Heliostat Project

MTM Scientific, Inc.

A Heliostat is a device which is capable of automatically reflecting sunlight towards a stationary target, independent of the apparent position of the sun in the sky. Traditionally, Heliostats have used mechanical gears and clock mechanisms to accomplish this task. In the age of computers, the aiming function is now most often accomplished by using computer control. This webpage describes the construction of a simple Heliostat based upon a surplus Pelco security camera actuator. The actuator is controlled using an older Dell laptop with a parallel port and serial port, running a simple computer program written in Basic.

Heliostat Project using Pelco Actuator

The first step in building a Heliostat is to calculate the current position of the sun in the sky for your location and time. This actually turns out to be relatively simple to do. For example, there is an open source computer program written in Basic for just that purpose.  The name of the program is SUNALIGN.BAS and it was written by David Williams. Here is a link to it: SUNALIGN.BAS  This program calculates the Azimuth and Elevation of the sun's position in the sky based upon your location and time. (Azimuth is the compass heading: 180 degrees is due south. Elevation is the angle from the horizon: 45 degrees is halfway between the horizon and directly overhead.)  You should be able to download and experiment with the program using any old computer capable of running BASIC.

In addition to calculating the position of the sun, the SUNALIGN program also has a Heliostat feature. The user simply enters the aiming coordinates and the program responds with aiming coordinates for redirecting sunlight to the desired target.  There you have it!  That leaves the challenge of devising a method to aim a mirror automatically at the given coordinates.

There are all kinds of approaches to aiming a machine using motors, gears and actuators.  We actually presented and discussed some of these challenges in our book "Build a Solar Tracker".  In our book we described using linear actuators and a mechanical frame built of an aluminum extrusion, called "80/20".  In this project we are going to keep things especially simple and use a surplus actuator intended for the 2-axis motion of security cameras, manufactured by Pelco.

STMAX dual axis tracker
Figure 1. The STMAX Dual Axis Solar Tracker

Electronic Control and Positioning

With the Pelco unit we basically need to accomplish two objectives: 1) We need to be able to move the actuator in Azimuth and Elevation by computer control, and 2) We need to know what direction the Pelco actuator is actually pointing in real time. In our case the Pelco actuator's movements are controlled by two 24VDC motors. One motor controls the up and down motion (Elevation), and the other motor controls the East and West motion (Azimuth). A DC motor's motion can be either direction, simply depending on the arrangment of the "+" and "-" electrical wire leads. We chose to control the motors by using electromechanical relays.  Although this may sound complicated, it actually is quite easy to use a computer's parallel port to control relays by using a kit for the purpose. We are using a kit from Electronic Kits, called "Parallel Port Relay Board Kit CK-1601".  Here is a photo of the Relay Board Kit being used for this project.

Parallel Port Relay Board

A more challenging design challenge is adding sensors to the Pelco Actuator to report the current aim direction. Pelco actually offers position sensors on some of their actuators as an option. The surplus unit we had available for this project did not have built-in sensors, so we had to add position sensors.  A very simple approach to this challenge is to install linear potentiometers on both axes of motion. This required some careful thought, and the liberal use of silicone sealant, as shown in this photo. 

Potentiometers attached to Pelco Unit

When a linear potentiometer is supplied with a stable supply voltage (e.g. 5 volts) the rotary position of the wiper is reported as a voltage proportional to the angle. By measuring the voltage reported at different positions you can easily establish the voltage-position relationship for both axes. Then the formulas for position vs voltage can be deduced and used in the BASIC program coding to determine the current Heliostat aim.

This approach leaves us with the challenge of measuring the output voltages of the sensors using a computer. Well, once again there are many different approaches to this problem.  We chose to measure the voltages using a Picaxe A/D circuit which reports voltages to the computer using the serial port. We have described this approach elsewhere: Serial A/D using a Picaxe.  At one time we considered offering a kit for A/D using a Picaxe called the "Dataport".  Here is a Zip file containing all the design information for the data acquisition module DATAPORT.ZIP 
  (You can see a Dataport in the photo with the relays.)  Note: Write to us at MTM@MTMSCIENTIFIC.COM if you are interested in purchasing a Dataport Kit for $35, as we have several left in stock.

With the aiming direction determined by software, and with the current aim of the actuator reported by sensors, all we need to do now is write a computer program in BASIC to move the Pelco to the correct coordinates. This is where you have to put on your thinking cap and think about how to accomplish that! The basic strategy is pretty simple. For example, take the challenge of East-West aiming (Azimuth). You must code the program to measure where you are, and what direction the Pelco needs to move. And then, while moving, measure the aim and stop when the motion is sufficient to hit the target.

Here is a link to the BASIC program we wrote for our setup: PELCO.BAS
 Of course, this program is specific to the calibrations and wiring arrangements we used in our setup. But the general outline of the program gives the basic idea of what is needed.

Heliostats are quite amazing devices, and it is especially fun to see how well this simple arrangement works! By mounting a mirror on the Pelco platform sunlight can be directed to any desired target. For example, note the beam of light being projected onto the interior wall in the garage photo above.  A Heliostat could certainly be used to direct sunshine into a window during Winter. Another use might be to focus sunlight on a steam boiler, or some other thermal type engine. And of course, there is nothing to prevent this identical setup from being using as a regular dual axis solar tracker!