Next large robot

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  • Its been a while since I last posted on the Hexapod (still haven't come up with a name...) status thread. Actually there has been very little work done on it this summer, too much work, bicycling, and preparing for retirement to allow enough time in the basement shop. However, with the advent of colder weather I expect to start spending time on the coding once again.

    As I left the code the last time, there is a problem with the amount of 'jerkiness' that the leg experiences during movement under IK. When I reduce the size of the movement steps to reduce the jerk, then the slow nature of the Spin code becomes a factor, and the leg moves unacceptably slow. I also believe that the position sensors in the motors are not sensitive enough (too much noise in the potentiometers) for the PID loop and contribute their own delays to the movement.

    That leaves me with a couple of choices. One would be to re-write the PID in assembly (since I haven't done assembly for years and never on the prop) which could take quite a bit of time. The other choice would be to continue to see if I can continue to optimize the current spin version of the PID loop to get a better response (lots of testing, pretty much what I was doing until summer started). The bad part of option 2 has been the lack of progress causing a corresponding lack of motivation. Additionally I have been looking at magnetic position encoders that could be attached to the leg and use them as a better feedback mechanism instead of the internal potentiometers. That would require some re-design work on the legs to mount them and then coding them in.

    I hope to get back to regularly posting progress reports again however up until my summer break there had not been a whole lot of progress which was sapping my motivation. Any suggestions from anyone would be appreciated if you can think of what could be done to get this rolling again.

    Thanks for any help that comes this way! I may be a bit slow in responding to questions since I started night shift last week and will be on it through most of December (lots of work at the plant going on this time).

    Bob Sweeney
  • Did some testing of the 3 axis and monitored the movement data. Still seeing a lot of 'noise'' in the output from the position potentiometers. This causes confusion in the PID loop and is leading to motor overshoot and increased reaction time. So I've decided to add encoders to each axis to help fix this issue. The limitations I have are that I only have a single cog unassigned on each leg controller and I will have to figure out how to install the encoder. The preferable method would be to remove the old pot inside the motor housing and replace it there but the available area is pretty small. Another option is to use a angular encoder and just measure the actual angles that the leg is at instead of motor rotations. That allows more space for an encoder but it isn't as clean for mounting as the other option. Plus, the mechanical design will have to be tweaked to get the angles right.
    So tonight I will be doing some studying, I remember seeing some capacitive angular encoders a while back that measured down to either a tenth or hundredth of a degree. The options for mounting will be a factor also. Any recommendations, has anyone tried these out yet with a Prop?
  • After a few days of research I decided to use a AS5045 rotary magnetic encoder. This is a 12-bit (0-4096 over 360 degrees or ~0.08 degrees per count) encoder that uses a small magnet rotating over the chip to output either PWM or SSI. This should give the resolution needed to more precisely control the PID loop.

    Once I figured out what chip I wanted, next I looked for any commercial solutions that incorporated the chip, circuit board and magnet into a single, easy to use package. There are several available however at a cost that was beyond my budget. I was starting to figure out how to do my own circuit boards and housing when I ran across the HH-12. This solution is being used to control motorized telescopes and a gentleman in Germany is creating these. He houses everything in a potentiometer case making for a small and neat package. I ordered 6 to try out on the coxa to replace the potentiometer I am currently using. If this works out, then I will have to see if it is possible to replace the potentiometers in the linear motors with them also.

    Once they arrive I will be diving into the data sheets for the chip and see if the SSI communications protocol it uses is compatible with anything in the Propellor library otherwise I'll have to write something up myself.

    Bob
  • RS_JimRS_Jim Posts: 1,265
    edited 2016-10-14 - 13:44:06
    Bob,
    I assume the HH-12 is based upon the AS5045. Duane Deign developed som code to run on the prop for that device I will see if I can find it and link to it for you.
    Jim
    Edit attaching link: [url="http://http://forums.parallax.com/discussion/138736/magnetic-encoder-project-as5055/p3"]http://forums.parallax.com/discussion/138736/magnetic-encoder-project-as5055/p3[/url]
    Hope this gives you a jump start.
    Edit 2: I just found the HH-12 and yes it is based upon AS-5045. I used Duane's code as a basis for what he calls the HH-05 and it worked well. My problem with the the sensor was the same as yours, the housing. Look into the code as he was able to get them to function in yhe cascade mode which could get you 6in 1 cog.
    Jim
  • Thanks for the link.I had seen the thread with the project back in 2012 and his work at making circuit boards during my recent research. That's what helped me look into this chip and subsquently finding the HH-12. I hope that these will have enough resolution to solve my problems! When they arrive I will do some testing and figure out how to mount them.

