Interfacing with accelerometer

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sorenriis

Joined: 04 Sep 2008
Posts: 11

04 Sep 2008 13:25 interfacing accelerometer with mux

I would like to create an interface between my accelerometer and an embedded computer.

The accelerometer is of this type:
- Measurement Specialities ACH-01: http://www.meas-spec.com/vibration/documents/datasheets/OEM_ach01.pdf

The embedded computer is this one:
- Atmel STK1000 (http://www.atmel.com/dyn/products/tools_card.asp?tool_id=3918)

Anyone who knows something about this topic?
Also i'm very interested in books, articles etc. about it.

Thanks in advance.

Added after 5 hours 17 minutes:

Another question is:
What is the easiest to interface to?

Are we talking the serial-port, LAN, USB or connecting it directly to the A/D-converter of the STK1000?

DrBob13

Joined: 26 Mar 2008
Posts: 140
Helped: 36
Location: UK

04 Sep 2008 13:30 usart ads8320

Hi sorenriis,
You need to incorporate some sort of amplifier, usually a charge amplifier is used and then digitize the AC signal that is generated for reading by your microcontroller unless the microcontroller has an integrated ADC, this link has some useful application data on your accelerometer together with a suggested interface amp.

http://www.msisensors.com.cn/manage/sensortypes/ACH_01.pdf

I am not familiar with that ATMEL microprocessor board but a quick look at its schematics suggests that it will need either a little slave microcontroller such as the Analog Devices ADuC848 (which has a built in ADC) or a bit of FPGA state machine logic to control a separate ADC and output the data as a digital stream to the USART or to the USB with a suitable interface.
Hope this helps.
Bob.

sorenriis

Joined: 04 Sep 2008
Posts: 11

05 Sep 2008 12:32 low pass filter iphone accelerometer

Thanks alot Bob.

You are right that the STK1000 does not have a integrated ADC - I was wrong on that point.

My thoughts about it right now is to interface to the SPI- or the I2C-bus. A guy told me that there aren't many decent RS232 ADC's, so I've skipped the serial-port option. But maybe thats not true?

The requirements for sampling from the accelerometere are (at least):
- samplingfrequency: 5 kHz
- resolution: 16 bit

Added after 13 minutes:

Also - actually it's not just one accelerometer I would like to interface with.
Actually I need 3 (x-,y-,z-axis), but how is that done? Smile

Again i'm very interested in references to book, articles etc.

raptor1981

Joined: 08 Jun 2008
Posts: 17
Helped: 5

05 Sep 2008 12:47 Interfacing with accelerometer

Sensors (MEMS) can not be compared with other standard blocks because sensor can not give output directly as given by analog or digital circuit. Consider the caes of of accelerometer, It can be ,odelled as a variable capacitor. Its capacitance is a function of the external force applied(assuming force feedback).Hence a analog circuit has to be used where the variable capacitor is used in negative feedback. The output of complete analog block is related with the responce of sensor.
For further details refer to uc berkley website

DrBob13

Joined: 26 Mar 2008
Posts: 140
Helped: 36
Location: UK

05 Sep 2008 13:30 Re: Interfacing with accelerometer

Hi again,
Yes I agree, either the I2C or SPI port is the best way to go to talk to your ADC device(s), I usually use SPI as it can be faster, it does however require external enable signals for each SPI slave but that is easy with a microcontroller like yours. I have not seen one with a RS232 serial interface for a while. The ADuC848 device that I mentioned earlier would not be good enough for your application as its onboard ADC it is not fast enough. You need a four channel ADC with a sampling frequency of better than 10kHz per channel. The Linear Technology LTC2487 looks like a possible candidate (with I2C interface) for your app.

As described in the previous post by raptor1981 you will need to incorporate an analog interface similar to the one in my previous post FOR EACH accelerometer. Accelerometers with the signal conditioning amps built in and even some with digital interfaces are available but these are a lot more expensive than the ones that you have identified.

