MEMS Devices

Micro-electro-mechanical Systems (MEMS) Technology is one of the most advanced technologies that have been applied in the making of most of the modern devices like video projectors, bi-analysis chips and also car crash airbag sensors. This concept was first explained by Professor R. Howe in the year 1989. Since then many prototypes have been released and revised and has thus become an integral part of the latest mechanical products available in the market today.

During its early stage, the MEMS chip had two parts. One part included the main structure of the chip and the other part included everything needed for signal conditioning. This method was not successful as the total space taken by the device was larger, and thus the different parts of a single chip needed multi-assembling procedures. The output obtained from such a device had less accuracy and the mounting of such a device was difficult.

As the technology became more advanced the idea of integrating multi-chips was applied on to produce a single chip MEMS with high performance and accuracy.

The main idea behind this technology is to use some of the basic mechanical devices like cantilevers and membranes to have the same qualities of electronic circuits. To obtain such a concept, micro-fabrication process must be carried out. Though an electronic process is carried out, an MEMS device cannot be called as an electronic circuit. MEMS duplicate a mechanical part and have holes, cantilevers, membranes, channels, and so on. But an electronic circuit has a firm and compact structure. To make MEMS from silicon process, the manufacturer must have a deep knowledge in electronics, mechanical and also about the materials used for the process.

Advantages

1. MEMS device are very small and can be applicable for many mechanical purposes where large measurements are needed.
2. The small size of the device has also helped in reducing its cost.
3. If two or three different devices are needed to deploy a particular process, all of them can be easily integrated in an MEMS chip with the help of microelectronics. Thus, data reception, filtering, storing, transfer, interfacing, and all other processes can be carried out with a single chip.

Applications

1. The device is highly applicable as an accelerometer, and thus can be deployed as airbag sensors or in digital cameras in order to stabilize the image.
2. Can be used as a pressure sensor so as to calculate the pressure difference in blood, manifold pressure (MAP), and also tire pressure.
3. It is commonly used in a gyroscope, DNA chips and also inkjet printer nozzle.
4. Optical MEMS is used for making projectors, optical fiber switch and so on.
5. RFMEMS is used for making antennas, filters, switches, relays, RAM’s microphones, microphones, and so on.

MEMS Accelerometer

An accelerometer is an electromechanical device that is used to measure acceleration and the force producing it. Many types of accelerometers are available in the market today. They can be divided according to the force (static or dynamic) that is to be measured. Even today, one of the most commonly used one is the piezoelectric accelerometer. But, since they are bulky and cannot be used for all operations, a smaller and highly functional device like the MEMS accelerometer was developed. Though the first of its kind was developed 25 years ago, it was not accepted until lately, when there was need for large volume industrial applications. Due to its small size and robust sensing feature, they are further developed to obtain multi-axis sensing.

Working

One of the most commonly used MEMS accelerometer is the capacitive type. The capacitive MEMS accelerometer is famous for its high sensitivity and its accuracy at high temperatures. The device does not change values depending on the base materials used and depends only on the capacitive value that occurs due to the change in distance between the plates.

If two plates are kept parallel to each other and are separated by a distance‘d’, and if ‘E’ is the permitivity of the separating material, then capacitance produced can be written as

C₀ = E₀.E A/d = E_A/d

E_A = E₀EA

A – Area of the electrodes

A change in the values of E, A or d will help in finding the change in capacitance and thus helps in the working of the MEMS transducer. Accelerometer values mainly depend on the change of values of d or A.

A typical MEMS accelerometer is shown in the figure below. It can also be called a simple one-axis accelerometer. If more sets of capacitors are kept in 90 degrees to each other you can design 2 or 3-axis accelerometer. A simple MEMS transducer mainly consists of a movable microstructure or a proof mass that is connected to a mechanical suspension system and thus on to a reference frame.

The movable plates and the fixed outer plates act as the capacitor plates. When acceleration is applied, the proof mass moves accordingly. This produces a capacitance between the movable and the fixed outer plates.

When acceleration is applied, the distance between the two plates displace as X1 and X2, and they turn out to be a function of the capacitance produced.

From the image above it is clear that all sensors have multiple capacitor sets. All upper capacitors are wired parallel to produce an overall capacitance C1 and the lower ones produce an overall capacitance of C2.

If V_x is the output voltage of the proof mass, and V₀  is the output voltage produced between the plates, then

(V_x +V₀) C1 + (V_x -V₀) C2 = 0

We can also write

V_x =V₀ [(C2-C1)/(C2+C1)] = (x/d) V₀

The figure below shows the circuit that is used to calculate the acceleration, through change in distance between capacitor plates. The output obtained for different values of acceleration is also shown graphically.

When no acceleration is given (a=0), the output voltage will also be zero.

When acceleration is given, such as (a>0), the value of value of V_x changes in proportion to the value of V₀.

When a deceleration is given, such as (a<0), the signals V_x and V_y become negative. He demodulator produces an output equal to the sign of the acceleration, as it multiplies both the values of V_y and V₀ to produce V_OUT, which has the correct acceleration sign and correct amplitude.

The length of the distance, d and the proof mass weigh is surprisingly very small. The proof mass weighs no more than 0.1 microgram and the output capacitance is approximately 20 aF and the plate distance is no more than 1.3 micrometers.

We must select the device in reference to its noise characteristics. If the acceleration value at low gravity condition is to be found out, the noise characteristics could easily affect its accuracy. An MEMS accelerometer is said to have three noise producing parameters – from the signal conditioning circuit, from the vibrations produced by the springs, and from the output measuring system.

