Hi guys,
I am a noobie in robotics and I need help on servos.
I want to get position feedback from the servos. I want the position information of the servos to be directly read by the computer *(in addition of it being read internally by the control circuitry inside the servo). Is there anyway to get that information directly?
Thanks in advance for all the advice, thanks!
Unfortunately standard RC hobby servos only have an internal feedback path.
However as long as the movement of the device you are building is unobstructed and the torque ratings of your servos have not been exceeded you should be able to assume that the servo has completed the requested command.
Alternatively you could hack the servo to access the wiper pin of the pot and feed that into a high-impedance analog to digital converter to read the actual position as a voltage. Of course you would be voiding your servo warranty and hooking up a nice antenna to a rather ticklish spot.
For a real solution if you don’t mind voiding your servo warranty [and getting some groovy digital features as a bonus] you might try the Openservo project.
You will need some circuit that can translate I2C into something your PC can understand. There may be a standard tool for this.
It depends on what price you want to pay for servos. The top range robotics servos from Hitec, Kondo and Robotis have position feedback, though differing interfaces, and some non standard mounts.
The open servo board fits to standard servo cases and can be controlled by i2C using a controller or an USB to I2C converter.
Many servos especialy digital have the internal position pot as a simple potential divider, so you might bring out a single wire to an AD converter if noise can be avoided.
There was a thread somewhere for reading the position of an standard RC servo by reducing the power so it could not move then applying pulses while reading the current to find where it would try to move.
There are different threads in these forums on each of these options.
The Open Servo board will also accept standard PWM for control. In fact, you could control the Open Servo using PWM and read status information via I2C. 
VERY flexible.
8-Dale
What is the advantage to also use PWM on the Openservo, if I need I2C anyway ? Isn’t it rather slow `and a greater load on the controller by comparison ?
You could take advantage of the capabilities of a great servo controller, such as the SSC-32 if you have one, and still be able to get status information from the servo.
8-Dale
Another idea–not sure how well it will work. Attach a rotation sensor like:
trossenrobotics.com/store/p/ … ensor.aspx
to the axis of the servo and anchor the circuit board somehow. Then the sensor output should reflect rotation angle. I just ordered a couple of these sensors to give it a try.
–patrick
