Announcement

Collapse
No announcement yet.

Magnetic Stirrer Project Reset

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

    Magnetic Stirrer Project Reset

    So based on some of the comments in my other posts, there seems to be some confusion or maybe some assumptions, I'm not sure exactly, so I wanted to hit the reset button on the entire discussion and make a few things clear that might not have otherwise been obvious.

    First of all, the reason I post in this forum is that I simply do not have enough knowledge in electronics to do some things that I would like to do. So I post my questions in here and I try to be as thorough as possible MOST of the time, so that those of you who do take the time to respond, will be as informed as possible so that you can offer your knowledge from an informed position.

    I may not always succeed in being thorough enough, or I might make a mistake in a schematic symbol ... but I assure all of you that I have no intentions on making this process difficult, nor should anyone assume that I am locked into anything that I say concerning the parameters of a given project that I'm working on.

    I know enough to be what I would consider a trainee technician if my skills were to be placed into the real world where peoples money is on the line. Thankfully, I am far better with network engineering than I am with electronics, and because of that, electronics is something I hobby with and I do not use these skills for any sort of monetary gain.

    That being said, I am ALWAYS open to new ideas or different ways of accomplishing what I might be trying to accomplish.

    You wont hurt my feelings by telling me that I'm an idiot for trying to do something a certain way because if I weren't an idiot in a lot of this stuff, I wouldn't be here in the first place. Fortunately, ignorance can be corrected, and I am often ignorant on many things but always willing to learn in order to correct it.

    OK, now that that's out of the way. I took some time to make an animated GIF that should clearly show what it is I'm trying to do with this project, and I included in the animation, the way that I envision how the magnetic energy should be distributed in order to attain the results I need. That doesn't mean that this is the only way I intend to do this, it just means that this is the only method that I can think of in order to get the results I need. I am always open to someone showing me a better way - on all levels of this project.

    However, if I didn't describe to you the method that I think would work, then you would not have a clear picture of what it is I'm trying to do. So I offer the method that I think will work with an open mind to be taught better methods whenever that would apply.

    SO, the way that I envision making this magnetic pill spin inside a viscous solution, can be seen in this animated GIF where I show the lines of magnetic flux as it needs to change over time and I indicate the reverse polarty being applied to the coils by the color changes in the north and south letters.

    There are four coils in this project, but I wired them in a similar fashion to how coils are wired in a stepper motor. Two sets of two coils per set and they are wired in series so that there are only two leads per coil set. When I energize the leads, one coil will light up with magnetic north polarization while the coil opposite of it will light up with magnetic south polarization. Then if I reverse the polarity on the leads they will flip (obviously).

    Here is how I envision making this happen using four phases wherein each phase I will gradually increase the voltage in one coil pair while decreasing it in the other coil pair ... then reverse polarity as indicated and repeat the graduating voltage, etc. And the reason why I believe it is necessary to gradually increase in one set while simultaneously decreasing in the other set is to keep the swing of the pill as fluid as possible so that it doesn't snap around ... picture an ocean wave gradually reaching up to the shore then gradually receding back into the ocean ... its a timed event with consistent fluid motion with no significant jumps in voltage happening anywhere along the travel path of the magnetic pill.

    It should make sense by looking at the animation



    Here is what the coils look like in terms of the leads coming off of each coil pair:



    Now, since I'm thinking that the best way to accomplish what I'm wanting to do here would be through the utilization of an Arduino, I am thinking that I need essentially two H-bridges where I can sweep the input voltage at each input of the bridge from 0 to 5 volts and at the output, I would get a 1 to 1 linear amplification of the voltage that will drive the coils so that when I code the driving of the H-bridges, I can use some consistent sweeping of voltage values using consistent timing. And I don't know how to design the H-bridge so that I can get that kind of consistency between input to output voltage with a solid linear relationship between them at every step of the way from 0 to 5 volts.

    I'm looking for advice, or someone to point me in the right direction so that I can learn what I need to learn to accomplish what I'm trying to accomplish. If there is an IC package with H-bridges that will do this, that would be amazing because I'm not looking forward to building two H-bridges by hand. It would be messy on the breadboards and prone to error in too many ways, but if an IC like that does not exist then I have to do what I have to do ...

