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Trim motor speed control

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donjohnston

Joined: 13 Dec 2009
Posts: 231

Posted: Thu Nov 14, 2013 9:45 am Post subject: Trim motor speed control

I have two trim motors that I need to control. The motors provided are 24v motors (3-5amps). But they are intended for with a 12v electrical system. This is done because the 12v motors on a 12v system turn too fast and you keep overshooting when you try to adjust the trim.



This is the elevator trim motor.

http://www.grainger.com/product/WARNER-LINEAR-Linear-Actuator-18G685?s_pp=false



But my electrical system is 24v. Which means the motors will turn too fast. I looked for 48v versions and they're either too expensive or not available at all. So I'm going to have to slow  down the existing motors. And to that end, I have come up with 3 possibilities:



1) Put in a DC-DC step down converter. I can get one for about $250. But it is a rather large box that I'll have to mount somewhere and put in the necessary breakers and wiring. I may need 14v for other things in the future so this wouldn't be the worst solution.

http://www.lonestaraviation.com/Step-Down-Conveter-24V-to-13.8-Volts.html



2) I had picked up a motor speed controllers for the heater fan speed controller.  But it's only $10! You know the old saying about something being too good to be true?

http://www.amazon.com/dp/B009HKFAIQ/ref=pe_385040_30332190_pe_175190_21431760_M3T1_ST1_dp_1



3) Finally, I could get a dedicated trim motor controller.  But I would need their "HD" version and I would need two them... at $245 each for a $490 total.  

http://www.tcwtech.com/safety_trim.html



I've pretty much removed option #2 because I have to think that there's this $10 solution either generates noise, is unreliable, or... something else. 



So the question(s) are: Does anyone have any experience with a situation like this and if so, which path did you chose? Or does anyone have any opinions on which would be the best solution? And finally, is there an option that I haven't considered? (and changing my entire electrical system to 14v is NOT an option)   



Thanks,

Don

tim2542(at)sbcglobal.net
Guest

Posted: Thu Nov 14, 2013 10:08 am Post subject: Trim motor speed control

I would call TCW. I sent my Firgelli linear actuator (pitch trim) to them for approval. It was rated well over his specification, but in this application never sees much of a load. He approved it and it I'm using it now. 

I would recommend his trim controller for just the stuck trim switch over ride protection alone. Speed control is also available and 2 speeds if you want that. 

I'm not sure if they work with 28 v systems.

Tim

 	  Quote: 	
		   On Nov 14, 2013, at 9:45 AM, "donjohnston" <don(at)velocity-xl.com> wrote:

 

 

 

 I have two trim motors that I need to control. The motors provided are 24v motors (3-5amps). But they are intended for with a 12v electrical system. This is done because the 12v motors on a 12v system turn too fast and you keep overshooting when you try to adjust the trim.

 

 This is the elevator trim motor.

 http://www.grainger.com/product/WARNER-LINEAR-Linear-Actuator-18G685?s_pp=false

 

 But my electrical system is 24v. Which means the motors will turn too fast. I looked for 48v versions and they're either too expensive or not available at all. So I'm going to have to slow  down the existing motors. And to that end, I have come up with 3 possibilities:

 

 1) Put in a DC-DC step down converter. I can get one for about $250. But it is a rather large box that I'll have to mount somewhere and put in the necessary breakers and wiring. I may need 14v for other things in the future so this wouldn't be the worst solution.

 http://www.lonestaraviation.com/Step-Down-Conveter-24V-to-13.8-Volts.html

 

 2) I had picked up a motor speed controllers for the heater fan speed controller.  But it's only $10! You know the old saying about something being too good to be true?

 http://www.amazon.com/dp/B009HKFAIQ/ref=pe_385040_30332190_pe_175190_21431760_M3T1_ST1_dp_1

 

 3) Finally, I could get a dedicated trim motor controller.  But I would need their "HD" version and I would need two them... at $245 each for a $490 total.  

 http://www.tcwtech.com/safety_trim.html

 

 I've pretty much removed option #2 because I have to think that there's this $10 solution either generates noise, is unreliable, or... something else. 

