Impulsing the Pendulum: Escapement Error by Dr

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Impulsing the Pendulum: Escapement Error by Dr. George Feinstein (NY) A version of this article was originally published in the NAWCC Horological Science Chapter 161 Newsletter—Issue 2005-3, June 2005.

I. INTRODUCTION Table 1. Circular Error for 1-Second The purpose of this article is to investigate Nondamped Pendulum, Series Solution the following questions: What is the ideal location to impulse a pendulum? What are the consequences of not impulsing at the ideal location from the viewpoint of accurate timekeeping? Over the years a lot of attention has been given to circular error and not enough to the so-called “escapement” error, which is actual- ly impulse error. Impulse error is the change in the period of a pendulum due to the act of impulsing. Impulsing is required to maintain the amplitude of the pendulum and to take a synchronizing signal from it. In our space-time continuum three dimen- sions are required to define a point. The ideal location for impulsing a pendulum is defined by the swing plane, the center of percussion, and the position of equilibrium of the pendulum. The position of equilibrium is the position the pendulum takes when it is at rest. II. CIRCULAR ERROR An item to consider for precision timekeeping with a pendulum is maintaining a constant amplitude of swing. Most present-day pendulums follow a circular path, so they have what is called circular error. Circular error is the change in the period of a pendulum due to the fact that a freely swinging pendulum, following a circular path DESCRIPTION OF TERMS under the influence of gravity, takes longer to Nominal period - The desired period of a pendulum in seconds, the period is traverse a large arc than a small one. twice the time of the pendulum designation (i.e., for a 1-second pendulum the As can be seen in Table 1, the rate of period is 2 seconds). change of circular error increases with an g - Acceleration due to gravity, in centimeters per second per second. Length - Theoretical pendulum length for the nominal period, in centime- increased amplitude. Thus, to minimize the ters. See Eq. (B-1). problems with circular error, it is best to keep 1 & 2. Semiamplitude - The chosen angle of swing from the position of equi- the amplitude of the pendulum as small as librium to the position of maximum displacement, given in degrees and min- possible. Later we will use the results of Table utes. 1 to try to cancel out the effect of the change 3. Period - The actual period of the pendulum, including the circular error. in circular error with the change in the See Eq. (E-3). 4. Circular error - The increase in the period of a circular pendulum due to impulse error. It should also be noted that in an increase in the amplitude of swing, given in seconds per day. See Eq. (E-2). both cases (i.e., circular and impulse errors), 5. Linearity of circular error - Compared to the 1-minute value. the “error” itself causes no problem in time- 6. Rate of Change - The rate of change of the circular error, which is the slope keeping because the pendulum length can be of the circular error vs. semiamplitude curve, in seconds per day per second of adjusted to compensate for them; the real arc. See Eq. (E-4). problem is the change in the error due to 7. Linearity of rates of change - Rate of change of circular error compared to the semi amplitude of the pendulum. slight variations in the amplitude of the pen- 8. Maximum velocity - The maximum velocity of the pendulum, which occurs dulum. at the position of equilibrium, in centimeters per second. See Eq. (B-3).

