Case Study: Pendulum Clock

This case study illustrates how to mix gvar.GVars with numerical routines for integrating differential equations (gvar.ode) and for finding roots of functions (gvar.root). It also gives a simple example of a simulation that uses gvar.GVars.

The Problem

The precision of a particular pendulum clock is limited by two dominant factors: 1) the length of the pendulum (0.25m) can be adjusted with a precision of at best ±0.5mm; and 2) irregularities in the drive mechanism mean that the maximum angle of swing (π/6) is uncertain by ±0.025 radians. The challenge is to determine how these uncertainties affect time-keeping over a day.

The angle theta(t) of the pendulum satisfies a differential equation

d/dt d/dt theta(t) = -(g/l) sin(theta(t))

where g is the acceleration due to gravity and the l is the length of the pendulum.

Pendulum Dynamics; Finding the Period

We start by designing code to integrate the differential equation:

import numpy as np
import gvar as gv

def make_pendulum(theta0, l):
    """ Create pendulum solution y(t) = [theta(t), d/dt theta(t)].

    Initial conditions are y(0) = [theta0, 0]. Parameter l is the
    length of the pendulum.
    """
    g_l = 9.8 / l
    def deriv(t, y):
        """ Calculate d/dt [theta(t), d/dt theta(t)]. """
        theta, dtheta_dt = y
        return np.array([dtheta_dt, - g_l * gv.sin(theta)])
    y0 = np.array([theta0, 0.0])
    return gv.ode.Integrator(deriv=deriv).solution(0.0, y0)

Given a solution y(t) of the differential equation from this method, we find the period of oscillation using gvar.root: the period is the time at which the pendulum returns to its starting point and its velocity (y(t)[1]) vanishes:

def find_period(y, Tapprox):
    """ Find oscillation period of pendulum solution y(t).

    Parameter Tapprox is the approximate period. The code finds the time
    between 0.7 * Tapprox and 1.3 * Tapprox where y(t)[1] = d/dt theta(t)
    vanishes. This is the period, provided Tapprox is correctly chosen.
    """
    def dtheta_dt(t):
        """ vanishes when dtheta/dt = 0 """
        return y(t)[1]
    return  gv.root.refine(dtheta_dt, (0.7 * Tapprox, 1.3 * Tapprox))

Analysis

The last piece of the code does the analysis:

def main():
    l = gv.gvar(0.25, 0.0005)               # length of pendulum
    theta_max = gv.gvar(np.pi / 6, 0.025)   # max angle of swing
    y = make_pendulum(theta_max, l)         # y(t) = [theta(t), d/dt  theta(t)]

    # period in sec
    T = find_period(y, Tapprox=1.0)
    print('period T = {} sec'.format(T))

    # uncertainty in minutes per day
    fmt = 'uncertainty = {:.2f} min/day\n'
    print(fmt.format((T.sdev / T.mean) * 60. * 24.))

    # error budget for T
    inputs = dict(l=l, theta_max=theta_max)
    outputs = dict(T=T)
    print(gv.fmt_errorbudget(outputs=outputs, inputs=inputs))

if __name__ == '__main__':
    main()

Here both the length of the pendulum and the maximum angle of swing have uncertainties and are represented by gvar.GVar objects. These uncertainties work their way through both the integration and root finding to give a final result for the period that is also a gvar.GVar. Running the code results in the following output:

period T = 1.0210(20) sec
uncertainty = 2.79 min/day

Partial % Errors:
                   T
--------------------
        l:      0.10
theta_max:      0.17
--------------------
    total:      0.19

The period is T = 1.0210(20) sec, which has an uncertainty of about ±0.2%. This corresponds to an uncertainty of ±2.8 min/day for the clock.

The uncertainty in the period is caused by the uncertainties in the length l and the angle of maximum swing theta_max. The error budget at the end of the output shows how much error comes from each source: 0.17% comes from the angle, and 0.10% comes from the length. (The two errors added in quadrature give the total.) We could have estimated the error due to the length from the standard formula 2π sqrt(l/g) for the period, which is approximately true here. Estimating the uncertainty due to the angle is trickier, since it comes from nonlinearities in the differential equation.

The error budget tells us how to improve the clock. For example, we can reduce the error due to the angle by redesigning the clock so that the maximum angle of swing is π/36 ± 0.025 rather than π/6 ± 0.025. The period becomes independent of the maximum angle as that angle vanishes, and so becomes less sensitive to uncertainties in it. Taking the smaller angle reduces that part of the period’s error from 0.17% to 0.03%, thereby cutting the total error almost in half, to ±0.10% or about ±1.5 min/day. Further improvement requires tighter control over the length of the pendulum.

Simulation

We can check the error propagation analysis above using a simulation. Adding the following code at the end of main() above

# check errors in T using a simulation
Tlist = []
for i in range(100):
    y = make_pendulum(theta_max(), l())
    T = find_period(y, Tapprox=1.0)
    Tlist.append(T)
print('period T = {:.4f} +- {:.4f}'.format(np.mean(Tlist), np.std(Tlist)))

gives the following additional output:

period T = 1.0209 +- 0.0020

The new code generates 100 different values for the period T, corresponding to randomly chosen values for theta_max and l drawn from the Gaussian distributions corresponding to their gvar.GVars. (In general, each call x() for gvar.GVar x is a new random number drawn from x‘s Gaussian distribution.) The mean and standard deviation of the list of periods give us our final result. Results fluctuate with only 100 samples; taking 10,000 samples shows that the result is 1.0210(20), as we obtained in the previous section above (using a tiny fraction of the computer time).

Note that the gvar.GVars in this simulation are uncorrelated and so their random values can be generated independently. gvar.raniter() should be used to generate random values from correlated gvar.GVars.