MEMS Accelerometers – Applications

1. MEMS sensors are being used in latest mobile phones and gaming joysticks as step counters, user interface control, and also for switching between different modes.
2. Used in mobile cameras as a tilt sensor so as to tag the orientation of photos taken.
3. To provide stability of images in camcorders and also to rotate the image to and fro when you turn the mobile.
4. A 3D accelerometer is used in Nokia 5500 so as to provide easier tap and change feature by which you can change mp3’s by tapping on the phone when it is lying inside the pocket.
5. Used to protect hard disk drives in laptops from getting damaged when the PC falls to the ground. The device senses the free fall and automatically switches off the hard disk.
6. Used in car crash airbag sensors, where it senses the sudden negative acceleration and determines the correct time to open the airbag.
7. Used in real-time applications like military monitoring, missile launching, projectiles, and so on.

What is an Accelerometer?

An accelerometer is a transducer that is used to measure the physical or measurable acceleration that is made by an object. Co-ordinate acceleration cannot be measured using this device. (Refer: Acceleration Transducer)

Accelerometer

An accelerometer is an electro-mechanical device that is used to measure the specific force of an object, a force obtained due to the phenomenon of weight exerted by an object that is kept in the frame of reference of the accelerometer.

In the case of static acceleration, the device is mainly used to find the degrees at which an object is tilted with respect to the ground. In dynamic acceleration, the movement of the object can be foreseen.

Working

The most commonly used device is the piezoelectric accelerometer. As the name suggests, it uses the principle of piezoelectric effect. The device consists of a piezoelectric quartz crystal on which an accelerative force, whose value is to be measured, is applied.

Due to the special self-generating property, the crystal produces a voltage that is proportional to the accelerative force. The working and the basic arrangement is shown in the figure below.

As the device finds its application as a highly accurate vibration measuring device, it is also called a vibrating sensor. Vibration sensors are used for the measurement of vibration in bearings of heavy equipment and pressure lines. The piezoelectric accelerometer can be classified into two. They are high impedance output accelerometer and low impedance accelerometer.

In the case of high impedance device, the output voltage generated in proportion to the acceleration is given as the input to other measuring instruments. The instrumentation process and signal conditioning of the output is considered high and thus a low impedance device cannot be of any use for this application. This device is used at temperatures greater than 120 degree Celsius.

The low impedance accelerometer produces a current due to the output voltage generated and this charge is given to a FET transistor so as to convert the charge into a low impedance output. This type of accelerometers is most commonly used in industrial applications.

Types

In industrial applications, the most commonly used components to convert the mechanical action into its corresponding electrical output signal are piezoelectric, piezoresistive and capacitive in nature. Piezoelectric devices are more preferred in cases where it is to be used in very high temperatures, easy mounting and also high frequency rang e up to 100 kilohertz. Piezoresistive devices are used in sudden and extreme vibrating applications. Capacitive accelerometers are preferred in applications such as a silicon-micro machined sensor material and can operate in frequencies up to 1 kilohertz. All these devices are known to have very high stability and linearity.

Nowadays, a new type of accelerometer called the Micro Electro-Mechanical System (MEMS) Accelerometer is being used as it is simple, reliable and highly cost effective. It consists of a cantilever beam along with a seismic mass which deflects due to an applied acceleration. This deflection is measured using analog or digital techniques and will be a measure of the acceleration applied.

Accelerometer Specifications

1. Frequency Response – This parameter can be found out by analyzing the properties of the quartz crystal used and also the resonance frequency of the device.
2. Accelerometer Grounding – Grounding can be in two modes. One is called the Case Grounded Accelerometer which has the low side of the signal connected to their core. This device is susceptible to ground noise. Ground Isolation Accelerometer refers to the electrical device kept away from the case. Such a device is prone to ground produced noise.
3. Resonant Frequency – It should be noted that the resonant frequency should be always higher the the frequency response.
4. Temperature of Operation – An accelerometer has a temperature range between -50 degree Celsius to 120 degree Celsius. This range can be obtained only by accurate installment of the device.
5. Sensitivity – The device must be designed in such a way that it has higher sensitivity. That is, even for a small accelerative force, the electrical output signal should be very high. Thus a high signal can be measured easily and is sure to be accurate.
6. Axis – Most of the industrial applications requires only a 2-axis accelerometer. But if you want to go for 3D positioning, a 3-axis accelerometer will be needed.
7. Analog/Digital Output – You must take special care in choosing the type of output for the device. Analog output will be in the form of small changing voltages and digital output will be in PWM mode.

Device Selection

Selection of the device depends on the following factors.

1. Selection depends on the range of frequency you need.
2. Depends on the size and shape of the object whose acceleration is to be measured.
3. Whether the measurement environment is dirty or clean.
4. Depends on the range of vibration that is to be measured.
5. Depends on the range of temperature in which the device will have to work.
6. Depends on whether the device is to be case grounded or grounded isolated.

Applications

1. Machine monitoring.
2. Used to measure earthquake activity and aftershocks.
3. Used in measuring the depth of CPR chest compression.
4. Used in Internal Navigation System (INS). That is, measuring the position, orientation, and velocity of an object in motion without the use of any external reference.
5. Used in airbag shooting in cars and vehicle stability control.
6. Used in video games like PlayStation 3, so as to make the steering more controlled, natural and real.
7. Used in camcorder to make images stable.