Month 2006 NAWCC BULLETIN 1 III. SWING PLANE many factors, not measured by Q, that affect the time- The impulsing force should be perpendicular to the keeping ability of a pendulum. For example, Q would axis of the pendulum and in the swing plane of the pen- not be directly affected by temperature error, errors dulum. The axis of the pendulum is the line perpendic- due to buoyancy effects, circular error, or errors due to ular to its axis of rotation and passing through its cen- phase angle changes in the impulsing. ter of mass. The swing plane is the plane generated by From the above we see that although it is desirable rotating the axis of the pendulum about the axis of and maybe even necessary to have a large Q for good rotation. timekeeping, it is not sufficient. A pendulum with a Any force that is not in the swing plane and perpen- high Q can be a good or bad timekeeper depending on dicular to the axis of the pendulum produces one or its attributes not measured by Q, but a pendulum with more of the following effects: twisting, bending, and a low Q will never be a great timekeeper. stretching or shortening of the pendulum. These effects VI.“ESCAPEMENT” - IMPULSE ERROR2,3,4,5,6,7 cause extraneous movements of the pendulum that Escapement error was first described by George affect the regularity of its motion. Biddell Airy as Example 7 in his 1826 article “On the IV. CENTER OF PERCUSSION Disturbances of Pendulums and Balances, and on the (CENTER OF OSCILLATION) Theory of Escapements.” There have been qualitative discussions of the prob- A ball hitting the “sweet spot” of a baseball bat or lem since then, but no exact and comprehensive quan- tennis racket causes no sting or jarring to the hands titative presentations, due to the extensive computa- gripping the bat or racket. The sweet spot is at the cen- tions required. ter of percussion of the bat or racket. If a pendulum is impulsed at its center of percus- VII. IMPULSE ERROR, QUALITATIVE REVIEW sion, there is no reaction at the pivot point or center of To understand the cause of impulse error we will rotation. For a compound pendulum (a real-life pendu- examine five cases of impulsing the pendulum: lum) the distance from the center of rotation to the cen- A. Impulse at the position of equilibrium ter of percussion is equal to the length of a theoretical B. Impulse at the end of swing, away from the posi- simple pendulum of the same period. Another relation- tion of equilibrium ship between the center of rotation and the center of C. Impulse at the end of swing, toward the position percussion is that they are conjugate to each other. of equilibrium That is, if the pendulum is suspended from its center of D. Impulse at intermediate phase angle, away from percussion, its period remains the same, and the origi- the position of equilibrium nal center of rotation becomes the new center of per- E. Impulse at intermediate phase angle, toward the cussion. For this reason it is also called the “center of position of equilibrium oscillation.” Any impulsing force applied at other than the center For this exercise use a 1-second isochronous pendulum (i.e., no circular error). of percussion will disturb the pendulum by causing the A. The velocity of the pendulum is increased by the pendulum rod to flex and vibrate. The closer the loca- impulse at the position of equilibrium. This increases tion of the impulsing is to the center of percussion the the semiamplitude of the pendulum, but there is no lighter the required pendulum rod. change in the period because the pendulum is isochro- V.“Q” IS FOR QUALITY?1 nous. Q is called the Quality factor and is a measure of the B. The pendulum takes 0.5 second to travel from the efficiency of a pendulum in maintaining its amplitude. position of equilibrium to the end of the swing, at From the literature it would seem that a “free” pendu- which point it is impulsed in the direction away from lum requiring no impulsing and therefore having an the position of equilibrium. This causes the pendulum infinite Q would be the perfect pendulum. But from the to travel some additional distance from the position of timekeeping point of view this is not true. equilibrium, before stopping again, which takes some Δ It is true that “other things being equal” the higher additional time; let’s call it tB. After the pendulum the Q, the more constant the period of the pendulum, reaches this point, it reverses direction and returns to but the corollary that any pendulum with a high Q has the position of equilibrium in 0.5 second because it is a more constant period than any other pendulum with isochronous. Thus, the total time of travel is 1.0 second Δ Δ a lower Q is untrue. + tB. The term + tB is the impulse error for this case. Q is a measure of the relative magnitude of the C. The pendulum takes 0.5 second to travel from the impulse required to maintain pendulum amplitude; position of equilibrium to the end of the swing, at thus, it is also a measure of the magnitude of the vari- which point it is impulsed in the direction toward the ous energy losses the pendulum experiences. There are position of equilibrium. This causes the pendulum to

