Mass - Spring - damper in Parallel

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Discussion Overview

The discussion centers around determining the transfer function for a mass-spring-damper system, specifically focusing on the relationship between input force and output displacement. Participants explore the methodology for deriving the transfer function, the implications of different damping scenarios, and the challenges of obtaining the output in the time domain.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant presents the problem of finding the transfer function and seeks feedback on their methodology.
  • A later post corrects the initial assumption about the configuration of the system from parallel to series.
  • Another participant confirms the correctness of the differential equations but expresses uncertainty about the transformation process.
  • Discussion includes the challenge of determining the output displacement in the time domain using the inverse Laplace transform, with one participant noting the introduction of complex values.
  • Another participant suggests multiplying the numerator and denominator by a specific term to simplify the transfer function.
  • Questions arise regarding the implications of having a damping ratio greater than one, particularly in relation to complex numbers and their graphical representation.
  • One participant proposes using the quadratic formula and partial fractions to address the complexities introduced by the damping ratio.

Areas of Agreement / Disagreement

Participants express varying levels of confidence in different aspects of the solution process, with some agreeing on the correctness of the differential equations while others remain uncertain about the transformation and its implications. The discussion reflects multiple competing views on how to handle complex values in the context of the transfer function.

Contextual Notes

Participants note potential limitations in their approaches, including missing assumptions about the system configuration and unresolved steps in the mathematical transformations. The discussion does not resolve these issues.

Who May Find This Useful

Students and practitioners interested in control systems, mechanical engineering, and dynamics may find this discussion relevant, particularly those working on similar transfer function problems involving mass-spring-damper systems.

kjay262
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Homework Statement


The problem is to determine the transfer function where force F is input and displacement x is output in the mass-spring-damper mechanism.

Homework Equations


Spring Force = kx [k:spring constant]
Damping Force = Cx [C:damping coefficient]
Force = (Mass)(acceleration)

The Attempt at a Solution


Attempt at solution is in picture. I am interested to know if I am following the correct methodology and if I am missing anything.
 

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  • 10273955_10154109711405307_3235781433148029459_n.jpg
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****EDIT**** not in parallel, IN SERIES
 
The differential equations look correct. That was the hard part. I haven't look over the part about the development of the transform, but that shouldn't have been a problem.

Chet
 
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If the input force is a unit impulse the transfer function is equal to the output displacement in the complex domain. I'm struggling to determine the x(t) that is the output in the time domain, which is basically the inverse laplace of the transfer function. I using the identity (attached below), however the answer introduces a complex value, am I missing something.
 
Last edited:
Inverse Laplace Identity

Using the identity attached to determine the inverse laplace of the transfer function
 

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  • Screenshot 2014-05-21 13.11.30.png
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Multiply numerator and denominator by cs+k. See if this simplifies things.

Chet
 
What if you have a situation where \xi is greater than one (according to the identity image attached above) this would result in a complex number, yes? How would that be represented in graph?
 
kjay262 said:
What if you have a situation where \xi is greater than one (according to the identity image attached above) this would result in a complex number, yes? How would that be represented in graph?
Factor the denominator using the quadratic formula, and then resolve the transform into partial fractions, and you will then be able to answer your own question.
 
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