Inertial Speedometer/Dual Prop/Memsic/GPS/HC-OSD exercise

rjo_

Hi Guys,

I have a Proto board with a memsic2125 attached (now available at RadioShacks everywhere[noparse]:)[/noparse]
I am generating a graph to TV, which I then feed into an overlay board from Hitt Consulting... the HC-OSD (with a Parallax GPS attached) takes the video, which shows a graph of MPH and the most recent value·and overlays GPS data onto that and sends it to TV out.· And it·actually works... and I haven't destroyed anything for almost 3 hours.

I have two problems...

1.· There is a slow drift in the output of the memsic after it gets excited... if the average value at start up is in the minus, the drift will be toward the minus and if the average value at startup is in the plus then the drift will be toward the plus.· I think this is a change in the zero point of the memsic, but since I don't actually know what it is doing internally, I am really just guessing.· There is a pin on the memsic... which reports chip temperature... anyone know how to use it?

2.· If I use tv_text rather than graphics, then the text being generated by the OSD looks fabulous...· but I prefer graphics and the OSD text is very dim... any way to improve it?

Rich

rjo_

Just a few comments. The Memsic IS accurate to 3 places, quite an amazing little device. I handle the accuracy issue more or less correctly in the normalization step (overkill?) but then forget about it in the rest of the code. AND there are a variety of other issues that aren't being dealt with at all...yet (too many toys/not enough time) but even after I resolve the known issues, I am still going to need to know more about that extra pin.

If you want to try this in your car, wait a few seconds after you reset your Prop to give the normalization enough time to complete. On a flat surface, you should be accurate while accelerating the first time... if you accelerate at a modest pace (car tilt isn't in the mix yet[noparse]:)[/noparse] To make this practical will require some other sensors and (of course) more than one axis[noparse]:)[/noparse]

As a matter of fact, I've looked through the sensors that Parallax carries and we are one short[noparse]:)[/noparse] I'm planning to factor in all of the others[noparse]:)[/noparse]

Reality can be so complicated.

Thanks.

Rich

ILMP (I love my Propeller)

rjo_

I guess I should have gone to bed BEFORE I destroyed my nice little display from 4DSystems[noparse]:)[/noparse]

Anyway, as to problem number 2 listed in my original message, the answer is: when I generate the graphics on the Proto board... I need to color fill the areas that the OSD will be using to write text... duh[noparse]:)[/noparse]

I know this is true, because I left the system running last night and my velocity went from 26 (where I put it) to -46 MPH... filling the areas in the graphic which are overlaid by text (by the OSD), which I couldn't read last night, looks fine!

That just leaves Problem number 1.

I have two good discussions about the Memsic 2125... a N&V article and the Parallax technical document, which states that the output of the suspect pin changes by 5 millivolts per degree and suggests using LTC1298. Ok... I have an LTC1298... and I have Parallax's AppKit http://www.parallax.com/Portals/0/Downloads/docs/prod/appkit/ltc1298.pdf

It looks to me like I need 5 Volts (stable)... and I just don't know. (If I use the 5 volts on the Proto board and then do a lot of other stuff is that 5 volts really going to be stable?)

I don't suppose anyone would like to build a little board? This is a fairly basic issue... seems like a good opportunity for some refined engineering.

Any experiences?... suggestions? Code?




Rich

rjo_

By the way, my inertial speedometer is now reading -26MPH (the room is getting warmer)... so if nothing else it looks like I have a pretty good thermometer here... except it reads in MPH.

Perry

Hi Rich

the use of MEM accelerometers in INS is when there is lack of sufficient GPS signal so "dead reconing" is used until the GPS recovers from its problem ( canyon, tunnel, orientation). It has been shown that you will always get drift with a simple accelerometer solution.

the speed from the GPS is very accurate, consider using it to calibrate the "inertal Spedometer" when available.

I have made a daughter board for my "On Screen Drone" so that I can mount a triple axis accelerometer right on the Hitt's OSD.

since it is for model planes I do not worry about loss of GPS signal.

The only other project I am investigating is using the accelerometer , GPS and OSD for recordings of 1/4 mile "snow sled" drag racing.

Perry

Bean

Rich,
The brightness of the OSD is controlled by the resistors R5 and R8.
By making R5 lower and/or R8 higher SHOULD make the text brighter.

Bean.

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