2 NAWCC BULLETIN Month 2006 start traveling toward the position of equilibrium at ing away from the position of equilibrium increase the some finite velocity rather than zero speed, as if it had period. started from some point further from the position of Because damping forces act throughout the motion equilibrium. Thus, the time to return to the position of of a pendulum and are generally symmetric (i.e., acting Δ equilibrium is 0.5 second - tC, and the total travel time half the time toward the position of equilibrium and Δ Δ is 1.0 second - tC. The term - tC is the impulse error the other half away from the position of equilibrium), for this case. they have practically no net effect on the period of the D. The pendulum swings from the position of equi- pendulum. librium to the impulse phase angle, at which point it is However, the positive impulsing force generally acts impulsed in the direction away from the position of repetitively over a small portion of the pendulum’s equilibrium and continues to the end of the swing. motion and usually in the same direction, either After the pendulum reaches this end point, it revers- toward or away from the position of equilibrium, es direction. Because the pendulum is isochronous, it depending on the escapement, therefore producing an returns to the position of equilibrium in 0.5 second impulse error. However, the initial travel from the position of equilib- VIII. IMPULSE ERROR, QUANTITATIVE EVALUATION rium to the end of the swing took 0.5 second + Δt D The values of the impulse errors and rates of change because the semiamplitude traveled was for the pen- were calculated on a Quattro Pro spreadsheet by using dulum with the impulsed energy state; however, ini- the equations derived in the Appendices. The Quattro tially the pendulum was traveling at the lower pre- Pro spreadsheet carries 15 significant figures. It would impulse velocity until it reached the point of impulsing. have been better to use a program that carries more Thus, the total time of travel is 1.0 second. + Δt . The D significant figures, such as FORTRAN, but with care Δ term + tD is the impulse error for this case. the Quattro Pro program was adequate for the job. E. The pendulum swings from the position of equi- First, the impulse errors and rates of change were librium to the end of the swing in 0.5 second After calculated for the following four cases (see Table 2): reaching this point the pendulum reverses direction 1. Nondamped circular path pendulum using a and continues to the impulse phase angle, at which series solution of elliptic integrals. This was the first point it is impulsed in the direction toward the position case used to calculate impulse error; the equation is of equilibrium and returns to the position of equilibri- the only pendulum equation of motion that has an um. The trip from the end of the swing to the position exact solution and is the easiest to use. It is the classic Δ of equilibrium took less than 0.5 second by - tE equation of motion of a pendulum following a circular because the semiamplitude traveled was for the pen- path. The one deficiency of this equation is that it is a dulum with the preimpulsed energy state; however, mathematical equivalent to a perpetual motion after the impulse the pendulum was traveling at the machine. Because there is no damping to reduce the higher postimpulse velocity from the point of impuls- amplitude of swing, any impulse will increase the ing until it reached the position of equilibrium. Thus, amplitude of the pendulum. There was a question Δ Δ the total time of travel is 1.0 second - tE; the term - tE whether this would affect the results. is the impulse error for this case. 2. Linearly damped circular path pendulum using Even though the above discussion was done for a 1.0 fourth-order Runge-Kutta method. This case includes second isochronous pendulum, the conclusions are damping and a reduction in amplitude, which is coun- valid for all pendulums, balance wheels, and torsion tered by impulsing. But this case requires numerical pendulums. integration for solution and thus a much greater Impulse (escapement) error is due to the time differ- amount of calculation. ence in the travel of the pendulum from the position of 3. Nondamped cycloidal path pendulum using equilibrium to the point of impulse, with the initial and numerical linear integration impulsed energy states. 4. Linearly damped cycloidal path pendulum using All interference to the motion of a pendulum is a an algebraic solution. Cases 3 and 4 are for a cycloidal form of impulsing. There is both positive and negative path pendulum, which is isochronous and thus has no impulsing. Positive impulsing (in the direction of circular error and were used to ensure that circular motion of the pendulum) is energy transferred to the error was correctly eliminated in cases 1 and 2. pendulum to maintain its energy level, whereas nega- These calculations were done for Q = 10,000 and tive impulsing (opposed to the direction of motion of semiamplitude = 240 minutes. All cases were calculat- the pendulum) is energy taken from the pendulum. ed at the same energy levels, and the circular path It should be noted that any forces applied to the pen- solutions were corrected for circular error. The calcula- dulum and acting toward the position of equilibrium tions for the four cases used totally different mathe- reduce the period of the pendulum, whereas forces act- matical techniques; however, the results came out vir-

