RC Filtering and Schmitt trigger Question

Electronics related to CNC

RC Filtering and Schmitt trigger Question

Postby gera229 » Fri May 25, 2012 6:05 am

Upon reading the specifications and details of this board: http://www.buildlog.net/blog/2011/08/op ... er-driver/

I came across this:
"Filtering.
All step and direction signals are filtered with a RC filter and a schmitt trigger. This is ideal for a noisy environment like a laser cutter or CNC machine. The RC filter frequency is high enough to allow 1uS pulse control of the drivers."


I wonder why filtering is necessary for laser cutter or CNC router machines?

By "noisy environment" what kind of noise are they referring to? The noise that you hear?
What does this "noise" cause? What would happen if filtering was no there?

I am concerned why other boards used for CNC router machines do not mention filtering in their specifications. Probably because they don't have problems with it? Or because they use a different schematic setup that does not require filtering and still works good without it?
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Re: RC Filtering and Schmitt trigger Question

Postby RSWeaverAz » Fri May 25, 2012 7:33 am

Thats probably electrical noise caused by ... just about anything :o
but most likely turning large electrical loads (laser's etc) on-off.
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Re: RC Filtering and Schmitt trigger Question

Postby gera229 » Fri May 25, 2012 8:17 am

So what if I used a manual controlled router that is not operated by the program and uses a separate power source? Would it be fine to run it that way without filtering?

Now if the router was connected to the board and run by the program, then filtering would be necessary right?
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Re: RC Filtering and Schmitt trigger Question

Postby RSWeaverAz » Fri May 25, 2012 10:32 am

There are several controller boards out there (I'm most familiar with ones that control stepper motors.)
I'm assuming that most controller boards that have some kind of CPU have some filtering on the DC Power input side
so that when a heavy load like a laser pulsing or a heatbed cycles, the control board is somewhat protected from "noise".
Individual pololu driver carrier boards like the ones used on the Azteeg X1 or Barts "open-source-4-axis-stepper-driver"
have built in filtering for VMM (a 4.7 uF and a 0.1 uF capacitor) and for VDD (a 0.22 uF capacitor).
Seehttp://www.pololu.com/catalog/product/1182 for example, the filters are connected to JP1 & JP2.

Now you're talking about a schmitt trigger (used in a laser application?)
I would assume the chip/board you're using should have some filtering examples on the Schematic, data sheet or application note.
If you're rolling your own, then the filters connected to JP1 and JP2 would be a good place to start.

BTW, electrical noise can come from AC powered stuff as well,
air conditioners, drill press, and even the CFL bulbs can output electrical noise.
That kind of noise should be handled at the AC input end of your AC-DC power supply,
but you might want to check that as well.
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Re: RC Filtering and Schmitt trigger Question

Postby bdring » Fri May 25, 2012 12:15 pm

When that board was first installed in my laser cutter, I was getting occasional false steps when the laser was pulsed at high power. I would manually pulse the laser and I could hear the steps in the machine.

Just like when lightning strikes in your area and you hear a buzz in the TV or radio, electrical noise can affect the stepper drivers. They are looking for short low voltage pulses as signals for the steps. That is what electrical noise looks like. Electrical noise can also cause false limit switch triggers and other problems on CNC machines

Good wiring techniques like shielding wires, shortening wires, twisting wires with the return signal and separating the high voltage can go a long way to fix that.

Another way to limit noise affects is filtering. RC filters are low pass filters, meaning they filter out high frequencies. Stepper motor pulses are fairly short, so you need to be careful not to filter them out. Schmitt triggers enforce that the signal voltage is closer to the full voltage and won't trigger on lower voltage noise.


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Re: RC Filtering and Schmitt trigger Question

Postby gera229 » Fri May 25, 2012 8:44 pm

So electrical noise can still come in and interfere even if like a Dremel tool is not connected to the PCB, but connected to a separate power source like an AC wall outlet?
Looks like wire shielding is what protects it from any problems in this case right? But filtering can too correct?

I bought a HobbyCNC Pro chopper board which does not claim to have filtering anywhere in the specs, and claim they have had no problems and feel it's not necessary.
I feel like I made the wrong choice to buy the HobbyCNC Pro board and should have gotten a board with filtering such as like the one I linked to.
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Re: RC Filtering and Schmitt trigger Question

Postby TLHarrell » Fri May 25, 2012 9:06 pm

I would wire it up and try it out before getting too worried about it. Do your best to not run AC power and low voltage signal wiring parallel to each other, or in close proximity to reduce or eliminate induced current and line noise. You may find out that there is no problem. If there is, then start procedures to eliminate it from the system.
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Re: RC Filtering and Schmitt trigger Question

Postby Robtbldr » Sat May 26, 2012 2:52 pm

I have built a hobbycnc board to run in my cnc machine and I have never had problems with noise. I use a 2.5 hp router on the same strip outlet and have run both for hours. The main reason to use the hobby cnc board is if you want to run larger stepper motors nema 23s at 3a. The pololu can handle 1.5-2a at most before heat becomes a problem.
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