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Caps on inputs
« on: August 02, 2011, 11:39:28 PM »
I apologize if I overlooked this in searching, but I'm trying to resolve a bit of a noise issue and need some help. I have a C10 with the input pins set to pull down and Mach3 set to Active Low. We had some estop/limit issues a while back and put a .1uF cap on the estop input. But we also put them on the motor inputs as well for X, Y, and Z. Now, on that board, there is +5v next to the input pins e're using (10, 12, 13). We put the caps between the inputs and the +5v right next to it, but is that maybe the wrong way to go on this? Should we instead be putting it between the input and ground? Overall, the machine works okay at the start of a run but after a short bit, starts to drift and get real ugly. We're pretty sure it's interference (got better after eliminating some wireless need it), but can't figure out to clean it up completely.

Any help would be greatly appreciated!

Offline Tweakie.CNC

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Re: Caps on inputs
« Reply #1 on: August 03, 2011, 02:51:30 AM »
The 0.1uF caps are used for the reduction / elimination of electrical noise on the switch inputs and are connected between the appropriate pin and GND.
It is not a good idea to fit them on the step and direction pin inputs.

Tweakie.
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Re: Caps on inputs
« Reply #2 on: August 03, 2011, 06:46:10 AM »
The 0.1uF caps are used for the reduction / elimination of electrical noise on the switch inputs and are connected between the appropriate pin and GND.
It is not a good idea to fit them on the step and direction pin inputs.

Tweakie.

The 0.1uF caps are used for the reduction / elimination of electrical noise on the switch inputs and are connected between the appropriate pin and GND.
It is not a good idea to fit them on the step and direction pin inputs.

Thanks Tweakie. Fortunately, we don't have them on the step/direction pins, just the inputs. The thing that maybe threw us off a little is that when we had a limit switch problem (all wired through estop), we put the cap there from the estop pin to +5v (we use NC switches) and that solved our problem. Did we just get lucky there? Or should that cap have been to ground? We have the same configuration on the inputs as well - should we change that too?

Thanks!
Re: Caps on inputs
« Reply #3 on: August 03, 2011, 07:32:42 AM »
Let me state this better...

We use NC limit switches, so we have pull down set on pins 10-15 with Active Low in Mach 3. That means all my axis inputs are Active Low and set to pull down, since they are all in the same bank of pins.

When we had the estop problem, we put the cap between estop and +5v and that issue went away. First question: Principally, would this still work if we went from the cap to ground? If the pin is high normally and we put a cap from there to ground, it should "hold" an actual stop condition just for a moment while the cap discharges through the pin - right?

Now, on to the inputs. The same setup occurs here with the caps going to +5v and the pin being normally high. Shouldn't this still work for it's purpose - noise reduction? Or does this simply have to have the cap going from the normally high pin to ground in order to avoid interfering with the motor operation?

I ask all this because the machine works fine at the beginning of a run, but as it goes on, it loses more and more of zero util it becomes a mess, so I'm trying to sort things out.

Thanks!

Offline Tweakie.CNC

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Re: Caps on inputs
« Reply #4 on: August 03, 2011, 07:58:57 AM »
Not sure about this statement.
 
Quote
We had some estop/limit issues a while back and put a .1uF cap on the estop input. But we also put them on the motor inputs as well for X, Y, and Z.

I really don't think you should be putting caps across your motor inputs (unless that is a recommendation by the boards manufacturers).

In my opinion the estop cap should connect between the input pin and GND.

Tweakie.
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Re: Caps on inputs
« Reply #5 on: August 03, 2011, 04:10:46 PM »
I haven't had any noise problem with my CNC machine, but I used to race RC cars, and we put caps from the positive motor input to the motor can, from the negative motor input to the motor can, and from the positive input across to the negative input.  (strictly DC battery power to the motor)

This completely did away with noise from the motor, don't know enough about steppers and driver boards to know if that would work or not.

jim  ;)
Re: Caps on inputs
« Reply #6 on: August 03, 2011, 10:15:56 PM »
Well, we switched the caps to go from the input pins to ground and the test pattern worked perfect!! but...then a longer pattern flipped out. We definitely have interference issues. Gonna rewire with shielded cable - is this stuff any good or is there some that would be better? http://homeavcables.com/22108.html

Offline Tweakie.CNC

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Re: Caps on inputs
« Reply #7 on: August 04, 2011, 02:07:08 AM »
Quote
I haven't had any noise problem with my CNC machine, but I used to race RC cars, and we put caps from the positive motor input to the motor can, from the negative motor input to the motor can, and from the positive input across to the negative input.  (strictly DC battery power to the motor)

This completely did away with noise from the motor, don't know enough about steppers and driver boards to know if that would work or not.

Jim,

You are quite right about the brush type RC car motors but Stepper Motors are different in some quite important ways. During operation the bipolar type motors have current reversal through their windings and although the magnetic field has to collapse and re-establish - to ensure that steps are not missed, this usually happens within 2nS. Fitting any capacitors to the motors will compromise this timing and could totally destroy performance - It may also cause damage to the motor driver circuit.

(Bit of a co-incidence really, but yesterday I was measuring current reversal times in 20 differently constructed coil types to establish the so called 'point of stall' when driving solenoids and relays from bipolar stepper motor drive circuits).

Tweakie.
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Re: Caps on inputs
« Reply #8 on: August 04, 2011, 12:41:13 PM »
Ha,

Foolish jim, I didn't realize they were brushless motors, I've wound a few of them for helicopters and short track cars, compeletly different animals with special controllers.

Thanks for the info.

jim  :D

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Re: Caps on inputs
« Reply #9 on: August 04, 2011, 12:52:40 PM »
Quote
Well, we switched the caps to go from the input pins to ground and the test pattern worked perfect!! but...then a longer pattern flipped out. We definitely have interference issues. Gonna rewire with shielded cable - is this stuff any good or is there some that would be better? http://homeavcables.com/22108.html

You would be better using a cable which has a braided shield (more surface area) the foil stuff is OK but far from Ideal.

Tweakie.
KEEP SAFE !