Drive motor forward on power on, backwards on power off

Discussion in 'The Projects Forum' started by men8th, Oct 5, 2012.

  1. men8th

    Thread Starter New Member

    Jul 7, 2011
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    Hi all,

    I have a project which requires a motor to
    • go forwards for ~1 second when the power supply is turned on,
    • go backwards for ~1 second when the power supply is turned off, i.e. the system is de-energiesed, and
    • remain stationary for the rest of the time.
    The power supply is 240 vac. The motor is ~6 vdc, < 400mA.

    I can think of a couple of ways of achieving this, all of which are more complex than I think it needs to be. What is the simplest way of achieving this reliably? I'm not bothered if the timing is accurate as long it is repetable and won't drift over time.

    Looking forward to your suggestions
     
  2. cork_ie

    Member

    Oct 8, 2011
    348
    58
    Can you elaborate a little , what do you mean by forwards and backwards?

    I assume it is a rotary motor. Is the load fixed or varying i.e. do you require position feedback or not?
    Would a fixed period of activation after power on & power off suffice?

    If so I would look at 555 in monostable and a double pole double throw relay or H bridge.
    You will need some form of reserve power to reverse the motor after the 240 Supply is removed.
     
  3. gerty

    AAC Fanatic!

    Aug 30, 2007
    1,153
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    Or a time delay to turn the power off after motor moves
     
  4. cork_ie

    Member

    Oct 8, 2011
    348
    58
    That is one possibility, we shall see his answers first and look at the various options.
     
  5. men8th

    Thread Starter New Member

    Jul 7, 2011
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    0
    The load is fixed and the motor is rotary. By forwards and backwards I mean that the motor should rotate in one direction and then the opposite direction. There is no particular requirement for position feedback or precise control, a fixed period of rotation would be fine.

    Am not close to internet at moment so sorry if my answers are slow.
     
  6. men8th

    Thread Starter New Member

    Jul 7, 2011
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    Hi, anyone any suggestions on this? In particular, what to use to provide the backup power supply for when the power goes off. Would like to avoid batteries if possible as don't want the hassle of designing the charging circuit.

    Tim
     
  7. John P

    AAC Fanatic!

    Oct 14, 2008
    1,634
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    Look up "supercapacitor" or "ultracapacitor". If you wanted to supply 400mA for 1 second at 6V, you'd need well under 1 Farad, but the current would start high and taper off as the voltage dropped, so I'm not sure what it would really take. Would one big current pulse do what you need?

    But then, if your AC power supply is a transformer, rectifier and filter capacitor, maybe you could detect when the input has been lost, and use the remaining energy in the filter capacitor to revere the motor.
     
  8. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    397
    Just a few mA drain when power is off. Maybe a little more complex than you wanted, but it's a start.
     
    men8th likes this.
  9. men8th

    Thread Starter New Member

    Jul 7, 2011
    11
    0
    Wow Bernard, that's really impressive that you spent the time to draw that out, thanks. I think I will proceed on those lines but use a h-bridge rather than relay - I already have experience with a suitable h-bridge, library parts drawn up etc. For backup power I think I'll use a supercap, for my application the 6v power supply will be disconnected at the same time as the rest of the circuits, i.e. the power will be cut to the whole thing all at once.

    Thanks again,

    Tim
     
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