    Also, if my approach to solving these problems is lacking, then please help me correct that.

    Thank you for your time,

    #2
    Re: Magnetic Stirrer Project Reset

    what is wrong with using a motor like one i worked on recently ?

    Comment


      #3
      Re: Magnetic Stirrer Project Reset

      Use two STK stereo audio power amplifier modules, chinese clones will work too. This is your "linear amplifier". To make it easier to understand, use the two amplifiers inside each of the modules in "BRIDGE" mode, and you have your H-tree.

      Again, this is dumb. What you're building is a stepper motor and we don't care about the "snapping" - at low speeds sure you'll see this happening but as the speed increases, the fluid you're mixing will dampen the snapping and it will go away. You could even put in more electromagnet poles to smoothen the turn and also increase torque due to the poles being closer together.

      That being said - you're making a stepper motor - there are plenty of stepper motor drivers out there that you can use that have built in H-trees. They even have step timing built in so you don't even need to worry about that.

      ---

      heck, if you really want to drive sinusoidally, get two class D stereo [subwoofer] audio amplifiers and run each module in bridge mode, Solves the power consumption problem too...
      Last edited by eccerr0r; 11-30-2020, 09:43 AM.

      Comment


        #4
        Re: Magnetic Stirrer Project Reset

        Originally posted by petehall347 View Post
        what is wrong with using a motor like one i worked on recently ?
        I've already built one with a motor and the strongest neodymium bar magnet I could find and I attached it to the shaft of the motor etc. The problem with that setup is that the magnets are not strong enough to keep the pill tracked to the bar magnet while it's spinning through the solution, which is a thick solution. It worked fine for water or things of that viscosity but as soon as the solution that i was mixing got thicker. the pill would just jump around.

        So if I use electromagnetics, I know I can create a magnetic field stronger than an earth magnet which should easily keep the pill pulled down tight while its spinning.

        Comment


          #5
          Re: Magnetic Stirrer Project Reset

          Originally posted by eccerr0r View Post
          Use two STK stereo audio power amplifier modules, chinese clones will work too. This is your "linear amplifier". To make it easier to understand, use the two amplifiers inside each of the modules in "BRIDGE" mode, and you have your H-tree.

          Again, this is dumb. What you're building is a stepper motor and we don't care about the "snapping" - at low speeds sure you'll see this happening but as the speed increases, the fluid you're mixing will dampen the snapping and it will go away. You could even put in more electromagnet poles to smoothen the turn and also increase torque due to the poles being closer together.

          That being said - you're making a stepper motor - there are plenty of stepper motor drivers out there that you can use that have built-in H-trees. They even have step timing built in so you don't even need to worry about that.

          ---

          heck, if you really want to drive sinusoidally, get two class D stereo [subwoofer] audio amplifiers and run each module in bridge mode, Solves the power consumption problem too...
          I have different stepper motor drivers, my favorite being the DRV8825, and I did test it many times using different libraries and even direct manipulation of the pulse pin but I could not get the pill to do anything other than jump straight off the platform and I'm not sure why.

          I just assumed that it was designed to pulse stepper coils in a way that works for them but for whatever reason, it won't work for this application.

          I guess I should look at the output of the stepper driver on a scope and see what it's actually doing ... because using a stepper driver would be ideal.

          Comment


            #6
            Re: Magnetic Stirrer Project Reset

            the positioning of your coils are probably not ideal for the pill. The magnetic fields visible to the pill are everywhere, unlike a real stepper where the fields are very compact and close to the magnet.

            real stepper motors' steps are usually like 1/100th or 1/200 of one turn. Your step is 1/4 or 1/8 of one turn, this is huge difference that needs to be accounted for. Or you could make your stepper more like a real stepper - you can do this by putting in more coils.