 

 So the question(s) are: Does anyone have any experience with a situation like this and if so, which path did you chose? Or does anyone have any opinions on which would be the best solution? And finally, is there an option that I haven't considered? (and changing my entire electrical system to 14v is NOT an option)   

 

 Thanks,

 Don

 

 

 

 

 Read this topic online here:

 

 http://forums.matronics.com/viewtopic.php?p=413041#413041

nuckolls.bob(at)aeroelect
Guest

Posted: Thu Nov 14, 2013 10:18 am Post subject: Trim motor speed control

At 11:45 AM 11/14/2013, you wrote:




I have two trim motors that I need to control. The motors provided 

are 24v motors (3-5amps). But they are intended for with a 12v 

electrical system. This is done because the 12v motors on a 12v 

system turn too fast and you keep overshooting when you try to adjust the trim.



This is the elevator trim motor.

http://www.grainger.com/product/WARNER-LINEAR-Linear-Actuator-18G685?s_pp=false

   First, why such a 'horse' of an actuator?

   What are the force, speed and travel requirements

   for moving the trim mechanism on your airplane?

But my electrical system is 24v. Which means the motors will turn too 

fast. I looked for 48v versions and they're either too expensive or 

not available at all. So I'm going to have to slow  down the existing 

motors. And to that end, I have come up with 3 possibilities:



    With a full load current rating of 2.3A (assuming

    that it's moving a 100# load), your voltage

    control task is pretty simple. But with a PM motor,

    inrush current is pretty high compared to the 2.3A

    max running load.



1) Put in a DC-DC step down converter. I can get one for about $250. 

But it is a rather large box that I'll have to mount somewhere and 

put in the necessary breakers and wiring. I may need 14v for other 

things in the future so this wouldn't be the worst solution.'



   How about this one?



   Rated for 8A (plenty of headroom) but

   in addition to an adjustable output voltage

   (controls speed) there's an adjustable

   current limit (controls inrush).



   Use this dc/dc converter upstream of your

   dpdt center off polarity control switch.

   Set voltage for max desired speed. Then

   set the current limit to some benign value

   well above max rated load for the actuator . . .

   say 3A.

   Bob . . .

nuckolls.bob(at)aeroelect
Guest

Posted: Thu Nov 14, 2013 10:29 am Post subject: Trim motor speed control

[img]cid:7.1.0.9.0.20131114122807.01f1ed78(at)aeroelectric.com.0[/img] 



 



 

   Bob . . .

dd32b5.jpg

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kuffel(at)cyberport.net
Guest

Posted: Thu Nov 14, 2013 11:12 am Post subject: Trim motor speed control

Don,



Agree with the others about the heftiness of your actuators.  100 pounds and 

4 inch travel sounds like something for a jet.  Finding a lower current 

actuator would ease your problem.



But assuming you can't change actuators, here is a full featured 2 axis trim 

controller:



Start with the circuit published in the June 2013 issue of Kitplanes 

(schematic attached).



Modify the circuit to use an LM338 (40 volt, 5 amp, 7 amp surge, $2.03 from 

Mouser) instead of the LM317 shown.



Power the low current logic part of the circuit with a separate regulator 

(LM317) set to supply 12 volts.  The heat sink specified is more than 

adequate for both jobs.



This gives you all the features of the fanciest commercial controllers for 

less than $35 in parts.



If you do not have access to the Kitplanes issue email me off list for a set 

of figures, photos and text.



Tom

KPTrimAFig1.pdf

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nuckolls.bob(at)aeroelect
Guest

Posted: Thu Nov 14, 2013 11:45 am Post subject: Trim motor speed control

My link to the suggested power supply

didn't paste . . . her 'tis again.

 	  Quote: 	
		     How about this one?


	


http://tinyurl.com/mk6vs4l

 	  Quote: 	
		     Rated for 8A (plenty of headroom) but

   in addition to an adjustable output voltage

   (controls speed) there's an adjustable

   current limit (controls inrush).



   Use this dc/dc converter upstream of your

   dpdt center off polarity control switch.

   Set voltage for max desired speed. Then

   set the current limit to some benign value

   well above max rated load for the actuator . . .

   say 3A.

