**LOW** Power application

I have come accross these accelerometers

With a digital output

Or

with analogue output

The help I am getting is invaluable, but I am showing my Greennyness :-) This is fun though. I expect by weekend I will be buying my prototytping stuff

Thanks George

You are talking about a **LOW** power data logger and then you are suggesting sensors with current consumption of 1.3 to 11 mA :-). Since you need 10 of these and constantly need to monitor the acceleration, the total sensor consumption will be 13 to 110 mA.

The required operation period was 12 weeks or about 2000 hours, the charge required is 26 Ah to 220 Ah. Compensating for the voltage, we are talking about a motorcycle battery or a car battery. This is just for the 10 sensors, the microprocessor consumption would add to the energy storage requirement.

Even if the mass and volume penalty for such large battery would be acceptable, ordinary lead batteries must be kept in a horizontal orientation, while some sealed lead batteries can be used in any orientation, however, I doubt that the sealed lead batteries do not tolerate too much punishment from acceleration.

I thing that you should review the requirements and your system design again.

Paul

Wow! Those sensors are pretty durn pricey as well at $7 to $28. That could put a dent in a budget. I think a simple piezo sensor will give you decent signal level with a much lower cost and no static power drain other than the driver circuit which you can control.

Yes, MEMS are expensive indeed, especially the good ones. However a piezo does not have DC response, therefore it can't be used for the static acceleration measurement.

Vladimir Vassilevsky

DSP and Mixed Signal Design Consultant

What is your point? Do you think this application requires "static acceleration" measurement? It appears to me that they are concerned about the dynamic aspects of acceleration.

Since this is in an operating vehicle, there is no reason not to tap into the main alternator/battery. However, there are several precautions. You want to draw power from the alternator only (i.e. when vehicle is operating). Assuming that you have your own small NiCd or NiMH. You can draw a little bit of power to keep the Ni* from self-discharging while the vehicle is parked.

Charging NiMH is tricky. You need to monitor and maintain the charging cycles. When charging, you need to dump couples of amps for an hour. Perhaps you need to inform the vehicle operator not to turn off the engine for an hour.

There appears to be some problems with my usenet message delivery, since I have not seen any such messages from the OP.

Could you please provide the message-ID of that message, so I can try to check from Google, what he actually said and do my calculations accordingly ?

Paul

I don't think the OP ever said there would be a supply from the vehicle. My understanding is that this is intended for a package that will be shipped. Perhaps there was some confusion about the analogy to an air bag sensor. I don't think the OP said it WAS an air bag sensor, but rather a shipping sensor.

In that case, I would design the motion sensors with mercury switches. They draw no power when off and as little as you want when on. You can set any of the sensors to wake up the MCU (microcontroller unit). A deep sleeping AVR can last for months in couple of Ahrs.

Congratulations, you've just designed a product that can't be sold or imported in the EU. Mercury is strictly prohibited by the RoHS & WEEE regulations, which were ratified by the EU parliament in January '05. Lead is also on the list, so without special exemption you need to switch to a different solder, also.

Mercury switches are being banned state-by-state in the USA, with differing phase-out dates:

"States participating in the Interstate Mercury Education and Reduction Clearinghouse (IMERC) are coordinating their mercury reduction initiatives to:

• promote consistency among the states in implementing product bans, and

• provide a single point of contact for manufacturers.

IMERC was established by state environmental officials from Connecticut, Maine, Massachusetts, New Hampshire, New Jersey, New York, Rhode Island, and Vermont to help them implement laws and programs aimed at reducing mercury in consumer products, the waste stream, and the environment. The State of Washington joined IMERC in 2003, followed by Illinois in 2004. Minnesota and California joined IMERC in 2005. IMERC is a program of the Northeast Waste Management Officials' Association (NEWMOA). For more information on IMERC see:

--Gene

There would be no supply from the vihicle. The sensor woudl be more like a shipping sensor. It has to rely on it's own power for the length of 12 weeks.

I am expanding the specs a little. I thaught I would put this in another email and I still may.

How do I differentiate between inclination and low acceleration ?

George

You can't. Gravity and acceleration are indistinguishable.

Robert

Following up to myself. The good news is your shipment can't tell the difference either. Just how sensitive is this package?

Robert

You might be able to tell the difference if the direction of the accelleration and the orientation of the sensors is right. Gravity will be a constant 1G in one direction. Assume three orthogonal sense axes. If the vector sum of your sensor outputs changes from 1G or changes direction, you can assume that accelleration is the source of the difference. The accelleration will need to be enough that the change exceeds the thermal drift of the sensors or does not match the already measured temperature of your system. All this is adding calculation complexity that will increase your average current consumption.

Good Luck, Bob

you need a graviational model, the earth's gravitation can be defined to various levels of detail and complexity depending on the accuracy you require. this is a common problem in inertial navigation systems, and probably something you don't want to tackle

Not entirely true. As long as you are near the surface of the Earth you will feel the same amount of gravity. If you are also subjected to acceleration the total force you feel will be different in magnatude. It may take calculations that are a bit complex for this application, but it can be done.

The other thing is frequency. I don't think anyone cares if the box is tilted as long as the forces involved don't exceed some magnitude. A shock will have a higher magnitude and likely high frequency compared to a box just being tilted.

Point taken. Although there are certainly combinations of acceleration that will have the same magnitude if a different direction. You'd need to distinquish tilt separately with something like a gryoscope. I'm not sure any of those combinations are sustainable though. The main issue will be if you are concered about distinguishing tilt from accel you are into fractional G issues. Just what is that fragile? Unless of course it's really tilt that's the concern and we're back to the original observation. Tilt or accel, not really separable in effect.

I agree. For real shocks tilts are likely to be lost in the noise. On the other hand if the OP is asking about distinguishing them that suggests a concern about sustained low acceleration values. Maybe they just wandered down a blind alley of thought.

Robert

:-) Good call, although I guess a shock would have a different profile to a tilt. Shipping a box stacked with the "This way up" indicator horizontal would be different to a nock when the forklift drops it.

No, there is only one acceleration that can mimic gravity and that would be twice the force of gravity in the opposite direction. Let's face it, that would be an incredibly rare case to find. Maybe if the package were blown out of an airplane with a rocket on top pushing it down?

I agree. Likely this just needs to be considered more. Or maybe the OP is actually concerned about tilt. For example if the container holds an engine with oil, maybe tilt beyond a certain angle would cause it to spill out?