3 ms·

There are many projects like this, but a lot of them are done wrong: they use a straight main bolt and they don't compensate for the error in the driver motor.

There are many projects like this, but a lot of them are done wrong: they use a straight main bolt and they don't compensate for the error in the driver motor. This one is done right - the main bolt is curved, so no error (or very little error) is introduced while tracking. Good job.

StavrosK 12y ago
Where is the error from? I don't understand why connecting the motor directly would be different.
- Florin_Andrei 12y ago
  In most cases, the motor has constant rotation speed, so the speed at which it advances on the bolt is also constant. If the bolt is curved, the radius of the motion, relative to the hinge, is constant through the whole range of motion. Constant linear speed + constant radius = constant angular speed. Some folks prefer to "simplify" the design and just make a straight bolt instead. Again, there's constant linear speed. But this time the radius, relative to the hinge, is not constant (you have a triangle, instead of a circular slice). So the angular speed is not constant anymore. You could vary the motor speed to compensate for the straight bolt, but why bother. Curving a threaded piece of thin metal rod is not that hard.
lutorm 12y ago
There is one thing missing though: a means of accurately aligning it to the pole.
cgearhart 12y ago
"Wrong" is a strong word. There are compromises in every design. As pointed out by another poster, accurate pole alignment will be the dominant source of error for any reasonably accurate barn door. The short focal lengths, wide field of view, and relatively short "long" exposures of DSLRs (5 minute exposures will induce thermal distortion on my D60 vs >15 minute exposures for film) common in DSLRs don't require extreme accuracy, especially if stacking the final images. Barn door trackers can have very low error without curving the rod. Modern microcontrollers also allow digital compensation compared to older barn doors that often used analog synchronous motors, and variations on this design (e.g., the double barn door [1]) can provide tracking accuracy at the limit of hobby applications. [1] http://davetrott.com/inventions/double-arm-barn-door-drive/ http://davetrott.com/inventions/double-arm-barn-door-drive/
- Florin_Andrei 12y ago
  Well, curving the rod is a pretty simple way of eliminating one source of errors - why not do it when it's so easy. Of course you still have to do polar alignment and so on.

StavrosK 12y ago

Where is the error from? I don't understand why connecting the motor directly would be different.

Florin_Andrei 12y ago
In most cases, the motor has constant rotation speed, so the speed at which it advances on the bolt is also constant. If the bolt is curved, the radius of the motion, relative to the hinge, is constant through the whole range of motion. Constant linear speed + constant radius = constant angular speed. Some folks prefer to "simplify" the design and just make a straight bolt instead. Again, there's constant linear speed. But this time the radius, relative to the hinge, is not constant (you have a triangle, instead of a circular slice). So the angular speed is not constant anymore. You could vary the motor speed to compensate for the straight bolt, but why bother. Curving a threaded piece of thin metal rod is not that hard.

lutorm 12y ago

There is one thing missing though: a means of accurately aligning it to the pole.

cgearhart 12y ago

"Wrong" is a strong word. There are compromises in every design. As pointed out by another poster, accurate pole alignment will be the dominant source of error for any reasonably accurate barn door. The short focal lengths, wide field of view, and relatively short "long" exposures of DSLRs (5 minute exposures will induce thermal distortion on my D60 vs >15 minute exposures for film) common in DSLRs don't require extreme accuracy, especially if stacking the final images. Barn door trackers can have very low error without curving the rod. Modern microcontrollers also allow digital compensation compared to older barn doors that often used analog synchronous motors, and variations on this design (e.g., the double barn door [1]) can provide tracking accuracy at the limit of hobby applications. [1] http://davetrott.com/inventions/double-arm-barn-door-drive/ http://davetrott.com/inventions/double-arm-barn-door-drive/

Florin_Andrei 12y ago
Well, curving the rod is a pretty simple way of eliminating one source of errors - why not do it when it's so easy. Of course you still have to do polar alignment and so on.