I just graduated and my senior project is complete! Here are the details…
My project is called the A.P.T.T. or Automatic Pointing and Tracking Telescope. I wanted to build a telescope that would automatically point at and track objects (stars and planets) in the sky so you would not have to spend all night trying to find them. There are already commercial versions of systems like this from many companies, but they are fairly expensive. I bought a regular telescope (from Orion) with an altazimuth mount and I modified the mount by adding bearings and servos for motion control. I used a PIC 18F2550 microcontroller on a custom designed and built board to make it all work.
- Calculate object positions like star software does
- Point at and track objects
- Implement USB for updating catalog list and advanced control
- Make it easy to use
Main Board Top
I decided from the beginning that after having made several printed circuit boards using the toner transfer method, I wanted to make my senior project board this way. I designed the board using Cadsoft Eagle, a schematic and PCB layout tool available for free (with some restrictions). Along the way I realized that there was no way I was going to be able to route the board on a single layer, so I went ahead and tried my hand at a double layer board. Also, along with the main controller board, I had to design and build three other boards. There is what I call the sensor board, that has the GPS and the magnetic sensor on it that physically attaches to the telescope base, the hand-held remote control board that has the LCD and the buttons for i/o and the power board that connects to the battery and charger and has dual linear regulators for the servos and circuits. I had to use two regulators because the microcontroller would reset ever time the servos moved!
Main, Servo/GPS/Compass and Remote Control Boards
The dual layer PCB is actually two single layer PCBs strapped back to back because I was worried I would not be able to align the toner masks well enough on a real two layer board. This worked okay… they are essentially held together by all the vias that I put wire through and soldered on both sides as well as the component leads. I ran into difficulties with the connectors, the USB and the serial, because their leads were too short and I had to extend them with some wire, but they don’t feel very secure. Also, I had to do some various hacks and tricks to get eagle to do what I wanted. It automatically assumes that all leads can be soldered both on the top and bottom layers, even for things that have them completely covered on top like the USB connector, so I had to route those manually.
How it works
To calculate the position of a star in the sky, you must have three pieces of information: The current time, your current position (latitude & longitude) and the coordinate of the star. Also, you must be aligned with true north. In my system, I get time and position from the on-board GPS module. I store a list of stars in the micro controller memory that has their coordinates as right ascension and declination. Finally, I find true north by finding magnetic north using my on-board magnetic sensor and then add the magnetic declination for the area, which unfortunately is hard coded in the system right now since I have no way of getting it other than looking it up online. As it works right now, you simply put the scope down on a level surface and it finds north and adds that as an offset in the servo position so it does not actually have to be aligned with north, it does so automatically. To actually get the angles you need to point the servos, you must calculate sidereal time, find the hour angle, and finally convert from the equatorial coordinates to altitude and azimuth. For my system, since I somewhat stupidly decided to use servos for some reason, there is an extra step since the servos only go about 180 degrees. Since, both servos can go 180 degrees, you can point in all directions, but if you hit the limit on the azimuth for instance, it must rotate 180 degrees, while the altitude servo must also rotate to the opposite side to keep pointing in the same direction. This is kind of goofy but it does work. Also, I am not using any position feedback, and so my system is open loop. I calibrated it pretty well though and so it has not been too much of a problem.
Remote Control Board
The board also has USB that allows the user to control it and hopefully update the star catalog from a computer (although the updating does not work yet). It also has a serial port for testing that allows the user to look at the current state of the system as well as GPS data. I implemented a menu system for the user I/O that displays the object, its magnitude (brightness) and the current time. The magnetic sensor on board is a two axis sensor that I initially hoped I would be able to read the values and do a four quadrant arctan on two find the angle of magnetic north. This however proved to be difficult since the sensitivity in each axis seems to be different, and each time you turned it on, there was a different offset in the output. Since I only need to find north, I actually put the sensor on the rotating part of the scope itself and when you turn it on, it goes through a calibration routine that turns the bottom servo through 180 degrees and finds where the maximum field strength occurs. As long as the front side of the scope is within 180 degrees of north, it works.
Schematic of Main Board
The main thing I would have done differently, and probably will since I plan on using this is not to use servos. I used high torque metal gear hobby servos because I knew how to use them and I thought they would be good enough for my purposes. Actually, they do work, but they are the main source of error in my system. Instead I would like to use just regular DC motors or steppers geared way down to get high torque and better positional accuracy with an optical encoder or just a potentiometer. Secondly, I probably would have chosen an Atmel controller instead of the PIC for many reasons… Finally, implementing the USB on my USB PIC was a huge pain in the ass and almost certainly not worth it. I should have just used one of these and been done with it. It provides a virtual COM device on the PC side, exactly like what I am doing with my PIC using the CDC device firmware from microchip, and you get regular serial I/O to your micro, all with no overhead on your micro.
If any one wants to look at my code to find out how to do things like calculate sidereal time (kind of tricky), here it is
APTT Project Source Code