DC Motor - How Do I Make it 1RPM?

[Benno85]

Howdy Folks :)

After being mountless for over a month, I'm really starting to have withdrawl symptoms from being unable to track my DSLR. Current circumstances now dictate an interim solution of a motorised barn door mount now that Orion season is fast approaching :)

I have found some plans at http://www.garyseronik.com/?q=node/52 which seem fairly straightforward given my limited electronic experience. The site makes use of a 3V 4RPM DC motor with 2 gears to reduce the speed to 1RPM. My question for all the gurus here is:

If I cannot find the same motor mentioned in the plans, is there a method to workout the gears needed to reduce the speed to 1RPM?


Cheers Ladies and Gents,

Ben :thumbsup:

[bojan]

DC motor and exact 1RPM (required for tracking) doesn't go together well without some sort of motor speed regulation.

In principle, the gear reduction could be anything, provided the speed of the motor is regulated to the required value.
For example, if you have a motor with 1000RPm, you need 1:1000 (more likely value fore DC motor) reduction to get 1RPM.

For this design, you do not need 1RPM - the arms of the barn door mount (shown at that website) and the driving screw can be tailored to accept wide range of motor rates.

DC motor speed regulator must use feedback (PLL, for example) - otherwise the rate will not be adequately precise.

I would suggest to you to try and find AC timing motor (used in washing machine timers for example). Their speed is determined by the mains frequency (50Hz, which is VERY accurate) an the output (after gearbox) is usually 1RPM.
Something like this (just make sure it is for 50Hz.. or you need to adjust the barn door lenght by shortening it for 20% to compensate for 60Hz motor runninh at50Hz):
http://cgi.ebay.com.au/Synchron-620-Clock-Motor-220V-60CY-3W-1RPM-Pack-2-/220662676412?pt=LH_DefaultDomain_0&hash=item3360853bbc
http://cgi.ebay.com.au/MOTOR-1-REVOLUTION-PER-HOUR-SYNCHRON-TIMING-MOTOR-/310251381546?pt=LH_DefaultDomain_0&hash=item483c6c632a
http://cgi.ebay.com.au/SYNCHRON-AC-TIMING-MOTOR-1RPM-110VAC-60HZ-3WATTS-/260674769870?pt=LH_DefaultDomain_0&hash=item3cb16d53ce


Another (and much better, more flexible) option is /uProcessor controlled stepper motor. This way, you can have any rate you need.

[wasyoungonce]

You can probably use one of these (http://cgi.ebay.com.au/12V-DC-60RPM-Powerful-High-Torque-Gear-Box-Motor-/220677427282?pt=AU_B_I_Electrical_T est_Equipment&hash=item3361665052) (60RPM servo motors) and some a 10:1 gearing. This would keep up torque. If you use a PWM for motor speed control then it won't be an accurate control modulation due to variations.

Best to use gears.

edit:

Ahh beaten.

[Benno85]

Thanks Bojan, I had hoped that you would offer your knowledge :)

Could you kindly explain further this uProcessor? Does this need to be computer controlled via a laptop? Ideally I would like all the electronics mounted on the underside of the barn door mount for ex tra portability :thumbsup:

[bojan]

No, it could be self-contained unit, no need for computer, battery powered.


It is a /uProcessor that generates necessary signals for stepper motor, and the rate is determined by crystal oscillator - which means it is accurate enough (actually more than enough) for this mount.

This is what I have done:
http://www.iceinspace.com.au/forum/showthread.php?t=45079&highlight=bintel+equatorial

[Benno85]

Thanks buddy, the price on that little beast is definitley right up my alley (except for the postage :().

So if constant frequency results in constant revolution, would a well regulated portable power supply be ok for accurate-ish tracking? I'm confussling myself :shrug:

[Benno85]

PWM sounds interesting, but where would I be able to source components for both options (PWM with gears, or the /uProcessor)?

[bojan]

I don't think the regulated DC power supply is good enough..
Given the constant voltage, DC motor speed will depend on load, temperature (grease in gears will be changing consistency with temperature), age of the motor (condition of brushes and bearings) and so on.

All this must be compensated for.

Or if you go for AC motor, rotation rate will depend only on mains frequency.

[bojan]

I have couple of spare boards, processors.. (as shown on previously mentioned thread)
But first, you have to build the mount, with suitable stepper motor.. This will determine the rate for the stepper. The firmware must be compiled for each individual case.... Very easy to do but, considering the processors I am using are OTP (very old Motorola KJ705), the rate must be known in advance, because once programmed, processor can't be re-programmed again.

[Benno85]

Based on the sizes in my link to the plans I'm using, how could I determine a suitable stepper motor (specs etc)? I think it's time I put together a shopping list :D

[bojan]

You just build the mount ... with bent rod and that gear reduction...

I will supply the rest ;)

[bojan]

BTW...
This is how I designed DC motor PLL control for telescope back in '80s..
It uses 4046 as oscillator and PLL.
The speed of the motor is sensed by the tone head from cassette player, which is picking the signal from cylindrical magnet mounted on the motor shaft (10 poles - taken out from rotor of AC timing synchronous motor). This signal is compared with VCO (internal to 4046) in phase comparator and the motor is pulsed according to the phase difference between two signals.
VCO frequency is controlled by potentiometer (connected to one of connectors at left side of the schematic). The circuit also accepts external stable frequency reference, however the jumper setting need to be re-arranged for alternative signal path.

With this arrangement, I managed to regulate the DC motor rotational speed down to 10rps minimum (lower that that and the movement became jerky because the speed was lower, and so was the pulses voltage from head... so motor was pushed with full speed for the moment. This could have been prevented by using optical encoder, but this would have meant higher power consumption - and I didn't want that).
Power consumption was negligible: 20mA or so.

The circuit was published in British "Wireless World" in early '80-ties - I don't remember when exactly..
I even received payment (30 pounds) for this contribution but realised only couple of months later that they attached the cheque to the reply letter sent with the magazine issue with the circuit published ... so I never cashed it :screwy: