Is My DC Motor Differential Equation Setup Correct?

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Discussion Overview

The discussion revolves around the setup of differential equations for a DC motor system in order to derive its transfer function. Participants explore various aspects of the motor's modeling, including input and output types, and the implications of these choices on the system's behavior.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant questions whether the gearbox setup is correct for deriving the transfer function.
  • Another participant emphasizes the importance of defining the input (current or voltage) and output (speed or position) for the transfer function.
  • A participant suggests that the armature inductance can be ignored, arguing it does not affect the modeling for this question.
  • There is a contention regarding the significance of input and output types, with one participant asserting that changing these types alters the motor's behavior.
  • One participant specifies the input as ea(t) and output as θL(t), indicating a lack of additional information.
  • Another participant proposes that setting the armature inductance to zero results in a second-order transfer function.
  • Discussion includes a mention of parameters Ka and Kb having the same value but different units, with a claim that this can be proven.

Areas of Agreement / Disagreement

Participants express differing views on the relevance of input and output types, with some arguing it is crucial while others believe it does not significantly impact the modeling process. The discussion remains unresolved regarding the implications of these choices on the transfer function.

Contextual Notes

Some assumptions about the system, such as ignoring armature inductance, are stated but not universally accepted. The discussion also reflects a dependency on the definitions of input and output types, which are not fully clarified.

Scott77
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question 1.jpg


This question involves finding the transfer function for the system, but I first need to get the differential equations correct. Have I set up the gearbox correctly?

question 1 answer.jpg
 
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Scott77 said:
This question involves finding the transfer function for the system, but I first need to get the differential equations correct.

Well, the transfer function is a differential equation. Using Laplace transform it may be written:

H(s) = output(s)/input(s)

When expressing the transfer function by its Laplace transform, it becomes much easier to calculate at controller.

I have some questions:

- Is it a DC-motor ?
- What is the input: Current or voltage?
- What is the preferred output: Speed or position?
- Is the armature damped ( Da )?
- What about the self induction in the armature, La?

If it's a DC-motor, you should come up with something like this, as for the transfer function:

Image8.gif

Use Masons rule to determine the transfer function as for the motor: ω(s) / Va(s)
 
This is a system modeling question. That diagram and the hints is all the info we got, but we were told to ignore the armature inductance. I don't think it matters what the input type and output type are for this type of question...
 
Of course the input and output type matters. If you change the type of input or output, the motors behaviour will change.

In the model, choosing ω(s) as output, the transfer function will be H(s). Choosing θ(s) as output, the transfer function will be H(s)/s:
θ(s) = ω(s)/s.
 
The input is ea(t) and the output is θL(t) like the question states, that is all I know.
 
Ok, then use these types as input/output.

Setting La = 0, you will get a 2. order transfer function.
 
So is does this look correct?

question 1 answer.jpg
 
Ka and Kb have the same value, but the units are [Nm/A] and [Vs] respectively.

There are people that can prove it.
 

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