This paper discusses the development of a multidirectional force sensor for the investigation of the flight dynamic of a tethered fly. The proposed sensor combines the well-understood concepts of piezoresistive force sensing with a unique design that allows for the measurement of forces with more than one degree of freedom (DOF). In addition, the system has been fabricated to support the fly inside a virtual reality arena. The sensor is fabricated on a wafer-level using standard MEMS technology, By directly measuring the thrust, lift, yaw and side slip generated by the fly, complex aerodynamics mechanisms due to rapidly rotating and flapping wings can be better understood

Dickinson, Michael

Liepmann, Dorian

Nasir, Mansoor

Caltech Authors - Main

2E4.26 
MULTIDIRECTIONAL FORCE AND TORQUE SENSOR FOR INSECT FLIGHT RESEARCH 
Mansoor Nasir a, Michael Dickinson and Dorian Liepmann a 
a Dept. of Bioengineering, Berkeley Sensors and Actuator Center, University of California at Berkeley, USA 
Sioengineering, California Institute of Technology, Pasadena, CA ,USA 
e-mail: mnasir@socrates. berkeley.edu 
ABSTRACT 
This paper discusses the development of a 
multidirectional force sensor for the investigation of the 
flight dynamic of a tethered fly. The proposed sensor 
combines the well-understood concepts of  piezoresistive 
force sensing with a unique design that allows for the 
measurement of forces with more than one degree of 
freedom (DOF). In addition, the system has been 
fabricated to support the fly inside a virtual reality arena. 
The sensor is fabricated on a wafer-level using standard 
MEMS technology. By directly measuring the thrust, lift, 
yaw and side slip generated by the fly, complex 
aerodynamics mechanisms due to rapidly rotating and 
flapping wings can be better understood. 
Keywords: Force sensor, Strain gauge, Fruit fly, Flight 
aerodynamics 
lNTRODUCT10N 
Flying insects have developed sophisticated and 
unrivalled flight mechanisms permitting them a 
remarkable range of maneuvers. Fruit flies ( rasphila 
melanoguster are a model organism for biologists and for 
insect flight research as within them we find the extremes 
of organismal design. They represent a superb model 
system for elucidating general principles that govem 
complex biological systems and these principles are 
critical in the realization of any biomemtics inspired flying 
microrobotic systems. 
Flying insects incorporate unsteady aerodynamic 
principles, which are highly complex due to dynamic 
nature of forces in both time and spatial domains. Until 
now the most important tool for understanding flight 
mechanisms has been high speed videography using a 
wing beat analyzer from which wing beat frequency and 
amplitude are deduced [I] .  Yaw force measurements were 
made by optical tracking tethered fruit flies inside a 
virtual reality arena [2]. However, apart from the elaborate 
setups required for such experiments, it is difficult to get 
enough information using such methods to elucidate 
fundamental issues like the number of independent 
controls and the coupling between different orthogonal 
and rotational forces in flying insects. Measuring these 
forces directly is, therefore, critical to understanding 
different flight maneuvers and the neural, sensory and 
mechanical feedbacks that affect the flight motor control. 
MEMS technology combines integrated-circuit 
microfabrication techniques with macroscopic 
instrumentation principles, makes it possible to develop 
highly sensitive microsensor systems that are adapted to 
the requirements of biomimetic studies. 
There are many different ways used to measure small 
forces including, but not limited to pieozoresistive, 
capacitive, optical and piezoelectric sensing, with each 
having its advantages and drawbacks, Generally 
multidirectional force sensing is difficult to do using 
optical and piezoelectric methods both from design and 
fabrication point of view. Capacitive force sensing has the 
advantage of being highly sensitive but multiaxis 
combdrives have to be used to measure multi DOF forces, 
again requiring complex fabrication methods. In addition, 
the requirement for having a physical tether for the fly 
poses significant problems in all practical designs. 
Nevertheless, capacitive sensing methods have been used 
to measure the forces with one DOF (lift) generated by 
tethered fruit flies with high accuracy [3]. 
