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I've never used one and had a couple questions.
Do they act like servos or motors?  
Are they some how settable or programmable such that its normal balanced position is half of the stroke or is that something done via software?  And if it doesnt act like a servo how does it know how far out it has extended?
I see some actuators say they have X amount of push/pull force which leads me to believe that some actuator ratings are only for push or pull?  Or are they all the same strength going in and out and thats marketing?

im looking at something like this:
http://www.ebay.com/itm/Heavy-Duty-8-Linear-Actuator-Stroke-200-Pound-Max-Lift-12-or-24-Volt-DC-/320948646093?pt=LH_DefaultDomain_0&hash=item4aba07b0cd
http://www.ebay.com/itm/6-Linear-Actuator-225lb-Adjustable-Stroke-12-Volt-DC-/130631704400?pt=Motors_Car_Truck_Parts_Accessories&hash=item1e6a419f50&vxp=mtr
« Last Edit: August 14, 2012, 04:46:57 am by jointtech » Logged

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Those I've seen are simply a lead screw driven by a DC motor with limit switches at each end. There may be other types but based on that...

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Do they act like servos or motors? 
Motors.

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Are they some how settable or programmable such that its normal balanced position is half of the stroke or is that something done via software?
No help from the device, all done in software.

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And if it doesnt act like a servo how does it know how far out it has extended?
Timing?

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Or are they all the same strength going in and out and thats marketing?
Same both ways.

As I said this is just for the ones I've dealt with, there may be smarter ones around.

The ones you linked to only appear to have two wires, so I'd say they are as as I described.

______
Rob
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thanks for the reply.  I found a couple more.  Im going to call on them in the am and see.  It does look like most are as you describe.
Im going to use this for steering so not sure timing the motor movement will be good enough to reliably center the shaft when there are other forces applied to the wheels.  It ranges from x seconds per inch at no load and half that at full load.  Thats why im looking for position feedback.
 
cheaper
http://www.ebay.com/itm/NEW-XSCORPION-6-Linear-Actuator-12-Volt-DC-450-LB-Static-w-Adjustable-Stroke-/350585835964?pt=LH_DefaultDomain_0&hash=item51a08b51bc
Potentiometer: 10K

i like these guys I just got a servo from them and the customer service was really good:
http://www.servocity.com/html/115_lbs__thrust__linear_actuat.html
 Built in potentiometer can be easily accessed for position feedback but is not necessary for operation.
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The first is a brushed DC motor geared to a screw-thread and a slipping clutch - that's all.  No position sensing.  Typically that would be separate sensors (microswitches).

The second has adjustable limit switch(s) of some sort from the description (probably fixed at one end, other end adjustable).  Not clear how the switches are cabled.
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Some linear actuators have built in potentiometers (typically linear) that can be used to detect position; in such actuators, you monitor a change in voltage from the potentiometer (set up as a variable voltage divider), which is proportional to position.

Other linear actuators use either an optical slot interrupt detector, a hall-effect switch, or a reed switch, which output a series of pulses. The way these actuators are used is to drive them to one end of their travel (depending on how things are set up, you can monitor the motor current, or an external limit switch to detect end-of-travel), then reset a counter and count every pulse of the detector (via an interrupt most likely) as you drive the actuator to the other end (until you detect the other limit switch). Once you have the count in one direction, you can use that (or count backwards as you drive again in the opposite direction - then take the floor() average).

Some linear actuators have built-in limit switches to shut the motor off at the end of travel for the actuator (which may or may not be the same as the end of travel for -your- application!); if that length is the same as your application, you can detect end of travel by looking for the current being drawn by the motor to rapidly fall to "0" (vs rapidly rising beyond a certain threshold when detecting a stall condition).

Finally - there are linear actuators that can be controlled digitally ($$$) via RS232 or other serial communications method, as well as ones that can be controlled just like a common R/C servo ($$).

Oh - a couple more things to be aware of: 1) Some linear actuators move at different speeds depending on whether they are extending or retracting, and 2) some linear actuators have a special (usually manually adjusted) mode to allow the "ram" to be decoupled from the gear mechanism, and be extended or retracted manually. You generally find both kinds of features on actuators meant for medical equipment, like beds and chairs.

...and just for sake of completeness:

http://www.firgelli.com/
http://www.firgelliauto.com/
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nice great reply and links.  Funny ive been searching around for the last few weeks and never hit on them.  Prices look to be not astronomical.
Just converting newtons to pounds though even the biggest one with the highest gear reduction is only about 44 pounds.  And thats at about .5"/s.  slow.  But maybe useable if I make my angle shorter so it doesnt have to push out very far.

