Arduino_heliostat

Model alt/az heliostat using Arduino

This is a complete, functioning model of a heliostat, typically used to position one mirror of a solar concentrator. The mirror is continuously adjusted according to the geographic location and time of day, so that the sun's rays are always focused on a fixed spot.

The model uses a home made alt/az mount, with geared stepper motor driver for the azimuth setting, and a high resolution, 2000 step per revolution five phase motor as direct drive for the mirror altitude.

The Arduino portion of the project consists of a serial link command intepreter to align the device, the Arduino SolarPosition library to calculate the sun position at any geographic location and UTC time, and TimeLib.h to keep track of the UTC and local time.

Method of operation:

The basic mechanism consists of an alt/azimuth drive that can accurately point to a given angle, i.e. a telescope mount. To create a heliostat, the mirror needs to be positioned according to the Law of Reflection: the vector perpendicular to the mirror surface must bisect the angles to the sun and to the target. Once the device is oriented so that azimuth zero points to True North and altitude zero is perfectly horizontal, the sun angle can be calculated for any time of day at any point on the Earth's surface.

What remains to be determined is the angle to the target, but that is trivial: the mirror is simply positioned "by hand" via serial commands, so that sunlight is reflected onto the target. Since the sun angle is known, one can back-calculate from the mirror position to determine the angle to the target. From this point on, the mirror can be automatically adjusted to track the sun as accurately as desired.

Here I make use of the Arduino SolarPosition and TimeLib libraries to track the sun. The only remaining data required is to set the correct geographic location and time of day.

Construction:

1. The azimuth ring is a mixture of commercial and home made parts. The basic mechanism is a modified pan platform from Servocity: https://www.servocity.com/gear-drive-pan-kit-for-37mm-spur-gear-motor/ However, instead of the D.C. motor drive, I fitted a 28BYJ-48 stepper with a spur gear chosen for 5.25:1 reduction, so the azimuth resolution is 10698.9 steps/revolution. Movements to correct for gear backlash are included in the azimuth stepper code.

2. Mirror normal positioning (altitude): This is direct drive, using a 1000 step/revolution five phase Vexta PX33M-A-C6 0.21A, 33 Ohms per phase, 0.36 degrees/step, (ten wires) with a custom made mirror mount. Driven in half-step mode 2000 steps/revolution can be achieved. Azimuth positioning of the mirror normal is thus accurate to (+/- 0.18)/2 degrees. For information on construction of the five phase driver and corresponding code, see https://github.com/jremington/Five-phase-stepper-driver The mirror mount was machined to align the mirror surface (front surfaced mirror) with the rotation axis of the stepper motor.

3. Power supplies: Arduino Pro Mini is USB powered from laptop, using an FTDI serial-USB converter. Motors are powered by a 5V 2A power brick.

4. Timing: Currently the program uses the Arduino crystal for timing, which is not particularly accurate. For long term tracking, you will want to add a DS3231 RTC or other source to synchronize TimeLib.h

5. Command interpreter:

This part of the code accepts serial port commands from a laptop to initialize the stepper motors, set the zero position, drive the steppers to desired altitude/azimuth coordinates, and can be used for two different continuous tracking options. The suntracking option is similar to telescope positioning, as it automatically aligns the platform to track the sun position across the sky. The heliostat option allows one to position the mirror so that sunlight is reflected in a desired altitude/azimuth direction, then automatically positions the mirror to keep sunlight focused on that spot as the sun moves across the sky.

For the above two options to work properly, the platform must be accurately leveled and aligned so that azimuth zero corresponds to true North, altitude zero is perfectly horizontal. I chose to align the mirror rotation axis with true North using a magnetic compass, with the mirror surface vertical, mirror facing due East (azimuth 90).

Serial commands: Sent to the Arduino from a laptop running Teraterm version 105, which also logs performance data.

General format CC NNNN where CC is one or two lower case ASCII characters, NNNN is an character string, integer or float constant.

Commands:

1. t hh:mm input local time in hours and minutes (colon required) Conversion from time zone to UTC done internally
2. d dd/mm set current UTC date (slash required). The present year is initialized in the source code.
3. ja NNNN jog altitude stepper by NNNN integer steps. NNNN may be preceded by a minus sign
4. jz NNNN jog azimuth stepper by NNNN integer steps. NNNN may be preceded by a minus sign
5. sa NNN.NN (set) reset altitude origin and define current altitude position to be NN.NN (degrees, floating point)
6. sz NNN.NN (set) reset azimuth origin and define current azimuth position to be NN.NN (degrees, floating point)
7. ma NNN.NN (move) orient altitude of platform to be NNN.NN (degrees floating point)
8. mz NNN.NN (move) orient azimuth of platform to be NNN.NN (degrees floating point)
9. a (autotrack) track sun in heliostat mode from this point in time forward. The code will not track when the sun is below the horizon.

Note Telescope tracking is possible, by activating the following two lines of code and disabling the heliostat code

// sun tracking
//      move_to_altitude(home.getSolarElevation(t_now));
//      move_to_azimuth(home.getSolarAzimuth(t_now));