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PWM ELEVATOR TRIM SERVO SPEED CONTROL

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user9253

Joined: 28 Mar 2008
Posts: 1925
Location: Riley TWP Michigan

Posted: Wed Feb 09, 2022 8:09 am Post subject: PWM ELEVATOR TRIM SERVO SPEED CONTROL

Van's RV-12 elevator trim servo speed is controlled by a PWM circuit.

The theory is that the trim motor sees full voltage and therefore its torque is not reduced.

Is that true?

Some RV-12 owners have trouble adjusting the trim servo motor speed. The motor either runs too fast or not at all.

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_________________
Joe Gores

nuckolls.bob(at)aeroelect
Guest

Posted: Wed Feb 09, 2022 12:03 pm Post subject: PWM ELEVATOR TRIM SERVO SPEED CONTROL

At 10:10 AM 2/9/2022, you wrote:

  	  Quote: 	
		  --> AeroElectric-List message posted by: "user9253" <fransew(at)gmail.com>



 Van's RV-12 elevator trim servo speed is controlled by a PWM circuit.

 The theory is that the trim motor sees full voltage and therefore its torque is not reduced.

 Is that true?

 Some RV-12 owners have trouble adjusting the trim servo motor speed. The motor either runs too fast or not at all.



 --------

 Joe Gores 	


   What are they using for speed control?

   Series control like a variable resistor

   isn't going to work. A PWM controller is

   better but it's not ideal. The recommended

   speed control management provides an

   ADJUSTABLE, CONSTANT VOLTAGE (REGULATED)

   SOURCE TO THE MOTOR. Here's one approach

   I crafted for a builder about 17 years ago:



  http://aeroelectric.com/PPS/Flight/Trim/trim6.pdf



   His pitch trim motor was rather hefty . . . it

   drew enough current to required one of the

   larger LM317 devices. If we're talking about

   the little MAC actuators, the smallest of LM317

   (plastic TO220) will suffice.



   There's a hangar full of bad science out

   there about motor performance but here's a

   place to start:



   The attached curve is an EXEMPLAR performance

   plot for a PM motor.  Note that TORQUE is

   directly proportional to CURRENT. RPM is

   proportional to VOLTAGE. Hence, assuming that

   the motor is operating within design limits,

   a slowing motor is starved for VOLTAGE. In

   the case of a trim system, torque loads are

   not a function of trim speed . . . so if

   speed regulation is poor, it's a prime notice

   that VOLTAGE regulation is poor . . .series

   resistors turn a constant voltage source

   into a wet noodle.



   Note that there is ONE plot for torque vs.

   current that's true for ALL voltages. No

   load RPM and STALL current are proportional

   to voltage (note 6v stall is 1/2 that of

   12v stall). 



   ACTIVE voltage regulators incorporated as

   suggested in the drawing are CONSTANT

   VOLTAGE sources irrespective of load. 

   Motors do not automatically 'draw more

   current' as their driving voltage is reduced.

   Using a series resistor to control speed

   makes speed regulation so poor that it SEEMS

   as if motor current is being modulated with

   speed . . . not so.



 



 

 

   Bob . . .



   Un impeachable logic: George Carlin asked, "If black boxes

   survive crashes, why don't they make the whole airplane

   out of that stuff?"

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nuckolls.bob(at)aeroelect
Guest

Posted: Thu Feb 10, 2022 5:51 am Post subject: PWM ELEVATOR TRIM SERVO SPEED CONTROL

 	  Quote: 	
		    ACTIVE voltage regulators incorporated as

   suggested in the drawing are CONSTANT

   VOLTAGE sources irrespective of load. 

   Motors do not automatically 'draw more

   current' as their driving voltage is reduced.

   Using a series resistor to control speed

   makes speed regulation so poor that it SEEMS

   as if motor current is being modulated with

   speed . . . not so. 	


    I've had some direct questions on this

    topic . . . perhaps I went too far into

    the weeds with the physics and offered

    insufficient practical approach to

    individual solutions.



    If anyone is having trouble managing

    their trim system performance, please

    have them join us here on the List or

    drop me a line personally. I've

    worked trim systems on everything from

    C182 to Hawker 4000 and most of those

    in between. There ARE proven solutions

    but they may require tailoring to a

    builder's specific situation.



    It CAN be fixed.



 

 

   Bob . . .



   Un impeachable logic: George Carlin asked, "If black boxes

   survive crashes, why don't they make the whole airplane

   out of that stuff?"

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user9253

Joined: 28 Mar 2008
Posts: 1925
Location: Riley TWP Michigan

Posted: Thu Feb 10, 2022 7:11 am Post subject: Re: PWM ELEVATOR TRIM SERVO SPEED CONTROL

Bob, thanks for the information.  If I ever redesign my trim circuit, it will contain a LM317.

You asked about the RV-12 trim circuit.  Attached is a schematic of the original trim circuit.

Van's has completely changed it in later models.  But they still use PWM.

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_________________
Joe Gores

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