Power window motors for sorvo's?

[FandZ]

Hi all,

I was wondering if salvaged power window motors could be retrofitted to be used in a cnc build. I searched the net and came buy this article

http://allaboutservo.blogspot.com/20...rvo-motor.html

"A servomechanism, or servo is an automatic device which uses error-sensing feedback to correct the performance of a mechanism. The term correctly applies only to systems where the feedback or error-correction signals help control mechanical position or other parameters. For example an automotive power window control is not a servomechanism, as there is no automatic feedback which controls position -- the operator does this by observation. By contrast the car's cruise control uses closed loop feedback, which classifies it as a servomechanism"

Now I know power windows have a lot of torque. Some are faster than others. Usually American cars have swift power windows. Are they considered a stepper motor since they are not closed loop?

Not that it is any answer but I have seen power window motors and some of them are a hefty size. Bigger than alot of the servo motors I've sen sold for a few hundred dollars each. They may be worthy of trip to a junk yard for 25 dollars a pop.

[harryn]

I don't know exactly what a car window motor is, but I am reasonably sure that it is not designed for precision positioning.

I have searched around various electronic junk yards and found stepper motors pretty cheap - in the $ 25 and under range. I do agree with you that a lot of home made routers could really use a bit more torque in their steppers. IMHO, 500 oz - in is kind of a minimum.

[NC Cams]

The car window is an EXAMPLE of a servomechanism-like device, it is not a servo. To some, a motor is a motor. THis is hardly the case when it comes to servo motors.

SErvo motors are a HIGH accuracy variant ot a starter motor or a car window motor. THey put out speed and torque for their size. THey do not have the resolution, as it armature pole count, to serve as a servo motor.- too much cogging potential which precludes one from getting good position accuracy and/or glass smooth rotation.

Since they are looking for max torque/speed from sized, they tend to have a larger diameter to length ratio that true servo. You can kluge them up to play as a servo, once you do, maybe you'll see their shortcomings as same. BTW. you generally can not solve the inertia problems of a poorly designed motor with electronic gimicry.

A horse tht pulls a milk wagon is a horse but not the type of horse that you'd want to try to race in the Kentucky Derby or Preakness..

[FandZ]

Thanks for the replies. It sounds like it would be more trouble that it is worth. After my machine is built I may give one a try just to see. But stepper motors are cheap enough.

[Geof]

The important thing missing on the window motor is an encoder; you need this for position information. It is possible to build an encoder, nearly 30 years ago I converted two electric hand drills into 'servo motors' by building a proper phase quadrature encoder between the output shaft on the drill and the feedscrew I was driving. They worked very well but only for one-direction positioning because the gear reduction in the drill meant they could not be controlled in a stable manner. I would never do something like that now but back then servos with built in encoders and big steppers where not on off-the-shelf item at a reasonable cost.

[Al_The_Man]

Originally Posted by FandZ Thanks for the replies. It sounds like it would be more trouble that it is worth. After my machine is built I may give one a try just to see. But stepper motors are cheap enough.

And so are DC servo motors on the surplus market, now with the advent of DCBL etc.
As Geof said the encoder is something you need, and most auto motors do not have a rear shaft to attach one to.
With a surplus motor, you get a true servo built motor c/w encoder for the price of a junk yard scrounged one and a purchased encoder.
Al.

[FandZ]

Originally Posted by Al_The_Man And so are DC servo motors on the surplus market, now with the advent of DCBL etc. As Geof said the encoder is something you need, and most auto motors do not have a rear shaft to attach one to. With a surplus motor, you get a true servo built motor c/w encoder for the price of a junk yard scrounged one and a purchased encoder. Al.

Thank you for you suggestion. At this point it is more like guidance to me.

I just took a look around the net for surplus dc servo's and their is quite a bit of range. I've seen them from the 200oz range on up to 1/2hp+. Can you tell me what I should be on the look for? I plan on working metals like aluminum and copper up to four inches thick. Perhaps some steel milling up to a quarter inch thick. The smaller things I'd like to make are water cooling heat sinks and the larger items would be intake manifolds, supercharger adapters, maybe parts for a custom motorcycle frame.

