Development of Force Monitoring Transducers Using Novel ... · Mechanics, Materials Science &...

Mechanics, Materials Science & Engineering, September 2016 – ISSN 2412-5954

MMSE Journal. Open Access www.mmse.xyz

Development of Force Monitoring Transducers Using Novel Micro-

Electromechanical Sensor (MEMS)

Dimitar Chakarov1, Vladimir Stavrov2, Detelina Ignatova1, Assen Shulev1, Mihail Tsveov1, Rumen

Krastev1, Ivo Vuchkov1

1 – Institute of Mechanics, BAS, Sofia, Bulgaria

2 – AMG Technology Ltd., Botevgrad, Bulgaria

DOI 10.13140/RG.2.2.20337.48487

Keywords: mechanical, transducer, force, monitoring, MEMS, piezoresistive, sensor, experimental, data.

ABSTRACT. MEMS piezoresistive sensors are a favourable and attractive option for strain detection due to a number

of key advantages such as high sensitivity, low noise, good scaling characteristics, low cost and their ability to have the

detection electronics circuit farther away from the sensor or on the same sensing board. This paper represents the results

obtained at characterization of novel transducers to be employed into force monitoring systems. Each transducer

comprises a coherently designed novel mechanical transducer and a positional MEMS sensor with very high accuracy.

The exploited positional MEMS microsensor and the mechanical transducer are presented in this paper. The particular

MEMS sensor provides a voltage output signal having sensitivity in the range of 240 µV/µm at 1V DC voltage supply.

The range of operation of the mechanical transducer is optimized to fit the 500 µm travel range of the microsensor. A

finite element model is constructed to simulate the system structure using the commercial FE package. Two prototypes

of force transducers are described and manner of used silicon MEMS sensor attachment is demonstrated. An experimental

set-up and experimentally measured load curve are presented in the paper. Diagrams force/voltage for two prototypes at

different supply voltage 1V and 2V are revealed.

Introduction. A load cell is a transducer that converts load acting on it into an analog electrical

signal. Typically, this conversion is achieved by strain gauges which are bonded into the load cell

beam and wired into a Wheatstone bridge configuration. Strain gauge load cells dominate the

weighing industry [1].

High-performance strain sensing systems, consisting of sensors and interface electronics, are highly

desirable for advanced industrial applications, such as point-stress and torque sensing, and strain

mapping. Conventional strain sensors made from metal foils suffer from limited sensitivity, large

temperature dependence and high power consumption. Further, the metal-foil strain gauges offer

flexibility and a potential for use in this format, but they suffer low gauge factor (GF) and limited

scalability to large areas due to lack of strategies for multiplexed addressing [2]. Therefore, they are

inadequate for high performance and low power consumption applications [3] and hence other strain

sensing methods, based on the Micro Electro Mechanical Systems (MEMS) technology, have been

proposed [4, 5].

New advances in the field of Micro Electro Mechanical Systems (MEMS) have broadened

considerably the applications of these devices. MEMS technology has also enabled the

miniaturization of the devices, and a typical MEMS sensor is at least one order of magnitude smaller

compared to a conventional metal-foil strain sensor that is used to measure the same quantity.

Consequently, MEMS devices can be batch-fabricated, which offers a high potential for cost

reduction. Moreover, proper design can solve problems related to power consumption, while

providing improved performance characteristics, such as accuracy, sensitivity and resolution. Finite

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Element Analysis (FEA) provides a reliable tool to carry out the required parametric studies in order

to optimize the sensor performance [6].

Several physical sensing principles have been explored in MEMS strain sensors including the

modulation of optical, capacitive, piezoelectric, and piezoresistive properties or frequency shift [7,

8].

More particularly, MEMS piezoresistive strain sensors are more favourable and attractive due to a

number of key advantages such as high sensitivity [1], low noise, better scaling characteristics, low

cost and their ability to have the detection electronics circuit farther away from the sensor or on the

same sensing board. Moreover, they have high potential for monolithic integration with low-power

CMOS electronics. Furthermore, piezoresistive strain sensors need less complicated conditioning

circuits [9].

