INDIAN INSTITUTE OF TECHNOLOGY ROPAR

MEL 419 MECHATRONICS

Term Paper

“HAIR TUNING: SOUND SENSOR” Submitted to: Dr. Ekta Singla (Course Instructor) Submitted by: P2009ME1074 Abhishek Ghosh P2009ME1081 Tahir Sheikh

ABSTRACT In recent times, several attempts have been made by man to mimic natural biomechanics. Natural sense organs have also been targeted. Some of these mechanisms show exceptional sensory performance. The sound sense organ in crickets comes under this category. The cricket intercepts vibrations using an array of hair strands. These hair strands oscillate in response to an incoming signal and produce electrical output for transmission to the cricket’s brain. The performance of this natural sensory organ surpasses that of the modern sound sensors with respect to sensitivity, dynamic range, frequency filtering, and selectivity. This paper brings up the recent development of a biomimetic hair flow sensor working on the same principle.

INTRODUCTION Bio mimetic hair flow sensors are being developed to act as sound sensors. These sensors work with micro level hair strands as their receptors. In addition to the resonant amplification obtained by the present day hair sensors, the new sensor aims to provide non resonant amplification. This will allow the sensors to provide selective gain and tunable filtering.

FABRICATION

Figure 1. MEMS hair flow sensors fabricated by surface micromachining and using SU-8

Lithography

The general procedure for making MEMS (Microelectromechanical systems) consists of Lithography. The hair strands for this biomimetic sensor are fabricated using a special type of lithography called SU-8 Lithography. Lithography consists of the transfer of a pattern onto a substrate by means of an etching process (figure 2). Resist lithography makes use of an irradiation source and a photosensitive polymer material to perform the pattern transfer. Selective irradiation initiates a series of photochemical processes in the resist which alter the physical and chemical properties of the exposed areas such that they can be differentiated in a subsequent image development step. Most commonly, the solubility of the film is modified by either increasing the solubility of exposed areas (yielding a positive image after develop) or decreasing the solubility to yield a negative-tone image. Development of the imaged substrate reveals a pattern on the resist layer which corresponds to the geometry of the mask.

Figure 2.

Photolithographic Steps

WORKING PRINCIPLE

The natural frequency of any system is given by

where, fn is the natural frequency k is the spring constant m is the mass of the system When the forcing frequency of the system matches with this frequency, the system tends to reach its maximum amplitude (limited by damping forces). This behaviour is shown in figure 3, fo represents the natural frequency of the system.

Figure 3.

Resonant Behaviour of system

Hence, when the incident sound frequency matches with that of the hair strand, high amplifications can be achieved. But this kind of amplification will allow the sensor to be operational in only a limited range of frequencies. The main objective of this sensor’s development is to increase this range by non-resonant amplification.

Figure 4 shows the structure of a single hair strand. The colours indicate the materials that have been used for making up the strand. A single hair strand is mounted over a variable capacitance plate. The incoming air flow variations acts as a forcing function for the system.

Figure 4. Modulating the torsional stiffness of the sensor by applying AC pump voltage to the

sensor’s capacitance The motion of a flow susceptible hair is governed by the second order differential equation:

θ rotational angle of hair flow sensor t time J moment of inertia R torsional resistance S torsional stiffness To periodic drag torque amplitude acting on the hair due to air flow ω angular frequency of air flow

The operational frequency mode for this configuration is shown in figure 5. This mode of vibration has been obtained by performing a modal simulation on a similar structure. The simulations were performed using the FEM software: Abaqus/CAE.

Figure 5. Vibration mode of hair strand (using Abaqus/CAE)

The stiffness of the hair base can be modulated by changing the capacitance of the plate on which the hair is mounted. Any change in capacitance of the plates will alter the distance between the plates and hence change the torsional stiffness of the hair. This is achieved by applying an AC voltage to the capacitative plates as shown on figure 4. This AC signal is called the pump voltage. In a nutshell,

S(t) = function (pump voltage amplitude, frequency and phase)

The torsional stiffness can be controlled as per the requirements. Under the small rotational angles normally encountered, the total torsional stiffness S(t) contains an intrinsic material-based stiffness , a time-independent softening term, and a frequency and phase-dependent softening term.

SENSOR BEHAVIOUR

Figure 6. Measurement setup for determining the sensor behaviour

In order to determine the sensor behaviour the setup shown in Figure 6 is utilized. The AC signal to the loudspeaker output is synchronised with that of the pump signal sent to the sensor. The rotational displacement of the hair strands is measured with the help of Laser Doppler Vibrometry**.

**Laser Doppler Vibrometer (LDV) works on the principle of Doppler Effect i.e. light

wave reflected from a moving object shows a frequency shift with respect to the source

wave. The laser beam from the LDV is directed at the surface of interest, and the

vibration amplitude and frequency are extracted from the Dopplerr shift of the laser beam

frequency due to the motion of the surface. The output of an LDV is generally a

continuous analog voltage that is directly proportional to the target velocity component

along the direction of the laser beam.

Figure 7 Amplitude of the periodic hair rotation angle as function of the pump voltage amplitude.

At flow frequency = pump frequency (200 Hz) The main objective of the sensor is to provide non resonant amplification. This can be achieved by synchronizing the pump frequency with the frequency of interest. Figure 7 shows the relationship between the pump voltage and the Rotational angle amplitude for the condition, flow frequency equal to the pump frequency. The phase is set to provide the maximum Rotational amplitude. As expected, increasing the pump voltage increases the amplification factor for the particular frequency. Equal frequencies for flow and pump give coherency in torque and spring softening, for which the pump phase determines whether the system will show relative amplification or attenuation. Therefore, it is possible to realize a very sharp band pass/stop filter, depending on the pump settings. Experiments show that significant amplification of the sensor’s response to the flow signal can be achieved for a suitable choice of pump parameters.

To investigate selective gain and filtering by the sensor, three frequencies were applied simultaneously to the loudspeaker and the response was captured. This response is shown in Figure 8.

Figure .8 Measured gain of 20dB for the airflow frequency component at 150Hz

Pump frequency = 150 Hz (FFT)

As seen from the figure, without any pump voltage all the frequencies registered almost equivalent amplification via the sensor. However, with the pump frequency set to 150 Hz, significant amplification can be seen for the air wave component which matches this frequency.

CONCLUSION Non-resonant parametric amplification and filtering have been demonstrated in the new biomimetic hair-based flow sensor. By selecting appropriate values for the AC pump voltage, selective gain and filtering are achieved. The responsivity to the incoming airflow can be improved by 20 dB, while having large selectivity with respect to non-matched frequency signal.

Reference: 1) Non-resonant parametric amplification in biomimetic hair flow sensors: Selective gain and tunable filtering H. Droogendijk, C. M. Bruinink, R. G. P. Sanders, and G. J. M. Krijnen 2) Physics of Directional hearing in the cricket (Gryllus assimilis) A.Michelson, A.V.Popov, B.Lewis 3) SU-8: a photo resist for high-aspect-ratio and 3D submicron lithography A del Campo and C Greiner 4) Mechanical Vibrations S.S.Rao 5) Abaqus/CAE [ Dassault Systems ] 6) Wikipedia: http://en.wikipedia.org/wiki/Laser_Doppler_vibrometer