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madriver(at)wildblue.net
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 Posted: Tue Dec 09, 2008 10:46 am Post subject: Trim Speed |
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Here's one for you electrical types. The elevator trim servo in my CH 701
( read MAC ) runs really fast. When checking out the wiring I used a common
9v battery to confirm the wiring and it ran substantially slower - what
resistor - gizmo do I put in series with the 12v to reduce the voltage to
about 9v and slow things down. What would the circuit look like in a schematic?
Thanks Phil
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docyukon(at)ptcnet.net
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| Posted: Tue Dec 09, 2008 12:39 pm Post subject: Trim Speed |
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Phil Ray Allen makes a speed controller just for that. kina pricy at app. $45.00 I think it is fully adjustable over a broad range. Doc
[quote] ---
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a.s.elliott(at)cox.net
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| Posted: Wed Dec 10, 2008 5:53 am Post subject: Trim Speed |
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No need for a fancy controller unless you want a continuously variable response. Try a diode or two in the line. Each diode will drop the voltage about 0.8-0.9 volts. Be sure you use diodes that can handle the load. Example - A 1N4001 might be rated for 1A at 50V. Should be fine, I'd think.
Andy Elliott, Mesa, AZ
N601GE,601XL/TD,Corvair
[quote][b]
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Eric M. Jones
Joined: 10 Jan 2006
Posts: 565
Location: Massachusetts
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| Posted: Wed Dec 10, 2008 8:18 am Post subject: Re: Trim Speed |
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The correct way to reduce the speed is to pulse-width modulate the input (kind of like jabbing the button fast). Reducing the voltage reduces the motor torque and thus the trim force. You might get away with it in some applications...but still...there is a point where applying low voltage to a stalled armature will just fry the brushes.
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_________________
Eric M. Jones
www.PerihelionDesign.com
113 Brentwood Drive
Southbridge, MA 01550
(508) 764-2072
emjones(at)charter.net |
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nuckolls.bob(at)aeroelect
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| Posted: Wed Dec 10, 2008 9:57 am Post subject: Trim Speed |
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| Quote: | The correct way to reduce the speed is to pulse-width modulate the
input (kind of like jabbing the button fast). Reducing the voltage
reduces the motor torque and thus the trim force. You might get away
with it in some applications...but still...there is a point where
applying low voltage to a stalled armature will just fry the brushes.
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This is not true. Motor torque is a function of average
armature current . . . whether the voltage is supplied
as a steady state or pulse-width modulated source. The
only way to reduce speed while RETAINING the actuator's
maximum torque capabilities is to add electronics that
maintains motor speed irrespective of load and voltage
(within bounds).
I did a tutorial on motor performance for a client last
week. Here's the set of power point slides I used in
that presentation.
http://www.aeroelectric.com/articles/Motors/DC_Motor_Performance_in_small_Actuators.ppt
There are three major drivers of motor performance.
(1) internal losses - i.e. total resistance of
brushes and windings. (2) Torque constant (Kt)
as a function proportional to average current.
(3) Counter emf constant as a function proporational
to motor RPM.
The first three figures in the PP presentation
speak to ways these parameters can be measured for
any particular PM motor.
Figure 4 is a compendium of motor performance
constants and their relationships with respect to
the speed that a motor can be expected to run
when the applied voltage and torque requirements
are known. Of particular interest is the last
expression in the top box where all constants
are combined in the Great Motor Performance
formula.
There are ways the algebraic models can be illustrated
and analyzed graphically. Figure 5 shows the speed
torque curve for a motor we were discussing for
a particular small actuator design.
Stall torque at 28v applied = 4.5 in-oz
Stall current at 28V = .74 amps yielding
a K(t) value of 6.1 in-oz/amp.
Counter EMF constant K(e) of 4.5v/1000RPM
Internal resistance of 37.8 ohms at room
temperature.
