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I am looking for a continuous function $\theta(t)$ to steer a particle on the trajectory $x(t)$.

$x$ and $\theta$ relate through this constraint

$\ddot x(t) = Tsin(\theta(t))$ where $T$ is a non-zero positive constant.

$\theta(t)$ must satisfy these initial conditions

$\theta(0) = 0$

$\theta(t_0) = 0$

$x(t)$ must satisfy these initial values.

$$x(0)=x_a, x(t_0)=x_b$$

$$\dot x(0)=0, \dot x(t_0)=0$$

$$\ddot x(0)=0, \ddot x(t_0)=0$$

The two functions $x(t)$ and $\theta(t)$ only need to be continuous between $x_a$ and $x_b$. $x_b$ can be greater or lesser than $x_a$. Further, the function $x(t)$ should be bijective on $[0, t_0]$

This is how I got a solution for $x(t)$. I just guessed what seemed right until I got it working. Perhaps there also exists a polynomial solution.

$$\ddot x=-rsin(t)$$

Thus $\ddot x(0)=0$ and $\ddot x(t_0) \implies t_0 \in 0,\pi,2\pi , ...$

$\dot x(t) = rcos(t) + C_1$

$\dot x(0) = 0 = r + C_1$

$\dot x(t_0) = 0 = rcos(t_0) + C_1$

Since $t_0 \in 0, \pi, 2\pi,...$. We can isolate $C_1$ if we force $t_0 = 2\pi$

Thus $\dot x(t_0) = 0 = r + C_1 \implies C_1 = -r$

$x(t)=rsin(t) - rt + C_2$

$x(0) = x_a = C_2$

$x(t_0) = x_b = rsin(2\pi) - r2\pi + C_2 \implies r = \dfrac{x_a - x_b}{2\pi}$

Therefore, $x(t) = rsin(t) - rx + x_a$, where $r = \dfrac{x_a - x_b}{2\pi}$

Now here is where I am stuck. I do not understand how I can find $\theta(t)$ from the given equation. I tried doing

$\ddot x(t) = Tsin(\theta(t)) = -rsin(t)$

but I'm not sure how to maintain the $x(t)$ constraints

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Here is a engineering type solution for some range of parameters. There is some flexibility in the choice of a function $f$, but I give a hard bound below based only on the information given. At the end, I draw a representative example.

Step 1: candidate path $x(t)$

Set $x_a=0, x_b=L$. $L=0$ is impossible by the bijection requirement. Set $L>0$ for notational simplicity. (If $x_b<x_a$, then let the particle go from $x_b$ to $x_a$ instead, and then go backwards in time.) Let $f$ be any function such that $$f:[0,1]\to[0,\infty), \quad f(0)=f'(0)=f''(0)=f''(1)=f'(1)=f(1) = 0,\\ x\in(0,1)\implies f(x)>0$$ A polynomial example exists;you can consider $-t^3(t-1)^3$. Or you can use the following $$f:[0,1]\to [0,\infty) , \quad f(t):= \begin{cases}\exp\frac{-1}{1-(2t-1)^2}& x\neq 0 ,1,\\ 0 & x=0,1\end{cases}$$ This is up to scaling, a classical function in the theory of PDEs known as a "bump function" or the "standard mollifier". It is $C^\infty$, bounded above, and verifies $$0=f(0)=f(1)=f'(0)=f'(1)=\dots=f^{(k)}(0)=f^{(k)}(1)=\dots$$ It follows that if we integrate it from $0$ to $t$ to get $g(t)$, then $g$ and all required derivatives vanish at 0, while all required derivatives of $g$ vanish at $1$ (but not $g(1)$). Now consider $f_{t_0} : [0,t_0] \to \mathbb [0,\infty)$, achieved by a rescaling $$ f_{t_0}(t) = cf(t/t_0)$$ where $c$ is chosen so that $\int_0^{t_0} f_{t_0} (s)ds = 1$. The same is true of $f_{t_0}$ now, but at $0$ and $t_0$. Now define $$x:[0,t_0]\to[0,L],\quad x(t)= L\int_0^t f_{t_0}(s) ds.$$ It follows from $f>0$ in $(0,1)$ that $x$ is a bijection, and satisfies all boundary conditions.

Step 2: satisfying the ODE

One now needs to check if $|x''(t)|\le T$ for all $t$. In terms of $f$ this is $$|Lf_{t_0}'(t)|=\frac{cL}{t_0}|f'(t/t_0)|\le T,$$ so if $$\sup_{s\in[0,1]}|f'(s)| \le \frac{Tt_0}{cL},$$ then this procedure works. (In snappier notation this is $\|f\|_{L^1}\|f'\|_{L^\infty} \le \frac{Tt_0}{L}$.) Since $c$ depends on $f$ you may get better results with other $f$.

In fact, the furthest you can go in the + direction is controlled by the solution to $x'' \equiv T$, which leads to $x(t) = Tt^2/2 + At + B$. $A=x'(0)=0$. $B=x(0)=x_a=0$. $x_b = x(t_0)= Tt_0^2/2 $. So this computation gives $L\le Tt_0^2/2 $. If this isn't satisfied, you can go home.

Step 3: path to angles

This part is easy. Given the above calculations we can invert the relationship for $\theta$. Under the above assumption, we have $x''/T\in[-1,1]$. Just define $$\theta(t) = \arcsin(x''(t)/T).$$ Here, arcsin is a function $[-1,1]\to[-\pi,\pi]$. There's a unique solution for $\arcsin(x)=0$ which is $x=0$ so the boundary conditions are met. This finishes the construction.

Here's a Desmos graph using the degree 6 polynomial I gave. I followed my construction exactly (but the variable names in Desmos are different). The blue line is $x$. The yellow line is $x'$. The dotted yellow line is $x''$. The green horizontal is $y=T$. The shaded region shows the parts where $x''>T$ ; so this is not a solution but would become one if $T$ were increased enough.

enter image description here In particular (response to comments) note that $x''(0)=0=x''(L)$.

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  • $\begingroup$ Thanks for the detailed response. However since $x(t_0) = x_b$ shouldn't $f(1)= L$? (Instead of 0 as you wrote) $\endgroup$ – Cedric Martens Jan 3 at 6:58
  • $\begingroup$ @CedricMartens no, I defined my $x$ using the integral of $f$. $\endgroup$ – Calvin Khor Jan 3 at 6:59
  • $\begingroup$ @CedricMartens in particular $L= x(t_0) = cL\int_0^{t_0} f(s/t_0)ds$ and the value of $f(1)$ is not the important number here $\endgroup$ – Calvin Khor Jan 3 at 7:05
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    $\begingroup$ I am sorry for this, but over an hour ago I edited my question to fix a conflict. It seems that you started before the edit. I changed $\ddot x(t) = Tsin(\theta(t))$ (before it was a cosine). It follows that at time zero the acceleration should be zero and not $T$. I'm sorry for the mistake and I greatly appreciate the answer. $\endgroup$ – Cedric Martens Jan 3 at 7:13
  • $\begingroup$ @CedricMartens Thank you very much for your apology. In fact I already know this, because right when you changed the question, I had already written an answer to the older version of the question. The acceleration at time 0 for my answer is 100% without a doubt, 0. This is stated at the beginning of step 1. I will make you a graph to prove this. $\endgroup$ – Calvin Khor Jan 3 at 7:17

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