MEMS load sensors capable of both steady-state and dynamic measurements are generally designed

as compliant structures. The device geometry and operating voltages can be optimized for maximum

force resolution and range, subject to a number of manufacturing and electromechanical constraints

[10].

In present paper, two force measuring systems including mechanical transducer and a new silicon

MEMS position sensor with sidewall piezoresistors [11] has been studied. Thus, the performance of

the entire force monitoring systems, such as high class electronic scales, can be strongly improved by

replacing the currently employed force transducers.

MEMS position sensor for detection of 500µm range displacement.

This approach is based on elaboration of contact MEMS device for displacement detection in the

range of 500 µm [12]. The envisaged position microsensor comprises of an anchored (1) and a

moveable (2) part. Both parts are connected with a monolithic flexure (3), shown in Fig. 1 (а). The

monolithic flexure (3) comprises two pairs of differential springs and two detecting cantilevers (4),

which are also attached to point C of the springs.

(а)

(b)

Fig. 1. (a): Optical micrograph of a positional microsensor with a single anchored (1) and a single

actuated (2) part and a flexure (3) comprising two cantilevers (4) oriented in X direction; four

sidewall piezoresistors (51–54), sensitive in Y direction are built-in at the fixed ends of cantilevers;

(b): Plot of the sensor signal showing 240µV/µm V sensitivity & 500µm travel range.

The cantilevers are oriented in X direction and the move of part (2) in Y direction is transduced to a

bending of cantilevers’ (4) by differential springs. In the sidewalls of the fixed end of the both

cantilevers (4) four piezoresistors are embedded (51 - 54). In more details, each cantilever possesses

0 50 100 150 200 250 300 350 400 450 500 5500

0.02

0.04

0.06

0.08

0.1

0.12

D [um]

U [V

]

U = 0.00024 D - 0.002

data

linear fit

1 2 3

4

4

51 52

53 54




a pair of two piezoresistors, 51 -52 and 53 - 54 respectively, which are electrically connected in two

voltage dividers. Both voltage dividers are connected in a full bridge configuration, to amplify each

other, when actuated part (2) moves in Y (horizontal) direction. This particular sensor has

demonstrated a displacement sensitivity of 240 µV/µm at 1V DC voltage supply of the bridge as

plotted in Fig. 1(b), and the travel range is limited to approx. 650 µm.

Since differential springs displayed in Fig. 1(a), are mechanically instable in compression mode, this

particular flexure (3) can be used in tensile mode, only. Additionally, the devices with sidewall

embedded piezoresistors exhibit very low noise and extremely low (i.e., non-detected, at all)

temperature dependence [13]. By means of a modification of the flexure layout the sensitivity and the

travel range can be tuned to meet optimization criteria [14].

As far, the detecting cantilevers (4) with sidewall piezoresistors ensure sensor signal having above

1,000,000 of intervals in the full scale range, there is a room to achieve a ppm (part per million)

resolution of the force transducers, if optimized mechanical transducers are developed.

Since the silicon flexures are extremely fragile, auxiliary mechanical parts having package features

for fixing and protecting the both MEMS parts and providing a relative displacement in the range of

from 50µm to 1.5mm, have been developed. They could be made of different materials and the

mechanical properties could be tuned to measure in desired force range. Based on experimentally

measured results, a method of force monitoring with ppm-accuracy, independently on ambient

conditions, has been proposed [14]. Thus, the performance of the entire force monitoring systems,

such as high class electronic scales, can be strongly improved by replacing the currently employed

force transducers.

Development of a mechanical transducers for force monitoring using MEMS sensor.

The goal of present study is to develop a new high performing mechanical transducers applicable in

force monitoring systems. To monitor the forces within specified limits it is necessary to develop a

flexible transducer mechanism, which transforms the force-load to a displacement of the MEMS

sensor, and the stiffness of the transducer determines the range of the measured forces. In order to

develop the targeted high performing force monitoring systems, the design approach exploiting

flexure mechanisms [15] has been proposed. Respectively, a flexible transducer mechanism, which

transforms the applied load to an elastic strain to be detected by the position microsensor, has been

created. There were developed two types mechanical transducers.