   Bob . . .



-----

No virus found in this message.

Checked by AVG - www.avg.com


	




   Bob . . .

donjohnston

Joined: 13 Dec 2009
Posts: 231

Posted: Thu Nov 14, 2013 1:12 pm Post subject: Re: Trim motor speed control

 	  Quote: 	
		   	  Quote: 	
		  This is the elevator trim motor.

http://www.grainger.com/product/WARNER-LINEAR-Linear-Actuator-18G685?s_pp=false 	




   First, why such a 'horse' of an actuator?

   What are the force, speed and travel requirements

   for moving the trim mechanism on your airplane?

	




One word... Canard.

kuffel(at)cyberport.net
Guest

Posted: Thu Nov 14, 2013 5:22 pm Post subject: Trim motor speed control

Don,



<<  why such a 'horse' of an actuator? ... One word... Canard.  >>



Must confess to still being confused.  My Defiant had very light control 

forces which were still heavier than any Eze I've seen.  Assume you are 

planning to use springs and not aerodynamic trim tabs?  Could you supply 

more information?



Tom

donjohnston

Joined: 13 Dec 2009
Posts: 231

Posted: Fri Nov 15, 2013 4:37 am Post subject: Re: Trim motor speed control

The aircraft is a Velocity XL. The trim motor is the one supplied by the factory.

donjohnston

Joined: 13 Dec 2009
Posts: 231

Posted: Fri Nov 15, 2013 4:37 am Post subject: Re: Trim motor speed control

The aircraft is a Velocity XL. The trim motor is the one supplied by the factory.

Eric M. Jones

Joined: 10 Jan 2006
Posts: 565
Location: Massachusetts

Posted: Fri Nov 15, 2013 5:56 am Post subject: Re: Trim motor speed control

Motor Speed Control--



An important point of trim motor speed controlling is that the torque should not be reduced when the speed is reduced (in fact, it could go up...but not down). This makes regular voltage controllers undesirable except where torque isn't an issue.



The second point is that trim speed should usually be reduced as a function of airspeed. Often this is critical and can give the pilot sudden difficulties. Tee-ing off the pitot line and using the pressure signal is easy to do.



Many speed controls use an LM3524 modulating PWM chip and a P-fet to drive the trim motor that will take a voltage-input based on the pitot pressure. This makes the motor go faster at slow speeds and slower at high speeds. No input from the pilot is needed.

_________________
Eric M. Jones
www.PerihelionDesign.com
113 Brentwood Drive
Southbridge, MA 01550
(508) 764-2072
emjones(at)charter.net

bakerocb

Joined: 15 Jan 2006
Posts: 727
Location: FAIRFAX VA

Posted: Fri Nov 15, 2013 9:48 am Post subject: Trim motor speed control

11/15/2013

  

 Hello Don, Thanks for the quick response. I’ve got the picture. Take a look  at the Firgelli Automations FA-SC24 Actuator Speed Controller seen here:

  

 http://www.firgelliauto.com/product_info.php?cPath=99&products_id=362

  

 Note the the 12V to 40V operating range.

  

 OC

  

 ==============================

  

    

  From: Don Johnston (don(at)johnston.org) 

 Sent: Friday, November 15, 2013 10:36 AM

 To: Owen  Baker (bakerocb(at)cox.net) 

 Subject: Re: Trim motor speed control

  



  

Owen, 

Thanks for reaching out to me. 

The aircraft is a Velocity-XL. I am trimming a canard-width  elevator.  I have been told by the factory and other builders that existing  trim is right at the upper limit of usable speed. The trim motor is connected to  a fiberglass leaf spring. 

Fon 

Sent from my Android mobile device.   	  Quote:

nuckolls.bob(at)aeroelect
Guest

Posted: Fri Nov 15, 2013 12:43 pm Post subject: Trim motor speed control

At 07:56 AM 11/15/2013, you wrote:

  	  Quote: 	
		  --> AeroElectric-List message posted by: "Eric M. Jones" <emjones(at)charter.net>



 Motor Speed Control--



 An important point of trim motor speed controlling is that the torque should not be reduced when the speed is reduced (in fact, it could go up...but not down). This makes regular voltage controllers undesirable except where torque isn't an issue. 	


