Three Ways to Improve iGaging DRO Scales Reliability

Tuesday, July 30, 2013
Shielded USB cables can eliminate most of the noise issues
and costs only a few Dollars

In the last post I covered the root causes of the two most common reliability issues with iGaging digital scales. Although those scales catch a good amount of flack of being unstable when mounted on a machine, the problems are rarely caused by the capacitive transducers. In my experience the problems with random resets and unstable readings can be traced to the wiring and are relatively easy to remedy. The mods described below can be done for under $10 total in less than an hour but can make a huge difference.

Eliminate Ground Loops

You might recall that a ground loop is an [undesired] condition where different components of the DRO receive their ground from different sources. In other words there are too many ground references that might lead to a difference in potential between different parts of the system. A lot of the do-it-yourself DRO’s live in home shops or garages with long extension cords that create long and noisy ground loops. Additionally, the scale cables create smaller ground loops when their grounds are connected to the machine’s frame. These issues can be addressed with two relatively easy mods.

First of all, if you’re using a transformerless power supply, switch to an old heavy and cheap wall wart. This will eliminate the giant loop running to the switch panel because the transformer isolates the two sides of the power supply. Second, isolate the scales from the machine’s frame. The “definitive” solution would be to replace the supplied metal brackets with ones made of hard plastic. If that looks like too much work, you can isolate the scale’s frame from the bracket by wrapping the last 0.5” or so with tape. To prevent metal swarf from bringing the gap apply a short piece of duct tape over the bracket.

Shorten the Cables

Remember, that powerful spindle motor is an electromagnet, creating strong dynamic magnetic field around it’s frame. As I mentioned in the previous post, cables act as small transformers and can pick up electromagnetic interferences. This effect is proportional to the length of the wire and can thus be reduced by shortening the cables. IGaging scales come with 6’ cables but chances are that on an average home milling machine you will need only about 2-3’. If you want to keep the Mini-USB plug intact, you can cut out the desired length from the middle of the cable and join the two shorter pieces together. Alternatively (this would be my suggestion) you might want to replace the Mini-USB connector with something that will provide a more mechanically-solid connection, such as 4-pin Din or Mini-Din connector etc. Both of those connectors (male and female) are available at Radio Shack and other stores that sell electronic components.

Use Better Shielded USB Cables

Better yet, you can replace the cheap stock unshielded USB cables with a heavier gauge shielded ones. This change helps in two ways. First, the shielding dissipates the interferences before the can get to the data lines. Second, larger gauge wire has lower resistance, so there will be less of voltage difference between the two grounds.

These braid-shielded cables cost under $4 shipped (from eBay)

To replace the USB cable you will need to take the cover off the iGaging reading head held in place by four small screws. Once the cover is off you will see another Mini-USB connector plugged into the board (and likely hot-glued in) that you need to [carefully] unplug. The new connector will likely be too thick and won’t fit; using a sharp hobby or utility knife shave off most of the plastic on the side closest to the board until it plugs in with a bit of friction.

To take the cover off, undo the four screws as shown

A regular USB Mini-B connector won't fit into the socket since it need to sit flush with the board. On the older scales iGaging used thinner connectors, but for the newer one the folks at iGaging appear to simply have ground off some plastic. To make your connector fit you can follow suit and either grind or cut a few millimeters off

iGaging supplied USB Mini-B connector is much thinner than the new one

iGaging scales use Mini-B to Mini-B cables that are very difficult to find, so the replacement cable will likely need to be USB A Male to Mini-B Male. If you wish to have the ability to use the scales with the iGaging remote readout, you can get a adapter. Alternative you can use this opportunity to upgrade to a more robust connector or even solder the wires directly into the board.

The new connector can be thinned by a few mm using a hobby knife

Please keep in mind that the shielding braid is connected to the frame of the connector. In order to dissipate the interferences it needs to be tied to the ground on one end only (presumably at the end connected to the controller). Leaving it floating might turn it into a transmitting antenna and tying both ends to the ground will create a ground loop.

If you want to keep the stock cables, you can get some metal braided sleeving, slip it over the cables (making sure to ground one end). This will probably be a bit more expensive, though.

