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Motor control diy

Discussion in 'Sensors and Actuators' started by masterarcher, Jul 8, 2012.

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  1. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    I don't remember if masterarcher mentioned where he located but I think the home centers and hardware stores still stock 18AWG & 16AWG zip cord. Yeah, that what we call it. Probably because it looks like a zipper! ;)

    Here's a brain teaser but totally insignificant to this thread.. Anyone know what the characteristic impedance of zip cord is?

    Kris, are you satisfied with the microswitch design? I'm really not that keen on them in this application. Optos or Reeds, as you suggested are an option but I've been leaning toward a totally different approach. That would be a PWM controller with current sensing. When the transit reaches the end of the lines power is cut due to the sensed increased stall current. Only switching the toggle switch direction would activate it again.

    Thoughts??

    Chris
     
    Last edited: Aug 26, 2012
  2. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Ah, "zip cord". That makes sense. I have no idea what the characteristic impedance would be.
    I really don't know Chris. I expect there will be problems with the microswitch design, and with optos or reed switches as well. I think detecting the stall current would be workable, without needing a PWM controller. But if you can provide a way to stop the cord at the two limit points, couldn't the same idea be used to operate a microswitch as well?

    For example, if the cord goes through a hole in a fixed panel, and there's some kind of stopper stuck around the cord that will hit that panel when the appropriate limit position is reached and stop the cord from going any further, then it should be possible for the stopper to trip the microswitch in the process of hitting the panel. For example, put a washer on the cable between the stopper and the panel, and attach the microswitch arm to the washer, so when the stopper approaches the panel, it trips the microswitch then hits the panel. A clutch in the motor drive would prevent problems due to motor overrun or control failure.

    Again we have to consider the issue that at least one stopper will be travelling with the cord, between the near and far ends of the travel, and won't be able to go over a pulley, and that the OP wants to have both switches at the near end. He could use a single stopper on the top cord, but that would require a limit switch at each end.

    I don't have much experience with this sort of thing.

    I'm trying to stick with the limit switch idea because it's simple. But it might not be too hard to implement your stall current idea. I think it would just need a current shunt resistor with a brief delay (so it doesn't detect the startup surge), activating a transistor that would trip one of two latching relays, which would be reset when the motor is switched to the other direction. With a bit of clever connecting, I think that's all it would take.

    So, what do YOU think? :)
     
  3. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    I need to sleep on this because I can't think straight with this storm scaring the crap out of me. It's making me nervous as a hatter! :eek:

    Chris
     
  4. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Isaac finally moved on. Three days of him! I thought he'd never leave!! If he's heading anyone's way you better make sure your bilge pumps are working! If he visits a drought state he's gonna fix that right quick.

    Kris, I suggested PWM because I figured if MasterArcher went with a current sense system he may as well have speed control too. :D

    Chris
     
  5. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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  6. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Isn't it amazing what you can get nowadays. A tiny, super-efficient, super-powerful module for a tiny price, that does things that just 10 or 20 years ago would have been either impossible, or extremely large and expensive.

    The synergy of many separate technology and process improvements enable smarter designs with higher performance, and smaller and cheaper parts, and the kitset manufacturers bring the advantages straight to the hobbyist.

    I'm going to have a go at designing an overcurrent trip for that circuit, using latching relays.
     
  7. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    You mean to say that man didn't steal this technology from extra terrestrial Aliens???? :rolleyes:

    Have fun with the design!
     
  8. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Here's my design for stall current detection as suggested by CDRIVE.

    I designed this more as an exercise to see whether it could be done tidily and efficiently. I personally think you should stick with limit switches and a loosely coupled drive, as in the earlier circuit drawings.

    [​IMG]

    If you use a hard limit on the cord travel, and a strong coupling between the motor and the cord, when a limit is reached, the load on the motor will suddenly increase, and so will the current drawn by the motor. This "stall" current will be detected by the transistor, which will trip one of the latching relays, which will interrupt the power to the motor. The relay will latch that state.

    You can unlatch that state by switching the control to the other direction. The motor will start in the other direction, because the other latching relay has not been tripped.

    The stall current trip method can be used in addition to the limit switches, as a backup, or without the limit switches.

