Back EMF, so many ways, what is best ?

Discussion in 'General Electronics Chat' started by howdoesthatworkguy, Sep 13, 2007.

  1. howdoesthatworkguy

    Thread Starter Active Member

    Jul 23, 2007
    39
    0
    I have seen ops amps, gain amps, simple resistor devider networks, capacitors been used as some type of filter and the list goes on. I have very limited PCB space. I am looking for a simple way to read my BEMF from my H-Bridge and feed that into a 10Bit ADC on my PIC. The supply voltage is anywhere from 9 to 17v, and can even spike sometimes up to 27v then a zener takes over as protection.



    With a supply voltage that changes how can I get some type of messurment ?
     
  2. techroomt

    Senior Member

    May 19, 2004
    198
    1
    i would think a current (loop) pick-up would work.
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Why is your supply voltage so variable? Nail that down and life becomes easier.
     
  4. howdoesthatworkguy

    Thread Starter Active Member

    Jul 23, 2007
    39
    0
    I don't have any control over the powersupply, this is a factor of circuit design, which is not my design. So I have to play with 9 to 17v, as far as I know I can use the voltage ref on the PIC since the supply rail varies so much correct ?

    But small and simple and cheap feedback is what I am after. PS, is current or backEMF feedback better ? I was told that BEMF does not work at small voltages because the motor does not really generate enough BEMF to be read ?
     
  5. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Back EMF is an apparent resistance in the motor that resists the flow of current. As current is dependant on the EMF, it can be used to indicate the state of the motor.

    Since you are trying to convert the reading, you will have to time your ADC to only convert during the time the H bridge is actually sending current through the motor. Sample timing will have to be delayed a bit after current is switched on. The motor's EMF will have to have some time to build and stabilize, so current will not be stable until that time has passed. An oscilloscope would be valuable to see how many microseconds after current starts before things have stabilized anough to make an accurate conversion.

    Lacking that, determine the shortest active period, and delay the conversion until close to the end of it. That might do as a fixed delay for all longer active periods if you are using PWM for speed control.
     
  6. howdoesthatworkguy

    Thread Starter Active Member

    Jul 23, 2007
    39
    0
    I don't have a problem with the software side of things when using the PIC and what is needed to get this side of things working. My issue is with the hardware interface cricuit. I have seen things as simple as two resistors coming off the motor directly feeding into the ADC to opp amps and gain amps. I am wanting to know if the simple resistor setup as a devider will be accurate enough or do I need to look at opp or gain amps ?
     
  7. ixisuprflyixi

    Active Member

    Sep 16, 2007
    52
    1
    The back EMF, being the electromagnetic field break down that occurs after the motor is no longer being sent current, or am I misunderstanding? Because if your looking to pick up on the magnetic field as it is "breaking down" a simple ferrite bead with a few loops of enamled wire around it would fit into your small PCB and would be simple.
     
Loading...