Joel Fortgang

Concurrent Design of Vibration Absorbers and Input Shapers

Systems with flexible dynamics often vibrate due to external disturbances, as well as from changes in the reference command. Feedback control is an obvious choice to deal with these vibrations, but in many cases, it is insufficient or difficult to implement. A technique that does not rely on high performance feedback control is presented here. It utilizes a combination of vibration absorbers and input shapers. Vibration absorbers have been used extensively to reduce vibration from sinusoidal disturbances, but they can also be implemented to reduce the response from transient functions. Input shaping has proven beneficial for reducing vibration that is caused by changes in the reference command. However, input shaping does not deal with vibration excited by external disturbances. In this paper, vibration absorbers and input shapers are designed sequentially and concurrently to reduce vibration from both the reference command and from external disturbances. The usefulness of this approach is demonstrated through computer simulations and experimental results.

Command shaping for micro-mills and CNC controllers

Micro-milling requires both high speed and high accuracy in order to economically produce parts with features on the scale of 1 micron. Because micro-mills are small, they are more flexible than traditional large scale machines and therefore vibration is a problem. Since they also require high positioning precision, even small vibrations of the cutting tool are also an issue. This paper presents a nonlinear command shaping technique to reduce the vibrations of a micro-mill which can be implemented with a standard CNC controller. The robustness of this technique to modelling errors and disturbances is investigated. Theoretical proofs and experimental demonstrations of the command shaping technique are presented. The improved performance from the command shaping technique enables higher throughput and improved accuracy of the machine.

Design of Vibration Absorbers for Step Motions and Step Disturbances

Mechanical systems with flexible dynamics often suffer from vibration induced by changes in the reference command or from external disturbances. The technique of adding a vibration absorber has proven useful at eliminating vibrations from external disturbances and rotational imbalances. Traditionally, vibration absorbers have been designed for systems subject to sinusoidal or random excitations. Here the applicability of vibration absorbers to systems with steplike changes in the reference command or similar disturbances is studied. This type of motion is more common in robotic applications. Here absorbers are designed using two methods; the first technique uses a weighting on peak overshoot and settling time to allow tradeoffs between the two performance criteria. The second simpler method utilizes an eigenvalue technique to predict the time constant. Both of these techniques provide the possibility of significant improvement in settling time. The performance of this absorber design strategies is compared with previously proposed vibration absorbers and experimental results verify its utility. @DOI: 10.1115/1.1825441#

Practical Implementation of a Dead Zone Inverse on a Hydraulic Wrist

This paper presents practical aspects of implementing a dead zone inverse on a hydraulic wrist. A dead zone occurs over a range of small input values for which a system does not respond. It was desirable to use the most straightforward method available to achieve improved system performance while requiring the least amount of modification to the controller. Thus a fixed parameter dead zone inverse (DZI) was added to an existing proportional-integral (PI) controller. First, the parameters of the dead zone were characterized from open loop testing. These parameters are the break points, or input values between which the system does not respond at all, and the slope of the system’s response just outside the break points. The DZI augments the PI signal input to the plant, effectively adding or subtracting a constant equal to the size of the dead zone break points and scaling the input by its slope. Simulations predicted perfect system tracking, but implementation on the hardware revealed several practical issues. First, the dead zone slope parameters vary throughout the robot’s workspace. Overestimation can lead to non-ideal system performance, but the more extreme problem is underestimation, which effectively increases control loop gain and can lead to system instability. However, performance is not affected significantly unless these parameters are off by an order of magnitude. Overall the system performance is relatively robust to modeling errors in the slope parameters. The second issue is that noise can be magnified by the dead zone inverse and cause chattering. This problem was very noticeable in the wrist when the estimated dead zone break points were used in the DZI. This problem can be eliminated by reducing the dead zone break points or reintroducing a small artificial dead zone back into the control loop to envelope the expected noise level. The requirements for successful implementation of the DZI were found to be a basic characterization of the dead zone and an understanding of practical system issues that can be accentuated by its use. The effectiveness of the technique was tested through simulations and experiments on the wrist.Copyright

