Jul 18, 2013

A Reverse Proof Mass?

posted by Bryon Moyer

This continues both the theme of “stuff at Sensors Expo” and non-traditional approaches to common sensors. Only this time, it’s the most ubiquitous of motion sensors, the accelerometer.

Most accelerometers use some sort of “proof mass,” a piece of silicon or metal or quartz or… whatever. Inertia makes the proof mass “move” in the opposite direction of acceleration, and you can measure that apparent movement.

Memsic (whose mag sensor we just looked at), does something different. The fundamental principle of inertia is still the same, but the proof mass, well, isn’t a mass. If you’ve ever carried a helium balloon in your car, you’ve seen the effect. (I haven’t, or I haven’t been perspicacious enough to notice and remember, so I’m taking their word for it.) When you accelerate your car, you’d expect the balloon to move backwards, just like those toys and stray French fries and the dog do.

But it doesn’t. It moves forwards. Why? Because the gas is lighter than the surrounding air (even compressed in a balloon), and the heavier air moves back, displacing the balloon forwards.

Memsic exploits this same behavior by heating gas in a cavity. They use nitrogen, although that’s not really critical. The point is that, by heating the middle of the chamber, you get this “ball” of warmer gas (I keep wanting to call it a “bolus” but I’m not sure if that word would apply). This heated mass is less dense – and hence lighter – than the gas on either side of it. So when the unit accelerates, it moves not back, like a normal proof mass would do, but forward, in the direction of acceleration. It’s like the proof mass is all the non-heated gas.

By putting temperature sensors at either end of the chamber, you can detect the approach and retreat of the heated gas and use that to signal acceleration.

The benefits of this are that you don’t get any of the messiness of a normal proof mass. There are no issues of shock, vibration, resonance, or stiction. Its calibration is more stable and it has better bias stability. The main drawbacks are that it’s not particularly responsive, so you can’t do high-G shock detection. And, of course, you need power for the heater, although they say it’s not that much – you could still use this in a phone.

The primary apps they’ve seen so far are for electronic stability control in cars and high-end inclinometers.

You can find out more on their website.

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Jul 17, 2013

Am I Spoiled Yet?

posted by Bryon Moyer

From the Sensors Expo files, I saw another interesting integration from ams for use with perishable products. It’s an RFID/temperature sensor combination with some smarts. It’s a tag you put on the packaging for a specific product to monitor the temperature history of the product.

In other words, this isn’t about alerting that, “It’s getting hot in here, so… turn up the A/C!” While you can specify high and low triggers, more interestingly, the mini-system has an Arrhenius “calculator” built in. Whoever installs a specific tag for a specific product initializes the activation energy for that product. This means that the tag can literally project the lifetime and declare when the product is too old.

The key here is that there isn’t some pre-determined lifetime; it depends on how much heat the product is exposed to. If the truck carrying the product is driving through the North Dakota winter, then the temperature sensor will accumulate less hot weather, and the product will last longer. A summer drive through Arizona with a dodgy reefer unit, by contrast, will register much faster degradation. The sensor can tell the difference, and there’s no (or less) guesswork involved.

You can find more on their website.

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Jul 16, 2013

A Move Towards a Magnetic MEMS Relay

posted by Bryon Moyer

Relays might seem amongst the most mundane of components, and yet even they are getting a miniaturization upgrade with the help of MEMS technology. Of course, you might reasonably ask, since reed relays are already mechanical devices, why not simply make them smaller? And there’s a very specific reason: The reed is encased in glass, and that glass is fused onto the leads, which have rhodium or ruthenium or iridium on the contacts. That fusing process is hot, and if you shrink the relay too much, that heat gets too near the contacts and the metal melts.

So you’re stuck with something bigger than you might want. Which suggests a MEMS alternative. And the obvious first approach would be electrostatic. Which, being the obvious first approach, has been tried. And, according to Coto, it was beset with issues, most notably stiction. In case you haven’t run into stiction, it’s a situation where, in this case, some sort of cantilever or see-saw structure gets stuck in a closed position due to low-level forces at the tip. An always-closed relay isn’t of much value. Evidently, a couple of companies have already gone out of business trying this.

So Coto is working its way towards a magnetically-actuated relay. Only they’re taking it one step at a time, starting with the switch only, which they call their Redrock switch. It’s a magnetically-actuated switch, so you essentially supply the magnet separately.

Where, you might wonder, might this be useful? They listed a few examples where you would not want an integrated actuator:

  • A brake fluid level sensor in a car: you float the magnet in the fluid and put several sensors along the fluid reservoir column so that, as the magnet floats down, you can get advance warning that the level is dropping. With a non-MEMS approach, the sensors in this “ladder” need to be far apart so they respond independently – meaning you need a deeper reservoir. By using MEMS, the whole assembly can be made smaller.
  • A switch inside some assembly where gases or liquids are flowing; you can actuate the switch from outside with a magnet.
  • A “capsule endoscope” is a small diagnostic “pill” that tours your insides, but it might be 18 months or so between the time it’s shipped from the factory until it’s actually used – you don’t want the battery to die during that time. So a magnet can be put in the enclosing box, keeping a switch open. Only when the box is opened and the magnet is no longer present will the unit turn on.

The unit is a surface-mount device formed by etching a ceramic substrate using X-ray lithography, seeding with a titanium seed layer, and then plating with nickel/iron. On a separate wafer, copper walls are grown; this “housing” is placed over the switch in a vacuum with a gold/gold seal that results in a perfectly hermetic enclosure.

The actual switch consists of two metal blocks and a cantilever that is attached to one and lies over the other. The two blocks are of opposite magnetic polarity and concentrate the magnetic flux in the gap between the cantilever tip and the block below it, bringing the cantilever tip into contact when actuated. The result is directional: you have to approach from the right angle with the magnet for it to work (or, at the very least, sensitivity is greatest from certain angles).

Why might directionality help? Well, consider the case of an insulin pump where there was a switch in the electronics and a magnet in the disposable insulin reservoir. Apparently there was a case where a stray magnetic field from an electric drill caused insulin to be injected at the wrong time. So the directionality provides a measure of selectivity, helping to make sure that the switch isn’t necessarily moved from any old magnetic field.

The next step is to integrate a magnet into a full-on relay. But apparently this isn’t trivial: you have to print a big enough magnet. They’re looking at proof-of-concept for this by the end of the year, with production a couple years out (assuming it works, of course).

You can find out more on their website.

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