Sensor for linear distance

[TAlholm]

So I'm embarking on a project. I am in need of linear sensors for distance traveled. I am looking at the string potentiometer from AndyMark, but would like to know if anyone has experience with any other form.
IR sensors are not going to work for the project as the risk of the sensors getting feedback from another is too high.

[Ernst]

Originally Posted by TAlholm

So I'm embarking on a project. I am in need of linear sensors for distance traveled. I am looking at the string potentiometer from AndyMark, but would like to know if anyone has experience with any other form.
IR sensors are not going to work for the project as the risk of the sensors getting feedback from another is too high.

Do you have more details on the application? Is the sensor attached to something moving or is it measuring something else that's moving? Are you driving an elevator or measuring distance driven on the ground?

I'd be inclined to go with a potentiometer or encoder if you have anything rotating relative to the linear motion, like a motor spinning a shaft to drive an actuator or belt, or a tracking wheel spinning on the ground.

Are you measuring to a moving plane? An ultrasonic sensor might work.

[TAlholm]

Originally Posted by Ernst

Do you have more details on the application? Is the sensor attached to something moving or is it measuring something else that's moving? Are you driving an elevator or measuring distance driven on the ground?

I'd be inclined to go with a potentiometer or encoder if you have anything rotating relative to the linear motion, like a motor spinning a shaft to drive an actuator or belt, or a tracking wheel spinning on the ground.

Are you measuring to a moving plane? An ultrasonic sensor might work.

I'm experimenting with a method that may allow faster and safer rehab for people with arm or leg injuries or who recently had surgery. For safety of the user, I am staying away from tracking rotation.

[GeeTwo]

We made our own "string potentiometer" for 2015 from a tape measure and a 10-turn pot from Jameco.

A roller as suggested by Ernst is a similar concept; make sure you know how many total turns it will make as the slide travels, and leave some spare for slippage.

Linear slide potentiometers are also available, mostly for much shorter lengths than the string unit.

An optical encoder can also be used directly on the linear slide. Use an LED and sensor module like you would use for tracking a line, then draw lines on the slide perpendicular to the direction of motion*, and count how often the surface becomes light and dark. You can even use two of these and make a quadrature encoder to keep track of the direction of motion.

If your counter is subject to slippage or loss of data due to missed transitions, you may want to add at least one limit switch to the slide to allow for an automated calibration.

* or print lines on a long label and stick it on the slide

Edit, if you don't mind doing way more processing than should be necessary, you can do what inspired our electric tape measure: Fix the loose end of the tape measure to the slide, and the housing to the fixed part of your device. Then, point a camera at the tape near the housing, and do a little OCR to read the tape measure. Probably not practical, but it led to a better idea.
Edit2: electronic calipers already do what you want to do - see if you can find some which will allow you to take a digital reading rather than drive a display.

[adciv]

Originally Posted by TAlholm

I'm experimenting with a method that may allow faster and safer rehab for people with arm or leg injuries or who recently had surgery. For safety of the user, I am staying away from tracking rotation.

Ok, I get what you're trying to do, but I'd like to know why tracking rotation is less safe. Is your end goal to measure the distance between two fixed points or is it to measure the angle of a joint?

Depending on details for straight linear distance there are also linear encoders. Do you require an absolute measurement or a relative measurement from startup? How much precision do you require?

We've had some use with the stringpots from Andymark. You'll need to protect your strings to ensure they do not get caught on anything. Also, you'll have to be mindful of sensor noise. Precision was something like 1mm before sensor noise dominated (RoboRio as voltage source & measurement). A question is will your system be able to keep the noise low enough you'll have the precision you require.

[Mike Schreiber]

Originally Posted by TAlholm

I'm experimenting with a method that may allow faster and safer rehab for people with arm or leg injuries or who recently had surgery. For safety of the user, I am staying away from tracking rotation.

I believe a string pot is just a normal pot with a torsion spring and string integrated (think internals of a badge holder). I've seen several teams make their own with badge holders or in one case a retractable dog leash.

[s1900ahon]

Originally Posted by Mike Schreiber

I believe a string pot is just a normal pot with a torsion spring and string integrated (think internals of a badge holder). I've seen several teams make their own with badge holders or in one case a retractable dog leash.

The string pot being sold by AndyMark started life as a Luminary Micro badge holder and a 10-turn pot from Mouser.