            Comment


              #7
              Re: Magnetic Stirrer Project Reset

              Originally posted by eccerr0r View Post
              the positioning of your coils are probably not ideal for the pill. The magnetic fields visible to the pill are everywhere, unlike a real stepper where the fields are very compact and close to the magnet.

              real stepper motors' steps are usually like 1/100th or 1/200 of one turn. Your step is 1/4 or 1/8 of one turn, this is huge difference that needs to be accounted for. Or you could make your stepper more like a real stepper - you can do this by putting in more coils.
              Yeah, I basically confirmed this by scoping the output of a DRV8825 driver board and what I noticed right off the bat is that there is no variation in voltage applied to the stepper motor from the driver board ... it either sends 0 volts or it sends the full voltage from the Vsource pin... ALSO, it steps very harshly and even when I set the code so that I could manually step it, it just jerks around and my thoughts were that since a stepper motor uses magnets that are attached to a shaft and tightly coupled, then stepping in this way works just fine, whereas in my application, the magnetic pill is just free standing and will react harshly to that kind of stepping which brings me back to the need to graduate the magnetic field so that the transition from coil to coil is much smoother.

              I thought about adding more coils, but doing that would not let me use a stepper driver at all ... and I would still be in the boat where I need to basically control the field with an analog "feel" to it ... so Im not sure that extra coils will help me at all unless they would give me the advantage of stepping coil to coil using digital on and off style switching because the travel distance between coils is shorter ...

              When I looked at a photo of the inside of a commercial stirrer that uses electromagnetic energy to spin the pill, it only had 4 coils ... so it must be doable somehow.

              I think my next step is to build an H-bridge but these MOSFETS have too much of a non-linear curve to them in terms of how they saturate from the gate voltage.

              Do you know of any transistors that might have more of a linear curve to them? Or what do I need to look for in the datasheets to check for that as I do have a box of different transistors that I can look up their datasheets and see if any of them would work?

              I need to see if my theory at least is on the right track where I graduate the field strength in an analog pulsating style ... if I can get the pill to spin in any way on a smooth path then I can look at other ways to accomplish it in the final circuit using some of the circuit designs you mentioned like the audio amplifier modules ... I've never worked with any of those but the logic seems right in what you're saying.

              Comment


                #8
                Re: Magnetic Stirrer Project Reset

                Any BJT or are you asking us to design a custom solution for you that's nonideal and uninteresting to us? Now we're talking about some design fees here, I could use some $ ...

                Comment


                  #9
                  Re: Magnetic Stirrer Project Reset

                  BTW tongue in cheek ridiculous response:

                  Find two STEREO audio amplifiers that supports bridging, brand doesn't matter, Denon, Yamaha, NAD, Pioneer, Realistic, I don't care.
                  One stereo you bridge L + R together (i.e. connect to one pair of your up/down coils)
                  Use one channel directly, use a 2n3904 to invert the signal and feed opposite polarity to the other channel.

                  Other stereo do the same thing except hook to the left/right coils.

                  Feed your PWMed 90 degrees out of phase sine waves to the line input of two stereos (with appropriate inverters). You may need to AC couple them to the amplifiers.

                  Adjust the volume on both stereos until you get proper behavior...

                  Now the question left as an exercise to the reader is how to simplify this to not use full audio amplifiers. This is what the design cost goes into.

                  Comment


                    #10
                    Re: Magnetic Stirrer Project Reset

                    Actually even easier, forget the 2n3904 inverter - use four different microcontroller outputs with the filtering you have.
                    Two outputs, 180 degrees out of phase of each other.

                    Other two outputs also 180 degrees out of phase with each other, 90 degrees out of phase with the previous two

                    Do all in software...

                    Alas the design aspect is still building the circuit without wasting your Marantz to power your stirrer.

                    Comment


                      #11
                      Re: Magnetic Stirrer Project Reset

                      Oh... and also it's quite possible that your Onyko is powerful enough on its own - since most power amplifiers are bipolar (this won't work with the cheap capacitor coupled single ended amplifiers!), there's no need to bridge outputs - just use one amplifier with AC coupled input to the microcontroller output. No need for the 180 degree opposite phase, just the 90 degree phase output on the second channel, and use left on vertical coils and right on horizontal.

                      If anyone thinks I'm kidding about any of this, no I'm not. There's no reason this shouldn't work, just that it's inefficient.

                      Comment


                        #12
                        Re: Magnetic Stirrer Project Reset

                        I built two H-Bridges with this configuration:



                        And they work in that when I bring pins J1 and J4 high while J3 and J2 are low, current flows through the coils, then if I reverse the high and low state on the four outer pins, current flows in the opposite direction through the coil.

