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autopilot servos

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ainut(at)knology.net
Guest

Posted: Fri Feb 17, 2012 11:50 am Post subject: autopilot servos

Guys, can you help guide me towards what the specs are for autopilot 

motors?  My idea is to use high-torque steppers so that they can be 

computer controlled by software that I write in a computer that I design 

(maybe an Arduino.)  This is being done mostly for fun but also to be 

safe and practical in my OBAM aircraft.  How much torque do I need?  The 

Mustang II takes very, very small stick movement once airborne and the 

faster you go, the less torque required.  Basically, once airborne, you 

weld your hand to your leg and just think about which direction you want 

to go and that is enough to turn or climb.



I think it would be easiest, in programming, to have feedback from the 

servo on it's current position and there are other methods but the 

encoded type sounds easiest to implement.  Thoughts?



Lastly, AIUI, most steppers become free-wheeling once power is removed 

so that gives me control back instantly in event that is necessary 

simply by turning them off, leaving no drag on the stick or ruddders.



I would like to control all 3 axes.



So which steppers and corresponding controllers do you guys recommend?  

Or am I barking up the wrong tree?



Once fully debugged, I may play with automated take-off and landing but 

that will be very, very closely watched if I ever get to that phase.  

Too easy to break something important (like me!)



Thanks,

David M.



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nuckolls.bob(at)aeroelect
Guest

Posted: Fri Feb 17, 2012 1:10 pm Post subject: autopilot servos

At 01:21 PM 2/17/2012, you wrote:

 	  Quote: 	
		  



Guys, can you help guide me towards what the specs are for autopilot 

motors?  My idea is to use high-torque steppers so that they can be 

computer controlled by software that I write in a computer that I 

design (maybe an Arduino.)  This is being done mostly for fun but 

also to be safe and practical in my OBAM aircraft.  How much torque 

do I need?  The Mustang II takes very, very small stick movement 

once airborne and the faster you go, the less torque 

required.  Basically, once airborne, you weld your hand to your leg 

and just think about which direction you want to go and that is 

enough to turn or climb.


	


      That's been the fondest wish of probably every a/p

      designer since WWII. Guys like Ed King, Honeywell,

      Bill Lear, Jim Younkin, et. als. can point to many

      decades of development spanning dozens of recipes for

      success.



 	  Quote: 	
		  I think it would be easiest, in programming, to have feedback from 

the servo on it's current position and there are other methods but 

the encoded type sounds easiest to implement.  Thoughts?


	


     What have you read about autopilot history? Have you

     studied any of the legacy design goals for failure

     mode effects?

 	  Quote: 	
		  Lastly, AIUI, most steppers become free-wheeling once power is 

removed so that gives me control back instantly in event that is 

necessary simply by turning them off, leaving no drag on the stick or ruddders.


	


     Sort of. Steppers have a small but noticeable

     "cogging" in back drive torque. Jim Younkin

     went to stepper motors with a step down gearing

     on the order of 10:1.  Typical torque needed at

     the aileron bell crank on an RV is 30 oz inches.

     So the stepper would need to supply about 3 oz

     inches of torque. At the same time, gearing magnifies

     the backdriving forces for the de-energized motor.



     10:1 is probably a golden goal for trading off physical

     size of motor and tolerable back-drive forces.

     Gene Brown and I discussed a stepper motor driven

     a/p design probably 25 years ago at Electro-Mech.

     It's the easiest to implement both mechanically

     and electrically. No disconnect clutch needed.



     Have you considered an RC servo driving a tab

     on the surface to be controlled? RC servos are

     cheap and easy to integrate. They need some 'help'

     in the EMC department but that's doable.



     Driving a tab keeps you totally disconnected

     from primary flight surfaces and in absolute

     control for over-ride forces needed to deal

     with a tab that's jammed to the stop.



     Most AUTOPILOTS are designed to do pretty much

     everything a MANPILOT would do. But my idea of

     an small-letters autopilot is to keep the airplane

     pointed in the right direction with a minimum

     of maneuvering . . . very light fingered touch

     from the electronics.



     This suggests that you don't need fat servos

     capable of 2G loops and 360 degree per second

     aileron roles.

 	  Quote: 	
		  So which steppers and corresponding controllers do you guys recommend?

Or am I barking up the wrong tree?


	


     There's LOTS of trees to contemplate before you

     start barking. Exactly what do you want this

     product to do for you?

 	  Quote: 	
		  Once fully debugged, I may play with automated take-off and landing 

but that will be very, very closely watched if I ever get to that phase.

Too easy to break something important (like me!)


	


     I can tell you that we had some VERY talented folks

     working that problem on the AGATE program. There is

     a Bonanza fitted with standard controls and 14v

     hardware for the right seat and fly-by-wire, glass-

     cockpit, highway-in-the-sky hardware in the left

     seat that ran from a totally independent 28v

     system. We installed our own SAAS (small aera

     augmentation system) right on Beech Field to give

     us real-time, centimeter correction for GPS signals.

     I wont suggest that you can't do it . . . but I've

     seen it done and the task is  . . . shall we say

     . . . monumental?



