I want to point a telescope at certain coordinates that I put into a program. I have the telescope mount, I just need the motors.
Should I use:
12V geared DC motors with optical position sensor
OR
stepper motors with no position sensor, but using software to count the steps
My two worries are:
I don't know of an optical position sensor for arduino that is reliable. I've read they can miss steps. Although this would be turning very slowly. 10RPM.
If I use a stepper motor, it would have to be on all the time in order to hold the telescope in position. Also stepper motors seem really complicated .
I couldn't find the webpage, but someone was saying it was missing steps. If you think it's reliable that's great. I already have the geared dc motors, I just would need to find an wheel to put on the shaft for the sensor to read.
That sort of thing should work well - provided you have a suitable disc with slots in it of a size that are compatible with the device.
You can also get reflective optical detectors (I use the QRE1113 device) which will detect black or white stripes. Printing a disc on a PC might be more practical than making a slotted disc.
If the Arduino misses steps it will be due to faulty programming or the slots/stripes moving too fast past the detector. I doubt if a telescope would need anything fast.
I use my QRE1113 to detect the speed of a small DC motor and it works perfectly fine with pulses at 300 per second (18,000 RPM) and can probably work much faster.
That’s an optical slot switch.
Generally any optical switch won’t lose steps, that responsibility falls on the hardware and software design.
to use this on a motor shaft, you also need a slotted disk to interrupt the beam as the motor turns.
If you buy this combined in a single package, it’s called an optical shaft encoder.
OL, to address your questions.
Any (optical) shaft sensor has finite resolution. (pulses per revolution)
How well this works in your application depends on the resolution you need, and the reduction gearing of you motor.
Virtually any motor will work, but again the holding power is also a function of the gear reduction ratio.
Steppers move very precisely if implemented properly, but typically have a limited top speed, and if under load, need to be speed ramped.
Control isn’ta big deal.
DC servomotors (not servos) are somewhat trickier to position accurately, but the gear reduction will make that easier.
Beyond all this, your control circuitry needs to account for gear lash etc, or your motor will hunt around a bit as it reaches the target position.
Definitely use the stepper motors...they aren't as complicated as you would think once you start playing around with them. They are fantastic for open loop control. Yes - you would need to have a holding current supplied to them in order to hold position once you are no longer slewing; however, with a DC motor you would require a feedback control architecture (encoders) and then a brake of sorts to hold the axis in position once achieved.
Start with a simple stepper motor and integrated controller (you can get inexpensive solutions through Amazon) - you may have to tune your design based on torque requirements of the overall payload you are looking to position and how fast you need to move it.
How do you know you’ve reached the target position without feedback?
Yes, you can count steps, but that’s like counting fingers in a leper colony.
I don’t think anyone suggested that steppers are difficult, just an option, and both are valid for different reasons.
Speed profiling/ramping is much easier with DC motors, and a lot more torque with well managed drivers and gearing.
Gearing down is critical to this application - steppers to hold position, and to reduce RPM with servomotors. Although steppers may become too slow.
Do you want it to track, or just go to a position?
That is, adjust AZ/EL to continuously compensate for the earths rotation?
Tom....
I don't know. It's aimed at an airport 5 miles away, focal length is 450mm, f4 focal ratio. It's a webcam of an airport and I want to make it so viewers can point it at different locations.
I think a 2RPM dc geared motor would be good but it's hard to find one that slow that comes with an encoder. I have a 2RPM motor already, I'm trying to research if I can add an encoder somehow.
What you really need is a DC motor, gear box and encoder. It's probably a tall order to find a combination of all three that exactly fulfill your specs so look for the separate parts.
DC motors typically run at speeds in the tune of 10-20k rpm, a shaft encoder would work great for you (as described above - one side of the shaft white, the other side dark, and optical detector tells you how many turns the motor shaft made). You'll have to get a 1:5,000-1:10,000 gear ratio to go down to your 2 rpm.