Concurrent design of input shaping and vibration absorbers

Systems that exhibit flexible dynamics are susceptible to vibration caused by changes in the reference command, as well as, from external disturbances. Feedback control is an obvious choice to deal with these vibrations, but in many cases, it is insufficient or difficult to implement. Input shaping has proven beneficial for reducing vibration that is caused by changes in the reference command. However, input shaping does not deal with vibration excited by external disturbances. On the other hand, vibration absorbers have long been used to decrease vibration from external sources, particularly sinusoidal disturbances. In the paper, vibration absorbers and input shaping are designed concurrently to reduce vibration from both the reference command and external sources. The usefulness of this combined approach for dealing with both external and reference command disturbances is demonstrated through computer simulations.

The Combined Use of Input Shaping and Nonlinear Vibration Absorbers

Abstract Systems that exhibit flexible dynamics are susceptible to vibration caused by changes in the reference command, as well as, from external disturbances. Feedback control is an obvious choice to deal with these vibrations, but in many cases it is insufficient. Input shaping has proven very beneficial for reducing vibration that is caused by changes in the reference command. However, input shaping does not deal with vibration induced by external disturbances. On the other hand, vibration absorbers are an open-loop method that have long been used to decrease vibration from external sources, particularly sinusoidal disturbances. In this paper, non linear vibration absorbers are designed to reduce vibration from step disturbances. Furthermore, input shaping is combined with the absorbers in order to deal with the vibration induced by the reference command. The usefulness of this combined approach for dealing with both external and reference command disturbances is demonstrated with computer simulations.

Experimental verification of vibration absorbers combined with input shaping for oscillatory systems

Systems that exhibit flexible dynamics are susceptible to vibration caused by changes in the reference command and from external disturbances. Feedback control is an obvious choice to deal with these vibrations, but in many cases, it is insufficient or difficult to implement. Input shaping does not deal with vibration excited by external disturbances; however it has proven beneficial for reducing vibration that is caused by changes in the reference command. On the other hand, vibration absorbers have long been used to decrease vibration from external sources, particularly sinusoidal disturbances. Vibration absorbers can also be designed to reduce vibration from a known trajectory move, such as a step motion. To improve overall performance, a vibration absorber and input shaping scheme are designed to reduce vibration from step disturbances from both the reference command and external sources. The usefulness of this combined approach is demonstrated through experimental tests on a linear motor driven stage.

Scheduling of input shaping and transient vibration absorbers for high-rise elevators

This paper presents techniques for the reduction of vibration in high-rise elevator passenger cabs. Reduction in cab vibration improves ride comfort and enables the use of more aggressive motion profiles to shorten travel times. Vibration reduction is accomplished by input shapers in a scheduling algorithm based on elevator position. To deal with transient disturbances, a vibration absorber is used to complement the input-shaping control scheme

Input shaping for continuum beams under longitudinal vibration

The applicability of standard input shapers to continuum longitudinal beams is presented in this paper. The effectiveness of different input shapers, specifically the zero vibration, the zero vibration and derivative, and the unity magnitude-zero vibration shapers, is investigated. Also, the sensitivity of these schemes to modelling errors is presented. Through a derivation of the equation of motion of a longitudinal beam, single mode shapers are shown to have the same vibration reducing characteristics for the first mode of continuum systems as they do for lumped parameter systems. However, the contribution of higher modes is also shown to impact the response, especially when negative input shapers are employed

Genesis and Evolution of an Undergraduate Mechatronics Course

Abstract A mechatronics course provides an excellent forum for teaching introductory mechanical design concepts. The students can learn all the traditional materials such as planning tools, evaluation matrices, functional decomposition, report writing, etc. Furthermore, the product of their design activities is exciting and rewarding because it is a mechatronic device. However, in addition to the fundamental design curriculum, the students must also learn basic mechatronic concepts such as programming and electronics. Combining all of this material into a single semester course for undergraduates at the sophomore and junior level proves to be challenging. This paper documents the goals and structure of such a course and describes its evolution. Conclusions summarize the important lessons learned by the faculty.