Domenico Campolo

Towards a force-controlled microgripper for assembling biomedical microdevices

This paper presents recent results on the development and control of a microgripper based on flexure joints, fabricated by LIGA and instrumented with semiconductor strain-gauge force sensors. The microgripper is the end-effector of a workstation developed to grasp and manipulate tiny objects such as the components of a typical biomedical microdevice. The development of the force control in the microgripper is of fundamental importance in order to achieve the dexterity and sensing capabilities required to perform assembly tasks for biomedical microdevices. As a step towards the definition of the force control strategy, system identification techniques have been used to model the microgripper. Results indicate that a proportional integral (PI) controller could be used to assure, at the same time, closed-loop stability of the system, and a bandwidth suitable for the intended applications. The force control is based on strain-gauge sensors which have been integrated in the microgripper and experimentally characterized. Sensor response in the idling condition and during grasp showed that they can provide useful information for force control of the microgripper.

Efficient charge recovery method for driving piezoelectric actuators with quasi-square waves

In this paper, an efficient charge recovery method for driving piezoelectric actuators with low frequency square waves in low-power applications such as mobile microrobots is investigated. Efficiency issues related to periodic mechanical work of the actuators and the relationship among the driving electronics efficiency, the piezoelectric coupling factor, and the actuator energy transmission coefficient are discussed. The proposed charge recovery method exploiting the energy transfer between an inductor and a general capacitive load is compared with existing techniques that lead to inherent inefficiencies. A charge recovery method is then applied to piezoelectric actuators, especially to bimorph ones. Unitary efficiency can be obtained theoretically for purely capacitive loads while intrinsic losses such as hysteresis necessarily lower the efficiency. In order to show the validity of the method, a prototype driving electronics consisting of an extended H-bridge is constructed and tested by experiments and simulations. Preliminary results show that 75% of charge (i.e., more than 56% of energy) can be recovered for bending actuators such as bimorphs without any component optimization at low fields.

Dynamically tuned design of the MFI thorax

This paper presents an analysis of the major mechanical component (the thorax) of the micromechanical flying insect (MFI), a centimeter sized aerial vehicle currently in development at UC Berkeley. We present a description of the kinematics of the mechanism which converts piezoelectric actuation into complex 3D wing motion. A complete non-linear modeling of the system based on the Lagrangian energy technique is presented. A design methodology is presented in order to achieve optimal matching conditions. Two kinds of sensors which are presently utilized on the MFI are described. Experimental results are presented which validate some of the modeled non-linear aspects of the mechanism.

Fabrication of gecko foot-hair like nano structures and adhesion to random rough surfaces

In this work the effect of substrate roughness on the adhesion of gecko foot-hair like nano structures as opposed to solid elastic materials is described and models of both synthetic nano-hairs and hair-substrate interaction are developed. First, by combining linear beam theory and geometric constraints, a nonlinear elastic model for the hair is derived. Then it is shown how for a given random surface, once its Zero Order Hold (ZOH) model is acquired through Atomic Force Microscopy, only the height distribution is needed to compute pull-off forces. In the effort of replicating gecko foot-hair adhesive properties, we synthesized arrays of nano hairs by casting polyurethane into a nano-pore array. Hairs of controlled size, in the range of 20-60 microns long and 200 nanometers thick, were thus fabricated, imaged via Scanning Electron Microscope (SEM). Elastic properties of polyurethane are measured and then fed into a model, based on cantilever beam theory, which, together with the height distribution of sample surfaces, provides a prediction for pull-off forces as well as a description of the hysteresis phenomena arising in push-in/pull-off cycles.

Intrinsic Constraints of Neural Origin: Assessment and Application to Rehabilitation Robotics

Ideally, robots used for motor rehabilitation, in particular, during assessment, should minimally perturb the voluntary movements of a subject. In this paper, we show how a state-of-the-art back-drivable robot, i.e., a robot that can be moved by the user with a low perceived mechanical impedance, when used for assessment can still perturb the voluntary movements of a subject. In particular, we show that, despite its low mechanical impedance, a robot may still not comply with the intrinsic kinematic constraints, which are of neural origin and are adopted by the human brain to solve redundancy in motor tasks. Specifically, the redundant task under consideration is the 2-D pointing task, which is performed by a subject with the sole use of the wrist [3 degree of freedom (DOF) kinematics]. Wrist orientations during pointing tasks are assessed in two different scenarios. In the first experiment, a lightweight handheld device is used, which introduces no loading effect. In the second experiment, similar pointing tasks are performed with the subject interacting with a state-of-the-art robot for wrist rehabilitation. In the first case, intrinsic kinematic constraints arise as 2-D surfaces embedded in the 3-D space of wrist configuration. Such surfaces are typically subject-dependent and reveal personal motor strategies. In the second case, a strong influence of the robot is remarked. In particular, 2-D surfaces still arise but are similar for all subjects and are referable to a mechanical origin (excessive loading by the robot). The assessment approach described in this paper, including both the experimental apparatus and data-analysis method, can be used as a test for the degree of back-drivability of mechanisms and robots in relation to constraints of neural origin, thus allowing the design of robots that can actually cope with such constraints. The clinical potential impact is also discussed.

