Warning: This simulation requires ten integrators, 15 summers, 16 multipliers, and 17 coefficient potentiometers (-> eight THATs).*

# Simulating a double pendulum¶

 Figure 1: Double pendulum

This application note describes the simulation of a double pendulum as shown in figure 1 on an analog computer. 1

## Equations of motion¶

The equations of motion will be derived by determining the Lagrangian $$L=T-V$$ with the two generalized coordinates $$\varphi_1$$ and $$\varphi_2$$ where $$T$$ represents the total kinetic energy while $$V$$ is the potential energy of the system. This potential energy is just

(1)$V=-g\left((m_1+m_2)l_1\cos(\varphi_1)+m_2l_2\cos(\varphi_2)\right)\label{equ_dp_potential_energy}$

with $$m_1$$ and $$m_2$$ representing the masses of the two bobs mounted on the tips of the two pendulum rods (which are assumed as being weightless). As always, $$g$$ represents the gravitational acceleration.

To derive the total kinetic energy the positions of the pendulum arm tips $$(x_1,y_1)$$ and $$(x_2,y_2)$$ are required:

\begin{split}\begin{aligned} x_1&=l_1\sin(\varphi_1)\\ y_1&=l_1\cos(\varphi_1)\\ x_2&=l_1\sin(\varphi_1)+l_2\sin(\varphi_2)\\ y_2&=l_1\cos(\varphi_1)+l_2\cos(\varphi_2) \end{aligned}\end{split}

The first derivatives of these with respect to time are

\begin{split}\begin{aligned} \dot{x}_1&=\dot{\varphi}_1l_1\cos(\varphi_1),\\ \dot{y}_1&=-\dot{\varphi}_1l_1\sin(\varphi_1),\\ \dot{x}_2&=\dot{\varphi}_1l_1\cos(\varphi_1)+\dot{\varphi}_2l_2\cos(\varphi_2)\text{~and}\\ \dot{y}_2&=-\dot{\varphi}_1l_1\sin(\varphi_1)-\dot{\varphi}_2l_2\sin{\varphi_2}. \end{aligned}\end{split}

The kinetic energy of the system is then

(2)$T=\frac{1}{2}\left(m_1(\dot{x}_1^2+\dot{y}_1^2)+m_s(\dot{x}_2^2+\dot{y}_2^2)\right)\label{equ_dp_kinetic_energy}$

with

\begin{split}\begin{aligned} \dot{x}_1^2&=\dot{\varphi}_1^2l_1^2\cos^2(\varphi_1)\\ \dot{y}_1^2&=\dot{\varphi}_1^2l_1^2\sin^2(\varphi_1)\\ \dot{x}_2^2&=\dot{\varphi}_1^2l_1^2\cos^2(\varphi_1)+2\dot{\varphi}_1\dot{\varphi}_2l_1l_2\cos(\varphi_1)\cos(\varphi_2)+\dot{\varphi}_2^2l_2^2\cos^2(\varphi_2)\\ \dot{y}_2^2&=\dot{\varphi}_1^2l_1^2\sin^2(\varphi_1)+2\dot{\varphi}_1\dot{\varphi}_2l_1l_2\sin(\varphi_1)\sin(\varphi_2)+\dot{\varphi}_2^2l_2^2\sin^2(\varphi_2) \end{aligned}\end{split}

and thus

(3)\begin{split}\begin{aligned} \dot{x}_1^2+\dot{y}_1^2&=\dot{\varphi}_1^2l_1^2\cos^2(\varphi_1)+\varphi_1^2l_1^2\sin^2(\varphi_1)\nonumber\\ &=\dot{\varphi}_1^2l_1^2\left(\cos^2(\varphi_1)+\sin^2(\varphi_1)\right)\nonumber\\ &=\dot{\varphi}_1^2l_1^2\label{equ_dp_sp_1}\\ \end{aligned}\end{split}
(4)\begin{split}\begin{aligned} \dot{x}_2^2+\dot{y}_2^2&=\dot{\varphi}_1^2l_1^2\left(\cos^2(\varphi_1)+\sin^2(\varphi_1)\right)+\dot{\varphi}_2^2l_2^2\left(\cos^2(\varphi_2)+\sin^2(\varphi_2)\right)+\nonumber\\ &~~~~2\dot{\varphi}_1\dot{\varphi}_2l_1l_2\left(\cos(\varphi_1)\cos(\varphi_2)+\sin(\varphi_1)\sin(\varphi_2)\right)\nonumber\\ &=\dot{\varphi}_1^2l_1^2+\dot{\varphi}_2^2l_2^2+2\dot{\varphi}_1\dot{\varphi}_2l_1l_2\cos(\varphi_1-\varphi_2)\label{equ_dp_sp_2} \end{aligned}\end{split}

