The Barndoor Tracker Revisited

Well, I got a chance to put the Barndoor Tracker to use this summer. On August 21st, I was privileged to observe the Great American Eclipse from Glenrock, WY. The weather was sparkling, except for some high, wispy clouds that showed up just in advance of the eclipse and continued to grow a bit right after. However, photographic opportunity was perfect and the eclipse occurred right on schedule.

In advance of the eclipse, I did a considerable amount of testing and work to get the tracker properly aligned to track the sun. I learned a lot about both astrophotography and the tracker. One thing is that the tracker has got a bobble to it. It tracked ok but not perfectly. The sun stayed in frame but it would move a bit from one picture to the next. I was taking pictures every two minutes and the sun would move one way and then the other. I think that there were two things that contributed to this. One was that the geometry is too complex. I plan to rebuild it this winter to align the pivot points with the lead screw. This will get rid of some of the calculations needed and also will make it easier to get the pivot points straight. That leads to the other possible problem, that at least one of the pivot points is not straight, which makes the door open at an inconstant rate.

The other thing I learned about the tracker was how to get it aligned. I had to do it the night before by sighting on the North Star (Polaris) with my camera attached to the tripod and shooting a timed exposure of about 30 second. Then I zoomed in on the image and adjusted the tripod to get the star to align perfectly with the center of the image. When this was correct, and the tripod was completely locked down against movement, I retired and, in the morning, attached the tracker to the tripod without disturbing it. After this, it tracked quite accurately, except for the aforementioned wobble.

So the other day I was at the Fry’s here in Wilsonville, Oregon, where we are visiting now that the eclipse is memory and photos, and I bought a Raspberry Pi. I guess I will be exploring this next and figuring out what I can do with it. Fun.

Barndoor Tracking Mount Part 5

The tracker is pretty much finished now and has been field tested with excellent results. The electronics are mounted on the bottom and covered by a shell, the lighting has been added where it is visible from the top, and I have added bubble levels to aid in getting the platform level and at the proper angle for the latitude. I have run some test programs which run the platform through some known angles and it came out spot-on. The only difficulty I have had is in setting up the tracker on the exact polar axis. So far, I have not used it in a location where I could see Polaris. I have used a compass and measuring angles with my phone to set it up, with mixed results. There is a little movement of the images but not much, many because I performed mostly short exposures. However, to aid in this setup, I have added two bubble levels. One is attached along the long axis to get the bed level. The other is adjustable on a protractor so it can be adjusted for the current latitude. Then I should be able to use the bubbles to set the correct angle. I would still need Polaris to site on or a compass to get the azimuth angle correct. This would also have to be the way to set up in daylight (as I will be using this tracker during the solar eclipse this summer).

Here are some final pictures.

Barndoor Tracking Mount Part 4

I have been having some problems with the stepper motor and the Sparkfun stepper driver and have finally gotten it all straightened out. The motor was having periods when it would sputter, misstep, and sometimes not even run at all. I finally reached the conclusion that the motor or the controller was bad. Since the motor was from surplus, I decided to replace it. It was not very expensive and I was able to find an exact replacement online so I did it. The new one seemed to work better, for a while, but then I started having similar problems. I was looking into replacing the controller but, in doing this, I found some documentation on the board that I had not seen before. In the margin notes, I found the recommendation that some of the inputs that were not being used in control be either connected to ground on Vcc (+5vdc). The ones that were listed included one to put the controller to sleep and another that was a reset. The third was called PFD and I did not examine its function closely but, I had not seen anything in the sample hookup documentation about these pins. However, I tied all three to Vcc and now everything is working fine.

Now I have a nearly completed mount. I just have to put on the part that will hold the camera and I will be ready for some field testing. However, first some more bench testing was in order. I attached a limit switch to allow the door to have a seekable home position. This ties to one of the Arduino inputs and, by doing a little testing. I found this to be a highly repeatable location even though the button I used seemed to have a lot of play to it. A second button was attached to be a command switch. The command button switches the program between the different modes: STANDBY, which is the default start mode and where the controller is just waiting for input, RETRACT, where the door is seeking the home position, READY, where the door is home and waiting to start tracking, and TRACKING mode.

