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Help With Robotic Project — Parallax Forums

Help With Robotic Project

dmigadmiga Posts: 6
edited 2005-02-11 17:48 in General Discussion
Hello all,

To briefly explain what I am trying to do;
Use the Stamp and Board of Education to drive an RC car DC drive motor and steering servo. The project goal is to simply drive the vehicle (autonomously) straight 10', turn right 90 degrees, drive another 6', stop and launch three raquetballs towards some (bowling) pins.

I think it would be best if I did not drive the steering servo·directly from the BOE and keep·a separate supply for the servo,etc.·The DC drive motor is powered by the RC rechargeable battery pack (tested at 7.8V and 19.5A no load). We also decided to use a spring mechanism for each ball each having a "trigger" that would be controlled by the microcontroller. The people responsible for this part of the design thought that they would use a spring loaded plunger mechanism inside a tube that will launch the ball when ready. They intended the trigger that would release the plunger assembly to be a cotter pin that would be pulled out by a small electromagnet. I purchased a ULN2308A Darlington Array to take the signal out of the microcontroller and hoped to use it to control a relay that would activate the electromagnet that would pull the trigger.

We could not get the ULN2308A to work. We tried different power supplies for the chip and never got a current out of it when the input to the ULN from the Stamp I/O was high (we used four AA batteries and then tried a 9V). We tested the outputs of the ULN and thre was no (measureable) current. We were hoping to drive a relay with it. I heard that the ULN2308A safely isolates the microcontroller signal from possibly damaging current and EMF. Also, the small trigger electromagnets that were supposed to activated by the relays were not strong enough to pull the trigger pin. We never got the relay to work with the ULN2308A but tested the mall electomagnets directly with a supply.

I realize that this is not a robotics message board but you all seem to have a lot of well rounded experience using microcontroller for projects.

I need to power the microcontroller, a steering servo, drive motor (RC battery), ULN2308 chip, and some kind of trigger or ejection mechanism for the balls.·I would like to know the best way to power everything else so that the Stamp is used to accomplish this seemingly simple task. Is it possible to tap into the RC battery supply to power the servo, ULN chip, and trigger mechanisms? Should I try to power the servo again using a 9V battery (I do not believe there is enough amperage). I believe that I need to separate the microcontroller supply from the servo supply.

As if this post is not long enough (sorry all) ... if there are any suggestions from people for a mechanism that will either pull the trigger pin for the launch mechanism or a completely different approach, I would be grateful. I thought of replacing the spring/trigger mechanism with a plunger/electromagnet system similar to pinball machines. Or maybe some self-contained device that will allow you to compress its spring and apply a voltage to release it. I thought of using servos to pull the pins too but, no matter what is decided, there is still the power issues to address.

I am obviously not and electronics guru so please be gentle with your suggestions, if you have any.

~Dave

Comments

  • BeanBean Posts: 8,129
    edited 2005-02-05 15:37
    How about using a servo to pull the pin ?

    You have to be careful using alot of different voltages too.

    I would run the micro from a seperate supply (4 AA), and everything else from the 7.2 battery back.

    Bean.
  • dmigadmiga Posts: 6
    edited 2005-02-05 15:53
    Thank you.

    Any suggestions how? I believe the servo takes 5V and low amperage, the ULN chip requires (??? never got it to work) and the trigger mechanism may require a different voltage or amperage depending on the decided solution (relay to control something, solenoids ...). I would imagine a solenoid to require a lot of amperage to get it to "kick" the ball out or pull the launch pin/mechanism.

    Dave
  • bobledouxbobledoux Posts: 187
    edited 2005-02-06 15:51
    Could you set up a servo so that as it incremented across its rotational range it fired each ball in sequence? At 30 degrees rotation ball 1 fires, 60 degrees fires ball two.....

    Some servos seem to work well up to 7.2 volts. I'd use two, one for steering and one for fire control. The solenoid could create issues with current draw as well as other technical problems. I'd try to eliminate it.

    This design could eliminate the need for the Darlington array.
  • dmigadmiga Posts: 6
    edited 2005-02-06 15:58
    Thank you for your suggestions.

    I was able to finally get the Darlington array to work properly and drive a 5V relay but your servo solution has merit.

    I am going to consult with my team members and see if we can use 1 servo (as you suggested) to pull the pins 1 at a time. That might just work!

    I was concerned about the solenoid current draw and believe the servo to be a better alternative.
  • BeanBean Posts: 8,129
    edited 2005-02-07 01:24
    Yes, because the servo has gear reduction it SHOULD draw less current. And it will certainly cause less EMF on your supply lines.
    Bean.
  • Paul BakerPaul Baker Posts: 6,351
    edited 2005-02-07 18:04
    You can also use an Opto-isolator to get the switching isolation your looking for. There are smaller solenoids which require less voltage (but still require a fair amount of amps) heres one such: http://www.allelectronics.com/cgi-bin/category.cgi?category=575&item=SOL-89&type=store , but seeing as how you will not be operating you motors when operating the solenoid this shouldn't be a problem.

    I noticed you cross posted this to the robotics forum, this is usually a·frowned upon thing. Rather than post the entire message in both forums, you may want to sub-divide the post into two posts where the specific subject is germaine to each forum.
  • dmigadmiga Posts: 6
    edited 2005-02-08 21:37
    Sorry all about the double-posting. I thought I posted it here but did not see it immediately after submitting it. Then, I saw the robotics forum and thought it might be a better place to post.
  • Paul BakerPaul Baker Posts: 6,351
    edited 2005-02-08 21:56
    No-biggie it wasn't an admonishment just a heads up. For the future, theres a means for switching the forum a post is in, the Support Forum has the post discussing it.·Its the top sticky thread and it just entails sending an email to the moderator.

    Post Edited (Paul Baker) : 2/8/2005 9:59:10 PM GMT
  • DaggsDaggs Posts: 1
    edited 2005-02-11 17:48
    dmiga said...
    ·I thought of replacing the spring/trigger mechanism with a plunger/electromagnet system similar to pinball machines.
    ~Dave
    · The kickers on pinball machine are pretty powerful.· They activate when an opto pair is blocked by the ball, sending a high voltage (I think 60v) to the solenoid.· Try this site for more info....

    http://http://www.marcospecialties.com/MODERN/items.asp?CartId={8386EAFB-D5B8-4EA2-BD65-9D32E96B013EVEREST8}&Cc=COIL-SINGLE&Bc=

    Post Edited By Moderator (IT Guy (Parallax)) : 2/11/2005 8:34:26 PM GMT
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