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Member Since 05 Feb 2011
Offline Last Active Sep 17 2014 02:27 AM

Topics I've Started

Netduino Bot

18 February 2011 - 05:08 AM

I just completed my first simple Netduino project, a radio-controlled mini-bot platform. It leverages much of the technology I developed earlier for combat robots, such as the XBee PS2 controller, which kept the Netduino portion of the project simple.

* Netduino Plus (though no +plus+ features used)
* Sparkfun XBee Shield
* Sparkfun Ardumoto Motor Driver Shield
* PS2 XBee radio controller

Both Sparkfun shields required some minor modification to make them usable with the Netduino. Those details are on my website but I'll also post them to the "Compatible Shields" thread.

Details, photos and videos can be found at my website.

I have no particular plans for this project. The experience gained will be useful as I move on to my next project of controlling AX-12 Dynamixel servos (and eventually an AX-12 robotic arm) with the Netduino and the PS2 radio controller.

Integrated Help files and documents?

08 February 2011 - 06:06 AM

I just got my Netduino Plus, and did the blinking LED thing. Very nice! So now I'm wanting to explore all the classes available in the .NET MF and the Netduino SDK and get a grasp on the APIs and general architecture of the environment. There seems to be no integrated help in the editor (in Visual Studio 2010 Pro) as pressing F1 on the class name 'OutputPort', for example, comes up blank. Am I missing something, or is this just not there? I see the '.NET Micro Framework Platform SDK' and API reference online, but it's not integrated. I see nothing at all in the way of documentation for the Netduino-specific classes. I can see classes in the Object Browser, but there's none of the embedded (e.g. intellisense) documentation I'm used to in the full .NET libraries. So just how are Netduino developers exploring this environment and learning how to use it?

AX-12 servo interface

05 February 2011 - 08:10 AM

Hello all! I've not yet received my Netduino Plus, but I'm already looking forward to my first real project after I get some experience with the basics. I have considerable experience with AVR ATmega designs and PC-based .NET/C# development. So the Netduino seems like the answer to a lot of prayers for a great micro platform!! But I'm more of a software and digital electronics type, so for some of the more troublesome linear electronics issues I seek the advice of those with greater knowledge and experience.

First let me briefly describe my project: I already have developed a PC-based .NET/C# solution for driving AX-12+ "Dynamixel" servos using a USB2Dynamixel interface. My library is featured on the CrustCrawler site where the servos are available. I also offer a sample program to operate an AX-12 Robotic Arm using a PC joystick. Since I already have an XBee module embedded in a PS2 controller from a previous combat robot project, I'm hoping that using something like the Sparkfun XBee shield should get me up and running fairly quickly with a nice wireless joystick. But there are a few interface issues that I need to resolve to drive the AX-12 servos: (these are perhaps tightly-coupled issues)

(1) interface voltage: The AX-12 is 5v while the Netduino is 3.3v. I did find a nifty little level converter at Sparkfun that just might help. The TX side of this can apparently be used in a bidirectional way.

(2) interface logic: The AX-12 uses a single-wire half-duplex serial interface, with the following suggested driver circuit:
Posted Image
I've seen 3.3v device designs (using the Propeller) that drive the one line thru a level shifter. The Propeller can apparently use the one IO pin for both transmit and receive while still achieving the necessary high baud rate.
I've seen 5v designs (using ATmega's) that tie a TX and RX pin together directly to the AX-12 and rely on careful enabling in the software so that only one is active at a time. But the need to interface a 5v device complicates just a bit.

3) interface speed: The AX-12, by default, operates at 1M bps. It looks to me like the Netduino processor at 48MHz should handle that OK with a UART function. I'm not sure if the drivers and libraries might need some tweaking for the receive buffers to keep up at that speed. But I suspect it can be done. The incoming data arrives in short and sparse packets. I do have some concerns for the level shifter operating at that frequency. The AX-12 can be configured for lower speeds, but it's not recommended.

4) interface current draw: I don't see any specs for current draw on the AX-12 signal line, but they daisy chain indefinitely and I don't know if they boost the signal from one to the next. So I'm thinking I may need some kind of beefier driver/buffer. Perhaps I should just build the circuit as above and level-shift that final IO line. Perhaps there's a device that can do it all? I've not started looking into that yet.

So that's where I'm at. I don't think any of these issues are too terribly difficult. But any advice is appreciated. I hope to make a contribution to the community and have some fun!

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