The need to maintain optimal energy efficiency is critical during the drilling operations performed on future and current planetary rover missions (see figure). Specifically, this innovation seeks to solve the following problem. Given a spring-loaded percussive drill driven by a voice-coil motor, one needs to determine the optimal input voltage waveform (periodic function) and the optimal hammering period that minimizes the dissipated energy, while ensuring that the hammer-to-rock impacts are made with sufficient (user-defined) impact velocity (or impact energy).

Planetary Rover equipped with a rotary percussive drill.
To solve this problem, it was first observed that when voice-coil-actuated percussive drills are driven at high power, it is of paramount importance to ensure that the electrical current of the device remains in phase with the velocity of the hammer. Otherwise, negative work is performed and the drill experiences a loss of performance (i.e., reduced impact energy) and an increase in Joule heating (i.e., reduction in energy efficiency). This observation has motivated many drilling products to incorporate the standard bang-bang control approach for driving their percussive drills. However, the bang-bang control approach is significantly less efficient than the optimal energy-efficient control approach solved herein.

To obtain this solution, the standard tools of classical optimal control theory were applied. It is worth noting that these tools inherently require the solution of a two-point boundary value problem (TPBVP), i.e., a system of differential equations where half the equations have unknown boundary conditions. Typically, the TPBVP is impossible to solve analytically for high-dimensional dynamic systems. However, for the case of the spring-loaded vibro-impactor, this approach yields the exact optimal control solution as the sum of four analytic functions whose coefficients are determined using a simple, easy-to-implement algorithm. Once the optimal control waveform is determined, it can be used optimally in the context of both openloop and closed-loop control modes (using standard real-time control hardware).

Future NASA in situ exploration missions increasingly require extensive drilling and coring procedures that stress the demand for more energy efficient methods to accomplish these tasks. For example, when rover-based autonomous drills are controlled non-optimally for long periods of time, the energy loss can grow at a rate that cannot be sustained by the rover’s internal energy supply. Motorized percussive units can be especially energy-draining (when controlled non-optimally), making this technology especially relevant to this type of future NASA work.

This work was done by Jack B. Aldrich and Avi B. Okon of Caltech for NASA’s Jet Propulsion Laboratory. NPO-48467

Topics:
Drilling Energy conservation Hardware Machining processes Mathematical models Optimization Parts Simulation and modeling Software Tools and equipment
This Brief includes a Technical Support Package (TSP).
Document cover
Optimal Force Control of Vibro-Impact Systems for Autonomous Drilling Applications

(reference NPO-48467) is currently available for download from the TSP library.

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This article first appeared in the September, 2012 issue of Software Tech Briefs Magazine (Vol. 36 No. 9).

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Overview

The document titled "Optimal Force Control of Vibro-Impact Systems for Autonomous Drilling Applications" is a technical support package from NASA's Jet Propulsion Laboratory (JPL), detailing research aimed at improving the efficiency of drilling systems used in planetary exploration. The focus is on percussive hammering units, which are integral to the operation of drills and coring tools employed by planetary rovers.

The introduction outlines the motivation behind the study, highlighting that excessive control energy consumption can occur when the control waveforms of these hammering units are not properly tuned. The document delves into the fundamentals of vibro-impact systems, discussing how percussive spring-loaded hammer drills are modeled and controlled.

A significant portion of the research addresses optimal control strategies. It presents two key problems: the fixed-time optimal control problem, which seeks to shape control waveforms to maximize energy efficiency given a fixed period and desired impact velocity, and the optimal periodicity problem, which focuses on tuning the waveform period for enhanced energy efficiency. Additionally, the document explores how optimal state and force trajectories can be translated into an optimal voltage waveform for voice-coil actuators.

The closed-loop control architecture is also discussed, emphasizing the integration of feedforward and feedback control mechanisms to leverage optimal open-loop control strategies. This approach aims to enhance the overall performance and energy efficiency of drilling operations.

The document concludes with insights into the implications of this research for future autonomous drilling applications, particularly in the context of planetary exploration. By optimizing the control of vibro-impact systems, the research aims to reduce energy consumption and improve the effectiveness of drilling tools used in challenging extraterrestrial environments.

Overall, this technical support package serves as a comprehensive resource for understanding the advancements in force control for drilling systems, showcasing the innovative work being done at JPL to support NASA's exploration missions. It emphasizes the potential for these technologies to have broader applications beyond aerospace, contributing to advancements in various technological and scientific fields.