Linear actuator control circuit

Discussion in 'General Electronics Chat' started by Dimitris76, Jul 17, 2011.

  1. Dimitris76

    Dimitris76 Thread Starter Member

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    Hi all! I am new in electronics and need some advice.

    I need to build a circuit that will power a linear actuator with built-in end switches like this one:

    http://www.firgelliauto.com/product_info.php?cPath=79&products_id=31

    for as long as I hold a push button depressed.

    At the end of the actuator's travel the polarity to the motor should be reversed until the actuator retracts all the way to it's other end and then reverse polarity again to extend, and so on... until I release that push button.

    The end switches are internal and not accessible and the actuator has only a cable with two poles.

    I was thinking that measuring the voltage across the legs of a low value high wattage resistor that is connected in-line to the circuit output would be a nice way for the circuit to "sense" that the actuator has reached it's end of travel and switched off it's motor. No (high) current flow - no voltage drop across the resistor.

    Simple PIC solutions are welcome too - you'll have to right the program for me though... :)

    Thank you in advance!

    Dimitrios
  2. Dimitris76

    Dimitris76 Thread Starter Member

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    Do you thing that a 555 in bistable mode and either external NO reed end-switches or inline to the load resistors for trigger and reset would work?

    The output would control a DPDT relay reversing the motor...

    Comments?

    Dimitrios
  3. ifixit

    ifixit Well-Known Member

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    Hi Dimitris76,

    It seems like the internal limit switches turn off the motor when there respective ends are reached. So, all you need is a way to detect when no current is be used by the actuator and then reverse the polarity to the actuator, which will reverse the motion until the other end limit switch is opened and so on.

    Let's see who comes up with a circuit to do as you requested.

    Regards,
    Ifixit
  4. John P

    John P Senior Member

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    I looked at the company's wiring diagram and couldn't make sense of it. Yes, they could have wired the limit switches with diodes so you wouldn't have external access to them. Then you could only detect that the motor had hit a switch by whether it was drawing current or not. It sounds crazy, but maybe they did it that way to keep the user from damaging the unit (by making the motor overrun its safe travel).
  5. SgtWookie

    SgtWookie Expert

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    I'd taken a look at the site yesterday, and for some reason they have a "Click here for PDF Spec sheet" shown, but it's not a link. You can click until you're blue in the face, and nothing will happen.

    John P, I'm in agreement with you about diodes with the limit switches; I figure that internally it's more or less connected like this:

    [​IMG]

    For automation's sake, it's somewhat unfortunate that they wired it that way. However, it makes perfect sense from the aspect of a manually-operated system using a single DPDT switch to reverse direction; the limit switches make completing a manually reversing circuit simple.

    But, to make this circuit reversible automatically, you'd really want to add external limit switches.

    Attached Files:

  6. Dimitris76

    Dimitris76 Thread Starter Member

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    Thank you for the interest!

    But let's forget those internal end-switches for a while...

    I can easily mount a small magnet on the actuator arm and two reed-switches placed a few mm "inside" the travel endpoints.

    Could you post a schematic of a circuit that would have the actuator running back and forth for as long as I press that push button?

    Dimitrios
  7. Dimitris76

    Dimitris76 Thread Starter Member

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    Can I use this bistable 555 circuit


    [​IMG]

    with NO (normal open) end switches and a power on reset trigger with capacitor like this?

    [​IMG]

    The 555 output would control a DPDT relay with a shunt diode across it's coil.

    What do you guys think?

    Also, if this does work, how can I prevent damage to the DC motor and the actuator's drivetrain by the sudden polarity reversal?

    Dimitrios
  8. ifixit

    ifixit Well-Known Member

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    Hi Dimitris76,

    I have attached a schematic circuit idea, very simalar to what you have suggested, but with more details. Using the 555 as a FF sould work also, but I have never tried it.

    Regards,
    Ifixit

    Attached Files:

  9. Dimitris76

    Dimitris76 Thread Starter Member

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    Great idea!

    Your circuit uses 5 resistors, 2 capacitors, a transistor and an IC (4013D-FF).

    What's the advantage over the above 555 based schematic that uses only 2 resistors and an IC (555) - plus the relay, diode and the two end-switches?

    Also, how can I prevent punishing the motor/drive during polarity changes?

    Dimitrios
  10. Adjuster

    Adjuster Well-Known Member

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    The first step in limiting the "punishment" to the motor during a polarity change might be to require the current to have fallen to zero for some minimum period before reversing the polarity (you probably have to do that anyhow to avoid false triggering).

    A minimum "dead" time will give the motor time to stop properly before applying the reverse voltage. Next, you might want to consider a current-limit or soft-start circuit to limit the motor peak current on restart.

    Even if you do that, you may find that the motor overheats if you make it cycle continuously - is it continuously rated?
  11. Dimitris76

    Dimitris76 Thread Starter Member

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    The motor is rated for a duty cycle of max. 10%. I don't intend to hold that push button depressed for more than 1 min though.

    As for turning off the motor before reversing. Any ideas how to introduce approx. 0.5sec delay when the end point is reached?

    Dimitrios
  12. ifixit

    ifixit Well-Known Member

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    • I think the internal limit switches will turn it off for you. That's why they included them in the design.
    • There is nothing special about the 4013, it is just another idea. There are other flip-flops you could use as well.
    • Where does it say you need a dead time before reversal?
    • The RC filter after the limit sw is to protect the electronics from ESD and switch contact bounce transients.
    • The 10% dutycycle is for the max load of 5 amp situation. What is your max load (pounds of force)?
    • Email the manufacturer and ask them if their motor can withstand instant reversals.
    Regards,
    ifixit
  13. Adjuster

    Adjuster Well-Known Member

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    The OP was concerned about the motor getting "punished" by reversing the supply. My suggestion of a minimum period without power comes from experience of sudden reversing of DC motors provoking severe current surges and blowing fuses (or worse). This can be a good deal less severe if the motor is given time to come to rest, so that it is not generating an emf aiding the supply when it comes on in reverse.

    If the manufacturer can confirm that instant reversals are OK, then all well and good, but I would not want to take it on trust. Admittedly, a manual switch could apply reverse voltage pretty quickly, but electronics might do it in milliseconds, time and time again.
  14. Dimitris76

    Dimitris76 Thread Starter Member

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    iFixit

    I don't intend to use the internal end-switches.
    Like I said in my previous post I will mount external reed switches that will be tripped a few millimeters before the internal end points are reached.

    I don't believe contacting the manufacturer will give any usefull information. They couldn't even provide the appliance's physical dimentions - they kept refering me to an online incomplete drawing that does not state the shaft housing diameter....

    These things are made in China and marketed by many different online ventors under different names. I suspect it is just a plain vanilla DC brushed can motor.

    Having said that, I am still looking for away to introduce a little delay. Then I can place the reed switches exactly on the mechanical endpoints and take advantage of the internal end-switches to stop the motor.

    Dimitrios
  15. Dimitris76

    Dimitris76 Thread Starter Member

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    Pistnbroke,

    Sorry mate, you obviously don't comprehend this discussion...

    Dimitrios
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