Ozan Tokatli

Hands-On Learning with a Series Elastic Educational Robot

For gaining proficiency in physical human-robot interaction (pHRI), it is crucial for engineering students to be provided with the opportunity to physically interact with and gain hands-on experience on design and control of force-feedback robotic devices. We present a single degree of freedom educational robot that features series elastic actuation and relies on closed loop force control to achieve the desired level of safety and transparency during physical interactions. The proposed device complements the existing impedance-type Haptic Paddle designs by demonstrating the challenges involved in the synergistic design and control of admittance-type devices. We present integration of this device into pHRI education, by providing guidelines for the use of the device to allow students to experience the performance trade-offs inherent in force control systems, due to the non-collocation between the force sensor and the actuator. These exercises enable students to modify the mechanical design in addition to the controllers, by assigning different levels of stiffness values to the compliant element, and characterize the effects of these design choices on the closed-loop force control performance of the device. We also report initial evaluations of the efficacy of the device for pHRI studies.

Optimal Design of a Micro Series Elastic Actuator

We propose using series elastic actuation (SEA) in micro mechanical devices to achieve precise control of the interaction forces. Using μSEA for force control removes the need for high-precision force sensors/actuators and allows for accurate force control through simple position control of the deflection of a compliant coupling element. Since the performance of a μSEA is highly dependent on the design of this compliant coupling element, we employ a design optimization framework to design this element. In particular, we propose a compliant, under-actuated half-pantograph mechanism as a feasible kinematic structure for this coupling element. Then, we consider multiple design objectives to optimize the performance of this compliant mechanism through dimensional synthesis, formulating an optimization problem to study the trade-offs between these design criteria. We optimize the directional manipulability of the mechanism, simultaneously with its task space stiffness, using a Pareto-front based framework. We select an optimal design by studying solutions on the Pareto-front curve and considering the linearity of the stiffness along the actuation direction as a secondary design criteria. The optimized mechanism possesses high manipulability and low stiffness along the movement direction of the actuator; hence, achieves a large stroke with high force resolution. At the same time, the mechanism has low manipulability and high stiffness along the direction perpendicular to the actuator motion, ensuring good disturbance rejection characteristics. We model the behavior of this compliant mechanism and utilize this model to synthesize a controller for μSEA to study its dynamic response. Simulated closed loop performance of the μSEA with optimized coupling element indicates that force references can be tracked without significant overshoot and with low tracking error (about 1.1%) even for periodic reference signals.Copyright

Stability of haptic systems with fractional order controllers

Fractional order calculus is a generalization of the familiar integer order calculus in that, it allows for differentiation/integration with orders of any real number. The use of fractional order calculus in systems and control applications provides the user an extra design variable, the order of differointegration, which can be tuned to improve the desired behavior of the overall system. We propose utilization of fractional order models/controllers in haptic systems and study the effect of fractional differentiation order on the stability robustness of the overall sampled-data system. Our results demonstrate that fractional calculus generalization has a significant impact on both the shape and area of stability region of a haptic system and inclusion of fractional order impedances may improve the stability robustness of haptic rendering. Our results also include experimental verification of the stability regions predicted by the theoretical analysis.

A Classroom Deployment of a Haptic System for Learning Cell Biology

The use of haptic systems in the classroom for enhancing science education is an underexplored area. In the education literature, it has been reported that certain concepts in science education are difficult for students to grasp and, as a result, misconceptions can be formed in the students’ knowledge. We conducted a study with 62 Year 8 (typically 12–13 years old) students who used a haptic application to study cell biology, specifically the concept of diffusion across a cell membrane. The preliminary analysis of the feedback from the students suggests opportunities for haptic applications to enhance their learning, and also highlights a number of points to consider in the design of the application, including the choice of haptic interface and the design of the virtual environment.

HandsOn-Computing: Promoting Algorithmic Thinking Through Haptic Educational Robots

Computational thinking lies at the intellectual core of computing. Promoting computational thinking ability requires that students are provided with a clear understanding of the fundamental principles and concepts of computer science, including abstraction, logic, algorithms, and data representation. We propose to use force-feedback educational robotic devices for hands-on teaching of computational thinking. The addition of haptic feedback for teaching abstract concepts of computer science offers several advantages, as haptic feedback (i) enables an effective means of data hiding, (ii) ensures a high level of student engagement by adding another pathway for perception and enabling active physical interaction, and (iii) improves student motivation through the novelty effect. Moreover, visually impaired students may benefit from replacement of visualization with haptic feedback. We present a force-feedback application for teaching sorting algorithms and report initial student evaluations of integrating haptics to promote computational thinking.

Using fractional order elements for haptic rendering

Fractional order calculus—a generalization of the traditional calculus to arbitrary order differointegration—is an effective mathematical tool that broadens the modeling boundaries of the familiar integer order calculus. Fractional order models enable faithful representation of viscoelastic materials that exhibit frequency dependent stiffness and damping characteristics within a single mechanical element. We propose the use of fractional order models/controllers in haptic systems to significantly extend the type of impedances that can be rendered using the integer order models. We study the effect of fractional order elements on the coupled stability of the overall sampled-data system. We show that fractional calculus generalization provides an additional degree of freedom for adjusting the dissipation behavior of the closed-loop system and generalize the well-known passivity condition to include fractional order impedances. Our results demonstrate the effect of the order of differointegration on the passivity boundary. We also characterize the effective impedance of the fractional order elements as a function of frequency and differointegration order.

Fractional order admittance control for physical human-robot interaction

In physical human-robot interaction (pHRI), the cognitive skill of a human is combined with the accuracy, repeatability and strength of a robot. While the promises and potential outcomes of pHRI are glamorous, the control of such coupled systems is challenging in many aspects. In this paper, we propose a new controller, fractional order admittance controller, for pHRI systems. The stability analysis of the new control system with human in-the-loop is performed and the interaction performance is investigated experimentally with 10 subjects during a task imitating a contact with a stiff environment. The results show that the fractional order controller is more robust than the standard admittance controller and helps to reduce the human effort in task execution.

Series elastic actuation for force controlled micro-manipulation

We propose using series elastic actuation (SEA) in micro mechanical devices to achieve precise control of the interaction forces. Using μSEA for force control removes the need for high-precision force sensors/actuators and allows for accurate force control through simple position control of the deflection of a compliant coupling element. In this work, we employ a design optimization framework to design this element. The proposed design framework ensures robustness of the design while simultaneously optimizing multiple objective functions. The robust design optimization method relies on the Sensitivity Region concept which minimizes the change of the objective function with respect to the small changes in the design variables. Once the optimal design is obtained, a non-overshooting controller is implemented for the μSEA to achieve accurate force tracking without ever exceeding the reference force input.

Non-overshooting force control of series elastic actuators

Whenever mechanical devices are used to interact with the environment, accurate control of the forces occurring at the interaction surfaces arises as an important challenge. Traditionally, force controlled systems utilize stiff force sensors in the feedback loop to measure and regulate the interaction forces. Series elastic actuation (SEA) is an alternative approach to force control, in which the deflection of a compliant element (orders of magnitude less stiff than a typical force sensor) placed between motor and the environment is controlled to regulate the interaction forces. The use of SEAs for force control is advantageous, since this approach possesses inherent robustness without the need for high-precision force sensors/actuators and allows for the accurate control of the force exerted by the actuator through position control of the deflection of a compliant coupling element. Here, a non-overshooting force controller is proposed to be embedded into the control structure of SEAs. Such controller architecture ensures safe operations of SAEs by making sure that the force applied to the environment are always bounded from above by the reference forces commanded to the controller.