    The coxa mount should be relatively simple since I'm already using a 20 turn precision pot, should just have to replace the parts. The only problem might be if the shaft is too short, then I will need to come up with some type of extender on the lathe.

    Replacing the pots in the linear drives might be trickier, its been a while since I've opened one of these up so I'm not sure how much space is available inside the housing. It might be possible to drill a hole through the housing and mount it outside with an extension inside the housing to the gear train. This would require changing the mounting bracket to allow more space for an external pot.

    Another way would be to mount the femur and tibia encoders to the main vertical support bars and tie the encoder shaft to the parts that are moved by the motors. This would take more work to figure out since it wasn't initially designed with this in mind and I don't want to have something that isn't visually neat or could be damaged easily.

    Bob

  • I got my HH-12 magnetic rotary encoders in the mail, now I need to try them out!
    IMG_5686.JPG
    It is a very compact unit, smooth rotation and looks very nice.
    Since I'm also looking to potentially use this to replace the potentiometer in the linear drives, I opened one of them up to see its configuration.
    IMG_5688.JPG
    The encoder will fit in the available area without having to modify the housing. The encoder uses 5 wires vs only 3 for the pot but that can be worked out. A bigger concern is that the shaft of the encoder is 6MM and the pot is 0.25", the encoder is 0.15" smaller so the gear doesn't fit. Also, the hole in the mounting plate is too large, making a very sloppy fit. The hole can be fixed by machining a custom washer to take up the slack. Increasing the shaft size will take a little more thought. The shaft is made of either brass or bronze so I need to do some research into possible methods for increasing its size without damaging the unit.
    Tomorrow I will start some coding tests and see how well it works and the speed of the data coming into the prop.
    3648 x 2048 - 1M
    3648 x 2048 - 2M
  • Try a piece of brass tubing from a hobby shop to make up the difference in the shaft diameter. You may have to sand and slit the tubing it to make it fit.
    In science there is no authority. There is only experiment.
    Life is unpredictable. Eat dessert first.
  • ercoerco Posts: 19,441
    edited 2016-10-26 - 17:40:26
    kwinn wrote: »
    Try a piece of brass tubing from a hobby shop to make up the difference in the shaft diameter. You may have to sand and slit the tubing it to make it fit.

    +1. That's not a high speed power application on those gears, so you should be able to retrofit a spacer on there with tubing and epoxy. Accuracy is important to avoid binding & backlash. Clamp the rig down and check the gear for runout with a dial indicator as the epoxy sets up.

    If you do this mod several times, your last one will the best one. :)

    What's the angular resolution on those magnetic encoders? Is it a full 360 degrees? Pretty cool stuff. I'm surprised Duane hasn't chimed in here yet, he's quite the encoder fanboy.

    "When you make a thing, a thing that is new, it is so complicated making it that it is bound to be ugly. But those that make it after you, they don’t have to worry about making it. And they can make it pretty, and so everybody can like it when others make it after you."

    - Pablo Picasso
  • erco,
    After a few days of research I decided to use a AS5045 rotary magnetic encoder. This is a 12-bit (0-4096 over 360 degrees or ~0.08 degrees per count) encoder that uses a small magnet rotating over the chip to output either PWM or SSI. This should give the resolution needed to more precisely control the PID loop.
    from his first post on the topic.

    Yeah, I too am surpised that Duane has not dropped in of the topic. I believe I sent him a PM.
    Jim
  • kwinn wrote: »
    Try a piece of brass tubing from a hobby shop to make up the difference in the shaft diameter. You may have to sand and slit the tubing it to make it fit.

    Using a brass tube was my initial thought. I found some .25 OD brass tube at the local hardware store, I will have to do some work to get it to fit over the 6MM shaft but that shouldn't be too hard to do, besides, that's what I have the machine shop in the basement for fixing things like this. Tonight I want to do some programming and see what kind of output I get from the unit. I'll be starting with Duane's code that he posted a few years back and modify it as needed. I like the fact that you can also daisy chain these together especially since I only have one spare cogs now!

