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mankku
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Hi everyone!
I'm stuck with an assignment in control systems and I need some opinions on what I've got so far.
The problem relates to an RC circuit as displayed:
The task is to form the differential equation and transfer function for the circuit, with V0(t) being the input and V2(t) being the output. The values for the components are given in the image. It is assumed that initial conditions (currents, voltages) are all zero.
Well, basic electrical theory such as voltage, current, impedance as well as parallel and series combination of impedances, I guess.
So I started out using the Laplace (frequency domain) impedances, attacking the problem from a voltage divider point of view. Thus, the resistor and capacitor on the left in the schematic were "combined" into a single impedance Z1, and the resistor and capacitor on the right were combined into Z2. The output voltage would thus be
[tex]V_{2}(s)=V_{0}(s) * \frac{Z_{2}(s)}{Z_{1}(s)+Z_{2}(s)}[/tex]
Thus the transfer function is obtained through dividing by V0(s) and is thus
[tex]G(s) = \frac{V_{2}(s)}{V_{0}(s)} = \frac{Z_{2}(s)}{Z_{1}(s)+Z_{2}(s)}[/tex]
The two impedances used are formed as:
[tex]Z_{1}(s) = R_1 || \frac{1}{sC} = \frac{\frac{R_1}{sC}}{\frac{sR_{1}C+1}{sC}} = \frac{R_1}{sR_{1}C+1}[/tex]
[tex]Z_{2}(s) = R_2 + \frac{1}{sC} = \frac{sR_{2}C+1}{sC}[/tex]
The sum of these two becomes:
[tex]Z_{1}(s)+Z_{2}(s) = \frac{sR_1C}{sC(sR_{1}C+1)}+\frac{(sR_{2}C+1)(sR_{1}C+1)}{sC(sR_{1}C+1)} = \frac{s^{2}R_{1}R_{2}C^{2}+s(2R_{1}C+R_{2}C)+1}{sC(sR_{1}C+1)}[/tex]
Forming the transfer function now then, we invert the above calculated sum and multiply by Z2(s) to obtain:
[tex]G(s) = \frac{Z_{2}(s)}{Z_{1}(s)+Z_{2}(s)} = \frac{sR_{2}C+1}{sC}*\frac{sC(sR_{1}C+1)}{s^{2}R_{1}R_{2}C^{2}+s(2R_{1}C+R_{2}C)+1}[/tex]
The sC eliminate each other and I am left with:
[tex]G(s) = \frac{(sR_{1}C+1)(sR_{2}C+1)}{s^{2}R_{1}R_{2}C^{2}+s(2R_{1}C+R_{2}C)+1}[/tex]
Which given a bit of cleaning turns into
[tex]G(s) = \frac{s^{2}R_{1}R_{2}C^{2}+s(R_{1}C+R_{2}C)+1}{s^{2}R_{1}R_{2}C^{2}+s(2R_{1}C+R_{2}C)+1}[/tex]
In case someone wants to plug in the values given for the components, the result becomes
[tex]G(s) = \frac{0.0000027s^2+0.0037s+1}{0.0000027s^2+0.0064s+1} = \frac{s^2+1370.37s+370370.37}{s^2+2370.37s+370370.37}[/tex]
4. The real question
The reason why I'm asking is that this transfer function seems quite ok to start with. The zeroes and poles are all real, which means from a stability point of view that the system does not oscillate, as would be expected by an RC circuit. Also, the DC gain (i.e. s -> 0) is 1, which holds true seeing that if the input signal is DC, then the capacitor on the right is an open circuit and the resistor R_1 pulls the output to equal the input. So from this point of view, I'm satisfied.
On the other hand, is it possible to have a transfer function with numerator of equal degree as the denominator? I know that Simulink does not like that, and I don't know how to work around this. I have done a long division on the two polynoms and sure, it gave me 1+<something> where something had a numerator with degree less than that of the denominator, but IMO that does not help in this situation.
So to get the differential equation I would be tempted to use the expression for G(s) that I have, multiply with denominator and V0(s) and do an inverse Laplace transform. The problem is that I'm unsure whether I am allowed to do that and if that will yield the correct answer.
The diff.eq. I would get would thus be:
[tex]\ddot{v_2}(t)+2370.37\dot{v_2}(t)+370370.37v_{2}(t) = \ddot{v_0}(t)+1370.37\dot{v_0}(t)+370370.37v_{0}(t)[/tex]First of all, if anyone actually read through this in detail, thank you.
If you have any thoughts on the above, please reply because I am in desperate need of assistance. The assignment is due on Tuesday so there is still time, however I would like to get this sorted out of the way because it has been bothering me for several days now.