The Lagrangian $$L$$ results from equations (1) and (2) with (3) and (4) as

(5)\begin{split}\begin{aligned} L&=\frac{1}{2}l_1^2\dot{\varphi}_1^2(m_1+m_2)+\frac{1}{2}m_2\dot{\varphi}_2^2l_2^2+m_2\dot{\varphi}_1\dot{\varphi}_2l_1l_2\cos(\varphi_1-\varphi_2)+\nonumber\nonumber\\ &~~~~g(m_1+m_2)l_1\cos(\varphi_1)+gm_2l_2\cos(\varphi_2).\label{equ_dp_lagrangian} \end{aligned}\end{split}

Now the Euler-Lagrange-equations

(6)\begin{split}\begin{aligned} \frac{\text{d}}{\text{d}t}\frac{\partial L}{\partial\dot{\varphi}_1}-\frac{\partial L}{\partial\varphi_1}&=0\label{equ_dp_eula_1}\\ \end{aligned}\end{split}

and

(7)\begin{aligned} \frac{\text{d}}{\text{d}t}\frac{\partial L}{\partial\dot{\varphi}_2}-\frac{\partial L}{\partial\varphi_2}&=0\label{equ_dp_eula_2} \end{aligned}

have to be solved. The required (partial) derivatives are

\begin{split}\begin{aligned} \frac{\partial L}{\partial\varphi_1}&=-m_2\dot{\varphi}_1\dot{\varphi}_2l_1l_2\sin(\varphi_1-\varphi_2)-g(m_1+m_2)l_1\sin(\varphi_1)\\ \frac{\partial L}{\partial\dot{\varphi}_1}&=l_1^2\dot{\varphi}_1(m_1+m_2)+m_2\dot{\varphi}_2l_1l_2\cos(\varphi_1-\varphi_2)\\ \frac{\text{d}}{\text{d}t}\frac{\partial L}{\partial\dot{\varphi}_1}&=l_1^2\ddot{\varphi}_1(m_1+m_2)+m_2l_1l_2\left[\ddot{\varphi}_2\cos(\varphi_1-\varphi_2)-\dot{\varphi}_2\sin(\varphi_1-\varphi_2)(\dot{\varphi}_1-\dot{\varphi}_2)\right]\\ \frac{\partial L}{\partial\varphi_2}&=m_2\dot{\varphi}_1\dot{\varphi}_2l_1l_2\sin(\varphi_1-\varphi_2)-gm_2l_2\sin(\varphi_2)\\ \frac{\partial L}{\partial\dot{\varphi}_2}&=m_2\dot{\varphi}_2l_2^2+m_2\dot{\varphi}_1l_1l_2\cos(\varphi_1-\varphi_2)\\ \frac{\text{d}}{\text{d}t}\frac{\partial L}{\partial\dot{\varphi}_2}&=m_2\ddot{\varphi}_2l_2^2+m_2l_1l_2\left[\ddot{\varphi}_1\cos(\varphi_1-\varphi_2)-\dot{\varphi}_1\sin(\varphi_1-\varphi_2)(\dot{\varphi}_1-\dot{\varphi}_2)\right]. \end{aligned}\end{split}

based on (5). Substituting these into (6) yields

\begin{split}\begin{aligned} 0&=l_1^2\ddot{\varphi}_1(m_1+m_2)-m_2l_1l_2\ddot{\varphi}_2\cos(\varphi_1-\varphi_2)-m_2l_1l_2\dot{\varphi}_2\sin(\varphi_1-\varphi_2)(\dot{\varphi}_1-\dot{\varphi}_2)+\\ &~~~~m_2\dot{\varphi}_1\dot{\varphi}_2l_1l_2\sin(\varphi_1-\varphi_2)+gl_1(m_1+m_2)\sin(\varphi_1). \end{aligned}\end{split}

Expanding $$(\dot{\varphi}_1-\dot{\varphi}_2)$$ yields

\begin{split}\begin{aligned} 0&=l_1^2\ddot{\varphi}_1(m_1+m_2)+m_2l_1l_2\ddot{\varphi}_2\cos(\varphi_1-\varphi_2)+\\ &~~~~m_2l_1l_2\dot{\varphi}_2^2\sin(\varphi_1-\varphi_2)+gl_1(m_1+m_2)\sin(\varphi_1). \end{aligned}\end{split}

