Time waits for no motor (stepper motor help!)

Hi,

I am trying to emulate the minute hand on a large clock using a stepper motor.

I have been researching this for about a week but I just can't get a handle on so many variables.

Can I use these 3 items to move a 2 foot rod (as light as I can make it) with a 4oz ball at the end?

Having no experience with stepper motors, I have no idea if this will work in a real world situation.

Motor:
Bipolar, 200 Steps/Rev, 42x48mm, 4V, 1200mA, 44 oz/in

Motor Controller:
A4983 Stepper Motor Driver Carrier with Voltage Regulators

Power supply:
AC/DC Power Supply Single- Output 7.5 Volt 20A 150W

20amp power supply because I will be running a few other things at the same time.

The minute hand will rotate 359 degrees and then rotate back to 0 degrees every hour.

4oz at 24" would need 96oz torque for the motor. On paper at least.

I'd like to use a stronger motor, but the stronger they are, they begin to exceed the 2amp limit on the controller board. (which at $20 is affordable) Plus in real world conditions I'm not sure if this is overkill? (or underkill?)

Any advice on the proper motor, controller, and power supply combination would be very welcome.

Thanks for any help : )

Forum would not let me post links. Here are the links:

Motor:
Bipolar, 200 Steps/Rev, 42x48mm, 4V, 1200mA, 44 oz/in

Motor Controller:
A4983 Stepper Motor Driver Carrier with Voltage Regulators

Power supply:
AC/DC Power Supply Single- Output 7.5 Volt 20A 150W
http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_123415_-1

20amp power supply because I will be running a few other things at the same time.

:o :o :o :o :o
That is a lot of other things at the same time you could run into interference problems.

to move a 2 foot rod (as light as I can make it) with a 4oz ball at the end?

To my mind this is 24 * 4 = 96 oz/inch if the motor is only 44 oz/in I would say no.

The minute hand will rotate 359 degrees and then rotate back to 0 degrees every hour.

Stepping motors continuously rotate why rotate back?

Balance the hands (as is quite often done in mechanical clocks) and they require very little power to turn.

I also don't see why the minute hand must rotate back... unless you are putting a wire out onto it to light LED's or something?

Although it was done with a PIC, you might be able to get some ideas from the LiniClock:
http://techref.massmind.org/techref/io/stepper/linistep/LiniClock/index.htm

If nothing else, you can download a CNC file for a balanced hand.

Since the minute hand only needs to move at .016 RPM, you could use a small stepper, run it at a few RPM, and gear the bleep out of it to get lots of torque.

Thank you very much for the excellent suggestions.

People like you make life better : )

My main issue is how to figure out the correct balance of motor, controller, and power supply.

For example here is an inexpensive high torque motor:
http://www.alltronics.com/cgi-bin/item/28M012/search/Lin-Engineering-5618S-58-01-Bipolar-stepper-motor

It runs at 2 amps so the Pololu board should handle it?

The torque is 130oz so it should have the power needed.

But how do I figure out what power supply I need for this motor?

From the specs:
http://www.alltronics.com/mas_assets/acrobat/28M012.pdf

RATED CURRENT/PHASE 2Amps
RESISTANCE/PHASE 2.6Ù ±10%
INDUCTANCE/PH 7.83mH±20%

Is there some calculation from the spec sheets that people use to figure out the correct power supply needed?

What volts does this motor need?

Thanks for any help : )

Why is the ball at the end 4 Oz? A ping-pong ball is only 2.7 g (0.095 Oz).

What volts does this motor need?

The motor needs about 5V, but the motor controller will need a little (maybe a lot) more, depending on its design.

Bipolar chips like the L293 or L298 need about 2.5V more than the motor, because of the drop across their Darlington transistors. But, because they waste so much energy (about a third of the total, in this case), you'll almost certainly be better off shopping for a controller based on FETs. The datasheet of the controller will tell you what the voltage drop across the drivers is, so you can calculate how much "extra" it needs to give the motor its 5V.

I'm also using the 4983 driver on a Pololu board to control a 6 volt stepper. The 4983 has FET outputs. It requires voltages between 8 - 35 volts applied to the 'VMOT' pin. The Pololu board has a current limit adjustment, so that you can drive the motor with a higher voltage than specified, and it will actively limit the current. I would suggest you apply about 10 volts to the 'VMOT' pin of the Pololu board, then follow the Pololu procedure to adjust the current (you need a meter). Steppers only have current applied to one phase at a time, so the current per phase is the max current the stepper requires. In your case that's 2 amps, the upper limit of the 4983. You get the voltage by multiplying the current by the resistance V = I x R = 5.2 volts. But double it if you're using a 4983, and then limit the current to 2 Amps. So you need a 10 volt 2 amp supply for that stepper.