Help choosing motors for small dish rotor/pan&tilt

[rwgast_lowlevellogicdesin]

I have had an 18ft Lband dish for quite a while but im living somewhere I cant put it up, so im opting for something a little more compact. I scored two dishes last night, an 18 inch DirectTV offset feed with single LNB, and a bigger more oval shaped gregorian style offset feed. I dont have any tripods around so I just mount antennas on to booming mic tripods, which is ok because I don't leave them out. The stands look like this http://www.musiciansfriend.com/accessories/musicians-gear-tripod-mic-stand-with-fixed-boom?cntry=us&source=3WWRWXGP&gclid=Cj0KEQjw0_O-BRCfjsCw25CYzYoBEiQAqO9BDDj2rEISz1H96-LbyqbqgsUVN9mjDtKMFelT2c0FBBcaAuep8P8HAQ&kwid=productads-adid^92666429307-device^c-plaid^146599741482-sku^451051000001000@ADL4MF-adType^PLA.

So anyways I would like to make one of these motorized that way I can use a PC to control it for sat and moon tracking. I have done plenty of motor work on mid size robots before, the problem is im just not sure what kind of motors I should be looking for here that will have the torque to spin either dish. I was thinking something like a 60RPM gear motor and home made quad encoder, but I am not sure how much torque I need. I have not worked with steppers very much but that is also an avenue I am looking at, would they be accurate for this kind of thing? I know they work well on CNC's and stuff but any error will add up from here to space. If steppers would be accurate enough would a nema17 work? Im just trying not to break the band here and even scrap the motor if possible, maybe 12v drill motors with there gears in tact, or windshield wiper motors??

[bitslice]

I just mount antennas on to booming mic tripods

OK for an antenna, but that's not going to hold up a dish

I was thinking something like a 60RPM gear motor and home made quad encoder

look on ebay for a couple of "Linear Actuator Motors"
they crop up all the time, salvaged out of motorised recliners and stuff.

Mount the dish on a balanced pivot and it will only need a tiny amount of torque to move.

[rstofer]

Your problem is going to be how to go slow enough.  Celestial bodies don't move very fast, I don't know about satellites.

There are a lot of telescope tracking mounts on the Internet and a number of DIY projects.  Most use stepper motors because they can microstep the motor and get a couple of thousand steps per shaft revolution.  Then figure on some gears.

Stepper motors within a given frame size have different torque ratings depending on how long the motor housing is.  I use NEMA 23s for the X and Y axis of my CNC Mill.

http://www.automationtechnologiesinc.com/products-page/nema-23/nema-23-three_eighths-inch-dual-shaft-with-a-flat-570-oz-in

I don't mess around and build my own drives, I use these for the X & Y axis:
http://www.automationtechnologiesinc.com/products-page/digital-stepper-motor-driver/digital-stepper-driver-kl-8056d-heat-sink-is-included

 

[kc8apf]

What are you tracking? The gimbal requirements are quite different for MEO vs LEO. Speed and accuracy tend to be opposing design goals.

In either case, holding torque of the whole system is important so wind won't blow it off target. You'll still have deflection but it shouldn't matter unless you are doing something like E-band to LEO.

Sent from my Nexus 5X using Tapatalk

[rwgast_lowlevellogicdesin]

Honestly I want to be able to track anything as my needs grow, Im not sure why speed is such an issue here 60rpm seems pretty fast, from there you can just use pwm and slow the motor down. I was thinking doing this with hi res quad encoders is going to cover any case, but the motors still have to stay strong with low pwm values.

[kc8apf]

Generally there is a tradeoff between speed and torque.  To get high torque, a gearbox is used so the motor runs at really high speeds but moves the output shaft slowly with higher torque.  If you want to track something in LEO, you need rather high speeds.  If you've designed for high torque (either to get high holding torque or because you are moving a large dish), you'll need the motor to go _really_ fast which they just can't do.  The system I built last year for work topped out at 20 deg/s on the azimuth axis but had 500 lb-ft of torque.

There's another tradeoff between speed and accuracy.  The same gear reduction used to build up torque also provides lots of motor resolution.  Typically, you want the encoder to be the limiting factor rather than the motor step size.  The system referenced above had 0.01 deg encoder accuracy and much finer step size.

Axis configuration is another concern.  Elevation over azimuth assemblies can't track directly overhead.  Azimuth over elevation can't track at two points on the horizon.  If you're tracking satellites, this usually isn't a problem as the orbits rarely pass directly overhead and tracking near the horizon has too much signal degradation.  Tracking aircraft is a different matter.

[bobaruni]

You would need stepper motors that are geared down to get the required torque and accuracy.
Tracking a bird (satellite) or a celestial body is no easy task even with purpose built commercial equipment, sneeze and you lose your target.
Perhaps try commercial motorised dish, most of these only position in 1 axis as most birds are sitting in a straight line in geostationary orbit around the equator so only single axis of movement is required to point at many birds.
You would then need to figure out how to interface to it but no way you could use one of these to track a celestial object or non geo object, they are way too slow.