Please help to choose hardware

[Klon]

Hi
Im a comp sci. student, having a major problem with one of my assignments, if anyone would be able to help me a bit, id be extremely grateful.
Ive been reading quite a lot tutorials and whole robotics got me really interested, straight after this assignment im gonna buy the equipment to build the $50 robot

Ok, so lets start from the beginning. Im supposed to design (not to build, btw) a small device which would be used in suitcases to measure all accelerations that they're exposed on. A small device would have to last 7 days on battery, and store data on whatever storage. So key parts, as far as I can see it, would have to be some microcontroller, storage device (flash memory?), accelerometer, and a battery ofc. I will have to design software to program this device, so it would be able to save and forward the data.
I was thinking about 18F4550, plus additional 3axes accelerometer, but ive no idea which one :/
If it is not the forum for questions like that, please let me know
Thanks in advance

[TrickyNekro]

well... you should have posted that on electronics section but no worries....
Well.... I think a ADXL303 or something equal would be nice...
For what accelerations are we talking???

[hazzer123]

So you'll need a lot of storage i guess to capture 7 days worth of data with enough resolution to find sharp spikes. This means you could look at using an SD card (http://www.sparkfun.com/commerce/product_info.php?products_id=204).

Or maybe you could just store the interesting readings along with a timestamp? Then you could probably use a smaller, more common serial Flash or EEPROM memory.

That microcontroller is plenty powerful enough for this application, maybe too powerful, you should look into lower end ones in order to save energy and increase the battery life. A suitable PIC16F could handle this no problem.

For the accelerometer, this one looks nice - http://www.sparkfun.com/commerce/product_info.php?products_id=692

[Klon]

ok, thanks for links.
Well, i think i would use a few megabites, the only think I would have to write down is, line by line: time, acceleration and direction of an event. Eventhough it have to last for a couple of days, during a regular trip forces are used on the suitcase mostly during the airport time, in the hotel the device would have to go to sleep mode (by the way, i have no idea how to wake it up if a suitcase is pushed, does accelerometer need additional power supply, or it would be able to get energy from the main battery, without a use of controller? any idea how to solve it?)

[hazzer123]

An analogue voltage is outputted for each axis on the accelerometer. You could use the PICs comparator module to put a threshold on each of these analogue voltages, which generate an interrupt when reached.

The interrupt could wake the PIC from sleep or idle mode and you could start sampling the data. It may be too late however (you could miss the initial spike in acceleration readings) if the threshold is too high or wake up transition time is too long. Maybe running at a lower voltage and lower clockrate is a more sensible approach to conserving battery power.

[Admin]

Well, i think i would use a few megabites, the only think I would have to write down is, line by line: time, acceleration and direction of an event.

Why is it measuring acceleration? I mean, what even are you looking for?

You can have it discard any old processed data that doesn't show the desired event.

For example, lets say the event is dropping the suitcase from a height of 3 feet. Simply don't record any data until that happens. And stop recording after. The EEPROM on a microcontroller can store a lot, and doesn't erase when power turns off.