Servo Load?

Comments

• Paul Baker Posts: 6,351

  2005-03-15 22:28 edited 2005-03-15 22:28

  At least with DC motors, the current drawn goes up when the load does, below is a link to an article describing a circuit which compares motor load with a set threshold.
  
  http://forums.parallax.com/showthread.php?p=525320
  
  I would assume that a servo would undergo a similiar surge in current during high loads, you would have to modify the circuit and likely experiment with it to make it·work for a servo, I would·input the feedback into the stamp and write software to adjust the servo's behaviour based on the signal.
• allanlane5 Posts: 3,815

  2005-03-15 23:15 edited 2005-03-15 23:15

  Yes, a servo is rated in a certain torque, typically in Ounce-Inches, or Ounce-Centimeters. (oz-cm). A 'Torque' is a certain force (weight) sitting at a certain displacement from the center of rotation (distance).
  
  Oh, but you are saying it's rotation is blocked -- how can you detect that?
  
  One way is with a rotation sensor -- if you've commanded the servo to move, and the rotation sensor (rotation encoder) says its not, then it's blocked. Another way is to monitor the current as above -- I'm sure the servo's 'blocked' current is much higher than its moving current -- like 10 mA moving, 100 mA blocked, for example.
• Beau Schwabe Posts: 6,547

  2005-03-15 23:23 edited 2005-03-15 23:23

  You can also monitor another component within the current..... What I mean is, the amount of error within the servo is proportional to the
  "pulse width" being sent to the motor. Since the current will pulsate, you can measure the width of these pulses to determine how much
  servo error there is between where you want to be vs. where you actually are.
  
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• Chris Savage Parallax Engineering Posts: 14,406

  2005-03-16 00:03 edited 2005-03-16 00:03

  ·· Speaking of rotation sensors...Allan pointed out a good way to detect the wheels stopping without having to detect current change in the supply.· Parallax actually carries an Optical Encoder designed for the BOE-Bot that does this very thing.
  
  http://www.parallax.com/detail.asp?product_id=28107
  
  ·· For non-BOE-Bot applications you could use the same principle using your own IR LED and Phototransistor, and detecting the pulses through an I/O pin.· Simple and elegant.
  
  
  
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  Chris Savage
  
  Parallax Tech Support
  csavage@parallax.com
  
  Post Edited (Chris Savage (Parallax)) : 3/16/2005 12:08:18 AM GMT
• Jonb Posts: 146

  2005-03-16 04:43 edited 2005-03-16 04:43

  Another option for encoding could be the use of Hall effect sensors( or reed switch) with small rare earth magnets. The simplest form is would be to use 1 magnet and reed switch to mark each revolution. By expecting a pre-determined value at a given speed, you can determine if there is resistance or blockage and perhaps even guage performance. I like the idea of monitoring current though.
  
  Jon