Thursday, 5 June 2014

Steering a Dish Antenna

Motor Drives

Firstly, before I write too much more, I should apologize for taking so long to get another article on this blog. I have a new radio astronomy project thats taking a lot of time but is proving interesting. I'll try writing about that shortly.

In the meantime I've worked out a cheap and readily available method of azimuth drive on my dish antenna. Last year I was looking for a way to effectively mount and turn a TVRO type mesh dish and use it as a point-able Radio Astronomy antenna. Thanks to the amazing developments in 3D printing technology there are some very useful devices available through Ebay.

I decided that I needed a cheap and usable stepper motor to control the azimuth (from 0 to 360 degrees). With careful attention to mounting hardware in order to reduce friction and the amount of torque needed I think I have a reasonably practical method of control.

The stepper motor I'm now using is a Nema17, 2 phase, 1.8 degree step device with a 5mm shaft. This is fitted with a 12 tooth aluminium timing pulley and drives a timing belt of two meters in length. The pulley and belt are available on Ebay as part of a kit for position control on a 3D printer.

The parabolic dish mounting is a circular disk 600mm in diameter around which the timing belt is wrapped and fixed at either end. Some slack in the belt is needed to go around the timing pulley.

In the picture above, taken from a random seller on Ebay, the timing belt is a rubber belt but reinforced with fibreglass to give it a lot of strength. The belt wont stretch therefore wont lose tension once fitted to the azimuth drive table.

The best part about this is the ability (I hope) for the Arduino and associated stepper controller shield to drive the stepper motor (rated at 2 Amps) without too much effort. I have read a lot of reports where the stepper shield H-Bridge L-298 overheats very quickly so a stick-on heat sink and reduction in voltage may be needed to reduce heat.

The other important part of this will be a brake system. When power on the stepper motor is off, the motor turns easily and will result in the dish rotating in a breeze. I think the best option will be a relay controlled device that will hold the azimuth table while the stepper shield is powered off.

The next post will hopefully show the stepper motor, toothed pulley and belt with the azimuth table all mounted together.