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Linear actuator vs stepper motor?

Linear actuator vs stepper motor?

Postby milz » Wed Sep 15, 2010 2:53 pm

heho

did anyone use a Linear actuator ?
im not sure how the linear actuator are contolled
but it seems a linear actuator are more precise then
a stepper...

http://en.nanotec.com/linearactuators.html
http://en.wikipedia.org/wiki/Linear_actuator

mfg milz
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Cam : USB2 CCD 1024x768 / USB3 1280x900
Laser : LC532-5-3-F(16x65) Focusable Green
Projector Acer X110P / Acer K132
dControl one David control box + Turntable + automatic texture lights
mooooom bathroom!!!!!!
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Re: Linear actuator vs stepper motor?

Postby WalterMo » Wed Sep 15, 2010 4:14 pm

Hi,
I think such a linear actuator will be controlled in the same way like a stepper. But instead of a rotating shaft has this actuator a non-rotating spindle with a fine thread. The rotor of the motor is equipped with a female screw in its centre and pushes the spindle forwards and backwards due to rotation.
May I ask you what you intend to build? Or no, let it be a surprise. :wink:

Walter
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Re: Linear actuator vs stepper motor?

Postby milz » Wed Sep 15, 2010 5:27 pm

heho

i was a little bit confused. stepper valvue are in degree and Linear actuator in mm/step
i think Linear actuator are interesting cause the made for more precious then a stepper
which is made for power(nm). there are Linear actuator with 0.0025 mm/step (0.9degree)

from nanotec
Highly reproducible resolutions (<1 μm) and fast feeds (>1000 mm/sec.) for the same construction volume achieve uniform construction platforms.

so i think about to replace my motor with one from the linear actuator series

and yes deep in my mind is a small voice which say "you need to build a linear scanning system and a CNC machine-)"
but my wallet cries NOOOO :D

mfg milz
Xeon x5650@4ghz
Cam : USB2 CCD 1024x768 / USB3 1280x900
Laser : LC532-5-3-F(16x65) Focusable Green
Projector Acer X110P / Acer K132
dControl one David control box + Turntable + automatic texture lights
mooooom bathroom!!!!!!
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Re: Linear actuator vs stepper motor?

Postby julienrl » Wed Oct 06, 2010 12:54 am

I hope this is not too far off topic, but could we not get the advantage of a linear scan with the ease of a stepper motor by using the motor to rotate the object (when it is small enough to be rotated) and having the laser held in place to project vertically?

I am aware that if you use the backpane everything would need to be rotated 90 degrees and that the software would need to be aware of the object displacement to properly construct the object... so I am not sure if this belongs here or in the wishlist, but I just wanted to know if it was a reasonable assumption.

thank you
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Re: Linear actuator vs stepper motor?

Postby JD_Mortal » Sat Nov 06, 2010 10:10 am

To Julienrl: I believe you are detailing a column scanner. That has limitations when surfaces project, blocking the view of the laser from the camera. In turn, that would require more re-positioning of the vertical scan line to be on the other side of the object, or demand real odd mounting and rotations of the object. (Possible, but ineffective, as the old scanners once were. Those scanners now use multiple lines and cameras, and have many other issues, compared to "V" triangulation with fixed targets.)

To Milz: The only difference between linear and rotor actuators, is the addition of hardware. If you add a screw-drive to a servo/stepper, it is now linear. The precision has nothing to do with being linear or rotor style. Moving with more precision is simply done by reducing the gearing or by adding more, and smaller coil-steps. A stepper with 360 degrees of rotation can move a linear drive system 1mm/step, or 0.000000001mm/step, or 10000mm/step, as can a pre made linear stepper. The more it moves, the less power it has, or the more power it needs to move. It will also move faster with a longer motion gearing, as opposed to taking days to span an inch, at 0.0000000001mm/step. In the end, it comes down to... How long do you have to wait for it to scan a one-foot tall area?

I have found that a linear motion yields the best results with scanning, however, that requires a large mounting rig and produces jitter in the laser. (Jitter results in bad scans, due to frame-alignment issues, as images in video are not whole, like a photo. The jittered laser can produce false readings as it also moves side to side, altering the "V" shape, making the program think the laser is at a different angle than it is.)

Using a single fixed pivot point, yields great results because the laser is secure at the most important location, the pivot. You can now counter-balance the entire weight, and use a mini-servo/stepper, since you are no longer lifting the whole device, and fighting the resistance of the glide rig. Using a linear actuator to control a pivoted laser will result in an orbital sweep as the actuator moves in a fixed-line at a linear speed. (EG, it moves the laser faster when near the horizon and slower when nearly vertical. Since you are moving the laser at a fixed point, by a second moving point.) However, with a rotating servo/stepper the result is exactly the opposite, on a linear wall. The beam target sweeps slower on a horizontal alignment on a vertical wall, while it sweeps faster as it nears a vertical position. (Neither single action is ideal for these setups, since the camera is usually aligned to the center of the object. Technically, you need four laser scans on each rotation to get the best complete scan. Two from the top, above the camera horizon, left and right. Two from the bottom, below the camera horizon, left and right. You can cheat this by aligning the camera so it looks DOWN at an angle, on the item being scanned, with a false horizon now being at the top of the camera. Now you only need two scans, one from the left and one from the right, from under the camera's horizon. It will not scan the top of the object that you focus on, use a separate scan for that.)

Perhaps it would be better to know the speed required for a scan, as a starting point. Followed by knowing the force required to move your laser-scanner that appropriate distance. That will give you a good idea of your requirements for "which" setup to choose.

In the end, I say that you should get a simple rotary stepper. It can be converted to linear, if needed. However, you can't convert a linear device to a rotary device, unless you take it apart.
DIY laser-line, Hand sweep scanning, Generic driverless webcam (640x480), Cardboard background rig, Adequate results.
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