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Angle Encoder Selection

Discussion in 'Robotics & Mechatronics' started by jpanhalt, Jan 18, 2015.

  1. jpanhalt

    jpanhalt Well-Known Member Most Helpful Member

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    I am fiddling on paper with a design to help guide me control the boom and dipper of a small excavator when leveling a surface. For context, as one pulls the dipper toward the operator, the boom needs to be raised until the dipper is vertical and then lowered after that to keep contact with the surface level. That is, the dipper swings in an arc. One can calculate the relationship knowing the angle of the dipper to the boom and the boom to the machine plus a few constants and assumptions (one being that the bucket to dipper angle is fixed). I have modeled it to estimate the accuracy that I need. For example, with the dipper almost fully extended, if I pull it 2" toward the cab (about 4° of arc), the boom needs to be raised about 1.65° to keep the track of the dipper on the same plane and level.

    My prior experience with angle sensors is limited to using an accelerometer to measure "level." I don't think that is the best sensor for this purpose. Rotary optical encoders would seem ideal, but are expensive compared to magnetic sensors like the AMS AS5xxx series (http://ams.com/eng/Products/Position-Sensors). One of my concerns with a magnetic sensor is that they will be attached to a lot of steel.

    What are our thoughts on using a magnetic versus optical sensor in this application?

    Regards, John

    PS: I realize experience is key to doing the leveling operation. One purpose of this project is just to keep me busy during the Winter.
     
  2. cowboybob

    cowboybob Well-Known Member Most Helpful Member

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    John, if I may ask, is your intent to give yourself some sort of feedback indicating the position of the bucket or is it to control the bucket's position for you?

    And, in either case, what sort of "slop", i.e., the + or - variability of the bucket's desired vertical position, will be tolerable?

    Not exactly what you've asked for, but... First thought I had was mounting a laser level device (like what you'd use to, for instance, put up the supports for a "dropped" ceiling) on the front of the excavator? And then putting numbered horizontal lines on the front boom attached to the bucket.

    Once you've established the desired level of the bucket, just keep the laser light on that mark on the boom.
     
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  3. jpanhalt

    jpanhalt Well-Known Member Most Helpful Member

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    Thank you. That laser idea might work and is certainly worth a try. I happen to have a self-leveling one that should work (Leica). By the end of the season, I was getting better at leveling. The hardest part for me was estimating where the cutting edge of the bucket was as dirt pilled up on it. I also need to tighten up the controls a little. It was inside a barn, and light was not that good.

    In my dreams, I would splice into the boom control and use a proportional, servo hydraulic valve to make the boom raising/lowering fully automatic. The chance of me actually doing that is almost nil. A new valve valve costs a significant proportion of what the excavator cost. More likely, I will have a display like a glideslope to track where the boom needs to be while I move the dipper with my right hand.

    Right now, though, I am just looking for something electronic that will be fun to concentrate on during the Winter.* I did buy a cheap goniometer last week to tear apart and study.
    upload_2015-1-18_11-4-57.png

    John

    *Learning C would be more useful, but not as much fun. ;)
     
  4. dave

    Dave New Member

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  5. Ian Rogers

    Ian Rogers Super Moderator Most Helpful Member

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    I use this..
    SCA61T-FAHH1G

    http://uk.rs-online.com/web/p/inclinometer-ics/6912381/

    They do a 160 degree one and its pretty linear.... The device take 5v and spews out a linear voltage according to angle..

    they have a serial output if you need greater control as there is an onboard temp sensor to help with deviations in angle due to temperature...
     
  6. cowboybob

    cowboybob Well-Known Member Most Helpful Member

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    I looked at the As5045 encoder from the link you provided. I should say I have no personal experience with this device.

    But, from the PDF datasheet for it (page 13, section 7), last paragraph, the device is "tolerant" of magnetic misalignment and stray magnetic fields.

    And since the device and it's associated 2-pole magnet, linkage and PCB will have to be enclosed in a protective steel enclosure, that should (should) dampen to insignificance any influences caused by the steel of the booms.

    And interfacing them to uC would be a price of cake. The PWM output is sweet.

    For less than $7 a pop, I'd at least give them a try.

    <EDIT> Ian's suggestion looks mighty good also...
     
    Last edited: Jan 18, 2015
  7. jpanhalt

    jpanhalt Well-Known Member Most Helpful Member

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    That was my conclusion too. Also, you don't have to separate very much from the sense magnet to lose the signal. I already am using a PWM output from a accelerometer/inclinometer for my remote level. So, the sensor and sensor to remote display is pretty much already done.

    John

    Edit: Here is a YouTube link provided on another site of the AMS device:

    I thought it was pretty impressive. The real question is repeatable accuracy, which seems to be 0.5°, if I read the datasheet correctly.

    Edit1: In this morning's chat, I mentioned my remote level. Here is a short clip (my very first one) showing how it performs. I am doing the usual test of setting an angle, then flipping the level sensor to see how symmetrical the response is. The sine table is quite accurate for the nominal angle. The floors in my log cabin are a little wobbly. I can easily detect walking across the floor. Unfortunately, I needed to upload as a zip file rather than an mp4.
     

