Hey guys,
Its been awhile since I have been active here on CD, I am currently working full time and taking part time college classes towards my engineering degree. Anyways, today at work my coworkers and I ended up talking about FIRST, and they are getting interested. Both of them have many, many years of experience in the electrical workforce. I was never a real integral part of my old team’s Electronics Sub-Team, but I knew enough today to talk with them about FIRST’s control systems and regulations. They are genuinely interested and frankly brought up some good questions that just left me stumped. Somewhere in our discussion they asked about the GROUNDING system inplimented, and I told them. Historically Rule R38 (2012) has kept teams from incorporating their frames from being a ground point.
They balked at me like I had just told them our company was going out of buisness. I have known many teams over the years, and it seems that some always have issues with electromagnetic interference and RF Noise, while other teams dont. These gentlemen are Electrical Engineers for a living, designing and producing automotive wiring harnesses on a daily basis for the last… few decades. They dont know the rules that well, and in all honestly while I do, the electrical sections have always been somewhat of a “Black Magic” type of thing for me. Now that I understand how exactly a control system works, I want to know…
How do other teams deal with RF Noise, and other types of interference from the motors? Wire shielding? Capacitors? Some slick grounding harness? What’s legal to do and what’s not? Along with your stories and experiences, pictures and all other resources are greatly appreciated!
Thanks for everything you all do!
Matthew Petrie
Powertrain Control Solutions
Cars are a whole different world from FIRST robots…
As far as I know, all we’ve been allowed to do to control EMI is to add a small capacitor to each motor, and route wires carefully. I suppose we could add shielding to the wires from the speed controllers to the motors, too, but I’ve never seen it done (probably because it’s not necessary).
The computers we use on the robots seem to be rather robust.
Shielding and such? As far as our team goes, we’ve just been mounting on either polycarb or last year used some HDPE (Man that stuff is heavy). As far as interference between power and signals - space. A solid 2" or so seems to be enough to solve any noise between wires. Any crossing is done at a parallel.
Grounding means a low impedance electrical connection to the really large mass the robot is ultimately sitting on. Not practical with rubber wheels. Connecting one leg of the battery to the First robot chassis is not useful in the electrical noise suppression sense.
What works for us is to keep control wiring well separated from the power wiring. Also mounting the Ethernet bridge in the clear away from the motor controllers is a good practice.
Speak engineering to them. Tell them that first implements a single point dedicated ground which uses dedicated wires as the return path instead of the robot frame. There isn’t an awful lot of noise to begin with, some of it is managed by using a twisted-pair, and we are allowed to use small capacitors for sparking motors.
Many of our systems are noise resistant to begin with, for example PWM, which doesn’t depend on the voltage but the signal timing which is inherently RF noise resistant. Our motors don’t really care what’s riding on the DC signal from a jaguar for example. If they are car guys then they should also know that we use CAN bus technology.
Compared to even a 1960s automobile our robot wiring is actually quite simple.
R-F interference from external (i.e. not from things on the robot) sources and/or other electrical devices and/or electromagnetic fields from the motors. I assume most of the COTS components would be standards certified for this kind of interference. Any custom circuits might have to take this into account, but like [CalTran] says, providing some distance between the electrical components and sources of strong EM fields would probably be sufficient. Alternately, shielding (tin foil?) tied to ground I would think would probably be effective. Running longer wire runs in twisted pairs can also help with this.
The other would be voltage transients in the power system caused by inductive properties of the motors. I assume there is some filtering being done on the digital sidecar and the analog bumper as part of the 12V->5V conversion to help with this, but somebody more familiar with the design of these devices would have to comment on specifics.
I don’t see how grounding the robot frame would help with either of these (correct me if I’m wrong). In an automobile, the frame is encompassing enough to perhaps provide some Faraday cage-like effects, but I don’t see this being the case (in general) with FRC frames - if nothing else because they’re specifically designed to use as little material as possible. There is existing discussion on Chief Delphi as to why automotive manufacturers use the chassis as ground this but FRC explicitly forbids it.
I suppose that, if we’re being completely ideal here, that would be the best kind of crossing! But, unfortunately, things don’t usually work out that way. Cross at a perpendicular, and if you can spare the space, put some distance between them when they do cross.
Marko,
In terms of noise, our systems are very small in the world of RF interference. The use of low impedance inputs and short wires makes most RF interference a non-issue. One of the worst RF environments at an FRC event occurs at the Midwest Regional. The venue is a mere 1.5 miles from one of the largest RF generators on the planet (Sears/Willis Tower) and in the opposite direction is in the path of the 10kW pulsed radar at Midway airport. Over the many years it has been held in this venue, I cannot remember any issues with RF interference. The Crio is well designed for RF environments and the shell is connected to battery common. The reason FRC rules require that the Crio be isolated is for the double fault that could occur when two robots with voltage on their frames come in electrical contact with one another. The potential is a 24 volt jolt to other electronics that may not be able to withstand the fault. This is to save teams from having to expend additional money on replacing valuable components.
Connecting one leg of the battery to the First robot chassis is not useful in the electrical noise suppression sense.