What would it take to use encoder feedback for positioning?

[Bowman]

I am trying to think through an idea I have but I am not sure what all options I might have available. Looking for any input, ideas, links etc to help me on my way.

Can the signal from a optical encoder be used to send a stepper motor to a preset location? I realize encoders are used to report position. What I am in need of accomplishing is a way to read the position of a manually adjusted optical focus wheel some 4-5" in diameter and have that position then be sent back to the stepper to index a spindle. I do not need fast motion but I need minature/micro size.

Maybe I need to be looking at optical positioning and some way to set marks an optical eye could read and then send the appropriate pulse to the stepper for indexing.

Any ideas much appreciated. I have not worked with encoders before and not sure what all they are capable of these days.

TIA


Bo

[Al_The_Man]

The two pulses from an encoder can be used as step and direction pulses for input into a stepper motor controller, if the encoder is attached to a Hand Wheel.
The commercial hand wheels have a detent every division, these commonly use 100pulses/turn encoders in them.
Any scaling would have to be done by either selecting the correct pulse/turn encoder or by digital circuitry, multiplier/divider circuit etc.
Al.

[keebler303]

using an lsi chip from us digital, you can convert the quadrature from the encoder into an direction and clock signals. It would be pretty easy to use a counter ic to scale down the number of clock counts to get the desired ratio between the encoder and the stepper.

http://www.usdigital.com/products/lfls7083-lfls7084/

http://hyperphysics.phy-astr.gsu.edu...c/counter.html

Matt

[One of Many]

Originally Posted by Bowman I am trying to think through an idea I have but I am not sure what all options I might have available. Looking for any input, ideas, links etc to help me on my way. Can the signal from a optical encoder be used to send a stepper motor to a preset location? I realize encoders are used to report position. What I am in need of accomplishing is a way to read the position of a manually adjusted optical focus wheel some 4-5" in diameter and have that position then be sent back to the stepper to index a spindle. I do not need fast motion but I need minature/micro size. Maybe I need to be looking at optical positioning and some way to set marks an optical eye could read and then send the appropriate pulse to the stepper for indexing. Any ideas much appreciated. I have not worked with encoders before and not sure what all they are capable of these days. TIA Bo

There are some stepper drivers and servo drives that act like stepper motors with encoder feedback. Both types use the encoder to track position at the end of a run, be that step and direction signals, or proprietary programming language via RS-232. The application will depend on how complex of a motion control the system needs are. Straight line point to point would be simple. Once you enter into interpolation of the axi's, then it becomes exponentially complex to have the controller mix signal ratios to follow a geometric path. Which as far as I know, cannot be achieved with a RS-232 fed ASCII drive.

This will take some research to find a unit that fits the need, then look around ebay to find it affordable! LOL!

Delta Tau comes to mind. Compumotor and one other that I had seen around the zone, used with EMC2....Ahha?

What application did you have in mind?

DC

[Bowman]

Thanks for all the input everyone I think I have some direction now. Maybe lol.

I will make a rough model of what I am trying to accomplish with a more detailed explaination of how it needs to work if it will work at all and post it here in the next day or two. It really seems like it should be doable to me but I know I am lacking the experience in some areas that might be required to pull it off.

Bo

[pminmo]

It sounds to me like your in search of a non closed loop system? i.e. the indexer has no effect on the optical focus. If so there are a couple of things you need to think about. There are two different types of encoders, absolute and incremenental. Using an incremental encoder to determine absolute position requires a mechanical method to establish a "home" or absolute location. If you are talking of closing the loop with a stepper and encoder, then this becomes more problematic if rates of acceleration and motion are involved.

[Bowman]

Correct the indexing stepper has no affect on the optical focus rather the opposite. I started a rough 3D model last night but the wife dragged me away.

Hopefully I will get a picture of the real life current setup and the model of what I am working to accomplish posted here tonight.

The task I am trying to eliminate is the dialing of the elevation knob on a rifle scope that uses a side focus adjustable objective wheel. This is for air rifles and maybe small caliber rimfire where range finding from shot to shot is critical.

In the sport of airgun Field Target the targets are small metal animals with small kill zone holes from 1/4"-1.5" cut into the face. You have to hit a paddle behind the hole to make the target fall and score a point. The targets are set at unknown ranges and at the moment in order to dial your elevation to the propper setting you have to take a reading off a homemade scale located on the focus wheel then reach over and turn the elevation knob to match for that distance.

My thought is that with some sort of small location sensor or encoder feedback on the focus wheel the signal could be fed to a stepper to index the elevation knob automatically based on the final stop location of the focus wheel. Then when you go to focusing on the target the stepper will always adjust accordingly to the correct distance being read from the encoder.

The focus wheel will not be turning more than 1/2 - 3/4 of a revolution and in some instances for very close or far targets this translates into a little more than one revolution of the elevation knob. Many times it will only need to move a few steps up or down. All of this is done rather slowly so no need for fast speeds or high torque. Trying for small size and low power consumption.

It would be great if each focus distance was equal to the same amount of travel but as the distance gets further out the amount of travel required for the focus wheel gets smaller. IE 10-----------20--------30-----40---50--60 yards.

Thanks again for the thoughts and input. Will post later tonight a better idea of what I want to do visually.

Bo

[One of Many]

To keep it compact, you might consider a simple audio taper potentiometer ratio from the focus wheel to span of the elevation motors feedback linear potentiometer. Al-a-antenna rotating base circuit aka wheatstone balanced bridge?



I couldn't find the basic circuit, but I know I have seen it somewhere out there......maybe my old electronics books. There may be some work in getting the proper ratio, but this would be a very simple circuit. Repeatability? Dunno, but gearing could add resolution and torque!

DC

[Bowman]

I am open to all options but would like to KISS as best as I can. This would have to have good repeatablility to work shot to shot over a 60 shot match.

Here are a few screen shots of the rough model and a pic of the actual setup in use now to give you an idea of what I am working with.

Hope this sheds some light on the problem I am trying to work out.

Bo

[One of Many]

Instead of a belt, possibly a small worm wheel gear reduction to adjust the elevation knob?

That would also provide plenty of torque and fine control for repeatability.

DC

[Bowman]

Thats another thought One of Many. Trying to keep it as light as possible but it that was small enough it could possibly even be made of delrin for this application.

This project it out of my scope of knowledge to be sure. I was hoping to come up with the perfect little system that you could simply program the corresponding location of an encoder count to then tell a stepper how many clicks I need it to go for that stop versus another that requires twice as many clicks in the opposite direction. There is plenty of room to put a battery pack and run wiring electronics in the knee riser/rest as well.

Looks like I will just take the couple ideas floated here and ponder the possibilities, do a bit more research.



Bo

[One of Many]

Bo,

I don't know what experience you have as far as creative design, machining or electronics.

From your solid models, I'd say, if you had the internals, I'm sure you would be capable of creating a housing, mounts and mod's to suit.....if you have access to the equipment.

There are toys and other junk(power tools, small kitched appliances etc) out there with worm gear units. All Electronics or a few other surplus outlets may have something you can modify. Don't forget Stock Drive Products for cool little drive requirements. Considering the application and replacement cost of the sights, planning ahead for run-away should be included.

There are PIC servo and stepper drive kits that might suit the I/O with analog and digital capability. There is quite a learning curve in programming what could be a custom non-linear output, that is scaled to the input. Basic Stamps by Parallax at least have PBasic which can cut the learning curve some compared to learning assembler or other high level languages.

You will need to be very conservative with power if you plan to use this under battery power and a stepper motor. Those things still consume power at idle. A PWM servo is much better in that regard.

Looks like a fun project.

DC