Active Model-Based Hysteresis Compensation and Tracking Control of Pneumatic Artificial Muscle

The hysteretic nonlinearity of pneumatic artificial muscle (PAM) is the main factor that degrades its tracking accuracy. This paper proposes an efficient hysteresis compensation method based on the active modeling control (AMC). Firstly, the Bouc–Wen model is adopted as the reference model to describe the hysteresis of the PAM. Secondly, the modeling errors are introduced into the reference model, and the unscented Kalman filter is used to estimate the state of the system and the modeling errors. Finally, a hysteresis compensation strategy is designed based on AMC. The compensation performances of the nominal controller with without AMC were experimentally tested on a PAM. The experimental results show that the proposed controller is more robust when tracking different types of trajectories. In the transient, both the overshoot and oscillation can be successfully attenuated, and fast convergence is achieved. In the steady-state, the proposed controller is more robust against external disturbances and measurement noise. The proposed controller is effective and robust in hysteresis compensation, thus improving the tracking performance of the PAM.

DNA in Motion

Students often struggle to understand the full implications of some basic chemical concepts of DNA structure and function, especially how DNA’s directionality and antiparallel nature determine key functional features of replication and molecular recombination. Visualizing the complexities of these processes requires a working knowledge of how DNA’s nucleotides are assembled and how these components interact. This article describes a simple activity that can be used to visualize how nucleotides join together, how base pairs form, and, most importantly, how the active processes of replication and recombination are related to DNA chemistry. In this activity, students model DNA structure, with each student representing a single nucleotide, then join together to form a polynucleotide with 5′ to 3′ directionality. Two chains then pair to form the antiparallel DNA duplex. The activity not only illustrates the basic chemistry of DNA but also allows students to participate in active modeling of leading-strand and lagging-strand replication and in the formation of the Holliday junction molecule, the basic intermediate of recombination events including crossing over and gene conversion. The demonstrations can be videotaped from above to make a permanent copy of these events for teaching and study purposes. Example illustrations and links to videos are included.

If Best Execution Is a Process, What Does That Process Look Like?

The CFA Institute defines Best Execution for securities trading as a process, not an outcome. For many, this is a disquieting definition, for it does not lead to easy yes/no answers. Instead, it leads to an active modeling and analysis of what goes into trading. We apply the work of Peter Drucker to the execution process, with an emphasis on: 1] establishing goals (typically to increase returns by lowering costs), 2] defining the process (including the roles of the portfolio manager, broker, and commission directing clients), 3] analyzing the data (measuring costs, but with context) to identify problems, 4] proposing solutions. This is not a simple exercise, for the process is both complex and filled with nuance. But it takes the random element out of the measurement of best execution. More importantly, it also leads to improved results over time.