2

I am in the process of building my own sous vide machine and like to replace the pump typically found in most commercial machines with a magnetic stirrer unit. I figured the easiest way would be to use a motor to spin a permanent magnet (instead of creating a strong rotating magnetic field electrically). I first started experimenting with some 12V DC fans leftover from an old computer. By either lowering the input voltage or via PWM I was able to obtain appropriate rotational speed (don't ask me about exact speeds, I guess around 300rpm or so). With the project maturing I am not sure which motor (type and/or exact model) to use. I hoped some of you might point me in the right direction for the type of motor or even better, could suggest a specific model. I considered stepper motors or continuous servo motors. My criteria for the motor are:

  • easy to control rotational speed between 0 and around 500 or so rpm (for ramp up and variation of speed to increase turbulent flow)
  • ability to rotate both clockwise and counter clockwise
  • enough torque at all speeds to spin a disk with magnets attached (weight and inertia probably negligible, the to be used alnico magnets create a field strong enough to lift 0,4kg each)
  • small axial dimensions
  • reasonably priced and long lasting
  • quiet

The following picture shows a commercial magnetic stirrer found in your average laboratory. The blurry white object in the glass beaker is the the so called stir bar or "fish", a PTFE coated permanent magnet. It spins due to the rotating magnetic field created by the actual device underneath the beaker. Photo of a commercial magnetic stirrer used in laboratories

  • After reading more than the first few questions on this site (and reading some meta as well) I wonder if my question should by moved to EE since it isn't really a arduino specifiy question. Reading and answering on another topic led me to believe asking this rather broad question was still okay-ish. (Maybe it is still interesting for the community since there appears to be a lot of confusion about motors and controlling them in general). – Sascha Apr 29 '14 at 19:19
  • What's the purpose of the magnets? Waterproofing? – Anonymous Penguin Apr 29 '14 at 20:28
  • I am sorry if my initial question was unclear on the details: The magnets are glued to a disk that is than spun by the motor to create a rotating magnetic field. This field is used to spin a small magnetic rod inside a bowl atop the device. The vortex created by the rod stirs the fluid in the bowl. I added a picture to the question to clarify the matter – Sascha Apr 29 '14 at 20:36
  • Ahh, I understand now. Writing an answer as we speak... – Anonymous Penguin Apr 29 '14 at 20:38
2

I'd personally recommend a continuous rotation servo or a brushed DC motor. Personally, for this kind of load, I think it would be easier and cheaper to go with the brushed DC motor. You have to remember a continuous rotation servo is a DC motor with some circuitry so you can send pulses to it to control the speed (although it's not directly proportional to the signal you send).

A servo is the only other option besides a brushless motor; both require a little more circuitry than the brushed motor/continuous servo. The additionally cost more. This blog post covers the differences between brushed and brushless motors.

For driving a continuous, you hook it up to power [depends on your servo voltage], ground, and another Arduino pin and use the servo library to send a signal as a degree (90 = no speed, 180 = full speed forward, 0 = full speed backward).

A brushed you would need a h-bridge that allows you to either send voltage, send no voltage, or send reverse voltage (for backward). This chip (~$5) does that for two motors and allows PWM for different speeds. It also includes a brake function, although that doesn't seem necessary for you. It can handle up to a "2.4 A continuous (4 A peak)." I talk about this in this answer of mine.

Quick note, if you want to control the exact RPM of the device and not just manually adjust it to a good rate, you'll need some sort of a rotary encoder or similar. A reed switch (sends current depending on magnetic fields) would work perfectly for this purpose, just mount it below the magnets and figure out the degrees rotated by the motor for each "pulse."

  • I feel kind of biased towards something brushless since the whole thing is a gift and thus eliminating parts that wear and tear is probably a good idea. One thing I didn't mention in the question is that I have to control up to three motors (big bowl of water). Do you think in that case using servos to minimise surrounding electronics is the way to go? – Sascha Apr 29 '14 at 20:55
  • @Sascha You have to pick that, I can't pick that for you. From experience, steppers (like brushless, just a little differently configured) are 5 times as much as a brushed. Ignoring money, brushless are better, but they are more complex to run. Although this might not work, I personally might just say If a motor goes out, I can fix it or whatever, but I don't know your situation exactly. – Anonymous Penguin Apr 29 '14 at 21:02
  • currently I am checking ebay (germany) but can't find anything useful since almost every motor appears to be either way to overpowered or just build for insanely high rpm. Any great resource for that kind of stuff? – Sascha Apr 29 '14 at 21:07
  • @Sascha I don't really know how to read German but this motor is about 20 euros and ships free to Germany... but max 80 RPM and too much voltage for the driver. I just listed that as a reference; you might need a bigger driver. – Anonymous Penguin Apr 29 '14 at 21:20
  • What I am unsure of at the moment is how a geared DC motor would behave if used with lower input voltage or PWM to rotate slower than the rated speed. See for example: ebay.com/itm/… – Sascha Apr 29 '14 at 21:27
0

If you need to rotate huge loads, a permanent magnet coupled to the axis of a brushless DC motor, driven by an electronic speed controller (used for model aircrafts, multicopters, etc.) could be an option. This is the mechanical principle used in standard magnetic stirrers used in the lab, particularly when the stirrer has an additional heating.

Small (and flat) magnetic stirrers without heating, similar to the one in your image, however often have no (rotating) mechanical parts at all, but use a set of coils periodically powered in sequence. Here, the only mechanically rotating part is the stirring bar itself. A commerically available model would be this one.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.