A. Mechanical transducer type "O-ring", as illustrated in Fig.2 (a). Tensile load is applied to one

side of the "O-ring" and a support reaction occurs at the opposite side. The MEMS position sensor is

placed and fixed in the middle of the ring along the axis of the applied force, thus it monitors the

displacement directly. The constructed 3D CAD model of the mechanical transducer is shown in Fig.

2(b). Admissible deformations of MEMS sensor are determined - 0.5 mm, and the upper limit of the

measured force is specified: -1000 N.

To meet the specified range of monitored forces, it is necessary to provide a relevant stiffness of the

mechanical transducer. Commercial CAD system exploiting finite element modelling (FEM) has

been used for caring out a simulation of a static load. Computer simulations were conducted,

assuming that the load has been attached at the upper end of the transmission mechanism and the

lower end is immobilized. Load with a static force of 1000 N was simulated, using a model with

varying thickness of the transducer plate. After a series of experiments, plate with thickness of 11

mm has been selected to achieve a suitable stiffness. Steel alloy with Young’s modulus of E = 210

GPa and Yield strength of σ = 0.620 GPa has been chosen as a raw material. The screen plot with

calculated results for effective displacements is shown in Fig. 3 (a). At so selected stiffness of the

transducer, a load of 1000 N generates a displacement of 0.300 mm, which is to be measured by the

MEMS position sensor. The screen plot with effective stress is shown in Fig. 3 (b).




F

F

(а) (b)

Fig. 2. Mechanical transducer as an "O-ring" type: (a) kinematic scheme; (b) CAD model.

(а) (b)

Fig. 3. Simulations of mechanical transducer mechanism: (a) screen plot with effective

displacements, (a) screen plot with effective stress.

A prototype of the mechanical transducer has been manufactured with the help of wire electro-

discharge machining. Photos of both sides of the force transducer prototype are shown in Fig 4. Both

parts of the piezoresistive MEMS positional sensor are firmly bonded to chip-carriers which are

further fixed by screws to both loaded sides of the "O-ring", as shown in Fig.4. (a). The maximum

travel range of the transducer is constrained to 0.300 mm, by cutting into the housing a gap with same

clearance - the gap G in Fig. 4 (b), thus, limiting the monitored force-load to 1000N. This constrain

prevents mechanical overload and damage to the MEMS position sensor, as well as keeps the tensile

and bending stresses in the transducer bellow yielding stress with a safety factor of 1.2.

B. Mechanical transducer type double symmetric parallelogram mechanism has been created.

To meet the specified range of monitored forces, it is necessary to achieve the relevant stiffness of

mechanical transducer. Since, the admissible displacement of the position microsensor is 0.5 mm and

the upper limit of the measured force has been specified to 100 N, the low-stiffness mechanical

transducer mechanism, as illustrated by structure scheme in Fig. 5 (a) and by 3D CAD model shown

in Fig. 5 (b), has been designed.

The transducer mechanism is made of a solid plate titanium alloy and it was processed to obtain the

elastic joints of the mechanism. The elastic joints are double-notched elastic beam type and they

possess a number of advantages compared to contact ones, as they are free from backlash, friction,

and hysteresis. This geometry allows the desired low stiffness in one directions of joint bending and

high stiffness in the remaining non-motional direction to be simultaneously achieved. The position




microsensor is attached at the middle of the mechanism. In this way there is transmission ratio

between the displacements along the two axes of the mechanical transducer.

(а) (b)

Fig. 4. Photo of a prototype of the mechanical transducer– (a) front side with mounted MEMS

position sensor, (b) rear side.

α

h

n

l

H

N

(а) (b)

Fig. 5. Mechanical transducer mechanism: (a) kinematic scheme; (b) CAD model.

The height of the device is denoted by H, and the width of the device in the sensor area is denoted by

N (Fig. 5a). Each symmetric branch of the transmission is considered as a rectangle with two sides n

and h. According to this geometry transmission ratio k of symmetric parallelogram mechanism is

determined as:

n

h

H

N

h

nk

(1)




Commercial CAD system exploiting finite element modeling (FEM) has been used for caring out an

experiment of a static load. The constructed 3D CAD model of the mechanical transducer shown in

Fig. 5(b) has been involved for computer simulations. The titanium alloy (Ti-8Al-1Mo-1V) with

Young’s modulus of E = 120 GPa and Yield strength of σ = 0.910 GPa has been chosen as a raw

material.