The proposed sensor uses piezoresistive sensing 
techniques to measure the flight force with greater than 
single DOF. Piezoresistivity is a material property due to 
which resistance of the material changes when it is under 
stress. Sensing techniques using this material property 
usually incorporate strain gauges located at high stress 
areas of the structures. Semiconductors strain gauges are 
up to two orders more sensitive than metal ones and are 
well-suited for bulk fabrication. Since the underlining 
theory is well understood and the sensing mechanism is 
inherently mechanical, novel designs can be used measure 
multidirectional forces with increased sensitivity. 
Strain gauge force sensors have been used extensively 
in MEMS engineering applications and recently strain 
gauge sensors have been used in biological applications 
such as the forces generated by cockroaches while 
walking and running [4j. Highly sensitivity has also been 
demonstrated using cantilever style structures IS]. 
FORCE MICROSENSOR DESIGN 
In developing a microsensor for measurement of 
flight forces, a number of requirements need to be 
satisfied. The sensor has to balance the material strength 
needed to support the weight of the h i t  fly while at the 
same time remain compliant enough to measure forces 
less than 50 N [2]. The design should also allow for easy 
tethering of the fly and placement inside the virtual flight 
arena [3]. Sensor reusability is important for reliable and 
repeatable experimental test results, 
Fig. 1 shows a solid model of the force sensor that 
uses piezoresistive strain gauges to measure the lift, thrust 
and yaw force. 
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2E4.26 
tethering 
Fig. 1 : A model of force microsensor. The fly is tethered at the 
tapered end of the L-shaped sensor. 
The novel design allows replaces the current fly tether 
and is therefore capable of simultaneously supporting the 
fly inside the fly arena and measuring the rotational and 
translational forces. The strain gauges are most frequently 
used in bridge configurations. The bridge is initially 
balanced with the strain gauges in the circuitry. When 
acted upon by a force, the resistance changes and therefore 
the bridge becomes unbalanced and an output voltage 
results, Our design has four strain gauges that are used in 
quarter bridge configuration. The wide base has contact 
pads to measure the voltage across the strain gauges. 
ANALYTICAL MODEL 
Linear beam theory can be used to find the governing 
equations defining the relation between the forces acting 
at the tip of the sensor and resulting strain across each 
strain gauges. The base of the sensor is essentially an 
anchor and therefore the forces only deform the L-shaped 
structure of the sensor. The three forces, lift ( F , ,  Thrust 
(FT) and Yaw (My) are shown to be acting at the bottom 
end of the vertical beam. With a free body diagram all the 
moments resulting from these forces can be visualized 
(Fig. 2). 
1 Mz(y) = Moment along z-axis as a fn of y 1 I: + 
+ 
Fig. 2 :  Free body diagram of the force sensor. The lengths of the 
vertical and horizontal beams are Lv and LH respectively. Right 
hand rule has been used to show the forces and moments acting 
on the ends of the beams. 
Mx(y) = Moment along x-axis as a fn of y 
My(x) = Moment along y-axis as a fn of y 
Mx(y) M4Y) 
Mz(x) = Moment along z-axis as a fn of x 
FT 
M ~ a w  FL 
I 
Using a force-moment analysis the four moments 
shown in Fig. 2 can be written for any point on the 
horizontal and vertical beams. The relationship between 
the stress 0, and moments is given by 
(1) 
bh3 where I =- M 
I 12 
s = - e  
where c is the distance from neutral fiber and I is the 
moment of inertia Using Hook's law, the strain E, at the 
four strain gauge location can related to the moments. 
These an be expressed as a matrix relation given by 
A is a (4x3) matrix given by 
cx, cy and cz are the distances of the four gauges from 
the center fiber along the x, y and z-axes respectively. 
The last two terms in the third column are for the torsion 
in the vertical beam caused due to the yaw force. Values 
for E, the young's modulus and Gr, the torsional modulus 
for silicon are given in literature to be around 160GPa and 
80GPa [7(a)]. a is the coefficient of rectangular bar whose 
value depends on the ratio of beam thickness and width 
