Physical Problem

The physical problem is to convert the acceleration data into the position data. As we know its formula is given like this:

Assuming the acceleration is constant over time the formula reduces to:

where are initial position and initial velocity, respectively.

Defining the Requirements Related with the Problem
The expected error is about 5 cm in 30 seconds. Using the formula above and assuming the initial velocity is 0, the error in the acceleration can be found as:

This means our resolution should be less than in order to reach 5cm error in 30 sec. Since for most of the sensors the resolution values are not available we should look at the noise densities. Noise determines the minimum resolution of the sensor and noise floor can be lowered by restricting bandwidth if the noise is Gaussian. [1]

So the required Noise density can be calculated like this:

You can see below the parking time vs. noise density for different error values.

The other very important parameters are range, frequency response, and non-linearity. Of course values can change from application to application but you can see the accepted values for navigation below.

The available sensors can be found on the web pages of producer like ADI, ST…

[1] Analog Devices FAQ 20.09.2007

 

Types of available Accelerometers

To be able to choose a suitable accelerometer, it would be nice to have a closer look at the types of accelerometers first. For more detailed information, read the reference [1].

1- Piezoresistive Devices
This type of accelerometers includes silicon piezoresistors in their suspension beam. The structure includes a support frame and the proof mass. As the support frame starts to move with respect to the proof mass due to movement of the object, the length of the suspension beams are chanced. The resistivity of the embedded piezoresistors is also changed because of this elongation. You can see one example for this type of accelerometer in Figure 1.

Figure 1: Piezoresistive Accelerometer [2]

One of the main advantages of the piezoresistive accelerometer is the simplicity of readout circuitry since it is only measuring the change in the resistance. The fabrication process and the structure are also simple which makes it easier to produce. But since this type of device usually has larger proof mass the overall sensitivity is lower and the temperature sensitivity is larger compared to the capacitive devices.

2- Capacitive Devices

When an external acceleration is driven to a capacitive accelerometer the support frame moves from its rest position and the capacitance between the proof mass and fixed conductive electrode will also change. In this type of devices we can see usually finger which increases the capacitance value. The advantages are better sensitivity, dc response, noise performance, and lover temperature sensitivity. However, capacitive accelerometers can be susceptible to electromagnetic interference (EMI). You can see a capacitive acceleration sensor in Figure 2.

Figure 2: Capacitive Accelerometer [3]

3- Tunneling Devices
As we can see from Figure 3 there is one tunneling tip, which is attached to the movable microstructure and one counterelectrode which is on the proof mass.

Figure 3: Tunneling Type Accelerometer [1]

A constant tunneling current flows from tunneling tip into the counterelectrode. “As the tip is brought sufficiently close to its counterelectrode (within a few angstroms) using electrostatic force generated by the bottom detection electrode, a tunneling current (I) is established and remains constant if the tunneling voltage (V) and distance between the tip and counterelectrode are unchanged.
Once the proof mass is displaced due to acceleration, the readout circuit responds to the change of current and adjusts the bottom detection voltage to move the proof mass back to its original position, thus maintaining a constant tunneling current.”

4- Resonant Devices
“Silicon resonant accelerometers are generally based on transferring the proof-mass inertial force to axial force on the resonant beams and hence shifting their frequency. The main advantage of resonant sensors is their direct digital output. “

5- Thermal Devices [4]
The proof mass of a thermal accelerometer is basically some gas. The gas is heated to a certain degree using a heat source in the middle of the sensor. There are some thermopiles which are placed equidistantly on all four sides of the heat source so that the temperature gradient is symmetrical. Once the sensor is moved, the heated gas also moves and the temperature gradient is changed. So the acceleration could be sensed. You can see the structure and how it is working from this figure more clearly.

Figure 4: Thermal Type Accelerometer [4]

6- Other Devices
There are some other types of accelerometer which are build using some other principles like optical, electromagnetic, and piezoelectric.

Summary
Because of the very nice properties and availability I will stick to capacitive type accelerometers and use this type of sensors in my project.

References

[1] Micromachined Inertial Sensors, Navid Yazdi, Farrokh Ayazi, and Khalil Najafi, Senior Member, IEEE

[2] http://www.multimems.com/Offer/Exp_Acceleration.htm 20.09.2007

[3]http://usami.princeton.edu/images/memsacc.png 20.09.2007

[4]MEMSic Accelerometer Fundamentals 17.08.2007

 

 

About MEMS

Since I’m studying Microsystems Engineering, I would like to write my first article about MEMS, which is a hot and developing topic.

First of all we have to give a brief description about what MEMS is. According to the memsnet.org: “Micro-Electro-Mechanical Systems (MEMS) is the integration of mechanical elements, sensors, actuators, and electronics on a common silicon substrate through micro-fabrication technology. Microelectronic integrated circuits can be thought of as the “brains” of a system and MEMS augments this decision-making capability with “eyes” and “arms”, to allow Microsystems to sense and control the environment. Sensors gather information from the environment through measuring mechanical, thermal, biological, chemical, optical, and magnetic phenomena. The electronics then process the information derived from the sensors and through some decision making capability direct the actuators to respond by moving, positioning, regulating, pumping, and filtering, thereby controlling the environment for some desired outcome or purpose.”[1]

Even we are not aware, everyday we are using some kinds of MEMS sensors. By taking a picture with your digital camera (Image stabilizer), or playing games in portable devices (Wii), or in your laptops (protection of hard disk against free fall), or in your car (active safety systems, navigation, Antitheft…). As I said before: Even we are not aware of them, they are there for our safety and comfort. Some more applications can be found here. [2]

Since the topic is very wide and it is impossible examine every MEMS related topic, I decided to choose one of them and I will try to solve a real-world problem with one of this MEMS based sensors.

My aim is to build an Inertial Measurement Unit (IMU) using MEMS accelerometer. Every week I will explain one small part of the problem.

[1]http://www.memsnet.org/mems/what-is.html 20.09.07

[2]http://www.st.com/stonline/products/technologies/mems/appli_table.htm 20.09.07