[techhelpbb]

String encoder?

Put a quadrature encoder on a badge holder pull out string assembly.
No noise floor, just a decoding issue which you can get electronics to solve like this:

http://www.avagotech.com/products/mo...r-ic/hctl-2022

[asid61]

Originally Posted by techhelpbb

String encoder?

Put a quadrature encoder on a badge holder pull out string assembly.
No noise floor, just a decoding issue which you can get electronics to solve like this:

http://www.avagotech.com/products/mo...r-ic/hctl-2022

The string encoder allows one to skip a lot of custom work and sense the distance directly, rather than going through extra layers like decoding encoder outputs. Not to mention that given the costs of shafts, bearings, machining, and the encoder itself, it might make more sense just to use the string pot.

[techhelpbb]

J

Originally Posted by asid61

The string encoder allows one to skip a lot of custom work and sense the distance directly, rather than going through extra layers like decoding encoder outputs. Not to mention that given the costs of shafts, bearings, machining, and the encoder itself, it might make more sense just to use the string pot.

Here are some cheap encoders:

Up to 256PPR:
http://www.grayhill.com/products/optical-encoders/

Up to 1000PPR:
http://m.ebay.com/itm/Allen-Bradley-...047?nav=SEARCH

2000PPR (put buffer on transistor output to get TTL):
http://m.ebay.com/itm/New-in-box-Omr...087?nav=SEARCH

There is almost no difference in the package as long as you ignore the shaft end style encoders like US Digital commonly provides for use in FIRST style gearboxes. Personally my success with that style encoder in FIRST and my MaxNC CNC machines is limited anyway.

So really all you need to add is the board to decode, and if you do not want to craft that:
http://www.superdroidrobots.com/shop...r-buffer/1523/

http://www.usdigital.com/products/in...cs/LFLS7366R-S

So if you can make this work with a 1/4" shaft potentiometer you can make this work with an encoder. Sure you need some extra electronic parts and cost, but the encoder provides clear signals.

Also the potentiometer is cheaper but wear can become an issue more quickly and then you have analog issues.

[TAlholm]

Alright, I got the chance to post a more in depth description.

The method for this is to track the human arm or leg and assist the recovery process by assist the movement of the arm or leg with an exoskeleton like frame. By slowing decreasing the assistance given to the arm or leg, the theory is that this could allow a way to possibly allow a faster recovery.

[asid61]

Originally Posted by TAlholm

Alright, I got the chance to post a more in depth description.

The method for this is to track the human arm or leg and assist the recovery process by assist the movement of the arm or leg with an exoskeleton like frame. By slowing decreasing the assistance given to the arm or leg, the theory is that this could allow a way to possibly allow a faster recovery.

This might be easier accomplished using a regular old absolute encoder on the rotating joints of the exoskeleton. That way there's no risk of missing counts, and that will probably be cheaper and easier than a string potentiometer.

[EmileH]

We used v1 of this sensor a few years ago and loved it. Would highly recommend.
http://www.robotshop.com/en/lidar-li...lsedlight.html

[adciv]

Originally Posted by TAlholm

Alright, I got the chance to post a more in depth description.

The method for this is to track the human arm or leg and assist the recovery process by assist the movement of the arm or leg with an exoskeleton like frame. By slowing decreasing the assistance given to the arm or leg, the theory is that this could allow a way to possibly allow a faster recovery.

I feel like I'm doing my thesis again. Stroke recovery, accident, CP, or what? I'm interested in following this. Alright, so what you're after is a power assist. There's a few options here. Any chance you looked at HAL-5 by Cyberdyne?

I can see a few ways to do this. If you're trying to track the human limb relative to the exoskeleton, a string pot is one way to do it. Another way would be to use a pressure sensor between the exoskeleton and the limb. If you only need position of the skeleton, then you should probably use an absolute encoder hard attached to the limb joints. There are a few options for these.

The andymark string pot will work as a good prototype string pot, but it's a bit bulky and can be fragile. You don't want to hit the housing and damage it. If you go with an absolute at or geared to the hinge, andymark also sells a US Digital absolute encoder as well. You can get these with a 0-5V signal output, similar to the stringpot. Pressure sensors come in enough types, I'm not sure what to post but are usually a variable resistance.

What your best choice is will also be based on how you decide to implement your control loop feedback for the power assist. Have you decided on your process variable?