Discuss About The Interface Capacitive Sensor Designing

Metal Oxide Semiconductor transistor

Question:

Discuss About The Interface Capacitive Sensor Designing?

Creating (MOS) Metal Oxide Semiconductor transistor (one of the essential building blocks) in the current electronic devices is one of the major achievements in the field of electronic engineering in the 20^th century (Rong Wu, 2013). Over past few years, gradual transistor miniaturization has resulted in the integration of huge electronic components to very small devices in a single chip.  Fabrication technologies for these IC later improved and were used to build very small features making these ICs to be very small until they can challenge human naked eyes (Hart, 2012). The normal operation of a synchronous demodulation has been tried by constructing a prototype (a typical example) on a PCB (printed- circuit board). This is tailored for hand gesture observing for user electronic components like mobile phones. 

To enable expansion of the dynamic circuit range a new response mechanism is put to the initial circuit.  In this our case study, the circuit was designed in exactly 0.35µm CMOS mechanism and it was then tried using a humidity sensor (Sze, 2012).  The rate of power consumption is a very vital criterion in this design and it is highly monitored and it is reduced further by using an interface circuit with charge transfer mechanism. In our case study design, a converter of a capacitance to pulse width is constructed with the help of building blocks (B, 2013).  The designed circuit was then fabricated in 0.35µm by the help of the CMOS technology which makes it easier to build this circuit in a small single chip.  The resultant small chip circuit was tested with a variable MEMS capacitor and a position sensor.

This micro sensor sense variation of chemical and physical stimulus, it achieves this by detecting the change in dielectric properties or sometimes displacement properties of a solid material. A lot of consideration is take in the designing of the structure of this sensing element to determine how the stimulus impact on capacitance value. The vital concepts of the capacitive sensors are illustrated further as below;

A meek configuration of this sensor is to have two electrodes which are parallel to one another having a distance of g between them and an overlapping cross sectional area A (Huddleston, 2011). Then we obtain the capacitance of this circuit as below

Where; £₀ is taken as vacuum’s permittivity, £_r is dielectric relative permittivity between the two parallel electrodes.

Synchronous demodulation

The most common method of capacitive sensing is grounded on varying the plate’s distance of separation g. And this is easily analyzed from the fact that the separation gap is inversely proportional to the capacitance. When the impedance of the capacitor is taken the behavior is linear but the output will be nonlinear if a direct measurement of the capacitance is taken (Shoop, 2011).  Thus the direct measurement always needs further conditioning of the signal to help compensate for that reciprocal (the inverse proportionality) between the motion of the electrodes and the capacitance. The diagram of this setup can be illustrated in the figure below

 The major bottleneck of this parallel plate capacitive sensor is that it has cross sensitivity along other axes to the motion. This bottleneck can be reduced by enclosing fully the electrodes edge by one another.  The different dimensions will help to make sure that the two plates are overlapping constantly leading to a reduction of errors generated by the motion along the edges.  There is also another source of nonlinearity in the parallel plate of the sensor occurs along the edge of the two parallel plates and it is known as fringe fields.  But this no linearity is reduced by adding some guard rings to the sensor resulting in a homogenous electric field between the parallel plates. This guard ring is an additional electrode which is separated by a non-conductor (Sanson, 2011).  This insulator (nonconductor) encloses the sensing electrode in the same potential.  The following diagrams show how the field lines are distorted in guard marketing but in the sensing electrode it remains uniform;

Even if these techniques decline the nonlinear effects in this capacitive sensor, the major challenge of using these capacitive sensors with changing gaps is the bound to useful Motion's range.  The sensing range of this capacitive sensor is highly limited by the reciprocal relationship the capacitance and the motion as seen above.

This is another group of sensors which operate on the basis of variation in the overlap between the areas of the electrodes. The common cross section area of the plates is varied by a horizontal movement of one of the two plates against the other plate (Baxter, 2013). Due to the values of capacitance and the area are proportionally linear, the measured capacitance will relate to the displacement linearly. The precision of these kinds of sensors depends on the mechanical precision of the electrodes. If the electrodes’ surfaces are rough, deformed, change the distance between the electrodes then there will be the nonlinear impact on any type of measurement. The diagram below shows parallel plate capacitor with a ring of the guard.

Interface circuit with charge transfer mechanism

The application of this type of sensor can be discussed best under two basic applications.

