Driving low-voltage geared stepper motor with Adafruit Motorshield

[JohnAnderson]

Hi all,

I am interested in driving a geared stepper motor using the Adafruit motor-shield. ActiveRobots (http://www.active-robots.com) supply a range of geared steppers (e.g. http://www.active-robots.com/motors-whe ... epper.html) and recommend using their motor driver (http://www.active-robots.com/phidgetste ... motor.html) in conjunction with these. However, I was hoping to be able to control them directly via the motor-shield. I notice that the coil resistance (1.73 Ohms) and supply current (1.6 Amps) may not be ideally suited to the motor-shield. Has anyone had any experience driving these motors or can recommend an alternative geared stepper that can be driven directly from the motor-shield.

Best wishes,

John

[adafruit_support_bill]

It is possible to increase the current handling capacity of the motor shield enough to handle a 1.6A motor. 2x SN754410's with a heat-sink should do it. See here for details: viewtopic.php?f=31&t=26873

With a 1.73 ohm coil, you will want a drive voltage of about 2.75v to achieve full torque. At that low level, the voltage drop through the H-bridge can't be ignored. Typically, there is about a 1.2v drop. So you will need a power supply in the 3.5-4v range.

The two biggest problems with low-voltage motors like this are that they require higher currents to achieve the same torque, and that they don't leave much room for variations in supply voltage. That looks like a nice motor. You might try to find if the manufacturer makes a version with higher coil resistance to run at a higher voltage.

[JohnAnderson]

Hi support,

Thanks for your reply. What I find slightly confusing is that the same stepper motor gear assembly is shown here (http://www.phidgets.com/products.php?ca ... _id=3317_1) with the recommended supply voltage of 12V, rated current of 1.7A and coil resistance of 1.7 Ohms. Given the coil resistance of 1.7 Ohms, I presume this would attempt to draw a very large current of around 7A from a 12V supply. Does this imply that the current supply needs to be limited in some way to the maximum rated value of 1.7A. Would the Adafruit motor-shield be able to limit the current supply when driving a motor of this kind, or simply burn out the SN754410? Is there are simple way of limiting the current supplied by the motor shield?

Best wishes,

John

[adafruit_support_bill]

The recommended phidgets controller does not appear to have any current limiting capability. If the coil resistance spec is right, I don't see how it can safely run the motor.

Maximum Speed of 118 RPM @ 12V

When running at very high step-rates, the actual current draw is limited by the the inductance of the coils. It could be that the 12v spec is only applicable at that speed.
High-performance industrial motor controllers with current limiting can use a much higher supply voltage to achieve higher speeds. At lower speeds, the current limiting will reduce the voltage to a safe level.

[edit]
I just noticed this warning on the Phidgets page:

Connecting the motor directly to a power supply will destroy the motor and void the warranty. If you want to check your motor make sure it is connected to a constant current / chopper drive controller.

[JohnAnderson]

Hi support,

Would it be feasible for me to simply place a large resister in series with each of the windings to increase the apparent winding resistance and drop the supply current to the operating level required by the motorshield. For example if I place a 5.3 Ohm resister in series with each winding this would limit the maximum current to 12/(1.7+5.3) = 1.7Amps. Admittedly that would require a 12*(5.3/7)*1.7 = 15.4Watt resistor which is pretty big. Is this a standard method for accommodating different types of stepper motor?

[adafruit_support_bill]

Hmmmm. It would probably work, but you would be running at 25% efficiency and dissipating a lot of heat. NEMA-17 is a standard size motor-frame. You would be better off fitting the planetary gear module to another NEMA-17 motor with better specs. Probably not much of a cost difference by the time you factor in the cost of the power resistors and an up-sized power supply.

These are NEMA-17 and run great on 12v @350mA : https://www.adafruit.com/products/324

[adafruit_support_bill]

The Phidgets motor is most likely a variant of this one: http://www.stepperonline.com/17hs191684 ... -p-40.html
The specs are largely the same and the warning text is identical.
However, note that they list the operating voltage as 2.8v.

[JohnAnderson]

Hi Support,

Thanks for your feedback. I already have several of these motors ( https://www.adafruit.com/products/324) and agree they run very reliably with the Adafruit motorshield. I didn't realise that it was that easy to mount the planetary gears on an alternative motor. They always seem to be sold as a complete motor assembly. Is the remounting relatively straightforward? And do you have recommendations for any planetary gears that would suit your stepper motors?

Thanks again for your help.

Best wishes,

John

[adafruit_support_bill]

I was actually thinking you could transplant the phidgets gear-train onto one of the 12v steppers. Gear-train modules are available separately, but all the suppliers I deal with charge much more for a gear-train than Phidgets charges for the whole assembly. http://www.anaheimautomation.com/produc ... oducts.php

The output shaft of the gear-train assembly is 8mm, but all the Phidgets motors have 5mm shafts, so I am assuming that the gear-train input mates to 5mm as well - although you might try to verify that with your supplier.

[JohnAnderson]

Hi support,

Thanks for your help with this matter.

Best wishes,

John