Month 2006 NAWCC BULLETIN 3 any of the lines of data in Table 2. Comparison of Impulse Error for Circular and Cyclodial Table 3, for a particular rela- Pendulums, Both Linearly Damped and Nondamped tive impulse location (column 1) the impulse error (column 2) is constant for all semi- amplitudes, whereas the RCIE (columns 4, 6, 8, 10, and 12) is reduced with an increase in semiamplitude. That is, the greater the semiamplitude the smaller the RCIE. We also see that the RCIE grows very rapidly near the very end of the swing. This is the reason that pendulums impulsed there are very poor timekeepers. During 90 percent of the swing of the pendulum, the impulse error is nearly linearly inversely proportional to Q with a deviation of less than 5 percent; however, near the end of the swing the relation becomes nonlinear and changes to a relation of the square root of the inverse of Q at the very end of the swing. The reason for the above is as follows: The impulse, no matter where in the swing it is regularly applied, has to provide the same amount of tually identical (see Table 2). Because of this a decision energy to maintain the amplitude of the pendulum. The was made to perform all the rest of the calculations by moving pendulum has two forms of energy: potential using the equations for the nondamped circular path and kinetic. The potential energy is due to the location pendulum because it was the simplest to work with. of the center of mass above some fixed datum, whereas The impulse errors and rates of change of the the kinetic energy is due to the motion of the pendulum impulse error (RCIE) were calculated for five values of and is a function of the square of the velocity of the cen- the quality factor Q, from Q = 100 to Q = 1,000,000. For ter of mass of the pendulum. The impulse increases the each value of Q, calculations were done for five values kinetic energy of the pendulum. When the pendulum is of the semiamplitude, from 30 minutes to 480 minutes traveling at a relatively high velocity (e.g., near the of arc, which covers the range for most presently used position of equilibrium), the change in velocity to pro- pendulums. vide the impulse energy is very small, and it has a very The impulse is provided once per swing by increas- small effect on the period of the pendulum. But when ing the velocity at the required point in the swing. the pendulum is at or near the ends of the swing (i.e., From these results it was found that impulse error moving very slowly), the change in velocity is much (see Figure 1) is a function of two variables: (1) the greater and so is its effect on the period of the pendu- quality factor Q, which defines the magnitude of the lum. The kinetic energy of the pendulum, as stated ear- impulse, and (2) the relative impulse location. The rel- lier, is proportional to the square of the velocity of the ative impulse location is obtained by dividing the pendulum, whereas the period of the pendulum is pro- phase angle of the impulse by the semiamplitude of the portional to the velocity. This is the cause for the pendulum (phase angle/semiamplitude). impulse error at the ends of the swing being propor- Because of this, even though the impulse error is tional to the inverse of the square root of Q. independent of the semiamplitude, the RCIE is For over 90 percent of the swing, mentioned earlier, inversely proportional to the semiamplitude (see the average impulse error is essentially the same Figure 2 and Table 3). We see this by noting that for whether the escapement impulses the pendulum once

4 NAWCC BULLETIN Month 2006 Figure 1. Escapement error for one impulse/swing. Figure 2. Relative rate of change of escapement error. in every swing, as in these Table 3A. Impulse Error for 1-Second Nondamped Circular Pendulum, calculations, or once every Eliptic Integral - Series Solution other swing, or even once in 30 swings as in some escapements. It is important to note that even though the average impulse error is practically the same, the actual impulse error only occurs in the swing that the impulse is applied. Impulse error can be positive or negative, depending on the direction of the impulsing force. An impulsing force acting away from the position of equilibrium produces a positive impulse error, whereas one toward the position of equilibrium produces a negative impulse error. A positive impulse error increases the period of the pendulum, but a negative one reduces the period.