Avoid Cable Loops and Sharp Bends

This modification is by far the easiest of the bunch but can make a big difference, especially in an Arduino setup. You might recall that real-world wires have some impedances. Without going too deep into the electronics theory you can think of impedance as resistance to pulsing current. Coincidentally the clock and position data is transmitted through the cables using a series of short pulses. In this case, inductive reactance accounts for most of the impedance, and sharp bends and loops in the cables increase inductance. Moreover, a cable loop around a piece of metal acts as a transformer and can create cross-talk and other sorts of issues. Straightening out the cables can make sizable difference in the pulse rise time and/or magnitude.

Conclusion

In my experience the few easy modifications outlined above take care of any noise-induced stability issues. I've used a set of iGaging scales with cables shown in the picture and the setup has been very stable. If you still experience issues with these mods in place, please drop me a line and I'd be more than happy to help.

11 comments :

  1. I had good luck using these specific cables from www.mouser.com:

    http://www.mouser.com/ProductDetail/Qualtek/3021009-06/?qs=yhNIDeTE%252bcaBsWw0NLAORQ==

    They required shaving the connector body as Yuriy shows, but they are much thicker and more shielded than the stock ones. I also had to enlarge the notch in the iGaging plastic body to allow this cable to fit through.

    I use 3 scales with those cables plugged into full size USB connectors on a custom pcb that stacks onto an arduino , all mounted in a die cast box with no noise issues. At least not yet, I have not yet mounted it to my actual machine.

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    1. These cables don't have the mini-B connector both ends. I found cables with MIni-B both ends at DCables, Richardson TX. They are not advertised as screened but in fact they are. Connector needs to be shaved but otherwise excellent.

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    2. Thank you for the heads up on DCables. I just ordered my new cables from them after not being able to find them anywhere else.

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  2. Yuri - great info for the AccuRemote SS scales. Didn't know about them; will soon be adding a set to my LMS 3960 minimill. I've been long waiting for scales that didn't have a display right on the scale!

    Just wondering - if you are going have to hack up the scale enclosure to put in a better USB cable - why not just solder direct and eliminate one more connector? Obviously there is a loss of easy replacement... but once installed, how often would one need to be changing cables?

    ReplyDelete
    Replies
    1. Alan,
      Mostly because undoing four bolts is much easier than desoldering a fine-pitch surface mount connector and soldering a cable in its place :)
      If you can do it, it's definitely a good mod but for most of people building the DRO's any soldering is too much soldering, let alone fine-pitch SMT "rework".

      Thank you
      Yuriy

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  3. Yuri, thank you for your work on these scales.

    I'm experiencing some inaccuracy, too with mine. Your analysis suggested to me that the accuracy could be improved by making a more rigid bottom shell out of aluminum.

    Your thoughts?

    -JW

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    Replies
    1. JW
      I think it would help a lot but I'd want to add some sort of bearing material between the metals. I.e. you don't want aluminum rubbing on aluminum. Possibly a thin strip of PTFE or Delrin on one side and a brass gib on the other.
      Thank you
      Yuriy

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  4. THANK YOU! I thought I would never figure this out. I kept having problems of resets and erratic readings. I did find on ebay shielded mini usb cables:

    http://www.ebay.com/itm/Shield-Mini-USB-Male-to-Mini-USB-Male-90-degree-USB-B-Mini-5-Pin-Right-/190675793233?pt=US_USB_Cables_Hubs_Adapters&hash=item2c6529b551

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    Replies
    1. Got the shielded cables and installed them. I had installed all 3 readouts in a box and supplied them with a 3v power supply. I turned the mill on and the X-axis readout was still fluctuating when I turned the motor on. The other 2 were fine. Only after much trial and error, and by shear accident, did I discover what the problem was. The pickup was not firmly seated against the bar. I solved this by stuffing the space with a few layers of felt so it would press the circuit board against the bar. I'm sure foam rubber would work but I just had the felt on hand.

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  5. Been keeping an eye on all of your projects. I can't wait to start my own soon. My machine both run off VFD's which I understand can create a lot if interference with other sensitive electrical components. Does anyone know if the use of a VFD to power the machine will have any effect on the DRO side of things?

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  6. I had exactly the same problem with 0.2" jumps on two DROs. Sent them back for replacement. No hassle from Anytime Tools.
    Just to be sure I decided to change the cables to screened variety.
    USB cables with Mini-B plugs both ends are hard to find. Only supplier I found: DCables, Richardson, TX. Their cables are not described as screened but in fact they are. Nice quality too. Plug needs to be shaved.

    ReplyDelete

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