                        HOWEVER, I began to notice that when I had 12 volts on Vcc, the two top mosfets would get fairly hot while the two bottom ones did not. I also noticed that there didn't seem to be much voltage applied to the coils as their magnetic strength was pretty weak. That's when I decided to put an amp meter at the coil node C1(J5) to see how much current was actually flowing in the coil.

                        What I discovered was that it did not matter how much voltage was applied to Vcc, the current through the coil was always around 250ma. So when I applied 5V, then 12V then 19V at Vcc, the current through the coil was always the same. The voltage across the coils (that is the voltage from J5 to J6) is always 2 volts regardless of the voltage at Vcc.

                        What I am assuming at this point, is that the MOSFETs that are connected directly to Vcc are for some reason, "absorbing" the voltage applied at Vcc as it increases causing the rest of the circuit to remain constant and even though the current remains constant, the increased voltage at those MOSFETs increases the wattage they are dissipating which is causing them to get hot.

                        What I don't understand, is why are they doing this? Seemingly acting like resistors? I would expect this behavior from resistors, not MOSFETs. I also would have expected the lower MOSFETs to behave similarly to the top MOSFETs and what I mean by that is when I built this circuit, I was under the impression that since the current flowing through the circuit is the same, if the top MOSFETs were going to warm up, then the bottom ones would as well, but this is not the case and I'm stumped. It's as if the MOSFETs at the top change their properties because they're closer to Vcc but I know that's not possible.

                        Any ideas?

                        Thank you,
                        Attached Files
                        Last edited by EasyGoing1; 12-16-2020, 11:42 AM. Reason: Uploaded screen shot into the post

                        Comment


                          #13
                          Re: Magnetic Stirrer Project Reset

                          Originally posted by eccerr0r View Post
                          Any BJT or are you asking us to design a custom solution for you that's nonideal and uninteresting to us? Now we're talking about some design fees here, I could use some $ ...
                          I'm not opposed to this ... it would depend of course on the cost and whether or not your role would be an isolated one in that ... would you insist that you just design a complete working circuit for a fee or would you be willing to educate me through the design process so that I can learn something?

                          Comment


                            #14
                            Re: Magnetic Stirrer Project Reset

                            Originally posted by EasyGoing1 View Post
                            would you be willing to educate me through the design process so that I can learn something?
                            I would be interested in this myself

                            I made this comment because I have no real use for this now but I have been keeping track of your progress with this project because I might have a need for something like this in the future so I try to keep an open mind for things like this and might learn something from the process you are going through to get where you need to be
                            Last edited by sam_sam_sam; 12-16-2020, 01:17 PM.
                            9 PC LCD Monitor
                            6 LCD Flat Screen TV
                            30 Desk Top Switching Power Supply
                            10 Battery Charger Switching Power Supply for Power Tool
                            6 18v Lithium Battery Power Boards for Tool Battery Packs
                            1 XBox 360 Switching Power Supply and M Board
                            25 Servo Drives 220/460 3 Phase
                            6 De-soldering Station Switching Power Supply 1 Power Supply
                            1 Dell Mother Board
                            15 Computer Power Supply
                            1 HP Printer Supply & Control Board * lighting finished it *


                            These two repairs where found with a ESR meter...> Temp at 50*F then at 90*F the ESR reading more than 10%

                            1 Over Head Crane Current Sensing Board ( VFD Failure Five Years Later )
                            2 Hem Saw Computer Stack Board

                            All of these had CAPs POOF
                            All of the mosfet that are taken out by bad caps

                            Comment


                              #15
                              Re: Magnetic Stirrer Project Reset

                              Well it looks like you've run into the MOSFET problem that all H-tree designers run into at one point or another - high side control. Easiest solution is to use P-channel devices on the high side, but you STILL NEED NEGATIVE FEEDBACK if you want to drive them sinusoidally. The problem with MOSFETs nonlinear inout curve still exists whether you're running single ended or H-tree.

                              As for amplifier design, I've already told the solution - you need an audio amplifier. Using an existing audio amplifier or copying an audio amplifier design is sufficient.