     Your probability of success for any phase of

     your endeavors begins with small goals with

     stopping places for evaluation of results. Suggest

     you start with a simple mod to one aileron to

     add a tab, drive it with a servo that you can fit

     into your aileron, then go fly it with manual

     control knob on the servo. Can you achieve a servo/

     tab combination that gives you the control authority

     you need to be a precision wing-leveler/heading-hold

     system in moderate turbulence? If the servo drives

     to either stop, is the airplane manageable without

     breaking a sweat?



     Tame that dragon to the extent he's not trying to

     eat you, then let's talk about ways to make him

     heel on a leash . . . and ultimately tap-dance.

   Bob . . .

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peter(at)sportingaero.com
Guest

Posted: Fri Feb 17, 2012 1:30 pm Post subject: autopilot servos

David,



I admire your courage in taking on one of the more difficult challenges 

in light aircraft avionics. An autopilot is primarily a flight 

control/flight dynamics problem - if you know little of this subject 

(for example if you are unfamiliar with stability derivatives) I would 

urge you to read up on the subject or get help from someone who does. 

There are also several non-intuitive gotchas that can sneak up on you. 

Could I suggest you read the relevant chapter of FAR23 - I think it is 

para 1329. I would suggest you ignore yaw until you have the other two 

axes sorted (or perhaps for ever).



Servo torque must the the minimum that will do the job - as the pilot 

must be able to over ride the servos and the autopilot must not be able 

to break the aeroplane if it decides to inject full deflection elevator 

(especially at max level speed). If possible I would measure the stick 

force that is required to pull 2 g throughout the flight envelope, and 

also the stick displacement. Then measure how much force and stick 

displacement to roll at 30* per second. Now calculate the mechanical 

advantage of the stick, and so the force the servo will have to exert at 

the place you decide to connect it to the control system (make sure the 

servo cannot under any circumstance go 'over centre'). I suspect you 

will have to use some rate feedback, as well as position feedback, to 

accurately apply the desired control surface angle. If you do not 

include autotrim in pitch how will you tell the pilot that the servo is 

about to become saturated so he can trim manually? How will you ensure 

there are no large out of trim transients when the autopilot is disengaged?



You will find quite a change of displacement required from circuit speed 

to fast cruise. CG position will also affect the stick force required.



The logic is also non-trivial, at what bank angles will you transition 

from heading hold to bank angle hold? 5* is usual. You will also have to 

decide which parameter to close on for altitude control. You only have 

control over aircraft pitch attitude, so how do you use that to control 

altitude? What will you control in the pitch axis if you aren't holding 

altitude? Speed (IAS, TAS or ground speed) or rate of climb/descent? 

Will you have a minimum speed that would won't let the airplane 

decelerate below? For example, you are holding 5000ft and the engine 

power slowly decreases (for whatever reason), at what point do you give 

up trying to hold altitude and let the airplane descend at a minimum 

airspeed - or will you hold altitude come what may and allow the 

airplane to stall? If you are trying to follow a route, and for some 

reason your are off the track line how will the autopilot regain track 

and what information do you require from the nav system (GPS?) - draw 

some pictures it will take me too long to type! For that matter, where 

will get heading information for for heading hold, if from GPS what do 

you do if the GPS quits? How will you tell the pilot that the autopilot 

is no longer in control and he should start flying the airplane again?



Over powerful autopilots have the ability to break airplanes and kill 

the crew, under powered servos will be no use at all. Best of luck with 

your project, please keep us informed of how you progress.



Regards, Peter



As an example a military autopilot I worked on only controlled one 

aileron through 2* (two degrees) and the elevator through about 15% of 

max travel - you will need more but will not need to operate over the 

full control surface travel.

On 17/02/2012 19:21, David wrote:

 	  Quote: 	
		   



 Guys, can you help guide me towards what the specs are for autopilot 

 motors?  My idea is to use high-torque steppers so that they can be 

 computer controlled by software that I write in a computer that I 

 design (maybe an Arduino.)  This is being done mostly for fun but also 

 to be safe and practical in my OBAM aircraft.  How much torque do I 

 need?  The Mustang II takes very, very small stick movement once 

 airborne and the faster you go, the less torque required.  Basically, 

 once airborne, you weld your hand to your leg and just think about 

 which direction you want to go and that is enough to turn or climb.



 I think it would be easiest, in programming, to have feedback from the 

 servo on it's current position and there are other methods but the 

 encoded type sounds easiest to implement.  Thoughts?



 Lastly, AIUI, most steppers become free-wheeling once power is removed 

 so that gives me control back instantly in event that is necessary 

 simply by turning them off, leaving no drag on the stick or ruddders.



 I would like to control all 3 axes.



 So which steppers and corresponding controllers do you guys 

 recommend?  Or am I barking up the wrong tree?



 Once fully debugged, I may play with automated take-off and landing 

 but that will be very, very closely watched if I ever get to that 

 phase.  Too easy to break something important (like me!)



 Thanks,

 David M.

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user9253

Joined: 28 Mar 2008
Posts: 1927
Location: Riley TWP Michigan

Posted: Sat Feb 18, 2012 3:21 pm Post subject: Re: autopilot servos

Here is a source of servos or linear actuators:

http://store.firgelli.com/

Here is an IC to control the speed of a motor or linear actuator:

http://www.mouser.com/Search/ProductDetail.aspx?qs=JaP0%252bruNJH%2F4ckdZw9AR1g%3D%3D

Joe

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_________________
Joe Gores

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