After installing the home position switch, I had to repeat some of my measurements to determine the lead screw leg distance for the home position (see the last installment to see where this measurement is used in the calculations). I then went back and ran the test to measure the angles of individual steps that should each open the door in one degree increments. I got much better measurements, although still not as perfect as I would expect. More testing and measuring might be in order.

Next, I created a test for the command button. The button throws an interrupt to the Arduino board. This sets a flag that is then detected and reset in the program. Everything checked out fine so I incorporated the command button into the final Barndoor program and implemented each of the modes of operation. Finally, I was able to run the door through its paces. The flowchart follows. Click on it for a bigger view.

Next, I will have to finish up the packaging for moving to the field. I think I will mount the circuitry on the bottom of the mount and then route the cabling so it does not get snagged. I will likely also mount the lighting where it is easily viewable. My power supply for the field will be an automotive jump starter and tire inflator which has both a USB power port and a 12V power port. The USB port will power the Arduino through its USB port and 12V will power the stepping motor circuit. I have already tested the door using this power source and it is robust enough for use in the field.

Barndoor Tracking Mount Part 3

So I had to back up on something I posted last time. I showed the stepper motor with an 8mm screw and two intervening shaft couplers. Well, after ruminating on it a bit I realized that the shaft coupler that was flexible would not work with my design. I needed one that was ridged, otherwise too much play would be added into the system and accuracy would suffer. I knew that the motor shaft is 1/4 inch and the lead screw is 8mm so I looked for a coupler that would match those sizes and, surprise, there is such a thing. $6 later I have one. So now I was ready to start building the mount. The two “doors” are 3/4 inch oak and there are three hinged areas that hold the two doors together and also allow the motor and lead screw nut to stay aligned as the door is opened and closed. I showed a picture of this design in the last post so I took to building. The RV resort where we live in the winter has a nice woodworking shop so I had everything I needed.

I started cutting and shaping and, with a couple of restarts due to misplaced components and the purchase of another piece of wood to make the hinges, I was able in a few days to have put together the mount. Here are some pictures of the components and the final assembly.

Next came a test of this set up. I created a program for the Arduino to move the door in one degree increments from the closed position to the maximum open position. I immediately found a problem. Measurements showed it to be opening too far, about 1.3 degrees per increment. I did some modeling of my code in an Excel spreadsheet and it looked good. After some more testing I finally discovered that the lead screw, which I could have sworn was a three-start thread, was actually a four-start thread. This meant that instead of moving 6mm per revolution, it was really moving 8mm per revolution. This made a big difference and I was able to just change a variable in the program to correct it.

I forgot to mention that a crucial part of getting the door to open correctly was making accurate measurements of the finished device. I need to know the exact dimensions from the main door hinge center to the motor block hinge and the lead screw nut hinge. I also need to know the distance from the lead screw hinge center to the center of the screw and the same from the motor hinge to the center of the motor shaft. With all of those dimensions I can there create a formula that gives me the angle between the two door halves and can compute how far the lead screw nut has to be pushed out to open that angle by a specific amount. The same formula then can be used to determine the rate at which to step the motor to get the door to match the sidereal angle of the earth.

Now, examine this diagram:

If the vertex of a-b is the main door hinge, the vertex of a-c is the lead screw nut hinge, and the vertex of b-c is the hinge of the motor block, then the angle C is defined by the formula:

So “a” and “b” are measured constants and “c” is variable as the lead screw moves. However, the screw does not pass thru these points so there is another formula to find “c” for different steps on the lead screw. The geometry of the lead screw with respect to “c” is described in the following diagram and formula:

The lead screw nut (“n”) leg and the motor base (“m”) leg are always at right angles to the lead screw (“L”). Note: this is the reason the flexible motor shaft coupler was not appropriate. So the Pythagorean Theorem is used to determine the length of “c” depending on the position of the nut on the lead screw. By combining these two formulas the angle “C” can be determined for any particular step of the motor and can be matched then to the sidereal angle of the earth starting at the home or closed position of the door.

Next, I have to attach a switch to mark the home position and program the tracking algorithm for the Arduino. More on that in the next installment.

Barndoor Tracking Mount Part 2

Ok, the lead screw and nut I ordered has arrived. The good news is it looks like it will work just fine for what I am doing although the shaft connector that came with it to go from the 8 mm shaft to the motor is too small for the motor, which has a 1/4 inch shaft. But, the shaft coupler I already had for the old lead screw is just the right size to match up with the other coupler so, with a cut off 3/16 inch screw, I was able to get it all together.

