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151  Using Arduino / Project Guidance / Re: GPS accurate to cm's? on: October 28, 2012, 09:51:48 am
I think the Due will be out in volume and have some kinks worked out before an RTK system was ready for prime time if we started today.  The Arduinos are great platforms for the right task, but sometimes they are vast overkill, then I go to the ATTINYs straight from AVR Studio and some times they just do not have the capabilities.

152  Using Arduino / Networking, Protocols, and Devices / Re: Wireless sensor network on: October 27, 2012, 04:47:17 pm
No guarantee that they all pulse at the "same" time but the spec says that iff all have 4 or more sats they will all receive a PPS pulse within +/1 10's of ns.  With the sensitivities and out of band rejection available on modern GPS receivers 4 sat fixes are easy today.  If one of the remote sensor nodes only has a lock on one sat the math says the error on the PPS pulse goes up to +/- 100 ns.

When I quoted a resulting timing resolution in the microseconds that assumed 4 or more sat fixes.  When one of more node had less than 4 sats I would assign a resolution probably in the neighborhood of 10's of microseconds, I have not worked up the math entirely for that statement so it is a little back of the envelope as even 10's of microseconds is good enough for my current needs of sound millisecond resolution.

Short story long: no they do not all arrive at the "same" time, that you can not guarantee for any system there will always be some uncertainty you just have to make the decision is that level of uncertainty small enough for the measurement being made.  That is the same thing that is done anytime you make measurements, we get away in a lot of cases today because the instruments available for small amounts of money are usually orders of magnitude better than what we need so the instrument uncertainty gets swept under the rug.

The PPS signal is used quite a bit as a means of data synching if the sites do not have internet connectivity or do not want to, or cannot, rely on internet connectivity.
153  Using Arduino / Networking, Protocols, and Devices / Re: Wireless sensor network on: October 27, 2012, 11:30:03 am
I tried to do the same thing with one base and 4 remote sensor with XBee on all of them, with no common clock signal.  I got all of the nodes talking to one another and all was happy on  the network.  My times were not making sense and one of the reasons was that the packets coming out from the XBee is not deterministic and that makes the latency different with each event from the sensors.  I found that I could get time resolution of maybe 0.050 seconds but would be happier saying it is 0.10 seconds.

My timing resolution needed to be about a millisecond so I needed some kind of time base that was common to all nodes and the base so the remote sensors could report the time of the event referenced to that common time base.  The time base I am implementing now is using the pulse per second (PPS) signal coming out of the GPS, the PPS signal is attached to an interrupt, the attached ISR sets the MCU timer to zero and records the time stamp from the GPS.  When an event occurs the remote nodes send a message that such and such happened at so many clock cycles from the time stamp reported.

When the message gets to the base it can store it and time correlate to all other messages.  This should give me time resolution to about a microsecond with no problems.

154  Using Arduino / Microcontrollers / Re: Fried another two programmers :-( on: October 27, 2012, 11:11:14 am
I know you can fry a USBtinyISP if you short an output and it draws too much current, better the USBtinyISP blows than your motherboard.  I love the little USBtinyISP they are cheap and they work, too bad AVR Studio does not support them directly. 

The ATMEL AVRISP Mk II is a great programmer too, I have two on my bench, but the cost is much more than the little USBtinyISP.

Just my 2 cents,

155  Using Arduino / Project Guidance / Re: Data syncing on: October 27, 2012, 11:05:32 am
Just a caution for those looking at using the PPS to sync remote data things, not all GPS modules or GPS shields provide PPS output so look at the docs carefully.  I am using Maestro GPS 1035 and 2035 modules about $20 and $18 from Mouser they work great and the price can't be beat.  The 1035 and 2035 are surface mount units and need to either go on a breakout board of a shield to use them with Arduinos, they skillet reflow very nicely but you will need to fab a board first.

156  Using Arduino / Project Guidance / Re: GPS accurate to cm's? on: October 27, 2012, 10:55:08 am
A couple of things:

- I do not think the 8-bit Arduino will have the computational horsepower to be able to handle the required calculations in anything close to real time and memory may be a problem also.  The Due probably has the guts to do it, if you can access some of the libraries that are out there for the Arm.
- The open source Real Time Kinetics has been done published and scrutinized see some of the links from sbright33.

This stuff works I have seen it in practice and it is pretty neat for those who have the need for it.  I wonder whether SA would be turned back on if there were cheaply available RTK systems.  The combination of RTK and some of the emerging and established technologies is a scary mix for some people.

157  Using Arduino / Sensors / Re: Mouse traps on: October 26, 2012, 07:27:41 pm
What no WebCam?
158  Using Arduino / Sensors / Re: What kind of sensure pressure for extreme environment? on: October 25, 2012, 10:55:09 am
It seems that when I have looked for inexpensive pressure sensors with good performance they employed MEMs and were all surface mount, although many of the surface mount sensors from Freescale are very easy to solder by hand.  Just because the sensor says MST in the catalog do not discount it immediately look at it and then Google surface mount hand soldering or something similar.
159  Using Arduino / Project Guidance / Re: Multiple Gyro Sensors on: October 25, 2012, 10:48:16 am
You will also need pullup resistors on the SDA and SCL lines, usually somewhere between 4.7k and 10k Ohms.
160  Using Arduino / Project Guidance / Re: PID Heating Control on: October 22, 2012, 11:09:48 am
In the far past I used low wattage light bulbs to heat up small boxes, I made a couple of incubators as projects in grade school.

Put the bulb in the middles of the box for even heating but also to keep bulb away from the walls for fire safety reasons.
161  Using Arduino / Networking, Protocols, and Devices / Re: IMU RS232 communication on: October 21, 2012, 10:18:41 am
The serial monitor speed is 9600 by default so if you do not change the baudrate of the serial monitor or the Arduino you will get nonsense characters.  This is a pretty common mistake.
162  Using Arduino / Project Guidance / Re: Obtaining Magnetic Declination on: October 21, 2012, 09:47:26 am
Maybe the easiest thing to do is to get g different GPS that outputs the declination.
163  Development / Other Hardware Development / Re: Found a place for cheap pcb manufacturing on: October 20, 2012, 11:08:18 pm
I have usually gotten my boards back from Itead in a month
164  Development / Other Hardware Development / Re: Found a place for cheap pcb manufacturing on: October 17, 2012, 10:02:30 am
One debugging step I have learned the hard way is to view only layer 19 the unrouted layer.  In a complex board you may not be able to see short unrouted air traces, by only showing the unrouted layer you of course can see them clearly if they are there.

Unrouted traces can occur if you route by hand or auto so I think this is a good debugging step.

165  Using Arduino / Networking, Protocols, and Devices / Re: gps trouble shooting on: October 17, 2012, 09:52:09 am
The GPS is transmitting data to the Arduino and it is being received, the 3rd to last column on the output is the cumulative number of characters received from the GPS, the 2nd to last is the cumulative number of sentences received.

The strings of asterisks are hat symbol that routine uses for invalid data, meaning the GPS has no fix possible reasons for the lack of fix
- no clear view of the sky
- the GPS has not been on long enough for the almanac to be loaded, the almanac gives it an idea what sats in that geographic area and time should be visible
- bad antenna connection
- bad GPS unit

Focus on the first and give it up to 30 minutes for it to get its cold fix.

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