# Tag Info

20

Because the mechanical devices are much more stable than their electric counterparts. Let's compare a crystal oscillator to an LC oscillator: Crystal: Has a very high Q. According to wikipedia, a crystal oscillator has a typical Q of 10,000-1,000,000. Stable with temperature. Many crystals are specified at <50ppm over their temperature range, and ...

13

Simply because: what else would you want to use? Chip-scale packages are typically out: MEMS devices typically need reliable enclosure; a layer of lacquer typically doesn't work. flip-chip BGA plastic packages minimize area of contacting (if you can't do chip-scale), but that's often not technically viable due to the MEMS structures being exactly where the ...

11

The answers and comments you're getting are excellent of course, but I can add a little color. For what its worth, our sensorineural system uses much the same tools, and doesn't always get the answer right! We have 3D accelerometers (the otolith organs) and 3D "gyros" (angular velocitomers, the semicircular canals), and yet we suffer from all sorts of ...

11

It is not true that they are only in QFN enclosures. Look, for example, to the Farnell website and see how many types of package there are: https://pl.farnell.com/c/polprzewodniki-uklady-scalone/czujnikowe-uklady-scalone/moduly-mems

9

The main issues with dead reckoning that I found while doing a senior design project similar to yours is that an accelerometer only measures acceleration. You have to integrate once to get velocity plus a constant C. Then you have to integrate again to get position + Cx + D. That means that once you calculate position from an accelerometer's data, you end up ...

9

If you meant your question as "why do we not integrate them into a full SOC", then I am afraid I do not really answer your question below. Else: In addition to the reasons already given here is that they not just require extra steps, but compromises on steps. In other words, your MEMS part will not be as good (or as cheap) when it is integrated with CMOS ...

8

The same reason why you do not find DRAM memory inside controllers: the process steps to make these are too different from the standard CMOS process. Even adding something like OTP to a device can mean an extra 4 or 5 process steps which makes the chips more expensive. I don't know if EEPROM is just about cost effective or if they add these because ...

7

You'll also have bias in the accelerometers and noise in the gyros to deal with as well. And gravity shouldn't introduce errors in the angle measurements; on the contrary, the gravity vector provides an "absolute reference" that helps you zero out the accumulated bias of the "pitch" and "roll" angles. Yes, what you want to do is possible, but the poor ...

7

It is possible to use gravity to calibrate an accelerometer but it has drawbacks. Typically, for any accelerometer the Bias and Scale factor are calibrated. In this case the accelerometer output is measured in the +1 and -1 g positions, that is, measuring up and down. The average of those measurements is the bias, and the scale factor (in units of output ...

6

What exactly are they? The error sources include zero-offset (bias) and scale errors (which tend to vary slowly) and noise. The prices of MEMS sensors vary from less than $10 to over$1000, and the magnitude of the error terms covers a wide range, depending on the quality of the sensor. The big problem is that integration is usually required to get from ...

5

I've flagged this question to be moved to a more appropriate SE site, but (to expand on what Eugene Sh. said) the answer can be deduced from the advice given to pro mic users: Vibrations will not only reach the microphone through the mic stand. They can also enter the microphone body through the XLR connector and the cable, particularly if it is quite ...

5

In a typical pure inertial nav solution, you must cancel out the acceleration due to gravity by subtracting it out. If the vehicle's down vector is incorrect, this error appears as a lateral acceleration that is equal to $\sin \theta_{error}$ -- and for small angles is close to $\theta_{error}$ itself. So if the gyro has an offset, $\theta_{error}$ ...

4

The short answer is "not exactly". The long answer is that you can form statements such as "Given my gyroscope readings, I am 95% confident that the device has been rotated between 28 degrees and 32 degrees since my last reading". The problem is that you end up collecting data about a noisy differential equation. For an angular gyroscope measuring angular ...

4

It's not really whether inductors and capacitors can be made more precisely than a mechanical oscillator. It's whether those components can operate in a stable manner over voltage/temperature ranges. Unless you want to design all of your circuits to have a band-gap voltage reference, a thermometer, and a heating circuit to keep voltage/temperature constant, ...

3

Something that's not covered in these answers yet is your specific application, which actually has been tackled at least a dozen times before by very smart people. The two keywords here are inverse kinematics and Kalman filters. By now it should be clear what the source of errors are for your application, and how to fix them. But when dealing with sensors ...

