Schematic illustration of the cart/pendulum system for simulating humans handling complex objects, such as coffee cups. (a) A conceptual model of a ball moving inside a cup; (b) A nonlinear mechanical model attached to a cart that has a pendulum attached, which is mathematically described using a series of differential equations. Credit: Brent Wallace, Ying-Cheng Lai, Arizona State University
Most people walk with coffee every day, without realizing the delicate balance it requires. There are many laws that prevent coffee from spilling.
Coffee, which is a thermally agitated fluid, can be found in a cup. The cup interacts with the coffee which in turn interacts with the human carrier.
"While humans possess a natural, or gifted, ability to interact with complex objects, our understanding of those interactionsespecially at a quantitative level, is next to zero," said ASU Professor Ying-Cheng Lai, an Arizona State University electrical engineering professor. "We don't have the ability to evaluate the effects of noise and climate on our interactions.
Understanding these external factors is an important issue in soft robotics.
Brent Wallace, who was a student of Lai's at the time and is now a doctoral student at ASU's Ira A. Fulton Schools of Engineering, said, "For example in design of smart prosthetics it is becoming increasingly important that you build in natural modes of flexibility that mimic the natural movement of human limbs." These improvements make the prosthetic more natural and comfortable for the user.
Lai believes that robots could be used in complex tasks such as controlling objects or moving people.
A robot that is designed to walk with a short stride length will be allowed to have relatively large variations in its walking frequency. If a robot is to walk with a shorter stride length, it should be chosen carefully.
Wallace's senior project in electrical engineering was the inspiration for a new paper in Physical Review Applied titled "Synchronous Transition In Complex Object Control." Wallace was awarded a NSF Graduate Fellowship and is now enrolled as a doctoral student at ASU's School of Electrical, Computer and Energy Engineering.
ASU's research builds on the groundbreaking virtual experimental study by Northeastern University. It uses the coffee-cup-holding paradigm with a rolling ball to explore how humans manipulate complex objects. Participants rotated the cup in a rhythmic fashion with the ability of varying force and frequency to keep the ball contained.
Northeastern found that participants are more likely to choose a low-frequency or high-frequency strategyrhythmic movement of the cupto handle complex objects.
It was remarkable to find that oscillations with low frequency strategies exhibit in-phase synchroization. However, when high-frequency strategies are used, antiphase synchronization occurs.
Wallace stated that "both the high- and low-frequency frequencies are effective, so it's possible that some participants in this virtual experiment switched strategies." "This raises questions.
Wallace asked Wallace, "How can a transition occur between in-phase synchroization associated with low-frequency strategies to antiphase synchroization associated with high-frequency strategies or vice versa?" "Is the boundary between in-phase synchronization and antiphase synchronization systems sharp, gradual, sophisticated, or simple in the parameter space?"
Wallace's curiosity prompted the ASU team to study the transition between in-phase and out-phase synchronization. They used a nonlinear dynamical model to simulate a pendulum attached with a moving cart that is subject to periodic external forcing to create a nonlinear dynamical model.
Researchers discovered that in the weakly driving regime, when the external driving frequency changes, the transition to the frequency of resonance is abrupt. This can be fully explained using linear systems control theory.
This regime is beyond the transitional zone, which is located between in-phase synchronization and antiphase synchroization. It is here that the motions of both the cart and pendulum do not coincide. Also, it was found that the low-frequency side has bistability.
The results show that humans can switch quickly and efficiently between synchronous attractors, which is a feature that can be used to design smart robots that can adapt to complex objects in changing environments.
"It's possible for humans to be able to use both anti-phase and in-phase strategies with skill and to seamlessly switch between them, sometimes without even realizing. These human skills can be applied to soft robots in other areas, like rehabilitation and brain-machine interface," Lai stated.
Even tasks that are as simple as running wires through a car's body on an assembly line, which humans do with great ease, can still be difficult for the most sophisticated machines.
Wallace stated that a systematic quantitative understanding of the dynamic interactions between humans and their environment will change the way we engineer our world. This knowledge could revolutionize the design and manufacturing of smart prosthetics, as well as bringing about a new era of automation and manufacturing. "By mimicking the dynamically-favorable behaviors adopted by humans in handling complex objects, we will be able to automate processes previously thought to be impossible."
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Further information: Brent Wallace et. al., Synchronous Transition In Complex Object Control, Physical Review Applied (2021). Brent Wallace et al, Synchronous Transition in Complex Object Control,(2021). DOI: 10.1103/PhysRevApplied.16.034012