    Bob
  • ercoerco Posts: 19,441
    K&S sells aluminum and brass tubing at our LHS in all nesting sizes. Use whichever material is easier to slit, even using a cutoff wheel in a Dremel.
    "When you make a thing, a thing that is new, it is so complicated making it that it is bound to be ugly. But those that make it after you, they don’t have to worry about making it. And they can make it pretty, and so everybody can like it when others make it after you."

    - Pablo Picasso
  • erco wrote: »
    K&S sells aluminum and brass tubing at our LHS in all nesting sizes. Use whichever material is easier to slit, even using a cutoff wheel in a Dremel.
    K&S brass tube is what I found. I used the lathe to drill the tube out to just under 6MM, now I will use sandpaper to get the final diameter. I'm thinking of getting a tight press fit with a touch of superglue as a means to prevent the tube from slipping.

    I've been testing the program from Duane and got an output pretty quickly. Unfortunately the output is wildly variable and constantly says it has an error in between readings. Sometimes I will get a whole series of outputs but again the numbers aren't even close to one another. Duane has an averaging routine built into the program but I thought that when the shaft isn't moving that the values would stay relatively consistant, I'm getting value swings of several hundred (the equivalent of around 30 degees of shaft motion).

    I will post on Duanes original thread if he has any insight for me.

    Bob
  • ercoerco Posts: 19,441
    Something isn't right with your software, wiring or circuitry if your values are changing that much. It's a precision pot, right?Are you RCtiming it, sigma Delta, or ADC?
    "When you make a thing, a thing that is new, it is so complicated making it that it is bound to be ugly. But those that make it after you, they don’t have to worry about making it. And they can make it pretty, and so everybody can like it when others make it after you."

    - Pablo Picasso
  • Bob,
    I was unsuccessful i finding my old files, I finally remembered that the computer that I was using at the time died a horrible and violent death and I may not have had the latest and greatest backed up. I lifted this link from the middle of Duane's thread that gave what I thought was stable code at the time. [url="AS5055Demo120526a - Archive [Date 2012.05.26 Time 15.21].ziphttp://"]AS5055Demo120526a - Archive [Date 2012.05.26 Time 15.21].zip[/url]. futher down in the thread is a video of it working.
    Jim
  • erco wrote: »
    Something isn't right with your software, wiring or circuitry if your values are changing that much. It's a precision pot, right?Are you RCtiming it, sigma Delta, or ADC?
    Duane sent me a copy of his old 3 wire file when I asked on his other thread, I'll be trying this out tonight.

    The test setup I'm using is outside of the robot, just an experimenter board connected directly to the encoder. I don't have anything else running that would cause this type of problem. The encoder uses SSI to communicate the results serially, the initial setup does output values but they are very erratic, not what I was expecting. Now the software was designed for connecting directly to the encoder chip and with the HH-12 I don't have that option, only the CLK, CS, and Data out line. I don't know how the internal circuitry is put together, it doesn't appear to easily disassembled so I can check out the circuitry inside the housing. (I'm going to send the vendor a request for the schematic, that may help also).

    Bob
  • Slowly working on the encoder code. More testing shows that the output is not consistent but for a given shaft position the output values fluctuate between around 4-6 specific numbers. Move the shaft some more and I get a different set of numbers. Not sure what that means yet but more coding and testing will be needed. Unfortunately I will be working the night shift pretty much for the rest of the year and that leaves little time to play. I'm looking forward to retirement even more now!
  • Success! I started from scratch and bit-banged out some SSI code based on the vendor's data sheet and now I'm getting very consistent and repeatable output from the HH-12 encoder. I am not seeing any jitter when the shaft is moving and the output is constant when stationary. The output changes from 0-4096 for a full 360 degree rotation. That comes out to around 0.087 degrees for each value change (360/4096). I can dedicate one cog to running this process and use it to input to the PID code.

    The first place I am going to use the encoder is the coxa, the side to side movement of the leg. The easiest mechanical installation is to replace the existing potentiometer with the encoder. I need to make an adapter for the HH-12 shaft as it is too short to engage the main gear. If I make the mechanical connection here then I have to take into consideration that the gearing results in about 2.5 turns of the pot shaft as the leg moves from one side to the other. Since this would result in 3 separate 0-4096 transitions, the code needs a counter since I couldn't get an absolute position from the leg for any given reading. This could cause some initialization issues where each leg would have to figure out its current position when power is applied.