1. Micro-scale application of capacitive sensors.
2. Macro-scale application of capacitive sensors.

These small sensors are employed in so many applications in micro and macro-devices for detecting both chemical and physical properties (Bracke, 2013). These properties include the humidity, proximity, strain, pressure, and acceleration. One of the advantages of this type of sensor is the ease of producing several such sensors at a relatively low cost.  Another vital merit of the capacitive sensor is its low power consumption. This advantage is realized since this sensor operates only on AC and they do not require any DC which makes them ideal for low power consumption.  Other major advantages of these type of sensors include the following;

1. Relative insensitivity to temperature
2. Good resolution, speed, and stability
3. They have simple structure
4. They are highly compatible with micro fabrication techniques

One major application of the capacitive sensor is to monitor the level of liquid in a container.  In this application, high sensing resolution is needed which can be given by capacitive sensing. The structure of the electrode has a long electrode and one is divided into insulated sections. The test electrode is connected at the same time to readout circuit while the rest are all earthed (connected to the ground).  The variation in the capacitance measured from each electrode the other gives info concerning the liquid level inside the tank. The following diagram shows a capacitive level detector in a liquid container.

The proximity sensor (capacitive) sense the presence the objects nearby without necessarily having the physical contact (Aezinia, 2012). Measurement of proximity includes a very big number of measurements taken from technology and science. For several practical applications, it is necessary to be capable to take a measurement of small variation in the distance between any two parts.  The figure below shows a capacitive proximity sensor in its cross sectional view.

From the above diagram of the proximity sensor, it operates on the principle of fringe capacitance which is between the electrodes having a ring shaped orientation. When an object is brought close to the field of fringing the value of the capacitance will increase (this shows the proximity of the object). It is not a must that the target object to be a conductor, it can be an insulator as well.

 In this application, the change in capacitance leads to the variation in dielectric properties, gaps, area of the material put between the two parallel electrodes. For the micro-scale applications, the capacitive sensors are usually referred to as capacitive MEMS (Microelectromechanical systems).  Pressure sensors (MEMS) works on the basis of the deflection of the membrane as pressure is applied.   In this capacitive sensor for detecting pressure, the capacitance will change due to the change in the gaps between a fixed electrode and the membrane (Aezinia, 2012). For the horizontal moving structure, the capacitive element area is highly limited due to the MEMS small thickness of between 1 µm to 100 µm.  Thus, the variation in capacitance will be small as the structure moves.

The integrated electrode is always employed in this type of application to help increase the actual area in between the electrodes. Integrated electrodes are always known as comb structure always increases the area of overlap between the two electrodes and it also helps to advance the linearity at the output.  If the top electrode moves to the fixed electrode at the lowest part the value of the capacitance will increase between the two parallel electrodes (Lindstrom, 2013).  And the value of the capacitance can be given by the following equation;

Conclusion

To summarize, the capacitive sensor a good design for this case study since it highly emphasizes on dynamic range, power consumption, an optimal resolution which are all positive in its design.  Where the synchronous demodulation offers a high resolution. The dynamic range of sensing of this type of sensor is increased by using a novel architecture (of synchronous demodulation technology). The circuit sensor was fabricated by using the CMOS technology in 0.35m. The most design criteria of this sensor are the low power consumption (Tur, 2013). The CMOS technology help to fabricate the whole circuit in a small single chip which helps to reduce the material used and also increase the production of this sensor.

References

Aezinia, F. (2012). Design of interface circuit for capacitive sensing application . Manchester: Newness.

B, J. (2013). EDN, Electrical Design News. Hull: Rogers psychology Company.

Baxter, L. K. (2013). Capacitive Sensors: Design and Applications. London: John Wiley & Sons.

Bracke, W. (2013). Ultra Low Power Capacitive Sensor Interfaces. Mumbey: Springer Science & Business Media.

Hart, J. (2012). Electronic Design. Washington DC: Hayden Publishing Company.

Huddleston, C. (2011). Intelligent Sensor Design Using the Microchip dsPIC. Amsterdam : Newnes.

J?h?, D. (2013). IEICE Transactions on Electronics. Tokyo: Institute of Electronics, Information and Communication Engineers.

Lindstrom, E. R. (2013). Proceedings of the 1987 IEEE Particle Accelerator Conference: Accelerator Engineering and Technology management. Manchester: IEEE.

Masten, M. K. (2012). Analog electronic: Acquisition, Tracking, and Pointing. Colorado: SPIE.

Rong Wu, J. H. (2013). Precision Instrumentation Amplifiers and Read-Out Integrated Circuits. Tokyo: Springer Science & Business Media,.

Sanson, L. D. (2011). CMOS technology and fabrication . accounting: Newness press.

Shoop, B. L. (2011). Photonic Analog-to-Digital Conversion. Chicago: Springer .

Sze, S. M. (2012). Semiconductor sensors. Manchester : J. Wiley.

Tur, D. (2013). Acquisition, Tracking, and Pointing. Chicago: The Society.