Do two steps per revolution stepper motors exist (aka help me identify this)? - Page 1
Forum section: electronics / beginners

author: ivaylo
author_rank: Frequent Contributor
timestamp: November 20, 2018, 11:26:59 pm
content: Came across the pictured device. The rotor is a solid cylindrical permanent magnet. The stator consists of two parts:a coil with just two wires coming out (and that goes straight out of the device)a permanent magnet with similar shape to the coil sitting on the same axis as the coil (the rotor goes through both)When I turn it by hand (not powered in any way) it has one distinct positions it prefers to stay in (the rotor and stator magnets just act with each other). There are no distinct teeth, or poles on neither the stator nor the rotor. Unfortunately I assembled it back before taking pictures, but the whole thing is super simple.The part number is C35MT02A12, by Portescap. Looking at the part numbers for steppers of that company -https://www.portescap.com/products/can-stack-motorsthe "02" part seems to be corresponding to 'steps per revolution'. I've messed with stepper motors, taken them apart, etc, but haven't seen anything this simple (two magnets, two wires and a coil).Reached out the manufacturer, Portescap. The first response I got was "this part doesn't exists". I sent them the pictures and got a "This part is obsolete and proprietary" back.I went the complete "for dummies" way and tried to see what is the minimum steps per revolution for which steppers exist. The minimum selection at Digikey is 20 -https://www.digikey.com/products/en/motors-solenoids-driver-boards-modules/stepper-motors/179.This article says it could be 4 -https://learn.adafruit.com/all-about-stepper-motors/types-of-steppersAny help appreciated... Could this be some very low steps per revolution stepper, or is it some sort of an actuator/servo/whatever? And how do you think it's driven?

author: james_s
author_rank: Super Contributor
timestamp: November 20, 2018, 11:50:58 pm
content: I had a similar motor when I was a kid, IIRC it also had an optical encoder on the back. I don't know for sure, but I believe it was a type of servo motor which would be driven by a closed loop analog circuit, a bit like the voicecoil head positioning used in hard drives.

author: emece67
author_rank: Frequent Contributor
timestamp: November 21, 2018, 12:04:57 am
content: .

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 21, 2018, 12:58:59 am
content: Two wires, a stepper it ain't.You need two axes (in three or more wires) to provide directional information. Otherwise it's just pulsing on and off, and some lucky timing, or contrivances, can make a net rotation, but not anything in the same general way that a multiphase motor does.Could be "single phase" BLDC?Or it's for angle, not rotation, and it's an oversized d'Arsonval movement, say?On multiphase:Single phase induction motors normally start up thanks to a "start" or "run" winding that is electrically phase shifted. The phase shift sets up a rotating component to the magnetic field, which drags the rotor along and makes it spin. The electrical means is usually an L/R or LC time constant: hence the shorting bar in "shaded-pole" motors (L/R), or the "run" capacitor in split phase motors, some induction motors, and rotary phase converters. These are frequency-dependent elements, so they only work over a modest frequency range, and the starting torque drops very quickly as frequency falls. Whereas a multiphase induction motor can run down to DC with maximum torque proportional to frequency (using a VFD to drive it), and a synchronous (permanent magnet) motor can run down to DC period -- a true "four steps per revolution" stepper, as such.You can't have less than 4 steps, because the two phases are driven in quadrature, and each step is 90 degrees, both electrically and mechanically. You can always have more steps, by dividing the magnet and coil into more poles.

author: ivaylo
author_rank: Frequent Contributor
timestamp: November 21, 2018, 01:36:55 am
content: Thanks, guys! I’ll look into those movements. If there was no stator magnet I can perfectly see how you could measure RPM or phase with this. The device actually looks exactly assembled from the body, front coil and rotor from this picture of the Portescap steppershttps://www.portescap.com/products/can-stack-motors/understanding-can-stack-motorsJust add another ring magnet on top of the stator coil and throw away everything else on the picture. If this isn’t a stepper (and it sounds like it probably isn’t) it would be impressive how they figured out how to sell a completely different product reusing the majority of parts from their steppers...

author: james_s
author_rank: Super Contributor
timestamp: November 21, 2018, 02:17:42 am
content: I suspect it's a servo actuator for something like the old washing machine sized hard disks or something else of that nature. It was likely intended for applications that may otherwise be served by a stepper motor.

author: ataradov
author_rank: Super Contributor
timestamp: November 21, 2018, 02:19:01 am
content: Can it be a two position switch/actuator, just made to order on a base of a real stepper?That may be a reason for denying its existence.

author: tpowell1830
author_rank: Frequent Contributor
timestamp: November 21, 2018, 04:06:54 am
content: Have you measured the voltage out of the 2 wires when you spin it by hand?

author: ivaylo
author_rank: Frequent Contributor
timestamp: November 21, 2018, 05:45:00 am
content: When I rotate by hand the voltage changes like the attached oscilloscope screenshot. It's "snappy" like this because half (?) of the cycle i am going against the forces of the two magnets. When I cross a certain point it quickly snaps 180 deg (?) ahead of my finger and does the decaying sinusoid thing.The discussion so far gave me an idea though. Thiscouldbe used as an actuator. If I apply DC voltage it does turn proportionate (so to speak, it's very non-linear) to the voltage (and polarity). The second attachment is the degree per voltage graph for voltages going up and down (the thing has a bit of hysteresis too). Excuse the swapped numbers, before realizing I actually recorded voltage per degree and was too lazy to repeat. Blue is voltage going up, green is voltage going down.So in total this could move +/- 60 degrees around its fixed point. May be only practical up to 15V/~50deg though (or in the linear part of the graph). The resistance of the coil is 100 Ohm, so at 30V it's sucking 300mA (I did push it)Coincidentally a couple of weeks ago I was restoring an ancient voltmeter (last picture) which had a design I haven't seen before, but very similar to this - the "rotor" was a permanent magnet moving inside a coil, the opposing force was provided by a spring. Here everything is the same, but the opposing force is provided by the magnet in the stator (and this can make a complete revolution, the voltmeter obviously could only go back). No, not planning to use this as a voltmeterThanks for the good ideas, guys! The true purpose of this device still escapes me, but I got my fair share of experimentation for the week.

author: james_s
author_rank: Super Contributor
timestamp: November 21, 2018, 07:24:30 am
content: I think in order to make it work well as an actuator you will need closed loop feedback of the position, then you would have a servo. You feed the control value and the feedback value into an error amplifier that drives the coil and it will "magically" hold at whatever position it is commanded to.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 21, 2018, 08:33:59 am
content: Ah, probably a 1-ph BLDC after all. AFAIK, the asymmetrical field is useful because that makes it more likely that a pulse to the winding causes it to rotate forward. From there, a sensor (may be back EMF based so nothing extra is required) detects position and pulses it in time.Tim