Servo shake

Earl FosterEarl Foster Posts: 185
edited 2005-02-03 - 20:36:47 in Robotics
I have starting working on my hexcrawler and I noticed that after running the HexCrawler Legs Version 1.1 by Parallax that all my servos shake.· There are no centering pins on these servos.· I think if I made the servos go low that will resolve problem but I don't understand the PSC yet.· Any suggestions?


  • Paul BakerPaul Baker Posts: 6,351
    edited 2005-01-11 - 18:28:30
    don't have experience w/ servos in almost a decade but i noticed a similarity w/ Dave's response is this post:·perhaps this whats going on.
  • Earl FosterEarl Foster Posts: 185
    edited 2005-01-11 - 20:34:58
    Thanks for the reply. I did look over those threads but this is the case. I believe it is the PSC that is still sending a pulse to the servos. Since this is my first time using the PSC I am not sure how to put the PSC is a low state and I haven't really found good documentation. I guess I could try to put the whole PSC in a low state. Hmmmmm I will try that approach.
  • dandreaedandreae Posts: 1,375
    edited 2005-01-15 - 00:32:33
    Depending on the servos that you use, the power supply that you use can make them jitter.· If you are using a 7.2v battery to power your PSC (Parallax Servo Controller), this can cause certain servos to jitter.· The closer you get to 5v the less jitter you should see.· Depending on firmware version of the PSC, the latest version 1.4 has been designed to reduce the jittering effect.· There is a PSC version code that will tell you what version you have.


    Dave Andreae

    Tech Support

  • Earl FosterEarl Foster Posts: 185
    edited 2005-01-18 - 15:25:08
    Thanks Dave - so what you are telling me is that I have a $700.00 shaking robot from Parallax. Can't wait to tell my friends about that one!
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2005-01-18 - 15:43:54
    Frustration is understandable, but it would help to solve your problem if you could let Dave know if the firmware version is 1.4 or not, and what voltage power source you are using.· That's where I would start...

    Chris Savage

    Knight Designs
    324 West Main Street
    P.O. Box 97
    Montour Falls, NY 14865
    (607) 535-6777

    Business Page:··
    Personal Page:···
    Designs Page:···
  • John BJohn B Posts: 82
    edited 2005-01-20 - 22:25:59
    Hi there,

    Servo jitter can occur when the servo supply voltage is high (>5VDC), the mass of the load is approximately equal to the rotational torque of the servo (as is on the horizontal portion of the leg of the HexCrawler) and if there is some stimulus.· This stimulus is commonly caused by signal jitter.

    Signal jitter is the inherent result of the indeterministic nature of microcontrollers.· When an interrupt occurs, the interrupt handler code will be executed between 3 and 6 cycles later.· This variability causes the signal to sometimes happen a little later than usual.· This manifests itself as signal jitter of about +-2uSec.

    We were able to restructure the PWM channels and the interrupt scheme on the PSC to avoid·signal jitter.· This new version is v1.4 and has not been released yet, but is presently in a Beta Test phase.··(The code restructure was rather pervasive, so we decided a Beta test phase was necessary.)· We are presently about half way through·testing but·welcome other participants if they are interested.· Presently, all indicators show that the new firmware works well and eliminates jitter.

    One of the nice things about the PSC is that it is re-programmable.· As soon as v1.4 is released, we plan on adding several commands and features and releasing that as v1.5.··Despite adding new features and commands, we plan on keeping the price of the PSC the same.· As soon as that version is released, I'm sure customers will request yet more commands and new features as well.· We do our best to please our customers.· Though we have yet to formalize a policy on this, customers are welcome to return their PSCs for re-programming (a free upgrade).

    Your remark concerns me.· If you feel you have been tricked,·slighted, or cheated in some way, please contact me directly and I will personally work with you to correct the error in an equitable fashion.

    Kind Regards,

    John Barrowman
    Parallax, Inc
    (916) 624-8333 x110
  • Earl FosterEarl Foster Posts: 185
    edited 2005-01-24 - 16:15:12
    I appreciate your explanation and I think it should be incorporated into the PSC documentation as opposed to just stating that you need a separate 5VDC power supply. I don't feel tricked, slighted, or cheated in anyway but I do feel that the documentation is lacking. I do think it is funny how after following all the documentation and downloading the latest version of code that the first time I turn on the crawler it stands there and shakes.

    If front-ending my PSC with a regulated 5vDC power supply will stop the jittering then I am fine with that course of action. If the jittering remains then perhaps I will request the re-programming of my PSC to the latest version. After all - nobody wants a $700.00 shaking robot.

    Thanks again
  • John BJohn B Posts: 82
    edited 2005-01-25 - 01:50:26
    Hi ebf306,

    Initial results of the Beta test confirm that v1.4 is solid.· If you would like your unit reprogrammed, just send it to me and I will take care of it.· Also, the manual has been updated (but not posted just yet) to reflect the changes made in v1.4 of the PSC firmware.· I would be happy to add any other information you think would be helpful.· Besides the explanation of servo twitch and the causes thereof, can you suggest some other items that you would like to see in the manual?

    At the request of a customer, I will be creating an interim version of the PSC that supports an additional command: group call.· This command will allow the user to control multiple servos with a single command.· If you are interested in this feature, I can reprogram your unit with this new command for your review.

    Kind Regards,

    John Barrowman


    Parallax, Inc.
  • Earl FosterEarl Foster Posts: 185
    edited 2005-01-25 - 14:36:15
    Thanks John - I believe I will take you up on your offer. The “group call” command sounds very useful and would be interested in reviewing it as well. Were do I need to send the PSC?
  • John BJohn B Posts: 82
    edited 2005-01-25 - 17:00:11
    Hello ebf306,
    Just send the unit to:

    John Barrowman
    Parallax, Inc.
    599 Menlo Drive Ste 100
    Rocklin, CA 95765
  • Earl FosterEarl Foster Posts: 185
    edited 2005-02-03 - 20:36:47
    You should have it now.
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