The screen with calculated results for effective displacements is shown in Fig. 6. The screen with

effective stress is not shown here, but normal stress in the mostly-loaded areas did not exceed the

allowable stress.. Computer simulations were conducted, when the load has been attached at the upper

end of the transmission mechanism and the lower end has been rigidly immobilized. Load with a

static force of 100 N was simulated, using a model with varying thickness of the titanium alloy plate.

The displacements H of the force application point, the deflection N between the attachment

points of the position microsensor, and calculated transmission ratio k are reported in Table 1.

Table 1. Calculated displacements and transmission ratio for mechanical transducer at load of

100N.

Plate

thickness 9 [mm] 10 [mm] 11 [mm] 12 [mm] 13 [mm]

N [mm] 1.138 0.879 0.707 0.598 0.498

H [mm] 0.695 0.537 0.432 0.365 0.304

k 1.637 1.637 1.6365 1.638 1.638

The plate thickness of t = 11 mm has been selected for prototyping of the force transducer, for

which additional simulations during load changes were carried out. A load respectively of 90N,

70N, 50N and 30N is applied. The calculated displacements H , N and transmission ratio k are

shown in Table 2.

Table 2. Calculated displacements and transmission ratio for different loads.

Load 90 [N] 70[N] 50[N] 30[N]

N [mm] 0.636 0.494 0.353 0.212

H [mm] 0.389 0.302 0.216 0.130

k 1.635 1.636 1.634 1.631

As a result of the experiment it has been concluded that the transmission ratio is maintained constant.

The range of operation of the force transducer depends on the width of the gap that limits the

transducer displacement noted by “G” in Fig.4(b). A gap of 0.500mm has been selected for which the

displayed force transducer will work in the range of 70N. The upper limit of the measured force

specified as 70 N provides load margin which avoid position microsensor damaging.




Fig. 6. Screens with effective displacements.

A prototype of the mechanical transducer has been manufactured with the help of wire electro-

discharge machining. A photo of the force transducer prototype is shown on Fig 7. It is enabled with

piezoresistive position microsensor, shown in the zoomed image, having a travel range of 650µm

limited by the width of the gap G of 500µm, also shown in additional zoomed image.

Fig. 7. Photo of the force transducer prototype having travel range of 500µm limited by the width of

the gap G.

Experimental set-up and experimental data.

The mechanical transducer prototypes are tested for load measurements by a precise loading test

machine TIRAtest 2300. The system exploits a load cell with a measuring range of 10kN, a digital

multi-meter PeakTech 3415 connected to a computer and the prototype of force transducer, connected

on both sides with loading machine traverses.

A. Mechanical transducer "O-ring" type.

The MEMS sensor is supplied with a stabilized DC voltage supply – one test has been carried out at

1V and other test - at 2V. The output voltage of the sensor is recorded using a digital multi-meter

G




Protek D470 connected to the computer. Fig. 8 shows the experimental set-up used for load

monitoring.

Fig. 8. Photo of the experimental set-up for mechanical transducer "O-ring" type.

The testing machine loads the prototype at a constant speed by measuring both: the loading force and

the output voltage of the position sensor. To obtain the force/voltage ratio, both registered signals are

time synchronized.

Measurements are made of two identical prototypes marked as P1 and P2, at power supply of 1V and

2V and loading/unloading from 0 to 1000 N and vice versa. There are three trials made to each

prototype under loading and unloading at a speed of mobile traverse of 0.48 mm/min. The obtained

force/voltage diagrams of these experiment are shown in Fig. 9. The resulting diagrams are linear,

which can be expressed by F = aU + b, where F is loading force and U is output voltage.

Fig. 9: Diagrams force/voltage for two prototypes P1 and P2 at supply voltage of 1V and 2V.