Just blathering here...  I'm going to use this for steering a ROV.  So I need my wheels straight position to be at the halfway point of the stroke.  To turn one direction I pull the shaft in and the other way I push it out and with no input on the joystick it should return to the "center".
I dont care about controlling the speed it goes in or out. 
I think I dont care about limit switches other than to stop from burning it up.  If it worked like a servo that would never be a problem because I would just never send it to end of travel in either direction.
I may need some type of controller to make it act like a servo.
Looking at http://store.firgelli.com/L16_Linear_Actuators_p/l16-p.htm
it shows an additional controller to "
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Turn your L16 -P into a Linear Servo using the LAC board listed in the "Controllers and Switches" section here. This combination has all the same functionality as the L12-I plus USB support, speed control, sensitivity control, position control, and adjustable end limits.
Hmmm.
Then I look at something like this which includes the controller:
http://www.servocity.com/html/heavy_duty_linear_servo__180__.html
vs
http://www.servocity.com/html/115_lbs__thrust__linear_actuat.html

which has no controller but does have position feedback.

I'm thinking that with a position sensor I should be able to make it a servo right?  Or is my whole problem I'll still need a controller because ITS NOT A SERVO and i cant swap polarity to run it in the other direction with out using a controller or creating some type of H bridge myself.  (which i just avoided by buying a motor controller for another part of the project.)

So now I think its sounding like I need an actuator with position feedback + arduino + the other half of my Dual motor controller  http://www.pololu.com/catalog/product/707
(I'm only using one side of it to control my forward and reverse drive motor) 

With that I think I can create a linear actuator that acts like a servo and I already have 2 of 3 of the peices.  And a prebuilt servo type actuator or one bundled with a controller is infinately more expensive than this combo that I'm thinking will work.

I have spent a lot of money trying to build things cheaper...

step 3: Profit

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The 110 lb ACME actuators below apparently have internal pots. With a tle5206-2 h-bridge and the internals of a $2.50 9g servo you probably could turn the linear actuator into a linear servo.

http://www.northerntool.com/shop/tools/NTESearch?storeId=6970&Ntt=linear
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if you have the hbridge and the internal pot and an arduino why do you need the 9g servo?
the hbridge gives you the in and out and the pot tells you where you are.  Then an arduino sketch to mash them together?
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if you have the hbridge and the internal pot and an arduino why do you need the 9g servo?
the hbridge gives you the in and out and the pot tells you where you are.  Then an arduino sketch to mash them together?

The circuit board of the 9g servo is what turns the linear actuator into a linear servo that uses the standaed servo library. No need to "mash" up a special sketch.
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ahh thats what they mean...  So I hook the pot on the actuator to the servo.  I've seen a couple youtubes on this but they were using it to control a DC motor.
thanks. 
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Hello,
this is Luigi,
from Rome, Italy.
I am a newby in the arduino world with great interest in any application of it.
My question is this:
I wish to operate an actuator 12V /5 A on board of my experimental airplane to control the flap system in an auto mode.
The flap has only 4 positions Up, 10, 20 and full Down.
Right now I command the actuator directly and when it reaches a reference on the wing I stop it. Then till full down an the retract during take-off.
The actuator has already the end of travel. I need a system that allows to reach the next position just sending an impulse and with a longer signal to reach the full travel. More or less like the windows of a car. I can install some switch during the run or a pot to feedback or eveything may be useful.
Which is the best way to have this ? In term of hardware and software, of course.
Thank you for the help may arise
Luigi
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Are you talking about a remote controlled aircraft, or one carrying a pilot?

If it's remote controlled, it'd make sense to look for an approach using an RC channel with a multi-position switch to let you select the flap position, and a controller based on a conventional servo to operate the flap mechanism.

If you're on board the aircraft it would make more sense to drive the actuator directly using position feedback. In this case it would be a safety-critical system and I would question whether an Arduino-based home made system will be safe and reliable enough - but it's your decision to make.
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Thank-you for the replay.
I fly an airplane and the safety is the priority in every action.
I need it only to command the flap, avoiding  to maintain the switch pushed for all the time needed to reach the position requested and being inattentive during this time to avoid the surface overtake the position.
With the system I imagine, and  the market is full of its, I push a switch and the flap goes to the next position.Kepping it for a longer time the flap goes to full travel.
The safety is assured from the control of the surface. I have reference on the wing for each position
When I land, the item "Landing Flap" is assured only if you check the real flap position not from a led or other trick.
This is the why I ask here.
Regards
Luigi
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