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Heliostats

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This section is for information that is indirectly related to heliostats and their uses, but which may be interesting to some readers.


Confused and Confusing Information in Reference Works

Heliostats are not widely known devices, even among the writers of dictionaries and encyclopedias. Many of these works contain information which is incorrect or outdated.

Some dictionaries and encyclopedias confuse Gay with sun-trackers. This is simply wrong.

Others have definitions and descriptions which essentially date back to the early 19th Century. At that time, the ancient Egyptian use of manually moved mirrors for daylighting was unknown. It was rediscovered after Egyptian hieroglyphic writing was deciphered, following the discovery of the Rosetta Stone in 1799. (This is a stone tablet, that still exists, which carries a lengthy inscription written in both hieroglyphs and Greek. It gave scholars an invaluable insight into the structure and meaning of Egyptian hieroglyphs.) In the early 1800s, all heliostats were laboratory instruments used for optical experiments. Like the ones made by Silbermann, they were driven by clockwork. Definitions of heliostats, written then, describe them as scientific instruments with mirrors driven by clockwork to reflect sunlight at stationary targets. At that time, these definitions were adequate. However, some of them are still used in modern works of reference. They make no mention of more recent designs, nor of present-day uses of the machines, nor of the ones in ancient Egypt. Extremely few of the heliostats now in use are correctly described by these old definitions and descriptions, which should all be updated as has been done here.


Unconventional Heliostats

The vast majority of heliostats fit into the categories described in "Types of Heliostat", above. However, machines are occasionally found that do not. Frequently, they have been constructed by amateur hobbyists.

Probably the most common are machines that are essentially the same as clockwork heliostats, but which are driven by other types of mechanism, such as electric or electronic clocks.

Devices that almost defy description also exist. For example, a few years ago the present writer met a retired carpenter who had a hobby workshop in his garage. He had a mirror outdoors to reflect daylight into it. The mirror was moved by an arrangement of electric motors, pulleys, and pieces of string. The carpenter had no relevant theoretical knowledge, did not know the word "heliostat", and was unaware that anyone else had ever done anything similar. Yet, by trial and error, he had made his device work, not well, but well enough for his purpose. Unfortunately, he was hoping to make his fame and fortune by selling his "invention".

Archimedes' Sun Weapon

The Greek scientist Archimedes, who is now best known for his work on buoyancy (Archimedes' Principle), lived in the city of Syracuse, which was then a Greek colony located on the coast of the island of Sicily. In about 215 BCE, Syracuse was attacked by Romans in ships. It is said that Archimedes helped with its defence by organizing people to hold large numbers of mirrors, using them to focus sunlight onto the ships, something like a field of heliostats focusing sunlight in a modern solar-thermal power station. The ships were set on fire by the concentrated solar heat. Modern tests of this idea have cast doubt on the story. The ships would have had to remain stationary close to the mirrors for long periods of time before igniting. Also, keeping all the mirrors accurately aligned by hand in the heat of battle would have been extremely difficult. If there is any truth behind the tale, it is more likely that the light from the mirrors served to dazzle the Romans, reducing their ability to fight.

Interestingly, Archimedes is thought to have studied in Alexandria, Egypt, which was then another Greek colony. There, he would probably have seen Egyptians using simple heliostats for daylighting their buildings. This could have interested him in different uses of mirrors.

Strange Experiences with a Heliostat

The following true story is included here since it may be instructive for anyone who is contemplating building a heliostat.


Back in the 1980s, I designed, built and programmed a computer controlled heliostat to bring more sunlight into the living room of my house. It was a simple machine, with the mirror in an alt-azimuth mount, so the principal axis of rotation was vertical and the other horizontal. The program that ran in its computer - an ancient Commodore VIC 20 - was developed from an earlier program I had written that calculated the position of the sun in the sky from astronomical theory.

Describing the heliostat in detail would be pointless. It was built from components that are now long out of date. Nowadays, duplicating it would be almost impossible. Anyone who wants to design a heliostat should start from scratch.

However, something happened when I was testing the machine that might be worth sharing...

I live in Canada, about mid-way between the Equator and the North Pole. Seen from here, the sun always moves clockwise in the sky, rising in the east, passing to the south around noon, and setting in the west. Since the heliostat mirror moves to keep reflecting sunlight in a constant direction, it was obvious to me that the azimuth drive of the machine would always turn clockwise. In fact, I contemplated using a motor for that drive which would turn only in the clockwise direction. I changed my mind only because it was convenient to use identical motors in the two drives. Since the elevation (altitude) drive changes direction as the sun rises and sets, I used motors, actually steppers, that could turn either way.

When I first set up the machine and started testing it, I encountered a couple of simple problems that I easily fixed. The next time I tried it, I was happy to see that it initialized itself and moved its mirror so as to reflect sunlight in the correct direction. It seemed to be working properly. But, as I watched it for a while, I saw to my dismay that the azimuth drive was turning anticlockwise. Obviously, that was wrong. I stopped the machine and started hunting in the hardware and software for the cause of the problem. In that, I had no success. Everything seemed to be as it should be, but the azimuth drive kept turning the wrong way.

Then I made another puzzling observation. Although the mirror was turning anticlockwise, it was continuing to reflect sunlight in the correct direction! The thing seemed to be working properly, but wrongly at the same time.

