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Boe Bot Servo Testing — Parallax Forums

Boe Bot Servo Testing

jplrobotjplrobot Posts: 12
edited 2013-06-12 11:36 in Learn with BlocklyProp
I am a beginner following the Boe Bot instruction manual and have a question arising from pulsing the servos at Pages 59 through 61.

1. I have successfully centered both servos
2. Both servos run as commanded in clockwise and counterclockwise direction
3. After a few cycles of testing, program says "scanning ports ......" and stays stuck in this mode
4. Exiting the program and reconnecting has not remedied the situation
5. I thought the battery pack was low but the pack shows 6V as the batteries are fresh

Any thoughts as to what may be the cause? I have gone back and run the earlier code snippets and they run without a glitch.

Thanks for your help.

Comments

  • ercoerco Posts: 20,257
    edited 2013-05-06 08:36
    Batteries are always suspect. Servos can draw a lot of current and even if your no-load voltage read 6V, they can still be low. Try brand new batteries, which will probably read ~6.4V initially.
  • AirbearAirbear Posts: 5
    edited 2013-06-12 09:36
    I have gone through the Robotics with the Boe-Bot book once now, and I am returning to go back and have some fun. I charged up a fresh new 9V battery, barely used, made sure it was fully charged and plugged it in. I ran a simple program that tests the servos. I used the command straight out of the book, It did everything like, "Program Running!", It was successful uploading to the Boe-Bot, but the servos never budged. I have done this before and they have worked but even with fresh out-of-package batteries, the servos never worked. I know they are centered and adjusted using the screwdriver correctly... Please help! Note: I am not using the prongs that plug into the breadboard, I am using the built in servo ports.
  • Mike GreenMike Green Posts: 23,101
    edited 2013-06-12 09:47
    Please do not use a 9V battery for driving servos, particularly a rechargable battery in a 9V package. These only provide 7.2V and, like the 9V alkalines, have a poor current capacity for this sort of high current use.

    If you want us to help further, you need to provide the program you're using. If it's "straight out of the book", provide a page number at least. If you've copied it out of the book, your copying is suspect. Make sure you've connected the servos properly. These examples do work (keeping in mind the severe limitations of 9V batteries for this use). Remember that the power switch has to be in position 2 for the servos to work and you need to have the voltage jumper set properly to use the +5V output of the regulator (Vdd) rather than the unregulated battery input (Vin). If you use a 6V or 7.2V battery pack, you'd use the Vin setting because that's low enough to not damage the servos in that case and that setting helps keeps motor noise out of the regulated supply.
  • AirbearAirbear Posts: 5
    edited 2013-06-12 10:06
    It is on page 59 In Robotics With The Boe-Bot by Andy Lindsay. The Boe-Bot has an slot for a 9V battery, I have tested this battery with a multi-meter, There is a proper amount of voltage and amperage. Anyways, I have used a 9V before, and they have worked, I am using the Board of Education Boe-Bot.
  • Mike GreenMike Green Posts: 23,101
    edited 2013-06-12 10:30
    It looks like you meant page 69 since there is a program for centering a servo on page 69 that is correct and no program on page 59. Like I said, check your wiring, make sure the servo plugs are inserted in the connector properly. Make sure you're using the right servo sockets. Make sure the voltage jumper is in place. If you've carefully done all that and it still doesn't work, there may be something broken. It's unlikely, but possible since the setup and program as described in the book do work. Consider calling Parallax Tech Support if you've checked everything and it all looks correct.
  • AirbearAirbear Posts: 5
    edited 2013-06-12 10:34
    ' {$STAMP BS2}
    ' {$PBASIC 2.5}


    DEBUG "Program running!"
    DO
    PULSOUT 13, 650
    PAUSE 20
    LOOP

    That is the program I used, servos ran off of that command a while ago, but now they just don't... Thanks for the help anyways.
  • W9GFOW9GFO Posts: 4,010
    edited 2013-06-12 10:45
    Airbear wrote: »
    I have tested this battery with a multi-meter, There is a proper amount of voltage and amperage.

    Just curious... what is the proper amount of amperage for a 9V battery and how did you test it?
  • AirbearAirbear Posts: 5
    edited 2013-06-12 10:59
    I have a multi-meter, set it on mA and then read the meter. One of the 9V batteries had 1200 mA, that was the Lithium one, the rechargeable NiMH was at 290, proper would've been 300.
  • AirbearAirbear Posts: 5
    edited 2013-06-12 11:01
    mAh was what I was looking at.
  • Mike GreenMike Green Posts: 23,101
    edited 2013-06-12 11:36
    Servos can draw 1000mA when just starting to move or when stalled they can draw even more. When fully charged, a 9V NiMH battery might last 10 minutes driving a servo if you're lucky. 9V Lithiums are optimized for long life with low current drain (like smoke alarm use). They're not designed to put out much current. If you're using a multimeter set to its current range, you're essentially short circuiting the battery and draining it rapidly. You can destroy the meter if the battery can put out enough current (like with an AA or larger battery pack). Lucky for you that the 9V batteries are relatively puny in terms of current output. To actually measure mAh, you have to put a specific current load on the battery and monitor the voltage as the battery discharges over time. The mAh rating varies with current drain and is specified for a particular current output.
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