motor to turn a valve

Hi,

I'm looking at using a motor to open/close a 3 way valve (see attached photo for reference) and have a few questions...

My thoughts were to use a stepper motor, to allow me to control how far to turn.

I've noticed all the motor I see have a "holding torque" listed. Is this the maximum torque that the motor can turn, or is it the most that it can hold? (or both?)

I have measured the amount of torque required to about 2.5Nm. I did this by hanging weight on the end of the handle and adding extra weight until it started to turn, so is is probably really not an accurate measurement, but at least it gives an idea.

I have found the following motor online, I has listed a holding torque of 121Ncm(1.21Nm), however it also lists the maximum permissable torque of 2Nm and a moment permissable torque of 4Nm.

http://www.ebay.com.au/itm/5-1-Planetary-Gearbox-Nema-17-Stepper-Motor-0-4A-for-DIY-CNC-Robot-3D-Printer-/171823951400

Would this be suitable, or do I need a more powerful motor? I have also found the following one that has 3Nm of holding torque.

http://www.ebay.com.au/itm/27-1-Planetary-Gearbox-Nema-17-Stepper-Motor-1-68A-for-DIY-CNC-Robot-3D-Printer-/171828613086

Thanks for your help.

The data in those links is confusing. I'm not sure if the holding torque refers to the motor without the gearbox or if it is the torque after the gearbox. My guess is that it is the torque of the motor alone.

It may be that the two motors use the same gearbox and that the larger numbers reflect the limitations of the gearbox rather than the theoretical motor-torque times gearbox-ratio.

If so, then the smaller motor with gearbox should be adequate or, perhaps, the larger motor with or without a gearbox.

You will also need a suitable stepper driver board that can handle the current required by the motor. For example a Pololu DRV8825 should be suitable as the larger motor seems only to need 1.68 amps.

You should also be aware that the running torque is always lower than the holding torque. The better (more expensive ?) motor manufacturers publish graphs of speed and torque. I'm guessing that your valve does not need speed.

You may find some useful stuff in Stepper Motor Basics

...R

Hi Robin,

Thanks for the info. I have sent an email requesting some clarification on the specs (in hindsight, I probably should have done that in the first place.)

Yeah, speed is not that important for what I'm doing, however I'd still like it to be able to open/close the valve with 10ish seconds.

I have read the link you provided, some very good info in there, thanks. I was going to use an h-bridge to control the motor, as that is what I found from searching for tutorials, however it sounds like a motor driver may be a much better solution.

wired67: however I'd still like it to be able to open/close the valve with 10ish seconds.

Anything above 1 sec is slowwww.

...R

wired67: I've noticed all the motor I see have a "holding torque" listed. Is this the maximum torque that the motor can turn, or is it the most that it can hold? (or both?)

Only what it can hold at stationary - if moving the practical torques can be a lot lower. The usual mode of mis-stepping is resonance, note, so controlling resonance will increase the usable torque. Microstepping helps a lot with this, as can mechanical damping.

I have measured the amount of torque required to about 2.5Nm. I did this by hanging weight on the end of the handle and adding extra weight until it started to turn, so is is probably really not an accurate measurement, but at least it gives an idea

So what is the length of the handle and the amount of the weight?

zoomkat: So what is the length of the handle and the amount of the weight?

The weight was just over 2.5Kg. The length of the handle is 11cm, however I hung the weight at the 10cm mark.

That makes 25Kg-cm, which converts to 2.45Nm, I rounded up to 2.5Nm

That first stepper motor can give upto about 1.2Nm, the gear reduction is 5:1, so the output torque will go upto 6Nm or so if the output is locked in place, which is above the rating of the gearbox.

Basically this motor can damage its gearbox if the output hits a hard endstop, which suggests you should run the motor at a somewhat lower current than nominal.

You also need some way to home the motor to a known position, which could be as simple as moving it until the valve hits its endstop - but could this cause damage? You'd need to be sure the max torque is safe for the valve.

With a DC gearmotor you can limit the current to the motor to limit torque fairly accurately. However you would need an encoder to determine position. If all you are interested in is turning to the endstops this is the best method - determine the endstop is reached by seeing the motor current rise (you'd have to measure it), and avoid physical damage by limiting the current in the first place.

DC motors are fast, so 100:1 reduction gearing and small motor might be plenty

The fun part will be the physical mounting of the motor to the valve. That is why I opted for a ready made motorised valve.

Weedpharma