I'm going to start a build thread soon but at this point I'm still gathering information for my design.

[FandZ]

I came across this article that talks about converting a DC Motor to a Servo by adding an encoder. http://www.truetex.com/servomod.htm

[Geof]

Originally Posted by FandZ I came across this article that talks about converting a DC Motor to a Servo by adding an encoder. http://www.truetex.com/servomod.htm

This is possible...but go back and read the post by NC Cams particularly the bit about 'cogging'; servos are designed to move smoothly when they are moving just a fraction of a rotation, regular DC motors are not.

[TOTALLYRC]

Originally Posted by FandZ Thank you for you suggestion. At this point it is more like guidance to me. I just took a look around the net for surplus dc servo's and their is quite a bit of range. I've seen them from the 200oz range on up to 1/2hp+. Can you tell me what I should be on the look for? I plan on working metals like aluminum and copper up to four inches thick. Perhaps some steel milling up to a quarter inch thick. The smaller things I'd like to make are water cooling heat sinks and the larger items would be intake manifolds, supercharger adapters, maybe parts for a custom motorcycle frame. I'm going to start a build thread soon but at this point I'm still gathering information for my design.

Motor are dependant on the size of the machine as well as the materials to be cut.
Based on what you are going to cut, you are looking at a good sized mill.
Let us know which machine you will converting or the specs of the machine you are going to be building.

Mike

[FandZ]

Originally Posted by TOTALLYRC Motor are dependant on the size of the machine as well as the materials to be cut. Based on what you are going to cut, you are looking at a good sized mill. Let us know which machine you will converting or the specs of the machine you are going to be building. Mike

Because of my limited available space I'm looking to build my first machine with a worakable area of about 18x26. Maybe a little smaller. None of the parts I want to build will have a size greater that for the time being. I'm thinking a gantry style machine. But that doesn't mean I haven't ruled out converting a small mill.

I live in a town house with a small back yard so it'll have to be light enough for me to move it in and out the house. I'd like to keep the weight around 125lbs. I'm thinking about a design similar to a desktop model that can bolt down to a heavy table that will stay outside. And then separate from the base for storage.

At first I was thinking about using 80/20 but I'm pretty sure I'll be using rolled steel. it'll be bolted together with various sections welded for added strength.

Later down the road I'd like to build a bigger machine when I have more space. So it is important to me that the motors and drivers can be transferred to a bigger machine. I'd rather go overkill on these items than just enough. But I am looking for the best value I can get.

Chris

[TOTALLYRC]

Originally Posted by FandZ Because of my limited available space I'm looking to build my first machine with a worakable area of about 18x26. Maybe a little smaller. None of the parts I want to build will have a size greater that for the time being. I'm thinking a gantry style machine. But that doesn't mean I haven't ruled out converting a small mill. I live in a town house with a small back yard so it'll have to be light enough for me to move it in and out the house. I'd like to keep the weight around 125lbs. I'm thinking about a design similar to a desktop model that can bolt down to a heavy table that will stay outside. And then separate from the base for storage. At first I was thinking about using 80/20 but I'm pretty sure I'll be using rolled steel. it'll be bolted together with various sections welded for added strength. Later down the road I'd like to build a bigger machine when I have more space. So it is important to me that the motors and drivers can be transferred to a bigger machine. I'd rather go overkill on these items than just enough. But I am looking for the best value I can get. Chris

I don't think that you are going to be able to machine steel on a homemade gantry style mill/router. Especially keeping it under 125lbs.

If steel cutting is definitely in the cards for this machine, then I would suggest looking at a small mill and converting it.

You may want to start a new thread and ask what small mill will do the job. Intake manifolds and supercharger adapters sound like parts for your second or third mill, as they can be large and complicated.