Abstract—This work demonstrates performance
improvement in motion control under a particular set of
machine system constraints. A high performance industrial
magnetic micro-manipulation system, the Minimag, is
introduced and modeled with both first principles and system
identification. The form of the closed loop controller is
constrained by operational bounds and the system software,
resulting in limits to achievable performance. A model-based
motion control enhancement is developed and implemented
using tools from Iterative Learning Control. The resulting
performance improvements indicate the benefits of motion
control even when the closed loop controller is fixed.
Experimental results at two different size scales (500 m and
4.5m) are given.
I. INTRODUCTION AND SYSTEM OVERVIEW
The use and impact of microscale machines have grown
tremendously in the past 20 years [1,2]. With increased
utilization of microscale machines, across diverse fields [3],
comes the demand for increasingly stringent manipulation at
commensurate length scales. Automation is a key enabler for
this microscale manipulation and there are emergent
applications in fields such as robotic microassembly [2] and
also cell mechanics [4]. Throughout the various fields of use
for microscale machines, significant opportunities exist for
control tools to contribute in their development and
performance [3]. This article focuses on one such specific
example.
Several different techniques have been proposed that can
servo microscale objects to localized targets; these include
both tethered actuators [5] and untethered systems.
Untethered actuation can be based on dielectrophoretic [6],
magnetic [7], fluidic [8], acoustic [9] or optically induced
forces [10]. Depending on the application, one approach
might be advantageous over the other. Magnetic forces are
favorable especially in several biological applications that
require a biocompatible, non-contact approach. In many of
these biological applications, magnetic fields are used to
manipulate small spherical objects; these objects are
available in range of sizes and are functionalized as the task
requires. The key to the object manipulation is the ability to
precisely (<1m) transport these systems over distances of a
few to hundreds of micrometers. A magnetic manipulation
system that is specifically designed for such a task is the
MiniMag [11,12]. The system consists of 8 electromagnets
A. Alleyne is with the Univiersity of Illinois, Urbana-Champaign, 1206
West Green Street, Urbana, IL, 61801 (phone: +1-217-244-9993; e-mail:
alleyne@illinois.edu)
S. Schurle and B. Nelson are with ETH Zurich. (email:
schuesim@ethz.ch and bradley.nelson@iris.mavt.ethz.ch)).
A. Meo is with the University of Pisa, Italy (email:
alessandro.meo@gmail.com)
arranged in a single hemisphere and can be incorporated into
either an upright or inverted microscope (see Fig. 1.a).
Magnetic samples are placed in a custom-made sample
holder on the x-y-stage with the lens is located underneath.
Figure 1: (a) Picture of microscope mounted MiniMag and
schematic of an inverted coil configuration; (b) screenshot of the
control panel Graphical User Interface (GUI).
Based on visual feedback the magnetic object can be
servoed to a target location in open or closed-loop fashion. A
typical scenario is depicted in Fig. 1.b. The position of the
object, a 500 µm large magnetic sphere, is detected based on
a visual servoing algorithm. The region of interest is
indicated by the blue box. Desired trajectories that specify
the task can then be loaded as system input or entered by the
user through the GUI. Here, two trajectory waypoints (grey)
are given as the input target signal. The interested reader is
referred to [11,12, 13], and the references therein, for further
details on the MiniMag system construction, use, and
applications.
Motion Control for Magnetic Micro-scale Manipulation
Andrew G. Alleyne, Simone Schurle, Alessandro Meo, Bradley J. Nelson
(b)
(a)
2013 European Control Conference (ECC)
July 17-19, 2013, Zürich, Switzerland.
978-3-952-41734-8/©2013 EUCA 784