You see,
hobby servos are extremely easy to use but they are also pretty dumb, you would
say them something like go to position “75 degrees” and then you will have
faith that it reaches that point; but you really don’t get any confirmation
from the servo. So if you are having a situation that can induce some bias on
the position, like a lot of torque applied against the rotor, you don’t get any
clue to where it actually is (besides visual feedback when you see the robot
arm smashing somewhere it shouldn’t smash).
I decided I
want to know where the servo rotor and horn actually are located at any time.
For this endeavor I used the potentiometer that is already built in the servo.
First I
opened the servo. Then I desoldered the connections from the servo PCB to the
motor, so I could remove the PCB and gain access to the potentiometer. Once I
was able to see the potentiometer I soldered a wire to the connection point in
the middle (there are 3 connection points). I made a small notch into the servo
case to put the new wire and later I soldered again the PCB to the motor.
Finally I closed the servo.
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| Opening servo case |
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| Servo PCB just desoldered from motor connections. Note the potentiometer on the right, I soldered a line to the yellow wire. |
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| Servo closed again with the position feedback line coming from the case. |
With this
method I measured the voltage from the moving part of the potentiometer (that’s
the middle connection point) to ground. I used the analog input in the Arduino,
therefore I had to convert the analog value (a number from 0 to 1024) to a
voltage value (a number from 0 to 5V).
Also to
characterize which voltage value corresponds to each position, I took apart
each servo from the arm and swept across all positions without any load. While
I was doing this, Arduino was also sending position and measured voltage values
to my PC by serial communication. With all that information, I graphed Voltage
vs Position, and fitted a linear tendency line to the graph, the resulting
equation from the best fit tendency line will be my model of servo position.
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| Shoulder servo graph with best fit line and equation. |
Next step will
be to use this position model into my arm code and develop some closed loop control
logic to correct position based on the position feedback.
