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Microsensor Performance and Applications

Section A

A1 (a) Define the term ‘microsensor’. [2 marks]
(b) The terms ‘accuracy’ and ‘resolution’ are often used to describe the performance of a microsensor. Clearly
distinguish the meaning of these two terms. [3 marks]

(c) All sensing materials are imperfect and exhibit nonideal behaviour as a result of inherent defects.

Describe the FIVE main categories of defect that exist and give some specific examples to illustrate your case.

(d) Four silicon strain gauges are connected in a full Wheatstone bridge arrangement. The power supply
across the bridge is 5V and a strain of 100 micro strains is applied to each gauge:

(i) Derive an expression for the output voltage of a full Wheatstone bridge, clearly stating any assumptions that you make.
(ii) Calculate the output voltage from the bridge if the gauge factor is +100.
(iii) If each strain gauge has a nominal resistance value of 10 k? with a tolerance of ±20%,
calculate the maximum DC offset voltage from the bridge. Suggest a practical way of reducing
the offset by using an additional fixed resistor.

Calculate the value of the required resistor.
A2. (a) With the aid of a diagram, describe what is meant by the terms ‘Seebeck’ and ‘Peltier’ effects. State how these effects may be exploited in microsensors. [5 marks]

(b) An automotive engineer is asked to monitor the temperature of a car engine by attaching a thermocouple to it. Suggest THREE failure mechanisms that could occur and propose suitable methods for identifying each of these.

(c) A thermistor is to be used as a temperature sensor in an automotive application. It has a resistance Ro=10 kΩ at To=273K and its resistance at its maximum operating temperature (523K) is 100Ω. Calculate the resistance of the thermistor at 323K. [5 marks]

(d) An agricultural company has developed a measurement system for monitoring the condition of crops in a remote field.

An array of sensor nodes is placed in the soil, spaced evenly at a distance of 10m between them. Each sensor node is required to measure the temperature of the soil and also its moisture content. Propose a cost-effective solution that allows continuous monitoring of the soil. You must account for the following factors:
• The lifetime of the measurement system is greater than 5 years.
• The sensor data are to be collected at a host computer located nearby.
• The system must be able to monitor the soil during day and night.
• There is no mains power supply available for the sensor nodes.

A3. (a) Define what is meant by a piezoelectric material and state why such materials are desirable as transducers.

## Microsensor Definition and Accuracy

(b) A sensor manufacturer develops a new type of piezoelectric material, which is inexpensive to produce and flexible. It is proposed that a new type of static (steady state) pressure sensor could be manufactured from this material. Is this approach feasible? Fully justify your answer.

(c) A lead zirconate titanate (PZT)-based sensor is to be used for measuring underwater acoustic signals (a hydrophone). The sensor is to be submerged in water from a boat to a depth of 10m. Describe, with the aid of a circuit diagram, how the hydrophone could be connected to the associated instrumentation panel (located on the boat) in order to avoid significant signal loss due to cable capacitance.

(d) A weighing sensor is used to measure the mass, m, of ampowder contained in a large vessel. The material is
discharged via a continuous screw-feed mechanism. A junior engineer has been asked to develop a way of measuring the rate (dm/dt) of discharge from the container. It is proposed that one way to do this is by taking successive measurements of the weight of the container (mi, mi+1, ...) at fixed periods of time, T, and calculating the discharge rate using the following equation:

Criticise the approach adopted and clearly demonstrate the limitations of this method.

B1 (a) Microsensors are a subset of MicroElectroMechanical Systems (MEMS). Identify an example microsensor
application and use this to explain the benefits of MEMS technology. Discuss how shrinking the mechanical component of your chosen sensor down in size affects its performance.

(b) Describe, with the aid of a cross sectional drawing and plan view, the fabrication process for a deposited
polysilicon piezoresistive strain gauge. Discuss what factors influence the gage factor for polysilicon and
compare the typical values with those of a single crystal silicon piezoresistor.

(c) Explain the principles of operation of an Anisotropic Magnetoresistive (AMR) sensor when used in the
saturated mode and explain how the AMR sensor could be used in this mode to monitor linear position. Use
diagrams to illustrate your answer.

B2. (a) Explain with the aid of a diagram the principle of operation of an accelerometer and what sensing mechanisms can be used to detect the motion of the inertial mass.

(b) Describe, with the aid of cross sectional drawings, the fabrication process for a single crystal silicon cantilever beam based accelerometer using a Silicon-on-Insulator (SOI) wafer. Explain how you would maximise the size of the inertial mass.

(c) Explain the operation of a MEMS gyroscope and why these are based upon vibrating rather than rotating structures. How can the MEMS gyroscope sensitivity be improved?

(d) You are required to design a sensor system that monitors rotational speed using Hall effect magnetic field microsensor.
Explain the basic operating principal of a hall sensor and describe with the aid of drawings how this could be used to measure rotational speed.

B3. (a) Sketch a triple beam tuning fork (TBTF) resonator, show its preferred resonant mode of operation and explain why this minimises structural damping.

(b) Describe how the TBTF resonator can be used to both tensile and compressive strain. List four possible
transduction mechanisms that could be used to drive the TBTF into its preferred mode of operation and detect its vibrations.

(c) Sketch a frequency versus amplitude plot of a TBTF resonator that has a resonant frequency of 132 kHz and a Q factor of 11,000. Show your calculation demonstrating the Qfactor of 11,000.

(d) Redraw the frequency versus amplitude plot showing the hard spring nonlinear effect and describe why this nonlinear behaviour occurs and how it can lead to hysteresis.

(e) A MEMS resonant strain gauge will be used to sensepressure by detecting the stress in a circular MEMS
diaphragm. Describe, with the aid of diagrams, the stress distribution across the diaphragm surface as it is pressurised and explain where the best location is for the resonant strain gauge in order to maximise sensitivity.