The screw turns out to have 3 starts and a 2mm pitch which results in 6mm travel per revolution. I will probably drive it in 1/4 steps from the EasyDriver so it will result in 800 steps per rev and so 0.0075mm per step. The step interval should be around 300 milliseconds, depending on what the actual final dimensions of the door are.

I should now be able to begin fabricating the door itself. I have a piece of 3/4 inch oak for that purpose. The working drawing is below. I will hinge the door on one side and will also hinge both the nut on the top and the motor on the bottom. I believe I have the formulas worked out correctly to compute the step interval to get it to follow the sidereal angle of the earth. I will do some testing after I have the structure built to see how accurate I am. The final proof will of course be in the pictures I take.

Barndoor Tracking Mount, Part 1

So in my last post, I mentioned I was going to build this DIY equatorial mount for photography and general viewing of the skies. I am using an Arduino UNO board as the controller for the device, which will drive a stepper motor to open and close the “door”. I purchased an EasyDriver card to interface between the Arduino and the stepper motor. I attached it to a shield card I already had for prototyping with the UNO and brought power and motor connections out to pins. The power for the motor and the Arduino will both come from a car starter/air compressor that has a 12VDC auxiliary port. The stepper motor came for the surplus house I mentioned before and is by Oriental Motor in Japan. So far, I have gotten all three parts connected together and have run the motor through some operations that show that I should be able to do the job with it.

I had also obtained a lead screw at the surplus store but have not been able to find a source for a lead screw nut to travel on it. The best source I could find for parts like I want to use for driving the door is parts that are designed for 3D printers. I took a chance and ordered some parts that should be able to work and they should arrive in the next week or two, so I should know how they will work when I write the next installment.

I will probably fabricate the hinging parts for the door since standard hinges may have too much play for the stability I will want. I think I can make some fairly accurate parts in the wood shop we have here, where I can use a drill press to make some nice, tight parts using wood and metal. The base door parts will be made with oak for a nice, hard surface and I have an old, sturdy tripod that should be able to hold the whole apparatus securely.

I have been running some spreadsheets looking at the calculation of the stepping timing for the door to make it match the sidereal angle of the earth. The geometry of the device determines that the speed of the stepping motor needs to vary as the door opens so as to maintain a constant platform. A formula can be easily derived for the stepping frequency and other implementations I have read about have used formulas and tables to minimize the error in the angle. However, I have come to realize that the computational speed of the Arduino processor is fast enough to be able to compute the next step interval well before the interval needs to occur, so accuracy should not be a problem assuming that my formula is correct and that I accurately position the device lined up on Polaris.

Next time I will show you the geometry of the device I am building, the formula I am using to determine the step intervals and, hopefully will know if the hardware I am getting for the stepper will work. Then I can get busy fabricating the platform

Back Again

It has been a long time since I have posted here. Sorry but life has had other things for me to do. However, I have been playing around with Arduino microcontrollers for a while and have just found a new application.

With the All American Solar Eclipse coming next August 21st, I have started thinking about filming it and have also been interested in filming other astral phenomena so I figure building a polar axis tracker for my camera and telescope needs might be interesting.

I found a design for a DIY Polar Axis Tracker based on the Barn Door design and started looking into the math used to determine how to drive it and the hardware. I was in Portland Oregon this passed Summer and was directed to Surplus Gizmos by my son. They have all kinds of electronic, electric and mechanical hardware for sale.

I was able to get a couple of stepper motors (which may or may not work), a lead screw with an 0.1 inch pitch and a 3/8 inch diameter. These were my first pieces of hardware for the device.

Let me drop back a little and describe the device. A Barn Door Tracker is essentially two pieces of wood with a hinge connecting them. The hinge allows the two parts to swing through an angle as a barn door might when it is opened with respect to the door frame. By anchoring one side of the door to a tripod as a “ground” plane and then moving the other side at a controlled rate, the moving side will remain relatively fixed from a cosmic standpoint as the other side moves through an arc as the earth turns below it.

So the trick is to make the “door” open at a rate consistent with the rotation of the earth (360 degrees in 23.9344696 hours). I will be posting the steps I go through here to make this thing work so stay tuned.