Cheers.
Bob.

sorenriis

Joined: 04 Sep 2008
Posts: 11

07 Sep 2008 11:16 Re: Interfacing with accelerometer

Thanks both of you - helpful answers.

I've been trying to simulate one accelerometer and the interface you mentioned, but somehow it's not working. For some reason the output-signal is just constant.
Could you tell me what i'm doing wrong here?

I've simulated the accelerometer as a simple AC-source (sinus formed), but maybe thats not correct?

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Last edited by sorenriis on 07 Sep 2008 13:07; edited 1 time in total

sorenriis

Joined: 04 Sep 2008
Posts: 11

07 Sep 2008 12:58 Interfacing with accelerometer

Problem seems to be solved. The reason was that somehow OrCAD didn't understand "2M", but was ok with "2000k".
Screenshot is updated to match the latest changes.

So now the "only" thing I have to do is to make 3 identical acc-gains and then connect them to the A/D converter?
Also - is the I2C-bus fast enough for managing 3 accelerometers (5kHz/16bit)?

halls

Joined: 12 Nov 2007
Posts: 107
Helped: 21
Location: Basque Country

07 Sep 2008 14:01 Re: Interfacing with accelerometer

sorenriis wrote:

Also - actually it's not just one accelerometer I would like to interface with.
Actually I need 3 (x-,y-,z-axis), but how is that done? Smile

Do you have to use that exact model of accelerometer?

The thing is that you could use a 3 axis accelerometer, and so, you simplify much more your circuit.

sorenriis wrote:

The reason was that somehow OrCAD didn't understand "2M", but was ok with "2000k".

Just a little point: "2M" means "two millis" for orcad, while what you would have to have written, was "2Meg" Smile

good luck!

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07 Sep 2008 14:01 Ads

sorenriis

Joined: 04 Sep 2008
Posts: 11

07 Sep 2008 15:26 Re: Interfacing with accelerometer

halls wrote:

Just a little point: "2M" means "two millis" for orcad, while what you would have to have written, was "2Meg" Smile

Hehe... Thanks for letting me know Very Happy

I'm aware that I could use 3-axis accelerometers, but because the ACH-01 is cheap and suits our needs almost perfectly we have decided to use that one.
Also in the future we might want to add a 4th accelerometer to be used as reference.

Based on the shown diagram - is it still the 'Linear Technology LTC2487' and the I2C-bus that I should go for?

halls

Joined: 12 Nov 2007
Posts: 107
Helped: 21
Location: Basque Country

07 Sep 2008 16:00 Re: Interfacing with accelerometer

From what i've read from the datasheet, i guess that sigma-delta ADC would be a good option. Actually it seems to fit very well in your application.

sorenriis

Joined: 04 Sep 2008
Posts: 11

07 Sep 2008 16:59 Interfacing with accelerometer

halls wrote:

...i guess that sigma-delta ADC would be a good option...

Thanks. I will stick with the LTC2487 then - unless someone else comes up with something better Smile

Added after 24 minutes:

Three more questions:
1) Does someone know if it's possible to get the LTC2487 into OrCAD for simulation-purposes?
2) In the datasheet it states that the LTC2487 has a programmable gain. Isn't it better to use that (much more compact circuit) instead of making the gain myself?
3) As far as I can see I need to add some offset to the signal before the ADC (no negative values). How is that done in the best way?

sorenriis

Joined: 04 Sep 2008
Posts: 11

08 Sep 2008 9:26 Interfacing with accelerometer

One thing about the LTC2487.
As far as I can read from the datasheet, it has a maximum of 15 sps as output.

If I need to sample at least 5kHz for each accelerometer, then that seems too slow?

halls

Joined: 12 Nov 2007
Posts: 107
Helped: 21
Location: Basque Country

08 Sep 2008 10:41 Re: Interfacing with accelerometer

15 Hz would actually be very low for sampling a 5 kHz signal of course, but as I understood from the datasheet, it is just a low power mode.

