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How to choose a good servo? — Parallax Forums

How to choose a good servo?

SRLASRLA Posts: 19
edited 2005-06-17 19:02 in Robotics
Hi
I'm starting about this, and I don't have experience with motors, I need to know how to choose the correct motor to a robotic arm, I need an amplifier?, I think so, but i don't know none of Parallax, I don't know what amplifier circuit. I realy thank something like a link to a device!. Is that, I'd like to show this machine in class, I have the arm and all but "In what I must base, what i have to consider?.
Thanks

Comments

  • dandreaedandreae Posts: 1,375
    edited 2005-06-11 15:06
    If you are looking for a servo that will work with your robotic arm, I suggest using a standard servo that is rated heavy duty.· Tower Hobbies at www.towerhobbies.com ·carries a wide range of servos that are heavy duty and are compatible with the BASIC Stamp 2.· We also have a great servo controller that can control up to 16 servos.· Here is a link for more informationregarding the servo controller:· http://www.parallax.com/detail.asp?product_id=28023·.· Here is a link for information regarding the BASIC Stamp 2 starter kit:· http://www.parallax.com/html_pages/products/kits/starter_kits.asp·.



    Dave


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    Dave Andreae

    Tech Support
    dandreae@parallax.com
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    Post Edited (Dave Andreae (Parallax)) : 6/11/2005 3:15:34 PM GMT
  • allanlane5allanlane5 Posts: 3,815
    edited 2005-06-11 15:22
    When making a robotic arm, there are a couple of considerations.

    1. How much torque is needed on the 'joint' to move it? A common servo (Futaba 148) will produce 44 inch-oz of torque.

    2. How long is the arm, and how heavy is the arm. Affects the amount of torque.

    3. How much weight should the arm pick up? Again, affects the amount of torque.

    4. How will you mount the motor?

    5. How will you detect the arm position? Servo's are good for this, as they have good repeatable positioning. They use internal feedback for this, so your BS2 doesn't have to read a position.

    6. How many joints do you have? This affects the number of servos you need. This is sometimes called 'degrees of freedom'.
  • SRLASRLA Posts: 19
    edited 2005-06-11 20:53
    thank Dave, this PSC or Parallax Servo Controller can control any servo?, because I read that to control the most of motors with Basic Stamp i would need an "interface's circuitry" but doesn't name none, then, is this PSC that circuit i need in combination with BS2 + a servo?.

    Hi Allanlane5
    I tell you, for example, if i've got an robotic arm with 5 axis to place motors that weights 1,3kgs without servos, almost 2kgs with this, and I'd like to pick up with this 3kgs, i must choose a servo with more torque than the weight of the arm loaded (5kgs)?. What happen if i use a motor with inferior or the same force?. OK thanks

    Carlos
  • allanlane5allanlane5 Posts: 3,815
    edited 2005-06-12 12:40
    Well, if you use a motor which cannot put out enough torque to move the weight, then the weight does not move. The motor sits there and hums and gets warm.

    Also, a "Servo" is a neat device for $20 or so which contains a motor, some reduction gears, a comparitor circuit, and a variable resistor connected to the gears. It is not itself a 'Motor', though it can be used in a robot drive train. It is particularly easy for a BS2 to control through the "PULSOUT" command.

    A 'motor' is typically a small device which you put +5 volts on the 'hot' lead, ground on the 'neutral', and it spins. A much more primitive device. To run it with a BS2 takes an 'H-Bridge', which allows the BS2 to switch on +5 and ground in both directions. Then you want speed control. This requires a "Pulse-Width-Modulated" signal (basically, switching power on and off quickly) to control the speed.

    Then, there are "stepper motors". These have multiple windings. Each time you power one set of windings, the motor 'jumps' forward a little bit. Then you power the next set of windings to take the next 'step'. Keep powering set after set after set, and the motor continues to rotate.

    So, to summarize: The PSC is an external board that lets you control multiple Servo's -- ONLY servo's.
    A 'Motor' requires an H-Bridge and a PWM circuit -- typically these are combined in to a 'motor controller board'.
    A 'Stepper' -- well, I think you can control a stepper with a few transistors.

    And if your motor or servo does not have enough torque, it can't move the load.
  • dandreaedandreae Posts: 1,375
    edited 2005-06-12 14:43
    SRLA said...
    thank Dave, this PSC or Parallax Servo Controller can control any servo?, because I read that to control the most of motors with Basic Stamp i would need an "interface's circuitry" but doesn't name none, then, is this PSC that circuit i need in combination with BS2 + a servo?.

    ····· The Parallax servo controller can control servos that are rated from 4v to···· 7.5v in general, make sure to use a supply that is adequate or else it may damage your servos.· We like to the 7.2v RC batteries, we find that they work best.

    Yes, a BASIC Stamp 2 and a PSC will allow to control your robotic arm with the proper servos as Alan has pointed out.· Parallax's standard servo is the Futaba· S-148.
    Dave

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    Dave Andreae

    Tech Support
    dandreae@parallax.com
    [noparse][[/noparse]url]Http://www.parallax.com[noparse][[/noparse]/url]

    ·
  • SRLASRLA Posts: 19
    edited 2005-06-17 18:38
    Hi
    I guess that to buy a 'servo' is better, yeahh. If "I connect the BS2 with PSC 'by means of the protoboard' to control a Servo with enough torque", say me please, Can I connect directly the BS2 module to computer or to a home power supply and it will not damage the servos or circuits?. Then, when I'm using the circuits without the computer, I must use a battery between 5 and 7V, no more truth?, OK ^_^, now I will try to construct this on this way. But I believe to begin I prefer to cut more aluminium and to model more acrylic putty and glass fiber to make a robotic leg. I'm watching that i need two servos only, a strong one and a more weak one, if everything comes out right, i will show you it.

    Carlos.
  • Rick KRick K Posts: 18
    edited 2005-06-17 19:02
    Look here for an explanation of torque. An understanding of this will save you some time, trouble and money on purchasing the wrong servos/motors or burning out the ones you purchase by stalling them.

    If you're just experimenting, you can absolutely build things light, use any decent servo and if it doesn't lift the weight of the mechanism itself, get a more powerful servo, but an understanding of the variables your working with is here in a mix of scientific and layperson's language.

    This becomes much more important if you're trying to design the machine for a specific application (i.e. pick up a soda can).

    http://www.solarbotics.net/bftgu/tutorials_mech_torque.html
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