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EMS Electro-thermo-mechanical analysis of MEMS Micro-gripper

Used Tools:

Introduction

MEMS based micro-grippers reveals excellent flexibility and adaptability in miniaturization devices in various engineering applications, such as micromanipulations, micro assemblies, etc...

The actuating integrated micro-grippers are the subject of several multi physics analyses. Such analyses are used to study the micro grippers mechanical manipulation under a low power consumption.

The studied micro-gripper (Figure 1) consists of two gripping tips attached to two U-shaped actuators. The device is used to hold micro objects by deflecting its both arms under an applied DC voltage.

The studied micro-gripper holdding a ball between its both tips [1]

Figure 1 - The studied micro-gripper holdding a ball between its both tips [1]

Problem description and design

The performance of the micro-gripper is modeled using EMS finite element tool, to estimate its displacement and temperature distribution. The schematic illustration and 3D model are shown in Figure 2.

Schematic illustration of the micro-gripper [1] a). 3D Model b).

Figure 2 - Schematic illustration of the micro-gripper [1] a). 3D Model b).

Table 1 - Model dimensions [1]

Parameter	Symbol	Value (mm)
Length of the hot arm Width of the hot arm Thickness of the hot arm	$L indice h$ $2 a indice h$ $2 b indice h$	4.5 0.21 0.21
Length of the intermediate arm Width of the intermediate arm Thickness of the intermediate arm	$L indice i$ $2 a indice i$ $2 b indice i$	0.8 0.27 0.25
Length of the cold arm Width of the cold arm Thickness of the cold arm	$L indice c$ $2 a indice c$ $2 b indice c$	3 0.9 0.63
Length of the flexure arm Width of the flexure arm Thickness of the flexure arm	$L indice f$ $2 a indice f$ $2 b indice f$	1.5 0.35 0.3
Total length	$L$	9
Initial gap	$g$	1

Simulation Setup

The Magnetostatic module of EMS, coupled to thermal and structural analysis, is used to predict and evaluate the thermal and mechanical behaviour of the micro gripper.
The simulation setup consists of the next steps:

Select the appropriate material.
Define the necessary electromagnetic inputs.
Define the necessary thermal inputs.
Apply the structural boundary conditions.
Mesh the entire model and run the solver.

Materials

In our case study, the following properties of material are used (Table 2):

Table 2 - Silver-Nickel composite properties

Property	Density (Kg/ $m au cube$ )	Electrical conductivity (S/m)	Thermal conductivity (W/m. K)	Thermal expansion coefficient (/K)	Elastic Modulus (GPa)	Poisson’s ratio
Silver-Nickel Composite (Ag-Ni)	2370	31903	66.7	120 E-06	21.5	0.3

Electromagnetic Inputs

Each extended tip of the micro gripper is defined as a solid coil carrying a voltage of 1.54 V where the entry/exit port are shown in Figure 3:

Figure 3 - Applied voltage input

Thermal Inputs

Thermal boundary condition of 27°C is applied to both anchored pads. A thermal convection is applied on the air body at ambient temperature with a coefficient set to 10 W/ m²K.

Structural boundary conditions

Fixed boundary conditions are applied to both sides of the anchored pads, as shown in the figure 4:

Figure 4 - Fixed boundary conditions

Meshing

The whole model is meshed inside EMS with a fine controlled mesh, as shown in the figure below, for more accurate results.

Figure 5 - Meshed model

Results

The simulation revealed the results below. Figure 6 shows the maximum temperature distribution which occurs at the hot arm for an input current value around 0.26 A.

Figure 6 - Temperature distribution

For the mechanical displacement results, each extended tip reaches a maximum deflection of 166 µm.

Figure 7 - Resultant displacement plot

For the same input applied voltage, Table 3 shows the comparison between measured and simulated results given by the reference [1] and the EMS tool.

Table 3 - Comparative table between EMS and Reference [1] results

Results	Simulation [1]	Measurement [1]	EMS
Max total Displacement (µm)	322	311	332
Max Temperature (°C)	155	123	135

Conclusion

EMS Multiphysics capabilities ensures accurate simulation of electrically driven micro-devices. In the presented example, a higher temperature, caused by Joule effect, produces a higher displacement in the micro gripper.

References

[1]. Feng, Yao-Yun, et al. "Fabrication of an electro-thermal micro-gripper with elliptical cross-sections using silver-nickel composite ink." Sensors and Actuators A: Physical 245 (2016): 106-112.