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The ability to obtain the position of a servo depends on the brand of servo that you are using.   When I need feed back from a servo mechanism, I use Dynamixel  servos and I get them from Trossen Robotics.

Here is a URL to look at:
http://www.trossenrobotics.com/c/robotis-dynamixel-robot-servos.aspx?gclid=CJv0-LyYy60CFcfe4AodsE1QiA

These servos are fully programmable and you can daisy chain them together so you only have to use three wires to control multiple units.

Robot shop  http://www.robotshop.com has the necessary USB adapter to communicate directly with a chain of servos.

Hope this information is useful.

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The ability to obtain the position of a servo depends on the brand of servo that you are using.   When I need feed back from a servo mechanism, I use Dynamixel  servos and I get them from Trossen Robotics.

Here is a URL to look at:
http://www.trossenrobotics.com/c/robotis-dynamixel-robot-servos.aspx?gclid=CJv0-LyYy60CFcfe4AodsE1QiA

These servos are fully programmable and you can daisy chain them together so you only have to use three wires to control multiple units.

Robot shop  http://www.robotshop.com has the necessary USB adapter to communicate directly with a chain of servos.

Hope this information is useful.

Useful if you don't mind spending $40.00+ USD per servo; I suppose if you absolutely need these kinds of servos, you pay what you have to. Even so, for an 18 servo biped, that would run you almost $800.00 in servos alone (I was recently thinking about how cheap you could make a 20+ servo biped using standard servos, provided you could get the feedback signal from the servos). Having a way to simply and easily (without modifications or other hacks) get the position of a standard R/C servo would go a long way to making small bipeds much more affordable. So far it seems like the only way is to hack the servo (and connect to the pot), or do the current monitoring method (a much better method, but still not ideal).
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Having a way to simply and easily (without modifications or other hacks) get the position of a standard R/C servo would go a long way to making small bipeds much more affordable.
Making small bipeds is relatively cheap, if you can get someone of the opposite sex to help you. Caring for them is another matter.
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Having a way to simply and easily (without modifications or other hacks) get the position of a standard R/C servo would go a long way to making small bipeds much more affordable. So far it seems like the only way is to hack the servo (and connect to the pot), or do the current monitoring method (a much better method, but still not ideal).

Why exactly is the feedback required or needed for 'biped' control? Lets take a typical R/C servo application, rubber control on a model airplane. The command send to the servo via a continous PPM pulse width stream commands the servo to go to a specific angle and stop. When one sets up the initial servo/rubber linkage it is adjusted so that 0% command is equal to say +45 degree deflection and 100% command stick goes to -45 degree deflection, and 50% command is rubber neutral position. The servo itself is a self contained PID closed loop controller where the command sent to the servo is the desired setpoint. As long as one sets up the initial travel calibration for each biped moving servo controlled element, why go to the trouble and expense to build another redundant 'closed loop control' around the servo's control loop?

Lefty
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The issue of knowing the position of a servo is somewhat sloved by moving the servo over its desired movement range in small position incerments. the servo is usually expected to be in the location where it was sent. If the servo is not there, then there are probably non recoverable mechanical issues involved. The usual setup is move the servo until an event occurrs at which time the servo is stopped and the last position sent to the servo is used as its current position. Below is a simple terrain adaption test for a servo.


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