    A better move may be to connect the encoder shaft directly to the leg so that it is only measuring the +/- 60 degree leg movement. That would be using only 120 degrees of a single rotation so the output would be the absolute location. The encoder output would be limited to a value change of 1365 on the output since it isn't using the full range of the encoder. Mechanically doing this will be harder since the leg was not designed this way and the pins that the leg rotate around are stationary and don't normally rotate with the leg. Time to put on the thinking cap again!

    Once I get through this problem then I'll get back to figuring out putting the encoders in the linear actuators.

    Bob
  • You are the machinest, why not redo the gearing.
    Jim
  • I am looking at all options right now. I prefer to not have to make major changes to the leg design as it works pretty well. I am leaning toward a modification to the upper support bracket. There are threaded holes in both the stationary and mobile sections that were originally intended as support locations for sensors that I haven't installed. I can re-purpose those locations and add on a encoder support mechanism. There are about 2-3 ideas I am considering on the design change.
  • After spending a lot of time staring at the leg support struture and devising several potential methods for translating the side to side motion of the leg to rotate the encoder, I think I've come up with the potentially least intrusive and simplified mechanical connection. The upper leg swing connection has 2 pre-drilled and tapped holes on either side of the pivot point. I plan on attaching model airplane push rod connectors to both of connection points and attach the pushrods to a two sided arm that attaches to the encoder shaft (like a servo in a model airplane used to move the elevator). This means only drilling one hole in the upper bottom stationary connector to mount the encoder and then fabricating the arm for the encoder shaft. This way as the leg moves, the linkages will push and pull the encoder arm causing it to turn. Since I only need the shaft to rotate 120 degrees, the limited rotation is acceptable. I will order some aircraft connectors and install them on my next day off and test to make sure it will work without interference.

    There are a couple of other ideas I have for making this work if this idea doesn't pan out, they are mechanically more complex and potentially prone to issues in the future. I like ideas that are simple, easy to maintain and still look elegant.

    Bob
  • I built an arm for the encoder and have tried 2 methods of connecting the arm to the coxa of the leg. I tried 2 springs, one on each side but I don't get consistant results for a given position. I think if I use shorter springs so they are always extended I might see better results. The other method was using some stiff wire instead of springs but there is some 'slop' now. Possibly a wire with a couple of coils in it to absorb any mechanical looseness might be the best of both worlds. I will try it out tomorrow and see how well that works. I didn't use the model airplane clevis or tie rods as I can't seem to find any that are the right size, they are either way too small or huge.
    I started integrating the magnetic encoder into the main leg program and determined that I didn't have an extra cog to play with. I ended up using what I thought was my spare cog on the 4 way serial port since I needed 2 of them. I could use the ADC cog in the future if I totally move away from the ADC (I don't want to do that as the ADC can be used for various sensors I want to add in the future). I could remove one of the 4 port serial objects for now until I figure out how to go on (maybe someone will write a 6 port serial object someday!).
    I should be able to post some photos soon on this new layout once I'm happy I've got a workable solution for connecting the encoder and the leg.

    Bob
  • DriverBob,

    I've been keeping a curious eye on your build :)

    How many serial links are you using simultaneously? (ie why do you need two four-port cogs?)

    Out of interest, can you list your cog usage?
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  • How many serial links are you using simultaneously? (ie why do you need two four-port cogs?)

    Out of interest, can you list your cog usage?

    I was wrong, I'm only using a single 4 port serial port on the leg controllers (the master computer uses 2, 4 port serial ports for communicating to 6 leg controllers). My cog list for the leg controller is as follows:
    - Main program
    - Microchip 3208 8 channel ADC uses 2 cogs (ADC_Input_Driver and I run the whole thing in its own cog continuously)
    - 4 port serial port (pcFullDuplexSerial4FC)
    - Coxa motor PID
    - Femur motor PID
    - Tibia motor PID
    - HB-25 controller (jm_servo8)
    I could see if there is a way to run the ADC code differently, right now I run it in its own cog and use it to continually update the feedback variables.