After some bemusement, I realized what was going on. It was about noon on a summer day, so the sun was high in the southern sky. The window through which I wanted the heliostat to reflect sunlight was roughly to the north of the mirror, and not much higher than it. The mirror had to be aimed in the direction that bisected the angle between the directions of the sun and the window, as seen from the mirror. At that time, the aim direction was high to the north. As the sun moved from east to west, this aim direction also moved from east to west. But since it was to the north of the zenith, this motion was anticlockwise in azimuth. My heliostat was absolutely correctly performing this anticlockwise rotation.

It was startling to realize that this machine, which I had designed and programmed, was apparently smarter than I was. It had correctly figured out which way to move, although my expectations were wrong.

Another realization was that only sheer good luck had allowed the machine to work properly. If I had used a motor in the azimuth drive that would turn only clockwise, which I had been sure would work properly, the machine would have been incapable of turning in the correct direction. I wonder how long it would have taken me to figure out what was wrong!

A few months later, as the noon-day sun sank lower in the sky as winter approached, a situation arose where the aim direction of the mirror was almost vertically upward at some time near mid-day. The angles of elevation of the sun and the window, as seen from the mirror, were equal, and their azimuths were 180 degrees apart, so the angle-bisector pointed vertically up. In that situation, a tiny movement of the bisector, as the sun moved westward, caused a large change in the bisector's azimuth. This made the azimuth drive of the heliostat turn very rapidly, about 180 degrees in just a few seconds! I hadn't expected the machine to move rapidly like that, since the sun moves very slowly in the sky. On one day, the drive spun anticlockwise, since the bisector was passing just to the north of the zenith. On the next day, the bisector passed just to the south of the zenith, and the drive spun clockwise. The reverse happened in the spring, as the sun moved northward. Of course, since the mirror was lying on its back, pointing upward, the rapid rotation about the vertical axis did not cause much change to the aim direction.

Heliostats can be very counter-intuitive machines!

DOwenWilliams 17:23, 5 July 2010 (UTC) David Williams


Some More Technical Details

For the sake of anyone who is thinking of designing and building a heliostat, here is a bit more about the VIC-controlled heliostat I built in the 1980s. (See "Strange Experience with a Heliostat", above.) The mirror was held in an alt-azimuth mount, so one of its two stepper motors turned the assembly about a vertical axis, i.e. in azimuth. The assembly included the second stepper motor and the horizontal axis about which it rotated the mirror, i.e. in elevation or altitude. There were two microswitches which closed when the two rotations reached specific positions. When I first set up the machine, I determined the compass bearing of the position where the azimuth switch closed and the angle of elevation where the altitude switch closed by experiment and measurement, and wrote these quantities into the software.

The software included an initialization routine which was executed whenever the machine was re-started, e.g. after a power outage, and also every morning at sunrise. The two drives stepped around until their respective switches closed. They then stepped slowly in the opposite directions, counting steps until the switches opened. This put the mirror into a known orientation, and also measured the backlashes in the drives. During the day, the computer sent requisite numbers of stepping instructions to the motors to turn the mirror to the required orientation. Also, when the direction of either rotation changed, additional stepping instructions were sent to take up the backlash. This meant that the machine automatically compensated for wear of its mechanical components.

At sunset each evening, i.e. when the software determined that the sun's angle of elevation became negative, the machine turned the mirror so it faced downward. This was to reduce the buildup of dust on its surface. The mirror remained in this position until sunrise, when the initialization routine was executed.

The wires leading to the elevation drive and its microswitch could have become twisted until they broke if the azimuth drive had turned many times in a single direction. To avoid this, the software automatically made the azimuth drive turn a full rotation in the opposite direction if it had previously turned more than a full rotation from its starting point, where its microswitch closed.

The VIC's internal clock was used to give the heliostat the time and date. Of course, like any other clock, it did not keep perfect time. The software therefore included a routine that allowed me to reset the clock without stopping the heliostat program. A feature of the VIC's clock was that its speed could be finely adjusted. My software made that adjustment automatically whenever I reset the clock. It kept a record of the date when each reset was done, and calculated by how much the speed should be adjusted to optimize the timekeeping. After two or three resets, the timekeeping was accurate to within a few seconds per year.

In order to make the machine capable of surviving brief interruptions of the AC power, I added a high-value capacitor essentially in parallel with the capacitor that the VIC had in its power supply. However, charging this large capacitor rapidly would have overloaded the transformer and rectifier, possibly damaging them. I therefore put a resistor and diode in parallel with each other and in series with the big capacitor. The charging current went through the resistor, which limited it to a low value. The capacitor took several minutes to charge. During a power interruption, the capacitor discharged through the diode as rapidly as required to keep the computer running. This arrangement eliminated computer crashes due to power flickers. It was still possible for the stepping motors to get slightly out of step, but this was automatically corrected the next morning when the initialization routine was executed.

I designed and built an interface unit that fed power to the stepper motors and connected them, and the microswitches, with the computer. This unit took power directly from the AC supply. It received small control signals from the computer, changed them to the necessary voltage, etc., and fed them to the motors. It was capable of producing two different voltages, a high one which was fed to the motors while they were executing steps, and a lower voltage which was used just to hold the motors stationary. Since the motors were stationary for most of the time, using the low voltage conserved power and prevented the motors from getting warm. The computer signalled which voltage should be produced. Just before steps were taken, the software made the computer select the high voltage, and return to the low voltage when stepping was temporarily completed.

None of this is of any theoretical importance, but it does show the kinds of things that must be considered when designing an operational machine.

DOwenWilliams (talk) 22:47, 31 October 2010 (UTC) David Williams

See Also

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