Need help reversing current

Discussion in 'The Projects Forum' started by HunterIsGr8, Sep 23, 2008.

  1. HunterIsGr8

    Thread Starter New Member

    Sep 23, 2008
    4
    0
    Hi, i'm new to this board and to circuits. i am trying to control 6 different arms of my "crabot" (it's a robot that walks like a crab) but it's only a small project between me and a few friends so i'm trying to keep it low budget.

    i found small actuators that are used to control automatic car door locks and i figured they would work for extending the legs, but the trouble is that to open they need current one way and to close the current needs to be reversed. i'm sure this is a simple thing for the most of you but the way i drew it out didn't work, there are 2 wires coming out of the actuators. lets call them a positive and a negative. the way i thought would be easiest would be to connect the positive from the actuator to both the positive and negative of the battery, with a transistor acting as a switch on the positive side, and then the same coming from the negative wire of the actuator. the problem is that when i turn it on there is a direct path from bat+ to bat-. i was wondering if there is a one way valve for electricity. because that would make sure the grounds didnt act as a route to short. i dont know what to call it really and i'm sure its in a previous post but i couldnt find anything.

    i have to wire 10 motors total. all have to have reversible current. i am eventually going to hook them all through a serial port. but i will cross that bridge when i get there.

    any help would be much appriciated.
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,347
  3. KMoffett

    AAC Fanatic!

    Dec 19, 2007
    2,574
    230
    Google: H-bridge
    This is the usual circuit in robotics to control forward, reverse, brake, and coast of motors.

    Ken
     
  4. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    At the risk of being repetitive, I too recommend that you explore the use of H-bridge drivers. You may also need to explore the topic of pulse-width modulation (PWM) since you will probably need to manage the speed of these motors.

    hgmjr
     
  5. HunterIsGr8

    Thread Starter New Member

    Sep 23, 2008
    4
    0
    thank you, that is exactly what i was looking for. do the solid state H switches allow me to control the motors with low voltage? and most importantly, where would you suggest buying components from? thank you very much for your help!
     
  6. scubasteve_911

    Senior Member

    Dec 27, 2007
    1,202
    1
    Be careful with the mechanical design. Depending on the construction of the doorlock, you might run into trouble. This is if there is no mechanical locking action after the solenoid. Solenoids have tremendous holding current, which will drain your batteries quickly. So, if the plunger is being held in an energized state for any length of time, your power draw can be very high. Also, these solenoids are not meant for continuous duty, otherwise they will overheat.

    If you require the actuator to be held at various points, you should consider an acme leadscrew or trying to keep it rotary and use a servo. Or, a pneumatic system might be good here.

    Steve
     
  7. HunterIsGr8

    Thread Starter New Member

    Sep 23, 2008
    4
    0
    The actuators i was looking at were based on an electric motor that will spin a bunch of reducing gears and then finally a stick with teeth on it (sorry i dont know what to call most of this stuff) so i wont have to worry about overloading them. however i still am going to have a big concern for overheating since i dont think they were meant to put up with as much torque as i plan on giving them. the only problem is cost. how much would i be looking at for pnumatic actuators?
     
  8. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    You really don't want to get into using pneumatic or hydraulic actuators. That would mean needing to add an air compressor to the load, along with an air tank, or a large air tank to omit the compressor. That would add a lot of bulk and weight; in the case of an air compressor would require a lot more electrical power, which means battery weight. Things would get spiraled out of control in a hurry, as you'd need larger actuators to carry the extra weight, which would require more air to operate. Small pneumatic actuators aren't very expensive (perhaps $8 each) but when you get into the larger versions, suddenly you're spending $50 or much more apiece.

    As far as your electric actuators, you haven't mentioned anything about how much voltage and current they require.

    If the requirements are low (less than 1 amp), you could look at L293 IC's; which are integrated H-bridge drivers.
    If the requirements are moderate (a few amps) you could look at using L297 and L298 IC's; the L297 drives the L298.
    If you have heavy-duty requirements (up to 30A, up to 41V) you could look at a VNH2SP30-E IC, which is an Automotive Fully Integrated H-Bridge motor driver.

    You could build your own H-bridge driver boards if you wish. There are a number of open-source H-bridge drivers out there. The Linistepper is one:
    http://www.piclist.com/tecHREF/io/stepper/linistep/index.htm
    However, kits to build them are $30/ea if you buy three. The VNH2SP30-E IC is around 1/3 that cost, and is completely integrated into a single IC.
     
  9. scubasteve_911

    Senior Member

    Dec 27, 2007
    1,202
    1
    Yeah, pneumatics have their place, not so much on smaller robots.

    Automotive stuff is usually designed knowing that they can hog a lot of current, especially on a short duty cycle. You definitely need to calculate your power budget carefully. If it is based upon a motor, then you should be okay to run it. I was worried about it being solenoid-based.

    How much power do you need from these? How big is this robot going to be?

    Steve
     
  10. HunterIsGr8

    Thread Starter New Member

    Sep 23, 2008
    4
    0
    i'm going for a fairly large robot, the highest point will be about 12 inches tall. front to back maybe about 3 feet. i will be able to change the load of torque on the motors by changing the distance of each step so hopefully i wont burn them all out. i dont really know how i'm going to power them. since it goes to a car i am going to assume it runs on 12v, i dont really know how many amps. they look like pretty small motors and theres enough gear reduction that i think it would have enough power even with low amps. so i think i'm going to try for a drill battery. i had one lying around last night and i tried to test it with a cheap multimeter but it overloaded it on the ampes. so i guess i could try the L297 and L298 IC's and fuse off the source at 2 ampes. are there some other types of batteries that you guys would suggest for this purpose. i figure i need to run at most 6 motors at once. all at atleast an amp. i'll work on drawing up a schematic to this after work tonight, thanks again for all the help.
     
  11. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
    1,438
    368
    central locking solenoids will not work well in this application as they are only designed to be powered occasionally for about a second and consume quite a lot of current when stalled. If you apply power for longer in order to hold the position the motor will burn out.

    A better option is RC servos which have built in motor drivers and position feedback.
     
Loading...