Thin Wafer Handling Robot

Jordan Hall, Fang Li, Joel Neff, and Alex Podust

• Background– Handling of thin silicon wafers for solar cells– Reducing handling stress and characterizing

residual stresses are part of manufacturing research

• Objective– Design a robot to move wafers between

conveyor and inspection station without dropping or imposing large gripping forces

– Minimize energy consumption and handling time

Overall System

Bernoulli Gripper

Bernoulli Gripper Model

• Used an mass flow input to create a vertical “lift” force– Gripper force depends on

distance to wafer– Rubber pads keep wafer from

impacting gripper frame

• The friction of the pads keeps wafer “stuck” to the gripper as the robot accelerates

-1 -0.8 -0.6 -0.4 -0.2 00

0.05

0.1

0.15

0.2

rrel

(m)

For

cegr

ippe

r (N

)

Gripper Force vs. Relative Distance

Gripper Force Balance

Friction force is only active when the wafer is contacting the posts

Gripper – Modelica Code

Gripper - Verification

0.0 2.5 5.00

10

20

30

40

50

60

Mas

s F

low

Rat

e (L

/min

)

Time (sec)

gripper_correct_lift.m_dot_in

0.0 2.5 5.0-0.05

0.00

0.05

0.10

0.15

0.20

[N

]

Time (sec)

gripper_correct_lift.bernoulliForce

Wafer separates

WaferPick-up

Gripper Model Challenges

• Required text-based Modelica modeling – No MultiBody sliding friction model– Required pneumatic->mechanical

energy conversion– Force balance and direction vectors

defined “from scratch.”

• Interaction between gripper and wafer was highly coupled; required in-depth Dymola experience

Fgripper

Ffriction

Actuator

Electric MechanicalEMFV Torque

DC MOTOR

OutputController(PID)

MotorCommand

+ -

AXIS CONTROLLER

Chassis

Base

Rotating Joint

Lin. joint

Axis 1 θ

Rotating Joint

Rotating Joint

Rotating Joint

Arm 1

Arm 2

Arm 3

Axis 2 θ

Axis 3 mm

Axis 4 θ

Axis 5 θ

Gripper position

ROBOT ARM

Model Limitations• Inflexibility - Path hard coded as joint angle

start and end values.

• Multiple simplifying assumptions s.a.– Motor– Gripper– Chassis– Cost of robot components not considered.

• Hard to determine safety factor for wafer flying off gripper.

Results

Energy usage (J) vs time (s)

Gripper friction and lateral force (N) vs time (s)

Lessons learned• Value of using simulation software to solve

engineering design problems

• Gained appreciation for information economics

• Do not underestimate complexity of modeling details of a system

• How to make good decisions under uncertainty