3

You asked what else could be added. A 3 axis magnetometer should be helpful. Earth's magnetic field tends to move around substantially slower than the average user does (fortunately). Look at the fabulous MPU6000/6050 One version provides SPI & IIC interface, the other IIC only. This contains a 3 axis gyroscope + a 3 axis accelerometer plus inputs to ...

3

You are asking the wrong question. What you should do is first define what you need a accellerometer to do to measure earthquakes and any other criteria that are important to you, like possibly size, power consuption and price. This is what we call a specification. Once you have the spec, compare it to accellerometers of any technology and see what fits ...

3

Your assumption may well be right. Quite possibly the MEMS is biased to 1/2Vdd +/-20%. But the datasheet does not give any guarantees. However a couple of 0603 NP0 capacitors for decoupling and DC rejection and two 0402 resistors are not going to increase your footprint that much. EDIT1: Also the DC output may depend on your local air-pressure, and as such ...

3

The short answer is yes; they do change characteristics with time - and I have measured them to do so. On advice from the manufacturer of the device I work with (in this case, VTI, now a division of Murata), most of the ageing occurs in the first year. As far as what changes, the most obvious is bias. I have seen drift from 3mg to 10mg on the SCA3100, ...

3

The sensitivity of -46dBV means that with an incident sound level of 94dB (SPL) at 1kHz, the device's voltage output is 5mV. For the record, 94dB SPL is 1 pascal RMS of sound pressure. If you read the data sheet it also tells you that the absolute maximum level of sound it can withstand is 160 dB SPL and this is very loud. At this level of SPL it is likely ...

3

Each sensor has specific characteristics: Accelerometer: Provides detailed information about linear acceleration. Can also measure direction of gravity (partial orientation) under some circumstances. High sample rate, but subject to scale and offset errors. Integration to get velocity and position leads to drift. Gyroscope: Provides detailed information ...

3

The answer is behind the characteristics: So, this part has a minimum frequency of 8 x 0.96 = 7.68MHz and a maximum frequency of 8 x 1.04 = 8.32MHz. You measured 8.09MHz which is in guaranteed range.

3

Yes, you can use multiple sensor units to improve the data, but only if their errors — biases, scale factors, noise, etc. — are independent (uncorrelated). In that case, you can expect the RMS error to decrease in proportion to the square root of the number of units used. For example, if you have four units, the random errors will be roughly cut ...

3

MEMS gyros are not (yet) accurate (and stable) enough to do that, but laser-based gyros (FOGs and RLGs) are capable of directly measuring the 0.004166°/sec rotation of the Earth and determining the axis of that spin, thereby aligning themselves to true North. If you'd like to learn more, search on the phrase "Strapdown Gyrocompass Alignment".

3

If you are performing a wet etch you use some other liquid, usually water, to remove residual etchant. Substrates will be in some etchant bath for the required amount of time, once that time is complete they are transferred to a rinse tank to stop the etch and remove residual etch chemicals. Dry etches are done in a vacuum chamber. When the etch is ...

2

Look up something called a resistor divider. That is something that scales down a voltage with a fixed gain between 0 and 1. However, that will scale all sounds the same amount. There will be no threshold between whatever you consider loud and soft sounds. To give detials of the divider would require a more detailed question, like what the output will be ...

2

In your first figure, the root AV curve appears to follow a slope of $-0.5$ from the start of the curve and all the way up to an integration time of a few hundred s. A simple way to extract angular random walk (ARW) is to make a straight line through the part of your curve with slope of $-0.5$ and find where this line crosses $\tau = 1$s. In some cases ...

2

I found my answer in Wikipedia: Spin coating is a procedure used to deposit uniform thin films to flat substrates. Usually a small amount of coating material is applied on the center of the substrate, which is either spinning at low speed or not spinning at all. The substrate is then rotated at high speed in order to spread the coating material by ...

2

Stability wise, there are very few MEMS accelerometer designs that would have their mechanical stability affected, until of course the cantilever beams break. But that is pretty noticeable. Parametrically you will expect some shifts over time. You can see that as the beam flexes that dislocations will tend to migrate to higher stress areas, so that will ...

2

I will focus on two specific topics of the original question, I am now suspecting manufacturer's fault. Either they managed to burn the chips during the reflowing or they somehow used some quick test-run lower quality setting on PCB production. I'm just guessing. So my question is: Are there any telltale signs, that a chip might be burnt or ...

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