The slope a of each diagram depends on the supply voltage. Both transducers have similar slopes at

the same supply voltage, but each transducer has a different offset b. This offset depends on the sensor

preload. The average values of coefficients a and offset voltage Uo=-b/a, at a given supply for each

prototype are as follows U=1V: a1= 21767 N/V, U01=0,0660 V, a2=22047N/V, U02=0,0315 V; U=2V:

a1=11027N/V, U01= 0,1318 V, a2=11226N/V, U02=0,0624 V. The charts show that the unloading

curve does not tally with loading chart but the hysteresis loop is fairly small. Hysteresis observed can

0

100

200

300

400

500

600

700

800

900

1000

0.000 0.050 0.100 0.150 0.200 0.250

F, N

U, V

1V, P1

2V, P1

2V, P2

1V, P2




be due to imperfections of the measuring system, i.e. miss-synchronization between the two

measurements channels when switching load/unload measurements, etc.

B. Mechanical transducer type double symmetric parallelogram mechanism.

In this experiment the MEMS sensor is supplied with a stabilized DC voltage supply of 1V and in a

similar manner the output voltage of the sensor is recorded using a digital multi-metre Protek D470

connected to the computer. Fig. 10 a) shows the experimental set-up used. There are four trials made

under loading from 0 till 70 N and unloading at a speed of mobile traverse of 0.48 mm/min. The

obtained force/voltage diagram of one trial is shown in Fig.10 b).

(a) ( b)

Fig. 10. (a) Experimental set-up and (b) Diagrams force/voltage for mechanical transducer type B

at supply voltage of 1V.

The resulting diagram is linear and the hysteresis get is fairly small. The average values of slope a

and offset voltage Uo=-b/a, at a given supply voltage of 1V have the values: a= 1119 N/V and U0=

0,0271V.

Development of a control unit for prototypes of mechanical transducers with original MEMS

sensor.

This control unit is computerized system, designed so that it is compatible with the original MEMS

sensors. It includes high-precision amplifier, that cover the functional characteristics of all

transducers. It includes also user display, setup keyboard, a channel for communication with host

computer for real time data transfer. Control unit can be used like smart transmitter. The overall look

of control unit for transducer prototypes is shown in Fig.11 (a). Presented experimental setup has

been used for calibration of the control system. The prototypes were loaded with constant speed and

both: loading force and measured force were recorded using “smart transmitter” mode. The resulting

diagrams after calibration are strictly linear. The relationship - loading force / measured force for

transducer "O-ring" type is shown on Fig. 11 (b). It can be expressed by equality: Fm = 1,0012F +

0,0085.

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

0.0200 0.0400 0.0600 0.0800 0.1000

F, N

U, V




(a) (b)

Fig. 11. (a) Photo of the control unit;(b) Relationship- loading force / measured force for

transducer "O-ring" type.

Summary. This paper represents the results obtained at characterization of novel transducers to be

employed into force monitoring systems. Each transducer comprises a coherently designed

mechanical transducer and a MEMS positional sensor with very high accuracy.

The MEMS positional micro sensor and the design of two type mechanical transducers are presented

in the paper. The range of operation of the mechanical transducer is optimized to fit the 500µm travel

range of the positional micro sensor. Respectively, the flexures’ stiffness corresponds to achieve the

maximum displacement at the upper limit of the measured load. A finite element model is constructed

to simulate the system structure using a commercial FE package. The force transducers range of

operation is limited by the width G of a gap, which also, avoids damages of the mechanical transducer

and MEMS positional sensor. Prototypes of two transducers are described and manner of usage of

the silicon MEMS position sensor attachment is demonstrated. An experimental set-up for measuring

the load curves are reported in the paper. Diagrams of force vs. sensor output voltage of the prototypes

at different supply voltage 1V and 2V are demonstrated and discussed.

As a result of the experiment it has been concluded that the force/voltage ratio is constant.

Piezoresistive MEMS position sensors can be successfully used for strain detection at force loading

of the mechanical transducer. Respectively, other than force values like: 3D strain, acceleration,

torque, temperature and other environmental parameters, as well as their combinations can be

accurately monitored, when suitable transducers are designed.

Acknowledgments. Authors gratefully acknowledge the financial supports of this work by grant 6IF-

02-13/15.12.2012 of the Bulgarian National Innovation Fund.

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Fmx10[N]




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