[7(b)]. Thc moments of incrtias are given by: 
where hH and hV are the widths of the horizontal and 
vertical beams respectively while h is the thickness of the 
sensor. Finally, the strain can be related to change in 
electrical resistance by 
1 A R  
G R  (9) 
e=--  
where G is the gauge resistance the value of which 
for Si ranges from 30-150 [6]. The high gauge factor of 
semiconductors is the main reason why they are so 
sensitive. A summary of how forces affect the strain gauge 
.,resistances is shown in Table I .  
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2E4.26 
Table 1 :  The effect of increasing forces on the resistance of 
four strain gauges. By simultaneously measuring the change in 
resistances the forces can be resolved. 
MICROFABRICATION 
Bulk micromachining has been used to fabricate the 
L-shaped sensor out of single crystal silicon wafer and 
requires only three masks (Fig. 3 ) .  
Fig. 3: Microfabrication process (a) A thin oxide layer (<I  m) 
is grown an a P-type single crystal Si wafer which will act as a 
passivation layer. (b) N-doped pnly silicon layer ( 400A) is 
deposited on the wafer after oxide has been removed from the 
back side of the wafer. The polysilicon deposition step is 
followed by an annealing step to drive in the dopants. (c) 
Polysilicon strain gauges are defined using an RIE etch (Maskl). 
(d) A thick layer of Aluminum (AI) ( 1.2 m) followed by a very 
thin layer of Titanium (1100A ) i s  sputtered on the front side of 
the mask. The Ti layer helps protect the A1 leads from getting 
scratched off. (e )  The following lithographic step defines the 
leads (Mask2). The leads are etched with a combination of dry 
and wet etches. (0 After a handle bond is attached a last 
lithographic step (Mask3) is used to R E  etch oxide around the 
boundaries of the wafcr followed by a DRIE through-etch froin 
the front side. 
A pair of doped polysilicon strain gauges is 
microfabricated on each arm of the sensor with AI leads 
for measuring the voltages across them. There are also 
four strain gauges on the base of the sensor which arc 
paired up with active strain gauges in the temperature- 
compensated quarter bridge configuration. The AI leads 
are much thicker than the polysilicon layer so that the lead 
resistances are negligible as compared to the voltage 
across thc strain gauge itself. This also helps to get better 
step coverage in the overlapping region around the strain 
gauge and helps reduce the contact resistance. The leads 
connect to pads that are designed spccifically for 
wirebonding to a printed circuit board (PCB) to which the 
force sensor is glued (Fig. 4).  
Connection Pads 
! 
i 1 Tapered End- 
Fig. 4: Microfabricated force sensor with one of the strain 
gauges highlighted. SEMs of the elbow comer of the force 
sensor and the tip where the fly is eventually tethered are also 
shown. The inuch magnified view shows the AI leads 
overlapping the piezoresistive n-doped poly strain gauge. 
PROTOTYPE SENSOR CALIBRATION 
Initial testing of the sensor with static loads has been 
done. A force transducer is used to apply a known force at 
the tip of the sensor after it has been glued and 
wirebonded to the PCB and the change in'voltage is 
measured across the four strain gauges while each is held 
in a quarter bridge configuration. The output voltage of 
the bridge is amplified using a low noise instrumentation 
amplifier and high-pass filtered with a cutoff frequency of 
2kHz. The whole test setup has to be placed inside a 
closed hood to prevent the voltage fluctuations due to air 
drafts, The results for static forces show that the strain 
gauge voltages vary quite linearly with the applied force 
(Fig. 5). 
Straln Gauge Respsone Vs. Thrust Force 
1 8  
-- .-- 
Gauge 2 
1 -  
I 
-4 35 .3 ~ 2 5  -2 i s  i -05 a 
Force (mN) 
Fig. 5:  Change in voltage as a function of applied force shows a 
relatively linear relationship between the two. The force, thrust 
in this case produces a different degree of response in the strain 
gauges depending on their location on the sensor arms. Similar 
response has been measured for the lift and yaw. 
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2E4.26 
Sensor is also calibrated to for temperature drift (Fig.6). 
Change In Voltage Vs. Temperature 
I 
-h -..,,..,,- :I---- 
No compensation -., ; 
' f  
I 
24 215 2 255  ;5 2G5 27 
Temperature (C) 
-4 .:1'~~'''' 5 