    In the meantime I disabled the tibia PID cog so I could troubleshoot code. I got the encoder to run with good output data in the coxa PID loop. I just had a thought that the encoders are unique for each motor, so there are only a few specific places that it is called within a PID loop. I could just not run it in its own cog and call the routine when needed while inside the CoxaPID cog. The only time the encoder is checked outside the CoxaPID cog is during initialization when the slope is calculated. I just can't initialize the CoxaPID cog until this step is complete. This might be a way to circumvent the need for another cog.

    Today's tests included using the current control method to move the leg back and forth and observing the raw data output from the magnetic encoder as I moved from one extreme to the other. The output tended to be consistent +/- 2 counts. Of course this was after I removed the springs (the output was very inconsistent using springs) and put in a stiff wire to connect the moving part to the encoder.

    I've been playing for a while so time for a break until tomorrow, I'll try out the idea above and see if that works. Then I can code in the encoder in place of the potentiometer.

    Bob
  • Bob,
    Thanks for the update. Since you have switched to the HH-12 encoder, do you still Need ADC?
    Jim

  • RS_Jim wrote: »
    Bob,
    Thanks for the update. Since you have switched to the HH-12 encoder, do you still Need ADC?
    Jim

    I still have the potentiometer feedback from the linear actuators that I haven't converted over yet and there are some future mods that will require an ADC input.

    I took out the encoder code from running in its own cog and am incorporating it into the cog for the coxa PID loop. Came up with some errors that were causing problems but after thinking about it I believe I know what I did wrong. I'll try again tonight and see if I can get it running right.

    Bob
  • The coding changes worked and I am running the PID loop using the magnetic encoder as the feedback device. Just with what I have seen so far I was able to greatly reduce the deadband around stopping at the final position so I should get better accuracy when moving. I have a couple of bugs to work out, the calibration routine isn't moving the coxa motor any longer (the other motors still work fine) and the math for calculating the initial coxa position keeps coming up with the wrong value. Manually calculating the position using a calculator and the supplied input values does give the right answer so that is a problem to ponder, but it isn't critical to operation. The leg is about 15 degrees off of where I want it to be for any given command but that could be just a calibration issue, and updating the minimum and maximum values should clear that up.

    I'm going to play with this a bit longer and tweak the PID code and see if I can get 0.1 degree movement out of the motor. It should be possible to get that degree of accuracy. Repeatability looks pretty good also but I need to add a second connecting rod to the encoder arm to take up any additional slop
  • Glad to hear it is working better!

    I think you could reduce the number of required cogs...

    1) as you already noted, you should be able to use a single cog for ADC, just change the driver to update an eight element array constantly with the readings

    2) I suspect you could use just one PID cog, time-multiplexed between the three motors, as I don't think one PID loop would take a large fraction of a cog's time
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  • Thanks for the suggestions Bill. I did have one experiment where I tried to combine the PID loop into a single cog for all three motors but it got so complex with all the variables for each motor that I got lost just trying to troubleshoot it! The ADC cog is the one I will be shooting for next, I would just modify the driver to run continuously and output any ADC input on command.
    In the meantime the encoder is working pretty good. I figured out that I had a math error that was giving me the 15 degree offset, that's fixed now. I found and corrected the calibration routine so it seems good now. Still have to add another 'arm' to the encoder from the leg to help take up any slack in the movement of the encoder.
    Next event will be to start running the gait sequence from the master computer and see if the overall leg movement has improved.
  • You are most welcome!

    Re/PID cog, there is a way to really simplify running multiple PID loops.

    Keep the cog mostly as is, with the following changes:

    hub byte variable UPDATE, value 0...3, 0=idle, 1/2/3 calculate PID for loop 1/2/3

    hub variables CV/SP/PV etc for each of the loops, in three consecutive blocks

    PID cog monitors UPDATE. When zero, keeps monitoring until non-zero

    When finally non-zero:

    - load variables for the loop (1..3 or higher) into the variables in the cog
    - process via PID
    - save variables
    - set UPDATE to zero (you can also set UPDATE1/UPDATE2/UPDATE3 if needed to tell other cog that PID is done for that loop)

    This way the PID cog only has to deal with a single set of variables, yet you can run three (or more) PID loops in one cog.
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