Month 2006 NAWCC BULLETIN 5 Table 3B. Impulse Error for 1-Second Nondamped Circular Pendulum, Eliptic Integral - Series Solution

IX. IMPULSE ERROR, CONCLUSIONS would need an escapement with two attributes: (1) The By comparing the rate of change of the circular error impulse must be toward the position of equilibrium (RCCE) (see Table 1) with the RCIE (see Table 3) we and (2) The relative impulse location must be see that the optimum semiamplitude is dependent on adjustable. the value of Q for the clock and the relative impulse Then we could choose a combination of amplitude location because increasing the amplitude increases and phase angle (relative impulse location) such that the RCCE but decreases the RCIE. when the amplitude varied, the increase or decrease in The ideal would be to eliminate or at least to mini- the circular error would be corrected by the decrease or mize the total change in the circular error plus impulse increase in the impulse error. error by matching the RCCE and RCIE by adjusting For example from Table 1 the RCCE for a semi- the semiamplitude for a pendulum with a specific amplitude of 4o is 0.00366 sec/day/asec, whereas from value of Q. Table 3B for a Q = 10,000, a semiamplitude of 240' (4o) We can theoretically eliminate the combined change for an impulse toward the position of equilibrium and in circular error plus the impulse error. To do this we a phase angle of 216' the RCIE is -0.00362 sec/day/asec;

6 NAWCC BULLETIN Month 2006 Table 3C. Impulse Error for 1-Second Nondamped Circular Pendulum, Eliptical Integral - Series Solution

a slight increase in the phase angle or decrease in the About the Author semiamplitude will make the two changes exactly can- A retired civil engineer by profession, Dr. George cel out each other. Feinstein has spent his career in various phases of X. APPENDIX mass transportation. He is very active in Electrical Horology Chapter 78 where he has been the co-editor of For the equations used to obtain the numerical the chapter newsletter since 1987. He has contributed results in this article and their derivation with a com- several articles to that publication, including descrip- plete set of results, see the online appendix at tions of the Fedchenko clock as well as a list of U.S. www.nawcc.org/pub/bulsupp.htm or contact the patents in electrical horology. NAWCC Library and Research Center.