Dividing by $$l_1^2(m_1+m_2)$$ results in

$0=\ddot{\varphi}_1+\frac{m_2}{m_1+m_2}\frac{l_2}{l_1}\ddot{\varphi}_2\cos(\varphi_1-\varphi_2)+\frac{m_2}{m_1+m_2}\frac{l_2}{l_1}\dot{\varphi}_2^2\sin(\varphi_1-\varphi_2)+\frac{g}{l_1}\sin(\varphi_1)$

which can be further simplified as

(8)$0=\ddot{\varphi}_1+\frac{m_2}{m_1+m_2}\frac{l_2}{l_1}\left[\ddot{\varphi}_2\cos(\varphi_1-\varphi_2)+\dot{\varphi}_2^2\sin(\varphi_1-\varphi_2)\right]+\frac{g}{l_1}\sin(\varphi_1).\label{equ_dp_motion_1}$

Proceeding analogously, (7) yields

(9)$0=\ddot{\varphi}_2+\frac{l_1}{l_2}\left[\ddot{\varphi}_1\cos(\varphi_1-\varphi_2)-\dot{\varphi}_1^2\sin(\varphi_1-\varphi_2)\right]+\frac{g}{l_2}\sin(\varphi_2)\label{equ_dp_motion_2}$

after some rearrangements.

To simplify things further it will be assumed that $$m_1=m_2=1$$ and $$l_1=l_2=1$$ in arbitrary units yielding the following two equations of motion of the double pendulum based on (8) and (9):

\begin{split}\begin{aligned} \ddot{\varphi}_1&=-\frac{1}{2}\left[\ddot{\varphi}_2\cos(\varphi_1-\varphi_2)+\dot{\varphi}_2^2\sin(\varphi_1-\varphi_2)+2g\sin(\varphi_1)\right]\label{equ_dp_motion_1_final}\\ \ddot{\varphi}_2&=-\left[\ddot{\varphi_1}\cos(\varphi_1-\varphi_2)-\dot{\varphi}_1^2\sin(\varphi_1-\varphi_2)+g\sin(\varphi_2)\right]\label{equ_dp_motion_2_final} \end{aligned}\end{split}

## Implementation¶

The implementation of equations (10) and (11) is straightforward as shown in figures 2 and 3. Both circuits require the functions $$\sin(\varphi_1-\varphi_2)$$, $$\cos(\varphi_1-\varphi_2)$$, $$\sin(\varphi_1)$$, and $$\sin(\varphi_2)$$, which are generated by three distinct sub-circuits such as the one shown in figure 4. These circuits yield sine and cosine of an angle based on the first time-derivative of this angle, so the simulation is not limited to a finite range for the two angles $$\varphi_1$$ and $$\varphi_2$$.

It should be noted that each of the two integrators of each of these three harmonic function generators has a potentiometer connected to its respective initial value input. When a simulation run with given initial values for $$\varphi_1(0)$$ and $$\varphi_2(0)$$ is to be started, these potentiometers have to be set to $$\cos(\varphi_1(0))$$, $$\sin(\varphi_1(0))$$ and $$\cos(\varphi_2(0))$$, $$\sin(\varphi_2(0))$$, respectively.

 Figure 2: Implementation of equation (10) Figure 3: Implementation of equation (11)

Since $$\dot{\varphi}_1$$ and $$\dot{\varphi}_2$$ are readily available from the circuits shown in figures 2 and 3, two of these harmonic function generators can be directly fed with these values. The input for the third function generator is generated by a two-input summer as shown in figure 5.

 Figure 4: Generating sin($$\varphi$$) and cos($$\varphi$$) Figure 5: Computing $${\dot{\varphi}_1}$$ - $${\dot{\varphi}_2}$$
 Figure 6: Display circuit for the double pendulum

With $$\dot{\varphi}_1$$ and $$\dot{\varphi}_2$$ and thus $$\sin(\varphi_1)$$, $$\cos(\varphi_1)$$, and $$\sin(\varphi_2)$$, $$\cos(\varphi_2)$$ readily available, the double pendulum can be displayed on an oscilloscope featuring two separate $$x,y$$-inputs by means of the circuit shown in figure 6. To yield a flicker-free display, this circuit requires a high-frequency input $$\sin(\omega t)$$ which can be obtained by two integrators in series with a summer, the output of which is fed back into the first integrators. This circuit is not shown here but can be found in [ULMANN 2017, p.67].

## Results¶

Figure 7 shows a long-term exposure of the movements of the double pendulum starting as an inverted pendulum with its first pendulum rod pointing upwards while the second one points downwards.

 Figure 7: Long-term exposure of a double pendulum simulation run

This simulation requires ten integrators, 15 summers, 16 multipliers, and 17 coefficient potentiometers.

## References¶

[MAHRENHOLTZ, 1968] Oskar Mahrenholtz, Analogrechnen in Maschinenbau und Mechanik, BI Hochschultaschenbücher, 1968

[Ulmann 2017] Bernd Ulmann, Analog Computer Programming, Create Space, 2017

1

This has also been done by [MAHRENHOLTZ, 1968, pp. 159–165] which served as an inspiration for this application note, especially the display circuit.