Cheers
// Mankku
I'm stuck with an assignment in control systems and I need some opinions on what I've got so far.
Homework Statement
The problem relates to an RC circuit as displayed:
The task is to form the differential equation and transfer function for the circuit, with V0(t) being the input and V2(t) being the output. The values for the components are given in the image. It is assumed that initial conditions (currents, voltages) are all zero.
Homework Equations
Well, basic electrical theory such as voltage, current, impedance as well as parallel and series combination of impedances, I guess.
The Attempt at a Solution
So I started out using the Laplace (frequency domain) impedances, attacking the problem from a voltage divider point of view. Thus, the resistor and capacitor on the left in the schematic were "combined" into a single impedance Z1, and the resistor and capacitor on the right were combined into Z2. The output voltage would thus be
[tex]V_{2}(s)=V_{0}(s) * \frac{Z_{2}(s)}{Z_{1}(s)+Z_{2}(s)}[/tex]
Thus the transfer function is obtained through dividing by V0(s) and is thus
[tex]G(s) = \frac{V_{2}(s)}{V_{0}(s)} = \frac{Z_{2}(s)}{Z_{1}(s)+Z_{2}(s)}[/tex]
The two impedances used are formed as:
[tex]Z_{1}(s) = R_1 || \frac{1}{sC} = \frac{\frac{R_1}{sC}}{\frac{sR_{1}C+1}{sC}} = \frac{R_1}{sR_{1}C+1}[/tex]
[tex]Z_{2}(s) = R_2 + \frac{1}{sC} = \frac{sR_{2}C+1}{sC}[/tex]
The sum of these two becomes:
[tex]Z_{1}(s)+Z_{2}(s) = \frac{sR_1C}{sC(sR_{1}C+1)}+\frac{(sR_{2}C+1)(sR_{1}C+1)}{sC(sR_{1}C+1)} = \frac{s^{2}R_{1}R_{2}C^{2}+s(2R_{1}C+R_{2}C)+1}{sC(sR_{1}C+1)}[/tex]
Forming the transfer function now then, we invert the above calculated sum and multiply by Z2(s) to obtain:
[tex]G(s) = \frac{Z_{2}(s)}{Z_{1}(s)+Z_{2}(s)} = \frac{sR_{2}C+1}{sC}*\frac{sC(sR_{1}C+1)}{s^{2}R_{1}R_{2}C^{2}+s(2R_{1}C+R_{2}C)+1}[/tex]
The sC eliminate each other and I am left with:
[tex]G(s) = \frac{(sR_{1}C+1)(sR_{2}C+1)}{s^{2}R_{1}R_{2}C^{2}+s(2R_{1}C+R_{2}C)+1}[/tex]
Which given a bit of cleaning turns into
[tex]G(s) = \frac{s^{2}R_{1}R_{2}C^{2}+s(R_{1}C+R_{2}C)+1}{s^{2}R_{1}R_{2}C^{2}+s(2R_{1}C+R_{2}C)+1}[/tex]
In case someone wants to plug in the values given for the components, the result becomes
[tex]G(s) = \frac{0.0000027s^2+0.0037s+1}{0.0000027s^2+0.0064s+1} = \frac{s^2+1370.37s+370370.37}{s^2+2370.37s+370370.37}[/tex]
4. The real question
The reason why I'm asking is that this transfer function seems quite ok to start with. The zeroes and poles are all real, which means from a stability point of view that the system does not oscillate, as would be expected by an RC circuit. Also, the DC gain (i.e. s -> 0) is 1, which holds true seeing that if the input signal is DC, then the capacitor on the right is an open circuit and the resistor R_1 pulls the output to equal the input. So from this point of view, I'm satisfied.
On the other hand, is it possible to have a transfer function with numerator of equal degree as the denominator? I know that Simulink does not like that, and I don't know how to work around this. I have done a long division on the two polynoms and sure, it gave me 1+<something> where something had a numerator with degree less than that of the denominator, but IMO that does not help in this situation.
So to get the differential equation I would be tempted to use the expression for G(s) that I have, multiply with denominator and V0(s) and do an inverse Laplace transform. The problem is that I'm unsure whether I am allowed to do that and if that will yield the correct answer.
The diff.eq. I would get would thus be:
[tex]\ddot{v_2}(t)+2370.37\dot{v_2}(t)+370370.37v_{2}(t) = \ddot{v_0}(t)+1370.37\dot{v_0}(t)+370370.37v_{0}(t)[/tex]First of all, if anyone actually read through this in detail, thank you.
If you have any thoughts on the above, please reply because I am in desperate need of assistance. The assignment is due on Tuesday so there is still time, however I would like to get this sorted out of the way because it has been bothering me for several days now.
Cheers
// Mankku
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