Month 2006 NAWCC BULLETIN 7 8 NAWCC BULLETIN Month 2006 Month 2006 NAWCC BULLETIN 9 10 NAWCC BULLETIN Month 2006 Month 2006 NAWCC BULLETIN 11 12 NAWCC BULLETIN Month 2006 Month 2006 NAWCC BULLETIN 13 14 NAWCC BULLETIN Month 2006 Month 2006 NAWCC BULLETIN 15 16 NAWCC BULLETIN Month 2006 Month 2006 NAWCC BULLETIN 17 18 NAWCC BULLETIN Month 2006 Month 2006 NAWCC BULLETIN 19 Notes 1931: March, pp. 129-130; Apr., pp. 147-149; Sept., pp. 1. D. A. Bateman, “Accuracy of Pendulums and Many 16-18; Nov., pp. 40-42; Dec., pp. 68-72 Factors that Influence It,” NAWCC BULLETIN, No. 290 1932: Feb., pp. 96-97. (June 1994): pp. 300-312. 5. E. C. Atkinson, The Proceedings of the Physical 2. G. B. Airy, On the Disturbances of Pendulums and Society, London. Balances, and on the Theory of Escapements, a. Escapement Errors of Pendulum Clocks, Vol. 42, Transactions of the Cambridge Philosophical Society, 1930: pp. 58-70. Vol. 3, 1830: pp. 105-128. b. The Relation Between Rate and Arc for a Free 3. Prof. R. A. Sampson, Studies in Clocks and Time- Pendulum, Vol. 48, 1936: pp. 606-617. keeping, Proceedings of The Royal Society of c. On Airy’s Disturbance Integrals and Knife-edge Edinburgh. a. No. 1. Theory of the Maintenance of Motion, Vol. 38, Supports for Pendulums, Vol. 48, 1936: pp. 618-625. 1918: pp. 75-114. d. The Dissipation of Energy by a Pendulum b. No. 2. Tables of the Circular Equation, Vol. 38, Swinging in Air, Vol. 50, 1938: p. 721-741. 1918: pp. 169-218. e. The Amplitude Deviation of Rate of a Pendulum: A c. No. 3. Comparative Rates of Certain Clocks,Vol. Second Experiment, Vol. 50, 1938: p. 742-756. 44, 1924: pp. 56-76. 6. Paul F. Gaehr, Airy’s Theorum and the d. No. 4. The Present-day Performance of Clocks,Vol. Improvement of Clocks, American Journal of Physics, 48, 1928: pp. 161-166. Vol. 16, 1948: pp. 336-342, 519-520. e. No. 5. The Suspended Chronometer, Vol. 56, 1935: 7. A. L. Rawlings, The Science of Clocks & Watches pp. 13-25. (British Horological Institute, 3rd Ed., 1993): pp. 163- f. No. 6. The Arc Equation, Vol. 57, 1936: pp. 55-63. 185. 4. Prof. David Robertson, “The Theory of Pendulums 8. V. M. Faires & S. D. Chambers, Analytic Mechanics and Escapements,” Horological Journal. (The Macmillan Co., 3rd Ed., 1952): pp. 359-363. 1928: Sept., pp. 11-16; Oct., pp. 45-48; Nov., pp. 70-73; 9. H. W. Reddick & F. H. Miller, Advanced Dec., pp. 101-105. Mathematics for Engineers (John Wiley & Sons, Inc., 1929: Jan., pp. 128-131; Feb., pp. 160-163; March, pp. 194-196; Apr., pp. 215-218; May, pp. 236-240; June, 3rd Ed., 1955): pp. 128-133. pp. 254-256; July, pp. 271-274; Aug., pp. 293-295; 10. I. S. Gradshteyn & I. M. Ryzhik, Table of Nov., pp. 51-54. Integrals, Series, and Products (Academic Press, Inc., 1930: Feb., pp. 116-119; March, pp. 139-140; Apr., pp. 5th Ed., 1994): pp. 907-913. 153-156; June, pp. 191-193; Aug., pp. 219-222; 11. B. O. Peirce, A Short Table of Integrals (Ginn & Sept., pp. 16-18; Nov., pp. 52-54. Co., 3rd Ed., 1929): pp. 66-67.

20 NAWCC BULLETIN Month 2006 12. C. H. Edwards & D. E. Penney, Differential 18. S. L. Loney, An Elementary Treatise on the Equations & Linear Algebra (Prentice Hall, 2001): pp. Dynamics of a Particle and of Rigid Bodies (Cambridge 564-566. at the University Press 1950): pp. 108-110. 13. S. C. Bloch, Classical and Quantum Harmonic 19. W. D. MacMillan, Theoretical Mechanics; Statics Oscillators (John Wiley & Sons, Inc., 1997): pp. 93-106. and the Dynamics of a Particle (Dover Publications, 14. W. E. Boyce & R. C. DiPrima, Elementary Inc., 1958): pp. 319-321. Differential Equations and Boundary Value Problems 20. L. A. Pars, Introduction to Dynamics (Cambridge (John Wiley & Sons, Inc. 5th Ed., 1992): pp. 444-445. at the University Press, 1953): pp. 214-217. 15. D. Greenspan, Discrete Numerical Methods in 21. B. Williamson, An Elementary Treatise on the Physics and Engineering (Academic Press, Inc., 1974): Differential Calculus (Longmans, Green, & Co., 9th p. 40. Ed., 1899): pp. 335-339. 16. F. B. Hildebrand, Introduction to Numerical 22. L. N. Hand & J. D. Finch, Analytical Mechanics Analysis (McGraw-Hill Book Co., 2nd Ed., 1974): pp. 29 (Cambridge University Press, 1998): pp. 90-94. 1-292. 17. P. A. Tipler, Physics for Scientists and Engineers (W. H. Freeman & Co. 4th Ed., 1999): p. 423-424, 43 1- 432.

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