Retro-fitting Stepper Motors

First experiment proves viability of replacing some of inmoovs Servos with stepper motors.

I am using a Planet Gear (blue-box) in the picture above .....fortunately it just squeezes into the chest cavity.

Its downloadable off

I am still awaiting a Nema 17 stepper motor to drive it (this too I hope will slide into chest cavity)

In the meantime I have tested it with a PM55L stepper motor (@12Volts), its being controlled (for test) by a picaxe with IRL520N Mosfets. I have on order a ramps 3D printer controller to be able to drive 5 such steppers.

It drives the Yes/No response and has enough power even with this wimpy printer stepper, so I am expecting even better results with the Nema steppers.

The unit contains skateboard bearings and gives a gear reduction of 4.444:1 (40:9)

Build in progress.... needs limit switches to detect end/start of movements.

...... thoughts and ideas welcome

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GroG's picture

I would assume the stepper

I would assume the stepper motors with the planetary gear are much higher torque than servo motors - but what of the speed?

I have to say I always enjoy listening to your different drum beating ... 

What pros / cons do you suspect the steppers + planetary gears will have vs servos

These are some of my guesses :

pro - high torque
con - low speed
? - expense ?  a servo vs stepper + mosfets  
? - control - (pro) potentially high torque with high accuracy - (con) possibly more complicated with dedicated picaxe for each ?

What are the Swis elves perculating?

Gareth's picture


Torque :- Steppers are not by default very Torque-y, hence the use of a planet gear, the ratio can be adjusted (when i find some magic gear elf to explain the ins and outs).

Planet gears have a linear orientation so you can simply stack them together for increasing the torque.

Speed :- From what I can gather Nema 17 Steppers can comfortably run at 50-60 revs/min. (circa 360°/second) .... my planet set up will be roughly 1/4 of this  (due to ratio) so 4 seconds/rev (for 180° = 2 seconds ballpark figure )

For comparison :- Assuming a servo has 180° arc (at best) - one of my metal geared motors is rated @ 0.13sec/45° 4*0.13 = 0.52 second (for 180°)......, under load this will be slower for sure.

Price :- my ordered Nema 17 costs £9.99 and ramp shield (arduino) £4.50..... plus Ramp chips 4*£1.00

Servo for Neck ... circa £30.00- £40.00

So for me

Pro's = ability to go beyond 180°, cost, flexibility, burning out a stepper means easy replacement of individual parts,gear ratios easy to fiddle, accuracy.

Cons = low_ish torque,low_ish speed .....

NB. this is all hypothetical, so the Formula 1 elves will have some data to collect in the real world...