The field of control engineering began with mechanical clocks thousands of years ago.
SUN – WATER & SPRINGS
II.A.1 SUN CLOCKS to fight starvation
Everyone is familiar with sun dials which can mark the hours of the day based on the position of the sun, but sun was also used to mark seasons. The Fajada Butte at Chaco Canyon, New Mexico uses a dagger of light filtered between boulders on a rock wall to show how close we are to the next equanox or solstice. Tracking the moon and sun over seasons was important for telling farmers around the planet when to start planting or when to harvest. Clocks were a tool used against the forces of starvation as entire seasons of crops could be destroyed by planting too early, or harvesting too late.
II.A.2 WATER CLOCKS for justice, industry and art
6000 Years ago in China, Egypt and Greece we used water clocks to measure the passing of hours. Ancient Greeks figured out how to use gears and wheels to make draining water consistantly drain, the clock was called a “clepsydra”. Why would early people need to know the passing of hours in what many people today view as a era with a “simpler” life “without all the technology”?
Early uses:
- Use in processing metal, to time the stages of production
- Use in court to ensure each speaker spoke for the same alloted time
- Use in diverting stream flow to give different land owners the same amount of irrigation water.
- Entertainment – making figurines and musical noises (Athens, Greece)
- Calendars – zodiacs, lunar and solar orbits, the first “computer” (Al-Jazari’s Castle Clock)
The “Invention” of Hours:
With the water clock the Egyptions divided the day into 12 units of daylight, and 12 of night time. This worked during the equanox (2 days a year) and the rest of the days the “hour” would vary in length. Hipparchus of anchient Greece proposed that the 24 parts of the day should remain equal in length, and the idea of the “hour” was born. There was a problem however, the average person was not located near or did not have a sophiscated water clock. With most people being unable to keep track of the consistant hour they did not adopt the idea. It was only 1400 years later that Europe changed to a true “hour” being exactly 1/24th of an earth’s single rotation.
Minutes! Who Needs Them?!
When Hipparchus took the sexagesimal system already used to divide up a circle into 60 parts to divide the hour into 60 parts, his idea was once again fell on deaf ears. Peasants and leaders alike didn’t see the need to be so detailed as to use Hipparchus’s 60 minutes and 60 seconds, so it took over a thousand years before these incriments of time became standardized and adopted by the masses.
II.A.3. SPRINGS for Kings, War and Cuckoo Sounds
If you remember from Unit 1 it was types of round springs which allowed us to first accurately measure the quantity of electricity in an electric meter. Two hundred years before that these springs gave us grandfather clocks and wind-up clocks.
Advantages:
- No need for sunshine or reloading a reservoir with water
- Increased reliability and accuracy over all previous time keepers
- Portability meant clocks could be used in war to more accurately record metrics and direct movements. Record keeping is a hidden important factor in military history as war technology required study of past events to evolve.
- Portability meant stop watches and other non-pendulum clocks could be used on ships which helped navigation accuracy
Pendalums:
Due to the experience of creating water clocks, the basic gearing of “clock hands” was already understood, so how could we make a predictable and uniform oscillation to replace the spinning water driven wheel? A pendalum was the answer.
Taking a rigid swinging rod with minimal resistance at the bearing was an obvious answer to creating an oscillation. What was not obvious was how to keep it swinging. The invention of the spiral spring would help.
Turn the knob (transfer energy from the human) this tightens the spring (stores a reservoir of energy), and through gearing keep giving a little “push” to a pendulum to keep it going. From here we can let each swing move a wheel that drives the seconds in the clock gearing.
The escape wheel turns and the anchor fork both nudges the pendulum to keep it going, and it stops the escape wheel. It’s necessary to stop the escape wheel so the springs don’t just use up all the energy.
See the video here for a demonstration: https://youtu.be/lDZHlcjQnOo?t=105
The problem with most pendulum type clocks:
- Will not work on a ship in constant motion
- Temperature variations cause the materials to expand or contract, therefore changing the length of the oscillation, therefore making the clock inaccurate so it has to be recalibrated frequently.
Portable time
The invention of the balance wheel was the last piece to the challenge of how to build a clock that could move. The pendulum clock was stuck in one place, carefully calibrated. To move it you had to stop it and set it up in a new spot. Instead of swinging back and forth, they figured out they could cause a weighted wheel to rotate back and forth.
The balance wheel uses a hairspring carefully aligned with a balance wheel. When the balance wheel swings it turns the spring until the resistance in the spring pushes the wheel back the opposite direction.
Just like with the pendulum we needed to give the balance wheel a nudge to keep it going, so we still used an anchor fork to push it. For a detailed old film on this see the film here:
The Chronometer:
In 1714 a clock in a vacuum was made to try to reduce the inaccuracies caused by warm/cold air causing gears to expand or contract. Jeremy Thacker developed this method which soon helped in ship navigation.
The Marine Chronometer:
Imagine being at sea far from the sight of land, how do you know which way to go? Precise time keeping was needed to calculate latitude and longitude. John Harrison’s “chronometer” of 1730 (H1, which used a pendulum and temperature control) was the start of 200 years of use of chronometers for ship navigation. The time pieces were later replaced by other devices that used radio frequencies to keep a more accurate time.
Latitude: The north star was an accurate way to determine latitude on the open ocean, however longitude was difficult to measure, this is where the clock helps.
Greenwich Mean Time and the start of a world standard:
Today we use an international system of time that counts Greenwich, England as the starting point. How did this happen? Well it’s linked to the marine industry of the 1700s.
When a ship left Greenwich, the clock would start.
Greenwich time – local time x 15 degrees of longitude = your longitudinal position
The clock that build an empire:
The idea of using two clocks was not new, but no clock was accurate enough over months under variable temperatures until 1730. While it took time for scientists to accept the clock Harrison, the inventor eventually became fabulously wealthy when the English Royal Navy adopted his designs. The custom built high accuracy chronometer of the 1700s could only be afforded by the highest admirals, and with time (no pun intended) saw mass production and widespread use.
Europe in the 1700s was fully occupied with war and expansion of trade routes around the world. The invention of the marine chronometer gave Britain a real edge over competitors and they quickly employed it to map the world. With accurate maps and navigation it wasn’t long before the sun never set on the British Empire.
The Pocket Watch:
We are all familiar with the gold pocket watches depicted in films or treasured as family heirlooms. Earlier we talked about how the clock revolutionized global trade networks through marine applications. The clock was also critical for overland travel.
In the 1800s a railroad grade pocket watch was created to keep thousands of railroad workers coordinated across the expansive continental railroads of the US, India and Europe. Workers would synchronize their watches on each shift change.
There are many forms of mechanical clocks, however they all suffer from inaccuracies due to expansion of materials and other issues. In the video below John Lowe from NIST tells us about the problems with mechanical clocks:
Now lets move on to staple in modern clocks, the quartz clock.