Barndoor tracker guidance needed.

Projects General Guidance
1

Hello all.

I am making what is known as a "barn door" tracker. Essentially it is a very basic equatorial mount for taking pictures of the night sky. It accounts for the Earth's rotation, holds the camera still on the stars to take long exposures.
I figured this would be a great opportunity to finally dive into Arduino.

At the very minimum, I would like to have 2, maybe 3 control buttons:

-Play/pause. This will start and stop a stepper motor. The motor needs to run at 1800 steps per minute.(9 RPM) Full steps should be OK at this point in time, I may need or want to do micro-stepping later on to smooth it out.

-Rewind. This just needs to be a faster speed to slew the device back to it's starting point. Ultimate speed and accuracy is not needed for this operation.

-Fast forward. This isn't totally necessary, but might be nice in some situations.

Essentially this is a clock drive. I have calculated it out to 1800 steps per minute, but will adjust rate this up or down a bit to tune it the device once it is actually built.

Eventually it would be nice to add all kinds of extra features like a screen etc, but for now I would be satisfied with the basic operation.

For Hardware I have a Genuino Uno, Adafruit motor-shield and a 200step/1.8 degree motor.
I made an error and selected the wrong motor/driver combination. My motor is a 2.38V whereas the driver board is the L/R type suited for 5v to 12v motors.
It appears that I might be able to add some "ballast" resistors to the motor and even potentially improve the performance by operating it with a higher voltage.
I will address this motor question in a different forum, just thought I should mention it. I realize now a chopper driver is more suitable for my motor, but would really like to work with what I have, I don't think my application is overly demanding.

Galactrack 1.jpg
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2

first off, your motor is a GREAT choice.
lower voltage = better performance.

second HALF stepping, or more is a huge improvement over full steps. full step are like cogging, half steps and more are preferred.

start here :

http://forum.arduino.cc/index.php?topic=284828.15

my friend Robin2 put together a great introduction to steppers.
( I joust with him on some bits, but this is a great contribution to the community)

3

a note on voltage.
there are two ways to drive a stepper, connect each power leg in turn with an of/off switch.
this puts full and unrestricted voltage and power to the stepper motor. careful control of voltage is needed and the nameplate voltage must be the maximum.

the other and preferred way, is to feed power to each leg, watch the current that is being delivered, then chop the power to that leg as soon as the current has been reached.

with a chopper driver, you can use any voltage. even 48 volts to your motor ! ( IF THE DRIVER IS RATED FOR IT )
power is proportional to voltage, so the more voltage the more power.

so, by design or by mistake, you got (mostly) good parts that will work well together. I would offer that a 12V power supply would work well with your parts.

the driver board has limitations. the A4988 chip is a true stepper driver, with microstepping and chopper circuit. it also uses only 2 pins, one for step, one for direction. RECOMENDED

as for speed, the single arm barn door will work well with a constant speed, the double arm will track better.
but with the Arduino, you can adjust the speed to track better than either.

getting started, the single arm, constant rate will give you great results. I would offer that if you can use 4 microsteps, you will be happy. 8 would be the most I would suggest unless you see any jitter in your photos.

the return speed could be half step, or just faster pulses.

4

dave-in-nj:
first off, your motor is a GREAT choice.
lower voltage = better performance.

second HALF stepping, or more is a huge improvement over full steps. full step are like cogging, half steps and more are preferred.

start here :

Stepper Motor Basics - Motors, Mechanics, Power and CNC - Arduino Forum

my friend Robin2 put together a great introduction to steppers.

Thank you, I will have a look.

[/quote]

dave-in-nj:
a note on voltage.
there are two ways to drive a stepper, connect each power leg in turn with an of/off switch.
this puts full and unrestricted voltage and power to the stepper motor. careful control of voltage is needed and the nameplate voltage must be the maximum.

the other and preferred way, is to feed power to each leg, watch the current that is being delivered, then chop the power to that leg as soon as the current has been reached.

with a chopper driver, you can use any voltage. even 48 volts to your motor !
power is proportional to voltage, so the more voltage the more power.

so, by design or by mistake, you got good parts that will work well together. I would offer that a 12V power supply would work well with your parts.

as for speed, the single arm barn door will work well with a constant speed, the double arm will track better.
but with the Arduino, you can adjust the speed to track better than either.

getting started, the single arm, constant rate will give you great results. I would offer that if you can use 4 microsteps, you will be happy. 8 would be the most I would suggest unless you see any jitter in your photos.

the return speed could be half step, or just faster pulses.