Figure 5 explores the effects of variable
system loads when the motor is supplied
with fixed voltages. The top set of data
points show that over an operating range
of .72 to 1.82 in-oz of torque, the speed
of this motor can be expected to run between
5340 RPM at (C) and 3780 RPM at (B).
Let us suppose we want to drive the actuator
at 1/6th nominal speed over the same ranges
of load torque. With a nominal load of 1.22
in-oz, the voltage needs to be reduced to
11.2 volts. Now as torque varies over the
same range, speed varies from 1430 RPM at
(F) and is incapable of carrying the expected
torque level. The motor will stall.
Figure 6 shows us what's necessary to maintain
a constant speed over full torque range. Here
we see that 11.2 volts gets us 1/6th speed
at nominal torque. At full torque (point H)
we need to increase applied voltage to
15.1 volts. At minimum torque (point J),
applied voltage can be reduced to 8 volts
applied. This functionality can be achieved
ONLY with active electronics that adjusts
motor applied voltage such that speed is
well regulated.
Smart DC motor controls for maintenance of
speed under variable loads is what butters
our bread these days.
It matters not how the voltage is adjusted.
It can be smart voltage regulator, duty-cycle
pwm, or even a series resistor. NONE of these
techniques will offer a STABLE speed with
respect to load.
Having said that, the effects of constant
applied voltage for the Ray-Allen actuators
is not too big a deal. The reason for slowing
them down is to effect good control over fine
changes in pitch at cruising speed. Torque
loads on the actuator under this condition are
not wildly variable. Set the voltage at the
most useful speed and you'll probably not notice
much variability of performance at cruise.
For approach to landing, running the motor wide
open is often the solution in order to achieve
rapid changes in combinations of descent rate/
airspeed/power during approach maneuvers.
There's a host of articles on motor performance
on the 'net for those interested in exploring these
critters in more detail.
Bob . . .
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nuckolls.bob(at)aeroelect
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| Posted: Thu Dec 11, 2008 9:15 am Post subject: Trim Speed |
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Some folks have asked about a clearer version of the Power
Point slides I published. Here it is:
http://www.aeroelectric.com/articles/Motors/DC_Motor_Performance_in_Small_Actuators.pdf
Bob . . .
----------------------------------------)
( . . . a long habit of not thinking )
( a thing wrong, gives it a superficial )
( appearance of being right . . . )
( )
( -Thomas Paine 1776- )
----------------------------------------
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marcausman
Joined: 08 Feb 2007
Posts: 70
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| Posted: Thu Dec 11, 2008 9:58 am Post subject: Re: Trim Speed |
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If you are interested in a modern power management system for your aircraft that also includes variable-speed trim (runaway trim protection and backup trim controls too) take a look at Vertical Power www.verticalpower.com
In the case of the trim, we have chosen to use PWM to slow down the Ray Allen servos.
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_________________
Marc Ausman
http://www.verticalpower.com "Move up to a modern electrical system"
RV-7 IO-390 Flying |
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deej(at)deej.net
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| Posted: Thu Dec 11, 2008 1:53 pm Post subject: Trim Speed |
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You may consider checking out the Safety Trim system. I bought one, but
have not installed it yet so I can't tell you how well it works.
<http://www.tcwtech.com/Safety-Trim Page.htm>
-Dj
--
Dj Merrill - N1JOV
Glastar Sportsman 2+2 Builder #7118 N421DJ KR-2 Builder N770DJ
http://deej.net/sportsman/ http://deej.net/kr-2/
"Many things that are unexplainable happen during the construction of an
airplane." --Dave Prizio, 30 Aug 2005
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deej(at)deej.net
Guest
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| Posted: Thu Dec 11, 2008 2:01 pm Post subject: Trim Speed |
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Dj Merrill wrote:
| Quote: | <http://www.tcwtech.com/Safety-Trim Page.htm>
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*sigh* I really hate it when people put spaces in web URLs. It
makes direct linking difficult.
Try going to <http://www.tcwtech.com/> and then click on the
"Safety-Trim" link if you want to see it.
-Dj
--
Dj Merrill - N1JOV
Glastar Sportsman 2+2 Builder #7118 N421DJ KR-2 Builder N770DJ
http://deej.net/sportsman/ http://deej.net/kr-2/
"Many things that are unexplainable happen during the construction of an
airplane." --Dave Prizio, 30 Aug 2005
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