# Finding the laplace transform of a wave?

I was going through the solutions of a past paper and came across this question where it is asked to determine the laplace transform of a transient waveform. I'm a bit confused with how it balances the waves and getting 10000 slope. Can anybody shed some light on it?

The equation obtained is as follows

$$i(t) = (5000 slope ramp at t=0) -(10000 slope ramp at t=2)+ (5000 slope ramp at t=4)($$ $$I(s) = \frac{5000}{s^2}(1-2e^{-0.002s}+e^{-0.004s})$$

In the interval [0, 0.002], the signal has a slope of $100/0.002= 50000$. Then in [0.002, 0.004] its $-50000$ and in [0.004, $\infty$] it is $0$.

Now let us try to represent the original signal as a sum of ramp (shifted ramp) signals. ie.,

$$y = y_1 + y_2+y_3$$

Interval 1, [0, 0.002]: The slope is 50000 so a ramp with slope 5000 is required. So $$y_1 = 50000t$$

Interval 2, [0.002, 0.004]: The slope here is -50000. So another ramp ($y_2$) starting at $t=0.002$ should be added here to make the slope = $-50000$.

$$y = 50000t+y_2$$

Taking the derivative on both sides

$$\frac{dy}{dt} = 50000 +\frac{dy_2}{dt} =-50000$$ $$\frac{dy_2}{dt} =-100000$$

So the slope of $y_2 = -100000$ and

$$y_2 = -100000(t-0.002)$$

Interval 3, [0.004, $\infty$]: following the similar analysis,

$$y_3 = 50000(t-0.004)$$

Then,

$$y = 50000t -100000(t-0.002) + 50000(t-0.004)$$

Taking Laplace transform, $$Y(s) = \frac{50000}{s^2}(1-2e^{-0.002s}+e^{-0.004s})\tag1$$

EDIT: Calculating Laplace transform obtained in (1)

If $F(s)$ is the laplace transform of $f(t)$, then by property of Laplace transform, $$f(t)\Leftrightarrow F(s)$$ $$f(t-t_0)\Leftrightarrow e^{-t_0s}F(s)$$

We know that: $$t \Leftrightarrow \frac{1}{s^2}$$ Using the property mentioned above, $$(t-0.002) \Leftrightarrow \frac{1}{s^2}\times e^{-0.002s}$$ $$(t-0.004) \Leftrightarrow \frac{1}{s^2}\times e^{-0.004s}$$

• Can u please explain a bit about how you converted the y(t) to Y(s)? I can understand how the first term (tranformation of 50,000t) was obtained but I can't understand the next two terms. – S.Dan Mar 5 '15 at 12:17
• I was not aware of a time shifting property before. Got it (y) – S.Dan Mar 5 '15 at 13:58

The easiest way is to define $f(t)$ as a piecewise function

$$f(t)=\begin{cases}\frac{100}{0.002}\cdot t,&0\le t\le 2\cdot 10^{-3}\\ 100-\frac{100}{0.002}\cdot (t-2),&2\cdot 10^{-3}<t\le 4\cdot 10^{-3}\\ 0,&\text{otherwise}\end{cases}\tag{1}$$

Then with the definition of the Laplace transform

$$F(s)=\int_{-\infty}^{\infty}f(t)e^{-st}dt$$

you just split the integral and use the top expression in (1) in the interval $[0,2]$, and the bottom expression in the interval $[2,4]$.

Another way to solve such problems is to compute the Laplace transform of the derivative of $f(t)$, which is a piecewise constant function. The derivative of $f(t)$ can be written using the step function $u(t)$:

$$f'(t)=k\cdot[u(t)-2u(t-2)+u(t-4)]\tag{2}$$

where $k=100/0.002$, and where I've used units of milliseconds. From (2) you can immediately write down the result by noting that the Laplace transform of $u(t-t_0)$ is $e^{-st_0}/s$. As soon as you have the Laplace transform of $f'(t)$, you just need to divide by $s$ to obtain the Laplace transform of $f(t)$.

And, by the way, $100/0.002=50000$, and not $5000$.

• I definitely wouldn't call your first method the easiest way - it's the most obvious way, but it's pretty messy and unnecessary. – Greg d'Eon Mar 4 '15 at 13:52
• @Kynit: It's easier than that cancelling ramp business in the OP. I meant easy in terms of defining $f(t)$. The easiest solution to the problem is the one with the step functions. – Matt L. Mar 4 '15 at 13:55
• The "cancelling ramp business" is exactly the same as your step functions. You just wrote the derivatives instead of the functions. – Greg d'Eon Mar 4 '15 at 13:59
• @Kynit: I know that, but it's exactly the derivatives that make it clearer and less error prone. But it's up to everybody to choose their favorite method. – Matt L. Mar 4 '15 at 14:05
• Meh. The way I learned it, time shifts are simple and seem like they require one less step. To each their own. Take a +1 for your trouble. – Greg d'Eon Mar 4 '15 at 14:08