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Unread 10-02-2008, 14:16
morpheous115 morpheous115 is offline
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Common Ground?

I am mounting a couple of sensors and solenoids in a certain area on our robot and was wondering if we were allowed to connect all of our grounds to a few specific wires that led back to the RC?

For example: For a double solenoid I would have two powers and one common ground and for a couple of momentary switches I would have a common ground etc.

I could not find it specifically in the rules whether we were allowed or not so if somebody could help me out I would greatly appreciate it.

-morpheous




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Unread 10-02-2008, 16:50
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Re: Common Ground?

I believe that the answer to your question is yes, you can have multiple low load components grounded through a single wire. Although I cannot find anything in the rules that states this is specifically allowed, I cannot find anything that specifically disallows it, either, which means that the general rule is "is it good engineering practice?" FIRST is pretty good about this and so long as you aren't doing something foolish or dangerous (which you aren't, here) you should get through tech just fine.

Specifically, however, for your momentary switches, I assume that you will be hooking them up to the RC's digital inputs. You may as well use servo cable to connect them, and each servo cable comes with a ground line anyways.

As for the solenoid valve, I believe the set-up you describe has been used many times.

Jason

If you want an official answer, however post a question to the Q&A. That and the rule book are the only official, take-it-to-the-bank sources of information.
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Unread 10-02-2008, 18:37
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Re: Common Ground?

last year we had our spikes (i think 3) all with common ground and the inspectors had no problems with it.
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Unread 10-02-2008, 18:42
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Re: Common Ground?

Everything that feeds off of the breaker panel all returns to a common ground anyway... so as long as your wire is guaged high enough to handle the load you are fine.
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Unread 10-02-2008, 19:34
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Re: Common Ground?

Quote:
Originally Posted by tseres View Post
last year we had our spikes (i think 3) all with common ground and the inspectors had no problems with it.
Can you describe this in a little more detail please? Where was the "star" point for the spike's ground? How did you connect them together?



I do not know the wiring rules off the top of my head. However, I would strongly recommend keeping the ground star points in the same topology as IFI originally intended. As an electrical/RF engineer, I spend a lot of time carefully planning my grounding scheme.

I saw one team that used CAT5 cable (normally used for ethernet) quite elegantly for their sensor wiring. It is ok to common the grounds here because they are all going to the same star point - the RC's analog sensor port.

It is also ok (unless the rules say otherwise) to common spikes together, as they are also going to the same star point.

However, I would not be comfortable commoning a ground for a spike and a sensor. The most obvious reason is that this would couple noise into the sensor. The more dangerous reason is that you probably just bypassed some of the RC's protection circuitry.

One easy way to look at it is to consider your ground path to be a tree, with your battery in the trunk. As long as it is still a tree when you are done, you are probably good.

Does it really matter here? Probably not, but it is a bad habit to get into.
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Unread 10-02-2008, 19:47
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Re: Common Ground?

we had (i think it was 10awg) wire running from ground on the fuse panel, with the crimp-on connectors going to the grounds on the spikes. so out of all but one of the spikes, there were 2 wires crimped together on the one female connector, and they continued in a chain until the last spike.
sorry, but it's hard to describe...
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Unread 10-02-2008, 20:52
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Re: Common Ground?

Why not just follow the wiring diagram so you dont have to explain any weird looking wiring to the inspectors?

-Mike
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Unread 11-02-2008, 07:40
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Re: Common Ground?

Quote:
Originally Posted by morpheous115 View Post
I am mounting a couple of sensors and solenoids in a certain area on our robot and was wondering if we were allowed to connect all of our grounds to a few specific wires that led back to the RC?
For example: For a double solenoid I would have two powers and one common ground and for a couple of momentary switches I would have a common ground etc.
-morpheous
Morpheus,
There is no specific rule related to wire returns for solenoids except if you think about the wiring for a single solenoid. That wiring requires a pair of wires from the Spike to the solenoid. (red & black) In the Spike user's manual there is a method for connecting a dual action solenoid using both Spike outputs with a common return for the dual action solenoid only. Since First rules do not require adding breakers to the common side wiring, common electrical practice would dictate returning each solenoid on a separate wire back to the breaker panel that feeds that Spike. This prevents some teams from overloading a single #18 wire with the returns from several devices fed from several 20 amp breakers.
Remember that the breaker capacity specifications are dictated by the wire size not the load.
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Unread 11-02-2008, 11:43
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Re: Common Ground?

In many cases commoning grounds together is permissible. Sensors and Double Solenoids are the most common places and the least problematic come inspection time. I am however surprised that the inspector allowed your daisy chaining of spikes together, as I recall we've been told in the past that wasn't allowed.

Differences in inspecting, I guess.
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