    This design is based on my earlier design. The differences are: the power supply is now rated for 2A to ensure that the stall current is high enough to detect reliably, a current shunt and detector are added in the positive supply rail, and two latching relays are added in the motor supply loops.

    The changeover switch operates as before. It connects the 12V supply voltage to either the "away" or the "back" loop, according to the position of the switch (or neither, if the switch is at the "centre off" position).

    If the selected loop draws too much current, voltage drop across the shunt resistor causes the transistor to conduct and supply positive voltage to the "OFF" coils of the latching relays. Whichever latching relay is connected (via the control switch) will be tripped into the OFF state, and will disconnect the power supply from the motor for that direction. The latching relay will be reset into the ON state when the switch is changed to the opposite direction.

    The characteristics of the current detection circuit depend on the shunt resistance and the R-C delay to the base of the transistor. With a shunt of 0.82 ohms, the circuit will trip at around 900 mA sustained load. The higher the overload, the quicker the circuit will trip.

    I'm fairly happy with this design. My only concern is that the control signal to trip the latching relay comes from the current that itself is interrupted by the latching relay, so the current to the relay's "open" coil will disappear as soon as it opens. I think latching relays have mechanical hysteresis that gives a snap-like behaviour, so they don't need the coil current to be sustained after the contacts have opened, so this shouldn't be a problem.

    Edit: The 1A fuse in the power supply circuit needs to be up-rated to at least 2A.

    Chris, what do you think?
     

    Attached Files:

    Last edited: Aug 28, 2012
  9. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Kris, nice job on the schematic. MasterArcher hasn't commented on it though. Perhaps you were spot on about keeping it simple. I hope we don't have to defibrillate him! :D

    Chris
     
  10. masterarcher

    masterarcher

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    Jul 8, 2012
    Hi

    Thanks for the schematics unfortunately a bit beyond what I need I shall stick with the easier option to start with, if I find this does not work how I need it to I shall have to go a bit more compicated.
    I am still waiting for a couple of bits to arrive before I start, I have to look for another couple of microswitches, the two turned up in the post I thought I had an empty envelope as they were that tiny they hide in the corner of the envelope.
     
  11. masterarcher

    masterarcher

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    Jul 8, 2012
    Need to work on the hardware side until I get all the electrical parts now. I shall let you know how I get on and if there are any further questions, I do have a concern that If I have to put a pulley on the motor I bought that it will go to fast and the target carrier will become unstable.
     
  12. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Yes, the most critical part of this system is the mechanics. When the contactors reach there prospective limit switch it must remain in contact until the transit completely stops.

    If tests prove the motor to be too fast your easiest fixes would be a different motor or an off the shelf PWM module. Even if it comes in a kit they're easy to solder up if you're skilled with an iron. On the up side PC Boards are easy even for novices. Personally, I think they were invented to help make even the worst wirer - solderer look competent. :D
     
  13. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Yes, I think it would. I suggested that motor on the basis that you would wind the cord around the motor shaft, which would give you a speed under 0.75 m/s. If you can't do that, you might be able to couple the motor drive onto a large pulley using a second large pulley next to the main pulley, with a belt running to a small pulley on the motor. Otherwise you could use a motor with built-in gearing, but you'll need something faster than the other two options you found earlier.
     
  14. masterarcher

    masterarcher

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    Jul 8, 2012
    Ok cheers for the additional info, I shall get this setup over the next few days and try it out.
     
  15. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Kris, I must have missed this somewhere up the pike. I thought the cord would be driven from a small V Pulley, as I drew it. I'm having a hard time getting my head around a 1/4" dia cord wrapped around a shaft that's probably << 1/4" diameter. V pulley's work well because forces press the cord deeper and tighter into the V. That and because of more surface area.

    Your thoughts..?

    Chris
     
  16. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    I don't remember seeing the cord thickness specified anywhere. I was imagining it a lot thinner than 1/4". The motor shaft diameter is 3.7 mm.

    I guess some kind of gearing will be needed. Either built into the motor, or between the motor and the drive pulley.
     
  17. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    I pulled that dimension out of a hat because the commercial transits I've used employed steel cable nearly that dia.

    Chris
     
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