Horologium, solo naturae motu, atque ingenio, dimetiens, et numerans momenta temporis, constantissime aequalia. (A clock that, by natural motions alone, indicates regularly equal divisions of time.) —Mateo de Alimenis Campani (1678) he escapement is a feedback reg- ulator that controls the speed of a mechanical clock. The first an- chor escapement used in a me- chanical clock was designed and applied by Robert Hooke (1635- 1703) around 1657, in London. Although there is Targument as to who invented the anchor escape- ment, either Robert Hooke or William Clement, credit is generally given to Hooke. Its application catalyzed a rapid succession in clock and watch escapement designs over the next 50 years that revolutionized timekeeping. In this article, I con- sider the advances this escapement design made possible and then describe how horolo- gists improved on this escapement in subse- quent designs. Before continuing, it is important to stress that the development of the escapement by gen- erations of horologists was largely an empirical trial-and-error process. As will be seen, this pro- cess was remarkably successful despite being based on only an intuitive understanding of physics and mechanical engineering principles. Even today, the understanding of the dynamics The author is with the Abbey Clock Clinic, Austin, TX 2001 CORBIS CORP. 78757, U.S.A. 0272-1708/02/$17.00©2002IEEE April 2002 IEEE Control Systems Magazine 41 of linkages under impact, friction, and other realistic effects tem is the potential energy of the driving weight, which falls is incomplete. Consequently, the explanations I give in this slowly during operation. In early clocks, the driving weight article concerning the evolution and operation of the clock could weigh as much as 1,000 lb, and large towers were con- escapement are based largely on kinematic, geometric, and structed to accommodate its range of motion. energy transfer principles. It is important to understand how the verge escapement An escapement mechanism is a speed regulator, and it works to appreciate the circumstances that led to the inven- uses feedback to obtain precision operation despite imper- tion of the anchor escapement. The oscillator consists of fect components. The presence of feedback is realized by the foliot, suspended at its center by a string, often made of the interaction between the escape wheel and silk. For the foliot to oscillate, accelerating and de- the escape arm, which interact according to celerating forces must be acting on it. their relative position and velocity. This interac- When a tooth of the escape wheel escapes, this tion can be seen in [1] and [2], where the wheel rotates freely by about 2° (called drop) un- verge-and-foliot escapement, one of the earliest til another tooth strikes an arm protruding from escapements, is analyzed. It is with this escape- the vertical shaft that is attached to the crossbar. ment that I begin this description of the evolu- The vertical shaft has two arms, called pallets, lo- tion of the anchor escapement. cated with about 100° of angular separation and with a vertical separation equal to the diameter of Prior to the Anchor the escape wheel. The pallets rotate by about 100° Escapement until a pallet releases an escape tooth. An instant The earliest record of a mechanical clock with an later, another escape tooth strikes the other pal- escapement, which is believed to date around let. As the pallets rotate, the escape tooth slides 1285, was a reference to a payment for a hired across the surface of the pallet, exerting a force on clock keeper at St. Paul’s in London. All the early it. The work done on a pallet is therefore the ap- mechanical timepieces are believed to have had a M. HEADRICK plied moment times the arc through which the es- verge and foliot as the control cape tooth moves during contact, mechanism for measuring the pas- and it is this moment that causes sage of time. The verge-and-foliot The Graham the foliot to accelerate and rotate in design was clearly based on the ala- one direction. In horology, the mo- rum (the alarm mechanism, with a escapement has been ment applied during contact is tra- hammer and a bell instead of a ditionally called impulse, although foliot), which was invented several the escapement of the applied torque is not necessar- centuries earlier. No one knows ex- ily impulsive in the usual engineer- actly when the mechanical clock choice in almost all ing sense. was invented or by whom. After another tooth strikes the First, let us consider a clock finer pendulum clocks other pallet, the foliot continues to consisting of a set of gears and a rotate in the same direction (as it driving weight, using the force of since 1715. was rotating in before the tooth gravity (see Fig. 1). In such a clock, struck the other pallet), causing