On normal performance i think you could reach even 30kHz sampling frequency. Just take a look at the graphs shown on the datasheet

sorenriis

Joined: 04 Sep 2008
Posts: 11

08 Sep 2008 11:06 Re: Interfacing with accelerometer

halls wrote:

On normal performance i think you could reach even 30kHz sampling frequency. Just take a look at the graphs shown on the datasheet

Could you tell me what graph you are looking at?

I only found this in the datasheet:

Code:

Output Data Rate
When using its internal oscillator, the LTC2487 produces
up to 15 samples per second (sps) with a notch frequency of
60Hz. The actual output data rate depends upon the length
of the sleep and data output cycles which are controlled
by the user and can be made insignificantly short. When
operating with an external conversion clock (FO connected
to an external oscillator), the LTC2487 output data rate
can be increased. The duration of the conversion cycle is
41036/fEOSC. If fEOSC = 307.2kHz, the converter behaves
as if the internal oscillator is used.

An increase in fEOSC over the nominal 307.2kHz will translate
into a proportional increase in the maximum output
data rate (up to a maximum of 100sps).....

What does this in practice?

DrBob13

Joined: 26 Mar 2008
Posts: 140
Helped: 36
Location: UK

08 Sep 2008 13:41 Re: Interfacing with accelerometer

Hi Sorenriis
You are correct,
The device is definitely too slow, sorry about that, when I read the datasheet again it is much too slow to capture data even in the low Audio region, this is only suitable for slow moving sensor data.

The analog devices AD7655 looks like a better choice, this has a SPI interface and is available from stock from RS components in the UK. Analog devices are usually good performing chips for af analysis.

I hope this helps.
Bob.

halls

Joined: 12 Nov 2007
Posts: 107
Helped: 21
Location: Basque Country

08 Sep 2008 14:19 Re: Interfacing with accelerometer

Hi Soenriis,

i'm sorry for my mistake, but I'm afraid DrBob13 is right. I didn't see the "up to a maximum of 100sps" thing, and got messed with the graphs...

Analog devices is a good choice, and for my experience, I would take a look at Texas Instruments too.

Sorry again.

sorenriis

Joined: 04 Sep 2008
Posts: 11

08 Sep 2008 16:34 Interfacing with accelerometer

Thanks for the reply to both of you.

The AD7655 that I can find only has two channels?
Also it's a 48-pin - remember I might have to solder myself Wink

I've looked around myself abit and found this:
Burr-Brown ADS8341EB
Datasheet: http://focus.ti.com/lit/ds/symlink/ads8341.pdf

Could that be working?
In Denmark I can get 2 of those for the price of one AD7655.

halls

Joined: 12 Nov 2007
Posts: 107
Helped: 21
Location: Basque Country

08 Sep 2008 17:04 Re: Interfacing with accelerometer

Except for a little inconvenience, I think this is a good ADC too. The inconvenience I see is the serial interface. It is not a standard serial interface, like I2C or SPI, so you will have to implement your own code for handling the connection between the ADC and the micro.

I encourage you to take a look at this one: http://focus.ti.com/lit/ds/symlink/ads8320.pdf

This last one has the disadvantage of not having 4 channels, like the one you proposed, but in the other hand is a bit cheaper and consumes much less power (I don't know if power consumption is an issue for your application).

Hope it helps

sorenriis

Joined: 04 Sep 2008
Posts: 11

08 Sep 2008 19:48 Interfacing with accelerometer

Thanks. It also says that the ADS8320 is made for vibration analysis, which is exactly what i'm going to use it for.

The fact that it's only a 2-channel. How do I solve that issue?
Does this mean that I will have to make 2 seperate circuits?

halls

Joined: 12 Nov 2007
Posts: 107
Helped: 21
Location: Basque Country

08 Sep 2008 20:50 Re: Interfacing with accelerometer

It comes to my mind a solution where you could use an analog multiplexer to multiplex 4 accelerometer outputs into one single channel to the ADC's input. For this you would use the micro to select the sampling channel.