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    Last edited: Jan 18, 2015
  8. jpanhalt

    jpanhalt Well-Known Member Most Helpful Member

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    Received the gadget (post #3) yesterday afternoon. It appears to use a capacitative rotary encoder, not unlike the CUI AMT102/103 devices (http://www.cui.com/product/resource/amt10-v.pdf ), and is quite compact. Accuracy is claimed to be ±0.3° . I will probably not try to extract any parts from it.

    John
     
  9. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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  10. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    I think the angle encoders are a good idea.
    I am thinking about another way.
    You want an excavator to have different controls. Up/down and In/Out. Not the two controls that just govern two joints.
    The real question is how high is the end of the arm.
    I have a 20 foot long clear flexible tube. It is mostly filled with water. There is a box on both ends. By looking at the water level on my end I can see how high the other end is. Mostly I use it for leveling something large like a building.
    I can see that by knowing the angle of two encoders you can find the height of the other end.
    I am thinking at only looking at the height of the operator verses the far end.
     
  11. jpanhalt

    jpanhalt Well-Known Member Most Helpful Member

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    A buddy has a Bobcat with an extendable dipper. It extends maybe 3' or so. It is great for reaching under something and digging. He uses it to dig graves, among other things. We used it to dig under a deck. Ironically, when we back-filled, we used my excavator, because the curl geometry was just a little more suited to the task. The Bobcat is a little like the In/Out you suggest.

    Agreed, that is not always obvious. A water level will work, but I like cowboybob's suggestion of a laser. My little Leica auto levels, gives an easily visible horizontal line, and can be locked at any angle. It has a magnetic base, which will work for testing. If it helps me, I will make a more permanent mounting bracket. (I makes me feel dumb that I didn't try it last year. I actually had it at the site for getting elevations.)

    Anyway, this (http://www.digikey.com/product-search/en/sensors-transducers/encoders/1966131?k=amt103) is already in my shopping cart. Even if it never makes it to a excavator, coding for it will be educational and fun.

    John
     
  12. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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  13. jpanhalt

    jpanhalt Well-Known Member Most Helpful Member

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    That is exactly what I will do when the snow is gone. The Leica Lino L2 I mentioned is a laser level with cross hairs and was considerably cheaper at about $200 (http://lasers.leica-geosystems.com/lino/l2-0?gclid=CPrz6I3bssMCFWsV7AodlQUAPg ). The cross hairs are easy to see in light.

    However, the capacitative gizmo is so neat, I just have to get one to play with will watching the snow melt.

    John
     
  14. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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  15. simonbramble

    simonbramble Active Member

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    I used to work for Austria Microsystems (AMS). Their encoders are good if you have something small and need very good accuracy. The magnet needs to be within about 5mm of the chip's surface and misalignment in the XY direction should be no more than a millimetre or so. If you can do this, then you should be able to get 12 bit accuracy. it will be a mechanical problem more than anything - keeping the motor aligned with the encoder, but if your project is a one off, it should be fairly easy to do. They are not affected by surrounding magnetic fields and you can hover hard disk drive magnets over them and they are still immune
     
  16. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    What I noticed is that you don't have to program them with a $1500 gizmo. I think it's Melexis that does this but they try to have one gizmo that works across the entire product line.

    A tangent:
    A Sunbeam model 10 Mixmaster (Kitchen mixer) uses centrifugal force (the shaft moves) to activate a contact, thus doing speed control. There is an adjustment of the length to the contact. As one of those "play with" projects. I just think about it, but haven't tried anything. I was wondering if one could put a magnet on the end of the shaft and a magnetic sensor to activate a triac instead of the switch.

    The other components inside the Mixmaster is a power resistor and capacitor. The cylindrical objects in the diagram that is off center near the knob has an insulator at one end and can be expanded, The insulator is attached to the contact block.

    magnets such as: http://www.kjmagnetics.com/products.asp?cat=1 and a switch such as this one http://www.melexis.com/Position--Speed-Sensors/Hall-Effect-Latches/MLX92211-BAA-827.aspx or a programmable one.

    On this page http://en.allexperts.com/q/Small-Household-Appliances-3559/2009/7/sunbeam-mixmaster-model-12.htm is a lousy picture of the mechanism. The contact distance is varied by the knob on the back. A combination of the spring force and the distance =F=kx and the force from V^2/r varies the speed.

    So, I'd need to solve some mechanical issues, but not sure what would work or what could be used where the distance is electrically programmable.
    I don't really have machine shop access anymore and haven't mastered 3D printing yet, but I do think it would be a cool project.

    Sorry to hijack, but i'm not putting any effort into the project, so it's only an idea that hopefully can get some commenting.
     
  17. jpanhalt

    jpanhalt Well-Known Member Most Helpful Member

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    Hijack not problem for me. Snow is piled head high. I ain't gonna be dig'n for awhile. Then the laser gets a test. I plan to get that capacitative sensor with my next DigiKey order. My SSD crashed a week ago Thursday, and I lost a lot of recent work.

    The only thing I use my Kitchen Aide mixer for is making ravioli dough.

    John
     
  18. jpanhalt

    jpanhalt Well-Known Member Most Helpful Member

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    UPDATE:

    Well, my AMT102-V (capacitative) and magnetic encoders arrived today. It is cold enough to stay indoors (-3°F this morning ...warmer that the last few days), but the continual snow keeps me busy. Can't wait to play with them

    John
     

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