04-02-2015 16:58

Lil' Lavery

Re: pic: A little teaser from 3571

That's a very aggressively spaced electronics board. So much so that I have to question why it had to be so compact. That wire routing is certainly beneath the minimum bend radius of 12 AWG wire.

04-02-2015 17:35

icq10

Re: pic: A little teaser from 3571

Looks quite nice, but I am always concerned that some of the thicker wires are breaking due to too much bending. We had one robot of the last years that we are about to revive for a fundraising event and it had connecting issues... Took us 3 days to figure out that it was one of the networking cable that was broken due to too much bending with cable ties.

But at least this is a nice overview, the wiring cleanup is always a big challenge!

04-02-2015 19:29

Jared Russell

Re: pic: A little teaser from 3571

Looks good to me.

04-02-2015 21:37

Briansmithtown

Re: pic: A little teaser from 3571

How water resistant is it? We need to know this for... future endeavors.

05-02-2015 14:05

Addison4300

Re: pic: A little teaser from 3571

I love the organization, So very perfect! That is also an excessive amount of Talons

05-02-2015 15:02

JustPlyZ

Re: pic: A little teaser from 3571

If you can guess the amount of cims we're using and what type of drive we are using, I will send you a button.

05-02-2015 18:29

TheThings

Re: pic: A little teaser from 3571

Can anyone say interference? Also, look to the fact that the roborio isn't supposed to be near things that are that powerful. Your talons also are going to get hot. Just a warning from our electrical lead.

05-02-2015 19:48

Alan Anderson

Re: pic: A little teaser from 3571

Can you clarify what you mean by that?

05-02-2015 20:10

TheThings

Re: pic: A little teaser from 3571

(this is the Electrical lead): By running the pwm signal wires next to large motor wires that could potentially have upwards of 40 amps through them, you risk electromagnetic interference across the pwm. For this reason we like to run our pwm cables as far away from our power cables as possible, and where they have to come near each other, we cross them at a 90 degree angle. This prevents the magnetic fields on the power cables from interfering with pwm communication.

05-02-2015 20:38

Foster

Re: pic: A little teaser from 3571

Oh how much I want to covet this board, hold it, whisper things to it. But sadly there isn't a single label on any of the cables or talons. So sad, a thing of beauty, struck down by such a minor thing like being able to trace wires.

05-02-2015 22:38

Caleb Sykes

Re: pic: A little teaser from 3571

Anecdote: I have never had a problem when routing PWM wires and power wires together.

Physics Note: Even if the magnetic field were substantial, routing PWM wires parallel to power wires should not have an appreciable effect on signal, since the magnetic field would only cause a Hall Effect on the signal wires. That is, the magnetic field would not speed up or slow down the electrons, it would just shift the electrons to one side of the wire or the other.

Just for fun though, let's calculate the magnetic field created by a 12AWG wire, and see how that magnetic field affects a 22AWG PWM cable next to the power cable.

Take:
I1 = current in the 12AWG wire
I2 = current in the 22AWG wire
r1 = radius of 12AWG wire = 1.03mm
r2 = radius of 22AWG wire = .32mm
d = distance from center of 12AWG to center of 22AWG wire

The magnetic field B(r) created by the 12AWG wire at a distance r (r>r2) from the center of the wire will be:
B(r) = (mu0)*I1/(2*pi*r)

The magnetic field will be perpendicular to the propagation of the electrons, which means the magnetic field will cause the electrons in the 22AWG wire to move either toward or away from the 12AWG wire (This is the Hall Effect). The difference in voltage VH between the near side and the far side of the 22AWG wire will be given as*:
VH ~= I2*B(r)/(n*t*e-)

Where B(r) is given by the first equation, n is the number density of charge carriers of conducting material, t is the thickness of the 22AWG PWM cable, and e- is the fundamental unit of charge.

Combining these equations, and taking mu0 = 4*pi*10^-7 (mks), n=8.49*10^28 m^-3 (copper), t = r2 = .00032m, and e- = 6.02*10^-19 C, gives:
VH = (4.6*10^-14*I2*I1/d (mks)) V

Plugging in some ridiculously high values for I1 and I2, and a ridiculously low value for d:
I1 = 1000A
I2 = 10A
d= .1mm

This yields a potential difference between the near and far side of the PWM cable to be 5*10^-6 V. Which could be measurable if you had a good sensor, but doesn't come anywhere near the 5V** high voltage transmitted by the PWM cable.

