Isn't there enough real intertia around?

Mechatronics in Design: Adding inertia to a system, that is, adding mechanical mass, is not usually desirable as it slows down system response. Electronic intertia through acceleration feedback improves performance.

08/17/2010


Keven C. CraigThe word inertia in everyday use suggests resistance to change and an unwillingness to act. This is hardly something we need in engineering practice to solve the urgent problems we all face. Even in a motion-system context, the idea of adding inertia to a system, i.e. adding mechanical mass, is not usually desirable as it slows down system response.

One familiar exception is adding a flywheel to an engine or machine to smooth out speed fluctuations. Two of the most important benefits of feedback control are command following and disturbance rejection. Usually the focus of attention in a control system is on command following, but in many situations the ability of a system to reject disturbances, i.e., have high dynamic stiffness, is paramount.

Mechatronics Page 58 A common industry motion-control system has three cascaded feedback loops: motor current, velocity, and position.

For a motor-velocity feedback control system, increasing inertia J reduces the high-frequency disturbance response, i.e., makes the system dynamically stiffer at high frequencies. But the closed-loop command-following is degraded. How do we add inertia without degrading command-following performance? A common industry motion-control system has three cascaded feedback loops: motor current, velocity and position. Newton’s 2nd Law says torque is proportional to angular acceleration, so if we can measure or estimate acceleration, we can scale the acceleration by inertia J to give units of torque, and then by 1/KT, the inverse of the
motor torque constant, to give current.

This is then multiplied by a gain KAFB, and subtracted from the current command
to the current-control loop. KAFB has a similar effect to increasing inertia J; hence the alternate name electronic inertia. To ensure that the command following performance remains the same, the velocity control gains must be scaled by the same factor (1 + KAFB).

The velocity command response is unaffected by the value of KAFB because the loop gain increases in proportion to the inertia, producing no net effect. So why are we adding electronic inertia? The real benefit of acceleration feedback is that the disturbance response is improved by acceleration feedback through the entire frequency range in proportion to the term (1 + KAFB), as shown by the block diagram and transfer function and frequency-response plot.

This improvement cannot be realized significantly above the bandwidth of the current loop, as the acceleration feedback signal cannot improve the system at frequencies where the current loop cannot inject current. Of course, a robust acceleration feedback signal is required. This can be accomplished through differentiation of a position sensor signal and filtering or through the use of an observer.

MechatronicsFor mechatronics engineers, here is one situation where adding
inertia is highly desirable. In this virtual world we live in, electronic inertia is almost
expected. Peter Schmidt, Rockwell Automation, and Robert Lorenz, University of Wisconsin at Madison, have done foundational work in this area and their work should be consulted.

For more information, go to bit.ly/adSOlI. This appears in August 2010 Control Engineering and Plant Engineering.

Kevin C. Craig, Ph.D., holds the Robert C. Greenheck Chair in Engineering Design and is professor of Mechanical Engineering, College of Engineering, Marquette University.



No comments
The Engineers' Choice Awards highlight some of the best new control, instrumentation and automation products as chosen by...
The System Integrator Giants program lists the top 100 system integrators among companies listed in CFE Media's Global System Integrator Database.
Each year, a panel of Control Engineering and Plant Engineering editors and industry expert judges select the System Integrator of the Year Award winners in three categories.
This eGuide illustrates solutions, applications and benefits of machine vision systems.
Learn how to increase device reliability in harsh environments and decrease unplanned system downtime.
This eGuide contains a series of articles and videos that considers theoretical and practical; immediate needs and a look into the future.
Controller programming; Safety networks; Enclosure design; Power quality; Safety integrity levels; Increasing process efficiency
Additive manufacturing benefits; HMI and sensor tips; System integrator advice; Innovations from the industry
Robotic safety, collaboration, standards; DCS migration tips; IT/OT convergence; 2017 Control Engineering Salary and Career Survey
Featured articles highlight technologies that enable the Industrial Internet of Things, IIoT-related products and strategies to get data more easily to the user.
This article collection contains several articles on how automation and controls are helping human-machine interface (HMI) hardware and software advance.
This digital report will explore several aspects of how IIoT will transform manufacturing in the coming years.

Find and connect with the most suitable service provider for your unique application. Start searching the Global System Integrator Database Now!

Infrastructure for natural gas expansion; Artificial lift methods; Disruptive technology and fugitive gas emissions
Mobility as the means to offshore innovation; Preventing another Deepwater Horizon; ROVs as subsea robots; SCADA and the radio spectrum
Future of oil and gas projects; Reservoir models; The importance of SCADA to oil and gas
Automation Engineer; Wood Group
System Integrator; Cross Integrated Systems Group
Jose S. Vasquez, Jr.
Fire & Life Safety Engineer; Technip USA Inc.
This course focuses on climate analysis, appropriateness of cooling system selection, and combining cooling systems.
This course will help identify and reveal electrical hazards and identify the solutions to implementing and maintaining a safe work environment.
This course explains how maintaining power and communication systems through emergency power-generation systems is critical.

(copy 5)

click me