help with a small project - precise 1rpm motor & circut

My thought is that you do not need 1rpm exactly since you can vary the motor speed with R1 to suit. The speed could vary with temperature and motor friction.

A better but much more complicated method would be to use a stepper motor controlled with a micro as timer.

You can get components from Maplin, Bowood Electronics, Cricklewood Electronics, RS, Farnell, etc

I would solder the components to some strip board (Vero) for neatness.

 

Hey, I always thought that a motor that turns a higher R would be better for a tracker plate... that if you geared it down exponentially, and then used a standard dimmer switch to increase or decrease the power to the motor, causing it to speedup or slowdown....
it only has to turn one way if its battery powered., you could use a celson(sp)selsen motor? if you want it to go backwards. anyway use two, one for declination and one for ascension... I was smart, bought an ETX 80...tracking built in...

 

Strip board, origionally made by Vero, is SRBP board which comes in various designs. The most common has holes in a square matrix, spaced at 0.1 in (2.54mm). One side of the board has copper strips running in one direction. Standard components can be placed with their leads through the holes and soldered to the strips.
The strips can be cut to restrict the connection using a small drill but I have the luxury of a Vero cutter which is a small drill with a plastic handle.

I attach a photo of a commercial Vero board, you can see there are some integrated circuits with the strips cut between the pins. I will cut this up to use for my projects

If you go for the Vero board method, then make sure that you get through hole components, not the modern diddy surface mount variety.
This method is very good for a one off design but I have trouble cutting the strips in the right place as all holes look the same. Remember that the pin numbers of integrated circuits are labeled clockwise from underneath and anticlockwise from above.
You must get the electrolytic capacitors installed the right way round.

I have never used a 'breadboard' as used today but this consists of a plastic block with holes to make connections to underside strips. The holes contain sprung sockets so that components can be plugged in and changed around quickly. This is not suitable for a permanent solution.

 

Since you're using this for star tracking, you clearly want something very accurate. I would use a stepper motor running faster than 1 RPM and gear it down to 1 RPM.

A stepper motor is controlled by a logic circuit, often a microcontroller, and driven by MOSFETs. Each time the control signal changes state, the stepper motor rotates ("steps") a certain number of degrees, depending on the design of the motor.

Say your stepper motor has 180 steps per revolution and you use a gearing of 10:1 to step it down to 1 RPM. The stepper motor needs to make one full rotation every 6 seconds, and every rotation requires 180 steps, so it needs to be stepped once every 33.333 milliseconds. A controller should be able to do this - Google stepper motor controller.

I suggest gearing the stepper motor down so you can get smoother rotation. The stepper motor moves in 2 degree increments, but after gearing by 10:1 your 1 RPM axle will move in 0.2 degree increments; much smoother.

The stepper motor will need to be crystal-controlled, for accuracy. (Or perhaps mains-synchronised.) If it's programmable, you will be able to get other rotation rates, which might be useful. (My astronomy knowledge is too limited for me to say.)

I suggest Googling stepper motor drivers and stepper motors, and seeing what stepper motors are available locally. There are several different types with different numbers of coils and different coil configurations; you need to make sure that your controller can drive the type of motor you choose. You will also need a power supply that can provide the voltage and current required by the stepper motor.

 

Agree about the stepper as a better solution, but this might be a bit out of range for a beginner.

Bob

 

just another thought

invariability is what you are after, you need to be more accurate than electronics alone can provide, and yes, you have to use a stepping motor, but I had this idea last night...

I remember something about building a stepping motor with caps.

why not cap the stepping motor for more accuracy? it would add about 16k?

and it will take a genius to figure out the actual constants!

 

Bob, I agree it would be too much for a beginner if he had to design it all, but I imagine there are stepper motor driver boards available with documentation on models of stepper motors they can drive, and how to get a particular speed, etc, so maybe there wouldn't be much design involved. Some of these drivers might include a power supply.

I haven't done this sort of thing before so I don't know, but I do know that stepper motors are widely used in hobby electronics, so off-the-shelf well-documented easy-to-use solutions are probably available. Perhaps a model-making forum, an Arduino forum, or a robotics forum would be a good place to ask.

Re stepper motor with caps, I don't know what you mean. You would need to post some links.

 

That's the kind of board you need, yes. But I wouldn't search on eBay because the information you get is too brief.

Look for web sites of companies that design and make stepper motor control boards. These will have detailed descriptions of what the board is capable of doing, and should have recommendations for models of stepper motor to use with it. Or the kit may include a stepper motor.

I think you should definitely go with a preassembled or kitset controller board. There's a fair bit of design work involved, and it makes sense for that design work to be done by people who already have experience with stepper motors and how to drive them, and the stepper motors that are available.

The second board you found on eBay needs to be connected to an Arduino, and the Arduino needs to do the timing and signal generation, i.e. to act as the controller. The board has just the stepper motor drive circuitry. I would recommend a board that has the controller on-board; that will be a lot tidier and will not require you to write your own software or get into the details of how the software works.

Perhaps I shouldn't have suggested looking in an Arduino forum. All you could find there are pointers to companies that manufacture stepper motor drive circuits; you would have to look at their web sites to see whether they make boards with the controller on-board. Robotics forums are probably a better place to look.

I Googled stepper motor controller driver kit and found a few companies right away:

http://www.oceancontrols.com.au/Stepper-Drivers.html
http://www.jaycar.co.nz/productView...WORD&ProdCodeOnly=yes&Keyword1=KV&Keyword2=KD (a PC-controlled board with software included... do you have Jaycar in the UK? Probably not)
http://www.robotshop.com/

Do some more Googling, you should find lots of possibilities.

There are people here with stepper motor experience. Anyone? Anyone? Bueller?

 

I don't think the Jaycar kit is what you want. I think when they say "manually" they mean that you can step the motor by pressing keys on the PC. I don't think there is any kind of controller on the board.

I could be wrong. There isn't enough documentation on the Jaycar site to be sure. I downloaded the support software and it's pretty old - MS-DOS and Win9x. Probably not appropriate.

There should be other options around. Try some Googling. If the documentation on the product doesn't describe it in enough detail for you to know whether it will do what you want, try another company. Stepper motor control is a common enough requirement nowadays that there should be properly documented options around.

Did you look at oceancontrols?

Sorry I can't help much. My internet access is limited for the next 10 days because I blew my monthly data allocation last week :-/

 

So far, it looks like all of these stepper controllers are intended to be interfaced to a micro or computer. Not much use for the purpose of the OP.

What you need is a stepper controller that let's you set the speed digitally (not via a pot), and I can't seem to find anything like that. Which makes sense, becuase stepper motors are not normally used that way. The are used in variable speed and positioning applications, where some external control would always be necessary.


Bob

 

No, you would most likey need to do it every time you use it.

The one you reference seems to be adjustable for speed, but not ditially, so it won't be any more accurate than your original design. I also worry that there are descrepancies between the description of the controller and that of the motor it is supposed to work with.

controller: 6 to 12V
motor: 24V!

Bob

 

No, I am saying that I can't find any such thing. All of the stepper controllers seem to be indended to use with either micro or a PC supplying the speed and direction information.

Bob