So, your ADC would be sampling at 20 khz (for example) which would be a 50 us period. You would switch channels every period, and when finished 4 periods (4 channels) you would start again with channel four. That would be a 200 us period for every channel, this is 5kHz.

I really haven't though much about it, so I might be missing something. I'll check it up later, now I gotta go, sorry.

equivalent

Joined: 08 Sep 2008
Posts: 7
Helped: 3

08 Sep 2008 21:31 Re: Interfacing with accelerometer

You seem to be making heavy weather of this...

sorenriis wrote:

The requirements for sampling from the accelerometere are (at least):
- samplingfrequency: 5 kHz
- resolution: 16 bit

So you'll need at least a 10ksps 16 bit a/d, for one channel (and you could use a 30 ksps a/d multiplexed around 3 channels, if you really wanted to).

halls wrote:

Quote:

From what i've read from the datasheet, i guess that sigma-delta ADC would be a good option. Actually it seems to fit very well in your application.

The trouble is that sigma-delta adc tend to be accurate, but slow. The LTC2487 mentioned earlier is about a thousand times too slow for the application, depending on whether you sample one ch or several.

Note that you can get semiconductor accelerometers which interface directly but don't quite meet your requirements (but are a whole lot closer than a factor of 1000 off). In general, semiconductor accelerometers are cheap and have integrated electronics, which allows the manufacturer to trim them in production. These are designed for things like airbags and the Wiimote/iPhone, etc.

Quote:

The accelerometer is of this type:
- Measurement Specialities ACH-01: http://www.meas-spec.com/vibration/documents/datasheets/OEM_ach01.pdf

The embedded computer is this one:
- Atmel STK1000 (http://www.atmel.com/dyn/products/tools_card.asp?tool_id=3918)

In general, any time that you design things this way around (select some of the parts without taking into account all of the specification), you'll find that the parts of the specification that you didn't take into account turn out to be problematic/expensive. So, if you were designing with an accelerometer with an analogue interface, selecting a board with a suitable ADC would have been a good idea. If, on the other hand, you were designing with a digitally interfaced accel, then having the appropriate serial interfaces would be vital.

On the other hand a AD7655 looks good; the data sheet I have seen is for a 4-ch part and can be selected for serial or parallel interface; its succesive approximation, precise enough and fast enough (good). I'm not quite clear how the serial interface works as it claims to be compatible with several different standards including SPI and I'm not sure how you select the one you want.

halls

Joined: 12 Nov 2007
Posts: 107
Helped: 21
Location: Basque Country

08 Sep 2008 21:55 Re: Interfacing with accelerometer

In my opinion the AD7655 is too big for the needs of this project, and the ADS8320 fits well in this.

I understood that the sampling frequency needed was 5 kHz (2.5 kHz max signal freq), not 10 kHz. Anyway, this ADC fits in as well.

In the other hand you should take a look at what analog multiplexers are there on the market which suit the requirements of the signal (let's talk about freqs again, because the mux might act as a low pass filter itself) and estimate the total cost. In terms of cost it might be cheaper to just use the AD7655 (remember to include hand work on the cost though).

I don't know, I guess it's up to you and your requirements.

sorenriis

Joined: 04 Sep 2008
Posts: 11

12 Sep 2008 22:25 Re: Interfacing with accelerometer

Again thanks for the answers.

Let me point out:
1) The samplingfrequency is 5kHz (max. signalfrequency is 2,5kHz)
2) I need 16 bit resolution
3) A 4-channel A/D-converter is not nessesary. If you have any good ideas for 2- or even 1-channel A/D-converters, please speak up Smile

I will then start working on a circuit based on 2 or 1 accellerometers and then expand later, if time allows it.