Fig. 6 :  Voltages from three strain gauges of the same sensor are 
shown for compensated and uncompensated quarter bridge 
configuration. The former helps in reducing voltage fluctuations 
due to change in temperature. However even with the 
compensated bridge, temperature drift is quite significant. 
Forces as small as 100 N have been successfully 
measured by hanging small weights at the tip of the 
sensor. The dynamic response is also under investigation 
currently and initial results have showed that the 
fundamcntal frequencies of all modes are higher than the 
fly wingbeat frequencies, typically around 200Wz. 
EXPERIMENT AND DISCUSSION 
Initial experiments with tethered fruit flies were done 
to detect flight forces and also check the feasibility of the 
experimental setup. Data was collected at lOkHz (Fig 7). 
Even though the signal from the fruit flies was observable, 
the actual forces were too small to be measured with the 
sensor. 
Fry Flying 
1 
Frequency 
Fig, 7: (Left) FFT of the signal from one of the strain gauges 
when a fruit fly tethered to the sensor (Right). There is a distinct 
peak at 200Hz which is known to be the wingbeat frequency of 
the flies. The large peak at 400Hz is possibly due to interaction 
between the first harmonic and the resonant frequency of the 
bending mode the force sensor. 
The calibration data corresponds well with the results 
of the analytical model and the proof of concept has been 
verified. However, In order to measure the flight forces 
applied by the fruit fly the sensor should accurately 
measure forces smaller then 50 N with a force resolution 
of at least 0.1 N or better. Currently this range of forces is 
not detectable mainly because of voltage fluctuation 
caused by temperature changes. Semiconductor strain 
gauges have a very high thermal coefficient of resistivity 
because of high thermal conductivity and low thermal 
expansion coefficient. Design changes are being 
implemented in which strain gauges are in close proximity 
and this will insure that the strain gauge pairs of the bridge 
are isothermal. Increased sensitivity will be accomplished 
by fabricating the sensors out of thinner wafers and local 
backside etching of areas where strain gauges are 
integrated. 
CONCLUSION 
A force microsensor system has been developed. The 
novel design of the sensor allows measurement of forces 
with greater than single DOF that are being applied by a 
fly tethered to one end of the sensor. Initial testing of the 
sensor has verified the proof of concept and forces as 
small as 100 N have been successfully measured. The 
sensor will be used in tethered fly experiments inside a 
virtual LED arena and by measuring multidirectional 
forces in real time, complex aerodynamics mechanisms 
due to rapidly rotating and flapping wings can be better 
understood. 
REFERENCES 
[l] Lehmann, F.-0.  and Dickinson, M. H. The Changes in 
Power Requirements and Muscle Efficiency during 
Eleveated Force Production in the Fruit Fly Drosophila 
Melanogaster. The Joumal of Experimental Biology, 1997, 
200, p- I133 1143. 
[2] Dickinson, M. H. and Lighton, J. R. B. Muscle Efficiency 
and Elastic Storage in the Flight Motor of Drosophila 
Science, April 7Ih 1995, Vol. 268, p. 87 90. 
[3] Potassek DP, Sun Y, Fry SN and Nelson BJ. Characterizing 
fruit flv flirrht behavior using a micro force sensor with a 
new c&bcdrive configuratik. .I~ of MEMS, February 
2005, Vol. 14, p 4 
Bartsch, M. S . ,  Partridge, A., Pruitt, B. L., Full. R. J. and 
Kenny, T. W. A Three-Axis Piezoresistive Micromachined 
Force Sensor for Studying Cockroach Biomechanics. 
MEMS, ASME, 2000, Vol. 2, p.  443 
J.A. Harley and Kenny, T. W. High-sensitivity 
piezoresistive cantilevers under 1000 A thick . Applied 
Physics Letters, Vol. 75, No. 2, pp. 289 
G. T. A. Kovacs, Micromachined Transducers Sourcebook. 
Boston: WCB McGraw-Hill, 1998 (Chapter 9). 
W. C. Young, Roark s Formulas for Stress and Strain, 6th 
ed., (a) Chapter 8, (b) pg. 243, McGraw-Hill, 1989, 
11. 
448. 
291, 1999, 
ACKNOWLEDGMENTS 
Foundation for funding this project. 
The researchers would like to thank Packard 
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558 


Multidirectional force and torque sensor for insect flight research

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Multidirectional force and torque sensor for insect flight research

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