Awesome. I am fine with starting out with micro-stepping. I had designed the device to move 0.5 arc seconds per full step, so anything extra would be gravy. I was however a little concerned that the full-step "cogging" might actually shake the device which may be visible, so the smoother option would be desirable.

Yes this will be a single-armed device with a curve threaded rod, so I don't have to account for the tangent errors of a straight threaded rod.

Now assuming I was to use the hardware I have, would there be much benefit it ballasting down 12V vs say 5V? I'm asking because I'm assuming would need higher wattage ballast resistors with the 12V but admit I haven't actually done the math yet.

To give you an idea of the load, it will spin a GT2 timing pulley at 3RPM to wind a 3/8"-24 tooth per inch threaded rod. It should have a pretty easy life I am thinking?

5

Number_5:
Thank you, I will have a look.

Awesome. I am fine with starting out with micro-stepping. I had designed the device to move 0.5 arc seconds per full step, so anything extra would be gravy. I was however a little concerned that the full-step "cogging" might actually shake the device which may be visible, so the smoother option would be desirable.

Yes this will be a single-armed device with a curve threaded rod, so I don't have to account for the tangent errors of a straight threaded rod.

Now assuming I was to use the hardware I have, would there be much benefit it ballasting down 12V vs say 5V? I'm asking because I'm assuming would need higher wattage ballast resistors with the 12V but admit I haven't actually done the math yet.

To give you an idea of the load, it will spin a GT2 timing pulley at 3RPM to wind a 3/8"-24 tooth per inch threaded rod. It should have a pretty easy life I am thinking?

unfortunately, the driver is not a chopping driver and it wants to run a 5v motor.
to use a resistor to drop the voltage will require a large wattage resistor.
a much better final result would be to get the A4988 driver from e-bay or some such.
but, in the interim, you have some choices. get a new motor, get a new driver, get a lower voltage power supply, get reistotors.
post the link to the motor, or better yet, the data sheet.
it is a simple calculation to get watts based on the power supply voltage, the motor voltage and the amp rating you want to run the motor.
you can run the motor on very few amps.
if you have an old 5-1/4 inch floppy, you could rob the stepper.
if you are too impatient to wait, you can try to wire in a small light bulb. the filament is a resistor of high power rating.
just solder to the base where the wires are already soldered, or put it into a socket and wire it up from the cord.

6

dave-in-nj:
unfortunately, the driver is not a chopping driver and it wants to run a 5v motor.
to use a resistor to drop the voltage will require a large wattage resistor.
a much better final result would be to get the A4988 driver from e-bay or some such.
but, in the interim, you have some choices. get a new motor, get a new driver, get a lower voltage power supply, get reistotors.

OK so in the meantime I will look into supplying it with 5v and ballast resistors. I might actually have some higher wattage resistors around as I tinker with vacuum tubes now and again. Probably not that low of resistance, but I can parallel some together.

I will look into those Pololu drivers, or a different motor to swap in later. I'm definitely not trying to be the difficult new-guy, just I would like to keep the project moving in the meantime. Adafruit was out of stock of the appropriate motor for the "motor-shield" and the stepper motor I do have took almost 2 months to receive when purchased on the bay.

I have no issue swapping something out later on.

dave-in-nj:
post the link to the motor, or better yet, the data sheet.

I have attached what specifications I do have below.

dave-in-nj:
it is a simple calculation to get watts based on the power supply voltage, the motor voltage and the amp rating you want to run the motor.
you can run the motor on very few amps.

The motor shield has a 1.2A per channel constant rating,(TB6612 MOSFET) so I will have to drop the motor's current somewhat. (1.7A motor)

dave-in-nj:
if you have an old 5-1/4 inch floppy, you could rob the stepper.

lol I almost ruined my perfectly good laser printer looking for stepper motors. lesson learned.
I would like to stick with the NEMA 17 size due to the size and 5mm shaft.

dave-in-nj:
if you are too impatient to wait, you can try to wire in a small light bulb. the filament is a resistor of high power rating.
just solder to the base where the wires are already soldered, or put it into a socket and wire it up from the cord.

Yes, this, I guess I am kinda impatient, on the other hand it took me 8 months to start this project lol.

Cheers.

NEMA 17 1.4Ohm - 1.7A.png
NEMA 17 1.4Ohm - 1.7A.png1219×953 73.4 KB

7

The motor shield has a 1.2A per channel constant rating,(TB6612 MOSFET) so I will have to drop the motor's current somewhat. (1.7A motor)

you can drop it a lot. all you want to do is to get it to spin.

