Re: Small low-cost embedded board for model airplane
- From: "PeteS" <PeterSmith1954@xxxxxxxxxxxxxx>
- Date: 24 Sep 2006 14:46:28 -0700
rickman wrote:
Albert Goodwill wrote:
Hello Embedded Experts,
For a real-time signal monitoring and logging application, I am loking
a small, low power and light weight microcontroller board. This board
will be carried on a remotely piloted model airplane. As the airplane
flies it will collect data from various sensors and save them to a SD
memory card as well as it will send selected data to the ground for
data logging and monitoring.
Can you recommend any embedded board which can satisfy the following
requirements?
THX,
Albert Goodwill
AlbertGoodwill @ yahoo . com
Requirements
============
* ADC (Analog to Digital Converter)
13 channel (simultaneous sample hold is desirable)
16 bits resolution
it will be used to capture 1000 samples/ second
You did not say if your signals are AC or DC. If you have AC signals
you need to specify the bandwidth, mainly the low end of the frequency
band. The sample rate sets the high end. I assume you need DC inputs.
Are they single ended or differential?
* I2C interface
* SPI interface
* 4 X UART
The only hard part of this is the 4 UARTs. That may require a separate
chip since most low size and low power MCUs only have 2 UARTs. A pair
of UARTs can be added with a small CPLD or if the bit rate is low
enough, you can bit bang it.
BTW, when you specify things like small, low power, light weight, you
have not really specified anything. Adjectives like these are not
useful in selecting boards or components. Numbers are very useful! Do
you have a size limit, a weight limit, a power limit? I expect you
will find that size and weight will be swamped by the size and weight
of the battery unless your mission time is very short. So it is likely
that the power spec is the most important. Can you provide info on the
mission time you require and how much power your sensors will be using?
You don't provide any info below on what the UARTs and SPI interfaces
will be doing. I guess the SPI might be for the SD card interface and
the UARTs might be for controlling the sensors.
* Free (or low-cost) C compiler and SW development environment
Not a problem for most MCUs as vendors can supply a code size limited
tool or open source tools can be used.
Following are the sensors to be connected to the embedded board
Sensors with analog outputs
* 3 axis accelerometers (0-5v or 0-3.3v analog outputs)
* 3 axis gyros (0-5v or 0-3.3v analog outputs)
* 3 axis magnetosensors (0-5v or 0-3.3v analog outputs)
* 2 pressure sensors (0-5v or 0-3.3v analog outputs)
* 2 current sensor (0-500mv outputs)
Can you give the accuracy needed of each type of input? An input range
of 0-5 volts is not an issue since it can be lowered using a divider.
But the low voltage of the current sensor, 0.5 volts, will require an
amp if you need the full resolution of the 16 bit converter. BTW, you
also need to spec the accuracy of the ADC. Few 16 bit converters are
16 bits accurate. This is an area where the accuracy required will
drive up the power consumption and thereby the weight of the battery.
RMP Sensor
* 0-10000 RMP sensor with 1 logic pulse per rotation
What do you need to do with this sensor, count the pulses within a
period of time or measure the time between the pulses? Most MCUs have
a timer/counter input that can help with this, but again, the range and
resolution of the signal is required. At the fastest end I believe
this works out to 6 ms per pulse which means you can use the timer to
measure the period and likely use software to catch the timer overflows
on the slow end. What is the maximum period you need to measure?
Fuel Consumption sensor
* 1 logic pulse per xx milliliter fuel flow
I assume the flow rate is not very high and these pulses will be rather
slow.
Sensors with I2C iterface
* 10 temperature sensors with I2C
What is the update rate? Do the sensors have an event output to
trigger a measurement or is it periodic?
Other Sensors
* GPS receiver (accepting DGPS correction) with RS232 interface
I have recently worked with these and there are some very nice units
available as well as some very much nicer units coming down the pike
shortly. I believe there are any number of "single chip" solutions
which means two chips, one RF and one digital. We settled on a very
small module from Fastrax while another very small module from uBlox
came in a very close second. If you really want small size, you can
use the ARM MCU inside the GPS module to do your other functions and
save the size and weight of the separate MCU. But the vendors charge a
rather stiff amount for the tools. IIRC Fastrax wants $10 large!
Please keep in mind that a GPS receiver requires an antenna and there
are positional requirements to be able to get a clear view of the
satellites.
In reality, your requirements are not all that hard to meet. It will
be important to know how much processing you need to perform or if you
just need to record the data. I expect you can do this all quite
easily with an ARM MCU with the exception of the 16 bit ADC with 13
inputs. That will require an external chip and you are not likely to
find this on a typical small board. This may have to be a custom
board, but that can be much less expensive than you may think.
I won't reiterate what others have already said, but there are extra
considerations at the A-D for starters:
What is the output impedance of the sensors? Most A-Ds require a low
impedance source for a number of reasons, which would possibly mean
adding buffers for each channel.
Then there's the current sensors. 0-500mV ? Is that the sense range at
the sensor? If so, is the measurement floating? high side? low side?
All these impact the choice of sensor and amp (high side amps are
available but you have to be careful - sometimes it's better to roll
your own with good instrumentation amps). A good amp such that you get
a transfer function of 0 - 10mA -> 1 - 4V is easy to design (Note the
offset).
As to GPS - I currently use a single BGA solution from Falcom (JP-13S)
that has all the necessary options on board (runs a SirfStar 4
chipset), but it's not particularly cheap. Going down-market a little
can save a bundle.
I *will* reiterate one thing a number of others have mentioned; the A-D
resolution. At 5V, a 16 bit converter has a nominal resolution of
76.3uV per bit where one might think the nominal quantisation error
would be about 31.15uV, but given system noise this is very hard to
achieve (it's difficult to get true 10 bit performance!) apart from
other issues common to such devices (DNL, gain error to name but two
standard specs). Indeed, to get this low an error, you would need a
very low source impedance indeed - you are starting to get close to the
thermal noise floor even at 10k bandwidth, 1k source impedance. This is
an area where you can really get burned - if you can get away with 10
bits, then do so. Most 16 bit converters have an ENOB of around 13.
So, as others have said; some real numbers would help. Low power can
mean 100mW or 50W - it depends on context.
Cheers
PeteS
.
- References:
- Small low-cost embedded board for model airplane
- From: Albert Goodwill
- Re: Small low-cost embedded board for model airplane
- From: rickman
- Small low-cost embedded board for model airplane
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