Forearm orientation guidance with a vibrotactile feedback bracelet: On the directionality of tactile motor communication

User-teacher interaction during the learning and the execution of motor tasks requires the employment of various sensory channels, of which the tactile is one of the most natural and effective. In this paper we present a wearable robotic teacher for predefined motor tasks, consisting of a localization system and a wearable stimulation unit. This unit embeds four vibrotactile stimulators which are activated in order to provide the user with a feedback about the movement direction of the forearm in the cartesian space. Stimulators were chosen in order to maximize tactile sensitivity and spatial resolution. Tactile interface performances in guiding 2 DOF forearm movements were comparatively evaluated with two different sensory modalities: visual and visuotactile, by using a Virtual Reality (VR) rendering of the motor task. The comparison among sensory modalities was based on two movement indexes ad hoc defined: positioning accuracy and directionality of motor communication. The experimental tests have shown that the system described hereafter is a valuable tool for human motor motion guidance, allowing a successful and useful weighting of concurrent sensory inputs without providing relevant sensory interferences. Compared to visually-guided trajectories, positioning accuracy was improved in visuotactile-guided trajectories. The comparative analysis of the directionality index in all sensory modalities suggests that increasing the number of stimulators could improve the directionality of tactile motor communication.

Controllability issues in flapping flight for biomimetic micro aerial vehicles (MAVs)

We explore controllability in flapping flight for micro aerial vehicles (MAVs), inch-size robots capable of autonomous flight. Differently from previous work, we focus on a MAV with very limited wing kinematics and simple input control schemes. In particular, in the first part we show how an MAV provided with a pair of wings, each with a single degree of freedom and passive rotation, can still ensure controllability. This is obtained by combining two ideas. The first idea is to parameterize wing trajectory based on biomimetic principles, i.e. principles that are directly inspired by observation of real insect flight. The second idea is to treat flapping flight within the framework of high frequency control and to apply averaging theory arguments in order to prove controllability. The results obtained set flapping flight as a compelling example of high frequency control present in nature, and shed light on some of the reasons of superior maneuverability observed in flapping flight. Then, in the second part we show that controllability is still guaranteed even when the wing-thorax dynamics is included and the electromechanical structure is driven by a pulse width modulation (PWM) scheme where only its amplitude, period and duty cycle are controllable on a wingbeat-by-wingbeat basis. However, in this case our modeling clearly shows some tradeoffs between controllability and lift generation efficiency, which seem consistent with observations in real insect flight.

Liftoff of a Motor-Driven, Flapping-Wing Microaerial Vehicle Capable of Resonance

This study presents the design of a novel minimalist liftoff-capable flapping-wing microaerial vehicle. Two wings are each directly driven by a geared pager motor by utilizing an elastic element for energy recovery, resulting in a maximum lift-to-weight ratio of 1.4 at 10 Hz for the 2.7 g system. Separate directly driven wings allow the system to both resonate and control individual wing flapping angle, reducing necessary power consumption, as well as allowing the production of roll and pitch body torques. With a series of varied prototypes, system performance is examined with change in wing offset from center of rotation and elastic element stiffness. Prototype liftoff is demonstrated with open loop driving a tethered prototype without guide wires. A dynamic model of the system is adapted and compared with the prototype experimental results for later use in prototype optimization.

Kinematic analysis of the human wrist during pointing tasks

In this work, we tested the hypothesis that intrinsic kinematic constraints such as Donders’ law are adopted by the brain to solve the redundancy in pointing at targets with the wrist. Ten healthy subjects were asked to point at visual targets displayed on a monitor with the three dof of the wrist. Three-dimensional rotation vectors were derived from the orientation of the wrist acquired during the execution of the motor task and numerically fitted to a quadratic surface to test Donders’ law. The thickness of the Donders’ surfaces, i.e., the deviation from the best fitting surface, ranged between 1° and 2°, for angular excursions from ±15° to ±30°. The results support the hypothesis under test, in particular: (a) Two-dimensional thick surfaces may represent a constraint for wrist kinematics, and (b) inter-subject differences in motor strategies can be appreciated in terms of curvature of the Donders’ surfaces.

Development of piezoelectric bending actuators with embedded piezoelectric sensors for micromechanical flapping mechanisms

This paper presents the fabrication and the testing of piezoelectric unimorph actuators with embedded piezoelectric sensors which are meant to be used for the actuation of the Micromechanical Flying Insect (MFI). First the fabrication process of a piezoelectric bending actuator comprising a standard unimorph and a rigid extension is described together with the advantages of adding such an extension. Then the convenience of obtaining an embedded piezoelectric sensor by a simple and inexpensive variation of the fabrication process is pointed out. A model for the sensor embedded into a unimorph actuator with rigid extension is derived together with its flat response band limits. Calibration steps are also outlined which allow, despite residual parasitic actuator-sensor coupling, the use of the actuator with the embedded sensor for measuring position and inertial forces when external mechanical structures are driven. An experiment is carried out which validates the model for the actuator/sensor device under desired operating conditions. Preliminary application of the fabricated device to the MFI is also presented where the mechanical power fed into the wing is estimated.