I have to laugh, I look in my junk boxes and have junk motors that I will probably never use from old floppies, printers and the like.

8

dave-in-nj:
you can drop it a lot. all you want to do is to get it to spin.

Indeed.

dave-in-nj:
I have to laugh, I look in my junk boxes and have junk motors that I will probably never use from old floppies, printers and the like.

It must be nice, lol. I've looked high and low for an appropriate motor for this project.

I suppose the defining parameter at this point would be the 5v minimum that the driver board likes to use. IF I had a 3 ohm ballast resistor, it would dissipate around 3.9 watts, running the motor at 1.13 amps. (On a 5v supply)

The other issue with choosing this ballast resistor is whether or not I should compensate for temperature. I realize that this motor will heat up during use and eventually reach somewhat of an equilibrium, I just don't know at what temperature.
Here is an article that has the math for temperature compensation for reference. Determining current limiting resitors for stepper motor L/R Drives

Anyhow, I appreciate your help, I am learning something and I don't necessarily mind the loss of efficiency, assuming I can cobble together a resistor for it without too much hassle.

9

Number_5:
Indeed.

It must be nice, lol. I've looked high and low for an appropriate motor for this project.

I suppose the defining parameter at this point would be the 5v minimum that the driver board likes to use. IF I had a 3 ohm ballast resistor, it would dissipate around 3.9 watts, running the motor at 1.13 amps. (On a 5v supply)

The other issue with choosing this ballast resistor is whether or not I should compensate for temperature. I realize that this motor will heat up during use and eventually reach somewhat of an equilibrium, I just don't know at what temperature.
Here is an article that has the math for temperature compensation for reference. Determining current limiting resitors for stepper motor L/R Drives

Anyhow, I appreciate your help, I am learning something and I don't necessarily mind the loss of efficiency, assuming I can cobble together a resistor for it without too much hassle.

if you want to use battery, then go with a DC motor and encoder. steppers are not very efficient.
if you are going to run your power supply at 5V, then your motor should not suffer. use a regular 25 watt light bulb as a resistor and see if the motor works as expected. the build should not light, you are using it as a resistor, not a lamp..nichrome wire from an old hair dryer ? 25 resistors, 100 ohm 1/4 watt... that would be the ticket ?

10

dave-in-nj:
if you want to use battery, then go with a DC motor and encoder. steppers are not very efficient.

I should have mentioned that this will be a battery powered device. I might have enough lithium batteries to run the device like this. While not ideal it is acceptable in the meantime. When I upgrade to a better driver, perhaps I will consider the encoder approach. I had not considered it previously.

dave-in-nj:
if you are going to run your power supply at 5V, then your motor should not suffer. use a regular 25 watt light bulb as a resistor and see if the motor works as expected. the build should not light, you are using it as a resistor, not a lamp..nichrome wire from an old hair dryer ? 25 resistors, 100 ohm 1/4 watt... that would be the ticket ?

Lol I was already checking my pre-soldered chains of 100 ohm resistors while you were typing I bet. lol Not enough of them.

Your nichrome wire idea was brilliant. I grabbed a 50w aquarium heater that I previously removed from service and removed the wire from it.
It looks like a relatively small length of wire will do the trick. I might even be able to spread the coil of wire out a bit to make it less inductive. Perhaps I can wind them on a chop-stick or perhaps dip them in epoxy. If I make them fairly big, it should be fairly easy to dissipate 4 watt.

11

OK back on track I think.

I was able to make a couple of 3 Ohm ballast resistors from some 1 1/2" lengths of nichrome wire.
They are fragile but should do the job for the time being. I was able to use some acid core flux to get a half decent joint on some little pieces of solid copper wire. I guess I will attach them to the proto area on the motor shield.

Anyhow, so now time to familiarize myself with a few things including the IDE and round up some cables etc.

12

I wanted to ask, If I order a chopper drive will it make much of a difference in code?

Do you guys see a suitable chopper driver on this page that would be appropriate for my motor? Choppers etc

13

This is the chopper I am thinking of getting.

It has micro-stepping and supports my 2.5v motor as is. It is similar to the recommended A4988 as far as I can tell.

Unless anyone objects I think this is the one I will order? Pololu DRV8834 Low-voltage stepper motor driver carrier

14

OK So I ordered that Chopper driver!! lol

Sorry for resistance. This will work well with my 10v lithium batteries and be more efficient then my current limiting resistors.

Thanks for the advice.