Please don't route your signal and power wires separately for this reason.


*At these small of distances, the magnetic field can not be assumed to be constant over the entire width of the cable, hence the ~ sign.
**Source needed

06-02-2015 08:16

arichman1257

Re: pic: A little teaser from 3571

6 cims. Swerve drive.

06-02-2015 14:39

philso

Re: pic: A little teaser from 3571

^
|
|
This is standard practice in industry and is described very thoroughly in literature on Electromagnetic Interference.


The PWM signals are "digital" and have a large noise margin, by design, and their response to noise is non-linear. Increasing the amount of noise induced on your PWM wires will not cause noticeable degradation in performance until you have used up all of your noise margin. Shifting your wires around may change the coupling enough to start causing a problem. Been there, done that, had to write reports about it, didn't get a T-shirt for it.


A current in a wire WILL induce a current in a wire that is parallel to it. That is why it is standard practice in industry to avoid running power wiring and signal wiring together, in the same bundle. Please DO route your signal and power wires separately for this reason.

You are correct in thinking that the Hall Effect is insignificant in situations like this. I have never seen any mention of it in any literature on Electromagnetic Interference or in any of the many courses my employers have paid to have me attended on the subject.

06-02-2015 15:53

Caleb Sykes

Re: pic: A little teaser from 3571

I'm really having trouble understanding how there would be any appreciable electromagnetic interference caused by a single wire with a constant (or at least, low frequency) current running through it. This situation would only cause a slight magnetic field around the wire, and should not be creating any electromagnetic waves. The situation would clearly be different if the current was AC, but we are not dealing with AC currents here.

It is quite possible I am in this way over my head and that I really don't understand what is going on. Could you possibly link to a more detailed description of this interference you are describing? Everything I am finding in searching relates to AC circuits.

06-02-2015 16:16

Alan Anderson

Re: pic: A little teaser from 3571

Actually, we are. The motor speed controllers work by "chopping" the power at high speed, and there really is significant EM produced. It almost never causes problems for PWM signals, but it definitely affects certain analog sensor measurements.

The pseudonymous poster from team 930 seemed to be warning about high power, not about EM interference, which is why I asked for clarification.

06-02-2015 16:21

philso

Re: pic: A little teaser from 3571

We really don't have just DC in the robot power system. The nominal voltage is 12Vdc but there are fluctuations due to a number of factors. The current flowing is mostly AC due to the PWM action of the motor controllers. Every time the output transistors in one of the motor controllers turns on and applies a voltage across the motor, current flows through the motor. This current comes from the battery. When the transistor turns off, the current stops flowing. You now have AC current flowing of a significant magnitude. The current is changing at the PWM frequency which is in the kiloHertz range. When you measure the current flowing from your battery with a meter, you are typically measuring the average (or RMS, depending on your meter) value of the current. This average/RMS current is what we think about when we are considering the trip rating of the breaker. The peak current (on a microsecond time scale) is typically much higher. Please keep in mind that when I am using the term AC, I am referring to a time-varying quantity where the waveshape is arbitrary, not the (nominally) sinusoidal 60 Hertz AC voltage waveform that one finds at a wall socket.


just one article that describes magnetic field coupling
www.learnemc.com/tutorials/Magnetic_Field_Coupling/H-Field_Coupling.html

Got to go to our build site now.

07-02-2015 12:50

TheThings

Re: pic: A little teaser from 3571

After a brief discussion with a couple of our mentors, we were informed that if you have to run wires, specifically signal and PWM wires, you should run them 3 in. apart to be safe, according to their calculations.

07-02-2015 20:13

Caleb Sykes

Re: pic: A little teaser from 3571

Could you please share those calculations?

07-02-2015 21:01

Sqrgz

Re: pic: A little teaser from 3571

They were done awhile ago, so I don't believe we have them anymore.

08-02-2015 01:26

philso

Re: pic: A little teaser from 3571

https://plus.google.com/u/0/10096186...7 19407130044

This is one page from the binder we were given at a 3 day course, Introduction to Electromagnetic Interference and Compatibility (EMI/EMC) Best Practices" given at the University of Wisconsin last October.

Assuming a motor controller switches 40A in 1 usec in one wire arranged in a loop with radius of 1cm, a second wire 3 inches away (0.0762m) also arranged in a loop with a radius of 1cm would have a voltage of 4mV induced in it.