   Not true. Speed and torque are not directly related

   in terms of motor performance. Speed of a motor is a

   function of terminal voltage AND load on the motor.

   Torque requirement for moving a flight control surface

   is a function of aero-dynamic loads. For example, the

   average torque needed to extend a flap from 0-10

   degrees is often a small fraction of that required to

   push it all the way out to 30 degrees.



   Assuming one wishes to slow flap motion as it approaches

   fully deployed, there is risk of having the motor STALL

   under some conditions of load . . . the risk is a function

   of motor resistance and a REDUCTION in available torque

   at voltages intended to simply slow it down. It has NOTHING

   to do with the manner in which voltage is controlled. You

   can set the terminal voltage of the motor by means of regulated

   power supply, battery, PWM controller, variable resistor,

   you name it. As long as you're holding the motor voltage

   constant, then it's possible to experience an unintended

   STALL which is illustrated in the following plot of

   an exemplar motor's performance in a pitch trim actuator.



 [img]cid:.0[/img]

    Folks that would like to explore all the details of

    my assertion are invited to download the data package

    at  http://aeroelectric.com/articles/Motors



    The figure above is an excerpt from one of the figures

    in that data package. In high speed trim, applied voltage

    was 26.5 volts which means that for ALL other conditions,

    the motor speed can be predicted right off the plots.

    Aerodynamic loads reflected to the motor ranged from .72 oz-in

    to 1.82 oz-in. With high speed voltage applied, the motor

    can be expected to run someplace between 3870 and 5304 rpm

    for a variation of +/- 16%.



    With low speed voltage applied, the same motor's speed

    variance from center was predicted to be +/- 100

    percent . . . with the motor stalling at max loads.



    The data package speaks to the characterization of

    motor stall current as (Eapplied-Ecemf)/Resistance

    of the motor. Motors being operated at FIXED voltages

    appear to 'lose torque' because their speed regulation

    is terrible compared to the higher speed operation.



    Note that torque-current demand on the motor at say 1.5 oz-in

    is exactly the same irrespective of speed (applied

    voltage). The motor  appears 'crippled' because the SAME

    variations in current (CAB) over designed torque range

    is a LARGER PERCENTAGE of motor's predictable

    behavior at the lower speed (FDE).



    Virtually ALL voltage control methodologies popular

    with the OBAM aviation community are simply modifications

    to applied voltage. The seemingly 'crippled' behavior

    of the motor will be the same no matter HOW that

    voltage is adjusted. LM317, pwm . . . you pick it.



    There ARE ways to close the loop on speed control

    by sensing motor current and inserting a derived

    correction of speed by increasing terminal voltage

    as load increases . . . or by sensing motor speed

    with some form of tachometer and closing the loop

    on measured speed.



    Assertions for expecting loads on a motor to

    go UP as shaft speed goes down are incorrect,

    the flight surface has no way to know that it's

    moving slower . . . it IS influenced only by change

    of dynamic pressure and moment arms.



  	  Quote: 	
		  The second point is that trim speed should usually be reduced as a function of airspeed. Often this is critical and can give the pilot sudden difficulties. Tee-ing off the pitot line and using the pressure signal is easy to do. 	


    I've designed a number of pitch trim controllers.

    The first two went on the Lear 55 and the second

    was a fleet retrofit to the 30 series. I

    proposed a continuously variable pitch trim rate

    based on IAS but it proved unnecessary. Trim rates

    optimized for cruising flight fell into one fairly

    narrow range, speeds for approach to landing in another

    narrow range. Turns out that approach:cruise trims

    in the Lear systems were 4:1 ratio. Use a flaps-not-stowed

    switch to select high speed and we were done.