the gears would spin uncontrolla- the other pallet to push the escape bly unless a control mechanism was applied at the other wheel backward as the foliot rotates. Since the escape end of the gear train. The control mechanism consists of an wheel exerts a force on the pallet, the pushing of the es- oscillating device that prevents the gear train from rotat- cape wheel backward causes a decelerating force, which is ing, except at specific intervals, when it releases one tooth opposite and equal in magnitude, to act on the pallet until of the last gear in the train. By controlling the rate of rota- the foliot stops. This backward action, called recoil, is tion of the gears, it is possible to use this device to measure equivalent to winding the clock by a small amount; in other time by incorporating an indicator and a scale at the end of words, energy is stored rather than wasted. After the foliot the shaft of one of the gears. has stopped, it changes direction, since the escape wheel The verge-and-foliot control mechanism consists of a continues to exert a force on the pallet, and the foliot be- shaft, called the verge, and a crossbar with a weight at- gins to accelerate in the opposite direction, continuing to tached at each end, called the foliot (Fig. 2). The weights can do so until it has rotated by about 100° and the pallet al- be moved to different positions on the crossbar, so that the lows the tooth to escape again. The escape wheel rotates radius (or distance) of the weights from the center deter- freely again by about 2° until another tooth strikes the mine the period of oscillation. The control mechanism is an other pallet. This process is repeated indefinitely. escapement because the energy is allowed to “escape” each Since it was difficult to control many of the factors that af- time a gear tooth is released. The stored energy of the sys- fected the period of oscillation of the foliot, the early clocks 42 IEEE Control Systems Magazine April 2002 were poor timekeepers, with errors exceeding several hours pulled in and had smaller diameters than before (tending to per day. The greatest problems were caused by changes in make the clock gain time and partially offsetting the effect of temperature and levels of friction. When the temperature in- time loss). This could be seen as a crude form of tempera- creases, the crossbar becomes longer due to the thermal ex- ture compensation. The wheel, or metal ring, that replaced pansion of the wrought iron, so the period increases, and the foliot is called a balance wheel, and it was introduced the clock loses time. Similarly, the clock gains time in colder around 1400. The foliot continued to be used as well, how- temperatures. ever, until around 1650. A warmer temperature causes the lubricants to become Another modification was the replacement of the weight thinner so that they create less resistance or drag, which re- with an elastic steel ribbon, called the mainspring. Its intro- sults in more energy reaching the foliot, and the clock gains duction around 1500 by Peter Henlein (1480-1542), a lock- time. The lubricants used in early clocks were primitive (an- smith from Nürnberg, is most significant because it made imal fats and fish or vegetable oils, especially olive oil) and possible the production of smaller and portable clocks (or did not have preservatives. Clocks needed to be lubricated very large pocket watches). It was extremely difficult to frequently because the lubricants were not hostile to bacte- make a steel ribbon by hand with the production methods ria, which accelerated their deterioration by causing the available at that time. formation of fatty acids that corroded metal parts and re- Mainsprings were relatively short and did not provide sulted in the formation of sludge, increasing resistance. constant power. Power levels were high when the clock (Since the foliot rotates by about 100° in each direction and was fully wound, decreasing gradually as the mainspring the pallets are almost continuously in contact with the es- unwound. Early spring-driven timepieces were extremely cape wheel, the action of the escapement is rather violent erratic timekeepers because they gained time drastically and requires a lot of energy to keep going. To reduce the re- at the beginning of the wind and lost time drastically to- quired power, it is necessary to reduce the levels of friction ward the end of the wind. Several devices were designed to involved, and thus there is a need for lubricants.) improve the moment-versus-angle curve of the mainspring, The first modification of the verge-and-foliot clock was but the spring-driven timepiece always remained an inferior the replacement of the foliot weights with a wheel in smaller timekeeper compared to an equivalent weight-driven time- (nonchurch) clocks.