                              Comment


                                #16
                                Re: Magnetic Stirrer Project Reset

                                It depends on how much torque you want and how deep the flask is.

                                Three coils, L298 H-bridge, Arduino: http://blog.dzl.dk/2018/02/26/magnetic-stirrer/
                                https://www.edn.com/electronically-g...gnetic-fields/

                                Every commercial stirrer uses a motor...

                                Comment


                                  #17
                                  Re: Magnetic Stirrer Project Reset

                                  Yeah, that's how it should be done, digital on/off signals driving the coils, not with analog signals driving the coil and certainly not analog signals driving MOSFETs to drive analog to the coils...

                                  Comment


                                    #18
                                    Re: Magnetic Stirrer Project Reset

                                    Originally posted by eccerr0r View Post
                                    Yeah, that's how it should be done, digital on/off signals driving the coils, not with analog signals driving the coil and certainly not analog signals driving MOSFETs to drive analog to the coils...
                                    So an interesting discovery ... I found a youtube video of a guy who is an EE who SUCCESSFULLY made a motor-less magnetic stirrer using what we all assumed would be the best driver - a stepper motor driver, specifically an A4983 which I've never messed with. I have tried using the 8825 and you mentioned the L298 H-Bridge, which I have also messed with.

                                    Here is one of his videos that he made to show how the stir bar cant get back on track if it gets decoupled - something he was talking about in a hobbyist chem forum where he kind of discusses his project in some detail. The main issue is, that he hasn't posted a damn thing since August of 2017 and he did mention wanting to mix "dangerous" chemicals ... explosive types ... so maybe he blew himself up or he ended up being a thwarted Unabomber and is in federal custody ... who knows ... but what that means for me is - no schematics and no Arduino code - he used an Arduino ESP-8266 to control his project - he wanted wifi for remote control ... obviously because of the nature of the stuff he wanted to stir.

                                    So I've been assuming so far, that my specific issues in this project have been related to not being able to graduate the voltage through the coils. However, if that EE managed to get it working with a stepper driver, and ASSUMING that stepper driver pulses the coils without changing the voltage levels on them as it ramps up the speed, etc., then maybe my thought process on this has been wrong the whole time.

                                    I'm going to look at the A4983 datasheet and see how it's different than the 8825 that I'm familiar with ... maybe something about it will jump out at me and explain why he chose that driver do do his project... which actually works.

                                    Something else that I thought was interesting ... he said that he only set the driver for 200ma and his coils were a lot smaller than what I'm using. They also had fairly high gauge wire on them and he talked about how smaller stir bars were better suited to this project ... although he did mention wanting to use larger coils to get more adhesion between the stir bar and the coils ... but as I think about it ... sometimes going smaller means you can do stuff using less energy ... sometimes having a bigger gun isn't always the right option...

                                    Mike

                                    Comment


                                      #19
                                      Re: Magnetic Stirrer Project Reset

                                      i suppose being called motor less is not really true as it must be a motor of sorts when its working .
                                      this sort of thing wants me to see what happens to iron filings on a sheet of paper when placed over the coils .

                                      Comment


                                        #20
                                        Re: Magnetic Stirrer Project Reset

                                        Problem is what the motor should do if the magnet N pole is near the S pole electromagnet and the S magnet S pole is far away from the electromagnet, you're going to be stuck with weird dancing in this situation no matter what and you'll need to manually align the rotor back on track or hope over time random movement will drop it back into proper alignment. I suspect the smaller stir bars the opposite end are closer to the "opposite" polarity electromagnet instead of being far away and have a higher chance of getting back to proper alignment.

                                        Yes this is still a motor that converts electrical energy to mechanical energy. Just that the rotor is not attached to the stator housing at all. The proper term is probably a stir plate with no moving parts.

                                        Here's another tidbit hint that probably can not be applied in this type of motor: Brushless fans have a hall effect sensor to know the position of the fan magnet so it can put the correct polarity near the poles so we don't have vibration instead of rotation when the fan initially tries to spin up. This is somewhat difficult to do with the no moving parts stir plate as magnet position would be difficult to feed back to the exciter.

                                        Comment

                                        Working...
                                        X