 

   Bob . . .

nuckolls.bob(at)aeroelect
Guest

Posted: Fri Nov 15, 2013 12:55 pm Post subject: Trim motor speed control

At 11:46 AM 11/15/2013, you wrote:

  	  Quote: 	
		  11/15/2013

  

 Hello Don, Thanks for the quick response. I’ve got the picture. Take a look at the Firgelli Automations FA-SC24 Actuator Speed Controller seen here:

  

   http://www.firgelliauto.com/product_info.php?cPath=99&products_id=362 

  

 Note the the 12V to 40V operating range. 	


   Note also the notation:



 The FA-SC24 is intended to control the speed of one Linear actuator or motor, Do not connect two actuators/motors to the same FA-SC24. The slowest the actuator/motor will run is about 10% of the full speed.  Reducing the speed can affect the maximum load the actuator can move.



   Motor current is proportional to load torque

   at any speed. There will come a time as voltage

   falls that STALL torque (Eapplied/Resistance of

   Motor) is some number smaller than load torque

   and the motor stalls.



   The load torque doesn't rise because the speed

   went down; ability to CARRY rated torque fell

   off because stall current came down proportionally

   with the drop in applied voltage.



   If we could wind the motor with unobtainium having

   zero resistance, the speed torque curve would not

   have that left-to-right, downward slope . . . it would

   be flat. This means that speed is absolutely locked

   to applied voltage no matter what load is applied.

   The same motor would be 100% efficient because it

   doesn't get hot!



   I've worked with designs that deliberately picked

   a too-large motor with lower values of internal

   resistance just to get a flatter speed-torque

   curve without use of external electronic controls.



 

 

   Bob . . .    [quote][b]

Eric M. Jones

Joined: 10 Jan 2006
Posts: 565
Location: Massachusetts

Posted: Mon Nov 18, 2013 12:52 pm Post subject: Re: Trim motor speed control

EMJ said: An important point of trim motor speed controlling is that the torque should not be reduced when the speed is reduced (in fact, it could go up...but not down). This makes regular voltage controllers undesirable except where torque isn't an issue.



BN said: Not true. Speed and torque are not directly related in terms of motor performance. Speed of a motor is a function of terminal voltage AND load on the motor. 



EMJ said: Thatâ€™s absurd and examples are so trivial I wonâ€™t even bother to note one. Besides, I didnâ€™t say they were related. The point is that motor speed for trimming and flaps should be reduced at high airspeed to prevent overstressing the pilot and the aircraft. This is common practice. I didnâ€™t invent this idea. Furthermore reducing the voltage is a poor manâ€™s way to reduce the speed of any actuator. I didn't invent that idea either.



BN said: Torque requirement for moving a flight control surface is a function of aero-dynamic loads. For example, the average torque needed to extend a flap from 0-10 degrees is often a small fraction of that required to push it all the way out to 30 degreesâ€¦. 



Folks that would like to explore all the details of my assertion are invited to download the data package at http://aeroelectric.com/articles/Motors ....



Virtually ALL voltage control methodologies popular with the OBAM aviation community are simply modifications to applied voltageâ€¦. 



EMJ said: And they are all wrong. Start with yours. The modern methods for changing motor speed without reducing torque â€¦ as I saidâ€¦should be used.



BN said: Assertions for expecting loads on a motor to go UP as shaft speed goes down are incorrectâ€¦ 



EMJ said: And WHO said that Bob? I said the torque should not be reduced as the motor speed decreasesâ€¦as is the case if the voltage is merely reduced.



EMJ said: The second point is that trim speed should usually be reduced as a function of airspeed. Often this is critical and can give the pilot sudden difficulties. Tee-ing off the pitot line and using the pressure signal is easy to do.



BN said: I've designed a number of pitch trim controllers. The first two went on the Lear 55 and the second was a fleet retrofit to the 30 series. I proposed a continuously variable pitch trim rate based on IAS but it proved unnecessary. Trim rates optimized for cruising flight fell into one fairly narrow range, speeds for approach to landing in another narrow range. Turns out that approach: cruise trims in the Lear systems were 4:1 ratio. Use a flaps-not-stowed switch to select high speed and we were done. 



EMJ said: Whatever... but Iâ€™m not sure Learjet experience (where the amount of trim motion needed at 500 knots is tiny and the motor are big)  is relevant for the stock RVs that have speed/trim problems using reduced voltage at high airspeeds.



