Igaging DROs, reducing sensitivity to noise

[homebrewed]

For awhile now I've been dealing with a noise problem affecting some of my remote reading Igaging DROs (attached to a mini mill). The mill's motor is a variable-speed version, controlled by PWM. The PWM noise sometimes gets into the DROs, causing them to lose their position. I put braided shielding around the cables going between the sensor and display units, which helped a bit (but did not completely solve the problem). I soldered a 1 microfarad capacitor between the sensor's Vcc and Ground, and that helped -- a bit more, but it didn't eliminate the problem, either. I made sure I had single-point grounds to avoid ground loops. Didn't improve things. Replaced the conductive rule mounting hardware with insulating ones. No improvement. Due to the fact that shielding didn't help much got me thinking that I was dealing with magnetic interference, not electric fields. Copper braid won't address magnetic field interference. And the effective source impedance of mag-field interference is very low, which would explain why a 1uf capacitor was not completely successful at fixing the problem.

So in a desperate move I did some surgery on the most problematic axis' DRO sensor (Z, the sensor closest to the motor). I got to the circuit board and severed the Vcc line coming from the USB connector, then soldered in a 1.5K resistor to restore the connection to Vcc. The idea was to filter Vcc. And this, so far, appears to have solved my problem.

My approach is not for those who are not experts at soldering. I had to use a fine tip soldering iron under a stereomicroscope to avoid solder bridges across the USB socket terminals. But later I figured out a less-demanding way to do the same thing, with no circuit board surgery. The cable that goes between the display and sensor can be modified. There's enough cable inside the sensor box to do it there -- cut the cable, insert a 1.5K resistor in series with the Vcc line and restore the other connections. The 1uF capacitor still must be added between Vcc and ground. And use heat-shrink tubing to insulate the connections. Yuiry's Toys indicates you can't depend on the wire colors being the same from batch to batch so I'm not going to indicate what wire color the Vcc line is for mine.

The sensor's low current draw doesn't produce much of a voltage drop across the 1.5K resistor so normal circuit operation is not affected. Power dissipation is not an issue so you can use a small resistor -- 1/10'th watt, if you've got 'em.

Some may ask why I'm going through this rather than buying "real" DROs that use magnetic or optical rules. 1. Budget (I already have these). 2. Fixed Income (retired). 3. Hobbyist (I've got the time).

 

[RJSakowski]

I added capacitors to my pickup heads as suggested in this thread. https://www.hobby-machinist.com/threads/question-about-blu-dro-touch-dro.69308/#post-607421.
Rather than connect to the ground plane as shown in the photo, I soldered directly to the USB socket. I used a .05mfd ceramic disk capacitor in parallel with a 4 mfd electrolytic. The reason for this is that ceramic dicks have a lower ESR and respond better to high frequency whereas the electrolytic capacitor can better buffer the internal voltage due to its higher capacitance. A tantalum electrolytic could be used instead of the pair.
I haven't used the DRO much since the modification but, to the extent that I have used it, I've not seen any more glitches.

 

[BaronJ]

Hi Guys,

So you have proved that the interference is coming down the DC power feed. Which in RJ's case is why the problem went away when running on batteries.

 

[whitmore]

For awhile now I've been dealing with a noise problem affecting some of my remote reading Igaging DROs (attached to a mini mill). The mill's motor is a variable-speed version, controlled by PWM. The PWM noise sometimes gets into the DROs, causing them to lose their position...

The cable that goes between the display and sensor can be modified. There's enough cable inside the sensor box to do it there -- cut the cable, insert a 1.5K resistor in series with the Vcc line and restore the other connections. The 1uF capacitor still must be added between Vcc and ground.

The inductive nature of the interference sugests that the 'cable inside the sensor' could also take a snap-on
common mode choke, which might do the trick without soldering. Something like this:
<https://www.digikey.com/catalog/en/...res-snap-on/9398?mpart=28A1507-0A2&vendor=240>

 

[homebrewed]

Hi Guys,

So you have proved that the interference is coming down the DC power feed. Which in RJ's case is why the problem went away when running on batteries.

The Igaging remote-reading DROs have a control/display unit and a separate sensor unit that reads the scale. Unfortunately, the sensor is powered by batteries in the display unit so there is no alternative to having power coming in via the cables....at least, not without some "interesting" modifications. I don't know about other remote-reading DROs but it seems likely they are similar designs.

Snap-on chokes might work. If I had one I'd try it, yu betcha. I do have lots of 1.5K resistors and 1uf caps. The legacy of 40 years of electronics work . I have some stuff I will never use, unless I throw it out & discover a need the next day.

 

[JPigg55]

For awhile now I've been dealing with a noise problem affecting some of my remote reading Igaging DROs (attached to a mini mill). The mill's motor is a variable-speed version, controlled by PWM. The PWM noise sometimes gets into the DROs, causing them to lose their position.

Curious as to what iGaging DRO model you're using ? Have you tried replacing the cables with good, shielded ones ?
I have the iGaging Absolute Origin scales and display on my mill and lathe and haven't had any issues, although I don't have variable speed on my machines and use the batteries to power them as that's my only option for now.
Mine are the model that DOESN'T work with Yuriy's App (still trying to crack that nut) and have only a battery power option. I hate it as they continually go into sleep mode to conserve power or batteries die and have to replace them. Can't wait to toss those displays in the trash.

 

[markba633csi]

Hi Homebrewed-good troubleshooting. What type of motor/controller on your mini-mill? DC brush type or brushless?
Mark

 

[homebrewed]

Curious as to what iGaging DRO model you're using ? Have you tried replacing the cables with good, shielded ones ?

I have 3 iGaging AcuRemote DROs. They are about 3 years old now. I checked Yuriy's web site for solutions to my problem, and it looked like it would be necessary to perform surgery on replacement cables to get them to fit, so I rolled my own shielded cable by buying some shielding braid from Jameco.

It was a bit tricky to feed the cable down the inside of the braid -- the connectors are much larger than the cable itself, so it looked like a snake swallowing a mouse. If you push the braid together (lengthwise), it gets shorter and, fortunately, larger in diameter. Otherwise I would have been out of luck.

I also got inside the display units and brought out a ground line so I could ground the shield. It's no good if it is floating.

 

[homebrewed]

Hi Homebrewed-good troubleshooting. What type of motor/controller on your mini-mill? DC brush type or brushless?

Hi Mark,

The one I have is the 500 watt brushless. I got mine from Micro-Mark when it went on sale a few years back.

Before I retired I performed failure analysis on electronics (IC's). In many ways that job was nothing BUT troubleshooting, so I guess you could say it's in my blood.

 

[JPigg55]

Reason I asked was a friend from work who told me about iGaging scales had the same noise issue. He has the AccuRemote models as well.
He told me it was noise issue due to unshielded cables. His solution (which worked great for him) was replacing the cables with shielded ones.
He got them from here http://www.dcables.net/, bit tricky to find on the site as they're mini-B to mini-B male to male cords. I can ask him if he still has the link for them.
He said he took his Dremel tool and cut down the thickness so it would fit inside the housing, told me was easy and worked great.
One warning, when he first disassembled his readhead to replace the cables, he took off the wrong side . It's spring loaded on one side and his springs went flying.