point where noise problems are greatly reduced. The difficulty is that the diode will also slow the speed with which the relay operates, which can lead to tack welding of the relay contacts as they spend too long in close proximity while opening and arcing across the small gap creates the tack welding effect. This phenomenon only affects normally open relay circuits, and the diode method is fine for normally closed.

More complex suppression methods may be helpful in preserving the dynamic response of the relay and eliminating this tack welding problem. A rectifier and Zener diode combination produces optimum results. However, it also turns out a resistor installed in parallel with the coil can do a better job than the diode will alone while preserving good dynamics. Resistances in the relatively small range of 82 to 220 ohms will work best for this application. Make sure your resistor is up to the current demands of the application.

Put Plasma Cutters On A Separate Ground

Plasma cutters throw out huge amounts of noise as you would expect. Make sure they're on a separate ground that doesn't meet up with the ground for the rest of your system until it gets back to the mains. If the plasma noise is bad enough, I would consider looking at something like the Vortex Technologies Ground Transient Eliminator.

Physical Separation When All Else Fails

If you can arrange to move noise producers further from the components hit by the noise, this can help. Don't set your plasma cutter right under the control circuitry for your CNC plasma table! Try to move your VFD further from the drive electronics. Put the VFD at the top of the mill near the motor on the left side and the drive electronics cabinet down low on the right side, for example.

Sometimes Your Breakout Board is the Problem

I read one account where a fellow was sent the wrong version of his breakout board which led to noise trouble. I'm not sure how they figured it out, but the problems went away when he swapped boards.

Ground all Your Power Supplies to the Same Ground

You've got separate chassis for VFD, PC, and the rest of your CNC electronics. You've grounded those chassis together, right? Now make sure the power supplies in each chassis are grounded together so that no supply is floating relative to ground. There is a great series of posts on this CNCZone thread about common grounds for power supplies.

Bypass Capacitors Can Help With Noise

US Digital HEDS encoders are very common on servo systems because they're cheap, and come with a lot of inexpensive servos. Unfortunately, they're very prone to noise sensitivity.

Mariss F. has been saying that there is a problem with servos that use HEDS encoders. These are the usual cheap US Digital encoders, and they're exactly what I have in my homeshopcnc servos. Some folks have said they had no problem with their encoders, others have said they have all kinds of problem. Mariss proposed a fix that involves some simple bypass capacitors, so I took it on myself to install the capacitors on my servos. Here is what it takes:

3 Allen-head bolts hold the rear cover on the servo...

Here's what the innards look like. The HEDS encoder is connected by a simple little harness...

We're bypassing the power supply to filter out noise, so the capacitor goes between pins 1 (Gnd) and 4 (+5V)...

The easiest thing is just to bend the capacitor leads, insert into the connector, and then reinstall...

The ceramic 1uF capacitor goes across the HEDS power supply terminals at the encoder. Use Digikey BC1151CT-ND capacitors. They cost $1.80 for a package of 10 pieces and Digikey was happy to send them to me without requiring a minimum order.

The result? I immediately noticed my servos were quieter when not moving--less dithering. Some odd glitches and faults that would happen every now and again while jogging went away. Best of all, my part was suddenly being more accurately made--I was getting some false feedback from the encoder due to noise.

This is a quick and easy fix: highly recommended if you have HEDS encoders!

I've heard other folks mention the same little caps work wonders for home and limit switches (put the cap at the switch across the contacts) and even for probing and electronic edgefinders.

Software Debounce in Mach3

Mach3 has a very nice capability to debounce contacts on a lot of its incoming lines. This is where the software looks at the signal and treats it as one single pulse even if it gets multiple pulses when the pulses are close enough together. You can specify an appropriate time period.

OddBall: To ground or not to ground the Gecko heat sink?

This fellow seems to feel that a scratch in the anodizing (hard anodizing provides isolation and doesn't conduct well) allowed his Gecko Drive to ground and create noise problems. I'm still not sure what I make of it. I would have thought you were supposed to ground the darned things.

 

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