Thanks...the loyal (occasional) opposition.

_________________
Eric M. Jones
www.PerihelionDesign.com
113 Brentwood Drive
Southbridge, MA 01550
(508) 764-2072
emjones(at)charter.net

nuckolls.bob(at)aeroelect
Guest

Posted: Mon Nov 18, 2013 1:54 pm Post subject: Trim motor speed control

At 02:52 PM 11/18/2013, you wrote:

 	  Quote: 	
		  



EMJ said: An important point of trim motor speed 

controlling is that the torque should not be 

reduced when the speed is reduced (in fact, it 

could go up...but not down). This makes regular 

voltage controllers undesirable except where torque isn't an issue.


	


   Help me out. Are you talking about available torque or

   demanded torque? In the examples I cited and data plots

   published, torque demanded is a function of the mechanism

   being powered. I'm aware of no piece of machinery that

   demands more torque as the speed is reduced.



   A speed/torque 'curve' is not really curved but a

   straight line that depicts motor speed/torque performance

   based on terminal voltage applied. The 'curve' has

   downward slope and is set by the motor's internal

   resistance. Internal resistance limits AVAILABLE torque

   at that voltage. My assertion is that for any applied

   voltage you choose, performance of the motor is

   predictable and easily illustrated by plotting the

   appropriate 'curve'.



 	  Quote: 	
		  EMJ said: And they are all wrong. Start with 

yours. The modern methods for changing motor 

speed without reducing torque â€¦ as I saidâ€¦should be used.


	


   Okay, suppose the target speed is satisfied

   by reducing applied voltage to say, 5 volts.

   Describe the mechanism by which a motor's performance

   is improved by using one form of voltage 'adjustment'

   over some other form. I.e., what is the "modern method"

   to which you refer?



 	  Quote: 	
		  EMJ said: And WHO said that Bob? I said the 

torque should not be reduced as the motor speed 

decreasesâ€¦as is the case if the voltage is merely reduced.


	


   Okay, you've answered the question. My apologies. Yes,

   AVAILABLE torque can be degraded to the point where a

   motor is at risk for stalling at the lower extreme of

   RPM vs. available torque for the voltage applied. My

   assertion is that 5 volts average applied voltage produces

   the same performance irrespective of the mechanism by

   which that voltage is produced.



 	  Quote: 	
		  EMJ said: Whatever... but Iâ€™m not sure Learjet 

experience (where the amount of trim motion 

needed at 500 knots is tiny and the motor are 

big)  is relevant for the stock RVs that have 

speed/trim problems using reduced voltage at high airspeeds.


	


   No argument that adjustable trim rates for aircraft

   with broader operating speeds is a good thing and

   may apply to airplanes of any size . . .



   Pilots would like to ask for and get very tiny movements

   in the trim system at cruise. If the motor speed is

   optimized for that condition, then it's painfully slow

   for setting the airplane up to land. The idea that

   there was value in crafting a continuously adjustable

   trim rate was rejected as unnecessarily complex. Lear's

   pilots were quite happy with the 4:1 ratio.



   Trim motor motion on a Beechjet at Mach cruise needed

   to be so small that servoing to a speed was impossible.

   The smallest trim adjustment offered by the autopilot

   is a mere 133 milliseconds long.



   The Lear systems were SERVOed speed controllers where

   motor characteristics did not impact available torque

   from the motors. Voltage was adjusted to hold target speed

   irrespective of voltage or load. The controller that

   finally went onto the airplanes held +/- 1% of target

   speed over the full range of load, voltage and temperature.



   In the Lear system, tachometers on the motor shafts

   provided speed feedback data. The motors used in

   OBAM aircraft are seldom fitted with such devices.

   The PM motor CAN be used as it's own tach generator.

   If you PWM the applied voltage 0ne can sample the Cemf

   of the motor and use that value to report motor speed

   to the servo-loop.



   With a servoed speed control loop, the motor's

   speed/torque curve no longer defines performance

   limits at slow speed. The controller drives voltage to

   maintain target speed even at the maximum expected load.



   Bob . . .

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